Polymer engineering based on reversible covalent chemistry: A promising innovative pathway towards new materials and new functionalities

Progress in Polymer Science 80 (2018) 39–93 Contents lists available at ScienceDirect Progress in Polymer Science journal homepage: www.elsevier.com...

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Progress in Polymer Science 80 (2018) 39–93

Contents lists available at ScienceDirect

Progress in Polymer Science journal homepage: www.elsevier.com/locate/ppolysci

Polymer engineering based on reversible covalent chemistry: A promising innovative pathway towards new materials and new functionalities Ze Ping Zhang a , Min Zhi Rong b,∗ , Ming Qiu Zhang b,∗ a Key Laboratory for Polymeric Composite and Functional Materials of Ministry of Education, GDHPPC Lab, School of Chemistry, Sun Yat-sen University, Guangzhou 510275, China b Materials Science Institute, Sun Yat-sen University, Guangzhou 510275, China

a r t i c l e

i n f o

Article history: Received 12 January 2017 Received in revised form 24 March 2018 Accepted 28 March 2018 Available online 31 March 2018

a b s t r a c t Reversible covalent polymers are able to change their bond arrangement and structure via reversible reaction triggered by external stimuli including heating, light and pH, while retaining the stability of irreversible covalent polymers in the absence of the stimuli. In recent years, more and more research has been devoted to utilization of reversible covalent bonds in synthesizing new materials, which not

Abbreviations: 4-OH-TEMPO, 4-hydroxy-2,2,6,6-tetramethylpiperidinyloxy; AAPBA, N-acryloyl-m-aminophenyl boronic acid; ABS, acrylonitrile-butadiene-styrene copolymer; AFM, atomic force microscopy; AIBN, 2,2-azodiisobutyronitrile; AOBA, 4-allyloxybenzaldehyde; APBA, 4-aminophenylboronic acid; APS, ammonium persulfate; ARES, advanced rheology expanded systems; ATRP, atom transfer radical polymerization; BADGE, bisphenol A diglycidyl ether; BFDGE, bisphenol F diglycidyl ether; BGPDS, bis(4-glycidyloxyphenyl) disulﬁde; BMA, butyl methacrylate; BMD, 1,12-bis(maleimido)dodecane; BME, 1,2-bis(maleimido)ethane; BMFDMS, bis(2-methoxyfuran) dimethylsilane; BMH, 1,6-bis(maleimido)hexane; BPO, benzoyl peroxide; BSPBA, 4-(3-butenylsulfonyl) phenylboronic acid; CA, citric acid; CANs, covalent adaptable networks; CBA, 4-carboxybenzaldehyde; CCS, core-cross-linked star polymer; CFRCs, carbon ﬁber reinforce composites; CLCPs, crosslinked liquid-crystalline polymers; CPBA, 4-carboxyphenylboronic acid; DA, Diels-Alder; DABBF, diarylbibenzofuranone; DART, Diels-Alder reversible thermoset; DATBDS, 1,3-diacetoxy-1,1,3,3-tetrabuty l-distannoxane; DBTDL, dibutyltin dilaurate; DBU, 1,8-diazabicyclo(5.4.0)undec-7-ene; DCDC, double cleavage drilled compression; DCM, dichloromethane; DETA, diethylene triamine; DHPM, N-(2,3-dihydroxypropyl) maleimide; DIW, direct-ink-writing; DMA, dynamic mechanical analysis; DMF, N,N-dimethylformamide; DMSO, dimethyl sulphoxide; DPDS, diphenyldisulﬁde; DPMBM, N,N’-4,4 -diphenylmethane-bismaleimide; DPOBM, N,N’-4,4 -diphenyloxide bismaleimide; DTDA, dithiodianiline; DTDB, dithiodibutyric acid; EG, ethylene glycol; ENR, epoxidized natural rubber; EPM, ethylene-propylene rubber; EPM-g-MA, ethylene-propylene rubber grafted with maleic anhydride; ESO, epoxidised soybean oil; ESR, electron spin resonance; FA, furfurylamine; FAol, furfuryl alcohol; FF, furfural; FFF, fused ﬁlament fabrication; FGE, furfuryl glycidyl ether; FM, furfuryl mercaptan; FMA, 2-furfuryl methacrylate; FTIR, Fourier transform infrared spectroscopy; GC, gas phase chromatography; HAc, acetic acid; HBA, 4-hydroxybenzaldehyde; HBPU, hyperbranched polyurethane; HDI, hexamethylene diisocyanate; HDPE, high-density polyethylene; HEBFMC, 1,6-hexamethylene-bis(2-furanylmethylcarbamate); 1 H NMR, proton nuclear magnetic resonance; HPLC, high performance liquid chromatography; IPDI, isophorone diisocyanate; LDPE, low-density polyethylene; LLDPE, linear low-density polyethylene; LMA, lauryl methacrylate; MAAPBA, 4-methacryloyl-m-aminophenylboronic acid; MA-Cu, copper(II) methacrylate; MDI, diphenylmethane diisocyanate; ME, 2-mercaptoethanol; MHHPA, methylhexahydrophthalic anhydride; MMA, methyl methacrylate; MPDBMI, 1,5-bis(maleimido)-2-methylpentane; NMRP, nitroxide-mediated radical polymerizations; PA, polyacrylate; PB, polybutadiene; PBA, poly(n-butyl acrylate); PBAG, poly(1,4-butylene adipate glycol); PBPSF2 , furyl-telechelic poly(1,4-butylene succinate-co-1,3-propylene succinate); PBS, poly(butylene succinate); PCL, poly(␧caprolactone); PCMS, poly(4-vinylbenzyl chloride); PCO, polycyclooctene; PDCP, poly(dichlorophosphazene); PDMS, poly(dimethyl siloxane); PDS, polydisulﬁde; PEA, poly(ethylene adipate); PEAm, polyetheramine; PEG, poly(ethylene glycol); PEGMA, poly(ethylene glycol) methacrylate; PEO, poly(ethylene oxide); PEO-PPO-PEO, copolymer of PEO and PPO; PETMP, pentaerythritol tetra(3-mercaptopropionate); PFMES, poly(2,5-furandimethylene succinate); PHAEs, poly(hydroxyaminoethers); PHUs, poly(hydroxyurethanes); PI, polyimine; PK, polyketone; PLA, poly(lactic acid); PMDETA, N,N,N’,N”,N”-pentamethyldipropylenetriamine; PMMA, poly(methylmethacrylate); POSS, poly(hedral oligomeric silsesquioxane); PPDO, poly(p-dioxanone); PPDO-PTMEG, poly(p-dioxanone)-poly(tetramethylene oxide); PPG, poly(propylene glycol); PPO, poly(propylene oxide); PR, polyrotaxane; PS, polystyrene; PS-b-PEG, polystyrene-block-poly(ethylene glycol); PSOE, polyspiroorthoester; PSOE-co-PAN, copolymer of PSOE and PAN; PTIL, poly(1,2,3-triazolium ionic liquid); PU, polyurethane; PUU, poly(urea-urethane); PVA, poly(vinyl alcohol); PVC, polyvinyl chloride; PVF, poly(vinyl furfural); PVU, poly(vinylogous urethane); PyPBA, 4-pyridineboronic acid; RAFT, reversible addition-fragmentation chain transfer polymerization; ROMP, ring-opening metathesis polymerization; Ru, catalyst ruthenium complex; SBS, poly(styrene-butadiene-styrene); scCO2 , supercritical carbon dioxide; SEM, scanning electron microscope; SLS, selective laser sintering; SMASH, shape memory assisted self-healing; SMPs, shape memory polymers; SMPU, shape-memory polyurethane; SOE, spiroorthoester; TAEA, tri(2-aminoethyl)amine; TBD, triazabicyclodecene; TBP, tri-n-butylphosphine; TDS, thiuram disulﬁde; TEA, triethylamine; TEMED, N,N,N’,N’-tetramethylethylenediamine; TEMPO, 2,2,6,6-tetramethylpiperidine-1-oxy; TFA, triﬂuoroacetic acid; TFPME, tris[(4-formylphenoxy) methyl]ethane; THF, tetrahydrofuran; TMAS, tetramethylammoniumsilanolate; TMEA, tris(2-maleimidoethyl)amine; TMPCA, 2,2,6,6-tetramethylpiperidinyl carboxamide; TPA, terephthaldehyde; Tri-HDI, tri-functional homopolymer of hexamethylene diisocyanate; TsOH, 4-methylbenzenesulfonic acid; TTC, trithiocarbonate; UV–vis, ultraviolet-visible spectroscopy; VBA, 4-vinylbenzaldehyde; VCR, vulcanized chloroprene rubber; VPB, vulcanized polybutadiene; VPBA, 4-vinylphenylboronic acid; WLF, Williams-Landel-Ferry. ∗ Corresponding authors. E-mail addresses: [email protected], [email protected] (M.Q. Zhang). https://doi.org/10.1016/j.progpolymsci.2018.03.002 0079-6700/© 2018 Elsevier B.V. All rights reserved.

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Z.P. Zhang et al. / Progress in Polymer Science 80 (2018) 39–93

Keywords: Dynamic covalent chemistry Reversible covalent polymers Polymer engineering Adaptivities Application

only overcomes disadvantages of permanent covalent polymers, but also brings in new functionalities. More importantly, a series of novel techniques dedicated to polymerized products with features such as properties regulation, self-healing, reprocessing, solid state recycling, and controllable degradation are developed, heralding the opportunity of upgrading of traditional polymer engineering. Although the exploration of this emerging topic is still in its infancy, the advances so far are encouraging and clearly directed to large scale applications. This review systematically outlines this promising trend, following a bottom-up strategy, taking into account both theoretical and experimental achievements. It mainly consists of four parts, involving design and preparation: (i) the basis of reversible covalent chemistry, (ii) rheology of reversible covalent polymers, (iii) methods of construction of reversible covalent polymers, and (iv) smart, adaptive properties offered by reversible covalent chemistry. The key elements for realizing reorganization of polymers containing reversible covalent bonds are covered. The advantages and weaknesses of representative reaction systems are analyzed, while the challenges and opportunities to engineering application of the equilibrium control based on reversible covalent chemistry for producing end-use polymers are summarized. In this way, the readers may grasp both the overall situation as well as insight into future work. © 2018 Elsevier B.V. All rights reserved.

Contents 1. 2.

3.

4.

5.

6.

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40 Reversible covalent chemistry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42 2.1. Thermodynamic and kinetic characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42 2.2. Reversible covalent chemistry involved in polymer solids . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46 Rheological properties of reversible covalent polymers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 48 3.1. Polymers with general reversible covalent bonds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 48 3.2. Polymers with dynamic reversible covalent bonds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49 Strategies of preparation of polymers containing reversible covalent bonds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52 4.1. From reversible moieties-containing macromolecules and monomer linkers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52 4.1.1. Reversible cycloaddition (DA reaction) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52 4.1.2. Reversible condensation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 56 4.2. From reversible moieties-containing macromolecules . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 56 4.2.1. Reversible cycloaddition (DA reaction) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 56 4.2.2. Reversible condensation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 57 4.2.3. Redox reaction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 59 4.2.4. Radical crossover exchange of reversible bonds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 59 4.3. From multi-functional monomers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 59 4.3.1. Reversible cycloaddition (DA reaction) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 59 4.3.2. Reversible condensation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61 4.4. From monomers containing reversible linkages . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63 4.4.1. Click chemistry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63 4.4.2. Step-growth addition polymerization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 64 4.4.3. Radical polymerization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 65 4.4.4. Controlled/“living” radical polymerization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 65 Polymer engineering driven by reversible covalent chemistry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 65 5.1. Properties regulation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 66 5.1.1. Mechanical properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 66 5.1.2. Functional properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 67 5.2. Intrinsic self-healing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 67 5.2.1. Healing based on general reversible covalent reactions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 69 5.2.2. Healing based on dynamic reversible covalent reactions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 72 5.3. Improvement of processability . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 74 5.3.1. Orientation, shape memory and welding after crosslinking . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 74 5.3.2. 3D printing and 3D photolithography . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 76 5.4. Recycling in bulk state . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 77 5.4.1. Recycling based on reversible addition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 77 5.4.2. Recycling based on reversible exchange . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 77 5.5. Controllable degradation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 81 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 85 Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 86 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 86

1. Introduction Polymer engineering plays a major role in advancement of polymer products and end-user applications. As a key aspect, polymer processing is responsible for converting polymers into ﬁnally ﬁn-

ished products with desired structure and properties [1,2]. There are different processing techniques speciﬁed for different polymers according to their ﬂow behaviors. For thermoplastics, which can be softened or melted by heating, solidiﬁed by cooling and remelted repeatedly because of the linear macromolecules, extru-

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Fig. 1. Examples of polymer engineering enabled by reversible covalent chemistry. (Note: Reversible covalent chemistry, also called dynamic covalent chemistry, refers to the chemical reactions carried out reversibly under thermodynamic controls).

sion, injection, blow molding and calendaring have been widely used. With respect to thermosets, which are shaped accompanying by crosslinking reaction and cannot be softened by heating for reprocessing due to the creation of permanent three-dimensional networks, compression molding, resin transfer molding, liquid casting, etc. are developed. The rapid growth of polymer industry has led to the fact that production and consumption of polymers in volume surpassed those of steel in early 1980s [1]. The consequent challenge is their recycling facing the crisis of increasing environmental pollution and decreasing fossil resources. Comparatively, recovering waste thermoplastics into useful materials is easier than thermosets because of their thermal plasticity, while the latter can only be treated by mechanical grinding, pyrolysis, solvolysis, etc., which are either environmentally unfriendly or energy intensive [3,4]. Meantime, the traditional processing techniques are gradually unﬁt for the requirements of modern society. When thermoplastic powders are handled by additive manufacturing through selective laser sintering, for example, strength of the products has to be low as the mechanism of compaction is incompatible with 3D printer. It remains unsolved that how to prepare high strength polymer bulk by 3D printing in the absence of in-situ pressurization. It is worth noting that the existing polymer engineering basically operates in line with the polymers assembled by irreversible covalent bonds. The polymers are thermodynamically more stable than the reactants (e.g., monomers) and the reverse reaction could not occur, which deﬁnes the advantages and disadvantages quoted above. In case reversible covalent bonds are introduced, however, polymers can be cleft and re-bonded in a controlled manner [5] because the free energy difference isolating the reactants from bonded product approaches to equilibrium. The boundary between thermoplastics and thermosets no longer exists to a great extent in this context. An entirely new discipline is evolving out of classic polymer engineering accordingly.

Interestingly, polymer networks built up by reversible covalent crosslinks acquire adaptivity to external stimuli [6–8], which differ from traditional covalently crosslinked polymers. More importantly, the polymer structure made of reversible covalent bonds is able to chemically respond to the applied stimuli even in solid state. Proactive manipulation of the advantageous stimuli-responsivities in turn has led to a series of new prototype techniques having potential for replication and large scale impact. As revealed by recent progresses [9–15], not only reprocessing, reshaping and recycling of traditionally non-reworkable thermosetting polymers, but also self-healing, structural modiﬁcation, processing of difﬁcult-processing materials of thermosetting and thermoplastic polymers are enabled mostly in solid phase without the need of solvent (Fig. 1). It is clear that these processes are energy saving and environmentally friendly as determined by the underlying mechanism. Furthermore, they are reversibly correlated with each other, so that life circle of synthetic polymers can be greatly extended. The new possibilities go beyond the scope of classic polymer engineering and enrich the measures of material diversiﬁcation. The hard technical problems (like reclaiming of scrapped vulcanized rubber [16,17] and in-situ strengthening of 3D printed items [18]), which are difﬁcult to be solved or cannot be solved by traditional approaches, would hopefully be solved with continuous improvement of the corresponding technologies. Eventually, sustainable development of the society is favored in the long run. In fact, processing based on reversible reaction of covalent bonds is different from the conventional reactive processing [19,20] and solid-state processing [21–23]. Reactive processing refers to shaping of polymers during their chemical formation associated by drastic increase in system viscosity. Liquid-to-solid transformation has to take place. As for solid-state processing, it involves either cold compaction of powdered thermoplastic polymers followed by sintering above their ﬂow temperatures [21], or extrusion just below the melting temperature of crystalline thermoplastics (or just above the glass transition temperature of amorphous thermo-

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Fig. 2. A schematic diagram showing the structural framework of this review.

plastics) [22,23]. Physical interaction rather than chemical reaction accounts for binding the polymer particles. Although the polymer engineering enabled by reversible reaction is taking shape, showing great potential of development and beneﬁts for next-generation industry, to the best of our knowledge, there has not yet been a review article combining the related topics together from an application perspective, except for quite a few high-level recent reviews on a number of subtopics [7,8,12,14,15]. Keeping this in mind, the authors collect the related publications appearing in scattered places, comprehensively clarify the fundamentals of reversible covalent chemistries, analyze the rheological property of reversible covalent polymers, summarize the ideas and methods of incorporation of reversible covalent bonds into polymers, discuss the derived new techniques and applications, and identify challenges, trends and future directions in this emerging area (Fig. 2). It is hoped that this paper will arouse increasing attention to the promising aspect, and more and more researchers will join the quest. 2. Reversible covalent chemistry 2.1. Thermodynamic and kinetic characteristics All the chemical reactions can be regarded as reversible ones in principle, but their degrees of reversibility are actually quite different, which are generally quantiﬁed by thermodynamic (or standard) equilibrium constant, K␪ . That is, when K␪ is in the range from 10−7 to 107 , the reaction is regularly referred to as reversible [24]. In the case of either K␪ > 107 or K␪ < 10−7 , the reaction is assumed to be irreversible, which occurs only unidirectionally as characterized by full consumption of reactants. However, there are exceptions to this empirical rule, such as alkylamine ﬁssion/recombination to be discussed later. For reversible reactions (CAB), as shown in Fig. 3a, products conversion macroscopically terminates when the system reaches equilibrium and the product distribution is determined by thermodynamic stabilities of the products. The main product possesses higher thermodynamic stability. Supposing the changes of standard entropy, S␪ , of parallel reactions are similar, the products ratios are approximately proportional to the corresponding K␪ . Here in

Fig. 3a, A gives B rather than C because of the higher relative stability of B (−GB > − GC ). Evidently, it is a thermodynamically controlled product. With respect to irreversible reactions (C ←− A → B), the products cannot be reversed once generated and the products ratios are proportional to the rate of formation. Product distribution depends on the relative difference between Gibbs free energies at the initial state and transition state. The lower energy barrier of the transition state, the faster the reaction. Since the energy barrier of the transition state [AC]* for the creation of C (GC ∗ ) is lower than that of transition state [AB]* for the creation of B (GB ∗ ), C becomes the main product, which is kinetically controlled (Fig. 3a). Accordingly, reversible covalent bonds can be deﬁned as a kind of covalent bonds being able to take part in reversible breakage and reformation. They combine the merits of the thermodynamically controlled synthesis, reversibility of non-covalent bonds, and the robustness of covalent bonds [25]. Under certain conditions, a thermodynamic equilibrium is allowed to be established between the reactants and products bearing reversible covalent bonds. Additionally, the breaking and reconstruction of reversible covalent bonds are thermodynamically driven and would respond to external stimuli (e.g., heat, light, pH, etc.), leading to shift of the involved equilibrium towards the selected products. In general, the relationship between thermodynamic equilibrium constant, K , and temperature, T, obeys Gibbs-Helmholtz equation: lnK = −H /RT + S /R

(1)

where H and R denote standard enthalpy change and gas constant, respectively. Meanwhile, the temperature dependence of rate constant, k, meets Arrhenius equation: lnk = lnA − E a /RT

(2)

For the exothermic reversible reactions (Diels-Alder (DA) reaction, for example), increasing temperature results in a rise of rate constant and a reduction in equilibrium constant (Fig. 3b), driving the equilibrium shifts to the inverse direction (endothermic). Eq. (2) also reveals that decreasing the activation energy, Ea , would effectively promote the reaction. A few slow reversible reactions, even those could not operate under general conditions (e.g. oleﬁn

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43

Fig. 3. (a) Free energy proﬁle of irreversible and reversible reactions under dynamic and thermodynamic controls, respectively. (b) Dependences of chemical equilibrium constant and reaction rate constant on reaction temperature.

metathesis) [26,27], can be conducted by incorporation of catalysts to reduce the activation energy, which makes the reaction more quickly under mild condition. Nevertheless, catalysts cannot change the initial and ﬁnal states of the reaction, so that the reaction free energy remains the same. In other words, catalysts can only deal with dynamics of the reaction path, and have nothing to do with the thermodynamic issues such as degree and direction of reaction. Reversibility of a chemical bond is determined by its reactivity for forward and reverse reactions. Reversible covalent bonds must be able to be cleft into reversible groups under certain conditions, while the reversible groups should also be active enough to be recombined together. For the bonds hard to take part in reversible reaction, structural design via introducing steric hindrance and electronic effect is an effective means to adjust their dissociation energy and reversibility, in addition to decreasing reaction activation energy. For example, Cheng et al. [11] proved that large steric substituents weakened the bond energy of C N bond making urea bond dissociable under a mild condition (37 ◦ C). Furthermore, the higher reactivity of the produced isocyanate and hindered secondary amine allowed them to reform urea bond. Nevertheless, the similar hindered amide bond remained difﬁcult to be reversible as the transient intermediate ketene resulting from ammonolysis was very active [11,28]. Miura et al. [29,30] found that the nitroxide radical bearing bulky spiro structures reduced bond energy of C ON and raised the equilibrium constant for homolytic dissociation/recombination. Otsuka et al. indicated that dynamic reversible equilibrium can even be established between carbon radical and the stable C C bond by using large steric hindrance [31–33]. However, excess spatial repellent is not good for the generation of reversible bonds. Triphenyl methyl radical, for example, has a “windmill” shape because the hydrogen atoms on phenyl ring repel each other [34]. It is very stable at room temperature. Its dimerization is an endothermic process according to theoretical estimation [35], so that the stable C C bond would be hard to be formed. Electronic effect is another important factor related to equilibrium of reversibility and stability of chemical bonds. McElhanon, Wheeler and Gandini [36,37] suggested that both dienophiles with electron-attracting groups (like furyl) and dienes with electrondonating groups (like maleimide) beneﬁt the creation of DA bonds. Zhang et al. [38–40] manipulated homolysis temperature of C ON bond through the electron-withdrawing effect of neighbouring substituents. The results revealed that with enhancement of the electron-withdrawing effect (e.g., from amido to cyano), homolysis temperature of C ON bond was decreased. On the other hand, stability of the C N bonds (i.e. imine, acylhydrazone and oxime bonds)

derived from condensation between aldehyde and primary amine, acylhydrazine and hydroxylamine is closely dependent on the conjugative effect of the adjacent groups. This is owing to the electron delocalization of oxygen and nitrogen, which greatly improves hydrolytic stability of the corresponding C N bonds [41]. In fact, the higher hydrolytic stability of aromatic imine bond (Schiff base bond) and aromatic boronic ester bond than the aliphatic counterparts proﬁts from the electron delocalization effect of benzene [42,43]. In brief, from the point of view of thermodynamics and kinetics, in order to develop reversible covalent polymer materials with engineering value, the reversible covalent bonds should meet the following requirements. (i) The forward and reverse rate constants are relatively fast, helping the realization of reversible reaction on a reasonable time scale. (ii) The dynamic reversible equilibrium inclines to the direction of combination in favor of formation of high molecular weight polymers. Cheng et al. [11] studied dynamic urea bonds and demonstrated that the urea bonds with large equilibrium constant (e.g., 7.9 × 105 s−1 ) and dissociation constant (e.g., 1.2 × 10−5 s−1 ) were conducive to the realization of catalyst-free dynamic properties. Guan et al. [44] also pointed out the importance of kinetics of boronic ester bonds. Compared to the boronic ester bond with slower exchange speed (1.2 × 10−2 s−1 ), the system with higher exchange speed (3.0 × 103 s−1 ) exhibited improved reversibility. Unlike the continuous equilibrium of supramolecular interaction [45], the triggerable reversibility of reversible covalent bonds suggests that the bonds could maintain their stability under ordinary circumstances in the absence of stimuli. The relatively strong covalent bonds further provide polymer materials with more superior mechanical properties. Besides, the resultant polymer system can gradually adjust the structure in virtue of crossover component recombination nature of reversible covalent reaction, and ultimately achieve the most stable thermodynamic state. So far, reversible covalent reaction could be divided into two categories (which may be related to the dissociative/associative mechanisms in covalent adaptable networks (CANs) [7,8,12]): general reversible covalent reaction (e.g., reversible addition, reversible condensation, reversible redox, etc.) and dynamic reversible covalent reaction (e.g., reversible exchange, reversible ﬁssion/recombination, etc.), as shown in Tables 1 and 2. The detailed thermodynamic and dynamic parameters of representative reversible covalent chemistries are summarized in Table 3. The key difference between these two groups of reversible covalent reactions lies in the fact that the types of starting reactants of the forward and backward reactions are different in the case of

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Table 1 General reversible covalent reactionsa . Reaction

Refs.

Reversible addition

DA reaction

[46–49]

Urazole formation

[50]

Urea formation

[11]

Reversible condensation

Imine bond formation

[42]

Acylhydrazone formation

[51,52]

Oxime formation

[53]

Aminal formation

[54]

Acetal formation

[55]

Aldol formation

[56]

Ester formation

[57]

Amide formation

[58]

Boronic ester formation

[59]

Others Disulﬁde bond formation a

[60]

Kc , concentration equilibrium constant. k1 , forward rate constant. k-1 , reverse rate constant.

general reversible covalent reactions. It means that at most of the time the forward and backward reactions are carried out under different conditions (temperature, pH, etc.). For instance, DA reaction involves a [4 + 2] cycloaddition between an electron-rich diene (e.g. furan) and an electron-poor dienophile (e.g. maleimide) to form a stable cyclohexene adduct at about 60 ◦ C, whereas the retro-DA uncoupling reaction becomes preponderant at a temperature above about 110 ◦ C [46]. In contrast, the forward and reverse reactions of a dynamic reversible covalent reaction occur under the stimulus of a single triggering factor, without completed dissociation of the reversible covalent bonds. During the crossover exchange reaction between the same dynamic reversible covalent bonds like ester bonds, for example [75,80], rate constant of the forward reaction equals to that of the reverse reaction as both reactants and products belong to the same species [7]. Variation under thermodynamic conditions (e.g., temperature) could only equally change the reaction rate (kinetics) without affecting the distribution of products. As for the equilibrium of homolysis and recombination of dynamic reversible covalent bonds represented by C ON bonds, change in temperature would lead to shift of the equilibrium (as the reaction

is endothermic as shown by Fig. 3b), but the dynamic equilibrium always tends to combination as the stable nitroxyl radicals produced by homolysis strongly desire to trap transient carbon radicals. Accordingly, the equilibrium constant of alkoxyamine ﬁssion is as low as 3.14 × 10−14 –7.45 × 10−8 [113–116] (Table 3) because of the very fast reverse rate, which does not obey the criterion mentioned at the beginning of this subchapter. Nevertheless, its homolysis rate (10−5 –10−3 s−1 [113–116]) is comparable to the forward reaction rates of other dynamic reversible covalent reactions [110,111] (Table 3). The different reversible habits of the covalent reactions discussed above would result in different engineering behaviors of polymers, while the same types of reversible covalent bonds can be involved in different reversible reactions. Imines, for example, are formed by the reversible condensation reaction of an aldehyde or ketone with an amine (primary amine, hydroxylamine, hydrazine, etc.), which is usually catalyzed by an acid, but are also hydrolyzed under higher acidity. Owing to the pH sensitivity, imine linkages have been widely used in stimuli-responsive materials, especially in biological systems [42]. Compared with aromatic imine bonds, aliphatic imine bonds have a rather low thermody-

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45

Table 2 Dynamic reversible covalent reactions. Reaction

Refs.

Reversible exchange

Exchange reaction of C N bond Transiminination

[61,62]

Transoximization

[63]

Hydrazone exchange

[64]

Exchange reaction of S S bond Disulﬁde exchange

[16,17,65–68]

Disulﬁde-thiol exchange

[69,70]

Thiuram disulﬁde exchange

[71]

Exchange reaction of C O bond Transcarbamoylation

[72–74]

Transesteriﬁcation

[75–82]

Transacetalation

[83,84]

Nicholas ether-exchange

[85]

Hemiaminal ether exchange

[86]

Alkoxyamine exchange

[87]

Exchange reactions of C C, C C and C C bonds Carbon radical exchange

[31,32]

Oleﬁn metathesis

[88–90]

Alkyne metathesis

[91,92]

Exchange reaction of C N bond Transamidation

[93–95]

Urea exchange

[11]

Transamination

[96]

Amine exchange

[97]

Pyrazolotriazinones exchange

[98]

Transalkylation

[99]

Others Boronic ester exchange

[44,100]

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Z.P. Zhang et al. / Progress in Polymer Science 80 (2018) 39–93

Table 2 (Continued) Reaction

Refs.

Trithiocarbonate exchange

[101,102]

Thiazolidines exchange

[103]

Siloxane equilibration

[104,105]

Alkoxyamine equilibration

[38–40,106–109]

namic stability because of the stronger hydrolysis tendency. In order to push the reaction toward the direction of the formation of aliphatic imine bonds, the small molecule by-product (water) must be continuously removed. In the meantime, imines can also undergo exchange reaction with the residual reactants (aldehyde and amine) [121,122] or other imine products directly (transiminination) [61,62]. By making use of the versatility of imine bonds and the like, multifunctional properties and processibility have been endowed to polymers [42,61,118,120]. 2.2. Reversible covalent chemistry involved in polymer solids The chemical reactions of macromolecules, which deal with reaction processes of intra-molecular and inter-molecular functional groups transformation, are basically devoted to polymer modiﬁcation, producing new polymers and new properties for various applications. Unlike traditional covalent polymers that are irreversibly formed, the ones containing reversible covalent bonds could reshufﬂe their inherent constitutions via reversible reactions within the macromolecular skeletons. Reversible reaction of polymers in solid state has to follow the thermodynamic limit that the change of Gibbs function of the whole reaction must be negative, which is similar to solution reaction. However, it is quite different from solution reaction in terms of dynamic behavior and inﬂuential factors of reaction. Additionally, the equilibrium composition also differs from that offered by solution equilibration owing to the kinetic effect [123]. Macromolecular chains in solution usually exist in the form of random coils. Chemical reactions of macromolecules occur in the localized region of random coils rich in functional groups. The reactant molecules stably exchange energy and easily collide with each other in the homogeneous environment. Reversible covalent reactions occurring in either aqueous or buffered solutions at near physiological pH seem attracting more and more attention over the last decade [124]. In contrast to solution reaction, the reversible covalent chemistry in solid polymers is a heterogeneous one. Generally, a number of physical and chemical processes are involved, and the physical processes include interfacial contact, diffusion and migration. Among these, intimate contact of solid state reactants is a prerequisite for chemical interaction and mass transport. Wool and O’Connor suggested ﬁve stages of crack healing of polymers [125], i.e. surface rearrangement, surface approaching, wetting, diffusion, and randomization, which are also valid for solid state reaction of reversible bonds. It has been well accepted that random walk chains diffusing at the rubbery–rubbery interface can be described by the reptation model proposed by De Gennes [126,127]. Each polymer chain is conﬁned by the neighboring chains in a tube-like region with diameter equal to the average distance between entanglements. This tube limits the lateral motion of the chain but allows its snake-like slither. Wool et al. [128] deﬁned that the chains that contribute to the interfacial strength straddle the interface plane

in the course of macromolecular interaction, so that the chains in the concentration gradient which have diffused further than their radius of gyration cease to be involved in the load bearing process at the interface. When motion time of the chains is longer than the reptation time, and diffusion distance is larger than the radius of gyration, the interface becomes fractal. The fractal nature of diffuse interfaces plays an important role in controlling the physical properties of polymer–polymer and interfaces. Macroscopically, the interdiffusion behaviors at rubbery polymer–polymer interface follow Fick’s laws undergoing case I diffusion because the rate of segmental relaxation is much faster than that of diffusion [129]. Thickness of interface is proportional to the square of diffusion time. Zhang et al. [38] checked the interface of a bilateral ﬁlm jointed by rigid polyurethane containing reversible alkoxyamine and its control by depth proﬁle analysis of confocal Raman microscopy. The results conﬁrmed that mutual diffusion of polymer chains were available at rubbery state yielding an interphase region ∼40 ␮m thick. More importantly, the newly formed C ON bonds at the interface due to dynamic reversible reaction, rather than chain entanglement, provided the interphase with the same mechanical strength as that of the bulk. Similar phenomenon was reported by the groups of Lehn and Barboiu, respecitively [130,131]. They found that reorganization and migration of components across the boundary could be carried out via reversible exchange reaction under certain circumstances, as visualized by a visible color change. Since the principal reversible covalent bonds that participate in the reaction are a part of the macromolecular chains, the aggregation states (crystalline region, glass state, etc.) of polymer have a great inﬂuence on the solid state reversible reaction. For instance, the macromolecules packed in the crystallization region are rather compact, leading to stronger intermolecular force, which is disadvantage for diffusion of macromolecules. Similarly, chain diffusion and migration at glassy state are difﬁcult as motion of the molecular chain segments is frozen. Therefore, the macromolecular chains must have sufﬁcient mobility to ensure diffusion and penetration for the solid state reversible reaction at the molecular level [38,132,133]. Even so, reversible behavior of polymers in solid state is different from that in solution. For example, Wang et al. [109] carefully studied solid state thermal reversibility of the polymers containing alkoxyamine. They found that the restricted macromolecular diffusion brought about a few incomplete recombinations of C ON bonds of the residual radicals. Fortunately, polymer solid also obstructed diffusion of carbon-centered radicals. Hence the useless side reaction was depressed even though recombination of the cleft C ON bond was slightly hindered. Another important issue of solid state reversible covalent chemistry lies in the speciﬁc mechanical responses of the corresponding polymers, which is closely related with applications. As exhibited by Table 3, the activation energies or bond dissociation energies of reversible covalent bonds (21.3–183 kJ mol−1 ) are lower than the bond energy of conventional irreversible carbon–carbon bonds

Table 3 Thermodynamic and dynamic parameters of representative reversible covalent reactions. Activation energya or bond dissociation energyb (kJ mol−1 )

Empirical equilibrium constant (mol L−1 c , or L mol−1 d )

Rate constant (s−1 e , L mol−1 s−1 f )

Reaction conditionh

Determination methodi

Refs.

DA reaction

47.0a 67.0a 156.7a 39 (amine = TMPCA)a N/A N/A N/A 183 ± 16a 38.6–46.6a N/A N/A 67.8–101.7 b 130.1a 80–180 a 172 b 106.02–240b 157.02a N/A 116 ± 8a N/A 23.1a 22.3 a 78.9 a 55.0 a 60.2–66.1a 111 ± 10a ; 148 ± 7a 113.6–157.9a 121 ± 9a 106a 68.2–97.3a 150 ± 3a 107.95a 59 ± 6a ; 60 ± 5a 140a 52.7a ; 518.8b 33.5–129a 21.3a

N/A N/A 1.17 × 10−5 –3.18 × 10−3 c 88 >107 d 4.5 × 10−6 –5.5 × 104 d 5.8 × 103 –8.1 × 104 d 1.0–57d N/A N/A N/A 8.02 × 10−4 –8.22 × 10−1 c N/A N/A 3.14 × 10−14 –7.45 × 10−8 c N/A N/A N/A N/A N/A N/A N/A N/A N/A N/A N/A N/A N/A N/A N/A N/A N/A N/A N/A N/A N/A N/A N/A

1.56 × 10−5 –1.34 × 10−4 f 3 × 10−5 –7 × 10−5 f 4.67 × 101 –1.18 × 103 e 3.0 × 10−7 –1.2 × 10−5 e 0.16–2.45f 2.8 × 10−4 –8.7 × 10−6 f 1.33 × 104 –3.00 × 106 f,g 7.1 × 10−7 –6.4 × 10−4 f 1.92 × 10−6 –2.31 × 10−5 f 3.6 × 10−9 –1.6 × 102 e,g 4.72 × 10−8 –1.07 × 10−4 e N/A 0.53 × 106 –4.84 × 107 e 10−5 –10−3 e N/A N/A N/A 7.9 × 10−4 0.6f N/A 2.1 × 10−5 f 6.120 × 10−5 f 1.085 × 10−2 f 0.48 × 10−3 –5.41 × 10−3 f N/A 1.67 × 10–1.67 × 102 f N/A N/A 2.33 × 10−5 f N/A N/A N/A N/A N/A N/A 1.2 × 10−2 –3 × 103 e N/A N/A

30–70 ◦ C 30–60 ◦ C 60–80 ◦ C 37 ◦ C pH = 1.89–5 25 ◦ C, pH = 7.4 25 ◦ C, pH = 6.4–8.3 120–170 ◦ C, TsOH 22 ◦ C, 0.5 mol L−1 NaOH 15–25 ◦ C 25 ◦ C, pH ≤ 7 −100–80 ◦ C 60–95 ◦ C 120–125 ◦ C N/A N/A 50–127.5 ◦ C 25 ◦ C Acid, 50–70 ◦ C 75 ◦ C TBP, 25 ◦ C CuCl2 , 0 ◦ C MA-Cu, 90–130 ◦ C 80–200 ◦ C N/A 170–210 ◦ C DBTDL, 110–150 ◦ C DATBDS, 122 ◦ C 120–160 ◦ C Zn(OAc)2 ·2H2 O, 80–320 ◦ C Sn(Oct)2 , 100–140 ◦ C Ru catalyst, 15–30 ◦ C 100–140 ◦ C 130–200 ◦ C 25 ◦ C 50–110 ◦ C KOH, 60–140 ◦ C

1

[47] [48] [49] [11] [51] [52] [53] [57] [110] [55] [111] [31,32] [112] [113–116] [117] [70,117] [118] [62] [63] [64] [67] [16] [17] [119] [70] [72] [73] [74] [82] [75,80] [78] [89] [96] [99] [44] [120] [105]

Retro-DA reaction Urea bond formation Acylhydrazone bond formation Oxime bond formation Ester bond formation Imine hydrolysis Acetal hydrolysis Boronic ester hydrolysis C C bond ﬁssion Thiuram disulﬁde ﬁssion Alkylamine ﬁssion Diselenide bond ﬁssion Disulﬁde bond ﬁssion Transiminination Transoximization Acylhydrazone exchange Disulﬁde exchange

Disulﬁde-thiol exchange Transcarbamoylation

Transesteriﬁcation

Oleﬁn metathesis Transamination Transalkylation Boronic ester exchange Imine exchange Siloxane equilibration

H NMR FTIR 1 H NMR 1 H NMR UV/VIS UV/VIS UV/VIS GC UV/VIS UV/VIS, etc. Titration ESR, UV/VIS Polymerization UV/VIS, ESR Calculation Calculation DMA 1 H NMR 1 H NMR UV/VIS HPLC HPLC HPLC DMA Summarized results ARES; GC DMA, GC GC ARES ARES ARES DMA 1 H NMR; ARES ARES 1 H NMR DMA Stress relaxation

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Reaction

i1 H NMR, proton nuclear magnetic resonance; FTIR, Fourier transform infrared spectroscopy; UV-VIS, ultraviolet-visible spectroscopy; GC, gas phase chromatography; HPLC, high performance liquid chromatography; ARES, advanced rheology expanded system; DMA, dynamic mechanical analysis; ESR, electron spin resonance. a Activation energy. b Dissociation energy. c Empirical equilibrium constant, mol L−1 . d Empirical equilibrium constant, L mol−1 . e Pseudo-ﬁrst order kinetics, s−1 . f Pseudo-second order kinetics, L mol−1 s−1 . g kH + , kH + = kobs /[H+ ], where kobs and kH+ denote apparent rate constant and reaction rate constant, respectively. h TMPCA, 2,2,6,6-tetramethylpiperidinylcarboxamide; TBP, tri-n-butylphosphine; TsOH, 4-methylbenzenesulfonic acid; TMAS, tetramethylammoniumsilanolate; MA-Cu, copper(II) methacrylate; Zn(OAc)2 , zinc acetate; Sn(Oct)2 , stannous octoate; Ru catalyst, ruthenium complex; DBTDL, dibutyltin dilaurate; DATBDS, 1,3-diacetoxy-1,1,3,3-tetrabutyl-distannoxane.

47

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Z.P. Zhang et al. / Progress in Polymer Science 80 (2018) 39–93

motion of the dangling chains generated by disconnection of the reversible C ON bonds of alkoxyamine, while the higher temperature one represents the undamaged networks. Owing to the random nature of the homolysis/recombination behavior of C ON bond, the dangling chain lengths change from time to time so that the position and height of the lower temperature peak are different for different tests. In contrast, the higher temperature peak is basically unchanged due to the relatively deﬁnite molecular weight between crosslinks of the undamaged networks. 3. Rheological properties of reversible covalent polymers

Fig. 4. Temperature dependence of tan ı of cured epoxy carrying dynamic reversible C ON bonds measured by cyclic dynamic mechanical analysis. [106], Copyright 2014. Reproduced with permission from the Royal Society of Chemistry.

(∼376 kJ mol−1 ) that constitute the basic skeleton of organic compounds. Clearly, the reversible covalent bonds belong to the scope of labile bonds. Different bond strengths, and thus different pKa , account for the different rates of reversible reaction in solids [69]. On the one hand, the dynamic property of the labile bonds endows polymers with attractive stimulus-responsiveness. On the other hand, the weak bonds would decrease mechanical strength of polymers, which is unfavorable for high performance usage. Nevertheless, the relationship between reversible covalent bonds and mechanical properties of ultimate polymers are not as straightforward as that reﬂected by bond energies. Unforeseen effects are being successively predicted and observed because the bonds open and close reversibly under applied force. Balazs et al. [134–137], for example, developed a series of computational approaches to examine the rupture and rearrangement of labile reversible bonds crosslinked polymer under tensile deformation. The simulations demonstrated that just a small amount of labile reversible bonds led to a ∼25% increase in the stress needed to induce fracture [134]. It means that the labile reversible bonds can signiﬁcantly improve tensile strength of material. Meanwhile, breakage of labile reversible bonds could improve the toughness of the material because it provided an energy dissipating mode when the material is stretched [137–141]. Moreover, the coiled chains can unfold under the applied stress before the sample fracture, which also increased ductility of the material [137]. In this way, the relatively weak and labile interactions (e.g., hydrogen bond and thiol/disulﬁde exchange) in intra- and intermolecular bonds can make positive contribution to the overall macroscopic performance of the materials. Recently, labile reversible bonds like metal-protein coordination bonds were introduced into double-network-type hydrogels as reversible sacriﬁcial bonds to provide high toughness and selfrecovery ability [138]. The failure of the brittle network made of reversible sacriﬁcial bonds dissipated large energy, while the elasticity of the second network allowed its rapid reformation afterwards. Monte Carlo simulations further indicated that topology of the sacriﬁcial bonds was correlated to the length of the loops deﬁned by the sacriﬁcial bonds [139]. Comparatively, solid state reversible covalent chemistry is easier to be inspected by dynamic mechanical analysis due to coupling of the mobile segments and testing conﬁguration. As shown in Fig. 4, for example, two peaks appear on the temperature dependence of tan ı of epoxy containing alkoxyamine in the backbone [106]. The lower temperature peak is contributed by segment

Rheology of viscoelastic polymers forms the basis of polymer engineering, because ﬂow and deformation of materials under applied stress are their common concerns. Knowledge of rheological properties of polymers is critical for the design and synthesis of proper molecular composition and structures, and optimization of processing techniques and equipments for the given products as well [142]. Since the introduction of reversible covalent chemistry gives rise to dynamicity of polymers, the corresponding rheological properties should be clariﬁed, which are the premise of application. In the following, rheological behaviors of the above-mentioned two types of reversible covalent polymers (refer to the classiﬁcations given in Tables 1 and 2) are brieﬂy discussed for better understanding and usage. 3.1. Polymers with general reversible covalent bonds Rheological properties of polymers are mainly performing as the change of viscosity, which is the reﬂection of intrinsic factors such as entanglement of macromolecular chains and variation in free volume. By taking amorphous thermoplastic polymer as an example, its viscosity decreases with a rise in temperature, leading to transformation from high-elastic state (solid) to viscous ﬂow state (liquid) above the viscous ﬂow temperature, Tf , which makes it convenient for molding and processing. Evidently, the melting viscosity is temperature dependent like small molecular liquids following Arrhenius equation. At the temperature below Tf , however, the temperature dependence of its apparent viscosity, a , does not agree with Arrhenius equation, as motion of chain segments is restricted by the decrease of free volume at relatively lower temperature [143]. In general, the relationship between viscosity of polymer, (T), and temperature, T, in the zone from Tg to Tg + 100 ◦ C can be described by Williams-Landel-Ferry (WLF) semi-empirical equation [144] (Fig. 5a): lg(T ) − lg(T g ) = − C 1 (T − T g )/[C 2 + (T − T g )]

(3)

where Tg denotes glass transition temperature, and C1 and C2 are empirical constants. As the lower limit temperature of macromolecule chain segments movement, Tg of amorphous polymer is commonly deﬁned as the temperature at which the apparent viscosity equals to 1012 Pa s [145]. In contrast, when the reaction of three-dimensional polycondensation is carried out to a certain critical extent, viscosity of the system would increase rapidly, leading to absence of ﬂuidity and transformation into an elastic gel, which is deﬁned as gelation phenomenon. Therefore, the ﬂow process of traditional thermosetting polymers has to be completed before the gel transition point owing to the infusibility after network formation, of which viscosity tends to be inﬁnitely large. The study of the critical reaction degree of gelation, gelation speed and system viscosity plays an important role in the processing and molding of thermosetting polymers [146–150]. Nevertheless, the situation becomes quite different when reversible covalent chemistry is involved. The networks built by reversible DA bonds, for example, on the one hand, have

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49

Fig. 5. Viscoelastic behaviors of (a) thermoplastic polymer, whose temperature dependence of viscosity above Tg follows WLF model ((T) = (Tg )exp {-2.303C1 (T-Tg )/[C2 + (TTg )]}); and (b) DA crosslinked networks, whose reverse sol-gel transition occurs between Tg and TrDA , resulting in decrease of viscosity with temperature that could be described by power law ( ∼ exp(Ea /RT)|ε|−s ) and further normalized by the activation energy.

many beneﬁts similar to thermosets such as solvent resistance and excellent mechanical properties at ambient temperature. On the other hand, the DA linkages disconnect above the temperature of retro-DA reaction, TrDA , leading to depolymerization and reduction of crosslinking density, followed by rapidly decrease of viscosity, which makes it able to ﬂow and process like thermoplastics [151,152]. As for the linear polymer containing DA bonds [153–155], viscous ﬂow is allowed above TrDA (∼100–120 ◦ C), which is lower than the processing temperatures of most thermoplastics. Owing to the thermally reversible equilibrium feature of DA reaction, both the ﬁnal conversion rate of reversible moieties (furan and maleimide) and crosslinking density of the polymer formed by DA reaction are temperature dependent. A reverse solgel transition from elastomer solid to viscous liquid occurs at the temperature, Tgel , between Tg and TrDA (Fig. 5b). The corresponding viscosity change above Tgel can be expressed by [156]: ∼ exp(E a /RT )|ε|−s

(4)

where ε is the relative distance from the gel point and s is the characteristic scaling exponents. It is thus convenient for the crosslinked polymer networks based on DA reaction to precisely and reversibly control the reverse gel transition. By measuring frequency dependence of storage (elastic) modulus, G’, and loss (viscous) modulus, G”, at various temperatures, different mechanical states would appear. When temperature is much lower than the gelation temperature, G’ is higher than G”, showing the characteristics of conventional crosslinked polymers. With increasing temperature, crossover between G’ and G” in the low frequency regime is observed above Tgel , which reveals the formation of sol state and resembles the case of exchange of dynamic reversible covalent bonds represented by disulﬁde metathesis [67]. Bowman et al. [156] revealed that retroDA reaction resulted in depolymerization and reverse gelation of propoxylate-based networks at about 92 ◦ C with gel-point conversion of 71%. The data coincide with the predicted ones in terms of Flory-Stockmayer equation (i.e. 92.5 ± 0.5 ◦ C and 70.7%). Additionally, activation energies of DA and retro-DA reactions in a DA bonds crosslinked epoxy were estimated to be 55.7 and 94.2 kJ mol−1 , respectively, by means of temperature controlled FTIR [157]. Actually, the initial ﬂow temperature and ultimate viscosity of the polymers containing reversible covalent bonds can be adjusted by varying the content of reversible moieties and molecular weight of the linear precursor polymers or small molecules. Wouters et al. [158] studied temperature dependences of complex viscosity, *, of DA bonds crosslinked acrylate copolymers and epoxy. As the precursor polymers of crosslinked acrylate copolymers were linear copolymers with higher number average molecular weights (Mn = 1.25 × 104 –7.2 × 104 g mol−1 ) and the reactants of cured

epoxy were DA bond-containing diglycidyl ether and low molecular weight polyether amine (Mn = 230 g mol−1 ), their complex viscosities at elevated temperatures were quite different. In the case of crosslinked acrylate copolymers, the * values due to retroDA induced decrosslinking were higher (102 Pa s < * < 105 Pa·s, 175 ◦ C). With respect to the DA crosslinked epoxy, it performed as a solid-like material at temperatures below 95 ◦ C, whereas like a true liquid (* < 10 Pa·s) at 150 ◦ C. Besides, since the formation and hydrolysis equilibrium of reversible condensation adducts (e.g., borate ester, imine and acetal bonds) strongly depend on pH value, covalent polymers with this type of reversible linkages display pH-responsive sol-gel transition behavior [159–163]. Xu et al. [159] studied the viscosity of phenylboronate-diol crosslinked polymer gel as a function of time during the sol-gel transition at 25 ◦ C. A drastic increase of viscosity was observed within 2 min (* ∼ 2 × 103 Pa·s) when the polyacrylate precursors were mixed, which led to the appearance of gel. Having been regulated to higher pH (≥7.0) by the addition of triethylamine (TEA), however, the gel was transformed into sol within 3 min accompanying by the decrease of viscosity to nearly zero. By further adjusting pH value to 6.4–6.7 with acetic acid, the sol was converted into gel again. Shibayama et al. [160] revealed that Tgel of poly(vinyl alcohol)-borate crosslinked gel was correlated to polymer concentration, molecular weight, boronic acid concentration and pH, which could be described by Eldridge-Ferry empirical formulae [164,165]. The measured Tgel mainly ranged between 0 and 100 ◦ C, and the enthalpy of formation of borate ester crosslinks was estimated to be about −29.3 kJ mol−1 . 3.2. Polymers with dynamic reversible covalent bonds Polymer networks containing dynamic reversible covalent bonds (e.g., ester bonds) as crosslinks are able to change their topology through reversible reactions without affecting the average crosslinking degree and functionality. It means that macroscopic viscous ﬂow due to depolymerization is prohibited, which helps to maintain operational stability of end-products. As proposed by Leibler et al. [76], a reversible transition of this type of polymers (named vitrimers) from viscoelastic solid to liquid occurs at a topology freezing transition temperature, Tv , controlled by reversible reaction kinetics. Similar to glass transition at nonthermodynamic equilibrium, here the topology freezing transition also shows dependence on cooling rate. Since the dynamic characteristics of reversible reaction follow Arrhenius law, the stress relaxation time and viscosity, , near Tv should also vary with temperature in the same way, which is similar to inorganic material such as silica: ln = lnA + E /RT

(5)

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Fig. 6. Angell plots of the logarithmic viscosity, lg, and segmental relaxation shift factors, lg˛T . The temperature (Tg or Tv ) at which the viscosity reaches 1012 Pa·s or apparent relaxation time equals 102 s [167] serves as the normalizing parameter. The data of the polymer networks crosslinked by dynamic reversible covalent bonds and silicon dioxide are quoted from ref. [75,168], respectively. The WLF shift factor of thermoplastics is calculated from the parameters of ref. [169]. PS, polystyrene; PMMA, polymethylmethacrylate; PVC, polyvinyl chloride.

where A denotes viscosity constant when temperature approaches to inﬁnity, and E is the apparent energy of activation of the ﬂow process. The Angell plot [166] in Fig. 6 is the most common representation of dynamic fragility of supercooled glass-forming liquids, which is generally expressed as the logarithm of viscosity (or relaxation time, , or relative magnitude of the respective relaxation time called shift factor, ˛T ) versus inverse temperature scaled with Tg . Most thermoplastics are classiﬁed as fragile liquids [168], which are greatly different from typical strong liquid − inorganic silicon dioxide. The viscosity (or ˛T ) of fragile liquid in a narrow temperature range close to Tg shows a non-Arrhenius behavior, which undergoes abruptly drastic drop beyond a couple of orders of magnitude with a rise in temperature. On the contrary, the polymers crosslinked by dynamic reversible bonds (e.g., epoxy/anhydride and epoxy/acid) belong to strong liquids [75], showing a slow gradual variation with a linear relationship between lg (or lg˛T ) and Tg /T. It means that they could be processed without precise temperature control similar to silica glass, which forms a striking contrast to thermoplastic polymers that require more stringent control conditions [12]. According to the relative magnitude of Tg and Tv , the polymers crosslinked by dynamic reversible covalent bonds can be divided into two groups with different mechanical states and viscoelastic behaviors [12]. As shown in Fig. 7a, the ﬁrst group [75–77] considers the case with Tv higher than Tg . With increasing temperature, the polymers ﬁrstly undergo a common transition from glassy state to rubbery state. Segmental motion is allowed. Because the rate of dynamic reversible reaction is rather slow at the beginning, the polymers structures remain stable until the temperature reaches Tv . Since then, the reaction rate speeds up, leading to the conversion into viscoelastic liquid that exhibits an Arrhenius behavior. Furthermore, Tv can be in turn determined by classical dilatometric method due to the higher expansion coefﬁcient of the dynamic networks above Tv [76]. The second group [96] deals with the case with Tv lower than Tg (Fig. 7b). Due to the absence of segmental mobility below Tg , the dynamic reversible reaction that used to result in change in network topology has to be restricted even if the polymers are heated to a temperature above Tv . In this context, Tv is only a hypothetical

parameter, which can be calculated via extrapolation of stressrelaxation experiments. When heating to a temperature above Tg , although the rate of dynamic reversible reaction is rather fast, the chain segment motion is just triggered. Therefore, the kinetics of topology structure rearrangement is diffusion-controlled at ﬁrst and the viscosity change agrees with WLF model. With further increasing temperature, the dynamic reversible reaction gradually becomes the main kinetic factor and Arrhenius viscosity behavior predominates. However, the exact deﬁnition of such a transition temperature and the corresponding measurement approach still remain open. An insight into the matter will favor polymer engineering enabled by reversible covalent chemistry in solid state. Once reversible reaction is triggered, the speciﬁc deformability of the polymers with dynamic reversible covalent bonds would lead to unique stress relaxation manner. Complete stress relaxation used to be detected for the polymers crosslinked by dynamic reversible covalent bonds (i.e. the stress relaxes to zero) [16,17,40,69,89,170,171], as the networks prefer to rearrange to reach a less stretched state. Owing to the same effect, plastic deformation after tensile loading-unloading cycles can be partially or fully recovered [172,173]. In some special cases, additional transient crosslinks were rapidly formed by dynamic reversible covalent bonds (e.g., catechol-Fe3+ and catechol-B3+ coordinate bonds) among neighbor macromolecules, which was even faster than relaxation movement of the molecular chains, so that the measured stress increased but not decreased with time showing the abnormal stress intensiﬁcation effect [174] (Fig. 8). Such a habit may prevent polymeric products from gradual ﬂowing under constant loading, which would become more and more stressed on the contrary. The characteristic relaxation time, *, deﬁned as the time required for the relaxation modulus to reach 1/e of its initial value, is the characteristic time scale for the dissociation and association of the dynamic bonds. It is related to molecular mobility and macroscale network mobility, and can be estimated from Maxwell model [75] (Table 4). Appropriate relaxation time range ensures ideal viscoelasticity for achieving macroscopic ﬂow. For shorter *, the macromolecules are able to rearrange, whereas a relative long * cannot provide enough dynamic behavior.

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Fig. 7. Viscoelastic behaviors of the polymers crosslinked by dynamic reversible covalent bonds: (a) Tv > Tg ((T) = Aexp(Ea /RT)), and (b) Tg > Tv (The solid part above the intersection of the two (T) ∼ T curves follows (T) = (Tg )exp {-2.303C1 (T − Tg )/[C2 + (T − Tg )]}, while that below the intersection obeys (T) = Aexp(Ea /RT)). Table 4 The mean relaxation times, *, of the polymers crosslinked by dynamic reversible covalent bonds at the topology freezing transition temperatures, Tv , and elevated temperature. Polymer

Mechanism of reversible reaction

* at Tv (s)

Tv (◦ C)

* at elevated temperature (s)

Ea (kJ mol−1 )

Refs.

Epoxy/acid Polylactide Polyhydroxyurethane Vinylogous urethane Epoxy Polytriazolium

Transesteriﬁcation Transesteriﬁcation Transcarbamoylaion Transamination Disulﬁde exchange Transalkylation

1.0 × 106 1.4 × 106 0.4 × 106 0.3 × 106 0.3 × 106 0.3 × 106

75 57 63 111 29 −13 98

<104 , 150 ◦ C <50, 140 ◦ C 9.6 × 103 , 160 ◦ C 85, 170 ◦ C 20, 200 ◦ C 69, 170 ◦ C

90 150 ± 3 111 ± 10 65 ± 5 55 140

[75,76] [78] [72] [96] [119] [99]

Fig. 8. Typical stress-time curves measured during tensile stress relaxation tests of crosslinked polymer with irreversible and reversible covalent bonds.

Similar to stress relaxation, creep is also an important tool to inspect viscoelastic manner of polymers, which mainly includes instantaneous elastic deformation, delayed elastic deformation and viscous ﬂow. Traditional thermoset only exhibits the ﬁrst two types of deformation and ﬁnally reaches the equilibrium of delayed elastic deformation when the loading time is much longer than the relaxation time. Furthermore, both the instantaneous and delayed elastic deformation can be completely recovered after removing the

applied force. In the case of dynamic reversible covalent bonding, however, creep behavior of the polymers resembles thermoplastics, which involves the viscous ﬂow at the temperature above Tf due to network rearrangement. Only the initial elastic deformation can be recovered, and the permanent plastic deformation caused by viscous ﬂow has to be remained even after unloading. For example, Denissen et al. [96] have prepared vinylogous urethane vitrimers that were easily deformed up to 45% permanent strain under 0.1 MPa at the temperature above Tf . In principle, the polymer networks that are able to carry out reversible reactions would quickly creep. This is supported by the epoxy elastomer with disulﬁde bonds [67]. As the material can be involved in metathesis reaction, reduced creep resistance as a result of network rearrangement was observed. Similar results were also found for catalyst-free dynamic exchange of aromatic Schiff base bonds and homolysis/recombination of C ON bonds [40,61]. Dual-crosslinking networks combining dynamic and permanent crosslinker have been exploited to overcome this weakness [175–177]. Poly(2-hydroxyethyl acrylate)-based polymer crosslinked by both rapidly exchangeable Upy hydrogen bond and slowly reversible DA linkages showed a limited extent of creep and stress relaxation, which reached a plateau at about 2.5 h and then retained almost unchanged. The phenomenon is attributed to rapid exchange of hydrogen bonds within minutes and the longer time-scale of DA reaction at ambient temperature [178]. Nevertheless, the deformability offered by networks rearrangement is not necessarily a bad thing. It can be used for remolding [67] and/or reshaping [17] of crosslinked polymer without decrosslinking, which factually brings about new technology unavailable for traditional thermosets and will be discussed latter. Besides ﬂowability measured under static loading conditions, rheological measurement of angular frequency dependence of storage modulus, G’, and loss modulus, G”, also provide information concerning structure evolution and viscoelastic status. The polymers with permanent crosslinking bonds exhibit that G’ is always higher than G” over the entire frequency range. Nevertheless, G’ of the polymers crosslinked by reversible covalent bonds usually intersects with G” with decreasing frequency [61,67,173]. Such a

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transition from elastic-like (G’ > G”) to viscous-like (G’ < G”) takes place in the low frequency regime because the networks tend to be continuously rearranged after initiation of the reversible reaction and the disconnected networks need sufﬁcient time to be restructured. Hence the system performs like viscous ﬂuid under the circumstances [161,179,180]. Meanwhile, the crossover frequency of G’ and G” is a reﬂection of dissociation rate constant of the reversible bonds [161] and gives a measure of the lifetime of the reversible bonds (like disulﬁde) [179,180]. Different dynamic reversible bonds have different lifetimes [181], affecting kinetics of network reconstruction. In-depth investigation would result in comprehensive understanding of the molecular mechanism and proactive control of dynamicity of the adaptive properties [44]. According to previous studies [182–185], favorable network mobility and reversibility of supramolecular polymers could be achieved for scratch healing when the average supramolecular bond lifetime was in the range of 0.1 and 100 s [179,180,184]. Relatively speaking, lifetimes of reversible covalent bonds also fall in the similar scope (e.g., 10 and 125 s for reversible boronic ester bonds [161,186,187], and 10 s for reversible Schiff base bonds [188]). It ensures that the covalent bonds are stable enough to hold molecular structures while capable of displaying dynamic behavior [15]. 4. Strategies of preparation of polymers containing reversible covalent bonds Chapters 2 and 3 are dedicated to the basis of reversible covalent chemistry and rheological properties of the polymers carrying reversible covalent bonds, respectively. Hereinafter a compilation of strategies for designing and synthesizing reversible covalent polymers are discussed, which determines the application of reversible covalent chemistry in practice. Reversible covalent chemistry in combination with polymerization is facile to build up various polymeric architectures. Compared to linear polymers, crosslinked polymer networks beneﬁt much more from reversible bonds as viewed from the revolutional structural and properties variations. Therefore, construction of reversible covalent polymer networks is the main concern of this chapter. So far, four strategies have been proposed for the preparation of reversible covalent polymer networks (Fig. 9). The speciﬁc synthesis processes and the resultant polymer structures are analyzed below along with their own advantages and weakness. (i) Introduction of reversible moieties-containing side groups or terminal structures into macromolecular chains, followed by the reaction with reversible moieties-containing monomer linkers (Fig. 9a). This is the most common way of synthesizing reversible covalent polymers and conducive to molding and controlling of reaction heat. In comparison with the system full of small-molecule monomers (refer to route (iii) shown below), which has too low viscosity for mold ﬁlling, the reactants utilized in the present case can be easily crosslinked to yield three-dimensional networks due to the appropriate ﬂuidity. Nevertheless, synthesis of functionalized macromolecules used to be tedious and complex, which have to be obtained from either bottom-up approach or modiﬁcation of existing polymer materials. (ii) Introduction of one or two types of pendant reversible moieties into macromolecules, which are then reacted with each other (Fig. 9b). This method provides pluralistic tunability of composition and functionality of the resultant polymers, and favors creation of polymeric hybrid materials with speciﬁc structures, such as multi-block copolymer, graft polymer and core crosslinked star polymer. Similar to the above strat-

egy, the shortcoming of this synthesis pathway is apparent, because preparation of the reversible moieties-containing macromolecules is complicated. Besides, the heterogeneity and complexity of the subsequent macromolecular reaction would affect the degree of reaction between reversible groups. (iii) Reaction among reversible moieties-containing monomers (Fig. 9c), towards relatively high content of reversible linkage and mobility of the back-reaction product. Compared with the above two strategies, which involve preparation of macromolecular precursors and need sophisticated synthesis and puriﬁcation processes, here the monomer precursors can be obtained through facile modiﬁcation or simply from market. Therefore, the method is quite convenient for generation of reversible covalent polymer networks. In addition to the inconvenience caused by the high ﬂowability of the starting materials as revealed above (refer to the discussion of route (i)), however, structures of the polymers produced by this method are hard to be precisely controlled, which usually have to behave like randomly crosslinked networks. (iv) Synthesis of reversible covalent polymers via the former three methods is based on reversible reactions, but irreversible reaction is mostly involved in the fourth one like the preparation of traditional polymer materials. In this case, the monomers containing reversible bonds are functionalized to carry reactive groups such as vinyl, hydroxyl, epoxy and bromine. Next, stepwise polymerization or chain polymerization proceeds according to the species of the reactive groups, simultaneously introducing the reversible bonds into the polymer networks (Fig. 9d). By combining different polymerization methods, different types of polymers with tailor-made topologies can be produced. Certainly, it is necessary to ensure stability of the reversible monomers in the course of polymerization, preventing the reversible bonds from dissociation and reverse reaction. Due to the constraint of reversible reaction equilibrium, the synthesis and puriﬁcation of the reversible bonds-containing monomers are rigorous, and the ultimate yield would inevitably be lowered.

4.1. From reversible moieties-containing macromolecules and monomer linkers 4.1.1. Reversible cycloaddition (DA reaction) Cycloaddition reaction refers to pericyc lic reaction, during which two or more unsaturated molecules react with each other through double bonds forming a cyclic adduct under light or heat. DA reaction between furan and maleimide is usually regarded as a thermally initiated [4 + 2] cycloaddition reaction, which has been widely used in constructing reversible covalent polymers. Besides, a few other reversible cycloaddition reactions, like photoinitiated [2 + 2] (e.g., cinnamate and coumarin) and [4 + 4] (e.g., anthracene) cycloaddition, can also be utilized for preparing reversible covalent polymers, but they would not be discussed in detail here. Formation of linear polymer chains bearing pendant furan or maleimide moieties (Table 5) is the key step prior to DA reaction. Afterwards, DA crosslinked networks used to be achieved by collaboration with small molecular maleimide and furan crosslinkers (e.g., 1-9, Fig. 10). Maleimide moieties are typically prepared through multi-step synthesis including amine-maleic anhydride acylation and cyclisation [189]. Nevertheless, furan moieties rather than maleimide moieties are preferentially incorporated into polymer backbone owing to the relatively simple modiﬁcation technique and numerous commercially available furan derivatives (Fig. 11). Paal-Knorr reaction is a classic organic name reaction that is carried out between 1,4-dicarbonyl and primary amine yielding

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Fig. 9. General synthetic routes for preparing reversible covalent polymer networks from (a) reversible moieties-containing macromolecules and monomer linkers; (b) reversible moieties-containing macromolecules; (c) multi-functional monomers; and (d) monomers containing reversible linkages.

a pyrrole unit. It was utilized to synthesize furan functionalized polyketone (10, Fig. 10) with furfurylamine (FA), which was then crosslinked by DPMBM (1, Fig. 10) via DA reaction [189]. Wang

and co-workers [47,153,190,191] prepared isocyanato terminated polyurethane pre-polymers from diisocyanate (e.g. methylene diphenyl diisocyanate (MDI)) and poly(1,4-butylene adipate glycol)

Table 5 Macromolecular architectures bearing furan or maleimide moieties obtained by different modiﬁcation methods and their partner crosslinkers for preparing DA networks. Polymera

Furan or maleimido derivative

Modiﬁcation method

Crosslinkerb

Refs.

PK PU EPM PBS PDCP PHAEs PEA PPDO, PTMEG PEAm PLA PVA PB, ABS PDMS POSS PU

Furfurylamine Furfurylamine Furfurylamine Furfurylamine Furfurylamine Furfurylamine Furfuryl alcohol Furfuryl alcohol Furfuryl glycidyl ether 4-(Furan-3-ylmethoxy)-4-oxobutanoic acid Furfural Furfuryl mercaptan Maleamic acid Furfurylamine N-(2,3-dihydroxypropyl) maleimide

Carbonyl-amine Isocyanato-amine Maleic anhydride-amine Carboxyl-amine Chlorine-amine Epoxy-amine Isocyanato-hydroxyl Isocyanato-hydroxyl Epoxy-mine Carboxyl-hydroxyl Hydroxy-aldehyde Thiol-ene Maleamic acid-amine Epoxy-amine Isocyanato-hydroxyl

DPMBM DPMBM BMD BME, TMEA BMH MPDBMI BME, TMEA TMEA DPMBM BMH, BMD, TMEA DPMBM, DPOBM, BMH DPMBM BMFDES DPMBM HEBFMC

[189] [47,153,190,191] [192] [193] [194] [195] [196] [197] [198] [199] [200] [201,202,206] [203] [204] [205]

a PK, polyketone; PU, polyurethane; EPM, ethylene-propylene rubber; PBS, poly(butylene succinate); PDCP, poly(dichlorophosphazene); PHAEs, poly(hydroxyaminoethers); PEA, poly(ethylene adipate); PPDO, poly(p-dioxanone); PTMEG, poly(tetramethylene oxide); PEAm, polyetheramine; PLA, poly(lactic acid); PVA, poly(vinyl alcohol); PB, poly(butadiene); ABS, acrylonitrile-butadiene-styrene copolymer; PDMS, poly(dimethyl siloxane); POSS, poly(hedral oligomeric silsesquioxane). b DPMBM, N,N -4,4 -diphenylmethane-bismaleimide (1, Fig. 10); BMD, 1,12-bis(maleimido)dodecane (6, Fig. 10); BME, 1,2-bis(maleimido)ethane (3, Fig. 10); TMEA, tris(2maleimidoethyl)amine (7, Fig. 10); BMH, 1,6-bis(maleimido)hexane (5, Fig. 10); MPDBMI, 1,5-bis(maleimido)-2-methylpentane (4, Fig. 10); DPOBM, N,N’-4,4 -diphenyloxide bismaleimide (2, Fig. 10); BMFDMS, bis(2-methoxyfuran) dimethylsilane (8, Fig. 10); HEBFMC, 1,6-hexamethylene-bis(2-furanylmethylcarbamate) (9, Fig. 10).

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Fig. 10. Chemical structures of maleimide monomers, and maleimide/furan functionalized macromolecules.

(PBAG), which were further end-capped by FA to obtain furylterminal polyurethane (11, Fig. 10). DA crosslinking was conducted between terminal furan groups and DPMBM. Similar approaches were employed to prepare PEA [196] and furyl-telechelic PTMEG [197], except that the capping agent was replaced by furfuryl alcohol (FAol). This method has the advantages of mild reaction condition, high yield and free of further puriﬁcation. However,

because the high activity of isocyanato group makes it easy to react with water in the air, the synthesis has to be strictly controlled. To obtain linear polyurethane bearing pendent maleimide moieties (12, Fig. 10), Yu et al. [205] exploited N-(2,3-dihydroxypropyl) maleimide (DHPM) as the chain extender of polyurethane prepolymer that was composed by MDI and PTMEG. DA bonds were

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Fig. 11. Furan derivatives utilized for preparing furan functionalized polymers.

introduced into the high-performance crosslinked polyurethane with HEBFMC (9, Fig. 10) as crosslinker. In general, DA crosslinked epoxy networks are produced by a two-step process. Firstly furan functionalized oligomer is synthesized via epoxy-amine irreversible reaction between certain macromolecules and furan derivatives. Then, dimaleimide is added to react with the attached furan via DA reaction. As shown by Scheltjens et al. [198], furan-terminated polyetheramine prepolymer (13, Fig. 10) was synthesized from amine-terminated polyetheramine and furfuryl glycidyl ether (FGE). Besides, PHAEs bearing pendent furan moieties (14, Fig. 10) was also prepared by epoxy-amine reaction between epoxy group of diglycidyl ether and amine group of FA [195]. Finally, the above two kinds of precursors were crosslinked by DPMBM and MPDBMI, respectively. In general, modiﬁcation of epoxy in this way can be easily completed, but organic solvent has to be added for lowering viscosity of epoxy oligomer in favor of DA crosslinking. Mechanical properties of the ultimate material may be affected by the remaining solvent, in addition to the environmental impact. Unlike polyurethane and epoxy that provide great freedom for designing DA crosslinked networks owing to the ﬂexibility of molecular composition and high activity of the functional moieties [201], synthetic rubbers (e.g., PB, SBS (poly(styrene-butadienestyrene)) and EPM) containing carbon–carbon double bonds are less reactive and relatively difﬁcult to be modiﬁed with functional moieties. Trovatti et al. [206] proposed a method to prepare PB (15, Fig. 10) containing furyl dangling chains via thiol-ene click reaction between furfuryl mercaptan (FM) and carbon–carbon double bonds on the skeleton of polybutadiene. DA crosslinked networks were created via DA click reaction between the furan modiﬁed rubber and DPMBM (1, Fig. 10). Bai et al. [201,202] also synthesized DA crosslinked PB and SBS elastomers (16, Fig. 10) following similar route. Polgar and co-workers [192] focused on the synthesis of DA crosslinked EPM from furan functionalized EPM (17, Fig. 10) and BMD (6, Fig. 10). The furan moieties were incorporated into the rubber backbone by acylation reaction between FA and EPM grafted with maleic anhydride (EPM-g-MA). Owing to the high molecular weight of the synthetic rubbers, the aforesaid modiﬁcation and crosslinking have to be performed in organic solvents (e.g., toluene, tetrahydrofuran (THF), etc.). Besides, the thiol reagents always cause unpleasant odor.

Under the concern of recyclability of biodegradable polymers, Watanabe and Yoshie [193] prepared furyl-telechelic PBS (18, Fig. 10) through amidation reaction of FA and PBS activated by 1,10-carbonyldiimidazole. They further studied chain extension and crosslinking of the prepolymer with BME (3, Fig. 10) and TMEA (7, Fig. 10), respectively. Inoue et al. [199] synthesized a fourarm furan-terminated PLA (19, Fig. 10), which was crosslinked by three kinds of dimaleimide linkers (BMH, 5, BMD, 6, and TMEA, 7, Fig. 10), forming a shape-memory PLA network containing DA linkages. Gaina and co-workers [200] proposed to introduce furan group into the skeleton of PVA by means of acetalization reaction, which further reacted with furfural (FF) to obtain poly(vinyl furfural) (PVF) (20, Fig. 10). Moreover, a variety of bismaleimide (DPMBM, 1, DPOBM, 2, and BMH, 5, Fig. 10) were utilized to react with PVF to prepare thermally reversible crosslinking polymers. Although these methods have successfully coupled biodegradable materials with reversible bonds, the cumbersome post-processing, like removal of catalysts and by-products, needs to be greatly simpliﬁed. Organic/inorganic hybrid polymers (e.g., polyphosphazene, polysilane and polysiloxane) have great application prospects owing to their high ﬂexibility coming from the polymeric components and excellent functional properties of the inorganic components. DA linkages were also incorporated into these hybrids to convert them into thermally reversible crosslinked networks. Because polydichlorophosphazene (PDCP) includes chlorine atoms side groups that are easy to be substituted by nucleophile leading to functionalized PDCP, for example, two kinds of poly[di(furfurylamine)phosphazene] (21, Fig. 10) were prepared by utilizing this feature followed by crosslinking with BMH (5, Fig. 10) [194]. Besides, Gheneim et al. [203] synthesized maleimide functionalized siloxane copolymers (22, Fig. 10) through amidation between pendent aminopropyl moieties of siloxane and maleic acid, which were crosslinked with furan linker BMFDMS (8, Fig. 10) via DA reaction. Recently, a POSS nanocomposite was prepared through DA reaction between furan-functionalized POSS (23, Fig. 10) and bismaleimide (DPMBM, 1, Fig. 10) [204], which can be thermally remended by the DA linkages. On the whole, preparation of organic/inorganic hybrid polymers containing reversible DA linkages from the precursors with furan or maleimide moieties is relatively simple, but the DA reactions are mostly carried out in

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Fig. 12. Aldehyde derivatives utilized for preparing aldehyde functionalized polymers.

high boiling-point solvents (e.g., N,N-dimethylformamide (DMF)), which are difﬁcult to be completely removed from the products. 4.1.2. Reversible condensation Condensation reaction refers to the one that creates large molecule through covalent combination between two organic molecules (sometimes more than two), followed by the production of small molecule by-product (e.g., water, hydrogen chloride and alcohol). As shown in Table 1, reversible imine, acylhydrazone, oxime, animal, acetal, aldol, ester, amine and boronic ester bonds are formed by condensation reaction, which is naturally employed for preparing reversible covalent polymers. Reversible imine bonds (including imine, acylhydrazone and oxime) are usually derived from aromatic aldehyde owing to the relatively high stability of aromatic imine linkage (Schiff base chemistry). Fig. 12 depicts a few aromatic aldehyde monomers dedicated to the preparation of polymers bearing aldehyde moieties. Lei et al. [61] reported the synthesis of aldehyde-functionalized polyethylene glycol methacrylate (PEGMA) (24, Fig. 13) through esteriﬁcation reaction between PEGMA and 4-carboxybenzaldehyde (CBA). The precursor was then copolymerized with butyl acrylate, and crosslinked by 4,4 -diaminodiphenyl methane (25, Fig. 13), forming catalystfree room temperature repairable and reprocessable aromatic Schiff base bonds crosslinked networks. Deng et al. [162] synthesized di-acylhydrazine telechelic poly(ethylene oxide) (PEO, 26, Fig. 13) through modiﬁcation of the corresponding hydroxyl functionalized monomer of PEO, and crosslinking with tris[(4formylphenoxy)methyl]ethane (TFPME, 27, Fig. 13) to obtain acylhydrazine crosslinked polymer gel. The challenge faced by the imine linkage formation reactions lies in the improvement of antioxidative capability of the derivatives of benzaldehyde and amino-containing crosslinking agents. Boronic acid is a typical Lewis acid that could form dynamic reversible borate ester bond with 1,2-diol or 1,3-diol [207]. When pH value is higher than pKa of boronic acid, production of boronate ester bond is favored. As for pH lower than the pKa , the reaction equilibrium favors the formation of boronic acid and diol. The boron atom is sp2 hybridized with an empty p-orbital, which is easy to be attacked by nucleophilic reagent (e.g., H2 O) with lone pair electrons, leading to hydrolysis of aliphatic borate ester. Owing to the charge conjugation effect of benzene ring, phenyl boronic acid derivatives (Fig. 14) have higher stability suited for preparing borate acid-containing polymers through amidation reaction

[208–210], quaternization reaction [211], and radical polymerization [212–215]. By adding appropriate polyols as crosslinking agent, crosslinked polymer networks with phenyl boronic ester linkages could be yielded. Besides, crosslinking of the polymers with multi-hydroxyl using phenyl boronic acid as crosslinker can also introduce boronic ester bonds into polymer networks. In fact, however, the types of commercial phenyl boronic acid are limited. Many other derivatives of phenyl boronic acid have to be home-made with the aid of Grignard reagents or precious metal catalysts (e.g., palladium, rhodium and cobalt), and the reaction conditions are harsh with inert solvent and oxygen-free atmosphere. Sumerlin et al. proposed a facile strategy to synthesize boronic acid-containing block copolymers (28, Fig. 13) with well-deﬁned topology and composition by using reversible addition-fragmentation chain transfer polymerization (RAFT). The products were further crosslinked with multifunctional diols (2932, Fig. 13) to obtain dynamic covalent star polymers [216]. Recently, Cromwell et al. [44] demonstrated that the adjustment of boronic ester exchange kinetics through changing monomers’ structures could tune the bulk dynamic properties (e.g., malleability and self-healing property). Their precursor with 1,2-diol (33, Fig. 13) was synthesized using ring-opening metathesis polymerization (ROMP) of cyclooctene-based monomers. Then, the authors prepared covalently tunable polymer networks containing boronic ester bonds by polycondensation between 1,2-diol in the polymeric skeleton and two kinds of diboronic ester crosslinkers (34 and 35, Fig. 13). 4.2. From reversible moieties-containing macromolecules 4.2.1. Reversible cycloaddition (DA reaction) As mentioned above (Fig. 9b), DA reaction between reversible moieties-containing macromolecules and monomer linkers has been applied for yielding reversible covalent polymers, but it is also valid for the reversible reaction between functionalized macromolecules. Generally, the furan/maleimide functionalized macromolecules have to be ﬁrstly dissolved in volatile solvent (e.g., chloroform and methanol) and mixed evenly for promoting collision among the molecules. Next, catalyst-free DA reactions are enabled at moderate temperature followed by vacuum drying to remove the solvent. Chujo and co-workers [217,218] synthesized the ﬁrst example of thermally reversible covalent bond crosslinked hydrophilic polymers (hydrogel) through DA reaction of maleimide

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Fig. 13. Chemical structures of monomers and functionalized macromolecules for preparing imine bond or boronate ester bond containing polymers.

Fig. 14. Phenyl boronic acid derivatives utilized for preparing boronic acid functionalized polymers.

and furan functionalized poly(2-methyl-2-oxazoline) (36 and 37, Fig. 15). Gheneim et al. [203] prepared copolymers bearing pendant furan groups (38, Fig. 15) by free-radical copolymerization of n-hexyl acrylate and 2-furfuryl methacrylate (FMA) using 2,2azobisisobutyronitrile (AIBN) as initiator. Then, a bis(maleimide) macromolecular crosslinker (39, Fig. 15) was synthesized from oligomer diamine precursor, and reacted with the furan functionalized copolymers by DA reaction. Toward the development of high toughness self-healable aromatic polyamides, Liu et al. prepared polyamides containing maleimide (40, Fig. 15) and furan pendent groups (41, Fig. 15), respectively. DA reaction between these two types of polyamides gave the desired crosslinked networks [219]. Similarly, Marref and co-workers explored a thermo-responsive epoxy network [220], which was prepared from furan side-chain-containing epoxy oligomer (42, Fig. 15) and terminal tris-maleimide functionalized polyether (43, Fig. 15). Deﬁze et al. [221] demonstrated that DA crosslinked poly(␧-caprolactone) (PCL) networks with recyclability and shape memory effect can be

formed via cyclo-addition between two functional polymers (44 and 45, Fig. 15).

4.2.2. Reversible condensation In analogy to the methods described in the subchapter 4.1.2., reversible imine bonds can also be introduced into polymer networks by reversible condensation between macromolecules. Because a wide range of imine bonds (which possess different stabilities and pH sensitivities of reversible transition) and functional polymers are available, there are many possible ways for obtaining the target materials. Nevertheless, acid catalyst (e.g., acetic acid (HAc) and triﬂuoroacetic acid (TFA)) or heating used to be required for accelerating the reaction. Wei and co-workers [121,188] synthesized di-acylhydrazine functionalized PEO (46, Fig. 16), which reacted with the amino moieties on chitosan (47, Fig. 16), offering dynamic gels with Schiff base bonds. Jackson et al. [222] proposed a method to prepare core-crosslinked star (CCS) polymers by formation of Schiff-base linkage between aldehyde-

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Fig. 15. Chemical structures of furan/maleimide functionalized macromolecules.

Fig. 16. Chemical structures of functionalized macromolecules for preparing imine bond, boronate ester bond and disulﬁde bond containing polymers.

and amine-functionalized polystyrene-poly(methyl methacrylate) based copolymers (48, Fig. 16). With respect to the production of networks consisting of borate ester linkages, reversible condensation between macromolecules proves to be a feasible pathway. Ivanov et al. [187] synthesized boronate-containing copolymers by radical polymerization of Nacryloyl-m-aminophenylboronic acid (AAPBA, 49a, Fig. 16) and N,N-dimethylacrylamide. Then, crosslinked water-soluble polymer gels were made by mixing the copolymer precursor and PVA in weak alkaline solution. Recently, in the lab of Deng et al. [186], selfhealing boronate ester-crosslinked hydrogels were prepared from

49b (Fig. 16) and PVA or catechol-functionalized copolymer (50, Fig. 16) by a similar strategy. Owing to the intramolecular coordination between carbonyl oxygen and boron (49b, Fig. 16), the hydrogels were allowed to be formed under neutral and acidic pH conditions. Furthermore, an ABA type tri-block copolymer (51, Fig. 16) bearing phenyl boronic acid dangling chain was prepared by aminoepoxy reaction between 3-aminophenylboronic acid (APBA) and the epoxy side group of poly(glycidyl methacrylate) segment. By adjusting the length of the linear polymer chain segment and pH of polymer aqueous solution, hydrogels with tunable mechanical

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thiolactone) [231], cyclic thiolimidate (e.g., 2-iminothiolane) [232], and dithiocarbonate group [233,234] could also react with primary amino group by aminolysis reaction to give thiol-grafted polymers. After that, disulﬁde crosslinked networks are allowed to be formed by necessary oxidation. In summary, on one hand, it is fairly simple to bring in disulﬁde bonds by oxidation of thiolated polymers with oxidant (even air, for example). On the other hand, because thiol groups are easily to be oxidized, preparation of thiolated polymers through amidation (or esteriﬁcation) reactions from thiol monomers has to be carried out in acidic condition with the protection of inert gas. Besides, thiol reagents (e.g., 2-iminothiolane) used to be expensive and difﬁcult to use to operate, which would affect their application.

Fig. 17. Thiolation agents utilized for preparing thiolated polymers.

properties were fabricated via condensation between boronic acid groups and hydroxyl groups on PVA [223]. In fact, due to the restriction of macromolecular reaction (i.e. the reaction between the functional groups has to mainly occur inside the random coils of macromolecules), phenyl boronic acid polymers are difﬁcult to effectively react with multi-hydroxyl polymers. For instance, hydroxyl groups are entangled along the PVA chains, so that the internal hydroxyl groups cannot be fully combined with phenyl boronic acid groups when a certain amount of phenyl boronic acid polymer is attached to PVA. 4.2.3. Redox reaction Redox reaction refers to the reaction in which atoms have their oxidation number changed through electron transfer [15]. Many important biological processes (e.g., cellular respiration, photosynthesis and protein folding) and electrochemical applications (e.g., cathodic protection and electroplating) involve redox mechanism, which includes both a reduction process and a complementary oxidation process. Among them, the formation of disulﬁde bonds from thiol oxidation is critical for building up bridging structures in biological systems, which plays an important role in the folding and stability of some proteins [60]. Redox reaction of thiol-disulﬁde has been extensively studied, which is closely related to the manipulation of bioactive substance as well as manufacturing, modiﬁcation and degradation of polymeric materials [70]. Accordingly, polymer networks with reversible disulﬁde crosslinks were prepared by utilizing oxidation of the thiol moieties on thiolated polymers [224]. In general, thiolated polymers are obtained by introducing thiol groups into polymer chains using thiolation agents such as cysteamine [225,226], cysteine [227] and thioglycolic acid [228] (Fig. 17). After that, different oxidation methods can be applied. According to the research report of Sun et al. [229], thioltelechelic poly(ethylene oxide-b-propylene oxide-b-ethylene oxide) (PEO-PPO-PEO) can be derived from the corresponding hydroxyl functionalized precursor through multi-step organic synthesis. The disulﬁde-containing multi-block copolymer (52, Fig. 16) was prepared by oxidation of thiol groups on PEO-PPO-PEO in the presence of oxygen. More recently, Michal et al. [230] prepared polydisulﬁde networks with photo-healability by oxidative coupling of the thiol groups between thiol-telechelic oligomer (53, Fig. 16) and pentaerythritol tetra(3-mercaptopropionate) (PETMP) using sodium iodide/hydrogen peroxide. The oligomer was obtained through photo-initiated thiol-ene reaction between 1,6-hexanedithiol and 1,5-hexadiene. In addition to the thiolation agents carrying sulfhydryl group, thiolactone (e.g., 4-butyrothiolactone and N-acetylhomocysteine

4.2.4. Radical crossover exchange of reversible bonds As a representative of nitrogen oxygen free radicals (stable radicals), 2,2,6,6-tetramethylpiperidine-1-oxy (TEMPO) is commonly used as free radical scavenger that can undergo fast coupling termination with growing chain radicals or primary radicals to form dormant species. Under certain temperature, dynamic equilibrium of the chain radicals and stable radicals could be achieved through homolysis of living (dormant) species, which has been used for nitroxide-mediated radical polymerizations (NMRP). Accordingly, alkoxyamine derivatives from TEMPO can serve as unimolecular initiators for NMRP [235]. Otsuka and co-workers [87] suggested that radical crossover reaction between alkoxyamine units could be useful for preparation of thermally reversible polymeric hybrid materials. The authors [236] prepared linear polyester (61, Fig. 18) and polyurethane (62, Fig. 18) containing alkoxyamine in the main chains by polycondensation or polyaddition of alkoxyamine diol (54, Fig. 18) with adipoyl chloride or hexamethylene diisocyanate (HDI), respectively. Then, mixture of the linear polyester and polyurethane was heated to carry out radical crossover reaction, achieving a scrambled polymer. Later on, a variety of macromolecular chains containing pendant or terminal alkoxyamines were synthesized via NMRP (63, Fig. 18), ATRP (64, 65 and 66, Fig. 18) and radical copolymerization (67 and 68, Fig. 18) from the corresponding vinyl monomers (55-58, Fig. 18). Consequently, polymers with different structures, such as graft polymer (obtained from 63 and 64, Fig. 18) [237], star polymer (from 66, Fig. 18) [238] and crosslinked polymer (from 64, 65, 67 and 68, Fig. 18) [239–241] were produced via crossover exchange reaction of C ON bonds between the alkoxyamine units. Wang et al. [109] also prepared two kinds of modiﬁed poly(4-vinylbenzyl chloride) (PCMS) with pendant alkoxyamine moieties (69 and 70, Fig. 18) through elimination reaction between chlorine atom on the side chain of PCMS and 59 or 60 (Fig. 18). Then, thermally reversible two-component crosslinked polymer blends carrying two types of C ON bonds in the side chains were synthesized by crossover reaction of the radicals between two PCMS chains, which served as ideal specimens for reversibility study of solid state radical reactions. Although the above approaches have proved to be useful, they suffer from several drawbacks, like high temperature, inert gas atmosphere, generation of irreversible crosslinks by the coupling of carbon free radicals and the accompanied redundant nitroxide radicals. Especially, the irrevesible alkoxyamine by-products would reduce yield and reversibility of useful products, which should be avoided. 4.3. From multi-functional monomers 4.3.1. Reversible cycloaddition (DA reaction) In early 1998, Goussé and Gandini [242] designed a single furanic-dienophile monomer 2-furfurylmaleimide (71, Fig. 19) containing both furan and maleimide groups, which was further

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Fig. 18. Chemical structures of alkoxyamine monomers and alkoxyamine functionalized macromolecules.

Fig. 19. Chemical structures of furan/maleimide multi-functional monomers.

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used for synthesizing DA bond-containing linear condensation polymer. Chen et al. [243] prepared a polymeric crosslinked network by DA cycloaddition reaction between multi-furan monomer containing four furan moieties (72, Fig. 19) and multi-maleimide monomer including three maleimide moieties (TMEA, 7, Fig. 10). The material sets the ﬁrst example of intrinsic self-healing polymer based on reversible DA reaction because of the controlled connection and disconnection of DA crosslinks. Later on, the multifunctional furan monomer (72, Fig. 19) was also utilized to construct DA crosslinked networks with various bismaleimides (73, 74 and 75, Fig. 19) [244]. Subsequently, Gong et al. [245] synthesized DA-based healable polymer from the monomer with four furan moieties (77, Fig. 19) and bismaleimide (1, Fig. 10). Liu and Hsieh [48] prepared crosslinked epoxy-like resin through DA reaction between tri-maleimido functionalized epoxy (76, Fig. 19) and tri-furan monomer (78, Fig. 19). Recently, Mineo and co-workers [246] reported a thermally reversible crosslinked polymer made from bismaleimido (75, Fig. 19) and tetra-furan monomers (77, Fig. 19) under microwave irradiation, which has potential application prospect as recyclable thermosetting material. Li et al. [247] synthesized modiﬁed novolac epoxy (79, Fig. 19) with furan groups by epoxy-hydroxyl ring-opening reaction between novolac epoxy and furfuryl alcohol, which was then crosslinked by bifunctional maleimide (DPMBM, 1, Fig. 10) via DA chemistry. Lehn et al. screened condensation reactions between various dienes and dienophiles [248,249], and found that functionalized fulvenes (82 and 83, Fig. 19) and bis(dicyanofumarates) (81, Fig. 19) or bis(tricyanoethylenecarboxylates) (80, Fig. 19) can rapidly and reversibly react in the temperature range from −10 to 50 ◦ C. The results paved the way for the generation of room-temperature selfhealing polymers. The above-mentioned methods of preparation of furan/maleimide moieties-containing monomers and the corresponding DA crosslinking networks are relatively simple as viewed from chemical synthesis. Because most of the functional monomers are solids, however, they have to be dissolved in organic solvents or melted at elevated temperature in advance for the DA curing reaction, which factually increases the difﬁculty of synthesis. 4.3.2. Reversible condensation Taynton et al. [118] developed a facile approach to prepare crosslinked polyimine networks by simply mixing terephthaldehyde (TPA), diethylene triamine (DETA, 84, Fig. 20) and tri(2-aminoethyl)amine (TAEA, 89a, Fig. 20) crosslinker in combined solvents according to the stoichiometric ratio (CHO/NH2 = 1). Similar preparation strategy was also applied to prepare imine crosslinked polymers with tunable mechanical property by changing the types of amines (85, 86a and 87, Fig. 20) and their proportions [250]. Chow et al. [251] synthesized linear dynamic polymers containing reversible imine bonds by polycondensation of dialdehyde monomers (91 and 93, Fig. 20) and diamine monomers (86b and 88, Fig. 20). This method comes with many advantages like availability of raw materials, easy operation and regulation of products structures. But it sometimes needs the addition of dehydrating agent (e.g., anhydrous sodium sulfate) or heating to promote the imine formation reaction and also involves mixed solvents. Lehn and co-workers [252–254] developed a series of dynamic polymers by acylhydrazone formation reaction between dialdehydes (90, 92 and 94, Fig. 20) and dihydrazides (95-103, Fig. 20), which allow the resultant polyacylhydrazones to exhibit properties different from original homopolymeric components. In the work by Nakazawa and co-workers [256], pH-dependent reversible polymers were synthesized through reversible boronate

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complexation between glucuronamide (105, Fig. 20) and boronic acid-based bolaamphiphiles (104, Fig. 20). Later in 2005, Niu et al. [255] prepared poly(dioxaborolane)s containing boronic ester linkage by polycondensation between 9,9-dihexylﬂuorene2,7-diboronic acid (106, Fig. 20) and pentaerythritol (107, Fig. 20), which could be hydrolytically degraded and re-assembled without the addition of catalyst. As proposed by Leibler et al. [75–77], epoxy cured by carboxylic acid or anhydride could generate ␤-hydroxy ester links, which enabled the networks (e.g., 108, Fig. 21) to participate in alcoholysis reaction (transesteriﬁcation) in the presence of proper catalysts (e.g., zinc acetate and zinc acetylacetonate). Generally, equivalent reactant ratio of epoxy and carboxylic acid moieties can generate equal amounts of hydroxyl groups and ester linkages, which is advantageous for transesteriﬁcation [75,257,258]. So far, epoxy/acid crosslinked networks containing ␤-hydroxy ester links have been made from diglycidyl ether and blended fatty acid [75–77,79,80], dicarballylic acid [259–261], and tricarballylic acid [81], respectively. Moreover, bio-based epoxy networks were produced from epoxidised soybean oil (ESO) and aqueous citric acid solution without catalyst or solvent [82]. Comparatively, the curing mechanism of anhydride is more complex, which has been suggested as selective ring-opening polymerization [262–265]. It used to be divided into two steps [265]. (i) Ring-opening of anhydride occurs to generate a monoester with a carboxyl moiety in the presence of hydroxyl or catalyst. (ii) The newly formed carboxyl moiety reacts with epoxy moiety, yielding a diester and regenerating another hydroxyl moiety. The hydroxyl moiety can continue to carry on step (i) to sustain the curing. The content of residual hydroxyl moieties in the cured material depends on the hydroxyl concentration within the reactant (e.g., hydroxyl moieties in epoxy oligomer, or water molecules in the metal catalyst for ester exchange) and mixing ratio of anhydride and epoxy moieties. Theoretically, one mole of anhydride group can consume twice epoxy group during curing. When the proportion between epoxy and anhydride moieties was increased from 1/1 to 1/0.5, gradual increase of residual hydroxyl moieties in cured material was detected [77]. Therefore, epoxy/anhydride crosslinked networks containing ␤-hydroxy ester structure were commonly obtained from higher epoxy to anhydride ratio [266]. These methods of preparing dynamic epoxy vitrimers have advantages of easily accessible starting materials and solvent-free, and are appropriate for large-scale production and application. Nevertheless, the curing mechanisms are rather complex. High reaction temperature and high dosage of catalyst are generally involved for promoting the curing process and the subsequent transesteriﬁcation. Other crosslinked networks that could undergo alcoholysis and aminolysis reactions were also prepared. Brutman et al. [78] synthesized polylactide (109, Fig. 21) with self-healability and renewability by ring opening polymerization of lactide and pentaerythritol, followed by crosslinking with MDI. Denissen et al. [96] synthesized poly(vinylogous urethane) (PVU) networks through condensation reaction between acetoacetates and slightly excessive amines to ensure the existence of residual free amines in the crosslinked polymer (111, Fig. 21). Crosslinked poly(hydroxyurethanes) (PHUs) containing carbamate bonds and free hydroxyl groups (110, Fig. 21) were prepared by ring-opening addition reactions between bis(6-membered cyclic carbonate) and TAEA (89a, Fig. 20), and tris[2-(methylamino)ethyl]amine (TMAEA, 89b, Fig. 20), respectively [72]. These approaches resemble traditional curing reaction except that a certain amount of reactive groups (e.g., amino and hydroxyl groups) and catalysts (e.g., alcoholysis and aminolysis catalysts) should be intentionally remained in the polymer networks. Nevertheless, the excessive reactive groups may have negative inﬂuences on the performance of the

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Fig. 20. Chemical structures of multi-functional monomers for preparing imine bond and boronate ester bond containing polymers.

Fig. 21. Chemical structures of the reversible covalent polymers carrying ␤-hydroxy ester bond, carbamate bond and vinylogous urethane bond, respectively.

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Fig. 22. Chemical structures of the monomers involved in click chemistry and step-growth addition polymerization.

resultant polymers. The residual amino groups, for example, would be easily oxidized by air. 4.4. From monomers containing reversible linkages 4.4.1. Click chemistry Click chemistry is an attractive synthesis concept introduced by Sharpless in 2001 [267,268]. Its main purpose lies in the development of a series of powerful, highly reliable and selective reactions for rapid preparation of new compounds, with special focus on the combinatorial chemistry based on carbon-heteroatom linkage. Owing to the unique advantages of click reaction, it has a wide range of applications in the synthesis, structural control, functionalization and modiﬁcation of polymers [269]. Production of 1,2,3-triazoles in terms of Huisgen 1,3-dipolar cycloaddition of alkynes and azides under the catalysis of Cu(I) is the most typical example of click chemistry [268]. Because terminal azide groups can be easily introduced into active polymer chains by ATRP, the combination of azide-alkyne click chemistry and ATRP

could provide a universal method to prepare functional polymer with narrow molecular weight distribution [270–272]. Telitel et al. [273] described a UV light-induced self-healing poly(n-butyl acrylate) (PBA) network based on alkoxyamines equilibrium, which was prepared by “click” coupling between dialkyne alkoxyamines (115, Fig. 22) and azide functionalized star-like oligomers synthesized by ATRP. Besides, disulﬁde bonds were also introduced into polymerized liposomes [274] and cationic polymers [275] by alkynes-azides click reaction from 113 and 114 (Fig. 22), respectively. Although Cucatalyzed azide-alkyne cycloaddition has shown the advantages of high efﬁciency, high selectivity and excellent group toleration, the metal catalyst is hard to be removed from products, which would certainly cause performance deterioration (e.g., photoelectric properties and biocompatibility) of the resultant polymers. Thiol-click reaction could be carried out through either radicalmediated process or base-catalyzed nucleophilic mechanism (e.g., thiol-epoxy, thiol-ene and thiol-isocyanate) under mind conditions, achieving relatively rapid reaction rate [276]. Disulﬁde link (112, Fig. 22) and alkoxyamine derivative (117, Fig. 22) were

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respectively introduced into epoxy elastomer networks via basecatalyzed thiol-epoxy ring-opening coupling reaction [39,69,277]. Crosslinked methyl methacrylate polymer containing reversible C ON bonds (116, Fig. 22) was also prepared by thiol-ene Michael addition initiated by 1,8-diazabicyclo(5.4.0)undec-7-ene (DBU) [40]. Besides, Cash et al. [278] synthesized di-vinyl functional boronic ester monomers (118, Fig. 22) and further incorporated these monomers into a polymer network via photoinitiated radical thiol-ene click chemistry. In general, the monomers qualiﬁed for thiol-click reaction involve complicated structural design. When the curing proceeds under solvent-free conditions, rapid gelation, severe heat accumulation and bubbling are the key hurdles to overcome. 4.4.2. Step-growth addition polymerization Epoxy has been extensively employed in many applications due to the excellent properties such as strong adhesion, electrical insulation, chemical resistance, high strength and easy processing [279,280]. So far, the curing agents of epoxy fall into two categories: reactive curing agent (e.g., primary amine, anhydride and carboxylic acid) and catalytic curing agent (e.g., tertiary amine, imidazole and Lewis acid). The former contains reactive hydrogen atom that could react with epoxy group via polyaddition, while the latter catalyzes curing of epoxy following cationic or anionic polymerization. Compared with ionic polymerization, addition polymerization is more convenient to introduce functional reversible monomers into polymers, and to regulate properties of the ultimate products by changing the ratio and structure of reactants. By using various reactive curing agents, epoxy can be cured in a wide temperature range so as to coordinate with the reversible temperatures of different reversible bonds, avoiding unnecessary side effects (e.g., irreversible combination of carbon-centered radicals of C ON bond [106]) and dissociation of reversible bonds. Basically, reversible bonds are incorporated into epoxy networks through two routes. The ﬁrst one lies in the preparation of glycerol ether containing reversible bonds, which is then cured by commercial hardener, while the second is the construction of appropriate hardener containing reversible bonds for curing commercial epoxy monomers. Actually, different preparation methods lead to different thermally reversible behaviors [281]. To obtain cleavable and removable epoxy material, Buchwalter et al. synthesized cycloaliphatic diepoxide containing acetal bond (123, Fig. 22), which was further cured with hexahydrophthalic anhydride at 150 ◦ C [282,283]. After that, diglycidyl ethers containing DA bonds (122a-d, Fig. 22) were also synthesized and cured by various curing agents [284,285]. At temperatures higher than 90 ◦ C, the DA bonds were taken apart, and the cured epoxy was able to be repeatedly produced and liqueﬁed [284]. Recently, Takahashi et al. [281] prepared degradable epoxy with disulﬁde linkage from the curing of bis(4-glycidyloxyphenyl)disulﬁde (BGPDS, 120, Fig. 22) with several diamines at 90–160 ◦ C. Tian et al. [286] synthesized an epoxy monomer containing both furan and epoxy groups. When it reacted with DMPBM (1, Fig. 10) and methylhexahydrophthalic anhydride (MHHPA), reversible DA links (121, Fig. 22) and irreversible crosslinks were successively created in the same network. The DA bonds offered reversible dynamicity, while the epoxy networks served as the ﬁxing phase. The cured version of this speciﬁc epoxy monomer had mechanical properties similar to those of commercial bisphenol-A epoxy. Lei et al. [67] synthesized a reversibly crosslinked polymer from the reaction between polysulﬁde diglycidyl ether (119, Fig. 22) and DETA hardener, which operated at room temperature for 3 days. In the work by Yuan et al. [106], alkoxyamine based diglycidyl ether (124, Fig. 22) was incorporated into epoxy through blending with bisphenol A diglycidyl ether (BADGE), which was then cured by DETA. The achievements in this aspect show that it is

beneﬁt to improve the content of reversible bonds of the resultant polymers by using reversible bonds-containing glycerol ether and commercial amine curing agent, but the synthesis and puriﬁcation are rather complicated. In comparison with the methods mentioned above, the reaction of the hardeners containing reversible linkages (e.g., carboxyl and amino groups) with commercial glycidyl ether is more convenient to prepare reversible epoxy material. This is because the curing agents can be easily synthesized, or even commercially available (e.g., dithiodibutyric acid (DTDB, 125, Fig. 22) and dithiodianiline (DTDA, 126, Fig. 22). Magana et al. [287] designed a carboxyl end-capped maleimide/furan adduct (128, Fig. 22) that was used as crosslinking agent of polyethylene-co-glycidyl methacrylate by epoxy-acid reaction at 140 ◦ C. Imbernon and co-workers [68] introduced disulﬁde groups into epoxidized natural rubber (ENR) using DTDB as curing agent, which performed at 180 ◦ C under acceleration of 1,2-dimethylimidazole. Moreover, diamines carrying reversible covalent bonds, like DTDA [119] and diamine DA adduct [288] (127, Fig. 22), were respectively synthesized as hardeners for commercial epoxy DGEBA. After curing reaction at relatively lower temperature (60–80 ◦ C), dynamic epoxy materials capable of self-healing and recycling were obtained. Polyurethane is a block copolymer composed of low glass transition temperature soft segments (e.g. polyethers, polyesters and polybutadiene) and high glass transition temperature hard segments of carbamate. It has many beneﬁts including abrasion resistance, chemical resistance, high elasticity, excellent mechanical and biocompatibility [289]. The unique micro-phase separation structure of polyurethane or polyurethane-urea ensures suitable molecular mobility for inter- and intra-macromolecular reversible interaction and mechanical strength of the materials. Usually, reversible monomer containing hydroxyl or amino group is ﬁrstly prepared and then introduced into polyurethane through reacting with the prepolymer of polyols and diisocyanates as chain extender. Owing to the different abilities of intermolecular association and crystallization, the choice of soft segments greatly inﬂuences overall properties of polyurethane. Because ester bonds possess higher cohesive energy, for instance, polyester-based polyurethane has higher mechanical properties than polyether- and polybutadienebased polyurethanes. Chen et al. [290] prepared a triol-functional DA crosslinker containing both furan and maleic moieties in one molecule (129, Fig. 22), which reacted with a prepolymer made from PCL and MDI giving a thermally recyclable polyurethane with higher strength (38–44 MPa). The approach is different from conventional one that incorporates furan and maleic moieties into different molecules in advance. Yuan et al. [107] synthesized crosslinked polyurethane containing C ON bonds by reacting tri-functional homopolymer of HDI (Tri-HDI) with poly(ethylene glycol) (PEG) and alkoxyaminebased diol (130, Fig. 22). Fukuda et al. [291] synthesized hydroxyl telechelic bis-imine monomer (132, Fig. 22) by Schiff base reaction between TPA and 2-(2-aminoethoxy)ethanol. Next, dynamic polymers were prepared by combining bis-imine monomers with bio-degradable oligomers (polybutylene adipate, PBS and PEG), and condensation with HDI. Zhang et al. [38] also produced a stiff linear polyurethane with thermally reversible alkoxyamine (131, Fig. 22) by utilizing crystallizability of PEG chain segments. It is worth noting that the crystalline polyether or polyester would decrease mobility of the chain segments, which may in turn affect the reversible reaction of the reversible bonds in solid state and the corresponding stimulus-response behavior. Polyurethanes with poly(propylene glycol) (PPG) as soft segment used to have relatively weak mechanical properties as compared with other crystalline polyether-based polyurethane, because PPG possesses poor molecular chain regularity that is difﬁcult to crystallize. Nevertheless, the macromolecular chain

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segments motion becomes easier, which beneﬁts the design of dynamic polymers workable at room or moderate temperatures. Rekondo et al. [65] reported a room temperature self-healing disulﬁde-containing poly(urea-urethane) elastomer, which was derived from the reaction between isophorone diisocyanate (IPDI)terminated PPG and amino terminated disulﬁde linkage (127, Fig. 22). Otsuka and co-workers [31,32] synthesized two kinds of PPG based polyurethanes with diarylbibenzofuranone bonds (133a and 133b, Fig. 22). Thiuram disulﬁde units (134, Fig. 22) were included in the polyurethanes through poly-addition [71]. Accordingly, these polyurethanes exhibited interesting reversibility at room temperature or temperatures slightly higher than room temperature. Besides, other reversible covalent polymers, such as disulﬁde crosslinked core-containing star polymers [292], disulﬁde functionalized dynamic polyester [293], and boroxine crosslinked PDMS networks [294], were obtained from bis-(2aminoethyl)disulﬁde (135a, Fig. 22), bis(2-hydroxyethyl) disulﬁde (135b, Fig. 22) and aryl-boroxine (136, Fig. 22), respectively. 4.4.3. Radical polymerization Free radical polymerization has been mostly used in polymer synthesis for chain polymerization of oleﬁns in the presence of radical initiators (e.g., azo, peroxide and redox initiation system). In comparison with ionic polymerization that requires harsh reaction conditions (e.g., high vacuum, high purity organic solvent and strong acid/base initiator), free radical polymerization can be conducted under much milder conditions with a wide range of monomers. Furthermore, the high sensitivity to acid and base conditions of some reversible covalent bonds (like disulﬁde and imine) makes them less suitable for participation in strong acid or base catalyzed polymerization. In fact, acrylate and acrylamido derivatives can be easily incorporated into hydroxyl or amino functionalized monomers, so that most of the reversible linkages are allowed to be functionalized with similar terminal groups and take part in radical polymerization. Aliyar et al. [295] prepared acrylamide hydrogels containing disulﬁde bonds by free radical copolymerization of acrylamide and N,N -diacryloylcystamine (137, Fig. 23) in aqueous ethanol, which was initiated by redox system composed of ammonium persulfate (APS) and N,N,N ,N -tetramethylethylenediamine (TEMED). Chien et al. [296] developed carboxybetaine hydrogels through free radical polymerization of carboxybetaine methacrylate and N,N-dimethacryloylcystine (138, Fig. 23) using APS/TEMED as the initiator. Compared with the peroxide and azo initiators, low activation energy of APS/TEMED redox system helped to reduce the aqueous polymerization temperature (to or below room temperature). Nevertheless, the method also had negative effects like slow polymerization rate, low molecular weight of the resultant polymer and high cost of removal of solvent. In 2011, Yuan et al. [108] synthesized self-healable polystyrene (PS) containing reversible C ON bonds as crosslinks through benzoyl peroxide (BPO)-initiated polymerization using alkoxyamine crosslinkers with terminal methacrylate (141, Fig. 23). Recently, Konkolewicz and co-workers [178] constructed crosslinked poly(2hydroxyethyl acrylate) containing both DA and UPy linkages by radical copolymerization of 2-hydroxyethyl acrylate (HEA), diacrylate functionalized DA monomer (139, Fig. 23) and acrylate functionalized UPy initiated by AIBN. Moon et al. [297] designed a methacrylate-terminated DA monomer (140, Fig. 23) acting as the crosslinker of PMMA initiated by BPO. The DA crosslinked PMMA eventually acquired chemical recyclability. Mostly, the BPO or AIBN initiator system can ignite bulk polymerization in the absence of solvent (or with a small amount of solvent to assist dissolution of the reversible bonds-containing monomers), while the thermal effect is relatively signiﬁcant that needs to be carefully controlled.

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As a special case, Pyun et al. [298,299] proposed a method called reverse vulcanization that polymeric sulfur could stably exist by copolymerization of a large amount of sulfur and a few diene monomers. Radical copolymerization was carried out between S8 (142, Fig. 23) and 1,3-diisopropenylbenzene initiated by the homolytic cleavage of S8 under molten state (180 ◦ C), achieving processable sulfur-rich copolymers that are potential to serve as an active substance in Li-S batteries. 4.4.4. Controlled/“living” radical polymerization Due to the existence of irreversible chain transfer and chain termination reaction, traditional free radical polymerization could not accurately control molecular structure and molecular weight. In contrast, controlled/“living” radical polymerization can establish a fast dynamic equilibrium between dormant species and active growth chain free radicals. Therefore, better molecular design and synthesis of polymers with speciﬁc structures (e.g., block copolymer, star polymer and comb polymer) and narrow molecular weight distribution become available. To date there are three kinds of controlled/“living” radical polymerization methods including NMRP, ATRP and RAFT, which have all been utilized to prepare polymers containing reversible covalent bonds. Comparatively, ATRP and RAFT have wider choices of monomers than NMRP, so that various types of reversible bonds-functionalized monomers (e.g., acrylate-type, styrene-type, etc.) can be purposely designed and synthesized. Matyjaszewski and co-workers [300,301] prepared PBA grafted star polymers by chain extension ATRP from crosslinked cores composed of poly(ethylene glycol diacrylate). Bis(2methacryloyloxyethyl) disulﬁde (143, Fig. 23) was further introduced into arms of the star as crosslinkage, producing disulﬁde crosslinked redox responsive star polymers. In addition, the authors proposed a new approach to synthesize dynamic covalent polymer networks [101,102]. That is, a dimethacrylate trithiocarbonate (144, Fig. 23) was used as both chain transfer agent and crosslinker to prepare crosslinked PMMA, PS and PBA via RAFT copolymerization in the presence of radical initiator AIBN. The reorganization of PMMA and PS crosslinked networks could be triggered by AIBN or CuBr/N,N,N’,N”,N”-pentamethyldipropylenetriamine (PMDETA) complex, while the PBA could carry out reshufﬂing under UV irradiation without other initiators because trithiocarbonate units could serve as photoinitiator. As a result, UV-induced rearrangement of the crosslinked PBA networks containing trithiocarbonate was available. Syrett et al. [302] synthesized two kinds of ATRP initiators (145 and 146, Fig. 23) containing DA bonds through multi-step reaction. Self-healing linear MMA polymers were prepared in the presence of the above DA-based initiators with cuprous bromide/pyridine imine as catalyst. Besides, ATRP initiator containing reversible boronic ester bond (147, Fig. 23) was prepared and used for ATRP polymerization of comb-like PS [303]. 5. Polymer engineering driven by reversible covalent chemistry The diversity of reversible covalent chemistry shown above clearly indicates the possibilities of developing new polymers or providing polymers with new functionalities through a novel path, i.e. reversible macromolecular reaction. The components in reversible covalent polymers are allowed to be removed or exchanged once the dynamicity of the well-deﬁned reversible junctions is triggered. The processes differ from traditional modiﬁcation of polymers based on irreversible covalent chemistry, and in many cases reaction medium like solvent is no longer necessary offering environmental and economic beneﬁts. Because of the appearance

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Fig. 23. Chemical structures of the monomers involved in free radical polymerization and controlled/“living” radical polymerization.

of the reversible covalent bonds, the synthetic macromolecules acquire increased freedoms of transforming inherent structure (including topology) and properties, which is factually a viable supplement to polymer synthesis. So far, there have been increasing reports of efforts in this promising and challenging area of polymer science and technology, which are brieﬂy reviewed in the following.

5.1. Properties regulation Polymers are macromolecules consisting of many repeating units. Not only the molecular weight and chain-like structure of macromolecules but also their higher levels of organization (i.e. morphology) account for the unique properties of polymers in solid state [304]. Any structural changes due to reversible molecular reaction ranging from nanometer, micrometer and up to millimeter scales would eventually affect the performance of polymers. In this context, reversible covalent bonds are effective means for tuning polymers properties as deﬁned by their speciﬁc feature.

5.1.1. Mechanical properties For structural materials, mechanical properties determine their applied range, such as rigid plastics or soft elastomers. Irreversibly bonded polymers have relatively stable structure and mechanical properties once they are prepared. On one hand, this guarantees usage safety of the products. On the other hand, however, it is not conducive to the change of properties of materials after polymerization. This is particularly true for thermosets full of permanent networks, which are hard to be modiﬁed even by the physical and chemical techniques applicable to linear polymers. Nevertheless, reversible covalent polymers may radically alter the situation as shown by the following attempts, which brings unprecedented convenience and efﬁciency to large scale production. Yoshie and co-workers [305] synthesized semi-crystalline crosslinked polymers with tunable mechanical properties by using reversible DA linkages in the absence of excessive chemicals and structural adjustments. The crystallizable furyl-telechelic prepolymer (PBPSF2 ) was polymerized with tris-maleimide (M3 , 7, Fig. 10) via DA reaction above or below the melting temperature of PBPSF2 . When DA reaction was conducted in the molten state, relatively soft polymer networks (PBPSF2 M3 -7025, Fig. 24a) were obtained. In contrast, the DA reaction in the semi-crystalline state gave relatively hard polymer networks (PBPSF2 M3 -25, Fig. 24a). The obvious difference in properties originates from different sizes and orders of the crystalline phases. By combining the reversible melt/recrystallization and depolymerization/repolymerization processes, the material can be freely changed between hard and soft (PBPSF2 M3 -25-70-25, Fig. 24a). Besides, molecular weight and substituent of the prepolymer were also found to inﬂuence such a hard-soft conversion [306,307].

Later on, Yoshie et al. [308] demonstrated that the mechanical property of DA crosslinked PCL can be adjusted by changing the curing temperature. Materials with various mechanical properties were obtained by simple thermal treatment that controlled the rates of crystallization and DA crosslinking reaction. Recently, Kuang et al. [309] further studied the dynamic competition between DA reaction and crystallization in DA bonds crosslinked biodegradable polyester at different annealing temperatures. They adjusted the kinetics of DA reaction and crystallization in terms of heat treatment. Since the crystallization process could be suppressed by DA crosslinking reaction at an annealing temperature lower than TrDA but higher than crystallization temperature, a soft and ﬂexible elastomer was obtained by the rapid DA reaction. When the annealing was conducted at a temperature closed to crystallization temperature, the simultaneous DA crosslinking and crystallization resulted in hard plastics with different crystallinities. As shown in Fig. 24b, tensile strengths of the polymers could range from 19 to 123 MPa, and Young’s moduli from 10 MPa to 2.1 GPa. However, the synergistic effect between DA cycloaddition rate and isothermal crystallization rate of the polyester chains at different annealing temperatures have not yet been systematically revealed, which leaves a large room for better understanding and manipulation of the matching of the two controlling factors. In addition to heat, light can also act as an effective stimulus to tune properties [310–313]. Fawcett et al. [312] incorporated coumarin along the backbone of PDMS, so that the silicone contained both thermally reversible hydrogen bonds and photoreversible covalent bonds. Under UV irradiation (365 nm) at a certain temperature, photodimerzation between coumarin moieties occurred, accompanied by the decrease of physical crosslinks of hydrogen bond. A transformation from thermoplastic to thermoset was observed as characterized by the increase of tensile strength and decrease in failure strain. By making use of the reversible photoinitiated retro-cycloaddition at shorter wavelength (254 nm), the covalent crosslink density and hence mechanical properties of the silicone can be reduced to a certain degree. The coumarin concentration was found to be critical for the overall properties. Although the interesting principle and prospect of the strategy has already been shown in literature, the photoinduced reversible property tunability of bulk polymer like this has not yet been reported before. Recently, Rivero et al. [313] incorporated a coumarin diol monomer into PCL-based polyurethane to create a translucent ﬁlm with poor mechanical properties. By taking advantage of photo-dimerization enabled by 354 nm UV light, a crosslinked tough elastomer was obtained, which displayed improved ultimate stress (from 1.9 to over 6.7 MPa) and fracture strain (from 2.1 to over 430%). When this elastomer was exposed to 254 nm light for 90 s, the ultimate stress was slightly reduced to 5.7 MPa due to photo-cleavage, while the fracture strain remained almost

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Fig. 24. (a) Stress-strain curves of PBPSF2 M3 specimens prepared under different conditions. PBPSF2 M3 -25: the melted sample of PBPSF2 M3 was quickly moved from hot press to an oven at 25 ◦ C and kept for 120 h. PBPSF2 M3 -70-25: the melted sample of PBPSF2 M3 was kept at 70 ◦ C for 6 h and then at 25 ◦ C for 120 h. PBPSF2 M3 -25-70-25: PBPSF2 M3 -25 was kept at 70 ◦ C for 10 min and then at 25 ◦ C for 120 h. [305], Copyright 2008. Reproduced with permission from the American Chemical Society. (b) Stressstrain curves of polyester samples with different processing treatments. The samples were ﬁrst cooled down from 150 ◦ C to 95 ◦ C or 80 ◦ C and annealed for different times, followed by quenching to room temperature in air. [309], Copyright 2015. Reproduced with permission from Elsevier.

unchanged. By further exposing the material to 354 nm for 60 min, its strength and failure strain became 7.5 MPa and 510%, respectively. Besides, the photo-induced crosslinking and de-crosslinking also affected crystallization of the PCL segment, leading to the decrease and increase of Young’s modulus, respectively. However, the photo-conversion efﬁciency was gradually reduced with increasing photo-dimerization/photo-cleavage cycles, and the adjustment scope of mechanical properties had to be narrowed accordingly. 5.1.2. Functional properties Based on the same principle, not only mechanical properties but also non-structural functional properties of polymers can be easily tuned. Lehn and co-workers [251] developed a series of dynamic covalent polymers containing acylhydrazone linkages or imine linkages, which were capable of interchanging their bonds in neat phase even at room temperature without external stimuli, leading to crossover component recombination between polymeric chains. Therefore, it offers the opportunity to develop methodologies for reorganization, modiﬁcation or hybridisation of polymers in the absent of solvents. For example, Ono et al. [130] demonstrated that color and ﬂuorescence changes could occur at the interface between two different polyhydrazone polymer ﬁlms by acylhydrazone bond exchange and crossover component recombination through the interface (Fig. 25a). The result was supported by the corresponding ﬂuorescence emission spectra dependence of heating time of the overlapping domains. Considering that the acylhydrazone bond-containing dynamic polymer ﬁlms are composed by linear macromolecules with lower molecular weight (i.e, 38400 and 13900 g mol−1 ), their application in optical devices may be limited. Further exploration of the polymers with higher molecular weight should be conducted. Similarly, Marin et al. [131] proved that the color transfer can occur at the interfaces between various hydrogels and solid state ﬁlms of imino-chitosan biological dynamic polymers. Compared to the traditional method of synthesis of ﬂuorescent polymers, the present one is rather simple and convenient without the complicated and stringent chemical processes. Nevertheless, trace amount of water was found to be the prerequisite for the color exchange. The combination of DA chemistry and block copolymers has been utilized for controlling dispersion and migration of nanoparticle within polymer matrix, and eventually properties of the polymer composite [314,315]. In principle, distribution of nanoparticles is determined by the compatibility between the external

shell of the particles and surrounding polymer. Accordingly, Au nanoparticles were functionalized by thermo-responsive thiolterminated polystyrene-block-poly(ethylene glycol) (PS-b-PEG) copolymer ligand, in which PS and PEG blocks were connected by DA bonds [315]. When the treated Au nanoparticles were dispersed in a micro-phase separation PS-b-PMMA block copolymer, they preferred to stay with PMMA domains due to the PEG shell. Subsequent heat treatment above the retro-DA temperature led to dissociation of PS-b-PEG, and migration of Au nanoparticles from PMMA to PS domains due to the immiscibility between PMMA and Au nanoparticles. The results may extend over controllable surface functionalization based on phase separation, and modulation of other properties of polymer nanocomposites. In fact, the thermo-driven nanoparticle aggregation feature can be employed to regulate optical clarity of PMMA ﬁlm. As reported by Costanzo and Beyer [316], ﬁrstly, SiO2 nanoparticles were tethered to PEG via DA bonds and uniformly dispersed in PMMA, leading to a transparent ﬁlm. Annealing of the ﬁlm above retro-DA temperature cleft the DA bonds and rendered the silica nanoparticles immiscible with the matrix creating particles aggregation. Consequently, the ﬁlm gradually turned to be opaque as the aggregated particles scattered light (Fig. 25b). Nevertheless, the authors did not consider the reversal process, which would be more attractive if the nanoparticles distribution related transparency can be reversible controlled. 5.2. Intrinsic self-healing Since inter- and intra-macromolecular disconnection and connection are involved in reversible covalent polymers, intrinsic self-healing strategies that enable autonomously re-bonding cracks have been proposed accordingly [318]. The research achievements on this topic would be eventually developed into a next generation technology that greatly improves reliability and durability of products. Unlike the extrinsic healing based on microencapsulated healing agent that used to be deactivated after depletion of the healing agent [319], intrinsic self-healing of the same place via reversible bonding can be conducted inﬁnitely in principle (Fig. 26). It has become the main application aspect of reversible covalent chemistry. In view of the properties to be recovered, self-healing polymers can be classiﬁed into two groups. One deals with restoration of mechanical properties (Table 6), while the other concerns regaining of non-structural functional properties (like electrical

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Table 6 Typical intrinsic self-healing polymers towards mechanical properties restoration based on reversible covalent chemistry. Mechanical strengtha−e

DA network DA network PEA PFMES

N/A N/A −30 (DMA) 7.8 (DSC)

∼150 ∼60a 25.9b 1.53 ± 0.3b

N/A N/A 1077 458 ± 30

Epoxy-anhydride

93.5–136.5 (DSC)

8.51–29.1c

N/A

PU

–

25–46.5b

250–450%

PA PMMA PU

4 (DSC) N/A −7.1, 41.2 (DSC)

∼0.25 44.3d 2.26b

>300% N/A 141

Dienes-thiol PDMS Epoxy-thiol Epoxy-amine PUU PDS

−3.5 to −16 (DSC) 65 (DSC) −35 (DMA) −35.5 (DMA) −50.39 (DSC) −30 (DMA)

1.5–4b 9.64 ± 0.28b ∼0.5b 0.23b 0.81 ± 0.05b ∼12b

N/A 9.72 ± 0.19 ∼70 113 3100 ± 50 50–160

VPB VCR PU PU

N/A N/A 29.8 (DMA) −26.9

∼3b ∼9b 9.66b 1.31b

∼400 ∼650 550 380

PA PS Epoxy-amine PU PU Epoxy-thiol Acrylate-thiol PBA

−21.2 (DMA) 125 (DMA) 52.2 (DMA) −60 (DMA) 20.3 (DMA) 12 (DMA) 27 (DMA) −50

0.19b 99.4c 0.38e 0.2e 6.78b ∼0.25b 0.42b 0.065 ± 0.011b

270 N/A N/A N/A 175.2 ∼55 266 N/A

PU PU PU PBD

−34 N/A -58 (DSC) N/A

0.392 ± 0.196b ∼0.08b ∼0.8b ∼0.6b

202 ± 46 ∼500 ∼800 ∼0.9

PCO PLA

N/A 50–57 (DSC)

2.85 ± 0.38b 48–60b

Epoxy-acid PU

∼20 (DMA) −52 (DSC)

∼0.6b 0.93 ± 0.06b

a b c d e f

a

b

Mechanism

Healing efﬁciency (%)

Refs.

150 C 115 ◦ C, 30 min; 40 ◦ C, 6 h R.T., 53 days R.T., 10 days

DA cycloaddition DA cycloaddition DA cycloaddition DA cycloaddition

[243] [244] [321] [322]

DA cycloaddition

345 ± 80 4.5–5.5

110–126 ◦ C, 20 min; 80 ◦ C, 72 h 120–130 ◦ C, 1.5–5 min; 55–60 ◦ C, 24 h 90 ◦ C, 7 h >280 nm, 10 min 254 nm, 1 min; 365 nm, 90 min. R.T., 3 days, 85% humidity 70 ◦ C, 12 h, water 60 ◦ C, 60 min 25 ◦ C, 24 h, air R.T., 24 h 320–390 nm, 2000 mW cm−2 , 5 min 110 ◦ C, 12 h 120 ◦ C, 5 h Sunlight, 4 h 0.6 W cm−2 , 457 nm bule laser, 35 ◦ C, 30 min. 25 ◦ C, 24 h, air 130 ◦ C, 2.5 h, Ar 90 ◦ C, 1.5 h, Ar 15–25 ◦ C, 48 h, air 80 ◦ C, 2.5 h, Ar 25 ◦ C, 24 h, air 80 ◦ C, 4 h, air Swollen in acetonitrile, 330 nm UV, 4 h Visible light, R.T., 24 h R.T., 24 h, air 50 ◦ C, 24 h 5–22 ◦ C, 10–30 kPa, 15 min–24 h 50 ◦ C, 16 h 140 ◦ C, 30 min, 4 MPa

50% recovery of fracture load 80% recovery of fracture load Average 45% recovery of strain at break 73.7% of the toughness calculated from the area underneath stress-strain curve 65.9–96% recovery of critical stress of DCDC test

∼0.3 301 ± 12

160 ◦ C, 2 h 37 ◦ C, 12 h

Failure strain (%)

Healing condition ◦

DA cycloaddition

[323] [153,190,191]

> 80% recovery of tensile strength DA cycloaddition Cinnamate dimerization Coumarin dimerization

85% recovery of tensile strength 0.1–7.8% recovery of ﬂexural strength 70.2% recovery of tensile strength

[178] [324] [325,326]

Boronic ester hydrolysis/formation Boroxine hydrolysis/formation Disulﬁde-thiol exchange Disulﬁde exchange Disulﬁde exchange Disulﬁde exchange

Complete recovery of tensile strength Complete recovery of tensile strength Almost fully recovery of tensile strength 91% recovery of tensile strength 97% recovery of strain at break Almost 100% recovery of tensile strength

[278] [294] [69,277] [67] [65] [230]

Disulﬁde exchange Disulﬁde exchange Disulﬁde exchange Diselenide exchange

75% recovery of tentile strength 92.0% recovery of apparent shear strength 92.2% recovery of tensile strength 84% recovery of tensile strength

[16] [17] [327] [328]

Schiff base exchange Alkoxyamine Alkoxyamine Alkoxyamine Alkoxyamine Alkoxyamine Alkoxyamine Trithiocarbonate

98.1% recovery of strength 75.7% recovery of critical stress of DCDC test 62.2% recovery of impact strength Over 90% recovery of impact strength 70.7% recovery of tentile strength 65.6% recovery of tensile strength Almost 100% recovery of tentile strength Almost 100% recovery of tensile strength

[61] [108] [106] [38] [107] [39] [40] [101]

Thiuram disulﬁde Diarylbibenzofuranone unit Diarylbibenzofuranone unit Oleﬁn metathesis

Average 97.4% recovery of tensile strength 98% recovery of strain at break Almost 100% recovery of tensile s strength Restoring most of the strength

[71] [31] [32] [89]

Boronic ester exchange Transesteriﬁcation

Average 94.6% recovery of tensile strength Maximum 67% recovery of strain at break and 102% recovery of tentile strength Almost 100% recovery of tentile strength Average 87% recovery of strain at break

[44] [78]

Transesteriﬁcation Urea bond

Compact tension facture load, N. Tensile stress, MPa. Critical stress measured by double cleavage drilled compression (DCDC) test, MPa. Flexural strength, MPa. Impact strength, kJ m−2 . PDS, polydisulﬁde; PCO, polycyclooctene; PFMES, poly(2,5-furandimethylene succinate); PUU, poly(urea-urethane); VPB, vulcanized polybutadiene; VCR, vulcanized chloroprene rubber.

[82] [11]

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Tg (◦ C)

Polymerf

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Fig. 25. (a) Color change and ﬂuorescence emergence by heating of two superimposed polyacylhydrazone ﬁlms P1 and P2. Originally, P1 was almost colorless but slightly turbid, while P2 was a light yellow. (Top) Schematic drawing. (Bottom) (i) before heating under sunlight, (ii) after heating, (iii) ﬂuorescence after heating under 365 nm. [130], Copyright 2007. Reproduced with permission from the Royal Society of Chemistry. (b) Photograph showing PMMA containing 15 wt% SiO2 nanoparticles with DA linkage before (left) and after (right) annealing at 120 ◦ C for 24 h. [316], Copyright 2007. Reproduced with permission from the American Chemical Society.

conductivity, transparency, corrosion resistance and superhydrophobicity) [320]. Hereinafter, a few typical intrinsic self-healing polymers belonging to the former group are discussed according to the types of reversible covalent reaction involved (Tables 1 and 2). Owing to the difference between general reversible covalent chemistry and dynamic reversible covalent chemistry (refer to chapter 2 for details), the corresponding healing processes are quite different. That is, the healing driven by general reversible covalent chemistry is conducted in a two-step fashion (e.g., at two temperatures), while that by dynamic reversible covalent chemistry proceeds in a one-step fashion (e.g., at a single temperature). The latter not only simpliﬁes the healing process, but also avoids completely losing load-bearing capacity and integrity of the materials due to disconnection of the reversible bonds during crack healing. 5.2.1. Healing based on general reversible covalent reactions 5.2.1.1. Thermally reversible cycloaddition. Many reported thermally reversible self-healing polymers are bonded by DA linkages. The healing principle lies in disconnection of DA bonds of a polymer through retro-DA reaction at higher temperature followed by reconnection of the disconnected DA bonds via DA reaction at lower temperature. Because of the randomness of the molecular chains at the crack interface, the broken parts can be sewed up by the suc-

cessive retro-DA and DA reactions. In 2002, Wudl and co-workers ﬁrstly applied DA cycloaddition reaction between multi-furan (72, Fig. 19) and multi-maleimide (7, Fig. 10) monomers to construct a thermally remendable crosslinked polymer [243]. The damaged specimen was treated at 150 ◦ C and then cooled down to room temperature, recovering about 50% fracture toughness, which proved the feasibility of solid state reversible covalent reaction for crack rehabilitation. No catalyst was needed for the entire healing process. To improve solubility of the monomers, the authors synthesized two bismaleimide monomers (73 and 74, Fig. 19) with lower melting points in a later work [244]. Higher healing efﬁciency was observed for the crosslinked polymer made from 72 and 74 owing to the higher mobility of the dangling chains. On the contrary, Tg of the crosslinked polymer derived from 72 and 73 was excessively reduced (30–40 ◦ C). The specimen had to be severely deformed before it reached the healing temperature, which made it hard to quantify the repair efﬁciency. Subsequently, Liu and co-workers prepared self-healable DA bonds crosslinked epoxy and high toughness polyamide [48,219], respectively. When the scratched epoxy surface was thermally treated at 120 ◦ C for 20 min and at 50 ◦ C for 24 h, the ﬁssure completely disappeared. However, the cracks on polyamide ﬁlm were only partly healed even the healing time was as long as 5 days due

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Fig. 26. Schematic drawing of self-healing mechanisms involved in polymers. (Note: Self-healing falls into two groups according to the ways of healing: autonomic selfhealing and non-autonomic self-healing. The former is carried out without any intervention, while the latter needs intervention/external triggering such as heating) [317], Copyright 2013. Reproduced with permission from Wiley-VCH.

to the low mobility of polyamide chains. The result displayed that the healing behavior was diffusion-controlled [151]. Considering the fact that the ﬁrst step of crack healing via DA reaction is retro-DA reaction, which would result in de-crosslinking and obvious ﬂowing of the polymer network (Fig. 5b), crosslinked epoxy networks containing both reversible DA linkage and irreversible C C linkage were designed by Tian and co-workers [286]. The latter is responsible for maintaining integrity and mechanical strength of the material when the former undergoes retro-DA reaction for crack healing. The DCDC tests indicated that the cracked specimen can regain about 65% fracture toughness after treatment at 125 ◦ C for 20 min and at 80 ◦ C for 72 h. By further increasing content of the DA bonds, the average healing efﬁciency could reach 96% [323,329]. Cheng et al. [194] extended DA chemistry to organic/inorganic hybrid materials with polyphosphazenes as matrices. Crack healing was performed by DA de-crosslinking at 120 ◦ C for 2 h and re-crosslinking at 60 ◦ C for 3–5 days. The authors suggested that appropriate content of the pendent furan moieties and density of DA crosslinks were important for achieving better healing effect. Lin and co-workers [204] reported a stiff and transparent POSS-based nanocomposite with crack-healing ability due to the thermally reversible reaction of furan and maleimide moieties. Moreover, DA crosslinked bulk polymers with crack healability, such as triple-shape memory poly(p-dioxanone)-poly(tetramethylene oxide) (PPDO-PTMEG) co-network [197] and novolac epoxy [247], were also synthesized. These works only qualitatively assessed the crack-healing ability and quantitative characterization remained open. In the works by Du and co-workers [153,190,191], linear and crosslinked polyurethanes containing DA bonds were developed. The thermally remendable PU displayed tensile strengths ranging from about 25–46.5 MPa, and the corresponding healing efﬁciency

could reach more than 80% for the ﬁrst time. Multiple healing was allowed with gradual decrease of the efﬁciency. Due to the poor heat resistance of polyurethane, thermal treatment at elevated temperature had to result in dissociation of hydrogen bonds and deterioration of mechanical properties [289], which would no doubt affect the healing efﬁciency. Generally, fast dynamic exchange rate not only means quick healing of polymer materials, but also rapid creep. To overcome this limitation, Konkolewicz et al. [178] explored the crosslinked acrylate polymer containing both multiple hydrogen bond (weak but fast) and DA linkage (strong but slow), which showed different self-healing behaviors and mechanisms depending on temperature. At room temperature, the exchange of dynamic hydrogen bond allowed for recovering 50% of strain at break within 7 h, while at elevated temperature (e.g., 90 ◦ C), the combination of physical hydrogen bond and DA linkage could recover majority of its initial mechanical property (90% of strain at break). Although most of the DA healing requires heating assistance, several systems that possess self-healing ability at room temperature have been developed. Early in 2009, Lehn and co-workers proposed such a DA crosslinked polymer despite that quantitative characterization of healing efﬁciency was not available [249]. It involved fulvenes (82 and 83, Fig. 19) as the dienes and bis(dicyanofumarates) (81, Fig. 19) or bis(tricyanoethylenecarboxylates) (80, Fig. 19) as the dienophiles, which was different from the conventional DA chemistry between furan and maleimide. Repairing of the elastomer ﬁlm was demonstrated by lapping followed by pressing to ensure microscopic contact, and the overlapping area was unable to separate by elongating. Then, Yoshie et al. [321] reported a DA crosslinked polyester, which allowed for autonomous mending of cracks at room temperature based on DA reaction between anthracene and maleimide. The average recovery due to healing at room temperature for

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53 days was 17 and 45% in terms of tensile strength and elongation at break, respectively. By increasing the healing temperature to 100 ◦ C, a better recovery was observed after 7 days. The healing efﬁciencies reached 45 and 79%, respectively. Compared with the DA system from furan-maleimide pair, the DA adduct between anthracene and maleimide is a potential replacement for designing self-healing polymers with high thermal stability, despite the fact that the healing time was obviously longer and the healing efﬁciency needed to be improved. Similarly, the authors also developed bio-based polymers from bis(hydroxymethyl)furan, which could undergo room temperature self-healing at bulk or with the aid of bismaleimide solutions [322]. Because the kinetic rate (Fig. 3b) and mobility of macromolecular chain segments decrease with decreasing temperature, however, the DA reaction rate has to be suppressed at ambient temperature. Consequently, healing efﬁciencies of the above polymers were not promising. On the whole, the DA reaction driven self-healing has to be carried out in a two-step fashion as a result of successive retro-DA and DA reactions. During the ﬁrst stage of healing (i.e. retro-DA reaction/de-crosslinking), creep deformation or even collapse of materials with DA crosslinked linkages often occurs [152,156] due to molecular cleavage. Evidently, it is a challenge for structural application, where distortion of end-use products is not allowed even if self-healing is proceeding. In fact, however, the low viscosity and high mobility of DA crosslinked networks at elevated temperature are ideal for application of self-healing coatings, where load bearing capacity is not the requisite property. Wouters and co-workers [158] synthesized methacrylate- and epoxy-based DA crosslinked coatings, which proved to be quite excellent for scratch healing even if small amounts of coating was scraped off. In comparison with methacrylate-based copolymers obtained from linear precursors bearing furan moieties and bismaleimide, the epoxy networks from DA adduct monomer had much lower initial ﬂow temperature (below 95 ◦ C) and ultimate viscosity (refer to subchapter 3.1 for details). Kötteritzsch and co-workers [330] described several DA pendent chains-containing methacrylate-based copolymers for the development of one-component coating, which exhibited diverse repair abilities as demonstrated by atomic force microscopy (AFM) and scanning electron microscope (SEM) measurements. Various methacrylate co-monomers (e.g., MMA, butyl methacrylate (BMA) and lauryl methacrylate (LMA)) with different chain lengths were used to tune ﬂexibility and mobility of the ﬁnal copolymers. Due to higher molecular mobility and sub-ambient Tg , the polymer with LMA as co-monomer was able to remend scratches within 2 min at 160 ◦ C or within much longer time (4 h) at the retro-DA temperature (110 ◦ C). Scheltjens et al. [157,198] reported a DA crosslinked epoxy coating (thickness = 200 ␮m) prepared by solution casting, which allowed for ﬂowing above 90 ◦ C. The manual scratches can be completely remended at 130 ◦ C within 2 min. Moreover, Amato et al. [331] used DA linkage-containing soybean-based polyurethane as a two-component automotive topcoat, which displayed remendability of abrasion above 95 ◦ C. 5.2.1.2. Photo-reversible cycloaddition. In addition to the thermally initiated [4 + 2] cycloaddition, photoinitiated [2 + 2] and [4 + 4] cycloaddition can also be used for photochemically healing of polymers, as reversion of the cycloaddition resultant to C C bonds can readily take place in solid state [332,333]. Self-healing based on photoreversible reaction is quite attractive because the use of light is clean, cost-effective and easily access. It is especially suitable for the repair of speciﬁc injured regions where thermal effect is unavailable. In general, UV light having shorter wavelength (␭ ≤ 280 nm) and that at longer wavelength (␭ ≥ 280 nm) are employed for successive photocleavage and photodimerization of the photoresponsive reversible bonds. Due to fast decay of incident

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light in bulk polymer, however, the cracks deep inside products could not be healed by this method. Therefore, the most promising application of photo-stimulated self-healing lies in coatings and ﬁlms. Chung and co-workers [324] proposed the ﬁrst example of photochemical crack healing of a rigid and transparent polymer based on cinnamate [2 + 2] photocycloaddition reaction. The material was ﬁrstly crosslinked by UV irradiation at ␭ > 280 nm. When it was subjected to impact load, cleavage of the resultant cyclobutane ring occurred due to its low bond strength. Re-irradiation with UV light at ␭ > 280 nm allowed recovery of the crosslinked networks within 10 min, but the healing efﬁciency in terms of ﬂexural strength was low. Froimowicz and co-workers [334] prepared a polyglycerol dendrimer grafted with anthracene groups, which showed photo-reversible crosslinking property by exposing to 366 and 254 nm UV lights, respectively. The crosslinked ﬁlm with an artiﬁcial scratch was ﬁrstly irradiated with UV light of 254 nm to regenerate the macromolecular building blocks, and then kept in darkness for overnight to reﬁll the damaged region. Finally, the crosslinked ﬁlm was recovered by anthracene [4 + 4] cycloaddition under 366 nm UV light. Obviously, the material is somewhat difﬁcult to be used in practice since the de-crosslinking procedure led to complete disintegration of the networks. In the works by Ling et al. [325,326,335], photocrosslinked polyurethanes containing coumarin in the main chain and side chain were respectively synthesized. By taking advantage of photo-reversibility of coumarin, repeatedly self-healing as characterized by restoration of mechanical strength under UV illumination at room temperature was observed. The inﬂuences of the soft and hard segments as well as the content of coumarin moieties were discussed. It was found that the rubbery domains originating from phase separation were necessary for improving the efﬁciency of photo-remending. More recently, UV-triggered self-healing crosslinked polyphosphazenes was explored by Hu et al. [336]. They introduced photo-responsive ethyl 4-aminocinnamate to the main chains of linear polyphosphazenes as pendant groups, which were then exposed to UV irradiation at 365 nm to obtain crosslinked networks. The retro[2 + 2] cycloaddition reaction at the UV of 254 nm and [2 + 2] cycloaddition reaction at the UV of 365 nm resulted in self-healing of surface cutting of the elastomer as revealed by UV–vis spectroscopy and SEM. Polyphosphazenes are known for their unique backbone consisting of alternating phosphorus and nitrogen, and the attractive properties like heat resistance and ﬂame retardancy. The photo-self-healability undoubtedly adds to it. In summary, although photochemically triggered cycloaddition has been successfully applied to self-healing polymeric materials, the high energy UV radiation used to result in irreversible side reactions and then decrease the reproducibility of the self-healability. In-depth fundamental researches have to be made in the future to solve the problem. 5.2.1.3. Hydrolysis-bonding equilibrium. Hydrolysis-bonding equilibrium of condensation reaction (e.g. borate ester linkage formation, refer to Table 1) has been exploited for self-healing hydrogels as they contain large amount of water. Self-healing hydrogels with borate ester bonds [159,186,337], for example, have enabled autonomous crack healing without external intervention, despite the relatively low mechanical strength. The underlying mechanism was attributed to the fact that borate esters were under hydrolysis equilibrium with boronic acid and diol constituents, providing free boronic acid and diol groups on the fracture surfaces for formation of borate ester bonds across the damage interface [186]. Sumerlin and co-workers [278] developed a self-healable boronic ester network, which could completely recover tensile strength of bisected specimens by healing at room temperature and 85% humidity for 3 days. The healing was proved to be depen-

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dent on water content, which shifted the equilibrium of boronic ester towards dissociation to generate more free boronic acid and diol moieties, followed by reformation of boronic ester linkages across the damage interface for remending. Meanwhile, Lai and co-workers [294] demonstrated that the boroxine/boronic acid equilibrium can be readily shifted by the addition/removal of Lewis bases or water. They provided a new mechanism of self-healing of boroxine-containing PDMS networks, which had both strong and stiff mechanical properties (Young’s modulus = 182 ± 15.8 MPa; compressive modulus = 142 ± 9.8 MPa). The self-healing was carried out by cutting the specimen into two pieces, which were then wetted with water and healed at 70 ◦ C for 12 h. Complete recovery of mechanical strength was detected after drying. Deng and co-workers synthesized an acylhydrazone crosslinked polymer gel (from 26 and 27, Fig. 13), which displayed self-healing ability based on the reversible breaking and regeneration of acylhydrazone bonds in the presence of acetic acid [162,338]. The free acylhydrazine and aldehyde moieties produced from reversible equilibrium could diffuse across the crack interface and reform acylhydrazone bonds, resulting in healing of two separated plates. Besides, almost identical results were obtained subsequently by Apostolides and co-workers [339] using tetrahydrazide-containing monomer and dialdehyde oligomer as building blocks. In fact, multiple reversible reactions (i.e. formation reaction and exchange reaction of C N bonds) may be simultaneously involved in the healing system containing C N bonds, which are rather complicated and sometimes difﬁcult to make a distinction. 5.2.2. Healing based on dynamic reversible covalent reactions 5.2.2.1. Exchange reaction of disulﬁde bond. Klumperman and coworkers [277] reported self-healing behavior of a commercially available epoxy elastomer containing disulﬁde bonds (20 wt%) for the ﬁrst time. Tensile strength of the broken specimens could be fully restored by treatment at 60 ◦ C for 1 h. It was believed that disulﬁde exchange accounted for the self-healability. But according to the follow-up research of the authors [69], the time scale of aliphatic disulﬁde bond exchange reaction appeared to be much longer, and the self-healing mechanism should be ascribed to pH dependent disulﬁde-thiol exchange reaction. Subsequently, Lei and co-workers [67] proved that room temperature air-insensitive disulﬁde metathesis can be triggered by the catalyst TBP under alkaline conditions. According to this ﬁnding, a cross-linked polysulﬁde containing the phosphine was prepared, which exhibited multiple self-healability in terms of restoration of tensile strength. Rekondo et al. [65] developed a catalyst-free room temperature self-healing poly(urea-urethane) (PUU) elastomer based on aromatic disulﬁde metathesis. The repaired sample showed 97% recovery of original strain at break, despite the fact that almost 51% of healing efﬁciency was contributed by hydrogen bonding. Very recently, Xiang and co-workers [16,17] added cupric chloride (CuCl2 ) and organic complex copper(II) methacrylate (MA-Cu) into vulcanized rubbers (polybutadiene and chloroprene rubber), respectively. Because disulﬁde metathesis can be activated in the inherent sulfur crosslinks at 110–120 ◦ C, the vulcanized rubbers can be repeatedly self-healed to recover most of the mechanical strength. Unlike the above works dealing with self-healing via exchange of disulﬁde bonds, the disulﬁde and polysulﬁde bonds in vulcanized rubber were not intentionally introduced for acquiring dynamicity but a must for transforming the plastic rubber into elastic one as invented by Goodyear over one hundred years ago. Otsuka and co-workers revealed that disulﬁde metathesis in aliphatic disulﬁde-containing polyester can be initiated by photoirradiation [293]. Subsequently, a semi-crystalline polymer containing disulﬁde bonds was developed by Michal and coworkers [230], which was self-healable under UV light, despite the fact that the power intensity of the lamp had to be as high

as 2000 mW cm−2 for partial melting of the sample to liberate the frozen disulﬁde bonds. To seek for replacement of UV light from artiﬁcial sources by that from nature, Xu et al. prepared a robust, transparent and yellowing-resistant crosslinked polyurethane carrying disulﬁde bond in the main chain [327]. The disulﬁde bond can participate in the exchange reaction under illumination of the low concentration UV component of sunlight. As a result, the damaged polymer was allowed to be repeatedly healed directly in the sun. Moreover, the conﬂicting requirements for facilitating reversible reactions in solid state and increasing mechanical strength of the material were united by comprehensive regulations of the macromolecular composition and architecture. Disulﬁde bond is one of the most widely used dynamic linkages in self-healing polymers, which combines both stimulus diversity (e.g., heat, ultraviolet light and sun shine) and the ﬂexibility of reaction mechanism (e.g., base-catalyzed disulﬁde exchange, metal ions-catalyzed disulﬁde exchange, thiol-disulﬁde exchange, and radical-mediated exchange). Nevertheless, the self-healing polymers based on disulﬁde bonds are facing some challenges, including relatively lower mechanical strength, compatibility and dispersion of catalysts, easy oxidation of thiol, and UV light induced aging.

5.2.2.2. Exchange reaction of C N bond. In recent years, dynamic hydrogels with self-healability based on imine and acylhydrazone bonds have been designed and prepared for biological applications [340]. A hydrogel made from Schiff base-containing chitosan [121], for example, could undergo dynamic exchange reaction between Schiff base bond and residual aldehyde and amine reactants under multiple biochemical-stimuli like pH, amino acids and vitamin B6 derivatives. Self-healing capability was demonstrated in terms of qualitative visual inspection of crack closure and quantitative rheological analysis of rapid recovery of elastic modulus. By further combination with ferroferric oxide [188], self-healing could even be driven by external magnetic ﬁeld. Chen and co-workers [341] synthesized a polysaccharide based hydrogel containing both acylhydrazone and Schiff base bonds with different pH response ranges. Generally, acylhydrazone bonds take part in reversible reaction only in a slightly acidic condition (pH = 4.0-6.0). Introduction of more reactive imine linkages can endow self-healing ability to the hydrogel under physiological condition. As a result, healing of the fragmented hydrogel was successfully conducted by immersion in phosphate buffer saline (pH = 7) at physiological temperature for 48 h, achieving 95% recovery of compression strength. Deng et al. [163] developed a multi-responsive dynamic polymer hydrogel containing both acylhydrazone and disulﬁde bonds, which exhibited self-healing property through diverse mechanisms adapting to different chemical stimuli. The exchange reaction of acylhydrazone and base trigged disulﬁde exchange were responsible for the self-healing in acidic (pH = 3 and 6) and basic conditions (pH = 9), respectively. Since acylhydrazone and disulﬁde bonds were dynamically locked in neutral condition (pH = 7), the hydrogel lost its self-healability in this case. Nevertheless, acylhydrazone exchange could be activated in the presence of aniline accelerator at pH = 7, which enabled similar healing manner [342,343]. In view of the fact that multi-stimulus responsive polymer gels possess unique properties and potential applications, Zhang et al. [344] developed a dual-responsive gel from benzhydrazide-containing poly(triazole) and disulﬁde-containing dialdehyde. When using aniline as catalyst, both acylhydrazone exchange and disulﬁde exchange reactions were triggered, leading to maximum healing efﬁciency of 85% in terms of compressive stress at break. Polymer hydrogels and organogels are able to provide sufﬁcient mobility for exchange reaction of C N bonds, so that healing could be carried out at ambient temperature. However, the gel-like mate-

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rials usually have poor mechanical strength and limited application range. Kuhl and co-workers [345] applied the mechanism of acylhydrazone bond exchange to crack healing of methylacrylate-based copolymer solid. When the 1:1 mixture of triethylene glycol methylether methacrylate and 2-hydroxyethyl methacrylate was copolymerized with 10 mol% acylhydrazone crosslinker in the presence of initiator AIBN, a bulk polymer (Tg = 45 ◦ C) was yielded, which was able to remend scratches up to one centimeter long through treatment at 100 ◦ C for 64 h. When aryl groups were attached to both nitrogen and carbon atoms of Schiff base bond, dynamic reversible exchange of the aromatic Schiff base bonds can be conducted at room temperature free of catalyst, as reported by Lei and co-workers [61]. Accordingly, polyacrylate with aromatic aldehyde groups (24, Fig. 13) reacted with 4,4 -diaminodiphenyl methane (25, Fig. 13), producing selfhealing networks (Tg = −21.2 ◦ C) crosslinked by aromatic Schiff base bonds. As Schiff base polymers are known for their diverse properties and stimulus-responsivities, triggering the dynamic exchange of the Schiff base bonds in a similar way would arouse a few novel functionalities in addition to self-healability. 5.2.2.3. Dynamic equilibrium of alkoxyamine. Homolytic cleavage of C ON bonds at certain temperature produces carbon-centered radicals and oxygen-centered nitroxide free radicals, which could be synchronously recombined [346]. It offers an effective damage repairing approach when C ON bonds are introduced to polymers. Accordingly, Zhang and co-workers developed a series of self-healing polymers based on the thermally reversible ﬁssion and recombination of C ON bonds [38–40,106–109]. Furthermore, homolysis temperature of C ON bond (i.e. self-healing temperature) can be adjusted within a wide range from sub-ambient temperature to elevated temperature by changing the steric effect and electronic effect of the substituent groups connected to the C ON linkage. Yuan et al. [108] ﬁrstly applied this mechanism for multiple self-healing of a hard solid material (crosslinked PS), which was performed in argon at 130 ◦ C for 2.5 h. More importantly, although the crack healing temperature was higher than Tg of the polymer, the specimen did not show creep distortion during healing as a result of synchronous covalent bond ﬁssion/radical recombination. The reversibly crosslinked PS at rubbery state had similar deformation resistance as irreversibly crosslinked PS, which ensured stability of the material when crack healing was proceeding. In the later works by the authors, tertiary butyl groups rather than ethyl groups (141, Fig. 23) were linked up with the C ON bonds to produce larger steric hindrance, which decreased homolysis temperature of the alkoxyamine derivatives (124 and 130, Fig. 22). Stiff epoxy [106] and polyurethane [107] were successively synthesized from 124 and 130, respectively. Both of them were able recover most of the mechanical strengths by treatment at 80–90 ◦ C in argon. To further decrease healing temperature and improve oxidative resistance of self-healable polymer containing C ON bonds, Zhang et al. [38] designed an alkoxyamine derivative (131, Fig. 22) with a nitrile group attached to the carbon atom of C ON bonds. As nitrile had a strong electron adsorbability and stabilized carbon radicals, 131 became active at lower temperature, and the stableness of both nitroxide and carbon radicals led to an oxytolerant reversible reaction at ambient temperature. When the alkoxyamine derivative was incorporated into a linear crystalline polyurethane, the latter acquired low temperature self-healability as characterized by repeated restoration of impact strength at 15 ◦ C in air. Similarly, crosslinked epoxy elastomer [39] that could selfheal at 25 ◦ C was developed by means of cyano group-containing alkoxyamine derivative (117, Fig. 22). On the contrary, when amido group was connected to C ON bonds (116, Fig. 22), which

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had weaker electron-absorbing effect, moderate homolysis temperature coupled with oxidation resistance was obtained [40]. Accordingly, the crosslinked methyl methacrylate polymer with embedded alkoxyamine moiety 116 can be repeatedly healed at 80 ◦ C in air. Meantime, Telite et al. [273] proposed a crosslinked PBA with photo-healability of cracks via photodissociation of alkoxyamine. The healing behavior was monitored by AFM under UV irradiation. Because C ON bonds in alkoxyamines frequently cleave but immediately recombine at certain homolysis temperature, the above mentioned two-step healing strategy via DA bonds is simpliﬁed to single-step one in the case of self-healing based on C ON bonds. It is conducive to maintain structural stability of products during service. Nevertheless, efforts are needed to hinder the irreversible coupling between carbon free radicals generated by homolysis of C ON bonds, which reduces reversibility of the C ON bonds. 5.2.2.4. Miscellaneous. To realize heat-/photo-stimulation of covalent self-healing polymer, Amamoto et al. [101,102] prepared several dynamic gels (from PBA, PS and PMMA) crosslinked by trithiocarbonate (TTC) unit (144, Fig. 23). Healing of the damaged PBA swollen in acetonitrile was completed in N2 protection as a result of networks reorganisation based on UV driven dynamic chain transfer reaction. Additionally, macroscopic fusion of shredded samples was obtained by UV irradiation under similar circumstances. Meanwhile, healing of PMMA and PS gels was conducted through heat-triggered network reshufﬂing in the presence of AIBN and CuBr/PMDETA complex as initiators, respectively. There is a distance between these ﬁndings and practical application as solvent swelling and inert gas protection serve as the prerequisites. On the other hand, Amamoto et al. [71] demonstrated that thiuram disulﬁde (TDS) moieties (134, Fig. 22) could undergo exchange reaction in air under visible light through two possible mechanisms: radicals transfer and crossover. Therefore, crosslinked polyurethane containing TDS in the main chain was able to self-heal under the stimulation of visible light through radical reshufﬂing of TDS units. Since TTC and TDS units are generally employed as chain transfer agents of RAFT polymerization, the selfhealing systems based on TDS and TTC provide a reference for synthesis of self-healing polymers through RAFT polymerization. Compared with disulﬁde bonds, diselenide bonds have relatively lower bond energy (Table 3), so that they could be involved in reshufﬂing reaction under milder conditions. In the work by Ji and co-workers [328,347,348], diselenide exchange reaction was enabled through radical mechanism under both visible-light irradiation and heating without catalyst. Accordingly, linear polyurethane containing diselenide bonds with visiblelight-induced self-healing ability was prepared. A signiﬁcant enhancement of healing efﬁciency could be achieved by applying pressure, but simultaneously leading to great decrease of Young’s modulus (by more than 20%). By using 0.6 W blue laser (457 nm) instead of table lamp, the healing time was greatly shortened from 48 h to only 30 min, achieving 84% recovery of breaking stress. Although visible light is a convenient and inexpensive means of stimulation, the dynamic linkages included in the visible-lightinduced self-healing polymer must be highly active, which would lead to the structural instability as a result of continuous stress relaxation similar to room temperature intrinsic self-healing polymers. Zheng and McCarthy [104] prepared a siloxane crosslinked polymer containing reactive tetramethylammonium dimethylsilanolate end groups. The “living” anionic species are stable to water, oxygen and carbon dioxide. Anionic equilibration of the networks allowed for self-healing of cut sample at 90 ◦ C. Fracture toughness measurements indicated that the critical strain energy release rate,

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GIC , of the healed sample was 70 ± 12 J m−2 , even surpassing the original value (61 ± 14 J m−2 ). To overcome the shortcomings of ref.[104] (i.e. elevated temperature and long time for exchange), Schmolke et al. [349] regulated the dynamic characteristics and mechanical strength of crosslinked PDMS by varying contents of the ionic initiator (i.e. tetramethylammonium hydroxide) and crosslinker (i.e. bis(heptamethylcyclotetrasiloxanyl)ethane), respectively. Consequently, stress relaxation and self-healing at room temperature through siloxane equilibration were detected. Besides, their results also revealed that there is a strong dependence of the stress relaxation rate on the concentration of “living” anion ends in the dynamic network. Because the reversible units (i.e. “living anionic species”) are not purposely introduced into the above polymers, and anionic ringopening polymerization of cyclosiloxanes is the most commonly used method of preparing polysiloxanes, all the polysiloxanes synthesized by anionic equilibrium polymerization should have the potential to achieve self-healing. Guan and co-workers [89] explored oleﬁn metathesis aided selfhealing of polymer through dynamic exchange of C C bonds. In their model material, the second generation Grubbs ruthenium catalyst was incorporated into crosslinked polybutadiene. Increase of catalyst concentration, pressure and temperature was beneﬁcial for promoting reshufﬂing of the crosslinking networks. Comparatively, the catalyst-free crosslinked polybutadiene showed minimal healability. Imato and co-workers [31] investigated the feasibility of self-healing based on exchange of C C bonds. To verify their idea, a chemical gel was made from PPG-based PU with diarylbibenzofuranone (DABBF, 133a, Fig. 22) as the reversible crosslinkage. The enormous steric hindrance near the center carbon–carbon bond of DABBF reduced C C bond energy, so that a dynamic equilibrium between carbon free radical and carbon–carbon bond can be established at room temperature. Tensile tests indicated that 98% recovery of strain at break was obtained by healing at room temperature for 24 h with the assistance of dimethyl formamide for wetting the fractured surfaces in advance. For overcoming the shortcomings of gels, which used to suffer from solvent volatilization and low mechanical property, the authors [32] prepared a crosslinked PU elastomer containing DABBF bonds (133b, Fig. 22), which was heablable in the bulk state without the help of solvent. Almost complete restoration of tensile properties was achieved after healing at 50 ◦ C for 12 h due to networks rearrangement originating from exchange of the DABBF linkages. It is worth noting that although stable and strong C C and C C bonds were included in the above dynamic polymers towards synthesis of strong self-healing polymers [31,32,89], tensile strengths of the resultants were not high enough (<0.8 MPa [32]) for structural application. Continuous study of the topic is meaningful as many oleﬁn-containing polymers may acquire healability accordingly. Transesteriﬁcation of ester bonds in covalent polymers is an effective method for healing but a considerable amount of catalysts has to be added. As a special case, Altuna and co-workers [82] produced a thermosetting bio-based polymer by crosslinking epoxidized soybean oil (ESO) with citric acid (CA) under the catalysis of the protons derived by the CA dissociation in water. Lap-shear test at 160 ◦ C for 2 h demonstrated the self-healing performance as a result of thermally activated transesteriﬁcation of ␤-hydroxyester groups created in the course of polymerization. Subsequently, Brutman and co-workers [78] prepared bioderived polylactide vitrimers (109, Fig. 21) with self-healing ability, which were synthesized from hydroxyl-terminated star-shaped PLA and crosslinked by MDI. Stannous (II) octoate (Sn(Oct)2 ) served as the catalyst for both crosslinking and transesteriﬁcation. Eventually, many of the materials exhibited full recovery of tensile strength due to transesteriﬁcation assisted healing at 140 ◦ C. Although Sn(Oct)2 catalyzed transesteriﬁcation allowed the polylactide vitrimers to

present shorter mean relaxation time at moderate temperature (Table 4), the catalyst is highly toxic and sensitive to oxygen and water. Cheng et al. [11] studied a dynamic covalent polymer containing hindered urea bonds, which could reversibly dissociate into isocyanate and amine under mild conditions without catalyst. The results showed that there was a direct relationship between dynamic characteristics of urea bonds and the polymer repair behavior. The sample containing hindered urea bonds with large Keq (7.9 × 105 M−1 , Table 3) and reasonable k-1 (0.042 h−1 ) was healed at 37 ◦ C for 12 h, recovering 87% of its initial strain. Cromwell et al. [44] developed two kinds of crosslinked covalent polymers containing dynamic boronic ester linkages (34 and 35, Fig. 13) with different kinetic exchange rates. As a result, completely different efﬁciencies of self-healing due to boronic ester transesteriﬁcation were measured. Only the sample bearing the crosslinker with fast exchange rate (35, Fig. 13) was able to effectively heal at 50 ◦ C, achieving almost the same strength as the original one. It was thus demonstrated that small molecule kinetics can be correlated with dynamic properties of bulk polymer, which would in turn be used to tune the performance of the latter. In consideration of the fact that wider crack would disable molecular interaction across the interface, shape memory assisted self-healing (SMASH) was proposed [350–353]. The method takes the advantage of shape memory effect, rapidly bringing cracked surfaces into intimate contact for the subsequent intrinsic selfhealing. The existing systems are basically coupled with one way shape memory effect, however, which means that after each operation the material has to be trained to regain shape memory performance. 5.3. Improvement of processability Once the reversible covalent bonds in polymers are activated, the latter behave like strong liquids (Fig. 6) [76]. The viscosity slowly decreases with a rise in temperature, offering wider processing window to the reversibly crosslinked polymer networks. In this context, introduction of reversible bonds into difﬁcult-toprocessing polymers is undoubtedly a solution, as they would acquire the aforesaid attractive processability while the number of crosslinks remains unchanged. In addition, when reversible reaction takes place at the interface between reversible covalent polymers, they can be welded together without the aid of adhesives, which has important engineering application value. 5.3.1. Orientation, shape memory and welding after crosslinking Crosslinked liquid-crystalline polymers (CLCPs) possess both the orientational order of liquid crystals and the elasticity of polymer networks. However, the two-step crosslinking alignment technique for manufacturing large-size mono-domain CLCPs is relatively complicated [354], and also impossible for preparation of three-dimensional shapes, which severely restricts practical application of CLCPs [355]. Recently, Pei et al. [356] prepared a liquid-crystalline epoxy elastomer containing exchangeable ester linkages. The included mesogens were allowed to be uniaxially aligned and ﬁxed by stretching above Tv even after crosslinking. Moreover, having replacing the permanent network crosslinks by reversible ester bonds, the liquid-crystalline epoxy elastomer can be remolded into different shapes above Tv , but performed as a traditional crosslinked network with robust mechanical properties below Tv . In a subsequent report of the same material [357], the changes of Tv in response to load were carefully explored. The authors found that the vitriﬁcation in the isotropic phase was independent of stress because Tv was determined by thermal activity of the catalyst. When temperature exceeded Tv , plastic extension

Z.P. Zhang et al. / Progress in Polymer Science 80 (2018) 39–93

increased with a rise in temperature with increasing stress, as crosslink dynamics predominated. Following the similar chemistry, Kawasaki et al. [359] tackled the processing issue of another crosslinked liquid-crystalline polymer based on polysiloxane backbone with side chain mesogens, which could undergo transesteriﬁcation between phenyl benzoates and hydroxyl groups. The results indicated that the crosslinked liquid crystalline polysiloxane acquired moldability at 80 ◦ C using triazabicyclodecene (TBD) as catalyst, which was lower than the thermal processing temperature of epoxy-based exchangeable networks [357]. Nevertheless, the microscopic mechanism involved in the molecular exchange between phenol benzoate moieties and phenolic hydroxyl moieties has not yet been discussed, which is different from that of the reaction between aliphatic ester and ␤hydroxyl studied in ref. [356,357]. Solid-state stretching proved to be an effective method to improve strength of thermoplastic polymers by creating ordered orientation. However, the technique is not applicable to crosslinked polymers as elongation of macromolecules is greatly hidered in this case. Based on the interesting habit of reversible covalent polymers, Zhang et al. [358] successfully induced tensile orientation of a reversible crosslinked polymer containing aromatic pinacol units. Due to the disconnection/reconnection of the C C bonds in aromatic pinacol units above the homolysis temperature, elongation of the macromolecules can be smoothly conducted when the drawing speed matched with the kinetics of the dynamic reversible equilibrium. It was found that the tensile strength and failure strain parallel to the stretcing direction can be adjustable from 27.3 MPa to 115.2 MPa and from 324% to 1501%, respectively. Interestingly, the tensile strength and failure strain in the direction vertical to stretching remained nearly unchanged because the crosslinks were synchronously reformed by the reversible C C bonds during stretching. This is greatly different from thermoplastics, whose tensile strength and failure strain in the direction perpendicular to stretching have to greatly decrease after the drawing treatment. More and more evidences suggest that reversible covalent polymer networks favor processing of devices with complex geometrical conﬁgurations by simple shape ﬁxing and heat treatment, which is difﬁcult to be done by traditional manufacturing methods. Shape memory polymers (SMPs), for example, take effect by elastic deformation based on the change of conformational entropy, which allows the polymers to recover to their permanent shapes from preprogrammed temporary shapes. By combination of reversible covalent chemistry controlled plastic deformation and elastic deformation in a single polymer, permanent shapes could be reset conveniently. Xie et al. [360] synthesized a crosslinked PCL network by radical initiated thiol-ene click reaction between PCL-diacrylate and a tetra-thiol crosslinker PETMP. Under the catalysis of transesteriﬁcation catalyst (TBD), the crosslinked polymer network could be plastically folded into complex 3D shapes above Tv . Furthermore, shape memory effect was obtained at relatively lower temperature between Tp and shape memory transition temperature (Ttran ). Recently, the authors [73] prepared a thermoset shape-memory polyurethane (SMPU) through polyaddition of HDI with PEG and glycerol in the presence of DBTDL catalyst. Similarly, the polyurethane networks were not only allowed for elasticitybased shape memory, but also plasticity-based shape change above Tv as a result of the exchange reaction between the inherent carbamate bonds with DBTDL as transcarbamoylation catalyst. Contrary to common belief, the results suggested that thermal plasticity is generally applicable to all thermosetting polyurethanes without requiring careful structure design. Moreover, highly complicated shape manipulations could be conveniently achieved by further taking advantage of scissor-cut technique. In fact, the intrinsic mechanism of plastic deformation involved in the above systems

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can be readily expanded to other shape memory polymers containing different reversible covalent chemistries. On the other hand, joining of thermosetting parts is a common step in polymer engineering. Because of the infusible and insoluble nature of the irreversibly crosslinked networks, adhesive bonding is mostly used, and the bonding quality is a function of the interface between adhesive and substrate. If fusion bonding, or welding, a long established technology in the thermoplastic industry [361] is applicable, stress concentration caused by curing of adhesive would be eliminated. By taking advantage of reversible covalent chemistry, Capelot et al. imparted weldability to cured epoxy in the presence of transesteriﬁcation catalyst [77]. They slightly increased the ratio of epoxy to anhydride in the epoxy-glutaric anhydride system, leading to increased amount of hydroxyl groups necessary for transesteriﬁcation in the cured networks. Lap-shear tests revealed that the epoxy-anhydride networks can indeed be welded by transesteriﬁcation after simple superimposition at 150 ◦ C and the welding time decreased with increasing zinc acetate content. Compared with organic catalyst (e.g., TBD), the metal catalyst (e.g., zinc acetate and zinc acetylacetonate) adopted here is more stable to resist the harsh conditions of processing. In addition, epoxy-acid networks cured by a mixture of dicarboxylic and tricarboxylic fatty acids can also be welded through the same mechanism. The welding strategy proposed in the two vitrimer systems is robust, cost effective and suitable for mass production. In a recent report, Chabert et al. further examined whether the above concept can be inherited by ﬁber composites [266]. They prepared epoxy-based continuous glass ﬁber reinforced composite plates (53 vol%) by resin transfer molding, in which the epoxy was cured by anhydride at 2:1 stoichiometry with zinc acetylacetonate as transesteriﬁcation catalyst. The composite specimens can be repeatedly welded at 160 ◦ C due to the exchange reaction (Fig. 27a). Surface conformity and chemical bonding at the jointed interface were greatly improved accordingly. Compared with the solid anhydride curing agent applied in ref. [77], here the liquid formula was more conducive to the ﬁber reinforced composites. Generally, a good contact of the overlapping areas must be ensured during the solid welding by applying an external pressure (e.g., about 25% compression for epoxy-acid networks [77]). Besides, the content of the active groups on the surface of the epoxy-acid vitrimers would decrease with increasing time due to oxidation and self-quenching [362,363]. Therefore, pretreatment like cutting and polishing [77,266] that produces a fresh surface has to be done, leading to enough active groups for bonding the interfaces. Towards surface welding without the need of applied pressure and surface pretreatment, Shi et al. [362] prepared an epoxy-acid network following the method of ref.[77], which can be welded with the assistance of ethylene glycol (EG). The strip specimens were ﬁrstly immersed in EG at 180 ◦ C for 30 min, so as to dissolve the surfaces through transesteriﬁcation between the crosslinks and hydroxyl groups of EG. Subsequently, two specimens were superposed at room temperature followed by vacuum heattreatment at 180 ◦ C for 3 h to remove the adsorbed EG and recover the crosslinking networks again. It was found that almost perfect welding was achieved, and the welding quality depended on the soaking time in EG solution. Such a solvent driven welding mechanism can be extended to the repair of surface damage as well as powder-based reprocessing. Meantime, Yang et al. synthesized an epoxy vitrimer using diglycidyl ether of bisphenol-A with adipic acid in the presence of transesteriﬁcation catalyst TBD [260]. Multiwalled carbon nanotubes were added to the reaction system so that the composite was able to be rapidly welded with other epoxy-based polymers without glue under the irradiation of infrared light. Unlike thermally-induced welding, light-triggered welding can easily achieve fast bonding, regional and remote control. Moreover,

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Z.P. Zhang et al. / Progress in Polymer Science 80 (2018) 39–93

Fig. 27. (a) (i) Exploded view of lap-shear specimens for evaluation of welding property: shims, glue, A and B: composites. Welding was performed at the central part; (ii) Composites slabs after welding; (iii) Principle of the test. [266], Copyright 2016. Reproduced with permission from the Royal Society of Chemistry. (b) SEM micrograph of freely rotating rings written using two-photon techniques. [367], Copyright 2012. Reproduced with permission from Wiley-VCH Verlag GmbH & Co. KGaA.

the strong penetration ability of infrared light makes it possible for developing specifc welding technology like transmission welding. On the other hand, as electric and magnetic energies can be transformed into heat by embedded carbon nanotubes, the corresponding electric ﬁeld-driven and magnetic ﬁeld-driven surface welding would be realized. With respect to the joining of liquid crystalline polymers, however, the valuable alignment would disappear if the whole specimen were exposed to high temperature for reversible reaction like the above. Yang et al. [261] added carbon nanotubes into liquid-crystalline epoxy elastomer containing exchangeable ester linkages. Light-induced welding within small area was completed through localized reversible transesteriﬁcation of the matrix activated by infrared light triggered photothermal effect of carbon nanotubes. Due to the ﬂexibility of monomer selection as well as replaceability of carbon nanotubes by other ﬁllers with photothermal effect (e.g., graphene and gold nanoparticles), strong expansibility is endowed to the light-induced joining of liquid crystalline epoxy elastomers based on the transesteriﬁcation. In addition to simple joining, welding via reversible bonding has been applied to more elaborate fabrication processes. As shown by Ji and co-workers [259], three epoxy vitrimers with different shape memory transition temperatures were integrated together by simply overlapping without glues and adhesives. Owing to the interfacial transesteriﬁcation reaction, multifunctional systems (2D and 3D structures) were obtained, possessing triple shape memory effect that could be selectively and spatially controlled stepwise. By further combination with functional additives such as carbon nanotubes and ﬂuorescent dye, photo-responsive multiple shape memory change was realized under the irradiation of different wavelengths. The strategy provides a convenient and feasible way for the manufacturing of 2D/3D multifunctional polymer materials without complex molecular design and synthesis. The works concerning improvement of processability up to now are mainly limited to epoxy vitrimers using transesteriﬁcation reaction. More attention should be paid to other difﬁcult-to-processing polymers such as polyimide, polytetraﬂuoroethylene, and ultra high molecular weight polyethylene or multi-functional polymers by using reversible covalent chemistry. 5.3.2. 3D printing and 3D photolithography Selective laser sintering (SLS) is a mostly used technique of additive manufacturing. For the moment, polymers suited for SLS are vey rare and nylon has taken the majority. To broaden the range of applicable polymer materials, reduce the processing difﬁculty,

and simultaneously improve mechanical strength and functionality of 3D printed products, Xia and co-workers [18] developed polyurethanes crosslinked by reversible covalent bonds (e.g., DA and metal-ligand coordination bonds). When the polyurethane powders were scanned by a laser beam, they would fuse together as the reversible bonds were dissociated, leading to reduced viscosity of the mass. This is critical for creating compact structure of the ﬁnal products, but is unavailable in the case of irreversibly crosslinked polymers. Then, the powder bed was lowered, a new layer of polyurethane powders was applied on top, and the laser scanning was repeated. During cooling after the laser scanning, the polymers in the same layer and neighbor layers would be crosslinked by the reformed reversible bonds. As a result, the printed products had higher strength and self-healing ability as well. The tensile stress and elongation at break of a 3D printed specimen made from DA bonds crosslinked polyurethane, for example, were 13.0 MPa and 413.4%, respectively. Fused ﬁlament fabrication (FFF) is another additive manufacturing technique, which enables 3D rapid prototyping of thermoplastic polymers. It suffers from poor surface ﬁnish, weak mechanical properties and anisotropy of the products. In light of these problems, Smaldone and co-workers [364] attempted to make use of Diels-Alder reversible thermoset (DART) as printing resin, which could be decrosslinked and melt-processed above reverse sol-gel transition (90–150 ◦ C, refer to Fig. 5b for details), and recrosslinked after cooling. As expected, the low viscosity and recrosslinking process provided smooth surface ﬁnish and smaller anisotropy (< 4% in toughness) for the resultants. Furthermore, DART was also used to blend with PLA for improving both strength and toughness of printing products [365]. Tensile strength and toughness of the resulting materials were 2.4 and 5.6 times higher than those of the original PLA, respectively. Recently, to effectively recycle thermosetting 3D print products, Qi et al. [366] proposed a recyclable thermosetting ink for direct-ink-writing (DIW), which consisted of epoxy-acid vitrimer (108, Fig. 21) and nanoclay (18 wt%). The printing product could be recycled into a new ink by transesteriﬁcation mechanism with glycol. To manufacture 3D complex structures required by emerging technologies like photonic devices and tissue-engineering scaffolds, Bowman and co-workers [367] proposed a 3D photoﬁxation lithography. A reversible DA crosslinked polymer was utilized as photoresist for overcoming the tradeoff between photoresist viscosity and the later removal in optical direct write lithographic technique. The DA networks had sufﬁcient mechanical strength serving as a scaffold to support the structure during the image

Z.P. Zhang et al. / Progress in Polymer Science 80 (2018) 39–93

transfer process. Meanwhile, they were easy to be depolymerized into the original low-viscosity monomers in the presence of heating stimuli. Accordingly, complex 3D shapes (e.g., freely rotating interlocked rings or log pile-type structures) were written into DA crosslinked polymer by selectively controlling thiol-ene click reaction to convert the DA adducts into irreversible crosslinks (Fig. 27b). A wide variety of DA crosslinked polymers have been proposed so far, and their mechanical strengths at ambient temperature and ultimate viscosities at elevated temperature can be easily adjusted, showing strong adaptivity to the 3D manufacturing technology. Because crosslinking density of DA crosslinked polymers is highly temperature dependent (refer to subchapter 3.1. for details), however, the thermal reversibility also brings about the disadvantage of lower operation temperature. 5.4. Recycling in bulk state Establishment of reversible covalent polymer networks brings a new dawn to recycling of thermosets. The traditional depolymerization can be replaced by reprocessing of bulk materials simply in solid state. This is undoubtedly an important advance towards development of environmentally friendly, energy saving and cost effective technologies. The latest research results indicate that the reversibly bonded polymers are mostly recycled or reprocessed in two ways. The ﬁrst one is carried out by reshaping of bulk materials based on their unique malleability. Shifting of permanent shape via plastic deformation can be ﬁxed by reformation of reversible covalent bonds. Accordingly, the effect of rearrangement of topology in response to external stimuli looks similar to the reshaping of thermoplastics despite that the underlying mechanisms are distinct from one another. As for the second approach, the polymers to be recycled are pulverized and then remolded at the temperature required by reversible reaction. The challenge is to ensure effective contact between the polymer particles, so that molecular chain diffusion, penetration and entanglement among interfaces could occur and assist the reversible reaction of the macromolecular chains. Moreover, because quite a few irreversible bonds have to be broken along with pulverization, it is difﬁcult to recover the original properties even after reorganization of the reversible networks. This is a shortcoming that should be addressed. Table 7 summarizes a few examples of recycling of crosslinked polymers with reversible covalent bonds. A brief analysis of the recycling via different reversible reactions is given below. 5.4.1. Recycling based on reversible addition The DA bonds crosslinked networks behave like a classic thermoset at ambient conditions, but can be reprocessed or remolded at elevated temperature like thermoplastics and then recover to crosslinked state after cooling [202]. Broekhuis et al. [189] found that due to the retro-DA reaction at higher temperature, the crosslinked polymer was depolymerized and displayed remeltability as linear polymers. In accordance with this ﬁnding, they remolded the granules of DA crosslinked polyketone by hot pressing above retro-DA temperature (120 ◦ C) for 20 min, which were then slowly cooled down to room temperature for re-crosslinking by DA reaction. Almost complete recovery of fracture load was measured for the recycled specimens. Since most of the reversible covalent cross-linked polymers are either stiff or weak in strength, Sun and co-workers [205] prepared a series of DA crosslinked polyurethanes, which were not only strengthened but also toughened by combination of hydrogen bonds and DA crosslinking. These high performance polyurethanes were recyclable through solution casting (dissolution-volatilization) or hot pressing. Compared with the complete recovery of mechanical properties by the solution cast-

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ing method, compression molding at 180 ◦ C for 10 min could only achieve ∼60% recovery of tensile stress. Furthermore, Sun et al. [290] synthesized thermo-reversible polyurethane using a single molecular DA adduct as triol-functional crosslinker (129, Fig. 22). The thermal recyclability of the crosslinked polyurethane was demonstrated by hot-press molding, injection molding and solution casting, respectively. Interestingly, the recycled samples derived from all the three techniques exhibited better tensile properties than the as-synthesized ones. The most attractive is that the DA crosslinked polyurethane was able to be melted at 145 ◦ C and then injected into dumbbell mold under 40 MPa. DA crosslinked epoxy networks with both recyclability and shape memory ability were prepared by Fan et al. [195]. The recycling was conducted by cutting the specimens into small pieces, which were then hot pressed under 10 MPa at 135 ◦ C for 5 min, naturally cooled down to room temperature and re-crosslinked at 50–70 ◦ C. The recycled material exhibited excellent mechanical properties and shape memory performance like the original version without signiﬁcant change. Kuang and co-workers synthesized reversible epoxy networks from a diamine DA adduct crosslinker (127, Fig. 22) and a commercial epoxy oligomer (BADGE) [288]. The as-cured epoxy was pulverized and hot-compressed at 120 ◦ C, and then annealed at 75 ◦ C for 1 h to obtain a renewed bulk material. Visible color darkening from light yellow to dark red was observed due to the formation of ␲ complex of maleimide. Both the ﬁrst and second recycled epoxy polymers showed similar dynamic mechanical properties like the as-cured version, which was measured in elastic range, but the recovery of mechanical strength was not discussed. Currently, sulfur vulcanization and peroxide curing are the main industrial crosslinking techniques of rubbers. The resultant crosslinks are hard to be recycled [14,369]. Polgar et al. [192] prepared a DA crosslinked ethylene propylene rubber that could be de-crosslinked at high temperatures, recycled by compression molding at 150 ◦ C and further annealed at 50 ◦ C for one week. Young’s modulus, tensile stress and failure strain of the recycled rubber were very close to their original values. Bai et al. [201,202] incorporated DA linkages into commercial PB and SBS elastomers, producing DA crosslinked networks coupled with good recyclability. The remolded PB and SBS elastomers possessed mechanical properties comparable to the original ones even after three generations of recycling. Actually, almost all the crosslinked polymers containing DA linkages could be recyclable through reversible de-crosslinking and re-crosslinking [193,196,197,206]. However, many researches in this aspect only studied thermal reversibility in solution disregarding mechanical properties of the recovered polymers. In the next step, recycling parameters have to be optimized for industrial application. 5.4.2. Recycling based on reversible exchange As mentioned above, Leibler and co-workers [75–77] introduced dynamic covalent interaction (i.e. transesteriﬁcation) into epoxy networks (108, Fig. 21). The materials factually acquired malleability at the same time. For instance, the hard networks of epoxy-anhydride containing 10% zinc acetate (serving as transesteriﬁcation catalyst) were ﬁrstly ground into powders, which were then remolded at 240 ◦ C for 3 min [75]. The recycled bulk sample had almost the same mechanical properties and insolubility as the original version. Qi et al. [79,80] studied the inﬂuential factors (e.g., pressure, time and particle size) of recycling of epoxy-acid thermosetting networks containing zinc acetate. Increase of the applied pressure and processing time were both beneﬁcial for improving the ﬁnal mechanical properties. The pulverous thermosets could be recycled for multiple times via reversible transesteriﬁcation at 180 ◦ C under 45 kPa with mechanical properties comparable to the original polymer.

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Table 7 Recycling of reversibly crosslinked polymers in bulk statea . Mechanism

Polymer

Catalyst

Tg (◦ C)

Tensile strength (MPa)

Failure strain (%)

Recycling condition ◦

PK

N/A

87–100

N/A

N/A

DA reversible reaction DA reversible reaction DA reversible reaction

PU PU Epoxy

N/A N/A N/A

−60; 148 (DMA) −23.5 (DSC) 34.9–56.0 (DMA)

49.8 ± 0.7 43 ± 4 15.6–27.4

304 ± 35 400 ± 10 21.8–173.0

DA reversible reaction DA reversible reaction DA reversible reaction Transesteriﬁcation Transesteriﬁcation Transesteriﬁcation Transamination

Epoxy EPR PB Epoxy-anhydride Epoxy-acid Epoxy-acid PVU

N/A N/A N/A Zn(OAc)2 Zn(OAc)2 TBD N/A

∼70 (DMA) N/A −88.6, 97.2 (DMA) 80 (DSC) 30.1 (DMA) 51 (DSC) 87 (DSC)

2.0 ± 0.2 ∼2.0 4.62 55 ≈2.75 N/A 90

N/A >200 46.1 <0.1 143.3 N/A 5.5–7.5

110–150 C, 10–30 min; R.T., 30–40 minb 160–180 ◦ C, 5–20 minb 135 ◦ C, 5 minb,c 135 ◦ C, 5 min; 70 ◦ C, 2 h; 60 ◦ C, 2 h; 50 ◦ C, 30 hb 120 ◦ C, 5 min; 75 ◦ C, 1 hb 150 ◦ C, 50 ◦ C, 1 week.b 160 ◦ C, 5 min, 10 MPa.b 240 ◦ C, 1 MPa, 3 minb 180 ◦ C, 45 kPa, 30 minb 200 ◦ C, 4 hb 150 ◦ C, 30 minb

Transcarbamolation Transalkylation

PHU PTIL

N/A N/A

61 (DMA) 3 (DSC)

72 ± 11 ≈ 1.2

6.9 ± 3.8 ≈ 20

160 ◦ C, 4 MPa, 8 hb 160 ◦ C, 20 MPa, 60 minb

Siloxane equilibration Disulﬁde exchange Disulﬁde exchange Disulﬁde exchange Disulﬁde exchange

Silicone Epoxy-amine Ion gel VPB VCR

TAMS TBP Ionic liquid/ copper salt CuCl2 MA-Cu

−129.5 (DSC) −35.5 (DMA) N/A N/A N/A

N/A 0.23 0.03 ∼3 ∼9

235 113 510 ∼400 ∼650

90 ◦ Cb R.T., 4.5 MPa.b 60 ◦ C, 0.04 MPa.b 110 ◦ C, 10 MPa, 3 hb 120 ◦ C, 5 MPa, 4 hb

Disulﬁde exchange Disulﬁde exchange Disulﬁde exchange Disulﬁde exchange

ENR PUU Epoxy-amine PU

N/A N/A N/A N/A

N/A −50.39 (DSC) 130 (DMA) 29.8 (DMA)

12–14 0.81 ± 0.05 88 9.66

5–5.5 3100 ± 50 7.1 550

Schiff base exchange

PI

N/A

56 (DSC)

40

47

Schiff base exchange Schiff base exchange Boronic ester exchange Alkoxyamine exchange Catechol-Fe3+ exchange

PI PA PCO Acrylate-thiol HBPUd

N/A N/A N/A N/A N/A

34.5 (DMA) −21.2 (DMA) N/A 27; 34 (DMA) −63, 3, −7.5

16.7 0.19 2.85 ± 0.38 0.42; 0.97 2.41

161.5 270 345 ± 80 266; 111 2900

180 ◦ C, 40 minb 150 ◦ C, 3 MPa, 20 minb 200 ◦ C, 10 MPa.b R.T., 10 MPa, 5 min; sunlight, 12 hb 80 ◦ C, 90 kPa, 40 min; water, 90 kPa, 24 hb 120 ◦ C, 10 MPa, 5 minb R.T., 4.5 MPa, 12 hb 80 ◦ Cb 80 ◦ C, 3 MPa, 2.5 hb pH = 4, 24 h; seawater, 24 h; R.T., 6 MPa, 48 hb

a b c d

PI, polyimine; PTIL, poly(1,2,3-triazolium ionic liquid). Compression molding. Injection molding. HBPU, hyperbranched polyurethane.

Refs.

∼100% (fracture load)

[189]

> 60% (strength) ∼100% (strength) ∼100% (strength)

[205] [290] [195]

N/A ∼85% (strength) ∼100% (failure strain) ∼100% (strength) 95.8% (failure strain) for the ﬁrst time –

[288] [192] [201] [75] [79] [368] [96]

> 100% for the ﬁrst time (strength) 74% (stress); 60% (failure strain) > 100% (failure strain) and ∼60% (strength) for the ﬁrst time N/A 100% (strength) 219.9% (strength); 84.3% (failure strain) 70% (strength, made from 60 mesh particles) 90% (strength, made from the 1:2 blend of 100 and 200 mesh particles) 80% (failure strain) ∼100% ∼100% 92.2% (strength) for the ﬁrst time

[72] [99] [104] [67] [234] [16] [17] [68] [66] [119] [327]

∼100%

[118]

∼100% (failure strain) for the ﬁrst time 99.5% (failure strain) for the ﬁrst time > 100% (strength) for the ﬁrst time 86.0% (stress); 73.9% (strength) 93.8% (strength)

[250] [61] [44] [40] [173]

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DA reversible reaction

Degree of recovery

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Furthermore, transesteriﬁcation was also utilized to provide recyclability for liquid crystalline epoxy networks constructed by azobenzene chromophores, liquid crystals and dynamic ester bonds [368]. The recycled ﬁlm obtained by hot pressing of the fragmented polymer at 200 ◦ C for 4 h exhibited great thermomechanical and liquid crystalline properties similar to the original material. In short, vitrimers based on transesteriﬁcation have displayed excellent reprocessability and robust properties, except the fact that an ideal processing could only be completed with high catalyst addition and high processing temperature. Besides, catalyst aging and hydrolysis of ester linkages would undoubtedly affect the long-term stability of the materials. Du Prez and co-workers explored catalyst-free transamination of vinylogous urethanes as an exchange reaction for dynamic reversible covalent polymer [96]. The networks (111, Fig. 21) could be recycled for four times through grinding followed by compression molding at 150 ◦ C without signiﬁcant decay of the mechanical property. Compared with transesteriﬁcation between ester linkage and ␤-hydroxyl, transamination between vinylogous urethanes and amines exhibited better chemical thermodynamic and hydrolytic stabilities, and faster exchange kinetics as well. The corresponding stress relaxation activation energy was determined to be 60 ± 5 kJ mol−1 , which was lower than the value of transesteriﬁcation reaction catalyzed by zinc acetate (88 kJ mol−1 [75]). However, the excess amine would have negative inﬂuence on the long-term durability of the material. For instance, it would be easy to be oxidized in air [12]. Recently, poly(thioether) networks that can be reshaped, remolded and recycled through swift transalkylation reactions have been developed by the same authors [370]. Poly(hydroxyurethanes) (PHUs, 110, Fig. 21) that could undergo network reconﬁguration through nucleophilic addition of free hydroxyl groups to the carbamate bonds (transcarbamoylation) in the absence of catalyst were described by Fortman and co-workers [72]. They were derived from conventional polyfunctional cyclic carbonates and amines, rather than the toxic isocyanates. Stress relaxation at elevated temperatures in the linear viscoelastic regime exhibited an Arrhenius activation energy (111 ± 10 kJ mol−1 ) lower than the simulation result of model compound, which was attributed to mechanical activation phenomenon and further proved by theoretical calculation. The ground sample could be reprocessed into bulk material at 160 ◦ C for 8 h (about 3 ∗ ) under 4 MPa, achieving 74% recovery of tensile stress and 69% recovery of strain at break. Thermal degradation test showed that minor decomposition of the polyurethane networks occurred after treatment at elevated temperature for long time, which resulted in incomplete recovery of the tensile properties. Liu et al. [371] reported a novel dynamic reversible poly(oxime-urethanes) obtained from multifunctional oximes and HDI. Based on the networks reconﬁguration via oxime-enabled transcarbamoylation, the crosslinked polymer showed catalystfree recyclability at 120 ◦ C. Obadia et al. [99] applied azide-alkyne cycloaddition and simultaneous alkylation to make a highly crosslinked ion-conducting networks, which could be recycled from fragments (2–3 mm) at 160 ◦ C and 20 MPa through transalkylation exchanges between 1,2,3-triazolium crosslinks and bromine-functionalized dangling chains. According to rheological study, the crosslinked networks displayed a viscosity activation energy of 140 kJ mol−1 and characteristic relaxation time of 69 s at 170 ◦ C, respectively. The irreversible scission of covalent bonds under the recycling conditions had to result in decrease of Young’ modulus with cycling times (50% for the ﬁrst time and 37.5% for the second time), but the strain at break remained almost the same as the original one. Although the poly(1,2,3-triazolium) networks can be conveniently prepared by using one-pot method without solvent and catalyst, the disad-

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vantages of high cost and low scalability need to be overcome for practical application. Zhang and McCarthy [104] prepared a living silicone rubber by tetramethylammonium silanolate-initiated ring-opening copolymerization of cyclic monomers. The “living” reactive anionic species (TMAS) could react with the macromolecular chains, providing the micro-mechanism (siloxane equilibration) for reorganization of the crosslinked networks and chemical stress relaxation. Actually, the related mechanism of the chemical stress relaxation behavior of PDMS elastomer in the presence of acid or alkali catalyst has been speculated to be the chain exchange reaction since 1954 [105]. In a demonstrative experiment, a dog bone-shaped silicone elastomer was cut into pieces and remolded into a dog-shape specimen at 90 ◦ C. The living networks could be further turned into permanent crosslinking version by heat treatment above 150 ◦ C due to the unstability of the living chain ends. Disulﬁde metathesis has been reported as one of the very few room-temperature dynamic covalent chemistries. In the work by Lei et al. [67], a crosslinked polysulﬁde containing catalyst TBP was synthesized. The material can be reshaped and reprocessed at 25 ◦ C under 4.5 MPa via compression molding as a result of TBP-mediated reshufﬂing of the macromolecular networks. The recycled specimen had almost the same mechanical properties and room temperature self-healability as the virgin one. Recently, Xu et al. [327] demonstrated a crosslinked polyurethane with disulﬁde bonds in the backbone. Photo-triggered reversible exchange of disulﬁde bonds made it possible to undergo multiple reshaping and reprocessing under sunlight. The powdered polymer was compressed into a blank (1 mm thick) under 10 MPa for 5 min and then clamped between two pieces of quartz plates allowing for sunlight shining for 12 h for each side (Fig. 28). Eventually, a transparent bulk polymer with 92.2% recovery of tensile strength was regained. Odriozola et al. [66,119] revealed that aromatic disulﬁde bond was able to provide recyclability for PUU and epoxy without any catalyst. Recycling of PUU was performed by pulverization followed by compression molding at 150 ◦ C and 3 MPa for 20 min. The dynamic epoxy had high mechanical property (e.g., tensile strength = 88 MPa) and allowed for both chemical degradation and mechanical recycling. A typical recycling process was enforced by compression molding at 200 ◦ C under 10 MPa, leading to almost complete recovery of tensile strength and elongation at break. In fact, vulcanized rubbers contain large amount of disulﬁde and polysulﬁde bonds. When the catalyst CuCl2 was compounded with polybutadiene and other usual additives following industrial formulation, reshufﬂing of the inherent sulfur crosslinked networks of the vulcanized polybutadiene can be triggered above 110 ◦ C [16]. On the basis of this mechanism, the vulcanized polybutadiene was allowed to be repeatedly recycled by hot pressing at 110 ◦ C for 3 h. In this case the reversible covalent bonds are not intentionally introduced but appear according to the design of Goodyear about 200 years ago. To improve the compatibility of the catalyst with the rubber matrix, an organic complex Cu-MA, rather than CuCl2 , was added into vulcanized chloroprene rubber [17], which can also initiate disulﬁde metathesis. During the recycling experiments, pulverized vulcanized chloroprene rubber was pressed under 5 MPa at 120 ◦ C for 4 h. Tensile tests of the recycled specimen showed 90% recovery of tensile strength for the blends of 100 mesh and 200 mesh particles at the ratio of 1:2. Besides, the rubber can be continuously reshaped at 120 ◦ C, which proved the recyclability of the rubber from another angle. Meanwhile, Imbernon et al. [68] explored the thermo-activated recycling ability of ENR with disulﬁde groups crosslinked by DTDB. It can be reprocessed without catalyst by hot-pressing at 180 ◦ C for 40 min, recovering 50% of the initial strength and 80% of the elongation at break. Disulﬁde bonds crosslinked ion gel with both high toughness and conductivity might be potentially applied in electrochemical

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Fig. 28. Sunlight activated recycling. The powdered polyurethane (a) was molded into a blank (b) and then clamped by quartz plates (c) for sunlight exposure, eventually resulting in transparent recycled sheet (d). [327], Copyright 2016. Reproduced with permission from the Royal Society of Chemistry.

ﬁeld [234]. Owing to disulﬁde rearrangement catalyzed by ionic liquid and residual copper salt from the click reaction for making the gel, the ion gel could undergo cyclic breaking-restructuring for multiple times at 60 ◦ C, showing good recyclability. More recently, Otsuka et al. [372] proposed a facile strategy to prepare poly(hexyl methacrylate) networks that contain thermally exchangeable bis(2,2,6,6-tetramethylpiperidin-1-yl)disulﬁde crosslinkers. The crosslinked polymer could be reprocessed by simple hot pressing at 120 ◦ C under 70 kPa. Imine exchange is another important reversible reaction. Taynton et al. [118] presented a crosslinked polyimine network, which behaved like a thermosetting polymer at ambient condition, and could be reprocessed in response to heat and water via catalyst-free imine exchange and imine condensation/hydrolysis, respectively. By simply molding the wet polyimine powers under 90 kPa for 24 h at room temperature, the recovered specimen possessed relatively lower tensile strength and elastic modulus than the one formed by heat pressing at 80 ◦ C under 90 kPa for 40 min. Moreover, it was found that the pliability of the polyimine network increased with increasing atmospheric humidity similar to

wood. Considering that the molecular chain structure of the polyimine in ref.[118] was relatively rigid, which would unavoidably result in unplasticized dots, Li et al. [250] prepared a series of crosslinked polyimine ﬁlms based on Schiff-base reaction between TPA and diamine using TAEA as crosslinking agent. By altering the proportion of diamine-functional monomer (87, Fig. 20) and triamine-functional monomer (89a, Fig. 20), crosslinking density of the polyimines can be adjusted so that their tensile strengths varied from 4.3 to 165.6 MPa. The recyclability was checked by hot pressing the pieces of broken specimens at 120 ◦ C for 10 min under 10 MPa. The remolded specimens exhibited good mechanical properties even after three generations of recycling. In the meantime, aromatic Schiff base bonds were incorporated into polyacrylate as the intermolecular crosslinking linkages, which enabled the crosslinked polymer to be recycled at room temperature through rearrangement of macromolecular networks without catalyst [61]. The above examples have shown the interesting malleability and processability of imine bonds-containing polymers, but the inherent hydrolysis sensitivity is an important factor that limits their application.

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As fast dynamic equilibrium between covalent bond and free radicals (carbon and nitroxide radicals) can be established by breaking and recombination of C ON bond at a certain temperature, thermal reversible reaction of alkoxyamine was also used for recycling of polymer in solid state. As shown in ref.[40], two kinds of crosslinked methyl methacrylate networks with different crosslinking densities were prepared by clicking reaction between vinyl terminated alkoxyamine and multifunctional thiol. The alkoxyamine moieties were attached by amido groups with proper electron withdrawing effect that greatly reduced oxygen sensitivity of the dynamic reversible C ON bonds. Consequently, the powdered polymer can be remolded at 80 ◦ C under 3 MPa in air, which is superior to other polymers containing reversible C ON bonds that had to take effect in inert atmosphere [106–109]. Coordination bond (also known as dative bond) is a special type of covalent bond descried as covalent bonding between two atoms in which both two shared electrons from the same atom. Xia et al. [173] synthesized HBPU with functional catechol and hydrophilic carboxyl end groups, which was crosslinked with Fe3+ at pH = 9. The catechol-Fe3+ coordinate bond became dynamic in the presence of water, which provided the polymers with both self-healing and recycling ability in seawater. Recycling of the polymer was performed by soaking the fragments in water of pH = 4 for 24 h to dissociate the catechol-Fe3+ coordinate bonds, and then transferring the mass into seawater for another 24 h to rebuild triscoordinated catechol-Fe3+ crosslinking bonds. The wetted polymer fragments were ﬁnally pressed under 6 MPa at room temperature for 48 h to obtain a fresh dynamic polymer network, leading to recovery of tensile strength and self-healing ability. To decrease pH sensitivity and increase adaptability to different aquatic environments of the underwater reversible lipophilic bulk polymer, the same groups of authors developed hyperbranched polyurethane bearing hydrophilic quaternary ammonium salts and dynamic reversible catechol-B3+ crosslinks [181]. The electron withdrawing effect of quaternary ammonium cations from catechol-B3+ hindered hydrolysis of the boronic ester bonds at lower pH. Consequently, recycling following the same way as mentioned above [173] showed that the polymer can regain its mechanical properties in the waters of pH of both 7 and 9. In addition, networks with boronic ester bonds can also be recycled by heating. Cromwell et al. [44] described two kinds of boronic ester linkages for constructing crosslinked polymer networks with different transesteriﬁcation rates. The polymer containing fastexchanging boronic ester linkages was reprocessed by pressing the millimeter-sized pieces at 80 ◦ C. The reformed bulk material had the same tensile strength and failure strain as the original one, and still retained good recovery after three cycles. Moreover, the localized hydrophobic environment of boronic ester-crosslinked networks can effectively prevent penetration of water and hydrolysis of boronic esters as observed in ref.[278]. Similar strategy was utilized for adjusting dynamic properties of silyl ether crosslinked polystyrene copolymer [373]. Moreover, Leibler and co-workers developed several high performance vitrimers from commodity thermoplastics (e.g., PMMA, PS, high-density polyethylene (HDPE)) through dioxaborolane metathesis, which can be reprocessed for multiple times by extrusion or injection molding [374]. The dioxaborolane metathesis showed thermal stability as well as rapid exchange without the aid of catalyst at moderate temperature during the reprocessing processes. Compared with the recycling based on reversible addition reaction, reversible exchange mechanism can be carried out under complete solid state without decrosslinking of polymer networks, which is more energy-saving and efﬁcient. Nevertheless, most of them generally require the addition of catalyst for completing recycling within reasonable time scale.

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5.5. Controllable degradation Conventional degradation of polymers involves random chain scission. The degradation processes as well as structure and composition of the degradation products are hard to be controlled, which hinders reuse of polymers [375–377]. The design of polymer materials that can be controllably degraded usually requires the incorporation of speciﬁc cleavable bonds, which are responsive to certain environmental factors such as light, heat and moisture, or microorganisms, or chemical reagents [378–385]. Nevertheless, most of controllable degradation of polymers reported so far are irreversible and rely on rigorous conditions (e.g., thermal degradation of ester bonds [378–380] and carbamate bonds [381,382]) with only the objective of translating the polymer wastes into low molecular compounds. The signiﬁcance of the ability of transforming into original reactants is ignored. For sustainable development, it is more preferable to convert the waste polymers to reusable raw substances (e.g., small molecules and macromolecular prepolymers) under relatively milder conditions, which can be subsequently reconstructed to yield new bulk materials without obvious sacriﬁce in properties. For purposes of designing such controllably degradable polymers following “cradle to cradle” philosophy, the introduced cleavable chemical bonds have to meet the following criteria. Firstly, the intermonomer linkages can be fully disconnected, leading to complete dissolution (or depolymerization) of the target polymer. Secondly, the chemical bonds should be tolerant of the preparation of the polymer. Finally, the cleavable bonds should keep stable during application without affecting performance of the material. It is clear that reversible covalent bonds are ideal candidates. For example, degradation can be conducted through exchange reaction between reversible crosslinks and small molecular reagents containing identical reversible bonds or exchangeable groups with the aid of solvent swelling. In the following, the concepts and realization toward controllable degradation via reversible reaction is discussed. There are not many works dealing with controllable degradation of polymers via reversible reaction up to now (Table 8), but cured epoxy has attracted substantial research interests probably due to its wide application. As a result of the excellent inherent strength and chemical stability, thermosetting epoxy is difﬁcult to be removed and degraded under common conditions. To solve the problem, reversible covalent bonds including disulﬁde bond [281,386–389], acetal bond [282,283] and DA bond [48,284,285,390,404–407] were successively introduced to prepare removable and degradable epoxy materials, which not only have mechanical properties similar to the conventional version with irreversible bonds, but also can be removed under controlled external stimuli (e.g., heating, pH and irradiation) [404,405,407]. As early as in the 1990s, Tesoro et al. [386–388] prepared reversibly crosslinked epoxy from epoxy 828 (BADGE) and disulﬁde-containing amine curing agent dithiodianiline (DTDA, 126, Fig. 22). It can be degraded by TBP-catalyzed reduction reaction in diglyme/HCl solution at 160 ◦ C and completely dissolved under the selected conditions except those highly crosslinked versions, whose average molecular weight between crosslinks, Mc , exceeded the threshold of 400–500 g mol−1 . The degradation product, thiol containing linear polymer, can take part in oxidation reaction or addition reaction with diepoxides and bismaleimide to reform crosslinked networks. Thereinto, Mc of the re-cured resins formed by oxidation with iodine and air slightly increased from 470 g mol−1 to 800 g mol−1 and 1180 g mol−1 , respectively. Similarly, epoxy networks obtained from conventional bisphenol F diglycidyl ether (BFDGE) and disulﬁde-containing amine (126, Fig. 22) were degraded in ME at 80 ◦ C through thiol-disulﬁde redox [389]. In order to enhance degradability of epoxy, Takahashi et al.

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Table 8 Controlled degradation of reversible covalent polymersa . Polymer

Solvent

Condition

Extent of degradation

Molecular weight of degradation products

Refs.

Disulﬁde redox Thiol-disulﬁde redox Disulﬁde exchange DA reaction DA reaction

Epoxy-amine Epoxy-amine Epoxy-amine Epoxy-amine Epoxy-amine

Diglyme ME 1,4-Dioxane n-Butanol DMSO

100% 75–100% 100% 100% 39.8–84.1%

Mn = 2400, Mw = 3600. N/A N/A N/A Mn = 48300–154500

[386–388] [389] [281] [285] [390]

Acetal hydrolysis

Epoxy-anhydrides

Butyrolactone/water (135:15, v/v)

100%

N/A

[282,283]

Thiol-disulﬁde redox Imine exchange Acetal hydrolysis Transesteriﬁcation Disulﬁde exchange Alkoxyamine exchange Alkoxyamine exchange Alkoxyamine exchange Alkoxyamine exchange Cross alkane metathesis

Epoxy-amine (CFRCs) Epoxy-amine (CFRCs) Epoxy-amine (CFRCs) Epoxy-acid (CFRCs) VNR PA copolymer PA copolymer PA copolymer Acrylate-thiol PE

DMF Alcohol THF/water (9/1, v/v) EG scCO2 Anisole Water Anisole DMF Hexane, petroleum ether, etc.

100% for matrix 100% for matrix 100% for matrix 100% for matrix 50% 100% 100% 100% 100% 100%

N/A N/A Mn = 780–821 Mn = 837 Mn = 14500 Mn = 17200 Mn = 66000 Mn = 23000 N/A Mn = 500–1200, Mw = 680–4320

[119] [120] [391,392] [393] [394,395] [239] [240] [241] [40] [396]

Oleﬁn metathesis

PMMA copolymer

DCM

50–90%

N/A

[397]

Acetal hydrolysis Imine hydrolysis Spiroorthoester polymerization equilibrium Spiroorthoester polymerization equilibrium Thiol-disulﬁde redox

PU PU PSOE

THF Water DCM

TBP, HCl, N2 , 160 ◦ C, 55 min 80 ◦ C, 5.5–7 h DBU, DPDS, 100 ◦ C, 30–60 min 90 ◦ C, Swelling, 4 h; sonication (30% intensity), 25–125 min 1.5 M H3 PO4, 105–106 ◦ C, <15 min ME, R.T., 24 h DETA, R.T., 24 h 0.1 M HCl, R.T., 24 h Zinc acetate, 180 ◦ C, 3 h 180 ◦ C, 10 MPa, 60 min Alkoxyamine, 100 ◦ C, 48 h Alkoxyamine, 100 ◦ C, 24 h Alkoxyamine, 100 ◦ C, 48 h 4-OH-TEMPO, 100 ◦ C, 6 h Iridium, rhenium, 175 ◦ C, 4 days Ethylene, Grubbs catalyst, overnight 1 M HCl, R.T., 24 h 23 ◦ C, 80 h 5 mol% TFA, R.T., 1 h

100% Insoluble in water 82%

Mn = 110–990 Mn = 8500 Mn = 448

[398,399] [291] [400]

PSOE-co-PAN

DCM

0.1 M TEA, R.T., 1 h

97%

Mn = 54000

[401,402]

PR

DMF

Thiol, 60 ◦ C, a few minutes–70 min

92–100%

N/A

[403]

a DPDS, diphenyl disulﬁde; ME, 2-mercaptoethanol; DMSO, dimethyl sulphoxide; scCO2 , supercritical carbon dioxide; CFRCs, carbon ﬁber reinforced composites; 4-OH-TEMPO, 4-hydroxy-2,2,6,6-tetramethylpiperidinyloxy; DCM, dichloromethane; PA, polyacrylate; PSOE-co-PAN, polyspiroorthoester-co-polyacrylonitrile; PR, polyrotaxane.

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Mechanism

Z.P. Zhang et al. / Progress in Polymer Science 80 (2018) 39–93

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Fig. 29. Schematic drawing of the differences in the structures and degradation mechanisms of (a) DTDA (126, Fig. 22)-cured and (b) BGPDS (120, Fig. 22)-based epoxy. [281], Copyright 2016. Reproduced with permission from Elsevier.

[281] prepared dynamic epoxy bearing large amount of disulﬁde bonds by using disulﬁde-containing diepoxide (120, Fig. 22) and various amine hardeners (Fig. 29). The corresponding epoxy networks could be completely dissolved in 1,4-dioxane in the presence of DBU and small molecule disulﬁde compound (diphenyl disulﬁde, DPDS) through disulﬁde exchange reaction between the crosslinks and DPDS. Meantime, a series of cycloaliphatic diepoxides with cleavable acetal/ketal bonds were designed and cured by cyclic anhydrides, giving cleavable epoxy that can be dissolved in acid-containing (e.g., acetic acid, phosphoric acid and p-toluene sulfonic acid) solvent (e.g., methanol, ethanol and ␥-butyrolactone) mixtures through hydrolysis [282,283]. The degree of dissolution was found to be a function of acid concentration and solvent polarity. For example, the cured epoxy containing acetal bonds was only partially dissolved in the mixed ethanol/water/acetic acid at 88 ◦ C, but a mixture of ␥-butyrolactone/water/phosphoric acid was effective for removing the same cured epoxy by reﬂux at 105 ◦ C even though the acid concentration was decreased. In addition, degradation based on acid-catalyzed transetheriﬁcation can also be performed when ethanol (or methanol) served as both reactant and solvent in the presence of organic acid p-toluene sulfonic acid. The results indicated that the transetheriﬁcation in a non-aqueous condition proceeded faster than either hydrolysis or transetheriﬁcation in an aqueous condition. In fact, depolymerization-repolymerization equilibrium of DA bond has long been recognized as a valuable means for preparation of removable epoxy adhesive and packaging materials [404–406]. Aubert and co-workers [285], for example, encapsulated low voltage electronic assembly by DA bond crosslinked epoxy foam. The latter could be removed by completely dissolving in a mild solvent such as 1-butanol at 90 ◦ C. The authors further demonstrated that the elastomeric epoxy adhesive containing DA linkages could undergo network degradation by simply heating to elevated temperature without solvent, as a result of retro-DA reaction and ﬂowing of epoxy network. Min et al. [390] proposed a sonochemical method for controlling degradation of epoxy thermosets containing reversible DA linkages at lower temperature (20 ◦ C). The forceinduced position-oriented retro-DA reaction in terms of combined swelling and pulling-out effects was performed by crushing the thermosets into granules (100 ␮m–5 mm) followed by swelling

in DMSO for 4 h and ultrasounding (30% intensity) for another 25–125 min, resulting in the increase of dissolution ratio from 39.8 to 84.1%. The corresponding Mn of the soluble polymers decreased from 154.5 to 48.3 kg mol−1 . Then, the recycled soluble polymers can be recurred to produce epoxy-amine networks via DA reaction at 70 ◦ C for 2 days, leading to slightly reduced storage modulus and Tg . The major advantages of this work lie in that no catalyst is required and the degradation conditions are moderate. Due to irreversible fracture of the chemical bonds induced by ultrasonic treatment and side reactions during re-curing, however, the epoxy thermosets can only be recycled for a few times. The controlled degradable epoxy based on reversible covalent bonds has been extended to ﬁber composites. Odriozola et al. [119] explored the application of disulﬁde metathesis in glass ﬁber- and carbon ﬁber-reinforced epoxy composites by using DTDA (126, Fig. 22) as hardener, which retained the reprocessing and recycling abilities of dynamic epoxy. The composites can be easily dissolved in a solution of ME in DMF at room temperature due to the reversible feature of the disulﬁde crosslinks. In this context, the embedded ﬁbers can be recovered without damage. Besides, acetal-containing glycidol ethers were also employed for preparing recyclable carbon ﬁber-reinforced epoxy by conventional laminated prepreg-based method [391,392]. Chemical degradation of the composite was carried out by immersion in a HCl-containing THF/water (9/1, v/v) mixed solvent for 24 h at room temperature as a result of acid-induced acetal hydrolysis. The concentration of HCl had a great inﬂuence on the degradation process. At the concentration of 0.01 mol L−1 of HCl, for example, the epoxy matrix was nearly not degraded, but considerable degradation took place at 0.05 mol L−1 . Moreover, when sterically hindered hexatomic ring was attached to the acetal bond, the acid-degradability of the corresponding composite was deteriorated under the same conditions. Very recently, Yu and co-workers [393] prepared a fully recyclable CFRCs with epoxy-acid matrix proposed by Leibler et al. [75]. The composite was able to be dissolved in EG solvent at elevated temperature (i.e. 160–180 ◦ C) in a sealed environment as a result of transesteriﬁcation between ester crosslinks and hydroxyl groups of EG. The reinforced fabric could thus be easily separated. The result revealed that dissolution rate of the epoxy matrix depended on temperature and catalyst content. Besides, the minimum amount of EG required for dissolving the epoxy matrix was consistent with

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the stoichiometric ratio (i.e., the molar amount of ester group is equal to that of EG). Afterwards, the mixed solution was heated to evaporate the remnant EG solvent. The reclaimed fabric and polymer solution were further utilized for re-polymerization, offering a recycled CFRCs with the same level of mechanical properties as the original version. Taynton et al. [120] described a fully recyclable composite made from woven carbon ﬁber sheets and malleable polyimine networks. By adding excess reactive monomer (e.g. DETA), polyimine could be degraded in neat DETA or DETA/alcohol via dynamic imine bond exchange, leading to recovery of the full-length ﬁbers and polyimine oligomers. Then, the recycled solutions were combined with the solution of complementary TAEA (89a, Fig. 20) and TPA to prepare polyimine networks again, which showed mechanical performance comparable to the original one. Besides cured epoxy, vulcanized rubber is another mostly used thermosetting polymer. The three-dimensional networks formed by vulcanization limit its recycling and reprocessing. Currently, industrialized recycling techniques for vulcanized rubber waste used to result in breakage of carbon–carbon bonds within the polymer chains, which deteriorates mechanical properties of recycled rubber. Therefore, a more selective degradation process for cleavage of the sulfur–sulfur bonds should be developed for improving the recovery effect [408]. Kojima and co-workers [394,395] used supercritical CO2 (scCO2 ) as reaction medium for recycling of unﬁlled vulcanized natural rubber. Through disulﬁde exchange between small devulcanizing reagent and disulﬁde bonds of the crosslinks, the vulcanized natural rubber was devulcanized in supercritical CO2 in the presence of DPDS under 10 MPa at 180 ◦ C for 60 min, yielding at least 50% sol fractions. The authors [409] further explored the degradation of vulcanized natural rubber ﬁlled with carbon black in the same way. 20–40% sol fractions were obtained regardless of the carbon black content. The real natural rubberbased truck tire product was also devulcanized, and blended with virgin rubber under different proportions (20–60%), which was then re-vulcanized to prepare renewed vulcanized rubber having mechanical properties comparable to the original one. Acrylate-type crosslinked copolymers have a wide range of industrial applications. Otsuka et al. [239–241] described a few reversibly crosslinked acrylate copolymers (from 64, 65, 66 and 67, Fig. 18) based on alkoxyamine, which could undergo decrosslinking reaction by swelling in certain solvent (e.g., water and anisole) with an excess amount of alkoxyamine monomer at 100 ◦ C for 24–48 h. This is because the structure and composition of alkoxyamine-crosslinked polymers depend on the stoichiometric and thermally reversible equilibrium conditions. By simply controlling stoichiometric ratio, as a result, the networks can be transformed into soluble linear molecules or small molecules according to the topology. The GPC results revealed that Mn of the de-crosslinked polymer was similar to that of the linear polymer before crosslinking. It implies that the de-crosslinked polymer could be re-crosslinked through alkoxyamine exchange reaction by removing the excess alkoxyamine monomers. Zhang et al. [40] synthesized crosslinked methyl methacrylate polymer carrying alkoxyamine and found that the networks can be completely dissolved in DMF within 6 h at 100 ◦ C by exposing to an excess amount of nitroxides (e.g., 4-OH-TEMPO). Nevertheless, the polymer was not swellable in pure DMF. The de-crosslinking mechanism was attributed to the dynamic exchange between alkoxyamine units of the crosslinked networks and nitroxides. Toward the fabrication of recyclable artiﬁcial marble, Moon and co-workers [297] prepared organic/inorganic composites consisting of DA crosslinked acrylic resin binder and aluminum hydroxide, which could be dissolved in DMF above 150 ◦ C through retro-DA reaction. As a result, the polymer matrix and inorganic ﬁllers were separated for diverse follow-up applications. Recently, Ellsworth

and Gravano-Doerfﬂer demonstrated that the PMMA copolymer containing pendant oleﬁn was able to be crosslinked in the presence of Grubbs catalyst via oleﬁn metathesis [397]. When the crosslinked networks were swelled by DCM and ethylene was bubbled into the system, decrosslinking took place due to metathesis between the crosslinked double bonds and ethylene, leading to a yield of 88% of linear polymer. Subsequently, Guan and co-workers [396] reported an efﬁcient strategy for degradation of polyethylene materials (e.g., HDPE, low-density polyethylene (LDPE), and linear low-density polyethylene (LLDPE)) through tandem catalytic cross alkane metathesis under mild conditions. Firstly, polyethylene and light alkane (like n-hexane, n-octane and petroleum ether) were dehydrogenated in the presence of iridium complex. Then, the resultant unsaturated polyethylene and light alkane underwent rhenium-catalyzed cross metathesis forming two new oleﬁns with lower weight average molecular weight, Mw , followed by hydrogenation to produce saturated alkane under the catalysis of iridium. By repeating the tandem reaction, different types of commercial polyethylene could be degraded into short alkanes in noctane within 4 days at 175 ◦ C. The authors also found that wasted polyethylene can be degraded into liquid fuels in more economic petroleum ether following the same mechanism. Since polyurethane backbone contains large numbers of cleavable linkages like urethane and urea bonds, it should be possible to be degraded into small molecules and oligomers through hydrolysis, alcoholysis and aminolysis [410]. Owing to the inherent chemical stability of urethane bonds, however, the abovementioned chemical degradation processes require relatively harsh conditions and also produce complex degradation products that are hard to be reused. Therefore, a new degradation mechanism based on acid-catalyzed hydrolysis reaction of acetal bonds was developed by Hashimoto et al. for chemically recycling of polyurethane elastomer [398]. Firstly, the authors prepared a novel hydroxyterminated polyacetal through the polyaddition of 4-hydroxybutyl vinyl ether, which was then utilized as the ﬂexible soft segment of polyurethane. Although the resultant polyurethane elastomer could be dissolved in THF containing 1 mol L−1 HCl at room temperature, the polyacetal segment was only degraded into small molecules of butanediol and acetaldehyde rather than polyols that could be directly reused. In the subsequent research of the same authors [399], di-acetal functionalized polyols (PTMEG) were introduced into polyurethane, and the latter could be degraded into polyols with high yield (80%) and high purity under relatively mild conditions. The reclaimed polyols must be able to be used again for synthesizing acetal bonds-containing polyols and degradable polyurethane. Furthermore, Fukuda et al. [291] designed a doubly-degradable polyurethane containing chemically degradable reversible imine bonds and biodegradable polyester. By simply immersing in distilled water at 23 ◦ C for 80 h, the polyurethane ﬁlm obtained from lower molecular weight polyester (2500 g mol−1 ) could be disintegrated into fragments owing to the hydrolysis of imine linkages. Moreover, the residual oligomers could be further biodegraded into CO2 and water. When the water soaked ﬁlm was dried at 80 ◦ C for 240 h, however, the imine bonds were restored allowing for recovery of the molecular weight and mechanical properties of the polyurethane. Endo and co-workers suggested to recycle polymer through equilibrium polymerization of spiroorthoester (SOE) derivative, which consisted of cationic single ring-opening polymerization and reverse depolymerization of poly(SOE) (PSOE) [400–402,411]. The bifunctional SOEs could undergo cationic polymerization in the presence of 2 mol% TFA catalyst at 0 ◦ C, and formed a crosslinked polymer capable of being recovered to the original monomers by treatment with 5 mol% of triﬂuoroacetic acid in DCM solvent at room temperature for 1 h. Furthermore, the copolymer bear-

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85

Fig. 30. Depolymerization of end-capped poly(benzyl ethers) upon cleavage of the end-cap from the polymer when it is exposed to a speciﬁc signal. [413], Copyright 2013. Reproduced with permission from the American Chemical Society.

ing SOE pendant chain obtained from vinyl-containing SOE and acrylonitrile was treated with 4.2 mol L−1 TFA in DCM at −10 ◦ C to get the corresponding crosslinked polymer, which could be degraded into linear copolymer again in low concentration TFA (0.1 mol L−1 ) at room temperature. The molecular weight of the degradation product (54000 g mol−1 ) approached that prior to crosslinking (46000 g mol−1 ). Takata and co-workers also prepared a crosslinked polyrotaxane network containing disulﬁde linkage in the axle [403]. Under the catalysis of equimolar thiol (e.g., benzenethiol), the heterogeneous mixture of polyrotaxane and DMF solvent could turn into homogeneous colorless solution within a few minutes as a result of decrosslinking driven by disulﬁde-thiol exchange, leading to the initial linear poly(crown ether). Moreover, Phillips et al. [412,413] described a new class of polymers (e.g., poly(benzyl ethers) and poly(phthalaldehydes)) that were capable of depolymerizing from head to tail when the endcaps were cleaved in response to a speciﬁc signal (e.g., ﬂuoride, DBU and UV light) (Fig. 30). 6. Conclusions The development trend of reversible covalent chemistry of polymers clearly shows that it is no longer limited to polymer synthesis (especially the steps in solution), but has become an indispensable approach to obtain new materials and new functionalities in bulk state. The reversible covalent polymers are capable of reversibly changing their constitutional structure under certain stimuli, while keeping the structural stability as the irreversible covalent polymers after removing the applied stimuli. By taking initiative to manipulate this valuable stimulus-response habit of reversible covalent polymers, properties regulation, self-healing, improvement of processability, solid state recycling, and controllable degradation, which are hard to be realized for conventional covalent polymers, are allowed to proceed as discussed hereinbefore and would be hopefully put in practice on industrial scale. More importantly, these newly acquired functionalities can be correlated with each other owing to the dynamic reorganizability of the polymers, which increases the possibilities and freedoms of cyclic utilization (Fig. 31). A series of circulation loops can thus be established in accordance to actual situations. All these not only reveal the brilliant prospects of the technique, but also form the prototype of a new branch of polymer engineering as mentioned in the Introduction. Although there has been much progress in preparation and proof-of-concept application of reversible covalent polymers, com-

Fig. 31. Multitask cyclic utilization of reversible covalent polymers.

mercialization of the proposed methodologies still has far to go. The main challenges are summarized below. (i) Enhancement of overall properties. Compared with irreversible covalent polymers, many reversible covalent polymers are gels and elastomers with lower mechanical properties, not suitable for serving as structural materials. It is mainly due to the lower bond energies of most reversible bonds (Table 3). The requirements for facilitating solid state reversible reactions and strengthening the materials are apparently conﬂicting. Efforts have been devoted to address this issue by exploration of reversible covalent bonds with higher bond energies [89], incorporation of assisted supramolecular interactions [140,141,205,327], elaborate control of structure of soft segments (by, for example, simultaneous coupling with sub-ambient Tg and higher crystallinity [38]) and soft/hard segments ratios as well, construction of microphase-separated system [414], and introduction of reinforced phases [415]. Nevertheless, packaged solutions with broad applicability are awaiting the continuing contributions of theoretical and experimental scientists. (ii) Synergy between hierarchical structures and reversible reaction. It is well known that hierarchical structures of polymers, like crystallization and micro-phase separation, determine their ultimate properties. When the formation and evolution of hierarchical structures of reversible covalent polymers are managed to coordinate with the reversible reactions, higher level diversiﬁcation would be available, expanding the family of new materials. The pioneer works correlating reversible crosslinking with crystallization [305–308] have made a good start, which reveal the fascinating breadth of potential of combined adjustment of macromolecular assembling, topology and constitution. (iii) Tailor-made equipments and processing techniques. The existing plastics industries consist of the following procedures: melting/plasticating of thermoplastics for chain disentanglement, shearing/mixing towards desired morphologies, and cooling/consolidation for ﬁxing the structures. As for reversible covalent polymers, disentanglement and re-entanglement of macromolecular chains can be simply completed (or replaced) by reversible bonds disconnection and re-connection without the necessity of melting/plasticating, so that even crosslinked polymers can be repeatedly remolded. In this context, speciﬁc equipments coupled with various external ﬁelds as stimuli for the reversible reactions and the related processing techniques should be designed, which comprehensively consider the chemistry and physics of chain interaction as well

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as thermodynamic and rheological characteristics of reversible covalent polymers. As a result, the strength of reversible reaction can be brought into full play. So far, however, processing of reversible covalent polymers is mostly carried out by the equipments peculiar to irreversibly bonded polymers. Compression molding aided by heating [16,17,118], sunlight [327,416] and water [118,173,181], for example, was the main measure for recycling of reversible covalent polymers, while injection molding was rarely used [75,290,374], which is not competent to the production of complicated architectures and high-throughput manufacturing of polymers with advanced properties. The applications of the techniques reviewed above might start with the existing polymers containing built-in reversible bonds [16,17] or the assistance of market available additives carrying reversible bonds [416]. This can temporarily keep away from the difﬁculty that most reversible covalent polymers have not yet been produced on an industrial scale. It is believed that when more and more examples of innovative materials and processing in this aspect are put in practice, more conﬁdence would be given to polymer synthesis and manufacturing industries for further promoting the polymer engineering based on reversible covalent chemistry.

Acknowledgements The authors thank the support of the Natural Science Foundation of China (Grants: 51333008, 51273214, 51673219, 51603235 and 51773229) and the Scientiﬁc and Technological Program of Guangdong Province (Grant: 2017A010103008).

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Polymer engineering based on reversible covalent chemistry: A promising innovative pathway towards new materials and new functionalities

Polymer engineering based on reversible covalent chemistry: A promising innovative pathway towards new materials and new functionalities

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