Controlled synthesis of azobenzene-containing block copolymers both in the main- and side-chain from SET-LRP polymers via ADMET polymerization

Journal Pre-proof Controlled synthesis of azobenzene-containing block copolymers both in the mainand side-chain from SET-LRP polymers via ADMET polymerization Liang Ding, Yadi Li, Hui Cang, Juan Li, Chengshuang Wang, Wei Song PII:

S0032-3861(20)30071-9

DOI:

https://doi.org/10.1016/j.polymer.2020.122229

Reference:

JPOL 122229

To appear in:

Polymer

Received Date: 15 November 2019 Revised Date:

21 January 2020

Accepted Date: 25 January 2020

Please cite this article as: Ding L, Li Y, Cang H, Li J, Wang C, Song W, Controlled synthesis of azobenzene-containing block copolymers both in the main- and side-chain from SET-LRP polymers via ADMET polymerization, Polymer (2020), doi: https://doi.org/10.1016/j.polymer.2020.122229. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2020 Published by Elsevier Ltd.

Graphical Abstract for the Contents Page

Controlled

synthesis

of

azobenzene-containing

block

copolymers both in the main- and side-chain from SET-LRP polymers via ADMET polymerization Liang Ding,* Yadi Li, Hui Cang, Juan Li,* Chengshuang Wang, Wei Song*

Controlled

synthesis

of

azobenzene-containing

block

copolymers both in the main- and side-chain from SET-LRP polymers via ADMET polymerization Liang Ding,* Yadi Li, Hui Cang, Juan Li,* Chengshuang Wang, Wei Song* Department of Polymer and Composite Material, School of Materials Science and Engineering, Yancheng Institute of Technology, Yancheng 224051, China *Corresponding authors. Tel.: +86 515 88298872; Fax: +86 515 88298251. E-mail: [email protected], [email protected], [email protected]

ABSTRACT: Novel ABA triblock copolymer contained azobenzene (azo) chromophores both in the main- and side-chain was designed and synthesized via combination of single electron transfer-living radical polymerization (SET-LRP) and acyclic diene metathesis (ADMET) polymerization. The α-bromoester end group of side-chain azo-polymer prepared in SET-LRP system using acrylate bearing azo group as monomer was reacted with potassium acrylate to yield azo-polymer with the chain end of α-acrylate, which acted as a monofunctional macromolecular chain stopper for subsequent ADMET polymerization of azo-functionalized α,ω-diene monomer to finally controllable synthesize ABA triblock azo-copolymer. The diluted solutions of main-chain ADMET azo-homopolymer, side-chain SET-LRP azo-homopolymer, and main-side chains azo-copolymer exhibited different photoisomerization behaviors (maximum absorption and rate of photoisomerization) under the irradiation of UV and visible light. All these interesting results could provide a guide for the design of photosensitive materials. KEYWORDS: Single electron transfer-living radical polymerization; acyclic diene metathesis polymerization; photoresponsive polymer

1. Introduction The photoisomerization of azobenzene (azo) chromophore is one of the most significant features of novel azo-polymer materials due to their unique photoinduced reversible trans–cis isomerization that can be efficiently conducted by alternating irradiation with UV and visible light [1−4]. As we know, the structure of azo-polymers switched by photoisomerization are completely different considering the location of the azo chromophore in the polymers. To date, remarkable examples in the synthesis of polymers containing azo chromophores in the main- or side-chain have been extensively reported based on diverse synthetic methods, especially controlled/living polymerization [5−9]. However, to our knowledge, only two classes of azo-containing polymers both in the main- and side-chain (main-side chains) have been reported. Zhang et al. recently designed and synthesized a novel copolymer contained visible-light-activated main-chain azo units and UV–visible activated side-chain azo units by utilizing Pd-catalyzed Suzuki coupling reaction and found the unique photocontrolled isomerization properties [10]. We reported that a comb-like copolymer with azo chromophores in the main-side chains was prepared via two sequential metathesis polymerizations in a one-pot procedure and exhibited the special photochemical trans–cis isomerization [11]. A major drawback for these main-side chains azo-polymers was relatively broad molecular weight distribution caused by uncontrollable synthesis methods. Therefore, efficient and controlled polymerization for constructing this particular structure is critical and of key importance. Olefin metathesis based step-growth polymerization, termed acyclic diene metathesis (ADMET) polymerization, is performed with α,ω-dienes, and proceeds by release of ethylene as a condensate [12,13]. Control over molecular architectures obtained via ADMET polymerization is limited by the step-growth nature of this technique but has also opened ways to prepare polyolefins with a number of different functional groups. Excitingly, the selectivity of olefin cross-metathesis between acrylates and terminal olefins is an established technique employed in ADMET polymerization to allow access to metathesis polyolefins with molecular weight control [14,15]. Moreover, the introduction of a monofunctional macromolecular chain stopper bearing an acrylate end-group to the polymerization of α,ω-diene could control the molecular weight and form telechelic ABA triblock copolymer [14a]. Since its invention, ADMET polymerization is expected to combine with other

controlled polymerizations for the synthesis of diverse functional polymer architectures. Single

electron

transfer-living

radical

polymerization

(SET-LRP)

is

a

Cu(0)-catalyzed controlled/living radical polymerization technique, which provides one of the most efficient polymerization methods to rapidly produce a wide variety of vinylic polymers with well-defined polymer chains under mild conditions using a large diversity of monomers with different polarity profiles, including (meth)acrylates, (meth)acrylamides, (meth)acrylonitrile, vinyl chloride and other type of monomers [16,17]. Moreover, SET-LRP of the oligomers with (meth)acrylate end-group has also been performed [18−20]. However, there are limited reports on SET-LRP of the sophisticated designed functional monomers and the properties of these polymers. Recently, Cu(0)-mediated SET-LRP of a series of new p- and n-type organic semiconductor monomers was exploited and displayed linear first-order kinetics up to high monomer conversion, which further broadened the types of monomer for SET-LRP [21]. More importantly, SET-LRP has been shown to produce polymers with near perfect retention of chain-end functionality even at high monomer conversion. The high halide chain end functionality of these polymers can be exploited to introduce specific functional groups [22] and further create complex structures by versatile synthetic route. As a result, this strategy would represent a step towards quantitative end-group modification in SET-LRP and provide the possibility of preparing the potential functional macromolecular chain stopper for ADMET polymerization.

Scheme

1.

Illustration

for

constructing

main-side

chains

ABA triblock

azo-copolymers via a combination of SET-LRP and ADMET polymerization.

Consequently, building on our experience with metathesis polymerization and radical polymerization, herein, we presented the practical protocol for preparation of the novel photoresponsive block copolymers in combination of SET-LRP with succedent end-group modification and ADMET polymerization as shown in Scheme 1. Azo-monomer for SET-LRP was prepared first by a simple two-step technique. Then, a thorough investigation of SET-LRP of this azo-monomer was presented to determine the optimum polymerization conditions for rapid, controlled, and quantitative production of well-defined azo-polymer. Later, acrylate terminated azo-polymer was conveniently prepared by merging the chain end of ω-bromo of SET-LRP polymer with the esterification reaction of potassium acrylate, which could act as a selective macromolecular chain stopper in the subsequent ADMET polymerization of azo-functionalized α,ω-diene monomer to finally produce the main-side chains azo-polymer.

2. Experimental section 2.1. Materials Acrylic acid (> 99%, TCI), 10-undecenoic acid (99%, Alfa Aesar), 2-bromoethanol (97%, Alfa Aesar), 1-[3-(dimethylamino)propyl]-3-ethylcarbodiimide hydrochloride (EDCI·HCl) (99%, Energy Chemical), 4-dimethylaminopyridine (DMAP) (98%, Energy Chemical), ethyl 2-bromoisobutyrate (EBiB) (98+%, Alfa), Cu(II)Br2 (99%, Alfa Aesar), copper(0) wire (20 gauge wire, 0.812 mm diameter, Fisher), dimethylformamide (DMF) (99.8%, Sigma Aldrich), dimethyl sulfoxide (DMSO) (99.9%, Fisher), benzylidene[1,3-bis(2,4,6-trimethylphenyl)-2-imidazolidinylidene] dichloro(tricyclohexylphosphine) ruthenium (Grubbs 2nd generation catalyst, C1) (Aldrich), and [1,3-bis(2,4,6-trimethylphenyl)-2-imidazolidinylidene]dichloro(o-iso propoxyphenylmethylene) ruthenium (Hoveyda–Grubbs 2nd generation catalyst, C2) (97%, Aldrich) were used as received without purification. 4-Hydroxyazobenzene (98%) and 4,4'-dihydroxyazobenzene (98%) were purchased from Shanghai DiBo Chemical Technology Co., LTD. Tetrahydrofuran (THF) (99.9%, Fisher),

dichloromethane (CH2Cl2) (99.5%, Fisher), and toluene (99%, Fisher) were distilled over drying agents under nitrogen prior to use. Potassium acrylate (KA) and hexamethylated tris(2-aminoethyl)amine (Me6-TREN) were synthesized as described in the literatures [22,23]. 2.2. Characterization 1

H (500 MHz) and

13

C (125 MHz) NMR spectra were recorded on a Bruker DPX

spectrometer using tetramethylsilane as an internal standard and using DMSO-d6 as the solvent at ambient temperature. Polymer samples were analyzed using a TOSOH EsoSEC HLC-8320GPC system, comprising an autosampler, two TSKgel SuperHZM-M (4.6 mm I.D. × 15 cm), TSKgel SuperH-RC, and TSKgel guardcolumn SuperHZ-L columns, UV-8320 detector and refractive index (RI) detector. GPC measurements were carried out at 40 °C using THF (Fischer, HPLC grade) as the eluent with a flow rate of 1 mL/min. Relative molecular weights and molecular weight distributions were calculated using PStQuick MP-M standards. The GPC Workstation EcoSEC-WS comprises system control and data analysis software for use with EcoSEC. Matrix-assisted laser desorption ionization time-of-flight mass (MALDI–TOF MS) measurement were measured on an Applied Biosystems Voyager DETM-PRO mass spectrometer. A 337 nm nitrogen laser at 20 kV acceleration voltage with a laser fluence of 161.5 mJ/cm2 was used. Each spectrum was acquired by an average of 50 laser shots. Samples were prepared from ~1.0 mg polymer in 1.0 µL THF and then were mixed with 1.0 µL of the MALDI matrix which were prepared by mixing 20 mg of 1,8,9-trihydroxyanthracene (dithranol) in 1.0 mL THF, aqueous silver trifluoracetate (5 g/L, 1.0 µL) and aqueous TFA (1 %, 10 µL). UV–Vis absorption spectra were measured on an Agilent Cary 60 spectrometer. UV irradiation was carried out with an 8 W × 4 UV lamp with the wavelength at 365 nm. Irradiation by visible light was performed using a 23 W Philips day light bulb (> 400 nm). LC– MS measurements were performed with an Agilent technology 1200 series at 45 °C using H2O or CH3CN as mobile phase, 50 m × 4.6 mm × 3.5 µm diffused column. Elemental analysis (EA) was conducted with an Elementar Vario EL. Polymerizations were carried out in Schlenk tubes under the dry nitrogen atmosphere. 2.3. Synthesis of azo-monomer (M1) for SET-LRP

4-Hydroxyazobenzene (3.96 g, 20 mmol), potassium carbonate (13.8 g, 100 mmol), and 50 mL of DMF were charged into a 250 mL Schlenk flask. The reaction mixture was heated at 80 °C for 6 h under nitrogen allowing for the potassium salt formed. A solution of 2-bromoethanol (3.75 g, 30 mmol) in 20 mL of DMF was then added dropwise to the above mixture. After 24 h of stirring at 60 °C, the reaction mixture was poured into excess water and the crude product was precipitated out, and further purified by chromatographic purification (silica gel, CH2Cl2/hexane = 1: 1~5: 1) to give an orange solid compound (3.92 g, 80.6% yield). Then, this as-prepared orange solid compound (3.63 g, 15 mmol), acrylic acid (1.62 g, 22.5 mmol), and DMAP (0.23 g, 1.8 mmol) were dissolved in CH2Cl2 (30 mL) and THF (6 mL), and the mixture was stirred at 0 °C for 15 min. EDCI (4.32 g, 22.5 mmol) was then added to the former solution, and stirred for 3 days under nitrogen flow after the solution was warmed to room temperature. The resulting solution was washed three times with deionized water (3 × 80 mL), and the organic layer was dried over anhydrous MgSO4. The solvent was then evaporated, and the crude product was purified by recrystallization by ethanol to give a brown crystalline powder M1 (3.41 g, 76.7% yield). 1H NMR (DMSO-d6), δ (ppm): 7.91–7.85 (m, 4H, o-ArH-N=N-ArH), 7.60–7.52 (m, 3H, p-ArH-N=N-ArH + m-ArH-N=N-ArH), 7.19–7.17 (m, 2H, m-ArH-N=N-ArH), 6.40–6.39, 5.99–5.97 (d, 2H, CH2=CH), 6.36–6.22 (m, 1H, CH2=CH), 4.51–4.49 (m, 2H, OCH2CH2OCO), 4.38–4.36 (m, 2H, OCH2CH2OCO). 13

C NMR (DMSO-d6), δ (ppm): 200.90, 196.39, 187.45, 167.50, 166.34, 163.50,

160.04, 157.73, 150.63, 101.67, 98.15. LC: single peak was observed. EI/MS: Calcd. for C17H16N2O3: 296.32; found: 296.52. Anal. calcd for C: 68.92, H: 5.40, O: 16.22, N: 9.46; found C: 68.94, H: 5.42, O: 16.20, N: 9.44. 2.4. Synthesis of azo-monomer (M2) for ADMET polymerization 4,4'-Dihydroxyazobenzene (2.14 g, 10 mmol), 10-undecenoic acid (4.42 g, 24 mmol), and DMAP (0.31 g, 2.4 mmol) were dissolved in CH2Cl2 (40 mL) and THF (10 mL), and the mixture was stirred at 0 °C for 15 min. EDCI (5.76 g, 30 mmol) was then added to the former solution, and stirred for 4 days under nitrogen flow after the solution was warmed to room temperature. The resulting solution was washed three times with deionized water (3 × 80 mL), and the organic layer was dried over anhydrous MgSO4. The solvent was evaporated, and the crude product was purified by chromatographic purification (silica gel, CH2Cl2/hexane = 1: 10~1: 1) and then

recrystallization by ethanol to give a light orange crystalline powder M2 (3.96 g, 72.5% yield). 1H NMR (CDCl3), δ (ppm): 7.96–7.94 (d, 4H, m-ArHN=NHAr), 7.25–7.23 (d, 4H, o-ArHN=NHAr), 5.85–5.79 (m, 2H, CH2=CH), 5.02–4.93 (m, 4H, CH2=CH), 2.61–2.58 (m, 4H, CH2OCO), 2.07–2.03 (m, 4H, CH2=CHCH2), 1.80–1.74 (m, 4H, CH2), 1.44–1.25 (m, 20H, CH2). 13C NMR (CDCl3), δ (ppm): 171.99, 152.75, 150.02, 139.15, 124.04, 122.23, 114.17, 34.40, 33.78, 29.27, 29.19, 29.06, 28.87, 24.86. LC: single peak was observed. EI/MS: Calcd. for C34H46N2O4: 546.73; found: 546.28. Anal. calcd for C: 74.69, H: 8.48, O: 11.71, N: 5.12; found C: 74.64, H: 8.50, O: 11.72, N: 5.14. 2.5. General procedure for ADMET polymerization In a nitrogen-filled Schlenk tube, a solution of Grubbs 2nd Generation catalyst, C1 (0.5

mol%

to

monomer)

in

0.1

mL of

degassed

toluene

with

three

freeze-vacuum-thaw cycles was added to a solution of azo-monomer, M2 (0.5 mmol) in 0.4 mL of toluene degassed with the same procedure. After the reaction mixture was stirred at 65 °C for 24 h, the polymerization was quenched by adding ethyl vinyl ether with stirring for 30 min. The solution was precipitated into an excess of cold methanol, and the precipitate was isolated by filtration and dried under vacuum to give the ADMET homopolymers. 2.6. Typical procedure for SET-LRP of azo-monomer 9.0 cm of a 20 gauge Cu(0) wire was soaked in concentrated HCl for 15 minutes to remove surface impurities, then washed with water followed by acetone. A Teflon coated stirring bar wrapped with above Cu(0) wire was placed in a 25 mL Schlenk tube under a strong flow of N2 and dried under vacuum before use. In a 25 mL Schlenk tube, the reagents were added in the following order under gentle stirring: organic solvent (DMSO, 0.9 mL), azo-monomer (M1, 0.4~1.6 mmol), water (stock solution containing the ligand (Me6-TREN, 1.5 µmol) and Cu(II)Br2, 1.6 µmol) and an initiator (EBiB, 0.02 mmol). After six freeze–pump–thaw cycles, the Schlenk tube was filled with nitrogen and the activated Cu(0) wire wrapped around a Teflon-coated stir bar was transferred to the Schlenk tube containing the reaction mixture under a positive flow of N2. Two more freeze–pump–thaw cycles were carried out while holding the stir bar on the top of the Schlenk tube using an external magnet. After that, the Schlenk tube was filled with N2 and the reaction mixture was

heated at 60 °C. Then, the stir bar wrapped with the pre-treated Cu(0) wire was dropped into the reaction mixture. This was considered as time zero. Samples were taken at different times using an airtight glass syringe and stainless-steel needles. During the sampling, the side arm of the Schlenk tube was purged with N2 for 2 min to avoid the introduction of oxygen inside the reaction mixture. Samples were dissolved in CDCl3 and quenched by air bubbling. After that, the monomer conversion was measured by 1H NMR spectroscopy. In order to determine the molecular weight and molecular weight distribution of the sample, the remaining solvent and the residual monomer were evaporated under vacuum. Finally, the sample PAzo-Br was dissolved in THF and filtered through a short small Al2O3 chromatographic column to remove residual copper and analyzed by GPC. 2.7. Chain-end functionalization of PAzo-Br with KA In a 10 mL Schlenk tube, KA (0.05 equiv.) was added to a solution of PAzo-Br (0.01 equiv.) in acetone (2 mL). The Schlenk tube was sealed with a rubber septum and placed into a 50 °C thermostated oil bath for 36 h. The resulting reaction mixture was precipitated into an excess of water, washed several times with cold methanol, isolated by filtration, and dried under vacuum to give the corresponding polymer, PAzo-Acrylate as a monofunctional macromolecular chain stopper. The degree of functionalization was determined by 1H NMR spectroscopy. 2.8. General procedure for ADMET polymerization in the presence of a selective macromolecular chain stopper In a nitrogen-filled Schlenk tube, M2 (546 mg, 1 mmol) and the desired amount of macromolecular chain stopper, PAzo-Acrylate in toluene (1 mL) was degassed by three freeze–vacuum–thaw cycles using a [M2]/[PAzo-Acrylate] ratio of 100: 1~100: 10. The mixture was heated to 60 °C while stirring and then a solution of Hoveyda– Grubbs 2nd generation catalyst (1 mol% to monomer) in toluene (0.1 mL) degassed with the same procedure was added. After the reaction mixture was stirred for 24 h, the polymerization was quenched by adding ethyl vinyl ether with stirring for 30 min at room temperature. The solution was precipitated into an excess of cold methanol, and the precipitate was isolated by filtration, dried under vacuum for 24 h to give the dark red-brown ABA triblock azo-copolymer.

3. Results and discussion 3.1. Azo-monomers synthesis Two types of monomers were designed and synthesized based on photoresponsive azo motifs found commonly in photochemical switching application. Azo-monomer, 1 (M1) for SET-LRP may be conveniently synthesized by simple etherification and subsequent esterification (Scheme 1), giving M1 as a brown crystalline powder in reasonable overall yield over two steps. Azo-monomer, 2 (M2) for ADMET polymerization is prepared directly through esterification from commercially available reagents. The chemical structures and purities of two azo-monomers were fully tested by LC–MS, elemental analysis, and NMR spectroscopy (Fig. S1–S4). 3.2. Cu(0) wire-catalyzed SET-LRP of azo-monomer Selecting an appropriate solvent for SET-LRP of M1 was crucial, as the highly polar solvents typically used to promote the disproportionation of Cu(I) to Cu(0) and Cu(II) in this polymerization. Considering the solubility of the azo-monomer, we determined that DMSO provided an adequate balance of solubility and proved to be an effective solvent for SET-LRP. Polymerizations were carried out in a nitrogen atmosphere and sampled throughout the reaction to monitor conversion, molecular weight (Mn), and molecular weight distribution. The combination of Cu(0) wire, Me6-TREN and Cu(II)Br2 was used as a catalytic system. Polymerization was conducted with 9 cm of 20 gauge Cu(0) wire, freshly cleaned by pre-treated with HCl for 15 min to remove surface impurities, then washed with water and acetone, and dried in vacuo. The Cu(0) wire/Me6-TREN catalyzed SET-LRP of M1 was carried out in DMSO to furnish an ω-bromo-terminated PAzo-Br by using EBiB as a monofunctional initiator. Using [EBiB]/[Me6-TREN]/[CuBr2] in 1: 0.075: 0.08 molar ratios, the certain amount of azo-monomer was chosen to give PAzo-Br with molecular weights of 0.6~24 kDa at full conversion. SET-LRP gave polymers with relatively narrow molecular weight distributions of 1.25~1.32 at high conversion (79~88%) after 72 h (Table 1).

Table 1

Characteristic for Cu(0) wire/Me6-TREN-mediated SET-LRP of M1 in binary mixtures of DMSO/H2Oa.

a

Entry

kpapp (h-1)

Time (h)

Conversion (%)b

Mn,(th)c (kDa)

Mn,(GPC) (kDa)

Dispersity

Ieffd (%)

1

0.0296

72

88

5.4

6.8

1.32

83

2

0.0237

72

83

10.1

11.3

1.28

88

3

0.0178

84

79

18.9

20.2

1.25

92

Reaction conditions: M1 = 0.4~1.6 mmol, DMSO/H2O = 9/1 (v/v, 1.0 mL);

[EBiB]0/[Me6-TREN]0/[CuBr2]0 = 1/0.075/0.08, HCl-activated Cu(0) wire 9.0 cm of 20 gauge wire. b

Obtained by 1H NMR spectroscopy using a formula: 1 – (S3H,Acrylate/3)/(S2H,CH2/2), methylene is

linked directly to acrylate group. c

Mn,th = [(Conversion %) × ([M1]/[EBiB]) × Mn,M1 + Mn,EBiB]/1000, where Mn,M1 = 296, and

Mn,EBiB = 195 are the molar masses of monomer OEOMEA and initiator EBiB, respectively. [M1]/[EBiB] = 20~80. d

Initiator efficiency.

Fig. 1. Kinetic plots, molecular weight, and dispersity evolution for the SET-LRP of azo-monomer in DMSO/H2O (9/1, V/V) with the ratios between monomer, initiator, and ligand equal to (a) 20: 1: 0.075, (b) 40: 1: 0.075, and (c) 80: 1: 0.075.

The living characteristic of SET-LRP was validated usually through the linear evolution of ln([M]0/[M]) along with polymerization time, the narrow molecular weight distribution, and the almost quantitative chain end functionality. Hudson et al. reported that Cu(0) wire-mediated SET-LRP of acrylates based on p-type or n-type organic semiconductors exhibited first-order kinetics up to high conversion, but above these limited linear behavior in plots of ln([M]0/[M]) are no longer observed because the combination of solvent (NMP or MeCN) and ligand (Me6-TREN) was unable to support a truly living polymerization [21]. Herein, the kinetic experiments of SET-LRP of M1 are shown in Fig. 1. All the samples were taken at different reaction times without purification, dissolved in CDCl3, and characterized directly by 1H NMR spectroscopy (Fig. S5), however, further purification by preparatory GPC to remove residual monomer gave the resulting polymers (Fig. S6). The polymerization exhibits a linear kinetics in the concentrations of monomer and propagating radicals with kpapp = 0.0296 h–1. This slow polymerization kinetic was attributed to the relatively poor solubility of the resultant polymer in DMSO because the reaction mixture gradually became cloudy as the polymerization proceeds. Moreover, we note that, M1 has nitrogen atoms capable of coordinating to Cu species in the reaction medium, the possible coordination of M1 to CuBr2 in the reaction may also interfere with the polymerization [21b]. When the degrees of polymerization increased (DP = 40 and 80), kpapp would reduce because the viscosity of the polymerization system raised evidently which influenced the stirring rate of polymerization, and thus led to a deceleration. However, Ieff increased evidently. Of course, some oligomers obtained from the relatively low monomer conversion at the initial polymerization stage limited linear behavior in the plots of molecular weights (Fig. 1a and b). Two reasons may lead to the deviation of the molecular weight as measured by GPC from the theoretical molecular weight predicted based on conversion: (1) GPC cannot accurately measure the molecular weight of oligomer (Mn = 0.1~0.3 kDa); (2) the possible copper-coordinating monomer may be acting as excess ligand in the reaction, leading to chain death early in the polymerization, accounting for the increase in observed molecular weights versus theoretical values [24]. The kinetic experiments revealed a linear evolution of molecular weight with conversion and narrow molecular weight distribution, associated with the “living” nature of the polymerization of this azo-monomer and showing good agreement between the theoretical and experimental values (Table 1). Importantly, GPC traces in Fig. S7

revealed a mono-modal polymer peak distribution shifting to higher molecular weights throughout the polymerization, while the molecular weight distributions were slightly reduced during the reaction with a final value. 3.3. End-functionalization of SET-LRP polymer via heterogeneous esterification Based on above results, a low [M1]/[EBiB] = 20 (Mth = 5.9 kDa at 100% conversion) was used in this study to prepare a low molar mass PAzo-Br sample suitable for a precise analysis of chain-ends by 1H NMR, GPC, and MALDI–TOF analyses before and after functionalization. Upon completion of the polymerizations of azo-monomer, the solution was precipitated into an excess of water and methanol, respectively. As can be seen in Fig. S8a, 1H NMR analysis cannot be used to faithfully assess the bromine chain end functionality (fBr) of the produced PAzo-Br due to the overlapping of signals but may confirm the structure of PAzo-Br with further purification. Then, functionalization of PAzo-Br via heterogeneous esterification with KA was exploited to introduce a specific acrylate moiety located at the ω-end group. Percec disclosed that the secondary α-bromoester end groups of relatively hydrophilic polymer can efficiently react with KA in acetonitrile or acetone [22]. The esterification reaction was investigated at 50 °C in acetone using molar ratios CH-Br/KA = 1/5 and could be determined by 1H NMR spectroscopy (Fig. S8b) by the new appearance of He′ corresponding to PAzo-Acrylate. More importantly, this low-field shift of the signal He′ after the esterification with KA allowed one to determine the fraction of PAzo-Br chains capped with bromine atoms (fBr = 96%). Meanwhile, the molecular weight could be calculated from the end-group analysis of the 1H NMR spectrum, which is then estimated by the ratio of Ar protons on azo-monomer units at 7.94–7.81 ppm to the terminal acrylate protons (h) on the polymer chain at 5.88 ppm as follows: Mn,NMR = (SAr/4)/Sh × M(M1) + M(EBiB) – M(Br) + M(acrylate). We were pleased to note that the observed value was close to the theoretical one (Table 1). Moreover, GPC elution curve of PAzo-Acrylate displayed a symmetric chromatogram and nearly the same molecular weight (Mn = 6.7 kDa) and molecular weight distribution (dispersity is 1.29) as those of PAzo-Br, suggesting that PAzo-Acrylate has a similar backbone structure to that of PAzo-Br.

Fig. 2. MALDI–TOF of PAzo-Br prepared by SET-LRP of M1 under the following conditions:

[M1]/[EbiB]/[Me6-TREN]/[CuBr2]

in

20:

1:

0.075:

0.08,

and

PAzo-Acrylate prepared by the reaction of PAzo-Br with KA.

MALDI–TOF analysis confirmed the end capping of bromine-terminated chains with acrylate moieties. As can be seen in Fig. 2, PAzo-Br isolated after SET-LRP process showed a single distribution of peaks, and the peaks were separated by 296 mass units, which corresponded to the molecular weight of the molar mass of the azo-monomer repeat unit. After substitution of a bromine atom with an acrylate moiety, a new series of peaks appeared approximately 9 mass units below the bromine-terminated polymer (Dotted line in the expansion showed the position of the original peak before esterification). The increase of 9 mass units in molar mass is consistent with the substitution of Br (79.9) – CH2CHOCO (71.0) = 8.9 at the polymer terminus. This is a remarkable result for a synthetic approach based on end group functionalization. 3.4. ADMET polymerization for preparation of azobenzene-containing ABA triblock copolymers Combining SET-LRP with the interfacial esterification of KA, end functional PAzo-Acrylate was conveniently prepared. Usually, electron-poor acrylates do not homodimerize or have a very low tendency to dimerize during olefin metathesis but do participate in a secondary metathesis reaction with homodimers of more reactive

olefins [25]. Therefore, acrylate could act as a selective chain stopper to react only with chain ends of the ADMET polymer. In this case, we may also consider PAzo-Acrylate

as

a

selective macromolecule chain

stopper in

ADMET

polymerization of M2 to directly prepare ABA triblock main-side chains azobenzene copolymer. 1H NMR spectrum in Fig. S10 gave a good indication that ABA triblock copolymer structure was formed as no terminal olefin signals were observed that should be observed if homopolymer or AB diblock copolymer was formed. Besides, a doublet of triplets, appeared at 6.97 ppm in the enlarged region, suggesting formation of internal acrylates after the termination. GPC result showed a clear-cut shift in the copolymer trace toward lower retention volumes (with respect to PAzo-Acrylate and homopolymer of M2) was observed while the molecular weight distribution became slightly broad (Fig. S7, Table 2). Importantly, the molecular weights can be tuned by adjusting the M2/PAzo-Acrylate molar ratio from 100: 1 to 100: 10 (Fig. S11, Table S1). Therefore, it is possible to prepare ABA triblock copolymer architectures with control over the molecular weight of B block by ADMET polymerization in a single step.

Table 2 Characteristics

of

azobenzene-containing

ADMET

homopolymer,

SET-LRP

homopolymer, and ABA triblock copolymer.

(Homo)ADMETa SET-LRPb Esterification

Yield/Conv. (%) 92 88 98

Mn (kDa) 13.2 6.8 6.7

(Co)ADMETc

93

24.8

Synthetic Route

a

Dispersity 1.89 1.32 1.29 1.77

kid (s /10-2) 0.87±0.03 4.26±0.01 – 3.92±0.02 0.69±0.01 -1

kre (s /10-2) 0.56±0.02 1.21±0.02 – 1.08±0.01 0.35±0.03 -1

Isomerization Efficiencyf (%) 93.6 98.9 –

λtransg (nm) 352 345 –

λcish (nm) 436 439 –

91.7

336

438

Reaction conditions: [M2] = 1.0 mol/L, polymerization temperature = 65 °C, [M2]/[Catalyst] =

100: 1. b

Entry 1 in Table 1.

c

Reaction conditions: [M2] = 1.0 mol/L, polymerization temperature = 60 °C, [M2]/[Chain

Stopper] = 100:1, [M2]/[Catalyst] = 100: 1. d

The rate constant of trans–to–cis photoisomerization.

e

The rate constant of cis–to–trans photoisomerization.

f

The ratio of Atrans–cis/A0.

3.5. Photoisomerization behavior of azobenzene-containing polymers The unique photoinduced reversible isomerization is one of the most important properties of azo-polymers under alternative irradiation with UV and visible light. In general, azo-polymers have trans and cis isomers, which show a high-intensity π → π* transition band located in the UV region and a relatively low-intensity n → π* transition band located in the visible region, respectively. In order to enrich the structural changes of azo isomer, we herein investigated the isomerization processes of the azo chromophores in main-chain (ADMET homopolymer, Fig. S9), side-chain (SET-LRP polymer), and main-side chains (ABA triblock copolymer) in diluted solutions. We dissolved the azo-polymers in THF and selected UV and visible light as the stimulus. The UV–Vis spectra for both main-chain ADMET azo-homopolymer and side-chain SET-LRP azo-polymer displayed that the π → π* absorption band around 352 and 345 nm gradually decreased and the n → π* transition near 436 nm gradually increased (Fig. 3). The photostationary state of the trans→cis photoisomerization of two types of azo-polymers was reached after 300 and 60 s, respectively, of UV light irradiation. The reversibility of the photoisomerization processes were achieved when the azo-polymer solutions were irradiated with visible light, during which the π → π* absorption band gradually increased and the n → π* transition gradually decreased. The photostationary state of the cis→trans isomerization were also reached after 400 and 200 s, respectively, of visible light irradiation. The photoisomerization rate constants of trans→cis (kis) and cis→trans (krs) of each azo unit were calculated and showed in Table 2. As we reported, photoisomerization rate for side-chain azo-polymer was faster than that of main-chain type.

Fig. 3. UV–Vis spectra changes in dependence of time for the diluted (a, b) main-side ADMET azo-homopolymer and (c, d) side-chain SET-LRP azo-homopolymer solutions with the concentration of 2 × 10-5 mol/L under UV and visible light irradiation.

The ABA triblock copolymer containing main and side azo units was firstly dissolved in THF and irradiated by UV light for 50 s. Considering the relatively fast photoisomerization rate for side azo units, the π → π* absorption band around 336 nm of the trans-isomer of side azo units gradually decreased, and the n → π* absorption band near 437 nm of the cis-isomer of side azo units gradually increased. The first photostationary state of the trans→cis photoisomerization of side azo units was reached, the π → π* absorption band of the trans-isomer of main azo units remained almost stable at this moment. After 90 s of continuous UV light irradiation, the π → π* absorption band around 336 nm of the trans-isomer of main azo units began to decrease gradually, but the n → π* transition near 437 nm was red-shifted to 500~550 nm and gradually increased, which was attributed to the cis-isomer of main azo units. The other photostationary state of the trans→cis photoisomerization of main azo units was reached after about 220 s of irradiation (Fig. 4a). In other words, all azo

units in both main and side chains underwent the trans→cis photoisomerization after 360 s of continuous UV light irradiation. Those changes in the UV–Vis spectra confirmed the trans→cis isomerization of main and side azo units was not simultaneously occurring even in the same polymer chain.

Scheme 2. Illustration of photoisomerization process of main-side chains triblock azo-copolymer upon irradiation in the order of UV and visible light.

Further, two cis→trans back-isomerization processes was also observed by visible light irradiation, and the trans-isomer of azo units were obtained finally, as shown in Fig. 4b. Unlike the photoisomerization of main-side chains azo-polymers we have reported earlier [11,26], herein, four isomers, the trans (main chain)-trans (side chain), trans (main chain)-cis (side chain), cis (main chain)-cis (side chain), cis (main chain)-trans (side chain) in the same polymer chain can be conveniently obtained by alternating photoirradiation in the order of UV and visible light. Based on these results, the possible photoisomerization process of the azo units in both main and side chains was proposed in Scheme 2. For this main-side chains azo-copolymer, the photoisomerization efficiency by calculating the ratio of Atrans→cis/A0 was 91.7%, which indicated that most of azo units underwent the isomerization process even in the special structure. The first-order kinetic plots and the values of kis and krs were deduced subsequently from Fig. 3 and 4, which were shown in Fig. 5 and Table 2. There are two values for the ki and kr, which were corresponding to the two types of azo structures. Although the value was slightly lower than that of homopolymer, both

the kis and krs values of side azo units are also distinctly higher than those of main azo units, which is attributed to the flexible side-chain structure.

Fig. 4. Changes in UV–Vis spectra of main-side chains triblock azo-copolymer in THF with a concentration of 2 × 10-5 mol/L under different irradiation times with (a) UV and (b) visible light.

Fig. 5. First-order kinetics for reversible isomerization of the main-chain, side-chain, and main-side chains azo polymers: (a) trans–cis photoisomerization and (b) cis– trans back-isomerization.

4. Conclusion In summary, a novel ABA triblock copolymer contained azo units both in the mainand side-chain was rationally designed and successfully synthesized by combining the SET-LRP of azo-functionalized acrylate monomer, the chain end of ω-bromo functionalization

of

this

SET-LRP

polymer,

and

subsequent

ADMET

co-polymerization of azo-functionalized α,ω-diene monomer. The photoisomerization processes of the copolymer and corresponding homopolymers prepared via SET-LRP and ADMET homo-polymerization were monitored using UV-vis spectroscopy. Upon alternating photoirradiation with UV and visible light, it was found that the isomerization of main-chain ADMET azo-homopolymer and side-chain SET-LRP azo- homopolymer happened mainly on one type of azo group, which only underwent the trans→cis photoisomerization and the cis→trans back-isomerization processes. While main-side chains azo-copolymer made two types of azo groups, which presented four structural isomers in the same polymer chain, including the trans (main chain)-trans (side chain), trans (main chain)-cis (side chain), cis (main chain)-cis (side chain) and cis (main chain)-trans (side chain). All these interesting results will provide the new prospects for the design of novel photoresponsive materials.

Acknowledgements The authors thank the National Natural Science Foundation of China (No. 21774107, 21801217, and 51903217), the Natural Science Foundation of the Jiangsu Higher Education Institutions of China (No. 18KJB430027), Qing Lan Project of

Jiangsu Province, and the Initial Scientific Research Foundation of Yancheng Institute of Technology (No. KJC2014002) for financial support of this research. The authors also would like to thank Professor Virgil Percec for providing unlimited support and enthusiastic help.

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Highlights 1. Novel ABA triblock copolymer contained azo chromophores both in the main- and side-chain was controllably synthesized via combination of single electron transfer-living radical polymerization and acyclic diene metathesis polymerization. 2. A thorough investigation of single electron transfer-living radical polymerization of acrylate bearing azo group was presented to determine the optimum polymerization conditions for rapid, controlled, and quantitative production of well-defined azo-homopolymer. 3. The main-side chains triblock azo-copolymer made two types of azo groups, which presented four structural isomers in the same polymer chain upon alternating photoirradiation with UV and visible light.

Credit Author Statement Author contributions Liang Ding: Conceived and designed the experiments, Writing-Original Draft. Yadi Li: Performed the experiments. Hui Cang: Performed the experiments. Juan Li: Performed the experiments, Data analysis. Chengshuang Wang: Data analysis, Writing: Review & Editing. Wei Song: Improved and corrected the style of the article.

Declaration of Interest Statement There are no conflicts to declare.

Liang Ding, Ph.D. Associate Professor