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Ultra-High Molecular Weights via Aqueous Reversible-Deactivation Radical Polymerization
Article
Ultra-High Molecular Weights via Aqueous Reversible-Deactivation Radical Polymerization R. Nicholas Carmean, Troy E. Becker, Michael B. Sims, Brent S. Sumerlin [email protected]
HIGHLIGHTS Aqueous, catalyst-free route leads to polymers with unprecedented chain lengths Polymerization control is mediated by mild UV or sunlight irradiation Synthesis of UHMW block copolymers could facilitate access to new advanced materials
Although progress in living polymerization methods has enabled access to polymers with controlled molecular weights and architectures, avenues to new ultra-high-molecular-weight (UHMW) materials are limited. Sumerlin and coworkers reveal a simple, catalyst-free route to well-defined UHMW polymers and block copolymers with degrees of polymerization above 85,000.
Carmean et al., Chem 2, 93–101 January 12, 2017 ª 2017 Elsevier Inc. http://dx.doi.org/10.1016/j.chempr.2016.12.007
Article
Ultra-High Molecular Weights via Aqueous Reversible-Deactivation Radical Polymerization R. Nicholas Carmean,1 Troy E. Becker,1 Michael B. Sims,1 and Brent S. Sumerlin1,2,*
SUMMARY
The Bigger Picture
Relying solely on mild UV irradiation of thiocarbonylthio compounds in the presence of vinyl monomers, a new avenue to well-defined ultra-high-molecularweight (UHMW) polymers has been developed. Through the use of aqueous conditions, well-controlled UHMW polymers that are unprecedented for controlled radical polymerizations have been achieved. This photomediated polymerization approach reaches number-average molecular weights in excess of 8.00 3 106 g/mol with degrees of polymerization above 85,000, making these, to our knowledge, the highest-molecular-weight polymers ever achieved via reversible-deactivation radical polymerization. In many cases, well-defined UHMW polymers can be obtained in minutes. The utility of the technique is further demonstrated through the synthesis of block copolymers, enabling access to a new field of well-defined UHMW materials.
Materials derived from ultra-highmolecular-weight (UHMW) polymers offer unrivaled mechanical strength but are often limited in composition and architecture. Although advances in living polymerizations, especially reversible-deactivation radical polymerization (RDRP), have enabled the design of welldefined polymers with controlled molecular weights and architectures, most methods enable control only up to modest molecular weights, despite the need for new robust materials. An ability to target complex UHMW polymers via RDRP could allow an unprecedented opportunity to investigate important fundamental principles in selfassembly behavior and phase segregation. Herein, we describe catalyst-free photopolymerization conditions that facilitate the synthesis of UHMW polymers in environmentally friendly aqueous solvents to achieve nearquantitative monomer conversion by requiring only a readily available and low-energy light source or, in some cases, only sunlight.
INTRODUCTION Conventional radical polymerization often results in high-molecular-weight polymers, but slow initiation and fast termination typically limit access to predetermined molecular weights, narrow molecular-weight distributions (MWDs), retained chain-end functionality, and block copolymer synthesis. Conversely, reversibledeactivation radical polymerization (RDRP) allows for excellent control of molecular weight and enables the synthesis of diverse architectures, including hyperbranched,1,2 star,3,4 brush,5,6 and block copolymers,7–10 while also providing narrow and tunable11 MWDs. However, applying RDRP techniques to achieve extraordinarily high chain lengths has proven challenging. Specifically, polymerizations targeting ultra-high molecular weight (UHMW), defined here as molecular weight greater than 1.00 3 106 g/mol, have been achieved through atom transfer radical polymerization (ATRP)12–16 and reversible addition-fragmentation chain transfer (RAFT)17–19 polymerization. However, these previous reports used rather specialized reaction conditions, such as high pressure12,13,18 or heterogeneous conditions.15,20 Herein, we outline a simple, aqueous-based route to UHMW polymers through the use of a mild UV light source and thiocarbonylthio compounds. We pushed the limits of high chain length to reach number-average molecular weights (Mn) in excess of 8.00 3 106 g/mol and number-average degrees of polymerization (DPn) above 85,000. The chain lengths achieved through this approach are well beyond those reported previously for other RDRP techniques. Furthermore, we demonstrate that with judicious thiocarbonylthio and monomer pairing, these polymerizations can be extremely rapid and lead to UHMW polymers in less than 5 min of irradiation with only low polymerization temperatures, environmentally friendly solvents, and no metal catalysts.
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RESULTS AND DISCUSSION Strategic consideration of key kinetic parameters enabled the synthesis of macromolecules with very high DPn. Specifically, carrying out polymerizations under conditions that lead to high propagation (kp) and low termination (kt) rate constants facilitates the production of polymers with high chain lengths. Enhanced kp is observed with increasing pressure,18,21 with increasing monomer concentration, and in many cases, under aqueous conditions.17,22–24 We exploited the latter by beginning our investigations with the polymerization of a hydrophilic acrylamide monomer, N,N-dimethylacrylamide (DMA), in water at temperatures near 35 C in the presence of a trithiocarbonate, 2-(2-carboxyethylsulfanylthiocarbonylsulfanyl)2-methylpropionic acid. Molecular weights of homopolymers prepared by this route ranged from 1.10 3 106 to 4.82 3 106 g/mol, corresponding to DPn values from 11,000 to 48,000 (Figures 1A, S1–S4, S10, and S11; Table 1, entries 1–3 and 9). It is believed that the thiocarbonylthio agent undergoes carbon-sulfur bond photolysis under UV and visible-light irradiation.25 Upon absorption, reversible photolytic cleavage of the carbon-sulfur bond results in carbon- and sulfur-centered radicals, the former of which initiate polymerization and the latter of which allow reversible termination with the propagating chain end.26–30 As the polymerization proceeds, activation of the dormant thiocarbonylthio-terminated chain ends continues to occur by photodissociation of the terminal carbon-sulfur bond. In this sense, the thiocarbonylthio group can operate as a chain-transfer agent or as an iniferter,28–33 and control is most likely achieved through a combination of degenerative chain transfer and reversible termination (Scheme 1).34 In contrast to traditional RAFT polymerization procedures, which require both a thiocarbonylthio compound and an exogenous initiating source, photoiniferter polymerizations operate without the addition of an external initiator.28–30 Our results suggest that this photolytic mechanism provides an excellent avenue to UHMW polymers. A typical RAFT polymerization would rely on radical generation by decomposition of an external initiator that continuously generates low-molecularweight chains that broaden the molecular-weight distribution and act as a constant source of low-molecular-weight radicals that can terminate polymerization by coupling with high-molecular-weight chains. Relying on photolysis of a thiocarbonylthio compound to generate radicals allows the majority of the radicals during the polymerization to have high molecular weights and therefore slower rates of termination. However, to our knowledge, this technique has yet to be used to target UHMW polymers. In fact, there have been reports that polymerization control is sacrificed at the low iniferter concentrations required for achieving high molecular weights.35 Yet, high thiocarbonylthio chain-end retention has been demonstrated through the photomediated synthesis of multiblock copolymers.36 To achieve UHMW polymers in a controlled manner, the rates of propagation and reversible termination should be high, whereas the rate of irreversible termination should be low. We decided to exploit the enhanced kp of many monomers in water and to combine this with the photoiniferter approach to limit background initiation of new low-molecular-weight chains, which are expected in traditional, exogenously initiated RAFT polymerization. Although both kp and kt depend on chain length, the effect of chain length on bimolecular termination, particularly that between two macromolecular radicals, is more pronounced.37–40 Indeed, the majority of chain termination in conventional RAFT polymerization has been attributed to coupling of long chains with low-molecular-weight radicals,41–43 an effect that exists largely because segmental and translational diffusion are inversely proportional to chain
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1George
& Josephine Butler Polymer Research Laboratory, Department of Chemistry, Center for Macromolecular Science & Engineering, University of Florida, Gainesville, FL 32611, USA
2Lead
Contact
*Correspondence: [email protected] http://dx.doi.org/10.1016/j.chempr.2016.12.007
Figure 1. Synthesis of UHMW Polymers via Trithiocarbonate Iniferter (A) DMA was irradiated with long-wave UV light (l = 365 nm, 7.0 mW/cm 2 ) in the presence of a trithiocarbonate iniferter to produce UHMW poly(DMA). Size-exclusion chromatography (SEC) curves are shown as a function of the targeted degree of polymerization; each trace represents a different polymerization. The void volume of this SEC system was 10 mL. (B) The polymerization of DMA to UHMW (M n,SEC 1.03 3 10 6 g/mol) displays near-linear pseudo-first-order kinetics, indicating a constant radical concentration up to >95% monomer conversion. (C) On/off kinetic plot demonstrates temporal control of the polymerization rate and suggests rapid reversible termination between carbon- and sulfurcentered radicals and that a similar polymerization rate is maintained through activation cycles. (D) During on/off polymerization, measured molecular weights closely matched theoretical values, and MWDs remained narrow, indicating rapid reinitiation and high end-group retention.
length.38 At high chain length, termination is dramatically decelerated by the large reduction in translational diffusion.44 We reasoned that this effect could be magnified when UHMW is targeted because of the high viscosities present during the polymerization, thereby resulting in a decreased probability of chain-chain interaction.45 Conceivably, the drastic reduction in polymer diffusion could also affect the deactivation mechanism during polymerization, such that reversible termination becomes overwhelmingly more likely than degenerative chain transfer at high solution viscosity. Although each polymerization required extraordinarily high monomer/iniferter ratios, the aqueous polymerizations of DMA exhibited clear RDRP characteristics, even under sunlight irradiation (Figure S10; Table 1, entry 9). In all of the
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Table 1. UHMW Polymerization of DMA Targeting a Number-Average Molecular Weight Greater Than 1.00 3 106 g/mol Entry 1
d
Iniferter
Time (hr)
% Conversiona
Mn, SEC (g/mol)b
Mn, theory (g/mol)c
Ð
TTC
10
96
1,034,000
1,065,000
1.12
2
TTC
10
97
2,520,000
2,040,000
1.03
3
TTC
8
94
4,820,000
5,107,000
1.39
4
TTC
8
89
2,670,000
2,540,000
1.28
5
XAN
0.5
93
1,300,000
1,340,000
1.36
6
XAN
0.5
93
2,960,000
2,320,000
1.18
7
XAN
1
95
5,200,000
5,170,000
1.43
8
XAN
2
89
8,570,000
9,930,000
1.17
TTC
7
62
1,600,000
1,300,000
1.17
9
e
a
Monomer conversion was determined with 1H NMR spectroscopy. b Absolute number-average molecular weights (Mn, SEC) were determined by SEC equipped with a multiangle light-scattering detector assuming 100% mass recovery. c The theoretical molecular weights (Mn, theory) were determined from the monomer conversion from 1 H NMR spectroscopy. d This polymerization was conducted with a 1 M monomer concentration, whereas subsequent polymerizations were performed at 2 M. The increased reaction viscosity early in the polymerization enabled the synthesis of higher chain lengths on reasonable timescales. e This polymerization was conducted on the roof of Sisler Hall on the University of Florida’s campus and relied on direct sunlight irradiation for chain growth.
photopolymerizations we considered, near-linear pseudo-first-order kinetic plots indicated a relatively constant radical concentration up to full monomer conversion (Figures 1B and S1–S4). These results suggest a high degree of chain-end fidelity because photolytic cleavage of the dormant end group is responsible for propagation. Irreversible decomposition of the thiocarbonylthio end group would produce a negative deviation in the pseudo-first-order kinetic plot because the radical concentration and, subsequently, the rate of polymerization would decrease. Furthermore, rapid reversible termination between the propagating chain end and the sulfurcentered iniferter radical was inferred from the narrow MWDs and through on/ off kinetic experiments (Figures 1C, 1D, and S4). During each polymerization, Mn increased with conversion (Figures 1D, S1–S4, and S10; Table 1, entries 1–3 and 9) and showed acceptable agreement with the theoretical molecular weights (Mn, theory). Although UV irradiation of the vinyl monomers we investigated resulted in slow background initiation that might give rise to hybrid iniferter-RAFT polymerization behavior, this adventitious radical source seemed to impart minimal effect on the molecular-weight control achieved during the polymerizations considered (Figure S12). In addition to the near-linear pseudo-first-order kinetic plot during each polymerization, chain-extension experiments provided insight into the extent of thiocarbonylthio chain-end retention while targeting UHMW. After full conversion of DMA during homopolymerization to reach an Mn of 1.11 3 106 g/mol, additional DMA was introduced directly into the polymerization reactor in a one-pot manner to yield PDMA-b-PDMA (Mn = 2.67 3 106 g/mol). A shift in the size-exclusion chromatography trace was observed, and the MWD of the block copolymer remained narrow (Figure 2; Table 1, entry 4). As far as we are aware, this UHMW PDMA block copolymer, each block of which is over 1.00 3 106 g/mol, is the highest-molecular-weight block copolymer reported to date for an RDRP process.
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Scheme 1. Comparison of Traditional Polymerization Approaches to the Photoiniferter Polymerization Technique for Synthesis of UHMW Polymers (A) RAFT relies on the decomposition of an external initiator to begin chain growth mediated through degenerative chain transfer. (B) The current work relies on photolysis of a thiocarbonylthio mediating agent to begin polymerization that is mediated through a combination of reversible termination and degenerative chain transfer.
Traditionally, photomediated polymerization rates can be tuned through modified light intensity;33 however, an alternative avenue to tailored polymerization rates is available through judicious selection of the thiocarbonylthio compound. A brief investigation into the absorption spectra of two iniferters, namely a trithiocarbonate and xanthate, revealed a dramatically different activation profile under mild UV light. The lowest unoccupied molecular orbitals of xanthates are higher in energy as a result of the increased electronic donation from the oxygen lone pairs to the carbon-sulfur double bond, which results in a blue-shifted absorption spectrum. The p-to-p* transition is most pronounced for both compounds, and an n-to-p* transition is also observed at longer wavelengths. Under long-wave, mild UV irradiation (peak emission near 365 nm), trithiocarbonate photolysis results from a p-to-p* transition, whereas the xanthate undergoes a spin-forbidden n-to-p* transition (Figures 3A and S5).36 The latter mode of activation translates to more rapid photolytic cleavage of xanthates,46,47 i.e., they demonstrate a higher quantum yield, which was confirmed in our case through a model trapping experiment. Low-molecular-weight PDMA that contained either trithiocarbonate or xanthate end groups was irradiated at 35 C in the presence of a large excess (20 molar equiv) of a persistent radical, 2,2,6,6-tetramethyl-1-piperidinyloxyl (TEMPO). Upon photolysis, the carboncentered radical underwent rapid irreversible termination with TEMPO to produce a ‘‘dead’’ chain end that was monitored by 1H NMR spectroscopy. After 2 hr of irradiation, PDMA-trithiocarbonate retained 87% of the thiocarbonylthio end groups, whereas only 25% of PDMA-xanthate chain ends remained intact (Figure 3B). Given the relatively slow photolytic cleavage of the trithiocarbonate during irradiation, one might expect molecular weights that significantly exceed theoretical values during photoiniferter polymerizations with the trithiocarbonate agent. However, as shown in Figure 1D, this is not the case, which suggests that degenerative chain transfer plays an important role in the consumption of the original small-molecule iniferter during photomediated polymerizations with trithiocarbonates, at least at the low viscosities present early in the polymerization. Faster photolytic cleavage results in a higher radical concentration and more rapid rates of polymerization (Figure 4B). The polymerization of DMA in the presence of the xanthate reached 93% monomer conversion in just 30 min, which was
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Figure 2. Synthesis of UHMW Block Copolymers UHMW block copolymer poly(DMA)-b-poly(DMA) was prepared upon full monomer conversion of the homopolymer poly(DMA) in a one-pot process. The near-linear pseudo-first-order kinetic plot indicates a constant radical concentration during chain extension to produce poly(DMA)-b-poly(DMA) in 8 hr. SEC of the chain-extended diblock copolymer shows a shift to shorter elution times and higher molecular weights, demonstrating good blocking efficiency and high end-group retention.
considerably faster than the analogous reaction conducted with the trithiocarbonate, which required 10 hr to reach a similar level of conversion. This result could also suggest that xanthate and trithiocarbonate iniferters are predisposed to controlled chain growth through slightly different polymerization mechanisms. Unlike the trithiocarbonate, the xanthate exhibits a high photolysis rate, resulting in a polymerization solution high in active carbon- and sulfur-center radicals. As described above, on/off kinetic investigations suggested rapid reversible termination between the active radical pair throughout the polymerization. Therefore, at the high concentrations provided by rapid xanthate photolysis, it seems reasonable that the contribution of reversible termination to deactivation could be enhanced with respect to degenerative chain transfer. Additionally, in traditional RAFT polymerization, these xanthate mediating agents are known to have low chain-transfer rates with more activated monomers such as acrylamides,48 further suggesting that reversible termination plays an important role in polymerization control. The overall result of the enhanced photolytic activity of the xanthate agent resulted in near-quantitative DMA conversion in just 30 min, achieving an Mn of 1.30 3 106 g/mol (Mw/Mn = 1.36; Mn, theory = 1.34 3 106 g/mol) (Figures 4B and S6; Table 1, entry 5). Compared with the trithiocarbonate-mediated polymerization conducted under otherwise identical conditions, this system led to a slightly higher Mw/Mn, but the MWD remained monomodal, and the measured molecular weight closely matched theoretical values. The utility of this rapid, xanthate-mediated polymerization was further demonstrated by targeting molecular weights near
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Figure 3. Alternative Avenue for Photolytic Cleavage (A) UV-Vis spectroscopy of the two iniferters reveals that xanthate has a blue-shifted absorbance spectrum in comparison with that of trithiocarbonate. This shift yields an alternative activation route under mild long-wave UV irradiation (l = 365 nm). (B) A low-molecular-weight model polymer, prepared with either a xanthate or trithiocarbonate iniferter, was irradiated in the presence of a large excess (20 molar equiv) of a persistent radical, 2,2,6,6-tetramethyl-1-piperidinyloxyl (TEMPO). By monitoring conversion of the iniferter-terminated polymer to the unreactive TEMPO-terminated product, photolytic trapping revealed that the xanthate-terminated polymer was consumed considerably faster than the trithiocarbonate-terminated analog. (i) Irradiation with long-wave UV light (l = 365 nm, 7.0 mW/cm 2 ) in DMSO at 35 C.
10.0 3 106 g/mol. With DMA/xanthate ratio of 110,000:1, high monomer conversion (88% by 1H NMR spectroscopy) was measured in 2 hr to produce a polymer with an Mn of 8.57 3 106 g/mol (Figures 4A and S9; Table 1, entry 8). Notably, during this polymerization, an Mn of 2.47 3 106 g/mol was reached in just 5 min. This measurement closely matches theoretical values (Mn, theory = 2.10 3 106 g/mol) and demonstrates a remarkable rate of polymerization while maintaining control over molecular weight. Furthermore, the polymerization rate is dependent on xanthate concentration, as expected for iniferter polymerizations, such that increased reaction times are required for higher monomer/iniferter ratios (Figures S6–S9 and S13).49 Chemical transformations triggered by electromagnetic irradiation often offer exclusive avenues to previously inaccessible materials. Rapid developments in the field of light-mediated RDRP have broadened the scope of advanced macromolecular design, typically through straightforward and facile synthetic procedures. However, these processes have yet to be realized for UHMW polymers, although this class of materials could offer tremendous promise for a number of materials applications. With the approach described here, we demonstrate the utility of photoinifertermediated polymerization for producing the highest-molecular-weight polymers ever reported via RDRP, thereby enabling the synthesis of precise UHMW architectures. Furthermore, through strategic monomer and iniferter pairing, it is possible to synthesize these polymers in minutes while maintaining predetermined molecular weights and narrow MWDs. The simplicity of this method facilitates the construction of UHMW macromolecules without the need for an external catalyst or exogenous initiator and can be expanded to address a broad class of materials.
EXPERIMENTAL PROCEDURES Full experimental procedures are provided in the Supplemental Information.
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Figure 4. Rapid UHMW Synthesis in Minutes via Xanthate-Mediated Polymerization (A) DMA was irradiated with long-wave UV light (l = 365 nm, 7.0 mW/cm 2 ) in the presence of a trithiocarbonate iniferter to produce UHMW poly(DMA). SEC curves are shown as a function of the targeted degree of polymerization; each trace represents a different polymerization. The void volume of this SEC system was 10 mL. (B) The polymerization of DMA to UHMW (M n,SEC 1.30 3 10 6 g/mol) displays near-linear pseudo-first-order kinetics, reaching 93% monomer conversion in 30 min.
SUPPLEMENTAL INFORMATION Supplemental Information includes Supplemental Experimental Procedures and 13 figures and can be found with this article online at http://dx.doi.org/10.1016/j. chempr.2016.12.007.
AUTHOR CONTRIBUTIONS R.N.C., T.E.B., and B.S.S. conceived and designed the research. R.N.C., T.E.B., and M.B.S. performed polymerizations and aided in the characterization of the resulting materials. R.N.C. and B.S.S co-authored the manuscript.
ACKNOWLEDGMENTS This material is based on work supported by the National Science Foundation (DMR-1606410). Received: November 1, 2016 Revised: December 12, 2016 Accepted: December 19, 2016 Published: January 12, 2017
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35. Wang, H., Li, Q., Dai, J., Du, F., Zheng, H., and Bai, R. (2013). Real-time and in situ investigation of ‘‘living’’/controlled photopolymerization in the presence of a trithiocarbonate. Macromolecules 46, 2576– 2582. 36. McKenzie, T.G., Fu, Q., Wong, E.H.H., Dunstan, D.E., and Qiao, G.G. (2015). Visible light mediated controlled radical polymerization in the absence of exogenous radical sources or catalysts. Macromolecules 48, 3864–3872. 37. Odian, G. (2004). Principles of Polymerization, Fourth Edition (Wiley-Interscience). 38. Tsarevsky, N.V., and Sumerlin, B.S. (2012). Fundamentals of Controlled/Living Radical Polymerization (Oxford University Press). 39. Barner-Kowollik, C., and Russell, G.T. (2009). Chain-length-dependent termination in radical polymerization: subtle revolution in tackling a long-standing challenge. Prog. Polym. Sci. 34, 1211–1259. 40. Kannurpatti, A.R., Lu, S., Bunker, G.M., and Bowman, C.N. (1996). Kinetic and mechanistic studies of iniferter photopolymerizations. Macromolecules 29, 7310–7315. 41. Hall, J.G., and Monteiro, M.J. (2008). Bimolecular radical termination: new perspectives and insights. J. Polym. Sci. A. Polym. Chem. 46, 3155–3173. 42. Hall, J.G., and Monteiro, M.J. (2007). Kinetic modeling of ‘‘living’’ and conventional free radical polymerizations of methyl methacrylate in dilute and gel regimes. Macromolecules 40, 7171–7179. 43. Adams, M.E., Russell, G.T., Casey, B.S., Gilbert, R.G., Napper, D.H., and Sangster, D.F. (2002). Bimolecular termination events in the seeded emulsion polymerization of styrene. Macromolecules 23, 4624–4634. 44. Mahabadi, H.K., and O’Driscoll, K.F. (1977). Termination rate constant in free-radical polymerization. J. Polym. Sci. A. Polym. Chem. 15, 283–300. 45. Lachinov, M.B., Simonian, R.A., Georgieva, T.G., Zubov, V.P., and Kabanov, V.A. (1979). Nature of gel effect in radical polymerization. Polym. Sci. Polym. Chem. 17, 613–616. 46. Turro, N.J., Ramamurthy, V., and Scaiano, J.C. (2012). Modern molecular photochemistry of organic molecules. Photochem. Photobiol. 88, 1033. 47. Coyle, J.D., and Carless, H. (1972). Selected aspects of photochemistry. I photochemistry of carbonyl compounds. Chem. Soc. Rev. 1, 465–480.
33. Otsu, T. (2000). Iniferter concept and living radical polymerization. J. Polym. Sci. A. Polym. Chem. 38, 2121–2136.
48. Nakabayashi, K., and Mori, H. (2013). Recent progress in controlled radical polymerization of N-Vinyl monomers. Eur. Polym. J. 49, 2808– 2838.
34. Khan, M.Y., Cho, M.-S., and Kwark, Y.-J. (2014). Dual roles of a xanthate as a radical source and chain transfer agent in the photoinitiated RAFT polymerization of vinyl acetate. Macromolecules 47, 1929–1934.
49. Pan, X., Tasdelen, M.A., Laun, C., Junkers, T., Yagci, Y., and Matyjaszewski, K. (2016). Photomediated controlled radical polymerization. Prog. Polym. Sci. 62, 73–125.
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Ultra-High Molecular Weights via Aqueous Reversible-Deactivation Radical Polymerization R. Nicholas Carmean, Troy E. Becker, Michael B. Sims, Brent S. Sumerlin [email protected]
HIGHLIGHTS Aqueous, catalyst-free route leads to polymers with unprecedented chain lengths Polymerization control is mediated by mild UV or sunlight irradiation Synthesis of UHMW block copolymers could facilitate access to new advanced materials
Although progress in living polymerization methods has enabled access to polymers with controlled molecular weights and architectures, avenues to new ultra-high-molecular-weight (UHMW) materials are limited. Sumerlin and coworkers reveal a simple, catalyst-free route to well-defined UHMW polymers and block copolymers with degrees of polymerization above 85,000.
Carmean et al., Chem 2, 93–101 January 12, 2017 ª 2017 Elsevier Inc. http://dx.doi.org/10.1016/j.chempr.2016.12.007
Article
Ultra-High Molecular Weights via Aqueous Reversible-Deactivation Radical Polymerization R. Nicholas Carmean,1 Troy E. Becker,1 Michael B. Sims,1 and Brent S. Sumerlin1,2,*
SUMMARY
The Bigger Picture
Relying solely on mild UV irradiation of thiocarbonylthio compounds in the presence of vinyl monomers, a new avenue to well-defined ultra-high-molecularweight (UHMW) polymers has been developed. Through the use of aqueous conditions, well-controlled UHMW polymers that are unprecedented for controlled radical polymerizations have been achieved. This photomediated polymerization approach reaches number-average molecular weights in excess of 8.00 3 106 g/mol with degrees of polymerization above 85,000, making these, to our knowledge, the highest-molecular-weight polymers ever achieved via reversible-deactivation radical polymerization. In many cases, well-defined UHMW polymers can be obtained in minutes. The utility of the technique is further demonstrated through the synthesis of block copolymers, enabling access to a new field of well-defined UHMW materials.
Materials derived from ultra-highmolecular-weight (UHMW) polymers offer unrivaled mechanical strength but are often limited in composition and architecture. Although advances in living polymerizations, especially reversible-deactivation radical polymerization (RDRP), have enabled the design of welldefined polymers with controlled molecular weights and architectures, most methods enable control only up to modest molecular weights, despite the need for new robust materials. An ability to target complex UHMW polymers via RDRP could allow an unprecedented opportunity to investigate important fundamental principles in selfassembly behavior and phase segregation. Herein, we describe catalyst-free photopolymerization conditions that facilitate the synthesis of UHMW polymers in environmentally friendly aqueous solvents to achieve nearquantitative monomer conversion by requiring only a readily available and low-energy light source or, in some cases, only sunlight.
INTRODUCTION Conventional radical polymerization often results in high-molecular-weight polymers, but slow initiation and fast termination typically limit access to predetermined molecular weights, narrow molecular-weight distributions (MWDs), retained chain-end functionality, and block copolymer synthesis. Conversely, reversibledeactivation radical polymerization (RDRP) allows for excellent control of molecular weight and enables the synthesis of diverse architectures, including hyperbranched,1,2 star,3,4 brush,5,6 and block copolymers,7–10 while also providing narrow and tunable11 MWDs. However, applying RDRP techniques to achieve extraordinarily high chain lengths has proven challenging. Specifically, polymerizations targeting ultra-high molecular weight (UHMW), defined here as molecular weight greater than 1.00 3 106 g/mol, have been achieved through atom transfer radical polymerization (ATRP)12–16 and reversible addition-fragmentation chain transfer (RAFT)17–19 polymerization. However, these previous reports used rather specialized reaction conditions, such as high pressure12,13,18 or heterogeneous conditions.15,20 Herein, we outline a simple, aqueous-based route to UHMW polymers through the use of a mild UV light source and thiocarbonylthio compounds. We pushed the limits of high chain length to reach number-average molecular weights (Mn) in excess of 8.00 3 106 g/mol and number-average degrees of polymerization (DPn) above 85,000. The chain lengths achieved through this approach are well beyond those reported previously for other RDRP techniques. Furthermore, we demonstrate that with judicious thiocarbonylthio and monomer pairing, these polymerizations can be extremely rapid and lead to UHMW polymers in less than 5 min of irradiation with only low polymerization temperatures, environmentally friendly solvents, and no metal catalysts.
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RESULTS AND DISCUSSION Strategic consideration of key kinetic parameters enabled the synthesis of macromolecules with very high DPn. Specifically, carrying out polymerizations under conditions that lead to high propagation (kp) and low termination (kt) rate constants facilitates the production of polymers with high chain lengths. Enhanced kp is observed with increasing pressure,18,21 with increasing monomer concentration, and in many cases, under aqueous conditions.17,22–24 We exploited the latter by beginning our investigations with the polymerization of a hydrophilic acrylamide monomer, N,N-dimethylacrylamide (DMA), in water at temperatures near 35 C in the presence of a trithiocarbonate, 2-(2-carboxyethylsulfanylthiocarbonylsulfanyl)2-methylpropionic acid. Molecular weights of homopolymers prepared by this route ranged from 1.10 3 106 to 4.82 3 106 g/mol, corresponding to DPn values from 11,000 to 48,000 (Figures 1A, S1–S4, S10, and S11; Table 1, entries 1–3 and 9). It is believed that the thiocarbonylthio agent undergoes carbon-sulfur bond photolysis under UV and visible-light irradiation.25 Upon absorption, reversible photolytic cleavage of the carbon-sulfur bond results in carbon- and sulfur-centered radicals, the former of which initiate polymerization and the latter of which allow reversible termination with the propagating chain end.26–30 As the polymerization proceeds, activation of the dormant thiocarbonylthio-terminated chain ends continues to occur by photodissociation of the terminal carbon-sulfur bond. In this sense, the thiocarbonylthio group can operate as a chain-transfer agent or as an iniferter,28–33 and control is most likely achieved through a combination of degenerative chain transfer and reversible termination (Scheme 1).34 In contrast to traditional RAFT polymerization procedures, which require both a thiocarbonylthio compound and an exogenous initiating source, photoiniferter polymerizations operate without the addition of an external initiator.28–30 Our results suggest that this photolytic mechanism provides an excellent avenue to UHMW polymers. A typical RAFT polymerization would rely on radical generation by decomposition of an external initiator that continuously generates low-molecularweight chains that broaden the molecular-weight distribution and act as a constant source of low-molecular-weight radicals that can terminate polymerization by coupling with high-molecular-weight chains. Relying on photolysis of a thiocarbonylthio compound to generate radicals allows the majority of the radicals during the polymerization to have high molecular weights and therefore slower rates of termination. However, to our knowledge, this technique has yet to be used to target UHMW polymers. In fact, there have been reports that polymerization control is sacrificed at the low iniferter concentrations required for achieving high molecular weights.35 Yet, high thiocarbonylthio chain-end retention has been demonstrated through the photomediated synthesis of multiblock copolymers.36 To achieve UHMW polymers in a controlled manner, the rates of propagation and reversible termination should be high, whereas the rate of irreversible termination should be low. We decided to exploit the enhanced kp of many monomers in water and to combine this with the photoiniferter approach to limit background initiation of new low-molecular-weight chains, which are expected in traditional, exogenously initiated RAFT polymerization. Although both kp and kt depend on chain length, the effect of chain length on bimolecular termination, particularly that between two macromolecular radicals, is more pronounced.37–40 Indeed, the majority of chain termination in conventional RAFT polymerization has been attributed to coupling of long chains with low-molecular-weight radicals,41–43 an effect that exists largely because segmental and translational diffusion are inversely proportional to chain
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1George
& Josephine Butler Polymer Research Laboratory, Department of Chemistry, Center for Macromolecular Science & Engineering, University of Florida, Gainesville, FL 32611, USA
2Lead
Contact
*Correspondence: [email protected] http://dx.doi.org/10.1016/j.chempr.2016.12.007
Figure 1. Synthesis of UHMW Polymers via Trithiocarbonate Iniferter (A) DMA was irradiated with long-wave UV light (l = 365 nm, 7.0 mW/cm 2 ) in the presence of a trithiocarbonate iniferter to produce UHMW poly(DMA). Size-exclusion chromatography (SEC) curves are shown as a function of the targeted degree of polymerization; each trace represents a different polymerization. The void volume of this SEC system was 10 mL. (B) The polymerization of DMA to UHMW (M n,SEC 1.03 3 10 6 g/mol) displays near-linear pseudo-first-order kinetics, indicating a constant radical concentration up to >95% monomer conversion. (C) On/off kinetic plot demonstrates temporal control of the polymerization rate and suggests rapid reversible termination between carbon- and sulfurcentered radicals and that a similar polymerization rate is maintained through activation cycles. (D) During on/off polymerization, measured molecular weights closely matched theoretical values, and MWDs remained narrow, indicating rapid reinitiation and high end-group retention.
length.38 At high chain length, termination is dramatically decelerated by the large reduction in translational diffusion.44 We reasoned that this effect could be magnified when UHMW is targeted because of the high viscosities present during the polymerization, thereby resulting in a decreased probability of chain-chain interaction.45 Conceivably, the drastic reduction in polymer diffusion could also affect the deactivation mechanism during polymerization, such that reversible termination becomes overwhelmingly more likely than degenerative chain transfer at high solution viscosity. Although each polymerization required extraordinarily high monomer/iniferter ratios, the aqueous polymerizations of DMA exhibited clear RDRP characteristics, even under sunlight irradiation (Figure S10; Table 1, entry 9). In all of the
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Table 1. UHMW Polymerization of DMA Targeting a Number-Average Molecular Weight Greater Than 1.00 3 106 g/mol Entry 1
d
Iniferter
Time (hr)
% Conversiona
Mn, SEC (g/mol)b
Mn, theory (g/mol)c
Ð
TTC
10
96
1,034,000
1,065,000
1.12
2
TTC
10
97
2,520,000
2,040,000
1.03
3
TTC
8
94
4,820,000
5,107,000
1.39
4
TTC
8
89
2,670,000
2,540,000
1.28
5
XAN
0.5
93
1,300,000
1,340,000
1.36
6
XAN
0.5
93
2,960,000
2,320,000
1.18
7
XAN
1
95
5,200,000
5,170,000
1.43
8
XAN
2
89
8,570,000
9,930,000
1.17
TTC
7
62
1,600,000
1,300,000
1.17
9
e
a
Monomer conversion was determined with 1H NMR spectroscopy. b Absolute number-average molecular weights (Mn, SEC) were determined by SEC equipped with a multiangle light-scattering detector assuming 100% mass recovery. c The theoretical molecular weights (Mn, theory) were determined from the monomer conversion from 1 H NMR spectroscopy. d This polymerization was conducted with a 1 M monomer concentration, whereas subsequent polymerizations were performed at 2 M. The increased reaction viscosity early in the polymerization enabled the synthesis of higher chain lengths on reasonable timescales. e This polymerization was conducted on the roof of Sisler Hall on the University of Florida’s campus and relied on direct sunlight irradiation for chain growth.
photopolymerizations we considered, near-linear pseudo-first-order kinetic plots indicated a relatively constant radical concentration up to full monomer conversion (Figures 1B and S1–S4). These results suggest a high degree of chain-end fidelity because photolytic cleavage of the dormant end group is responsible for propagation. Irreversible decomposition of the thiocarbonylthio end group would produce a negative deviation in the pseudo-first-order kinetic plot because the radical concentration and, subsequently, the rate of polymerization would decrease. Furthermore, rapid reversible termination between the propagating chain end and the sulfurcentered iniferter radical was inferred from the narrow MWDs and through on/ off kinetic experiments (Figures 1C, 1D, and S4). During each polymerization, Mn increased with conversion (Figures 1D, S1–S4, and S10; Table 1, entries 1–3 and 9) and showed acceptable agreement with the theoretical molecular weights (Mn, theory). Although UV irradiation of the vinyl monomers we investigated resulted in slow background initiation that might give rise to hybrid iniferter-RAFT polymerization behavior, this adventitious radical source seemed to impart minimal effect on the molecular-weight control achieved during the polymerizations considered (Figure S12). In addition to the near-linear pseudo-first-order kinetic plot during each polymerization, chain-extension experiments provided insight into the extent of thiocarbonylthio chain-end retention while targeting UHMW. After full conversion of DMA during homopolymerization to reach an Mn of 1.11 3 106 g/mol, additional DMA was introduced directly into the polymerization reactor in a one-pot manner to yield PDMA-b-PDMA (Mn = 2.67 3 106 g/mol). A shift in the size-exclusion chromatography trace was observed, and the MWD of the block copolymer remained narrow (Figure 2; Table 1, entry 4). As far as we are aware, this UHMW PDMA block copolymer, each block of which is over 1.00 3 106 g/mol, is the highest-molecular-weight block copolymer reported to date for an RDRP process.
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Scheme 1. Comparison of Traditional Polymerization Approaches to the Photoiniferter Polymerization Technique for Synthesis of UHMW Polymers (A) RAFT relies on the decomposition of an external initiator to begin chain growth mediated through degenerative chain transfer. (B) The current work relies on photolysis of a thiocarbonylthio mediating agent to begin polymerization that is mediated through a combination of reversible termination and degenerative chain transfer.
Traditionally, photomediated polymerization rates can be tuned through modified light intensity;33 however, an alternative avenue to tailored polymerization rates is available through judicious selection of the thiocarbonylthio compound. A brief investigation into the absorption spectra of two iniferters, namely a trithiocarbonate and xanthate, revealed a dramatically different activation profile under mild UV light. The lowest unoccupied molecular orbitals of xanthates are higher in energy as a result of the increased electronic donation from the oxygen lone pairs to the carbon-sulfur double bond, which results in a blue-shifted absorption spectrum. The p-to-p* transition is most pronounced for both compounds, and an n-to-p* transition is also observed at longer wavelengths. Under long-wave, mild UV irradiation (peak emission near 365 nm), trithiocarbonate photolysis results from a p-to-p* transition, whereas the xanthate undergoes a spin-forbidden n-to-p* transition (Figures 3A and S5).36 The latter mode of activation translates to more rapid photolytic cleavage of xanthates,46,47 i.e., they demonstrate a higher quantum yield, which was confirmed in our case through a model trapping experiment. Low-molecular-weight PDMA that contained either trithiocarbonate or xanthate end groups was irradiated at 35 C in the presence of a large excess (20 molar equiv) of a persistent radical, 2,2,6,6-tetramethyl-1-piperidinyloxyl (TEMPO). Upon photolysis, the carboncentered radical underwent rapid irreversible termination with TEMPO to produce a ‘‘dead’’ chain end that was monitored by 1H NMR spectroscopy. After 2 hr of irradiation, PDMA-trithiocarbonate retained 87% of the thiocarbonylthio end groups, whereas only 25% of PDMA-xanthate chain ends remained intact (Figure 3B). Given the relatively slow photolytic cleavage of the trithiocarbonate during irradiation, one might expect molecular weights that significantly exceed theoretical values during photoiniferter polymerizations with the trithiocarbonate agent. However, as shown in Figure 1D, this is not the case, which suggests that degenerative chain transfer plays an important role in the consumption of the original small-molecule iniferter during photomediated polymerizations with trithiocarbonates, at least at the low viscosities present early in the polymerization. Faster photolytic cleavage results in a higher radical concentration and more rapid rates of polymerization (Figure 4B). The polymerization of DMA in the presence of the xanthate reached 93% monomer conversion in just 30 min, which was
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Figure 2. Synthesis of UHMW Block Copolymers UHMW block copolymer poly(DMA)-b-poly(DMA) was prepared upon full monomer conversion of the homopolymer poly(DMA) in a one-pot process. The near-linear pseudo-first-order kinetic plot indicates a constant radical concentration during chain extension to produce poly(DMA)-b-poly(DMA) in 8 hr. SEC of the chain-extended diblock copolymer shows a shift to shorter elution times and higher molecular weights, demonstrating good blocking efficiency and high end-group retention.
considerably faster than the analogous reaction conducted with the trithiocarbonate, which required 10 hr to reach a similar level of conversion. This result could also suggest that xanthate and trithiocarbonate iniferters are predisposed to controlled chain growth through slightly different polymerization mechanisms. Unlike the trithiocarbonate, the xanthate exhibits a high photolysis rate, resulting in a polymerization solution high in active carbon- and sulfur-center radicals. As described above, on/off kinetic investigations suggested rapid reversible termination between the active radical pair throughout the polymerization. Therefore, at the high concentrations provided by rapid xanthate photolysis, it seems reasonable that the contribution of reversible termination to deactivation could be enhanced with respect to degenerative chain transfer. Additionally, in traditional RAFT polymerization, these xanthate mediating agents are known to have low chain-transfer rates with more activated monomers such as acrylamides,48 further suggesting that reversible termination plays an important role in polymerization control. The overall result of the enhanced photolytic activity of the xanthate agent resulted in near-quantitative DMA conversion in just 30 min, achieving an Mn of 1.30 3 106 g/mol (Mw/Mn = 1.36; Mn, theory = 1.34 3 106 g/mol) (Figures 4B and S6; Table 1, entry 5). Compared with the trithiocarbonate-mediated polymerization conducted under otherwise identical conditions, this system led to a slightly higher Mw/Mn, but the MWD remained monomodal, and the measured molecular weight closely matched theoretical values. The utility of this rapid, xanthate-mediated polymerization was further demonstrated by targeting molecular weights near
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Figure 3. Alternative Avenue for Photolytic Cleavage (A) UV-Vis spectroscopy of the two iniferters reveals that xanthate has a blue-shifted absorbance spectrum in comparison with that of trithiocarbonate. This shift yields an alternative activation route under mild long-wave UV irradiation (l = 365 nm). (B) A low-molecular-weight model polymer, prepared with either a xanthate or trithiocarbonate iniferter, was irradiated in the presence of a large excess (20 molar equiv) of a persistent radical, 2,2,6,6-tetramethyl-1-piperidinyloxyl (TEMPO). By monitoring conversion of the iniferter-terminated polymer to the unreactive TEMPO-terminated product, photolytic trapping revealed that the xanthate-terminated polymer was consumed considerably faster than the trithiocarbonate-terminated analog. (i) Irradiation with long-wave UV light (l = 365 nm, 7.0 mW/cm 2 ) in DMSO at 35 C.
10.0 3 106 g/mol. With DMA/xanthate ratio of 110,000:1, high monomer conversion (88% by 1H NMR spectroscopy) was measured in 2 hr to produce a polymer with an Mn of 8.57 3 106 g/mol (Figures 4A and S9; Table 1, entry 8). Notably, during this polymerization, an Mn of 2.47 3 106 g/mol was reached in just 5 min. This measurement closely matches theoretical values (Mn, theory = 2.10 3 106 g/mol) and demonstrates a remarkable rate of polymerization while maintaining control over molecular weight. Furthermore, the polymerization rate is dependent on xanthate concentration, as expected for iniferter polymerizations, such that increased reaction times are required for higher monomer/iniferter ratios (Figures S6–S9 and S13).49 Chemical transformations triggered by electromagnetic irradiation often offer exclusive avenues to previously inaccessible materials. Rapid developments in the field of light-mediated RDRP have broadened the scope of advanced macromolecular design, typically through straightforward and facile synthetic procedures. However, these processes have yet to be realized for UHMW polymers, although this class of materials could offer tremendous promise for a number of materials applications. With the approach described here, we demonstrate the utility of photoinifertermediated polymerization for producing the highest-molecular-weight polymers ever reported via RDRP, thereby enabling the synthesis of precise UHMW architectures. Furthermore, through strategic monomer and iniferter pairing, it is possible to synthesize these polymers in minutes while maintaining predetermined molecular weights and narrow MWDs. The simplicity of this method facilitates the construction of UHMW macromolecules without the need for an external catalyst or exogenous initiator and can be expanded to address a broad class of materials.
EXPERIMENTAL PROCEDURES Full experimental procedures are provided in the Supplemental Information.
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Figure 4. Rapid UHMW Synthesis in Minutes via Xanthate-Mediated Polymerization (A) DMA was irradiated with long-wave UV light (l = 365 nm, 7.0 mW/cm 2 ) in the presence of a trithiocarbonate iniferter to produce UHMW poly(DMA). SEC curves are shown as a function of the targeted degree of polymerization; each trace represents a different polymerization. The void volume of this SEC system was 10 mL. (B) The polymerization of DMA to UHMW (M n,SEC 1.30 3 10 6 g/mol) displays near-linear pseudo-first-order kinetics, reaching 93% monomer conversion in 30 min.
SUPPLEMENTAL INFORMATION Supplemental Information includes Supplemental Experimental Procedures and 13 figures and can be found with this article online at http://dx.doi.org/10.1016/j. chempr.2016.12.007.
AUTHOR CONTRIBUTIONS R.N.C., T.E.B., and B.S.S. conceived and designed the research. R.N.C., T.E.B., and M.B.S. performed polymerizations and aided in the characterization of the resulting materials. R.N.C. and B.S.S co-authored the manuscript.
ACKNOWLEDGMENTS This material is based on work supported by the National Science Foundation (DMR-1606410). Received: November 1, 2016 Revised: December 12, 2016 Accepted: December 19, 2016 Published: January 12, 2017
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