Color-Coding Visible Light Polymerizations To Elucidate the Activation

Feb 12, 2018 - We report mechanistic investigations into aqueous visible-light reversible addition–fragmentation chain transfer (RAFT) polymerizatio...
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Color-Coding Visible Light Polymerizations To Elucidate the Activation of Trithiocarbonates Using Eosin Y C. Adrian Figg,† James D. Hickman,† Georg M. Scheutz,† Sivaprakash Shanmugam,‡ R. Nicholas Carmean,† Bryan S. Tucker,† Cyrille Boyer,*,‡ and Brent S. Sumerlin*,† †

George & Josephine Butler Polymer Research Laboratory, Center for Macromolecular Science & Engineering, Department of Chemistry, University of Florida, PO Box 117200, Gainesville, Florida 32611-7200, United States ‡ Centre for Advanced Macromolecular Design (CAMD) and Australian Center for NanoMedicine (ACN), School of Chemical Engineering, UNSW Australia, Sydney, NSW 2052, Australia S Supporting Information *

ABSTRACT: We report mechanistic investigations into aqueous visible-light reversible addition−fragmentation chain transfer (RAFT) polymerizations of acrylamides using eosin Y as a photoinduced electron-transfer (PET) catalyst. The photoinduced polymerization was found to be dependent upon the irradiation wavelength and reagents, where either reduction or oxidation of the PET catalyst leads to inherently different initiation and reversible-termination steps. Using blue light, multiple mechanisms of initiation are observed, depending on the presence or absence of a sacrificial reducing agent. Using green light, both an oxidative and a reductive PET initiation mechanism can be pursued. Investigations into the role of PET catalyst, wavelength, and reducing agent demonstrated that precise polymers with predictable molecular weights are best realized under an oxidative PET-RAFT mechanism. Therefore, this study provides fundamental insight into visible-light RAFT photopolymerizations and the role of eosin Y as a photoredox catalyst.

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INTRODUCTION

Eosin Y (EY) presents an interesting opportunity to catalyze polymerizations in aqueous media, as the organodye is watersoluble and has been shown to effectively activate PET-RAFT polymerizations of methacrylates and vinyl ketones in organic solvents.25,26 Indeed, EY has recently been reported to catalyze aqueous polymerizations in the presence of cells,27 proteins,28 or oxygen (when ascorbic acid is added).29 To further expand the applicability of this photocatalyst, we sought to explore the polymerization mechanism and seek opportunities to optimize the initiation and reversible-termination steps. EY absorbs both blue and green light (Figure S1), with these absorptions being attributed to the dimer and monomer in solution, respectively.30 Therefore, we hypothesized that polymerizations may occur under irradiation at both wavelengths. The mechanism of initiation can be controlled through the addition of a sacrificial reducing agent, either through reduction of the excited-state EY (reductive PET-RAFT, Figure 1a) or oxidation of the excited-state EY (oxidative PET-RAFT, Figure 1b). Additionally, since trithiocarbonate (TTC) compounds absorb in blue wavelengths (Figure S2) and may undergo photolysis of the carbon−sulfur (C−S) bond,24,31 it was necessary to also consider a photoiniferter32 polymerization

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Reversible addition−fragmentation chain transfer (RAFT) photopolymerizations provide control over polymer molecular weight and structure predominately through reversible photolysis of photolabile bonds,2−6 light-induced degradation of photodegradable initiators,7 or electron/energy transfer via photocatalysts.8−12 Photopolymerization provides an opportunity for mechanistic and spatiotemporal control13 and allows reactions to be conducted at ambient temperatures, facilitating application to areas such as protein engineering using synthetic polymers,14,15 additive manufacturing,16 or photolithography.17−19 Most RAFT photopolymerization techniques are conducted in organic solvents, while only a few report employing aqueous conditions with either UV-irradiation,20 photoinduced electron/energy-transfer RAFT (PET-RAFT),21 or photoinitiators.22,23 However, developments in aqueous photo-RAFT polymerizations may facilitate access to polymeric biomaterials using environmentally benign and biocompatible conditions. Limiting the use of metal catalysts or high energy UV-irradiation will further aid in the development of biologically-friendly synthetic techniques. Additionally, excluding an external initiator during RAFT should result in better molecular weight (MW) control and end-group retention, as every polymer chain in solution is initiated from the chain transfer agent (CTA).24 © XXXX American Chemical Society

Received: November 29, 2017 Revised: January 29, 2018

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Figure 1. (a) Proposed photopolymerization mechanism where the excited-state eosin Y is reduced by a tertiary amine, leading to a reductive photoelectron transfer reversible addition−fragmentation chain transfer polymerization (PET-RAFT) mechanism. (b) Proposed photopolymerization mechanism where the excited-state eosin Y is oxidized by a trithiocarbonate, leading to an oxidative PET-RAFT mechanism. (c) Proposed photoiniferter mechanism where the trithiocarbonate species undergoes photolysis upon excitation. Figure 2. Trapping studies of 2-(ethyl trithiocarbonate)propionic acid using different ratios of chain transfer agent:eosin Y:4-dimethylaminopyridine under (a) blue light irradiation and (b) green light irradiation using 15 equiv of N-ethylpiperidine hypophosphite (EPHP) as a hydrogen source.

mechanism where the C−S bond homolytic cleavage occurs independent of catalyst (Figure 1c). Herein, we report our investigations into how polymerization conditions may affect the mechanism of visible-light PET-RAFT polymerizations and the resultant polymer properties. Specifically, the power of dictating mechanism via wavelength was found to greatly improve polymerization control. Therefore, this report shows that further understanding of redox processes during polymerization may lead to optimized aqueous polymerization conditions and access to a variety of well-defined polymer compositions using biologically relevant polymerization conditions.

cleavage of the C−S bond and subsequent radical quenching by EPHP. The slope of the pseudo-first-order kinetic plot of ETPA consumption (Figure 2a) was then evaluated to determine the apparent rate of radical generation from the C−S bond cleavage. For blue light, low equivalents of EY (Table 1, entry 3) led to a faster disappearance of the methine proton than photolysis (Table 1, entry 5), and higher concentrations of EY (Table 1, entry 2) led to even faster disappearance of the

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RESULTS AND DISCUSSION To study the mechanism for generation of carbon-centered radicals during photopolymerization with TTCs (ZSC(=S)SR), model trapping studies with 2-(ethyl trithiocarbonate)propionic acid (ETPA) and 15 equiv of the hydrogen source N-ethylpiperidine hypophosphite (EPHP)33 were performed (Figure 2), in a manner analogous to an end-group removal technique we recently reported.34 The monomethyl R-group was chosen to best emulate the sterics of a growing acryloyl chain end. The disappearance of the methine proton adjacent to the TTC (at δ = 4.55 ppm) was monitored using 1H NMR spectroscopy (Figure S3) and was attributed to homolytic

Table 1. Apparent Rate of Consumption of 2-(Ethyl trithiocarbonate)propionic Acid Using Different Ratios of Chain Transfer Agent:Eosin Y:4-Dimethylaminopyridine under Blue Light Irradiation

B

entry

ETPA:EY:DMAP

1 2 3 4 5

1:0.0004:1 1:0.004:0 1:0.0004:0 1:0:1 1:0:0

Apparent rates of ETPA disappearance (min−1) 5.58 2.31 1.81 5.49 1.52

× × × × ×

10−3 10−3 10−3 10−3 10−3

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Macromolecules Table 2. Results of Photoinduced Electron/Energy-Transfer Reversible Addition−Fragmentation Chain Transfer Polymerizations of N,N-Dimethylacrylamide with a DMA to CTA Ratio of 200:1 and a Solution pH of 8.4−9.0 entry 1 2 3 4 5 6 7 8

CTA:EY:DMAPa 1:0:0 1:0:1 1:0.0004:0 1:0.0004:1 1:0.0004:2 1:0.001:2 1:0.001:0 1:0.001:2

irradiation color e

blue bluee bluee bluee bluee bluee greenf greenf

monomer convb

Mn,theo (g/mol)c

Mn,MALLS (g/mol)d

molar mass dispersity, Đ

0.40 0.25 0.87 0.82 0.95 0.93 0.92 0.94

8320 5170 17500 16500 19000 18600 18500 18800

9540 7440 18500 19000 21500 20300 19400 20500

1.01 1.11 1.01 1.01 1.02 1.02 1.02 1.05

a

Molar ratio of chain transfer agent:eosin Y:4-dimethylaminopyridine. bDetermined using 1H NMR spectroscopy. cTheoretical number-average molecular weights (Mn,theo) calculated from monomer conversion. dNumber-average molecular weights obtained using gel permeation chromatography equipped with a multiangle light scattering detector with 0.05 M LiCl in N,N-dimethylacetamide as the eluent. e11 mW/cm2. f 6.2 mW/cm2.

could be used to initiate radicals and lead to reversible activation during polymerization. When EY, ETPA, and DMAP were present (ETPA:EY:DMAP ratio of 1:0.0004:1), faster consumption of the methine proton was observed, which confirms the excited-state EY can also induce TTC cleavage via the reductive PET pathway (Figure 1a) under green light. Furthermore, these results indicate that a sole initiation mechanism (oxidative PET-RAFT) can occur under greenlight irradiation, while a photoiniferter mechanism is unavoidable under blue-light irradiation, where both the ETPA and EY absorb. To test the effect of wavelength and reagents on aqueous photopolymerizations, N,N-dimethylacrylamide (DMA) was subject to different polymerization conditions at basic pH (8.4−9.0). First, blue-light irradiation was evaluated using 2(ethyl trithiocarbonate)-2-methylpropionic acid (CTA) and a DMA to CTA ratio of 200:1 (Table 2, Figure 3, and Figure S4). Since TTCs were observed to undergo photolysis under blue light during the trapping studies and in previous reports,3,24 an iniferter mechanism was first tested in which no EY or DMAP were added to the polymerizations. Monomer conversion reached 40% after 3 h, and experimental molecular weights were consistently ∼15% higher than the theoretical values (Table 2, entry 1). To test whether an amine would significantly affect the rate and control during polymerization, a reaction using 1 equiv of DMAP relative to CTA was performed (Table 2, entry 2). Slightly slower kinetics compared to the reaction undergoing an iniferter process were observed, and the polymer dispersity remained low (Đ = 1.01−1.11) across all the polymerization conditions tested. During control experiments (Table S1), DMAP was observed to inhibit the autopolymerization of DMA under blue-light irradiation, and a similar inhibition could be occurring during the PET-RAFT polymerizations as well. Overall, these results indicate that welldefined polymers can be attained in water using blue light to induce C−S bond cleavage and initiate polymerizations, corroborating other reports of using C−S photolysis under blue light to synthesize acrolyl polymers.3,35,36,39 Next, we investigated oxidative PET-RAFT polymerizations under blue-light irradiation. Initial polymerizations using a CTA:EY ratio of 1:0.0004 led to high monomer conversion after 3 h (Table 2, entry 3). Actual molecular weights were slightly higher than theoretical values, although the molecular weight dispersity remained low (Đ = 1.01). DMAP was then added to favor the reductive PET-RAFT polymerization mechanism. Using a CTA:EY:DMAP ratio of 1:0.0004:1

methine proton. This increased rate in disappearance in methine proton was consistent with EY-induced radical formation through an oxidative PET pathway under blue light (Figure 1b). When the tertiary amine (4-dimethylaminopyridine (DMAP)) was added in the absence of EY (Table 1, entry 4), consumption of the methine proton of ETPA was faster than in the absence of EY (when the ratio of ETPA to EY was 1:0, Table 1, entry 5), indicating that the tertiary amine can also undergo a redox reaction with the excited state TTC to induce radical formation. This observation is in agreement with previous studies by Qiao35−38 and Konkolewicz39 where a faster rate of polymerization was observed when only tertiary amine was added to a TTC. As expected, in the presence of both EY and DMAP (Table 1, entry 1), the rate of consumption of ETPA was faster than the other trials. Since the excited-state EY is gaining an electron from the tertiary amine, the difference between the redox potential of the radical anion EY (−1.06 V)40 and the ground state redox potential of the TTC (−0.6 V)10 is large, yielding a favorable electron transfer from the reduced EY to the TTC. Overall, these trapping studies indicate that multiple mechanisms of initiation, having similar rates of radical generation, may occur simultaneously when both EY and a tertiary amine are used as PET catalysts under blue-light irradiation. Next, the trapping studies were conducted under green light to further understand the impact of irradiation wavelength on C−S bond cleavage (Figure 2b). Unfortunately, due to the rapid degradation of EY under green-light irradiation (as evidenced by the decrease in rate of ETPA consumption after 3 h and loss of the characteristic pink color of EY), the apparent rates of ETPA degradation could not be determined using a linear regression. However, we observed important qualitative differences between radical initiation under green light versus blue light. The experiments containing only ETPA (ETPA:EY:DMAP ratio of 1:0:0) or ETPA and DMAP (ETPA:EY:DMAP ratio of 1:0:1) led to minimal disappearance of the methine proton adjacent to the TTC of ETPA. Low conversion is expected since TTCs show such a low absorbance at green wavelengths (Figure S2). Importantly, these results indicate the photoiniferter and tertiary amine reduction mechanism play minor roles during green-light irradiation. Trapping experiments that included EY and ETPA (ETPA:EY:DMAP ratio of 1:0.0004:0) showed moderate consumption of the methine proton after 20 h (43%), confirming the excited-state EY can undergo oxidation to induce C−S bond cleavage of TTCs under green light (Figure 1b), which C

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Figure 3. Pseudo-first-order kinetics of photoinduced electron/energy transfer reversible addition−fragmentation chain transfer polymerization with different ratios of chain transfer agent:eosin Y:4dimethylaminopyridine performed under blue-light irradiation (11 mW/cm2).

resulted in similar rates of polymerization, with 82% monomer conversion being achieved after 3 h (Table 2, entry 4). However, molecular weights were consistently ∼15% higher than theoretical values (Figure S4), possibly due to degradation of the CTA (vide infra). The concentration of amine was found to affect the apparent rate of polymerization (Figure S5), and using 2 equiv relative to CTA yielded full monomer conversion in 3 h with good control over dispersity, comparable to a previous report using organic solvents.25 Interestingly, keeping the equivalents of DMAP constant and increasing the equivalents of EY to yield ratios of CTA:EY:DMAP = 1:0.001:2 (Table 2, entry 5) and 1:0.002:2 did not significantly affect the rates (Figure S6). These results indicate that at these low concentrations of catalyst, EY is not significantly affecting the apparent rate of polymerization, but the amine can slightly affect the rate of polymerization. In conclusion, for PET-RAFT polymerizations under bluelight irradiation, at least four different mechanisms of trithiocarbonate activation occur when CTA, tertiary amine, and EY are present: (1) TTC photolysis, (2) tertiary amineaccelerated TTC photolysis, (3) oxidative PET-RAFT, and (4) reductive PET-RAFT using the tertiary amine co-catalyst. Consequently, we hypothesize the variety of mechanisms of radical initiation contribute to higher-than-predicted experimental molecular weights, although well-defined polymers with low dispersities are achieved (since degenerative chain transfer occurs during all four mechanisms). We then tested polymerization conditions using green-light irradiation at two different pH values (4.4 and 8.4) to compare the mechanism of reductive versus oxidative PET-RAFT (Figure 4). In addition to involving lower energy light that

Figure 4. Conversion, pseudo-first-order kinetics, and experimental molecular weight versus theoretical molecular weight for photo electron-transfer reversible addition−fragmentation chain transfer polymerizations containing different ratios of chain transfer agent:eosin Y:4-dimethylaminopyridine performed under green-light irradiation (6.2 mW/cm2).

may pose less problems during polymerization in the presence of sensitive systems (e.g., proteins, cells), the main advantage of using green light over blue light for RAFT polymerizations is that the contribution of homolytic photolysis to initiation should be dramatically reduced. Indeed, polymerization solutions containing only CTA or CTA and DMAP showed negligible monomer conversion after 3 h. Under basic conditions, when a CTA:EY ratio of 1:0.001 was used (Table 2, entry 7), high conversions (92%) were reached after 3 h, indicating the PET catalyst effectively activated the TTC without reducing agent present (i.e., oxidation of the excited state EY). In contrast to all other polymerization conditions discussed up to this point, experimental molecular weights closely agreed with theoretical values until high monomer conversion, where an increase in MW can potentially be D

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reasoned that degenerative chain transfer favors rapid consumption of the CTA prior to significant monomer propagation. Importantly, each initiation mechanism yields the same carbon centered propagating radical but results in different trithiocarbonyl species (Figure S8). Moreover, DMA was found to undergo initiation from EY, regardless of the wavelength or when tertiary amine was present, which introduces an unavoidable route of radical generation (Table S1). When a kinetic time point was qualitatively evaluated by electrospray ionization mass spectrometry, the chains synthesized under reductive PET-RAFT conditions showed a population of chains initiated from the DMAP reducing agent instead of the desired CTA, but a population deriving from DMA initiation was not observed (Figure S9). The background initiation route resulting from reducing agent intermediates44 demonstrates that reductive PET-RAFT conditions can cause a decrease in polymer chain-end homogeneity and reduced molecular weight control. While a lower than expected molecular weight should occur if chains are initiated from sources other than CTA-derived radicals, we hypothesize that a high concentration of radicals from the multiple routes of initiation very early in the polymerization may induce degradation of a non-negligible amount of the CTA, ultimately leading to higher-than-expected polymer molecular weights. Additionally, while we expect that only a small fraction of chains could be deleteriously affected by the differing mechanisms of initiation (e.g., initiation from DMAP), an overall beneficial increase in the characteristics attributed to controlled radical polymerizations was observed when the initiation mechanism was limited to oxidative PET-RAFT under green-light irradiation. Indeed, the oxidative polymerization conditions were calculated to have a lower radical concentration and fraction of dead chains due to irreversible termination events, although all of the polymerization conditions were suggested to yield polymers with very high retention of the CTA end groups (Table S2). Another key difference in the control over polymerization in each mechanism could result from the differences in reaction intermediates and deactivation steps. Since multiple mechanisms can occur simultaneously, multiple radical termination reactions (either reversible or irreversible) between the reaction intermediates are possible. These different modes of deactivation could lead to differences in polymerization control and discrepancies between theoretical and experimental molecular weights. Finally, additional termination events involving the reducing agent amine radical intermediates44 during reductive PET-RAFT conditions may lead to further reductions in polymerization control. Regardless of the irradiation wavelength, additional unavoidable termination events involving radicals that result from DMA initiation by EY could also be occurring. Our investigations of the kinetics of these polymerizations also revealed loss of EY during irradiation (most likely due to the formation of the leuco form of EY commonly formed under anaerobic conditions),45,46 as the polymerizations lost color and a noticeable decrease in apparent rate was observed over time. Previous reports attribute EY degradation to reaction with oxygen species47,48 or photobleaching in the presence of hydrogen sources or electron acceptors.49,50 To ensure EY degradation did not affect chain-end retention of the trithiocarbonate CTA, after 3 h additional EY was added to an oxidative PET-RAFT polymerization under green light irradiation at a pH of 4.4 (Figure 5). Linear pseudo-first-order

attributed to chain−chain coupling commonly observed in RAFT polymerizations in monomer-starved conditions. Additionally, these results indicate that tertiary amine is not necessary to achieve fast rates of polymerization under these conditions. To test whether the agreement between theoretical and experimental molecular weight values were due to wavelength or initiation mechanism, DMAP was added for a CTA:EY:DMAP ratio of 1:0.001:2 to access a reductive PETRAFT mechanism (Table 2, entry 8) at basic pH. Full monomer conversion was reached after 3 h, and a linear pseudo-first-order kinetics plot, linear increase in molecular weights with monomer conversion, and low molar mass dispersities were observed (Đ = 1.05−1.15). However, unlike the oxidative mechanism without DMAP, the actual molecular weights were ∼10−15% higher than predicted values (analogous to polymerizations under blue-light irradiation using reductive PET-RAFT conditions). To determine whether any molecular weight discrepancy was occurring due to CTA hydrolysis under basic conditions,41 polymerizations were conducted at acidic pH (4.4, Figure 4). The oxidative EY conditions (with CTA:EY = 1:0.001) yielded polymers with good agreement between theoretical and experimental molecular weights. However, the reductive EY conditions (CTA:EY:DMAP = 1:0.001:2) at a pH of 4.4 still yielded an ∼15% discrepancy between theoretical and actual molecular weights, suggesting that CTA hydrolysis is not the main cause of these differences. Although the rates of polymerization were generally slower at a pH of 4.4, increasing the EY concentration (i.e., CTA:EY = 1:0.003) led to a linear pseudo-first-order kinetic plot, slightly faster kinetics, and excellent agreement between theoretical and experimental molecular weights (Figure S7). The slower rates of polymerization were attributed to protonation of the excited state catalyst42 and a higher rate of bimolecular triplet decay,43 lowering the amount of active state catalyst and reducing the probability of electron transfer from the triplet state of EY to the CTA. From these results, we concluded that molecular weight control was best when the polymerization occurred exclusively via the oxidative PET-RAFT mechanism with green light. Overall, the polymerization and trapping results provide fundamental insight into the application of PET-RAFT polymerization in water using EY as the photocatalyst. Previous reports of aqueous polymerizations using EY involved tertiary amines as a reducing agent for the excited-state EY, effectively favoring the reductive PET-RAFT mechanism (Figure 1a). While this addition results in fast and linear polymerization kinetics and oxygen tolerance, the concurrence of photolysis (Figure 1c) and reduction (Figure 1a) of the excited-state trithiocarbonate (in blue wavelengths) and the oxidative PETRAFT mechanism (Figure 1b) in both blue and green wavelengths can lead to multiple mechanisms of initiation. It should also be noted that while electron transfer mechanisms are discussed, proposed energy transfer mechanisms of initiation could be occurring but are difficult to distinguish from PET pathways. Rapid consumption of the CTA is essential to achieve polymers of controlled molecular weights and low dispersities. Since all the polymerization conditions described here yielded well-defined polymers, we reasoned that a considerable amount of the CTA during polymerization is consumed from degenerative chain transfer events early in the polymerization.20 Even if TTC cleavage rates differ between the mechanisms, we E

DOI: 10.1021/acs.macromol.7b02533 Macromolecules XXXX, XXX, XXX−XXX

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C. Adrian Figg: 0000-0003-3514-7750 Cyrille Boyer: 0000-0002-4564-4702 Brent S. Sumerlin: 0000-0001-5749-5444 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This material is based upon work supported by the National Science Foundation under Grants OISE-1614040 (CAF) and DMR-1606410 (BSS). The authors also acknowledge Dr. Dominik Konkolewicz (Miami University) for helpful discussions.

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Figure 5. Chain-extension polymerizations of N,N-dimethylacrylamide under oxidative photoelectron transfer reversible addition−fragmentation chain transfer polymerization conditions under green-light irradiation (6.2 mW/cm2) after sequential addition of EY photocatalyst and monomer at a solution pH of 4.4.

kinetics were observed, theoretical molecular weights closely matched with experimental values, and good blocking efficiency was observed. EY and DMA were added a third time to the same flask, and good chain-end retention of the CTA was again observed by SEC with a symmetric shift to lower retention times, confirming catalyst degradation did not affect the resultant polymers.

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CONCLUSION In conclusion, we have investigated the initiation mechanisms available during aqueous PET-RAFT polymerizations. Because of the propensity of the trithiocarbonate species to undergo both catalytic reduction and independent photolysis, the only approach that is proposed to induce one mechanism of polymerization, with typical characteristics of controlled radical polymerization, is under oxidative PET-RAFT conditions during green-light irradiation with EY. The reliable control of molecular weights is paramount in controlled radical polymerizations, and elucidation that only oxidative PET-RAFT provided polymers with targeted molecular weights and fast polymerization kinetics suggests increased understanding of the role of the possible mechanisms of PET can lead to more welldefined polymeric materials.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.7b02533. Additional figures and experimental details (PDF)

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REFERENCES

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AUTHOR INFORMATION

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*E-mail [email protected] (C.B.). *E-mail [email protected]fl.edu (B.S.S.). F

DOI: 10.1021/acs.macromol.7b02533 Macromolecules XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.macromol.7b02533 Macromolecules XXXX, XXX, XXX−XXX