A Facile Route for Preparation of Comblike and ... - ACS Publications

Article Cite This: Macromolecules 2018, 51, 7776−7784

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Catalyst-Free Selective Photoactivation of RAFT Polymerization: A Facile Route for Preparation of Comblike and Bottlebrush Polymers Sivaprakash Shanmugam,† Julia Cuthbert,† Tomasz Kowalewski,† Cyrille Boyer,‡ and Krzysztof Matyjaszewski*,† †

Department of Chemistry, Carnegie Mellon University, 4400 Fifth Avenue, Pittsburgh, Pennsylvania 15213, United States Centre for Advanced Macromolecular Design (CAMD) and Australian Centre for NanoMedicine School of Chemical Engineering, UNSW Australia, Sydney, NSW 2052, Australia

Macromolecules 2018.51:7776-7784. Downloaded from pubs.acs.org by UNIV OF TEXAS AT EL PASO on 10/28/18. For personal use only.

‡

S Supporting Information *

ABSTRACT: We report a novel photoiniferter approach toward enabling chemoselective fragmentation of reversible addition−fragmentation chain transfer (RAFT) polymerization using two different RAFT agents. Methyl methacrylate (MMA) was polymerized in the presence of two different RAFT agents, 4-cyano-4-[(dodecylsulfanylthiocarbonyl)sulfanyl]pentanoic acid (CDTPA) and 2-(dodecylthiocarbonothioylthio)propionic acid (DTPA). Both RAFT agents contain the same Z-groups but different R-leaving groups. Under photoiniferter conditions, only one RAFT agent was preferentially fragmented while the other remained inert. This led to the synthesis of poly(methyl methacrylate) (PMMA) with close correlations between theoretical and experimental molecular weights and narrow molecular weight distributions (Mw/Mn). This approach was then employed to homopolymerize and copolymerize a RAFT inimer, 2-(2-(n-butyltrithiocarbonate)propionate)ethyl methacrylate (BTPEMA), under green light with CDTPA with the trithiocarbonate functionality on the inimer incapacitated for chain transfer. Switching to blue light irradiation allowed for photolysis of BTPEMA and grafting of a second monomer. This novel approach provided a facile route for the synthesis of complex macromolecular architectures such as comblike and bottlebrush polymers without the need for intermediate steps, such as postmodification.

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INTRODUCTION The past two decades has seen the advancement of reversible deactivation radical polymerization (RDRP) techniques such as atom-transfer radical polymerization (ATRP),1−3 reversible addition−fragmentation chain transfer (RAFT) polymerization,4 and nitroxide-mediated polymerization (NMP)5,6 enabling unparalleled access to diverse, well-defined polymer structures. RDRP techniques have been shown to be versatile and robust in the synthesis of precise polymeric architectures such block copolymers,7−9 stars,10−13 and surface grafted polymer brushes.14−16 Of the different architectures, molecular bottlebrushes, which are composed of a long polymeric backbone with densely grafted polymeric side chains, have generated recent interest as they possess promising potential in photonics,17 templating,18−20 lubrication,21,22 and drug delivery.23−26 Molecular bottlebrushes can be synthesized by coupling side chains to the backbone (grafting-to), grafting of side chains from initiator functionalized polymer backbone (grafting-from), or polymerization of macromonomers (grafting-through).27−30 As single RDRP techniques may sometimes be insufficient in accessing complex bottlebrush architectures, dual RDRP, which rely on the orthogonality of two polymerization reactions, has been developed to perform distinct RDRP processes sequentially without any chemical transformations between polymerization steps.31−33 Although myriad orthogonal reactions have been developed to synthesize © 2018 American Chemical Society

graft polymers, they often require combinations of different RDRP techniques as well as with other polymerization techniques such as ring-opening metathesis polymerization (ROMP)34 or postmodification of the polymer backbone to introduce additional initiators.28,31,35−38 These techniques often lead to stringent experimental conditions, unwanted metal contamination, and tedious synthetic and purification steps. Designing bottlebrushes using RAFT polymerization via grafting-from method can be performed via R-group approach or Z-group approach. Although Z-group approach provides better control over polymer grafting from a polymer backbone, this approach often suffers from steric shielding effects and shows limited control over grafting of branches.39 On the other hand, R-group approach suffers from the “entrapment” of active radicals within individual growing bottlebrushes leading to bottlebrushes with multimodal weight distribution.40 Zheng et al. implemented the use of low molecular weight chain transfer agent (CTA) that improved R-group approach for bottlebrush synthesis.41 In this shuttled CTA R-group approach, active radicals on bottlebrushes can transfer to the shuttle R-group CTA generating polymer radical that can further transfer to surrounding shuttles. Therefore, the active Received: August 7, 2018 Revised: September 9, 2018 Published: September 25, 2018 7776

DOI: 10.1021/acs.macromol.8b01708 Macromolecules 2018, 51, 7776−7784

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Scheme 1. Photoiniferter Mechanism under Visible Light and Chemical Structures of RAFT Agents Used in This Study

expensive.12,50,51,56 The use of RAFT agents as visible light iniferters58,59 is an attractive approach for mediating polymerization without the need for photocatalysts or initiators.60−62 In 2014, Boyer et al. reported the activation of trithiocarbonate RAFT agents under visible light via a photoiniferter mechanism.60 In this investigation, 4-cyano-4[(dodecylsulfanylthiocarbonyl)sulfanyl]pentanoic acid (CDTPA) RAFT agent was shown to be a versatile green light photoiniferter for polymerization of different functional methacrylates leading to polymers with narrow molecular weight distributions. However, methacrylate polymerization under blue light irradiation with CDTPA, despite being faster than under green light, led to broader molecular weight distribution. It is highly likely that the poor control of CDTPA under blue light can be attributed to high concentration of tertiary carbon radicals promoting faster termination. Furthermore, Qiao et al. showed that the photoiniferter approach can be employed to polymerize different functional acrylates with different degrees of polymerization.61 Polymerization of acrylates under blue light employing trithiocarbonates with secondary carbon radical leaving groups, such as 2-(nbutyltrithiocarbonate)propionic acid (BTPA) or 2(dodecylthiocarbonothioylthio)propionic acid (DTPA), has much lower concentration of propagating radical, resulting in much slower termination and better control over the polymer molecular weight and molecular weight distribution. Recent reports have explored the use of this approach for generating ultrahigh molecular weight polymers,63 polymer networks,64 and polymeric nanoparticles.65,66 Herein, we report the first implementation of chemoselective catalyst-free visible light RAFT polymerization for the facile synthesis of bottlebrush and graft copolymers. In contrast to previous work on selective activation of RAFT polymerization with photocatalysts,12 our study focuses on a novel photoiniferter route for activating RAFT agents by simple manipulation of light wavelengths between blue (λmax = 465 nm) and green (λmax = 520 nm) LED lights. Therefore, we utilize our discovery to provide a novel straightforward approach for designing complex architectures. We perform methacrylate polymerization under green light irradiation in the first step before switching to blue light irradiation in the

radicals can be efficiently transferred to surrounding bottlebrushes by these shuttles ensuring homogeneous propagation and narrow molecular weight distribution. Visible light mediated RDRP is currently emerging as a promising tool for preparing complex polymeric materials under benign conditions while imposing spatial, temporal, and sequence control.42−45 The use of photocatalysts has not only helped to mediate RDRP polymerizations but also aided the exploration of orthogonal chemistry to engineer complex macromolecular architectures by enabling coupling of radical reactions with step growth,46,47 cationic,48,49 and ring-opening50−52 polymerizations. For instance, two orthogonal photoROMP and photoinduced electron/energy transfer (PET)RAFT polymerizations were combined for the synthesis of one-pot diblock and graft copolymers. In this early example, the polymer architecture could be efficiently controlled by changing the wavelength of irradiation.50,51 Another method for controlling orthogonal reactions was demonstrated by manipulating photocatalyst concentrations where low catalyst concentrations (50 ppm) activate PET-RAFT polymerization of acrylates while higher catalyst concentrations (500 ppm) activate ATRP polymerization of methacrylates.32 Bottlebrush polymers have also been synthesized by coupling highly active ruthenium catalysts for controlled ROMP of norbornenefunctionalized RAFT agents followed by growing the side chains via RAFT polymerization.53,54 Recent studies demonstrated that orthogonal reactions can be introduced in RAFT polymerization without the need for coupling with other polymerization techniques. In this approach, careful selection of transition metal and organo-photocatalysts for selective activation of RAFT agents under different visible light wavelengths enabled the synthesis of graft polymers and sequence defined materials.32,55,56 In addition, interconversion of radical and cationic polymerization with RAFT agents through the use of two different photocatalysts absorbing under different wavelengths was also demonstrated.48,57 Orthogonal reactions in RAFT photopolymerization provide an interesting avenue to design complex materials. However, the orthogonal reactions designed so far rely on the use of photocatalysts especially transition metal catalysts that need to be removed at the end of the reactions which is laborious and 7777


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Table 1. Polymerization of Methyl Methacrylate under Photoiniferter Conditions with 4-Cyano-4[(dodecylsulfanylthiocarbonyl)sulfanyl]pentanoic Acid (CDTPA) as the Primary RAFT Agent in the Presence or Absence of 2-(n-Butyltrithiocarbonate)propionic Acid (BTPA) or 2-(Dodecylthiocarbonothioylthio)propionic Acid (DTPA) as the Secondary RAFT Agenta no.

exp conda [MMA]:[CDTPA]:[BTPA or DTPA]

additives

conditionsb

time (h)

αc (%)

Mn,thd

Mn,GPCe

Mw/Mne

1 2 3 4 5 6 7 8 9 10 11 12

200:1:0 200:1:1 200:1:5 200:1:10 200:1:0 200:1:1 200:1:5 200:1:10 200:1:0 200:0:1 200:0:1 200:0:1

none DTPA DTPA DTPA none BTPA BTPA BTPA none DTPA DTPA BTPA

green green green green green green green green blue blue green green

5 5 6 6 3 3 3 3 2.5 20 12 12

90 87 81 79 73 75 70 68 93 24 0 0

18400 17800 16700 16300 15100 15500 14500 14000 19000 4500

19000 15100 18500 18900 17400 16500 14200 13800 21800 8000

1.18 1.20 1.17 1.16 1.17 1.19 1.24 1.22 1.36 2.06

a Reactions were performed at room temperature with 50% v/v monomer concentration in DMSO. bReaction mixtures were irradiated under visible green (λmax = 520 nm, intensity = 4.25 mW/cm2) or blue (λmax = 465 nm, intensity = 6.5 mW/cm2) light LEDs. cMonomer conversions were determined by using 1H NMR spectroscopy. dTheoretical molecular weights were calculated using the following equation: Mn,th = [M]0/ [RAFT]0 × MWM × α + MWCDTPA, where [M]0, [RAFT]0, MWM, α, and MWCDTPA correspond to initial monomer concentration, initial CDTPA concentration, molar mass of monomer, monomer conversion determined by 1H NMR, and molar mass of CDTPA. eMolecular weight and dispersity (Mw/Mn) values were determined by THF GPC analysis calibrated using poly(methyl methacrylate) standards.

Scheme 2. Proposed Mechanism for Addition/Fragmentation of (I) CDTPA and (II) BTPA/DTPA in the Presence of PMMA Radical

need for external initiator source by employing photolysis of RAFT agents under the visible light wavelengths while generating narrow molecular weight bottlebrushes. Despite being a two-step procedure for bottlebrush synthesis, this approach requires no addition of chain transfer agents as shuttles to achieve narrow molecular weight distribution. In short, compared to the shuttled CTA R-group approach, the proposed photoiniferter approach for bottlebrush synthesis promotes atom efficiency as it requires no addition of initiator or shuttle CTA and produces less side product (i.e., free homopolymer).

second step to enable polymerization of acrylate and acrylamide. Comblike and bottlebrush polymers were designed by simple switch of visible light wavelengths and therefore omitting the need for additional chemistry on polymeric species as well as postpolymerization coupling reactions. In the aforementioned work, Zheng et al. implemented the use of low molecular weight chain transfer agent (CTA) that improved the R-group approach for bottlebrush synthesis; however, this approach has its own drawbacks.41 The shuttled CTA R-group approach requires the addition of low molecular weight CTA shuttle and the use of thermal initiator as an external radical source to initiate polymerization. Despite providing a better control over bottlebrush synthesis, the presence of CTA shuttle generates unwanted homopolymer chains that need to be separated from the targeted bottlebrush product. Our work highlights a simple and straightforward photoiniferter approach for bottlebrush synthesis that inherently embodies the principles of green chemistry. This work helps to improve bottlebrush synthesis by R-group approach by eliminating the

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RESULTS AND DISCUSSION The overarching goal of this study is to design a novel technique that selectively activates the fragmentation of a RAFT agent through a synergistic approach that involves simple manipulation of visible light wavelengths coupled with the RAFT addition−fragmentation process. Previous PET (photoinduced electron/energy transfer)−RAFT investiga7778


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Figure 1. Polymerization of MMA mediated by CDTPA under green light irradiation performed in DMSO in the presence of different concentrations of BTPA (λmax = 520 nm, intensity = 4.25 mW/cm2) with [MMA]:[CDTPA] = 200:1, 50% v/v monomer concentration. (A) Plot of ln([M0]/[M]t) vs exposure time in the presence of different concentrations of BTPA ([CDTPA]:[BTPA] = 1:0, 1:1, 1:5, and 1:10). (B) Plot of Mn vs conversion for different concentrations of BTPA. (C) Plot of Mw/Mn vs conversion for different concentrations of BTPA.

Figure 2. Characterization of purified PMMA (A) and supernatant (B) obtained from precipitation of final reaction mixture composed of [MMA]: [CDTPA]:[BTPA] of 200:1:10 via 500 MHz 1H NMR. (A) The molecular weight of PMMA was found to be Mn,NMR = 15 200 (Mn,NMR was determined using the equation Mn,NMR = (I3.6 ppm/3)/(I3.2 ppm/2) × MWM + MWRAFT where I3.6 ppm and I3.2 ppm correspond to integration of proton b and a, respectively), Mn,GPC = 13800, and Mn,theo = 14000, and (B) the supernatant was found to have unreacted BTPA.

tions demonstrated that the absorption of CDTPA (Scheme 1 and Supporting Information, Figure S1) in the visible region enables its photolysis under green (520−530 nm) and blue (460−470 nm) wavelengths without the addition of photocatalysts, while DTPA (Scheme 1 and Figure S2) could only be activated under blue light (Figure S3).60,61 An initial polymerization of MMA was conducted under green light irradiation (λmax = 520 nm, intensity = 4.25 mW/cm2) for 6 h with CDTPA (MMA:CDTPA = 200:1) in the presence of equimolar DTPA. The polymerization resulted in 85% monomer conversion with good agreement between theoretical and experimental molecular weights (Mn,th = 17400 and Mn,GPC = 18200). Inspired by these findings, kinetics of RAFT photopolymerization of MMA mediated by CDTPA under green light irradiation were then investigated in the presence of different concentrations of DTPA (Figures S4 and S5 and Table 1, entries 1−4). Close agreement between theoretical and experimental molecular weights was observed, which confirms that DTPA does not participate in the chain transfer process with propagating PMMA radicals (Scheme 2). To dismiss the effect of the Z-group of DTPA, such as steric hindrance from the long alkyl chain (12 carbons), another RAFT agent, 2-(n-butyltrithiocarbonate)propionic acid (BTPA) (Scheme 1 and Figure S6), with a shorter alkyl

chain (4 carbons) was tested. Despite the shorter alkyl chain on the Z-group, kinetics studies of MMA did not show a correlation to the molecular weight of PMMA with different concentrations of BTPA (Figure 1, Table 1, entries 5−8, and Figure S7). Interestingly, lower apparent propagation rate constants (kpapp) were also observed upon increasing the molar ratio of BTPA relative to CDTPA (Figure 1A). The presence of equimolar BTPA and CDTPA in the reaction mixture gave a comparable apparent propagation rate constant (kpapp = 7.6 × 10−3 min−1) to a reaction mixture with only CDTPA (kpapp = 7.3 × 10−3 min−1). However, reactions with 5- and 10-fold BTPA relative to CDTPA demonstrated lower apparent propagation rate constants (kpapp = 6.7 × 10−3 min−1 for CDTPA:BTPA = 1:5 and kpapp = 6.3 × 10−3 min−1 for CDTPA:BTPA = 1:10). A similar trend in the polymerization rates was also observed upon varying concentrations of DTPA relative to CDTPA (Figure S4A). Increasing concentrations of DTPA/BTPA relative to CDTPA led to a slower polymerization rate for MMA which was attributed to the presence additional reaction pathway for PMMA radicals (Scheme 2, pathway II). This pathway leads to the possible formation of intermediate adduct radicals between DTPA/BTPA and PMMA propagating radicals which will eventually fragment back to their respective components, but this reduces the radical concentration in the system.67,68 Inhibition periods 7779


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Scheme 3. Two-Step Synthesis of Comblike and Bottlebrush Polymers via RAFT Photoiniferter Polymerization under Two Different Wavelengths

the green region enabling continuous photolysis during the polymerization (refer to the section Photoinitiation Mechanism of CDTPA with MMA and MA in the Supporting Information). Under blue light irradiation, CDTPA (Table 1, entry 9) and DTPA (Table 1, entry 10) can undergo photolysis to generate radicals that initiate polymerization of MMA. Polymerization of MMA with CDTPA proceeds faster under blue compared to green light (Table 1, entry 4) with good control over molecular weight but poor control over molecular weight distributions observed under blue light. On the other hand, sluggish polymerization of MMA with poor molecular weight control was observed with DTPA due to mismatch of the initiating radical from DTPA (secondary) and propagating radical (tertiary carbon) and poor β-scission of the C−S bond. As photolysis is only promoted for CDTPA under green light irradiation, photolysis of BTPA and DTPA can be ignored under this wavelength during polymerization of MMA. Therefore, BTPA and DTPA should theoretically only participate in the addition−fragmentation steps during chain transfer reactions. The stabilities of RAFT agents and RAFT adduct radicals are mainly affected by the Z-group while the chain transfer efficiency is dictated by the R-group.70−73 As the Z-groups of DTPA and CDTPA are similar (dodecyl carbon chains), it is safe to assume that the Z-groups provide similar stabilities to both RAFT agents and RAFT adduct radicals. As the effect of the R-group is decoupled from the Z-group, the selectivity seen during chain transfer reactions can be attributed to the nature of the leaving group.74 The major influence on chain transfer enthalpy (ΔHCT) is the stability of the leaving group radical (R-group radical) which is dependent on relative magnitudes of the RAFT agent stability and of the leaving group radical stabilization energy (RSE).71,72 The release of the R-group radical from RAFT-adduct radical should be competitive to the release of the attached propagating radical, and the radical whose release is favored is often the one with greater stability (Scheme 2).70,75 Therefore, to promote chain transfer, RSE of the R-group radical should be greater than the RSE of propagating radical. Ab initio calculations performed by Coote and co-workers (Table S3) revealed that propionic acid R-groups on DTPA/

observed during initial stages of the polymerizations were attributed to the conversion of trithiocarbonates to single monomer adduct species prior to the polymerization reaction.69 The final polymers synthesized in the presence of various concentrations of BTPA or DTPA were then purified through precipitation in methanol. The precipitates and the supernatants collected during purification were analyzed with 1H NMR. In the case of [CDTPA]:[BTPA] of 1:10 (Figure 2A), the synthesized polymer, upon analysis with the equation f end‑group = I2.6 ppm/(I3.2 ppm/2) × 100, where I2.6 ppm and I3.2 ppm correspond to integration of protons c and a from Figure 2A, respectively, was found to have reasonable RAFT end-group fidelity (>95%). Furthermore, the close match between molecular weights calculated by 1H NMR and GPC values confirmed a living/controlled process. On the other hand, analysis of the supernatant revealed the presence of unreacted BTPA (Figure 2B). Similarly, polymers generated in the presence of various concentrations of BTPA or DTPA were found to have reasonable RAFT end-group fidelities (>85%) (Figures S8 and S10, respectively), and their supernatants containing unreacted BTPA or DTPA (Figures S9 and S11, respectively). Several control studies were performed to understand the mechanism of this polymerization, especially the absence of reactivity of BTPA and DTPA when introduced in the presence of CDTPA and MMA. No dependence on temperature was observed when similar polymerizations were performed under thermal conditions (Table S2). Therefore, the mechanism of this polymerization relies on two important parameters: wavelength of irradiation and nature of the leaving group (R-group) of RAFT agents. Photolysis of RAFT agents under the visible light wavelengths is made possible due to the excitation of the spin-forbidden n → π* electronic transition which enables β-scission of the C−S bond (Scheme 1).61 Under green light irradiation, no polymerization of MMA was observed in the presence of only DTPA and BTPA (Table 1, entries 11 and 12) which can be attributed to the lack of absorption of these RAFT agents in the green region. The addition of MMA units to CDTPA retains the absorption in 7780


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Figure 3. Copolymerization of methyl methacrylate (MMA) and 2-(2-(n-butyltrithiocarbonate)propionate)ethyl methacrylate (BTPEMA) in DMSO mediated by CDTPA under green light irradiation (λmax = 520 nm, intensity = 4.25 mW/cm2) with [MMA]:[CDTPA] = 200:1, 50% v/v monomer concentration. (A) Plot of ln([M0]/[M]t) vs exposure time for copolymerizations of MMA and BTPEMA with ratio of MMA:CDTPA:BTPEMA of 200:1:5 and 200:1:10. (B) Plot of Mn vs conversion for copolymerizations of MMA and BTPEMA with ratio of CDTPA:BTPEMA of 1:5 and 1:10. (C, D) Molecular weight distributions versus different polymerization times for copolymerization of MMA and BTPEMA with ratio of CDTPA:BTPEMA of 1:5 and 1:10.

MA7) but also incorporation of BTPEMA into the polymer backbone without the activation of the trithiocarbonate pendant group (Figures S13 and S14). The presence of signal at 4.8 ppm, which corresponds to −CH of BTPEMA in the adjacent position of trithiocarbonate, confirmed that the trithiocarbonate pendant group remains intact during polymerization of MMA. Subsequently, the purified copolymers were grafted with methyl acrylate (MA) to yield (PMMA-randPBTPEMA)-graf t-PMA copolymers. As polymerization of MA could be promoted by both CDTPA and BTPA (refer to discussion in Table S1), grafting of MA was performed under blue light irradiation to activate the RAFT moieties attached to polymer chains. Comblike polymers were synthesized under blue light irradiation (λmax = 465 nm, intensity = 6.5 mW/cm2) (Figure S15A and Scheme 3) with good correlation between theoretical and experimental molecular weights and fair molecular weight distributions (Figure S15B−D). 1H NMR of the purified comblike polymers revealed grafting of MA units from both copolymers (Figures S16 and S18) with the appearance of −CH3 and −CH peaks at 3.63 and 2.28 ppm of PMA, respectively. Upon comparison of the 1H NMR before and after grafting, protons in the Z- and R-groups of BTPEMA were shifted to a slightly lower chemical shift (Figures S17 and S19), justifying grafting of MA from BTPEMA. Given the successful attempt in generating comblike polymers, the versatility of this approach was expanded by generating bottlebrush polymers via facile manipulation of visible light wavelengths. Bottlebrush backbone was initially synthesized through homopolymerization of BTPEMA (Scheme 3) under green light irradiation with CDTPA for 22 h (Figure 5A and Figure S20) which led to an average degree of polymerization of 480 units in each polymer chain. The polymer was purified via precipitation and analyzed with

BTPA and PMMA initialized R-group on CDTPA have comparable stabilization energies.71,72 However, RSE of the Rgroup radical from PMMA initialized CDTPA is much higher than the RSE of the R-group radical from BTPA/DTPA. Hence, this results in higher chain transfer enthalpy and favorable fragmentation of the PMMA radical over the R-group radical of BTPA/DTPA (Scheme 2). This approach was then expanded as means for designing complex architectures, primarily comblike and bottlebrush polymers, through reduced synthetic steps (including chemical transformations and purifications, Scheme 3). To engineer complex graft polymer architectures through simple manipulation of visible light wavelengths, a RAFT inimer, namely 2(2-(n-butyltrithiocarbonate)propionate)ethyl methacrylate (BTPEMA) (Figure S12) was first designed. Copolymerization of MMA and BTPEMA mediated by CDTPA (Figure 3A) was then performed under green light irradiation with two different concentrations of BTPEMA ([MMA]:[CDTPA]:[BTPEMA] = 200:1:5 and 200:1:10). Both polymerizations led to pseudofirst-order kinetics with similar trends observed during polymerization of MMA with different concentrations of BTPA. Polymerization of MMA with a lower content of BTPEMA led to a slightly higher apparent propagation rate constant ([MMA]:[CDTPA]:[BTPEMA] = 200:1:5, kpapp = 8.6 × 10−3 min−1 in comparison to polymerization using [MMA]:[CDTPA]:[BTPEMA] = 200:1:10, kpapp = 7.5 × 10−3 min−1). Nevertheless, both polymerizations resulted in polymer products with a good agreement between their experimental and theoretical molecular weight values calculated if only CDTPA was activated. Further analysis with 1H NMR after purification revealed not only reasonable retention of CDTPA RAFT end-groups on the synthesized copolymers (PMMA158-rand-PBTPEMA4 and PMMA137-rand-PBTPE7781


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GPC and 1H NMR analyses (Mn,theo = 168700, Mn,GPC = 97 300, and Mw/Mn = 1.38). Poor correlation between experimental and theoretical molecular weights of PBTPEMA was attributed to the differences in hydrodynamic volumes between PBTPEMA and PMMA standards. PBTPEMA was then grafted in the presence of N,N-dimethylacrylamide (DMA) monomer by switching to blue wavelength irradiation (Scheme 3). Sampling during grafting of DMA (Table S4) revealed monomodal evolution of GPC traces (Figure 4A)

CONCLUSIONS In this work, we have successfully developed a chemoselective visible light RAFT polymerization that can be employed without the presence of photocatalysts. This approach was made possible through careful selection of wavelength of irradiation and RSE of the R-leaving group. This method was then expanded to design complex architectures such as comblike and bottlebrush polymers. Unlike the previous shuttled CTA R-group approach of synthesizing graft polymers, this work provides a facile, straightforward approach for bottlebrush synthesis with no postpolymerization coupling reactions. In comparison to bottlebrush synthesis via NMP and ATRP, the described photoiniferter approach can be performed at ambient temperature in the absence of an external initiator or metallic contamination. In addition, NMR characterization of the comblike and bottlebrush polymer reveal high grafting densities with complete initiation of the trithiocarbonate side chain upon grafting of second monomers, such as MA and DMA, under blue light irradiation. This selective fragmentation of RAFT agents under different wavelengths of visible light is currently being implemented in the development of structurally tailored and engineered macromolecular (STEM) gels.

Figure 4. Photomediated bottlebrush synthesis via grafting of DMA from PBTPEMA480 under blue light irradiation (λmax = 465 nm, intensity = 14.9 mW/cm2). (A) Grafting of DMA from PBTPEMA480, [DMA]:[BTPEMA480] of 100:1 and 52% v/v monomer concentration, represented by DMF GPC traces. (B) AFM image of the purified PBTPEMA480-g-PDMA38 bottlebrush polymer (Figure S21 shows length distribution of bottlebrush polymers).

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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.8b01708.

with relatively low molecular weight distributions. After 3 h irradiation, the synthesized bottlebrush was purified and analyzed with 1H NMR analysis (Figure 5B). The analysis revealed a complete shift in −CH signal at 4.8 ppm (proton e in Figure 5A) to 5.2 ppm (proton c in Figure 5B) upon addition of DMA confirming that all the pendant trithiocarbonates have undergone chain transfer reactions. The bottlebrush was then analyzed via atomic force microscopy (AFM) to reveal an average contour length of 119.9 nm (Figure S21). Assuming the length between each repeat unit in a bottlebrush is 0.25 nm (C−C bond), a stretched PBTPEMA chain (480 repeating units) is estimated to have a theoretical contour length of 120 nm, which is in good agreement with the values obtained by AFM (Figure 4B).

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Experimental details, GPC traces, and NMR spectra (Figures S1−S25 and Tables S1−S4) (PDF)

AUTHOR INFORMATION

Corresponding Author

*(K.M.) E-mail: [email protected]. ORCID

Tomasz Kowalewski: 0000-0002-3544-554X Cyrille Boyer: 0000-0002-4564-4702 Krzysztof Matyjaszewski: 0000-0003-1960-3402 Notes

The authors declare no competing financial interest.

Figure 5. 500 MHz 1H NMR spectrum of poly(2-(2-(n-butyltrithiocarbonate)propionate)ethyl methacrylate) (PBTPEMA, A) and PBTPEMA bottlebrush grafted with DMA (B) in CDCl3. 7782


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ACKNOWLEDGMENTS Support from the NSF DMR 1436219 and 1436201 and DoE ER 45998 is acknowledged.

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