Visible-Light-Induced Living Radical Polymerization (LRP) Mediated

Jul 28, 2015 - Visible-light-induced living radical polymerization of acrylates (MA, nBA, tBA), acrylamides (DMA, AMO), and vinyl acetate (VAc) at amb...
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Visible-Light-Induced Living Radical Polymerization (LRP) Mediated by (salen)Co(II)/TPO at Ambient Temperature Yaguang Zhao,† Shuailin Zhang,† Zhenqiang Wu,† Xu Liu,† Xianyuan Zhao,† Chi-How Peng,‡ and Xuefeng Fu*,† †

Beijing National Laboratory for Molecular Sciences, State Key Lab of Rare Earth Materials Chemistry and Applications, College of Chemistry and Molecular Engineering, Peking University, Beijing 100871, China ‡ Department of Chemistry and Frontier Research Center on Fundamental and Applied Sciences of Matters, National Tsing-Hua University, Hsinchu 30013, Taiwan S Supporting Information *

ABSTRACT: Visible-light-induced living radical polymerization of acrylates (MA, nBA, tBA), acrylamides (DMA, AMO), and vinyl acetate (VAc) at ambient temperature mediated by (salen)Co(II)/TPO was described. Effects of light intensity, feeding ratio of monomer and equivalent of TPO for the polymerization of MA were investigated. Well-defined homopolymers and block polymers with predetermined molecular weight and narrow polydispersity were obtained under mild conditions. The mechanism of the polymerization was proposed based on polymerization behavior and polymer structure analysis. The (salen)Co(II)/TPO system was suitable for both conjugated and unconjugated monomers under mild conditions.

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INTRODUCTION Living radical polymerization techniques have been widely used as a powerful tool to construct well-defined polymeric materials with predetermined molecular weight and architecture.1 Nitroxide mediated radical polymerization (NMP),2 atom transfer radical polymerization (ATRP),3 and reversible addition−fragmentation chain transfer (RAFT) polymerization4 are the most popular and widely applied techniques for polymers with precise tailor-made architectures. While in recent years, an alternate transition metal mediated LRP method, namely organometallic mediated radical polymerization (OMRP), has been well-developed.5 OMRP is based on temporary and reversible deactivation of the growing radicals by metallic species acting as persistent radical. Cobalt complex mediated radical polymerization (CMRP)5a,6 represents one of the most successful and efficient OMRP. Polymerizations of both conjugated monomers (acrylates, acrylamides) and unconjugated monomers (vinyl acetate, N-vinylpyrrolidone) are well-controlled by CMRP. Cobalt complexes, such as cobalt porphyrin complexes,7 bis(acetylacetonate)cobalt derivatives,8 1,3-bis(2-pyridylimino)isoindolatocobalt(II),9 alkylcobaloximes,10 and (ketoaminato)cobalt(II) complexes,11 were commonly used CMRP catalysts. Cobalt salen complex mediated polymerization of vinyl acetate (VAc) and methyl acrylate (MA) under thermal conditions was reported recently;12 however, a significant discrepancy between theoretical and experimental molecular weights was observed during MA polymerization due to the relatively weak Co−C bond (formation equilibrium constant Keq(333 K) = 2.4 × 107 M−1 for organo-cobalt(III)).12 The lability of cobalt−carbon bond under thermal treatment resulted in accumulation of © XXXX American Chemical Society

cobalt(II) during the polymerization process and variation of the concentration of the propagation radicals. Polymerization at lower temperature (such as ambient temperature) provided an alternate way to improve the controllability and maintain living character, but efficient initiation under milder conditions was required. In this aspect, photoinduced LRP was considered the ideal candidate because of its inherent advantage of low activation energy and mild polymerization conditions.13 In recent years, photoinduced LRP is receiving increasing attention and photochemistry has been employed in nearly all LRP methods including CMRP.14−17 Previously, we reported the visible-light-induced LRP of acrylates and acrylamides at room temperature mediated by organo-cobalt porphyrin complexes.18 Very recently, we demonstrated photopolymerization of monomers with various functional groups mediated by a structurally well-defined organo-cobalt salen catalyst (salen)CoCO2CH3; however, tedious preparation and purification process were needed.19 Herein, we report a facile photopolymerization process just by mixing commercial available complexes cobalt(II) salen with traditional photoinitiator (2,4,6-trimethylbenzoyl)diphenylphosphine oxide (TPO) under visible light irradiation, giving excellent LRP of acrylates and other monomers (Scheme 1).

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RESULTS AND DISCUSSION Control Polymerizations and Effect of Light Intensity. (salen)Co(II) (Scheme 1a) was used to mediate photoinduced Received: May 28, 2015 Revised: July 9, 2015

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required for the consumption of (salen)Co(II) to form (salen)Co(III)-P. These initial results indicated that visible light, photoinitiator, and cobalt complexes were all necessary for a well-controlled polymerization. The polymerization rates were effectively enhanced by increasing light intensity (Table 1, entries 6−8, and Figure 1a). When the light intensity of 50 mW/cm2 was used, the conversion of MA reached 55.3% after 2.5 h with formation of well-controlled PMA (Table 1, entry 8). With increasing light intensity, the fast dissociation of TPO led to higher radical concentration, shorter induction time, and faster polymerization rate (Figure 1a), however, at the expense of poor control (Mw/Mn > 1.35) at high conversion (>75%) due to the increasing of side reactions at high radical concentration (Figure 1b). Thus, the latter polymerizations were all irradiated with 3 mW/cm2 light intensity. Effect of Feeding Molar Ratio of MA. To investigate the capability of this system in synthesis of PMA with higher molecular weight, the kinetics of visible-light-induced polymerization of MA were studied by the 1H NMR with different initial feeding molar ratio of MA and (salen)Co(II) at room temperature. As shown in Figure 2a, the polymerizations all showed an induction time with molar ratio from 300/1 to 900/ 1. During the induction time, (salen)Co(II) was converted to organo-cobalt salen complexes through the reaction with MA and radicals derived from TPO. The length of induction time depended on the light intensity and molar ratio between TPO and (salen)Co(II). When the ratio of the concentration of TPO to (salen)Co(II) remained the same, similar induction time (19.4, 20.4, and 20.5 h) was observed under 3 mW/cm2 (Figure 2a). Linear first-order kinetics were all obtained after induction time, which indicated constant concentration of propagating radicals during the polymerization. The apparent rate constant values for the polymerization (kpapp, Rp = −d[M]/dt = kp[R•][M] = kpapp[M]) were determined to be 0.0581, 0.1278, and 0.1974 h−1 (corresponding to 300/1, 600/1, and 900/1 ratio, respectively), indicating faster polymerization rate with higher monomer concentration. Furthermore, molecular weight of the final PMA increased linearly with monomer conversion (Figure 2b). The corresponding polydispersity values remained narrow (Mw/Mn < 1.25) throughout the whole polymerization process up to high MA conversion. Increasing feeding ratio of MA led to higher molar mass, and the experimental molecular weights were all close to theoretical values. These results indicated that the (salen)Co(II)/TPO

Scheme 1. Structures of Cobalt Complexes, Photoinitiator, and List of Monomers Used: (a) Cobalt(II) [N,N-Bis(3,5-ditert-butylsalicylidene)-1,2-cyclohexanediamine], (salen)Co(II); (b) (2,4,6Trimethylbenzoyl)diphenylphosphine Oxide, TPO; (c) Methyl Acrylate, MA; (d) n-Butyl Acrylate, nBA; (e) tertButyl Acrylate, tBA; (f) N,N-Dimethylacrylamide, DMA; (g) N-Acryloylmorpholine, AMO; (h) Vinyl Acetate, VAc

LRP owing to its stability and commercial availability, which could be highly attractive for industrial applications. Initially, the photopolymerizations of MA were performed using (salen)Co(II) as catalyst and TPO as the photoinitiator irradiated by a 500 W xenon lamp equipped with a 420−780 nm filter. Control polymerizations indicated that no polymer was observed without irradiation or TPO at room temperature for over 30 h (Table 1, entries 1 and 2). Without the addition of (salen)Co(II), free radical polymerization of MA occurred rapidly, resulting in uncontrolled molecular weight and broad polydispersity (Table 1, entry 3). After addition of 0.25 equiv of TPO, no polymerization was observed for a substantial period of time (Table 1, entry 4). Encouragingly, irradiation of the mixture of MA with (salen)Co(II) and 1 equiv of TPO for 25 h gave PMA with predetermined molecular weight and narrow polydispersity (Table 1, entry 5). Under this condition, an induction time was observed corresponding to the time

Table 1. Visible-Light-Induced Polymerization of MA Mediated by (salen)Co(II)/TPO at Room Temperaturea entry

[MA]0/[CoII]0/[TPO]0

I (mW/cm2)

T (h)

convb (%)

1 2 3 4 5 6 7 8 9 10

600/1/1 600/1/0 600/0/1 600/1/0.25 600/1/1 600/1/1 600/1/1 600/1/1 300/1/1 900/1/1

0 3 3 3 3 10 20 50 3 3

30 50 1.0 30 25 11 6.5 2.5 30 20

0 0 76.0 0 65.0 60.5 55.6 55.3 69.3 50.9

Mn,thc

33600 31200 28700 28600 17900 39400

Mn,exd

Mw/Mnd

42400

1.97

34600 28500 23500 25200 17000 39300

1.17 1.17 1.18 1.20 1.20 1.16

a [MA]0 = 1.0 M, solvent was d6-benzene. bThe monomer conversion was determined based on 1H NMR analysis. cMn,th = MW(MA) × ratio × conv (%). dDetermined using gel permeation chromatography in DMF calibrated against poly(methyl methacrylate) standards.

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Figure 1. (a) Kinetics for the visible-light-induced polymerization of MA with different light intensities (3, 10, 20, and 50 mW/cm2) at room temperature in C6D6. [MA]0 = 1.0 M, [(salen)Co(II)]0 = 1.6 × 10−3 M, [TPO]0 = 1.6 × 10−3 M. (b) Evolution of molar mass and polydispersity versus MA conversion for polymers obtained under different light intensities.

Figure 2. (a) Kinetics for the visible-light-induced polymerization of MA with different feeding ratio of MA and (salen)Co(II) at room temperature in C6D6. [(salen)Co(II)]0 = 1.6 × 10−3 M, [TPO]0 = 1.6 × 10−3 M, I = 3 mW/cm2. (b) Evolution of molar mass and polydispersity versus MA conversion.

Figure 3. (a) Kinetic plots for the visible-light-induced polymerization of MA in the presence of different amounts of TPO in C6D6 at room temperature. [MA]0 = 1.0 M, [(salen)Co(II)]0 = 1.6 × 10−3 M, I = 3 mW/cm2. (b) Evolution of molar mass and polydispersity versus MA conversion.

Effect of Concentration of TPO. To further investigate the effect of the TPO on this system, kinetics for different equivalents of TPO were monitored by 1H NMR (Figure 3a). All of the polymerizations showed an induction time during which no polymer was formed. Induction time was often observed in CMRP systems for in situ generation of dormant organo-cobalt complexes from initiator radicals and the starting Co(II) complexes.6 When the concentration of total initiator radical is less than that of cobalt(II) salen, starting (salen)Co(II) could not be completely converted to organo-cobalt

Scheme 2. Photolysis Reaction of TPO under Irradiation

system could control the polymerization of MA over a wide range of feeding ratio. C

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Figure 4. 1H NMR spectrum of PMA prepared under visible light irradiation (3 mW/cm2) with Xe lamp in C6D6 at room temperature. [MA]0 = 1.0 M, [MA]0/[(salen)Co(II)]0/[TPO]0 = 600/1/1, t = 23 h, conversion = 58.5%. Mn = 29 800 g/mol, Mw/Mn = 1.09. Mn,NMR = MW(MA) × I2.3 ppm/ (I7.7 ppm/4 + (I6.8 ppm − 0.5I8.0 ppm)/2), where I8.0 ppm was the integral of signal at δ 8.0 ppm corresponding to o-phenyl protons of remaining TPO. CDCl3 was used as the solvent and tetramethylsilane (TMS) as the internal standard.

Figure 5. 31P NMR spectrum of PMA (Mn = 30 600 g/mol, Mw/Mn = 1.23, conversion = 75%) prepared under visible light irradiation (3 mW/cm2) in C6D6 at room temperature. [MA]0 = 1.0 M, [MA]0/[(salen)Co(II)]0/[TPO]0 = 600/1/2, t = 20 h. CDCl3 was used as the solvent.

complexes, which accounted for no polymerization observed after 30 h in the presence of only 0.25 equiv of TPO (Table 1, entry 4). The induction time could be reduced by increasing the amount of external initiator but often resulted in poor control due to excess radical concentration. In our experiments, induction time decreased from 20 to 13 and 6.5 h corresponding to 1, 2, and 3 equiv of TPO, respectively. Induction time for 2 and 3 equiv of TPO was nearly 1/2 and 1/ 3 of that for 1 equiv of TPO. Undoubtedly, the ratio of TPO/ (salen)Co(II) determined the induction time. The linear relationship, after the induction time, between ln([M]0/[M]t) and polymerization time showed that the concentration of propagating radicals remained constant in all cases. The apparent rate constants for the polymerizations were determined to be 0.1278, 0.2964, and 0.4590 h−1, respectively. As expected, the polymerization rate was enhanced when the amount of TPO increased. Furthermore, the polymerization rate for 2 equiv of TPO in current system was close to the polymerization process using trivalent (salen)Co−CO2CH3 with addition of 1 equiv of TPO (0.3013 h−1).19 For the (salen)Co(II)/TPO system, around 1 equiv of TPO might be consumed to convert (salen)Co(II) to (salen)Co−P during the induction time. Thus, current system could be considered as the (salen)Co−P/TPO mixture, and the remaining TPO was around 1 equiv to (salen)Co−P, similar to the (salen)Co− CO2CH3/TPO system. As a result, in both cases, nearly identical kpapp values were obtained.

Figure 6. MALDI-TOF-MS spectroscopy of PMA after modification of chain end (Mn = 2790 g/mol, Mw/Mn = 1.12).

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Figure 7. Simulation of isotopic pattern for peaks with m/z = 1359.8042 and 1391.8784.

Table 3. Synthesis of Block Copolymersa

Scheme 3. Proposed Mechanism for Visible-Light-Induced Polymerization of MA Using TPO as Photoinitiator and (Salen)Co(II) as the Mediator

entry block copolymerb t (h) 1 2 3

PMA-b-PnBA PMA-b-PtBA PMA-b-PDMA

2.0 1.8 1.9

convc (%)

Mn,thd

Mn,exe

Mw/Mne

45 40 64

47800 44000 51300

51900 43200 39300

1.17 1.16 1.21

a

For macroinitiator (salen)Co-PMA: [MA]0 = 1.0 M, [MA]0/ [(salen)Co(II)]0/[TPO]0 = 600/1/1, light intensity was 3 mW/cm2, solvent was d6-benzene, t = 21 h, conv = 25%, Mn,th = 12 900, Mn,ex = 13 200, Mw/Mn = 1.16. b[M]0/[(salen)Co-PMA]0 = 600/1. cThe monomer conversion was determined based on 1H NMR analysis. d Mn,th = MW(macroinitiator) + MW(M) × ratio × conv (%). eDetermined using gel permeation chromatography in DMF calibrated against poly(methyl methacrylate) standards.

all cases were monomodal and symmetrical throughout the whole polymerization process (Figures 3S−10S). These results undoubtedly demonstrated the living character and controllability of this (salen)Co(II)/TPO system. Investigation of Polymerization Mechanism. As described above, (salen)Co(II) was converted to the organocobalt complex through the reaction with initial radicals from TPO dissociation and monomers during the induction time. Irradiation of the reaction mixture led to the dissociation of the TPO into diphenylphosphine oxide radical and 2,4,6trimethylbenzoyl radical (Scheme 2). Both of these two radicals could initiate the polymerization although phosphorus-centered radical had higher reactivity.20 In order to investigate the mechanism of this system, the structure of PMA obtained was characterized. The 1H NMR spectrum of PMA obtained using the current method is shown in Figure 3. Resonances of remaining TPO were observed as no further purification was applied after polymerization. The chemical shift at 7.7 ppm (d in Figure 4) and 6.8 ppm (g in Figure 4) corresponded to the phenyl protons of the phosphorus and carbon centered group dissociated from TPO, respectively. Thus, both segments derived from photolysis of TPO initiated the polymerization process. Besides, the ω ends of polymer chains were capped with (salen)Co group as evidenced by typical resonances of salen ligand in the 1H NMR spectrum. No starting (salen)Co(II) was observed in 1H NMR, indicating complete conversion into the organo-cobalt salen complex. Furthermore, the molecular weight could be calculated from the integrals of methine group of main chain (2.3 ppm) and α chain end (Figure 4). The molar mass from 1H NMR analysis was 28 000 g/mol, which fitted well with theoretical value (30 800 g/mol) and experimental molecular weight (29 800 g/mol), indicating high initiator efficiency in the current system.

Table 2. Visible-Light-Induced Polymerization of Different Monomers Mediated by (Salen)Co(II)/TPO at Room Temperaturea entry

monomer

t (h)

convb (%)

Mn,thc

Mn,exd

Mw/Mnd

1 2 3 4 5e

nBA tBA DMA AMO VAc

29 9.5 23 29 95

77.2 49.2 32.6 27.4 45.8

59400 37800 19400 23200 21300

42600 41300 20000 23600 19500

1.19 1.22 1.18 1.19 1.25

a

[M]0 = 1.0 M, [M]0/[(salen)Co(II)]0/[TPO]0 = 600/1/1, light intensity was 3 mW/cm2, solvent was d6-benzene. bThe monomer conversion was determined based on 1H NMR analysis. cMn,th = MW(M) × ratio × conv (%). dDetermined using gel permeation chromatography in DMF calibrated against poly(methyl methacrylate) standards. ePolymerization in bulk, [VAc]0/[(salen)Co(II)]0/[TPO]0 = 540/1/2, light intensity was 60 mW/cm2.

Moreover, the number-average molecular weight of PMA linearly increased with monomer conversion in all cases (Figure 3b). The experimental molecular weights of PMA were in good agreement with theoretical values, and the molecular weight distributions remained narrow even in high conversion (1.1 < Mw/Mn < 1.3). The molecular weight of final PMA was determined by conversion and the feeding ratio of MA/ (salen)Co(II), yet independent of TPO concentration. So polymerizations with different amounts of TPO gave identical molar mass with similar MA conversion. GPC traces of PMA in E

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of 86.04, corresponding to the molar mass of MA monomer. The experimental values for isotopic mass of the main peaks agreed well with the theoretical values, as shown in the upper part of Figure 6. The main set of a series of peaks corresponded to the polymer chains end-capped with diphenylphosphine oxide, and the other set was attributed to polymers initiated by the 2,4,6-trimethylbenzoyl group. More specifically, the observed isotopic pattern (m/z) of the main peaks at m/z = 1359.8042 and m/z = 1391.8784 matched well with the simulated results (Figure 7). Phosphorus-centered radicals were reported to show extremely high reactivity toward vinyl monomers compared to carbon-centered radicals.21 Thus, although photolysis of TPO gave equal amount of phosphorus and carbon-centered radical, polymer chains derived from the phosphorus-centered radical dominated in the final product (Figure 6). In order to determine the proportion of polymer chains initiated by diphenylphosphine oxide radical, the MALDI-TOF-MS spectrum of PMA was quantitatively analyzed (Figures 13S−16S).22 The results showed that 69% of polymer chains were initiated by phosphorus centered radical, consistent with the ratio of integrals of diphenylphosphine oxide (7.7 ppm) and 2,4,6trimethylbenzoyl (6.8 ppm) groups in 1H NMR (Figure 4). These results further indicated that the reaction rate of the phosphorus-centered radical with MA was much faster than that of the carbon-centered radical.21 According to the experimental results, visible-light-induced polymerization of MA mediated by (salen)Co(II)/TPO occurred through the following steps, as shown in Scheme 3. (1) During the induction time, the phosphorus- and carboncentered radicals, generated from photolysis of TPO, reacted with monomers until trapped by (salen)Co(II) to form the dormant organo-cobalt complex. (2) Because of the different reactivity, phosphorus-initiated complexes were the major product. At the end of induction time, all (salen)Co(II) complexes were converted into (salen)Co−P. (3) During the chain propagation process, linear first-order kinetics started, and the polymerization was believed to mainly undergo degenerate transfer mechanism as the polymerization rate was dependent on concentration of external TPO (Figure 3a) while no (salen)Co(II) was observed in 1H NMR. Application to Other Monomers. To test the versatility of this visible-light-induced polymerization mediated by the (salen)Co(II)/TPO system, it was applied to several other monomers under the same conditions as the polymerization of MA. Table 2 summarizes the polymerization results. Acrylates with different substituent groups (Table 2, entries 1 and 2) were polymerized to give the corresponding polymers with controlled molar mass and narrow molecular weight distribution. Furthermore, this method also showed good controllability in polymerization of acrylamides, such as DMA and AMO. The molecular weights of final PDMA and PAMO were in very good agreement with theoretical values (Table 2, entries 3 and 4). More importantly, the current system could also be applied to unconjugated monomer VAc in bulk polymerization. However, the polymerization was rather slow when it was conducted with 1 equiv of TPO under 3 mW/cm2 visible light irradiation. So 2 equiv of TPO in total was added, and the light intensity was increased to 60 mW/cm2. Under this condition, monomer conversion reached 45.8% after 95 h, and the obtained PVAc showed predetermined molecular weight with narrow polydispersity (Table 2, entry 5).

Figure 8. GPC traces of PMA macroinitiator before and after block copolymerization with nBA, tBA, and DMA via visible-light-induced polymerization: (a) synthesis of PMA-b-PnBA; (b) synthesis of PMAb-PtBA; (c) synthesis of PMA-b-PDMA.

To further confirm the attachment of phosphorus centered group in the polymer chain, the 31P NMR spectrum of PMA is recorded in Figure 5. The remaining TPO after polymerization was completely removed by washing with methanol (Figures 11S and 12S). The resonance at 29.62 ppm in 31P NMR was ascribed to the phosphorus group in diphenylphosphine oxide at α ends. All these NMR analyses demonstrated that segments of TPO and (salen)Co moieties were attached at the chain end of final polymers. To confirm the exact initiating species in polymer structure, end-group functionalization was performed according to the oxygen insertion method and analyzed by MALDI-TOF-MS.19 Figure 6 shows the MALDI-TOF-MS spectrum of the PMA sample (Mn = 2790 g/mol, Mw/Mn = 1.12) obtained with feeding ratio of [MA]0/[(salen)Co(II)]0/[TPO]0 = 600/1/1 under visible light irradiation at room temperature. The results indicated two main series of peaks separated by regular interval F

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Block Copolymerization. To further demonstrate the living nature of this system, block copolymers were synthesized using the sequential visible-light-induced process. Irradiation of MA and 1 equiv of TPO in the presence of (salen)Co(II) afforded well-defined macroinitiator (salen)Co-PMA (Mn = 13 200 g/mol, Mw/Mn = 1.16). Subsequent visible-lightinduced copolymerization with nBA, tBA, and DMA led to formation of desired block copolymers (Table 3). The block copolymerization was much faster than macroinitiator synthesis because block copolymerization was initiated from trivalent (salen)Co-PMA, and no induction time was required. The molecular weights of final block copolymers were close to the theoretical values with narrow polydispersity. GPC traces of the block copolymers revealed a complete shift to higher molecular weight compared to the starting macroinitiator, and the amount of unreacted first block was negligible (Figure 8). The efficient block copolymerization confirmed the feature of living radical polymerization and provided additional evidence for the presence of cobalt complexes at ω chain ends.

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CONCLUSIONS In summary, we had developed a novel visible-light-induced polymerization mediated by commercial available (salen)Co(II) and TPO. Both of the conjugated acrylates, acrylamides and unconjugated VAc, could be controlled under mild conditions. The polymerization proceeded with an induction time, followed by linear kinetics through a degenerate transfer process. Polymers with narrow molecular weight distributions and controlled molecular weights were obtained. Efficient synthesis of various block copolymers further confirmed the living nature of this process. Both phosphorus- and carboncentered radicals generated from TPO could initiate the polymerization. The polymer chains end-capped with phosphorus accounted for 69% in accord with higher reactivity of phosphorus radical than that of carbon-centered radical. Development of neat process that combining LRP mediated by (salen)Co(II) with other processes to prepare novel hybrid copolymers is currently under investigation.

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ASSOCIATED CONTENT

S Supporting Information *

Experimental details; GPC traces of synthesized polymers. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.5b01149.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (X.F.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Natural Science Foundation of China (21171012 and 21322108). REFERENCES

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DOI: 10.1021/acs.macromol.5b01149 Macromolecules XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.macromol.5b01149 Macromolecules XXXX, XXX, XXX−XXX