A Highly Efficient Catalyst for the Synthesis of Alternating Copolymers

Jan 15, 2014 - ACS eBooks; C&EN Global Enterprise .... *(F.O.) E-mail: [email protected]. ... This paper reports an improved catalytic system fo...
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A Highly Efficient Catalyst for the Synthesis of Alternating Copolymers with Thieno[3,4‑c]pyrrole-4,6-dione Units via Direct Arylation Polymerization Masayuki Wakioka,† Nobuko Ichihara,† Yutaro Kitano,† and Fumiyuki Ozawa*,†,‡ †

International Research Center for Elements Science (IRCELS), Institute for Chemical Research, Kyoto University, Uji, Kyoto 611-0011, Japan ‡ ACT-C, Japan Science and Technology Agency, Uji, Kyoto 611-0011, Japan S Supporting Information *

ABSTRACT: π-Conjugated polymers with a donor−acceptor (DA) combination of repeating units possess a narrow HOMO−LUMO gap, thus resulting in a high device performance in solar cells. This paper reports an improved catalytic system for the synthesis of DA polymers containing 5-(2hexyldecyl)-5H-thieno[3,4-c]pyrrole-4,6-dione-1,3-diyl (TPD) group as the acceptor unit via palladium-catalyzed direct arylation polymerization. Although a related study has been reported (Angew. Chem. Int. Ed. 2012, 51, 2068), we attempted to reduce the catalyst loading because the palladium residue in π-conjugated polymers has been known to produce a detrimental effect on device performance. As a result, the amount of palladium could be reduced to 1/8 by using PdCl2(MeCN)2 and P(C6H4-o-OMe)3 (L1) as catalyst precursors. The polymerization smoothly proceeds at 100 °C in THF in the presence of pivalic acid and Cs2CO3 to afford TPD-based DA polymers 3a−3d containing the following donor units in almost quantitative yields: 4,4′-dioctyl-2,2′-bithiophene-5,5′-diyl (3a, Mn = 36800, Mw/Mn = 2.20), 4,8-bis(2-ethylhexyloxy)benzo[1,2-b:4,5-b′]dithiophene-2,6-diyl (3b, Mn = 31100, Mw/Mn = 2.44), 3,4-(2,2′-dioctylpropylenedioxy)thiophene-2,5-diyl (3c, Mn = 68200, Mw/Mn = 3.04), and 2,5-bis(2-ethylhexyloxy)-1,4phenylene (3d, Mn = 65500, Mw/Mn = 2.21). A detailed analysis of the structure of 3a is reported.

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INTRODUCTION π-Conjugated polymers with a donor−acceptor (DA) combination of repeating units possess a narrow HOMO−LUMO gap, thus resulting in a high device performance in solar cells.1 Such polymers are currently synthesized by polycondensation based on the Migita−Kosugi−Stille cross-coupling.2,3 However, this method requires prepreparation of organotin reagents, and produces a stoichiometric amount of toxic compound such as Me3SnBr. In this context, a novel polycondensation process via palladium-catalyzed direct arylation (so-called direct arylation polymerization) has attracted a great deal of attention as an easy and safe alternative.4−19 Recently, we reported a highly efficient catalyst for the synthesis of head-to-tail regioregular poly(3-hexylthiophene) via direct arylation.6 We also presented a remarkably improved catalytic system for producing the alternating copolymer with 9,9-dioctylfluorene-2,7-diyl and 1,2,4,5-tetrafluorophenylene units.7 Although direct arylation polymerization has been investigated by several research groups,8−19 most of them utilize conventional catalytic systems for direct arylation, where the use of highly polar solvents such as N,N-dimethylacetamide (DMA) and N,N-dimethylformamide (DMF) is essential for gaining high catalytic activity.4,5 However, it is reasonable that © 2014 American Chemical Society

DMA and DMF are not always suitable for polymerization, particularly in terms of the solubility of resulting polymers. Thus, we have developed novel direct arylation catalysts that are sufficiently reactive in THF and toluene.6,7 A key to this unique catalytic property is the use of P(C6H4-o-OMe)3 (L1) or P(C6H4-o-NMe2)3 (L3) as the supporting ligand. In this study, we examined the synthesis of DA polymers containing 5-(2-hexyldecyl)-5H-thieno[3,4-c]pyrrole-4,6-dione1,3-diyl (TPD) group, which has proven to function as a competent acceptor unit in DA polymers (Scheme 1).20 Although the synthesis of TPD-based polymers via direct arylation was examined by Leclerc et al.,18a a relatively large amount of catalyst (8 mol % Pd) was needed. Because high catalyst loading often causes deterioration in device performance,21 we attempted in this study to establish an efficient catalytic system that produces high molecular weight polymers in the presence of a small amount of palladium. Herein, we disclose that such a direct arylation polymerization system with high catalytic activity can be realized by using PdCl2(MeCN)2 Received: November 15, 2013 Revised: January 10, 2014 Published: January 15, 2014 626

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% Pd), P(C6H4-o-OMe)3 (L1, 1 mol %), Cs2CO3 (1.5 mmol), and PivOH (0.50 mmol). The initially light orange color of the solution gradually changed to dark purple. After 24 h, the reaction mixture was diluted with CHCl3, washed with water to remove inorganic salts, and poured into vigorously stirred MeOH. A dark purple solid thus precipitated was collected by filtration, washed with MeOH, and dried under vacuum (99% yield). The GPC calibration based on polystyrene standards at 140 °C using o-C6H4Cl2 as the mobile phase revealed the formation of 3a with Mn = 25500 (Mw/Mn = 1.88).22 The polymerization was little affected by the amount of L1 (2 mol %) (entry 2), but did not proceed in the absence of the ligand (entry 3). Entries 4−11 demonstrate the unique ligand properties of L1. The reaction using PPh3 in place of L1 provided an oligomeric product (entry 4). To our surprise, elimination of even one of the o-MeO substituents from L1 (i.e., L2) also resulted in oligomerization (entry 5). Moreover, neither L3 with o-Me2N groups nor L4 with o-Me groups caused polymerization (entries 6 and 7). The catalytic reaction using L1 was compatible with less polar solvents such as toluene and cyclopentyl methyl ether (CPME) (entries 8 and 9), but did not proceed in polar DMA and DMF (entries 10 and 11). The observed solvent effects are evidently opposite to those previously reported;4 namely, most of the direct arylation systems so far examined exhibit high catalytic activity in polar DMA and DMF, and the “ligandless mechanism” involving an arylpalladium intermediate coordinated with the polar solvent has been proposed.23 In contrast, the present catalytic system is sufficiently reactive in THF and toluene, where the use of L1 is an essential requirement for gaining the high catalytic activity. Entries 12−17 in Table 1 compare the catalytic performance of five kinds of palladium precursors in the presence of L1. While PdMe2(tmeda) was ineffective (entry 12), Pd(OAc)2 and CpPd(π-allyl) displayed slightly higher performance than

Scheme 1. Polycondensation of Dibromoarenes (1) and Thieno[3,4-c]pyrrole-4,6-dione (2) via Direct Arylation

and L1 as catalyst precursors in combination with pivalic acid (PivOH) and Cs2CO3.

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RESULTS AND DISCUSSION Optimization of Catalytic Conditions. First of all, catalytic conditions were optimized for the reaction of 1a with 2 (Table 1). On the basis of our previous study,7 Pd2(dba)3·CHCl3, Cs2CO3, and PivOH were chosen as a palladium precursor, a base, and a carboxylate source, respectively (entry 1). The carboxylate source is necessary for generating a palladium carboxylate, which causes C−H bond cleavage of heteroarenes in direct arylation.4 A 1:1 mixture of 1a and 2 (0.5 mmol each) was heated in THF (1 mL) at 100 °C in the presence of Pd2(dba)3·CHCl3 (0.5 mol %; i.e., 1 mol

Table 1. Effects of Catalyst Precursors and Solvents on Direct Arylation Polymerization of 1a and 2a entry

Pd complex

ligandb

solvent

yield (%)

Mnc

Mw/Mnc

1 2d 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17e

Pd2(dba)3·CHCl3 Pd2(dba)3·CHCl3 Pd2(dba)3·CHCl3 Pd2(dba)3·CHCl3 Pd2(dba)3·CHCl3 Pd2(dba)3·CHCl3 Pd2(dba)3·CHCl3 Pd2(dba)3·CHCl3 Pd2(dba)3·CHCl3 Pd2(dba)3·CHCl3 Pd2(dba)3·CHCl3 PdMe2(tmeda) Pd(OAc)2 CpPd(π-allyl) PdCl2(MeCN)2 Herrmann cat. Herrmann cat.

L1 L1 none PPh3 L2 L3 L4 L1 L1 L1 L1 L1 L1 L1 L1 L1 L1

THF THF THF THF THF THF THF CPME toluene DMA DMF THF THF THF THF THF THF

99 99 0 92 89 0 53 99 99 14 0 79 99 >99 >99 71 99

25 500 24 500 − 3800 3500 − 520 16 000 19 400 510 − 1900 29 300 29 300 36 800 1500 18 200

1.88 1.98 − 1.63 1.63 − 1.10 1.96 2.11 1.11 − 1.66 2.06 2.01 2.20 1.47 1.56

a Reactions were run at 100 °C for 24 h in solvent (1 mL) using 1a (0.50 mmol), 2 (0.50 mmol), Cs2CO3 (1.5 mmol), PivOH (0.5 mmol), Pd complex (1 mol %/Pd), and ligand (1 mol %) unless otherwise noted. bThe structures of L1−L4 are shown below. cEstimated by GPC calibration based on polystyrene standards (140 °C, o-C6H4Cl2). dReaction was run with 2 mol % of L1. eReaction was run at 120 °C.

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Pd 2 (dba) 3 ·CHCl 3 (entries 13 and 14). The complex PdCl2(MeCN)2 further improved the catalyst activity, yielding 3a with Mn = 36800 (entry 15). On the other hand, Herrmann catalyst24 provided an oligomeric product at 100 °C (entry 16). Although the catalytic activity was significantly improved at 120 °C, the molecular weight was still lower than that attained with PdCl2(MeCN)2 (entry 17). Substrate Scope. Next, four kinds of dihaloarenes (1b−1d and 1d′ in Scheme 1) were subjected to direct arylation polymerization with 2 under the catalytic conditions using PdCl2(MeCN)2 and L1. Table 2 summarizes the results. In

absorption at 611 nm, the value of which was almost identical to that reported for closely related alternating copolymers.25 The thermal properties were analyzed by TGA and DSC methods (Figures S2 and S3 and Table 3). TGA revealed that 3a, 3c, and 3d possess similar thermal stability with a 5% weight loss temperature (Td) over 400 °C. The thermal stability of 3b was somewhat lower (Td = 335 °C), but similar to that reported for alternating copolymers with almost identical constitution units.25b The DSC thermograms of 3b and 3c exhibited no obvious thermal transition between −50 and 300 °C,25a whereas those of 3a and 3d displayed melting and corresponding crystallization peaks. Melting and crystallization temperatures of 3a (Tm = 238 °C and Tc = 212 °C) were slightly higher than those of the same polymer previously prepared by direct arylation polymerization (Tm = 225 °C and Tc = 201 °C).18a Structure of 3a. Polymers 3a−d were also characterized by NMR spectroscopy. Figure 1a shows the 1H NMR spectrum of 3a formed by entry 1 in Table 1. Besides large peaks A−G arising from the main chain, small signals a−h are observed. Referring to the NMR spectra of model compounds (Figure S4), they were assigned to three terminal structures TA−TC. On the basis of peak integration of A (R: repeating unit), a

Table 2. Direct Arylation Polymerization of 1 and 2a entry dihaloarene 1 2 3 4 5 6 7 8

1b 1b 1c 1c 1d 1d 1d′ 1d′

polymer

L1 (mol %)

yield (%)

Mnb

Mw/Mnb

3b 3b 3c 3c 3d 3d 3d 3d

1 2 1 2 1 2 1 2

>99 >99 27 99 0 6 >99 >99

30 100 31 100 1900 68 200 − 1100 65 500 63 300

2.57 2.44 1.23 3.04 − 1.14 2.21 1.45

Reactions were run at 100 °C for 24 h in THF (1 mL) using 1 (0.50 mmol), 2 (0.50 mmol), Cs2CO3 (1.5 mmol), PivOH (0.5 mmol), PdCl2(MeCN)2 (1 mol %), and L1 (1 mol %). bEstimated by GPC calibration based on polystyrene standards (140 °C, o-C6H4Cl2). a

some instances, the polymerization was markedly affected by the amount of L1. 2,6-Dibromobenzodithiophene (1b) was successfully converted to 3b (Mn ≈ 30000) irrespective of the amount of L1 (entries 1 and 2). On the other hand, 2,5dibromo-3,4-dioxythiophene (1c) did not react with 2 in the presence of 1 mol % of L1 (entry 3), but provided the desired polymer (3c) with Mn = 68200 in the presence of 2 mol % of L1 (2 equiv/Pd) (entry 4). Although 1,4-dibromobenzene (1d) was poorly reactive, the desired polymer (3d) was obtained from 1,4-diiodobenzene (1d′) in almost quantitative yield (entries 5−8). Optical and Thermal Properties of 3. The optical properties of 3a−3d were examined by UV−vis spectroscopy in CHCl3 (Figure S1 and Table 3). The maximum absorption of 3a (λmax = 480 nm) was slightly red-shifted compared with the polymer previously prepared by direct arylation polymerization (λmax = 474 nm).18a Polymer 3b showed its maximum Table 3. Optical and Thermal Properties of 3 entry polymer 1 2 3 4

3a 3b 3c 3d

Mna

Mw/Mna

λmax (nm)b

Td (°C)c

36 800 31 100 68 200 65 500

2.20 2.44 3.04 2.21

480 611 574 476

429 335 407 400

Tm (°C)d

Tc (°C)d

238 e e 135, 241

212 e e 113, 229

a

Estimated by GPC calibration based on polystyrene standards (140 °C, o-C6H4Cl2). bObserved by UV−vis spectroscopy in CHCl3. c Decomposition temperature (5% weight loss) determined by TGA analysis under argon. dDetermined by second heating and cooling DSC traces with heating and cooling rates of 20 °C min−1 under argon. eNo obvious thermal transition was observed from −50 to +300 °C.

Figure 1. 1H NMR spectra of 3a in CDCl3 (a) before and (b) after end-group modification. Unidentified signals are marked with asterisks: see text. 628

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(TA), c (TB) and h (TC), the ratio of repeating and terminal units was estimated as follows [R: TA: TB: TC = 32.3: 0.7: 0.8: 0.5]. Given that the both termini have either of structures TA− TC, the molecular weight was calculated to be MnNMR = 25200, the value of which was in good agreement with the GPC molecular weight based on polystyrene standards (MnGPC = 25500). Further structural information was obtained from 3a′, prepared by end-capping of the Br terminus (TB) with 2 (Scheme 2). Thus, 3a formed under the catalytic conditions of

Table 4. Direct Arylation Polymerization of 1a and 2 in DMAa entry

ligand

yield (%)

Mnb

Mw/Mnb

1 2 3

none P(t-Bu)2Me PCy3

5 99 99

800 15 100 11 300

1.97 2.58 1.86

Reactions were run at 100 °C for 24 h in DMA (1.7 mL) using 1a (0.50 mmol), 2 (0.50 mmol), K2CO3 (1.25 mmol), PivOH (0.15 mmol), Pd(OAc)2 (2 mol %), and ligand (2 mol %). bEstimated by GPC calibration based on polystyrene standards (140 °C, o-C6H4Cl2). a

Scheme 2. End-Group Modification of 3a via Direct Arylation

for 24 h (entry 1). Although this “ligandless condition” has been shown to be effective for electron-rich thiophene derivatives,6−19 the present donor−acceptor combination of monomers (1a and 2) were not polymerized (entry 1). On the other hand, the polymerization proceeded in the presence of P(t-Bu)2Me and PCy3, and 3a with Mn = 15100 and 11300 were obtained, respectively (entries 2 and 3). In both cases, deposition of a solid of polymer was observed at the early stage of reaction (ca. 4 h). Figure 2 compares GPC curves of 3a formed by entry 15 in Table 1 and entries 2 and 3 in Table 3. The polymer formed in

Figure 2. GPC profiles of 3a formed with (a) PdCl2(MeCN)2/L1 in THF (entry 15 in Table 1), (b) Pd(OAc)2/P(t-Bu)2Me in DMA (entry 2 in Table 3), and (c) Pd(OAc)2/PCy3 in DMA (entry 3 in Table 3).

entry 1 in Table 1 was treated with 2 (0.5 mmol) in THF in the presence of Pd2(dba)3·CHCl3 (0.5 mol %) and L1 (1 mol %).26,27 Figure 1(b) shows the 1H NMR spectrum of the resulting polymer (Mn = 28100, Mw/Mn = 2.43). It is seen that the signals due to T B (b−d) are almost diminished [R:TA:TB:TC = 37.4:1.1:0.0:0.9]. The NMR molecular weight (MnNMR = 28900) was consistent with the GPC value. On the other hand, besides the signals of 3a′, broad and singlet signals are observed at δ 2.52 and 7.06, respectively. These signals are probably due to the occurrence of direct arylation at the 3 position of the 4,4′-dioctyl-2,2′-bithiophene-5,5′-diyl unit.8b On the basis of the peak intensity, the probability of this undesirable side reaction was estimated to be less than 2% of all bond formations in the polymerization process. Comparison with Other Catalytic Systems. The polymerization of 1a and 2 was examined in catalytic systems using DMA as the solvent.28,29 Table 4 lists the results. Compounds 1a and 2 (0.5 mmol each) were treated with Pd(OAc)2 (2 mol %), PivOH (0.15 mmol), and K2CO3 (2.5 mmol) in the absence of phosphine ligands in DMA at 100 °C

THF shows monomodal molecular weight distribution (chart a), whereas those obtained in DMA exhibit bimodal peak profiles with the major peak located in a low molecular weight region (chart b and c). Probably, the low solubility of 3a in DMA is the primary cause of the irregular distribution of molecular weight.

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CONCLUSIONS We have demonstrated that the catalyst precursors PdCl2(MeCN)2 and P(C6H4-o-OMe)3 (L1) in conjunction with PivOH and Cs2CO3 provide highly efficient polymerization systems to afford TPD-based DA polymers 3a−d with high molecular weight in almost quantitative yields. Although the synthesis of 3a via direct arylation was already achieved with Herrmann catalyst,18a the amount of palladium could be reduced to 1/8. The polymerization successfully proceeded in 629

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3b: A dark purple solid. 1H NMR (C2D2Cl4, 130 °C): δ 9.10−8.50 (br, 2H), 4.65−4.10 (br, 4H), 3.90−3.62 (br, 2H), 2.20−0.73 (br, 61H). 3c: A dark purple solid. 1H NMR (CDCl3, 25 °C): δ 4.13 (br, 4H), 3.56 (br, 2H), 2.00 (s, 1H), 1.34 (br, 8H), 1.28−1.25 (m, 44H), 0.89− 0.84 (m, 12H). 13C{1H} NMR (CDCl3, 25 °C): δ 163.6 (s), 148.5 (s), 134.6 (s), 129.5 (s), 116.0 (s), 78.6 (s), 44.5 (s), 43.7 (m), 37.2 (m), 32.6 (br), 32.5 (s), 32.4 (s), 32.1 (br), 31.2 (s), 30.7 (s), 30.4 (s), 30.2 (s), 30.2 (s), 32.4 (s), 30.0 (s), 29.9 (s), 27.0 (s), 23.6 (br), 23.3 (s), 23.2 (s), 14.7 (s). 3d: An orange solid. 1H NMR (CDCl3, 25 °C): δ 8.32 (br, 2H), 4.16 (br, 4H), 3.56 (br, 2H), 1.98−1.82 (m, 3H), 1.62−1.15 (m, 40H), 0.96−0.88 (m, 6H), 0.88−0.75 (m, 12H). 13C{1H} NMR (CDCl3, 25 °C): δ 149.6 (s), 141.6 (s), 130.6 (s), 121.8 (s), 115.4 (s), 72.7 (s), 39.0 (s), 31.9 (s), 31.9 (s), 31.7 (s), 30.4 (s), 30.1 (s), 29.7 (s), 29.6 (s), 29.3 (s), 28.9 (s), 26.5 (s), 26.4 (s), 23.8 (s), 23.1 (s), 22.7 (s), 14.1 (s), 14.1 (s), 10.9 (s). End-Group Modification. A 10 mL Schlenk tube equipped with a Teflon cock was charged with 1a (274 mg, 0.50 mmol), 2 (189 mg, 0.50 mmol), Cs2CO3 (489 mg, 1.5 mmol), PivOH (51.1 mg, 0.50 mmol), Pd2(dba)3·CHCl3 (2.6 mg, 2.5 μmol), L1 (1.8 mg, 5 μmol), and THF (1 mL). The mixture was stirred at room temperature for 0.5 h, and then at 100 °C for 24 h. A solution of 2 (189 mg, 0.50 mmol), Pd2(dba)3·CHCl3 (2.6 mg, 2.5 μmol) and, L1 (1.8 mg, 5 μmol) in THF (4 mL) was added, and the mixture was stirred at 100 °C for 24 h. The work up procedure is the same as above (383 mg, 100% yield, Mn = 28100, Mw/Mn = 2.43).

less polar solvents such as THF and toluene. A detailed structural analysis of 3a indicated the occurrence of a wellcontrolled polymerization process providing the polymer with monomodal distribution of molecular weight. In contrast, the polymerization conducted under conventional catalytic conditions using DMA formed 3a with bimodal distribution of molecular weight. The use of L1 was indispensable for the high catalytic activity in less polar solvents. Further application of this simple but highly efficient direct arylation polymerization system is in progress in this research group.

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EXPERIMENTAL SECTION

General Considerations. All manipulations were carried out under a nitrogen atmosphere using standard Schlenk techniques. Nitrogen gas was dried by passing through P 2 O 5 (Merck, SICAPENT). NMR spectra were recorded on a Bruker Avance 400 spectrometer (1H NMR 400.13 MHz and 13C NMR 100.62 MHz). Chemical shifts are reported in δ (ppm), referenced to 1H (residual) and 13C signals of deuterated solvents as internal standards. Analytical GPC was performed on a HLC 8120 GPC system with TSKgel GMHHR-H(S)HT (TOSOH). o-Dichlorobenzene was used as the mobile phase with a flow rate of 1.0 mL min−1 at 140 °C. The columns were calibrated against 9 standard polystyrene samples (Shodex; Mn = 1200−1410000). UV−vis absorption spectra were recorded on a JASCO V-560 spectrometer. Thermal gravimetric analyses (TGA) were carried out on a Shimadzu TGA-50 apparatus under a nitrogen atmosphere. After the sample was headed at 100 °C for 10 min, measurement was carried out with the scan rate of 10 °C min−1 in the temperature range 100−700 °C. The degradation temperature (Td) was reported as the temperature with 5% weight loss. Differential scanning calorimetry (DSC) was performed on a Perkin-Elmer DSC 7 instrument under an argon atmosphere. The instrument was calibrated with known standards: indium (Tm = 156.6 °C) and zinc (Tm = 419.5 °C). The sample was heated from 30 to 300 °C at the rate 50 °C min−1 and kept at 300 °C for 1 min and then cooled to −50 °C at the rate 50 °C min−1. After the sample was kept at −50 °C for 1 min, measurement was carried out with the scan rate of 20 °C min−1 in the temperature range −50 to +300 °C. Toluene and DMF (Kanto, dehydrated) were used as received. THF and CPME were dried over Na/Ph2CO, distilled, and stored over activated MS4A. DMA was dried over CaH2, distilled, and stored over activated MS4A. Cs2CO3 was dried overnight at 120 °C under vacuum and handled in a glovebox. Complexes 1a,30 1b,31 1c,32 2,33 Pd 2 (dba) 3 ·CHCl 3 , 3 4 PdCl 2 (MeCN) 2 , 3 5 CpPd(π-allyl), 3 6 PdMe 2 (tmeda), 37 Herrmann catalyst, 24 and P(C 6 H 4 -o-NMe 2 ) 3 (L4)38 were prepared according to the literature. The other chemicals were purchased from commercial sources and used without purification. Synthesis of 3. A typical procedure is reported for 3a. A 10 mL Schlenk tube equipped with a Teflon cock was charged with 1a (274 mg, 0.50 mmol), 2 (189 mg, 0.50 mmol), Cs2CO3 (489 mg, 1.5 mmol), PivOH (51.1 mg, 0.50 mmol), PdCl2(MeCN)2 (1.3 mg, 5 μmol), L1 (1.8 mg, 5 μmol), and THF (1 mL). The mixture was stirred at room temperature for 0.5 h, and then at 100 °C for 24 h. The mixture was cooled to room temperature, diluted with CHCl3 (100 mL) to dissolve the polymer deposited from the solution, and then washed with water (20 mL × 2). The solution was poured into vigorously stirred MeOH (400 mL). A dark purple precipitate of 3a was collected by membrane filter (0.5 μm), washed with MeOH, and dried under vacuum at room temperature overnight (381 mg, 100% yield, Mn = 36800, Mw/Mn = 2.20). The NMR data were in good agreement with those reported.17a 1H NMR (CDCl3, 25 °C): δ 7.16 (s, 2H), 3.53 (br, 2H), 2.80 (t, J = 6.0 Hz, 4H), 1.89 (br, 1H), 1.78−1.53 (m, 4H), 1.48−1.06 (m, 44H), 0.93−0.73 (m, 12H). 13C{1H} NMR (CDCl3, 25 °C): δ 162.6 (s), 145.6 (s), 138.3 (s), 136.2 (s), 130.3 (s), 127.4 (s), 125.0(s), 43.0 (s), 37.0 (s), 31.9 (s), 31.8 (s), 31.6 (s), 30.5 (s), 30.1 (s), 30.0 (s), 29.7 (s), 29.6 (s), 29.6 (s), 29.4 (s), 29.3 (s), 29.3 (s), 26.4 (s), 26.3 (s), 22.7 (s), 14.1 (s).

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ASSOCIATED CONTENT

S Supporting Information *

Experimental descriptions for end-group identification and analytical charts. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

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

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by KAKENHI (23350042, 24750088) from Japan Society for the Promotion of Science and the ACT-C program of Japan Science and Technology Agency. We are grateful to Profs. Y. Murata and A. Wakamiya (ICR, Kyoto University) for analytical GPC assistance, and Prof. Y. Tsujii (ICR, Kyoto University) for DSC and TGA assistance.

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REFERENCES

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dx.doi.org/10.1021/ma4023668 | Macromolecules 2014, 47, 626−631