Controlled Copolymerization of Acrylate and 6-Methyleneundecane by

Apr 2, 2012 - Eri Mishima†, Tomoki Tamura†, and Shigeru Yamago*†‡. † Institute for Chemical Research, Kyoto University, Uji 611-0011, Japan...
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Controlled Copolymerization of Acrylate and 6-Methyleneundecane by Organotellurium-Mediated Living Radical Polymerization (TERP) Eri Mishima,† Tomoki Tamura,† and Shigeru Yamago*,†,‡ †

Institute for Chemical Research, Kyoto University, Uji 611-0011, Japan CREST, Japan Science and Technology Agency, Tokyo 102-0076, Japan

‡

ABSTRACT: Copolymerization of acrylates, such as methyl acrylates (MAs) and trifluoroethyl acrylates (TFEAs), and 6methyleneundecane (6MU), under organotellurium-mediated living radical polymerization (TERP) conditions, was investigated. Structurally well-controlled copolymers, poly(MA-co6MU) and poly(TFEA-co-6MU), with predetermined numberaverage molecular weights (Mn = 3000−14000) and low polydispersity indices (PDI = 1.14 to 1.45) were obtained under thermal condition in the presence of an azo-initiator or photoirradiation. The addition of protic acids, such as 1,3C6H4[C(CF3)2OH]2 and (CF3)2CHOH, was effective to increase the insertion of 6MU into the copolymer. The structure of the ω-polymer-end group was analyzed by using labeling experiments, which revealed that the majority of the end groups were derived from 6MU. Although activation of dormant species derived from α-olefin is difficult, the copolymers were successfully reactivated and served as macro-chain-transfer agents for the synthesis of block copolymers. Poly(MA-co-6MU)-block-styrene and poly(MA-co-6MU)-block-(N-vinylpyrrolidone) with controlled structures were synthesized for the first time.

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INTRODUCTION Living radical polymerization (LRP) has become one of the most powerful polymerization methods for the synthesis of structurally well-controlled macromolecules with rich functionalities because LRP does not require stringent conditions like those required for living anionic, cationic, and coordination polymerizations.1 Extension of LRP from homopolymerization to copolymerization significantly increases the availability of polymers for macromolecular engineering because copolymerization can finely tailor the properties of polymer materials by changing the composition of the monomers. However, the controlled synthesis of copolymers by LRP has been limited to the combination of monomers having similar reactivities, and application of LRP for the controlled copolymerization of monomers having different reactivities, such as conjugated and nonconjugated monomers, for example, acrylates and α-olefins, has been limited. 2−12 For example, although random copolymerization of methyl acrylate (MA) and α-olefins has already been examined by using reversible addition−fragmentation chain transfer polymerization (RAFT),13−15 atomtransfer radical polymerization (ATRP),2,15−19 nitroxide-mediated radical polymerization (NMP),20 and iodine-mediated LRP,4 structurally well-controlled copolymers with low polydispersity indices (PDIs) were obtained only with low molecular weights and low α-olefin insertion ratios (mol fraction of α-olefin [MFalkene] < 0.26). When the targeted number-average molecular weight (Mn) was increased to greater than 10 000, PDI was greater than 1.5, which is a theoretical criterion for LRP. This is due to the accumulation of dormant species terminated with α-alkene monomer units, which less efficiently undergo regeneration to form polymer © 2012 American Chemical Society

end radicals than those terminated with acrylate monomers do.17,21 Furthermore, although the addition of Lewis acids, such as AlCl315 and Sc(OTf)3,22 and Brønsted acids, such as (CF3)2CHOH and (CF3)3COH,4 under RAFT and iodinemediated LRP conditions significantly increased MFalkene to 0.27 to 0.50, the PDI control decreased (>1.9) upon the addition of acids. This is probably due to the low compatibility of RAFT reagents and organoiodine compounds under acids, whereas acids increase electrophilicity of acrylate and the resulting polymer-end radical species upon coordination. Moreover, there is no report on the use of copolymers as macro-chain-transfer agents (CTAs) for the controlled synthesis of block copolymers. We have already developed organotellurium-, organostibine-, and organobismuthine-mediated LRPs (TERP, 2 3 − 2 9 SBRP,30−32 and BIRP,33,34 respectively), which mainly proceed by the degenerative chain transfer mechanism.35−37 One of the most characteristic features of these methods is their high versatility in polymerizing both conjugated and nonconjugated monomers by using the same CTA. This feature has been exemplified in the first controlled synthesis of a block copolymer of conjugated and nonconjugated monomers30,31,33,38 and controlled random and alternating copolymerization of (meth)acrylates and vinyl ethers.39 These results prompted us to investigate controlled random copolymerization of acrylate and α-alkene under TERP conditions with methyl 2-methyl-2-methyltellanylpropionate (1) as the CTA Received: February 19, 2012 Revised: March 15, 2012 Published: April 2, 2012 2989

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H), 1.42 (m, 4 H), 1.99 (t, 4 H), 4.69 (s, 2 H). 13C NMR (CDCl3): 14.30, 22.01, 27.70, 31.89, 36.24, 108.51, 150.70. Typical Procedure of Copolymerization under Thermal Conditions. Synthesis of Poly(MA-co-6MU). A solution of MA (0.26 g, 3.0 mmol), 6MU (0.50 g, 3.0 mmol), V-601 (4.6 mg, 0.020 mmol), and 1 (15.5 μL, 0.10 mmol) was heated to 60 °C for 12 h with stirring under a nitrogen atmosphere in a glovebox. A small portion of the reaction mixture was withdrawn and dissolved in CDCl3. The conversion of the monomer was determined by using 1H NMR

(Scheme 1). 6-Methyleneundecane (6MU) was used here because tertiary C−Te bond of the dormant species derived Scheme 1. Controlled Copolymerization of Acrylate and αOlefin under TERP

from 6MU is weaker and thus easier to regenerate polymer end radicals than secondary C−Te bond derived from monosubstituted α-alkene. We have recently reported that photoirradiation is effective for the activation of organotellurium dormant species possessing stable carbon−tellurium bonds by C−Te bond homolysis.40−43 Therefore, the effect of photoirradiation on the copolymerization was also examined.44 In addition, we report the synthesis of new block copolymers starting from random copolymer 2 as a macro-CTA.

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Figure 1. First-order kinetic plot of PMA-co-P(6MU).

EXPERIMENTAL SECTION

Materials. Unless otherwise noted, materials obtained from commercial suppliers were used without purification. Methyl acrylate (MA; >99%), 2,2,2-trifluoroethyl acrylate (TFEA; >98%), stylene (St; >99%), and N-vinylpyrrolidone (NVP; >99%) were washed with 5% aqueous sodium hydroxide solution and were distilled over calcium hydride under reduced pressure. Dimethyl 2,2′-azobis(2-methylpropionate) (V-601) was recrystallized from methanol. 1,1,1,3,3,3Hexafluoro-2-propanol (>99%) and 1,3-C6H4[C(CF3)2OH]2 (>97%) were distilled over CaH2 and deaerated by passing anhydrous nitrogen. Toluene and hexane were distilled over CaH2 and stored over molecular sieves. Methyl 2-methyl-2-methyltellanylpropionate (1) was prepared as reported.45 Measurements. 1H NMR (400 MHz) and 13C NMR (100 MHz) spectra were measured for a CDCl3 solution of a sample. 1H NMR spectra are reported in parts per million (δ) from internal tetramethylsilane or residual solvent peak and 13C NMR from solvent peak. MALDI-TOF mass spectra were recorded on a spectrometer equipped with a 337 nm N2 laser in the reflection mode and at 20 kV acceleration voltage. Samples were prepared from a tetrahydrofuran (THF) solution by mixing sample (1 mg/mL), dithranol (10 mg/mL), and sodium trifluoroacetate (1 mg/mL) in a ratio of 5:1:1. Gel permeation chromatography (GPC) was performed on a liquid chromatography equipped with two linearly connected polystyrene (PSt) mixed gel columns (Shodex LF-604), which were calibrated with poly(methyl methacrylate) (PMMA) standards using CHCl3 or DMF containing LiBr (0.1 mol/L) as an eluant. Preparative/recycling GPC was carried out with two linearly connected PSt mixed gel columns using CHCl3 as an eluent. Synthesis of 6MU. Sodium hydride (4.5 g, 60% dispersion in mineral oil; 0.36 mol) was placed on a three-neck, round-bottomed flask under a nitrogen atmosphere. The mineral oil was washed off with hexane (20 mL × 3) by using hypodermic syringe, and remaining hexane was removed under vacuum. Methyl triphenylphosphonium bromide (81.5 g, 0.23 mol) and 300 mL of toluene were added, and the mixture was heated to 60 °C for 1.5 h. 6-Undecanone (37.5 mL, 0.18 mol) was added, and the mixture was heated to this temperature for 15 h. The reaction mixture was poured into a 300 mL ice bath, and organic phase was extracted twice with hexane. The combined organic layers were dried over MgSO4, filtered, and evapolated. The residue was purified by column chromatography by using hexane as an eluant, followed by distillation (bp = 85 °C/10.4 mmHg) to give 22.9 g (74%) of 6MU as an oil. 1H NMR (CDCl3): 0.89 (t, 6 H), 1.26 (m, 8

spectroscopy (Figure 1). Conversion of MA and 6MU reached to 89 and 23%, respectively, after 12 h. PhSH (12.0 μL, 0.13 mmol) was added to the reaction mixture, and the resulting solution was stirred at 80 °C for 2 h.46 The mixture was concentrated under reduced pressure to give the crude product (0.64 g). The MF6MU = 0.22 was determined by using 1H NMR spectrometry. The Mn = 3400 and the PDI = 1.26 were determined by GPC calibrated against PMMA standards. Typical Procedure for Copolymerization under Photoirradiation Conditions. Synthesis of Poly(MA-co-6MU). A solution of MA (0.26 g, 3.0 mmol), 6MU (0.50 g, 3.0 mmol), 1,3C6H4[C(CF3)2OH]2 (1.2 g, 3.0 mmol), and 1 (15.5 μL, 0.10 mmol) was irradiated by 500 W high-pressure mercury lamp through a > 470 nm cutoff filter at 60 °C for 6 h. A small portion of the reaction mixture was withdrawn and dissolved in CDCl3. The conversion of the monomer (MA: 93%, 6MU: 46%) was determined by using 1H NMR spectroscopy. PhSH (12.0 μL, 0.13 mmol) was added to the reaction mixture, and the resulting solution was stirred at 80 °C for 2 h. The mixture was concentrated under reduced pressure to give the crude product (0.47 g). The MF6MU = 0.32 was determined by using 1H NMR spectrometry. The Mn = 5100 and the PDI = 1.24 were determined by GPC calibrated against PMMA standards. Synthesis of Poly[(MA-co-6MU)-block-St]. A solution of 1 (15.5 μL, 0.10 mmol), MA (0.26 g, 3.0 mmol), 6MU (0.50 g, 3.0 mmol), and 1,1,1,3,3,3-hexafluoro-2-propanol (0.49 g, 2.9 mmol) was irradiated by 500 W high-pressure mercury lamp through a >520 nm cutoff filter at 20 °C for 24 h to give PMA-co-P(6MU) with Mn = 5000 and PDI = 1.18. Conversion of MA (95%) and the MF6MU = 0.31 was determined by 1H NMR spectroscopy. St (3.1 g, 30 mmol) and V601 (4.8 mg, 0.021 mmol) were added, and the resulting solution was heated to 60 °C for 40 h. The monomer was removed under reduced pressure to give the block copolymer (2.7 g, 72% conversion of St) with Mn = 21 900 and PDI = 1.41. Synthesis of Poly[(MA-co-6MU)-block-NVP]. A solution of 1 (15.5 μL, 0.10 mmol), MA (0.26 g, 3.0 mmol), 6MU (0.51 g, 3.0 mmol), and 1,1,1,3,3,3-hexafluoro-2-propanol (0.51 g, 3.0 mmol) was irradiated by 60 W black lamp at 20 °C for 60 h to give PMA-coP(6MU) with Mn = 4000 and PDI = 1.22. Conversion of MA (96%) and the MF6MU = 0.31 was determined by 1H NMR spectroscopy. NVP (2.2 g, 20 mmol) was added, and the resulting solution was irradiated by 500 W high-pressure mercury lamp through a filter (>580 nm) at 60 °C for 110 h. The monomer was removed under reduced 2990

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pressure to give the block copolymer (3.6 g, 82% conversion of NVP) with Mn = 31 000 and PDI = 1.22. Synthesis of End-Deuterated and Protonated Poly(TFEA-co6MU). A solution of 1 (7.7 μL, 0.050 mmol), TFEA (0.12 g, 0.75 mmol), 6MU (1.3 g, 7.5 mmol), and 1,1,1,3,3,3-hexafluoro-2-propanol (0.13 g, 0.75 mmol) was irradiated by 60 W black lamp at 60 °C for 10 h. A small portion of the reaction mixture was withdrawn and dissolved in CDCl3. The conversion of monomer (TFEA: 80%, 6MU: 8%) was determined by 1H NMR spectroscopy. Volatile materials were removed under vacuum, and tributyltin deuteride (34 μL, 0.13 mmol) and V-601 (2.5 mg, 0.011 mmol) were added. The resulting solution was stirred at 80 °C for 2 h. Volatile materials were removed under reduced pressure, and the crude polymer was purified by preparative GPC to give a product (0.21 g) with Mn = 4800 and PDI = 1.24. The MF6MU = 0.46 determined by using 1H NMR spectroscopy was identical to the MF6MU = 0.47 determined by MALDI TOF MS spectra (Figure 2).

conversion of MA and 6MU followed first-order kinetics (Figure 1) and reached 89 and 22%, respectively, after 12 h. GPC analysis indicated that structurally well-controlled copolymer 2a (R = Me) with Mn = 3400 and PDI = 1.26 was formed. The mole fraction of 6MU (MF6MU) in the copolymer was determined to be 0.22 by using 1H NMR analysis after isolation, and the ratio is identical to the one estimated from the monomer conversion. We next examined the effects of acids because they increase MF alkene in the copolymerization of acrylates and αalkenes.47−49 TERP was compatible with various inorganic Lewis acids, such as Mg(OTf)2, Sm(OTf)3, Yb(OTf)3, Lu(OTf)3, and TiOiPr4, and copolymerization afforded structurally well-controlled copolymers with Mn values close to the theoretical values calibrated by PMMA standards (Mn = 2800−4100) and low PDIs (PDI = 1.18 to 1.31). However, no significant effects on MF6MU were observed (MF6MU = 0.17to 0.24). This is partially because these Lewis acids coordinate strongly with MA and the copolymers, causing the resulting complexes to phase-separate from the reaction mixture. Next, organic and liquid Brønsted acids 3, 4, and 5 (Chart 1), which have recently been used by Kamigaito and coworkers to increase MFalkene in the iodine-mediated copolymerization of acrylate and α-alkene,4 were examined. Copolymerization in the presence of 3 under otherwise identical conditions proceeded in a controlled manner giving a copolymer with Mn close to the theoretical value (Mn = 4400) and a low PDI of 1.17 (run 2), and MF6MU slightly increased to 0.28. MF6MU further increased to 0.32 and 0.42, when the amount of 3 (30 equiv to 1) was increased and the use of an excess amount of 6MU over that of MA (runs 3 and 4, respectively) while keeping the low PDIs. A longer reaction period was required as the increase in MF6MU probably due to the low efficiency of regeneration of polymer end radicals from the dormant species derived from 6MU. Acids 4 and 5 were also effective for increasing MF6MU, and the copolymerization gave the corresponding copolymers in a controlled manner (Mn values = 4800−5600, PDIs = 1.17 to 1.18) with a high MF6MU of 0.32 to 0.34 (runs 5 and 6). Other Brønsted acids, such as phenol and phosphoric acid, did not affect MF6MU, although the copolymerization proceeded in a controlled manner. We next examined the copolymerization under photoirradiation, which is effective for the activation of organotellurium dormant species having a stable carbon−tellurium bond.40−43 The copolymerization under photoirradiation with a 500 W high-pressure mercury lamp through a >480 nm cutoff filter or 60 W black lamp proceeded smoothly at 60 °C, and controlled copolymers with Mn close to the theoretical values, narrow PDIs, and high MF6MU (0.30 to 0.32) were obtained in the presence of acid 3 or 5 (runs 7 and 11). The copolymerization in the presence of excess amount of 6MU over that of MA also gave corresponding copolymer with a better MF6MU of 0.38 (run 8). Polymerization proceeded at low temperature, such as 20 °C, under photoirradiation because the generation of radicals from the dormant species does not require heating (run 9). A structurally well-controlled copolymer with a high MF6MU was obtained in the presence of 5. A copolymer with a high Mn of 13 100 and a low PDI of 1.19 was obtained by increasing the amount of MA and 6MU over that of 1 (run 10), and the result is consistent with the living character of TERP. TFEA was next employed as an acrylate monomer because the high electrophilic character of the TFEA-polymer end

Figure 2. (a) Full and (b) partial MALDI TOF MS spectra of the end deuterated copolymer.

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RESULTS AND DISCUSSION

MA (R = CH3, MA, 30 equiv) and 6MU (30 equiv) were polymerized in the presence of 1 and dimethyl 2,2′-azobis(2methylpropionate) (V-601, 0.2 equiv) at 60 °C for 12 h. The Chart 1. Structure of Brønsted Acids Used in This Study

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Table 1. Random Copolymerization of MA and 6MU in the Presence of 1 and Acida run

MA/6MU (equiv)

additive (equiv)

conditionb

time (h)

conv. of MA (%)c

Mn(theo)

Mn(exp)d

PDId

MF6MUe

1 2 3 4 5 6 7 8 9f 10e 11

30/30 30/30 30/30 30/150 30/30 30/30 30/30 30/150 30/30 100/100 30/30

none 3 (1) 3 (30) 3 (30) 4 (30) 5 (30) 3 (30) 3 (30) 5 (30) 5 (100) 5 (30)

A A A A A A B B B B C

12 24 8 26 6 6 6 13 18 83 32

89 90 93 100 89 92 93 95 92 83 93

3700 4100 4600 6200 4500 4600 5000 5400 4800 11700 4400

3400 4400 5700 5500 5600 4800 5100 3900 4700 13100 2800

1.26 1.17 1.14 1.32 1.17 1.18 1.24 1.38 1.20 1.19 1.32

0.22 0.28 0.32 0.42 0.33 0.32 0.32 0.38 0.34 0.28 0.30

a 1 (1.0 equiv), MA, and 6MU in the absence of presence of acids 3−5 were stirred at 60 °C. bA: V-601 (0.2 equiv), B: hν (500 W high-pressure Hg lamp through a >470 nm cutoff filter), C: hν (60 W black lamp). cDetermined by using 1H NMR spectroscopy. dMn and PDI determined by GPC calibrated against PMMA standards. eMole fraction of 6MU in the copolymer determined by using 1H NMR spectroscopy. fPolymerization was carried out at 20 °C.

Table 2. Photoinduced TERP of TFEA and 6MU Using 1a run 1 2 3 4e 5e 6e

monomer (equiv) TFEA/6MU TFEA/6MU TFEA/6MU TFEA/6MU TFEA/6MU TFEA/6MU

(30/30) (30/30) (30/150) (30/30) (100/100) (200/200)

additive (equiv)

time (h)

conv. (%)b

Mn(theo)

Mn(exp)c

PDIc

MF6MUd

none 5 (30) 5 (30) 5 (30) 5 (100) 5 (200)

20 24 24 66 220 384

92 98 94 75 80 81

6300 7800 7900 5300 19800 35300

4600 6400 5300 2300 10100 14200

1.18 1.15 1.19 1.15 1.07 1.14

0.31 0.40 0.49 0.35 0.35 0.33

a 1 (1.0 equiv), TFEA, and 6MU with or without 5 were stirred under irradiation with a 60 W black lamp at 60 °C. bConversion of acrylate determined by using 1H NMR spectroscopy. cMn and PDI obtained by GPC calibrated against PMMA standards. dMole fraction of 6MU in the copolymer determined by using 1H NMR spectroscopy. ePolymerization was carried out at 20 °C.

Scheme 2. Synthesis of Block Copolymers from Copolymer 2a

Figure 3. 2H NMR spectra of 2b-D and 2b′-D.

radical should increase the insertion of 6MU.50 When an equimolar amount of TFEA and 6MU (30 equiv each) was polymerized in the presence of 1 without acid under photoirradiation at 60 °C, the polymerization followed firstorder kinetics for both TFEA and 6MU and reached 92 and 46% conversion, respectively, after 20 h. The structurally wellcontrolled copolymer 2b (R = CH2CF3) with Mn = 4600 and PDI = 1.18 was obtained (Table 2, run 1). The value of MF6MU in 2b was 0.31, which is considerably higher than that in 2a (Table 1, run 1). The addition of acid 5, giving the corresponding controlled copolymer with an MF6MU of 0.40 (run 2), was also effective for increasing MF6MU. MF6MU further increased to 0.49 when the 6MU/TFEA ratio was greater than five in the presence of 5, suggesting that nearly alternating copolymerization occurred (run 3). MF6MU of this copolymer was estimated to be 0.47 by using MALDI TOF mass spectroscopy by comparing the peak intensities (Figure 2),

a

Reaction conditions: (a) 2b (Mn = 3800, PDI = 1.24, MF6MU = 0.44), TFEA (85 equiv), V-601 (0.2 equiv), 60 °C, 10 h, 98% conv. of TFEA. (b) 2a (Mn = 5000, PDI = 1.18, MF6MU = 0.31), St (300 equiv), V-601 (0.2 equiv), 60 °C, 40 h, 72% conv. of St. (c) 2a (Mn = 4000, PDI = 1.22, MF6MU = 0.31), NVP (200 equiv), 500 W high-pressure Hg lamp through a >580 nm cutoff filter, 60 °C, 110 h, 82% conv. of NVP.

and the value was almost identical to the one obtained by using H NMR spectroscopy. High-molecular-weight copolymers (Mn > 10 000) with low PDIs (