Article pubs.acs.org/Macromolecules
Controlled Copolymerization of 1‑Octene and (Meth)acrylates via Organotellurium-Mediated Living Radical Polymerization (TERP) Eri Mishima,† Tomoki Tamura,† and Shigeru Yamago*,†,‡ †
Institute for Chemical Research, Kyoto University, and ‡CREST, Japan Science and Technology Agency, Uji 611-0011, Japan S Supporting Information *
ABSTRACT: Copolymerization of 1-octene and (meth)acrylates, such as methyl acrylate, trifluoroethyl acrylate (TFEA), methyl methacrylate, and trifluoroethyl methacrylate, under organotellurium-mediated living radical polymerization (TERP) conditions was investigated. Polymerization under thermal conditions gave copolymers with considerably broad molecular distributions (polydispersity index [PDI] > 1.45), whereas that under photoirradiation greatly increased the PDI control. Structurally well-controlled copolymers with number-average molecular weights (Mn) of 3000−18 000 and low PDIs (1.22−1.45) were obtained. Addition of Brønsted acids, such as 1,3-C6H4[C(CF3)2OH]2 and hexafluoroisopropanol, increased the insertion of 1-octene into the copolymer. The molar fraction of 1-octene (MFoct) reached ∼0.5 in the copolymerization using TFEA as an acrylate monomer and excess amount of 1-octene in the presence of the acid. The copolymer was used as a macro-chain-transfer agent for the synthesis of block copolymers. This is the first example of the use of this type of copolymer as a macro-chain-transfer agent in the controlled synthesis of block copolymers.
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accumulation of dormant species terminated by α-alkene monomers, the reactivity of which in regenerating the polymer-end radicals is much less than those terminated by acrylate monomer.19,21 Furthermore, although the addition of Lewis acids, such as AlCl314 and Sc(OTf)3,22 and a Brønsted acid7 under RAFT and IRP conditions significantly increased MFalkene (0.27−0.50), PDI control decreased (>1.9). To the best of our knowledge, there is no report on the use of the copolymers as macro-chain-transfer agents 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).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 chain transfer agent. For example, the controlled synthesis of block copolymers from conjugated and nonconjugated monomers, such as N-vinylpyrrolidone,30,31,34,38 and controlled random/alternating copolymerization of (meth)acrylates and vinyl ethers39,40 have been reported. We have recently reported the controlled copolymerization of acrylates and α,α-disubstituted olefins and the use of the resulting copolymers in the synthesis of block copolymers.41 Since α-monosubstituted olefins (α-olefins) generate less stable polymer-end radicals than α,α-disubstituted olefins do (see Results and Discussion), the controlled
INTRODUCTION Precision synthesis of random copolymers has attracted a great deal of attention because the copolymer properties can be finely tuned by changing the type of monomer, functional group(s) on the monomers, and their compositions. The use of living radical polymerization (LRP) is particularly attractive because it enables the controlled polymerization of a wide range of vinyl monomers possessing polar functionalities, which cannot be compatible under anionic, cationic, and coordination polymerization conditions.1 There are many examples of copolymerization under LRP conditions by employing monomers with similar reactivities, which generate polymer-end radicals with similar stabilities, such as styrenes or (meth)acrylates. However, copolymerization using monomers with different reactivities, which generate polymer-end radicals with different stabilities, such as conjugated and nonconjugated monomers (e.g., (meth)acrylates and α-alkenes), has thus far been limited.2−12 Random copolymerization of acrylate and α-olefins under reversible addition−fragmentation chain transfer polymerization (RAFT),13−15 atom-transfer radical polymerization (ATRP),8,14,16−19 nitroxide-mediated radical polymerization (NMP),20 and iodine-mediated LRP (IRP)7 conditions has already been examined. However, structurally well-controlled copolymers with low polydispersity indexes (PDIs) were obtained only when low degree of polymerization and low αalkenes insertion ratios (molar fraction of α-alkene [MFalkene] < 0.26). When the degree of polymerization and thus the molecular weight increased (Mn > 10 000, where Mn refers to number-average molecular weight), PDI exceeded 1.5, which is the theoretical criterion for LRP. This is due to the © 2012 American Chemical Society
Received: July 26, 2012 Revised: November 1, 2012 Published: November 7, 2012 8998
dx.doi.org/10.1021/ma301570r | Macromolecules 2012, 45, 8998−9003
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copolymerization of α-olefin and (meth)acrylates is more challenging than that involving α,α-disubstituted olefins. Here we report the controlled copolymerization of an α-olefin and (meth)acrylates by using organotellurium chain transfer agent 1. 1-Octene was used as an α-olefin throughout this study because of its ease of handling (Scheme 1). In addition, we report the synthesis of a block copolymer starting from copolymer 2 as a macro-chain-transfer agent.
Typical Procedure for Copolymerization under Photoirradiation Conditions. Synthesis of Poly[MA-co-(1-octene)]. A solution of MA (0.258 g, 3.0 mmol), 1-octene (0.337 g, 3.0 mmol), 3 (1.23 g, 3.0 mmol), and 1 (15.5 μL, 0.10 mmol) was irradiated with a 500 W high-pressure mercury lamp through a >470 nm cutoff filter at 60 °C for 20 h. A small portion of the reaction mixture was withdrawn and dissolved in CDCl3. The conversion of the monomer (MA: 88%; 1-octene: 42%) was determined by using 1H NMR spectroscopy. The methyltellanyl group at the ω-polymer end was reduced by treatment with benzenethiol (12 μL, 0.13 mmol) at 80 °C for 2 h. The reaction mixture was concentrated under reduced pressure to give the product (0.4 g). Mn was 3200 and PDI was 1.45, determined by using GPC. MFoct (0.27) was determined by using 1H NMR spectroscopy after purification of the copolymer by using preparative GPC. Synthesis of ω-End Deuterated Poly[TFEA-co-(1-octene)]. A solution of 1 (15.5 μL, 0.10 mmol), TFEA (0.23 g, 1.5 mmol), 1octene (1.7 g, 15 mmol), and HFIP (0.25 g, 1.5 mmol) was irradiated with a 60 W black lamp at 60 °C for 43 h. A small portion of the reaction mixture was taken and dissolved in CDCl3. The conversion of the monomer (TFEA: 78%; 1-octene: 6%) was determined by using 1 H NMR spectroscopy. Unreacted monomers and HFIP were removed under vacuum, and the reaction vessel was flushed with nitrogen gas in a glovebox. Trifluorobenzene (1 mL) was added, and to the resulting homogeneous solution was added tributyltin deuteride (68 μL, 0.25 mmol) and V-601 (2.3 mg, 0.011 mmol). The solution was stirred at 80 °C for 2 h, and the volatile materials were removed under vacuum. From GPC analysis, Mn was 2800 and PDI was 1.21. Purification of the crude material by using preparative GPC afforded the product (0.12 g). MFoct (0.45) was determined by using 1H NMR spectroscopy. The polymer showed two broad singlets at 1.2 and 2.4 ppm in 2H NMR spectra measured in CHCl3 (Figure 2).
Scheme 1. Copolymerization of (Meth)acrylates and 1Octene under TERP Conditions
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EXPERIMENTAL SECTION
General. All reactions dealing with air- and moisture-sensitive compounds were carried out in a dry reaction vessel under a nitrogen atmosphere. 1H NMR (400 MHz) spectra were acquired using CDCl3 or CD2Cl2 as the solvent and are reported in ppm (δ) relative to the internal reference tetramethylsilane. Gel permeation chromatography (GPC) was performed on a machine equipped with two linearly connected polystyrene (PSt) mixed gel columns (Shodex LF-604) at 40 °C using UV and RI detectors. CHCl3 or DMF containing LiBr (0.1 mol/L) was used as an eluent. Poly(methyl methacrylate) (PMMA) was used as a standard. A 500 W high-pressure mercury lamp equipped with a >470 nm cutoff filter (Asahi Techno Glass), or a 60 W black lamp, was used as a light source for photopolymerization. Theoretical calculations were performed using the Gaussian 03 software. The geometry optimization was carried out using the B3LYP functional with the LANL2DZ(d,p) basis set for the Te atom and the 6-31G(d) basis set for the other atoms. Materials. Unless otherwise noted, materials obtained from commercial suppliers were used without purification. 1-Octene (>95%) was distilled over calcium hydride. Methyl acrylate (MA; >99%), 2,2,2-trifluoroethyl acrylate (TFEA; >98%), methyl methacrylate (MMA; >99%), and 2,2,2-trifluoroethyl methacrylate (TFEMA; >98%) were washed with a 5% aqueous sodium hydroxide solution and then distilled over calcium hydride under reduced pressure. Dimethyl 2,2′-azobis(2-methylpropionate) (V-601) was recrystallized from methanol. Trifluorotoluene was distilled over calcium hydride and stored over molecular sieves 4A. 1,3-C6H4[C(CF3)2OH]2 (3; >97%) and hexafluoroisopropanol (HFIP, >99%) were distilled over calcium hydride and deoxygenated by bubbling dry nitrogen. Methyl 2-methyl-2-methyltellanylpropionate (1) was prepared as reported.42 Typical Procedure for the Copolymerization under Thermal Conditions. Synthesis of Poly[MA-co-(1-octene)]. A solution of MA (0.258 g, 3.0 mmol), 1-octene (0.337 g, 3.0 mmol), V-601 (4.4 mg, 0.019 mmol), and 1 (15.5 μL, 0.10 mmol) was heated at 60 °C for 20 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 (MA: 91%; 1-octene: 18%) was determined by using 1H NMR spectroscopy. The methyltellanyl group at the ω-polymer end was reduced by treatment with benzenethiol (12 μL, 0.13 mmol) at 80 °C for 2 h.43 The reaction mixture was concentrated under reduced pressure to give the product (0.3 g). The number-averaged molecular weight (Mn = 3700) and polydispersity index (PDI = 1.47) were determined by using GPC. The MFoct = 0.19 was determined by using 1H NMR spectroscopy after purification of the copolymer by using preparative GPC.
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RESULTS AND DISCUSSION Estimation of the Stability of Polymer-End Radicals. The stability of the polymer-end radicals was estimated from the bond dissociation energies (BDE)44 of polymer-end mimetic organotellurium compounds 1 and 4−7 by using density functional calculations at the B3LYP/6-31G* (C, O, H) and LANL2DZ (Te) level of theory (Table 1). PMMA-mimetic Table 1. Bond Dissociation Energy (kJ/mol) of Organotellurium Compoundsa
a Obtained by the DFT calculation at B3LYP/6-31G*(C,H,O) + LANL2DZ(Te). bData taken from ref 23.
1, which was used as a chain-transfer agent in this study, had the lowest BDE (113 kJ/mol) among the compounds shown in Table 1. The BDE of poly(methyl acrylate)-mimetic 4 (142 kJ/ mol) was much higher than that of 1 because of the lack of the radical-stabilizing methyl group on the α-carbon. However, the value was much lower than those of 5−8, which mimic poly(α,α-dialkyl olefin), poly(vinyl acetate), poly(α-olefin), and polyethylene polymer-end structures, respectively, because ester is a powerful radical stabilizing group. We have already reported that 6 is effectively activated under thermal polymer8999
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Table 2. Copolymerization of Acrylate and 1-Octene under TERPa run
acrylate monomer
additive (equiv)
condition (°C/h)
conv (%)b
Mn (theo)
Mn (exp)c
PDIc
MFoctd
1 2 3 4 5 6 7e 8 9f 10 11 12e 13g
MA MA MA MA MA MA MA MA MA TFEA TFEA TFEA TFEA
none 3 (30) HFIP (30) 3 (30) HFIP (30) HFIP (30) 3 (30) HFIP (30) HFIP (100) none HFIP (30) HFIP (30) HFIP (30)
A (60/20) A (60/69) A (60/9) B (60/20) B (60/22) C (60/36) B (60/33) B (20/24) B (20/83) C (60/24) C (60/40) C (60/60) C (60/36)
91 100 94 88 75 78 58 79 73 81 70 97 73
3200 3900 3500 3900 2500 3200 4900 2500 8200 4600 4300 5000 5500
3700 3000 4900 3200 2900 3100 3500 2800 9000 2700 3900 3500 2200
1.47 2.06 1.62 1.45 1.34 1.45 1.39 1.39 1.31 1.21 1.22 1.37 1.28
0.19 0.28 0.26 0.27 0.24 0.24 0.35 0.16 0.19 0.24 0.32 0.43 0.47
1 (1.0 equiv), acrylate monomer, and 1-octene with or without fluoroalcohol were stirred under a nitrogen atmosphere. Conditions. A: thermal polymerization by the addition of V-601 (0.2 equiv); B: photochemical polymerization by irradiation with a 500 W high-pressure mercury lamp through a >470 nm cutoff filter in a pyrex tube; C: photochemical polymerization by irradiation with a 60 W black lamp. bConversion of acrylate determined by 1H NMR. cNumber-average molecular weight (Mn) and polydispersity index (PDI) obtained by GPC calibrated against PMMA standards. dMolar fraction of 1-octene determined by 1H NMR. e150 equiv of 1-octene was used. f100 equiv of MA was used. g300 equiv of 1-octene was used. a
out in the presence of acids 3 or HFIP, MFoct increased to 0.26−0.28, as expected (runs 2 and 3). However, the resulting copolymers were not well controlled (PDI > 1.6). The decreased PDI control is probably due to the increase of ωpolymer-end group terminated by a 1-octene unit, which is less reactive toward regeneration of the polymer end radical than that terminated by a MA unit. The progress of the polymerization under the condition in run 3 was monitored by 1H NMR. The monomer conversion followed the first-order kinetics in the first ∼2 h (∼75% and ∼30% conversion of MA and 1-octene, respectively), but it started to deviate from the first order upon further monomer conversion (Figure 1a). The results are due to the accumulation of the dormant species terminated by a 1-octene unit, as the progress of the polymerization (see below). Despite the rate retardation, the Mn of the copolymer increased linearly with the total monomer conversion (Figure 1b), suggesting the living character. The MFoct was also insensitive to the monomer conversion within an experimental error (Figure 1b; see also Figure S1 in the Supporting Information). We have recently found that photoirradiation is highly effective for the activation of organotellurium dormant species.46−48 Therefore, the copolymerization was carried out under irradiation with a 500 W high-pressure Hg lamp through a >470 nm cutoff filter (condition B) or a 60 W black lamp (condition C) under otherwise identical conditions (runs 4−6). The conversion of MA was greater than 75%, and copolymers with Mns of 2900−3200 and low PDIs of 1.34−1.45 formed while maintaining MFoct = 0.24−0.27. A copolymer with a high MFoct (0.35) and good PDI control (1.39) was prepared by increasing the amount of 1-octene over MA (run 7). The increase of PDI control must be due to effective photoactivation of the 1-octene-derived dormant species. MFoct decreased when the polymerization was carried out at 20 °C, but the level of PDI control was insensitive to the temperature (run 8). A copolymer with a high Mn and low PDI was prepared by increasing the amount of monomers (run 9). The similar kinetics behavior was observed under photoirradiation to that under thermal conditions; the monomer conversion followed the first-order kinetics at the early stage of
ization conditions, generating the corresponding radical species, whereas a dormant species with a primary alkyl−Te bond, e.g., 8, is not.45 Furthermore, in the controlled copolymerization of α,α-dialkyl olefins and (meth)acrylates, a tert-alkyl−Te bond, like that in 5, is sufficiently activated under thermal conditions.41 Therefore, it is of great interest whether the secondary alkyl−Te bond, such as that in 7, which had a higher BDE than that of 4 but lower than that of 8, is efficiently activated during the copolymerization involving α-olefins. Copolymerization of 1-Octene and Acrylate. 1-Octene (30 equiv) and MA (R1 = CH3, R2 = H, 30 equiv) were polymerized under thermal conditions at 60 °C (condition A) in the presence of 1 and V-601 (0.2 equiv) for 20 h. The copolymerization took place much more slowly than the homopolymerization of acrylates did. The conversion of MA and 1-octene reached 91 and 18%, respectively, after 20 h. Copolymer 2a formed with Mn close to the theoretical value (Mn(theo) = 3200 and Mn(exp) = 3700) and moderate PDI control (1.47), which was determined by using GPC analysis (Table 2, run 1). The MFoct of 0.19, determined by 1H NMR analysis after purification of the copolymer by using preparative GPC, is basically the same as that estimated from the monomer conversion. The rate retardation is probably due to the slower activation of the dormant species terminated by a 1-octene unit compared to that of the one terminated by an MA unit. The effects of alcohols 3 and HFIP were examined next (Chart 1) because Kamigaito and co-workers reported that these fluorinated alcohols serve as Brønsted acids and increase MFalkene in IRP of α-alkene and acrylate.7 We have also shown that these acids efficiently increase MFalkene in the copolymerization of α,α-dialkyl olefin and (meth)acrylates under TERP conditions.41 When the copolymerization was carried Chart 1. Structure of Fluorinated Alcohols Used as a Brønsted Acid
9000
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Figure 1. (a) First-order plot of monomer conversion vs time (◆: MA; ●: 1-octene; : approximated lines obtained from the data for the first 2 h) and (b) correlation between Mn (▲) or MFoct (■) and the total monomer conversion in the bulk copolymerization in the presence of HFIP at 60 °C.
Table 3. Copolymerization of Methacrylate and 1-Octene under TERPa run
methacrylate monomer
1 2 3e 4 5 6e 7f 8g
MMA MMA MMA TFEMA TFEMA TFEMA TFEMA TFEMA
additive (equiv) none HFIP HFIP none HFIP HFIP HFIP HFIP
(30) (30) (30) (30) (100) (200)
condition (°C/h) hν hν hν hν hν hν hν hν
(60/10) (60/12) (60/48) (60/36) (60/48) (60/60) (60/48) (60/110)
conv (%)b
Mn (theo)
Mn (exp)c
PDIc
MFoctd
93 88 72 98 88 73 72 76
3500 3800 6800 6100 5700 4900 12900 27100
4600 4200 2800 4100 3900 3100 11100 18000
1.61 1.43 1.45 1.63 1.48 1.41 1.36 1.40
0.06 0.07 0.22 0.08 0.13 0.32 0.10 0.08
a 1 (1.0 equiv), methacrylate monomer, and 1-octene with or without fluoroalcohol were stirred under a nitrogen atmosphere with photoirradiation with a 60 W black lamp. bConversion of methacrylate determined by 1H NMR. cNumber-average molecular weight (Mn) and polydispersity index (PDI) obtained by GPC calibrated against PMMA standards. dMolar fraction of 1-octene determined by 1H NMR. e300 equiv of 1-octene was used. f 100 equiv of TFEMA was used. g200 equiv of TFAMA was used.
copolymers with high MFoct (0.22−0.32) and low PDIs (
