Article pubs.acs.org/Macromolecules
Controlled Radical Polymerization of 3‑Methylenecyclopentene with N‑Substituted Maleimides To Yield Highly Alternating and Regiospecific Copolymers Daisuke Yamamoto† and Akikazu Matsumoto*,‡ †
Department of Applied Chemistry and Bioengineering, Graduate School of Engineering, Osaka City University, 3-3-138 Sugimoto, Sumiyoshi-ku, Osaka 558-8585, Japan ‡ Department of Applied Chemistry, Graduate School of Engineering, Osaka Prefecture University, 1-1 Gakuen-cho, Naka-ku, Sakai-shi, Osaka 599-8531, Japan S Supporting Information *
ABSTRACT: High-molecular-weight diene copolymers with a regiospecific repeating structure were produced in a high yield during the alternating radical copolymerization of N-substituted maleimides (RMIs) and 3-methylenecyclopentene (MCP) as the cyclic 1,3-diene monomer including a reactive exomethylene moiety. The eminent copolymerization reactivity of MCP was in contrast to the predominant occurrence of the Diels−Alder reaction of isoprene with the RMIs rather than copolymerization. The highly alternating structure of the copolymers was confirmed based on the monomer reactivity ratios for the copolymerization of MCP (M1) and N-phenylmaleimide (PhMI, M2), r1 = 0.010 and r2 = 0.0080. A mechanism for the highly controlled 1,4-regiospecific propagation, which consists of the addition of an RMI radical to the exomethylene group of MCP and subsequent 1,4-regiospecific propagation, was supported by the DFT calculations using model reactions as well as the precise structure determination of oligomers produced during telomerization in the presence of 1-butanethiol as a chain transfer agent. The resulting copolymers exhibited no weight-loss under 340 °C during heating in a nitrogen atmosphere and their glass transition temperature was over the wide temperature range of 66−159 °C, depending on the structure of the N-alkyl substituents. The transparent and flexible films were fabricated by a casting method. The optical properties of the films were as follows: the visible light transmittance over 95% at 380 nm, the refractive indices of 1.54−1.58, and the Abbe number of 42−45.
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conjugated 1,3-dienes27−29 because diene compounds and the RMIs predominantly give Diels-Adler adducts,30 not copolymers. A maleimide moiety is favorably used as a reactive functional group for the synthesis of self-healing materials,31−35 bioconjugated functional polymers,36−38 well-controlled polymer architectures,39−44 and controlled polymer networks45−49 by polymer couplings using the Diels−Alder and thiol−ene reactions.50−54 3-Methylenecyclopentene (MCP) is a cyclic diene compound including an exomethylene structure with a fixed s-tans conformation. Hillmyer et al. reported that MCP was synthesized from myrcene as a naturally occurring monoterpenoid by the one-step reaction using the Grubbs catalyst and that the cationic polymerization of MCP gave a polymer with a 1,4-regiocontrolled repeating structure,55,56 while low-molecular-weight oligomers were produced via a radical polymerization process. The structure of MCP is suitable for use as the comonomer of the radical copolymerization because the Diels−Alder reaction of MCP is entirely
INTRODUCTION Transparent polymer materials with a thermal stability are one of the key materials for various application fields, such as electronics, optoelectronics, and photonics.1−11 The transparent and heat-resistant polymers are industrially fabricated by condensation polymerization and metal-catalyzed polymerization processes, as seen in most cases of the mass-production of polycarbonates and cycloolefin polymers as the transparent polymers and polyimides as the high-temperature polymers.12 A radical chain polymerization process has a significant number of merits for the fabrication of polymeric materials due to the formation of high-molecular-weight polymers from commodity monomers, the design of polymer sequences using controlled polymerizations, and finely tunable polymer structures and properties by copolymerization.13−15 The radical copolymerization of electron-accepting N-substituted maleimides (RMIs) with electron-donating olefins readily produces high-molecularweight and alternating copolymers in a high yield.16−19 Especially, the copolymers of the RMIs with isobutene20,21 and other olefins22−26 exhibited an excellent thermal stability, transparency, high strength, and high modulus. In contrast, very few studies have reported the radical copolymerization with © 2013 American Chemical Society
Received: September 27, 2013 Revised: November 29, 2013 Published: December 12, 2013 9526
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Materials. 2,2′-Azobis(isobutyronitrile) (AIBN) (Wako Pure Chemical Industries, Ltd., Osaka) was recrystallized from methanol. N-n-Butylmaleimide (BMI) and N-cyclohexylmaleimide (CHMI) were synthesized according to the methods reported in a previous paper.57 Commercially available N-methylmaleimide (MMI) and N-phenylmaleimide (PhMI) (Wako Pure Chemical Industries, Ltd., Osaka) were used after recrystallization. Myrcene (Tokyo Chemical Industry Co., Ltd., Tokyo) and all solvents were distilled before use. Synthesis of 3-Methylenecyclopentene (MCP). MCP was synthesized by the ring-closure metathesis reaction of myrcene (Scheme 2).55 A 500-mL flask was charged with 500 mg of the
avoided. Furthermore, it was previously reported that exomethylenecycloalkanes readily copolymerized with the RMIs. 23,24 In this study, we investigated the radical copolymerization of MCP and the RMIs and found that the copolymerization occurred via a precisely controlled alternating and 1,4-regiospecific propagation mechanism (Scheme 1). We Scheme 1
Scheme 2
discussed a mechanism for the regiospecific propagation during the radical copolymerization based on the results of NMR spectroscopy and DFT calculations as well as the structure determination of the oligomers produced during the telomerization. The thermal and optical properties of the resulting copolymers are also described.
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second generation Grubbs catalyst (Sigma-Aldrich Co., Ltd.) under a nitrogen stream, then decahydronaphthalene (cis/trans mixture, bp 187 °C, 375 mL) and myrcene (bp 166 °C, 46.8 g) were added. The reaction mixture was stirred at 40 °C for 48 h and the produced MCP and isobutene as the volatile products were transferred to a dry ice− methanol trap under reduced pressure. The isobutene as the byproduct was removed by bubbling argon into the liquid products for 2 h at room temperature under atmospheric conditions. The liquid residue was purified by distillation before use [bp 73−76 °C].55 The structure and purity were checked by the NMR spectroscopies. MCP. Yield: 27%, colorless liquid. 1H NMR (300 MHz, CDCl3): δ 2.47−2.57 (m, CH2, 4H), 4.77 (s, CCHaHb, 1H), 4.86 (s, C CHaHb, 1H), 6.13−6.20 (m, CHCH, 2H). 13C NMR (75 MHz, CDCl3): δ 29.1 (CHCHCH2CH2), 32.4 (CHCHCH2CH2), 102.3 (=CH2), 134.6 (CH2CCHCH), 139.7 (CH2CCH CH)), 155.2 (CH2C). Copolymerization. The RMIs, MCP, AIBN, and 1,2-dichloroethane were placed in a glass ampule. After the freeze−thaw cycles, the ampule was sealed. The solution was heated at 60 °C for a given time, then the polymerization mixture was poured into a large amount of methanol to precipitate the copolymers. The copolymers were filtered out, washed, then dried in vacuo. The yield of the copolymers was gravimetrically determined. The copolymers were purified by a precipitation method using chloroform and cyclohexane as the solvent and nonsolvent, respectively. The precipitation process was repeated three times in order to remove a small amount of the MCP homopolymer, which was produced by spontaneous cationic polymerization during the radical copolymerization procedures. The copolymers of MCP with the RMIs were isolated by a centrifugal separator after the precipitation procedure. The composition of the repeating units in the copolymers was determined by 1H NMR spectroscopy. Poly(MCP-alt-MMI). 1H NMR (300 MHz, CDCl3): δ 0.78−3.60 (m, CH, CH2, and CH3, 12H), 5.01−5.59 (m, CH, 1H). 13C NMR (75 MHz, CDCl3): δ 24.8 (CH3), 25.9−28.1, 31.9−33.6, 34.0−35.6, and 40.9−42.8 (CH2), 45.9−47.3 and 47.9−49.5 (CH), 126.7−129.1 (CHC), 142.9−144.5 (CHC), 178.2−178.8 (CO). Td5 = 363 °C; Tg = 120 °C. IR (KBr): 3456, 2945, 1774 (CO), 1700 (CO), 1436, 1385, 1281, 1125, 1031, and 785 cm−1. Poly(MCP-alt-BMI). 1H NMR (300 MHz, CDCl3): δ 0.65−4.01 (m, CH, CH2, and CH3, 18H), 5.02−5.59 (s, CH, 1H). 13C NMR (75 MHz, CDCl3): δ 13.7 (CH3), 25.8−28.1, 27.0, 29.8, 32.4−35.4, 38.6, and 41.1−42.6 (CH2), 46.3−49.4 (CH), 127.0−129.2 (CHC), 143.2−144.4 (CHC), 178.2 −178.8 (CO). Td5 = 346 °C; Tg = 66 °C. IR (KBr): 3454, 2961, 1771 (CO), 1695 (CO), 1440, 1401, 1191, 1131, 923, 808, 750, and 620 cm−1. Poly(MCP-alt-CHMI). 1H NMR (300 MHz, CDCl3): δ 0.72−4.32 (m, CH and CH2, 20H), 3.75−4.24 (m, NCH, 1H), 4.94−5.94 (m, CH, 1H). 13C NMR (75 MHz, CDCl3): δ 25.1, 25.9, and 27.3−27.9
EXPERIMENTAL SECTION
General Procedures. The NMR spectra were recorded in CDCl3 using a Bruker AV300N spectrometer (Bruker-Biospin, Ltd., Yokohama). The number-average molecular weight (Mn), weightaverage molecular weight (Mw), and polydispersity (Mw/Mn) were determined by size exclusion chromatography (SEC) with tetrahydrofuran (THF) as the eluent using a CCPD RE-8020 system (Tosoh Co., Ltd., Tokyo) and calibration with standard polystyrenes. Preparative SEC was carried out using an LC-9201 (Japan Analytical Industry Co., Ltd., Tokyo) equipped with UV/RI dual detectors and JAIGEL-1H (exclusion limit, 1000) and JAIGEL-2H (exclusion limit, 5000) columns using chloroform as the eluent. The FT-IR spectra were recorded by a JASCO FT/IR 430 spectrometer using the KBr pellet method. The thermogravimetric and differential thermal analyses (TG/DTA) were carried out using a Seiko TG/DTA 6200 (Seiko Instruments Inc., Tokyo) in a nitrogen stream at the heating rate of 10 °C/min. The onset temperature of decomposition (Td5) was determined as the 5% weight-loss temperature in the TG curves. The differential scanning calorimetry (DSC) was carried out using a Seiko DSC-6200 at the heating rate of 10 °C/min. The UV−vis spectra were recorded using a V-550 spectrophotometer (JASCO Co., Tokyo). The refractive index and Abbe number (νD) were determined using a DRM2 refractometer (Atago Co., Ltd., Tokyo). The νD value is represented by the following equation.
nD = (nD − 1)/(nF − nC)
(1)
where nF, nD, and nC represent the refractive indices at 486, 589, and 656 nm, respectively. The nE values were also determined at 546 nm. The polymer films for the UV−vis measurements were prepared by casting the polymer solution (8 wt % in chloroform) on a quartz or Teflon plate (20 × 20 mm2) and slow drying at ambient pressure and temperature. The thickness of the obtained polymer films was 90−100 μm. Theoretical calculations were carried out using the Spartan’10 software package (Wave function, Inc., Irvine, CA, USA). The production of the initial molecular structure and the preliminary optimization of the structure were carried out using molecular mechanics (MMFF) and the semiemprical method (PM3), then the density functional theory (DFT) was used for the final structure determination and the calculations of the energy and electron densities at the B3LYP/6-311+G**//B3LYP/6-31G* level. 9527
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Table 1. Properties of Copolymers Produced during Radical Copolymerization of MCP with RMIsa RMI
Mn/105
Mw/Mn
RMI mol % in copolymer
Td5 (°C)
Tg (°C)
τb
n Dc
νDd
MMI BMI CHMI PhMI
1.04 1.65 3.45 1.97
2.03 3.11 2.22 2.32
54.3 50.5 51.9 50.1
363 346 378 364
120 66 142 159
95 96 97 96
1.541 1.535 1.542 1.577
45.3 43.6 43.9 41.6
Copolymerization conditions: [MCP] = [RMI] = 0.50 mol/L and [AIBN] = 1.0 mmol/L in 1,2-dichloroethane at 60 °C for 2 h. bTransmittance at 380 nm (%). cRefractive index at 589 nm. dAbbe number. a
126.5, 128.8, 129.3, and 132.0 (C6H5), 146.0 (CC), 175.8 and 178.3 (CO). BT/MCP/PhMI-1,4-Adduct (Isomer II). Rf = 0.92 (hexane/ethyl acetate =1/1). 1H NMR (300 MHz, CDCl3): δ 0.91 (t, CH3, 3H), 1.40 (ddd, CH2, 2H), 1.53 (m, CH2, 2H), 1.60 (m, CH2, 1H), 2.19 (m, CH2, 1H), 2.43 (t, CH2, 2H), 2.48 (m, CH2, 2H), 2.57 (dd, CH2, 1H), 2.88 (dd, CH2, 1H), 3.12 (ddd, CH, 1H), 3.22 (d, CH2, 2H), 3.35−3.50 (m, CH, 1H), 5.53−5.58 (m, CH, 1H), 7.10−7.76 (m, C6H5, 5H). 13C NMR (75 MHz, CDCl3): δ 13.9 (CH3), 22.2, 25.9, 31.4, 31.5, 32.0, 32.3, and 33.6 (CH2), 43.9 and 47.0 (CH), 126.6 ( CH), 127.8, 128.8, 129.3, and 132.0 (C6H5), 144.1 (CC), 176.0 and 178.2 (CO). Diels−Alder Reactions. The reaction of IP and PhMI was carried out using CDCl3 as the solvent at 60 °C for a given time, then an aliquot was analyzed by NMR spectroscopy in order to determine the conversion of PhMI and the yield of the resulting Diels−Alder adduct. IP/PhMI Diels−Alder Adduct: 1H NMR (300 MHz, CDCl3): δ 1.75 (s, CH3, 3H), 2.15−2.37 (m, CHCH2CH, 2H), 2.53−2.66 (m, =CCH2CH, 2H), 3.13−3.25 (m, CH, 2H), 5.55−5.64 (m, =CH, 1H), 7.14−7.52 (m, C6H5, 5H). 13C NMR (75 MHz, CDCl3): δ 23.5 (CH3), 24.5 and 28.8 (CH2), 39.2 and 39.6 (CH), 120.1 (CHC), 126.4, 128.5, 129.0, and 132.1 (C6H5), 136.5 (CH2C), 179.1 and 179.4 (CO). Monomer Reactivity Ratios. All the copolymers were recovered at a low conversion to allow the analysis of the copolymerization parameters using the Mayo−Lewis equation (eq 2)
(CH2), 28.8 (C6H11), 28.4−29.3, 32.3−33.8, and 34.3−35.2 (CH2), 40.9−42.3, 46.4−48.9, and 51.6 (CH), 127.3−129.0 (CHC), 143.2−144.4 (CHC), 178.3−179.0 (CO). Td5 = 378 °C; Tg = 142 °C. IR (KBr): 3452, 2933, 2855, 1770 (CO), 1700(CO), 1454, 1397, 1376, 1345, 1259, 1190, 1144, and 895 cm−1. Poly(MCP-alt-PhMI). 1H NMR (300 MHz, CDCl3): δ 0.45−4.15 (m, CH and CH2, 12H), 5.08−5.95 (s, CH, 1H), 6.45−8.85 (m, C6H5, 5H). 13C NMR (75 MHz, CDCl3): δ 26.2−28.3, 32.6−33.7, and 34.5−35.5 (CH2), 46.8−48.2 (CH), 48.3−49.8 (CH), 125.8−126.8 (C6H5), 127.4−128.2 (CHC) 128.2−128.8, 128.8−129.6, and 132.2 (C6H5), 143.2−144.5 (CHC), 176.7−178.0 (CO). Td5 = 364 °C; Tg = 159 °C. IR (KBr): 3473, 2936, 1777 (CO), 1713 (CO), 1598 (CC), 1500 (CC), 1384, 1179, 753, 692, and 619 cm−1. Poly(IP-alt-PhMI). 1H NMR (300 MHz, CDCl3): δ 0.60−3.81 (m, CH3, CH2, and CH, 9H), 4.89−5.95 (m, CH, 1H), 6.81−7.91 (m, C6H5, 5H). 13C NMR (75 MHz, CDCl3): δ 15.7−16.9 and 23.7 (CH3), 27.3−29.4, 32.4−34.1, and 40.1−42.4 (CH2), 42.4−45.9 (CH), 122.7−123.7 (CHC), 126.4, 128.7, 129.2, and 131.9 (C6H5), 134.9−136.3 (CHC), 177.0−178.3 (CO). IR (KBr): 3472, 2916, 1778 (CO), 1712 (CO), 1597 (CC), 1498 (CC), 1382, 1184, 756, 691, and 620 cm−1. Hydrogenation.58 The poly(MCP-alt-PhMI) (95 mg, Mn = 1.08 × 105, Mw/Mn = 2.89) and p-toluenesulfonyl hydrazide (1.23 g, 13 equiv), tri-n-propylamine (287 mg), and dibutylhydroxytoluene (5 mg) were stirred in 1,2-dichlorobenzene (5 mL) at 120 °C for 10 h under a nitrogen atmosphere, and then the reaction mixture was poured into a large amount of methanol. The precipitated polymer was filtered out, washed with methanol, then dried in vacuo at room temperature. The recovered polymer was purified by precipitation using chloroform and methanol. The conversion was determined by 1 H NMR spectroscopy. The Mn and Mw/Mn values of the hydrogenated copolymer were 1.04 × 105 and 2.93, respectively. Hydrogenated Poly(MCP-alt-PhMI). Conversion: 61%. 1H NMR (300 MHz, CDCl3): δ 0.40−3.80 (m, CH2 and CH), 5.15−5.78 (m, CH, 1H), 6.91−7.96 (m, C6H5, 5H). 13C NMR (75 MHz, CDCl3): δ 27.5−50.5 (CH, CH2, and CH3), 127.7−129.5 (CCH), 126.1− 126.8, 128.6, 129.2, and 131.8 (C6H5), 143.4−144.3 (CCH), 177.2−178.5 (CO). Td5 = 375 °C; Tg = 149 °C. Telomerization. PhMI (0.5 mol/L), MCP (0.5 mol/L), AIBN (0.01 mol/L), and 1-butanethiol (BT, 2.0 mol/L) in 10 mL of 1,2dichloroethane were placed in a glass ampule. After the freeze−thaw cycles, the ampule was sealed. The solution was heated at 60 °C for 1 h, then the reaction mixture was poured into a large amount of methanol to precipitate the copolymers as the byproduct. The filtrate was concentrated under reduced pressure and several oligomers with a BT fragment were isolated by preparative SEC and subsequent silica gel chromatography (hexane/ethyl acetate =7/3 in volume). BT/PhMI-Adduct. 1H NMR (300 MHz, CDCl3): δ 0.94 (t, CH3, 3H), 1.37−1.74 (m, CH2, 4H), 2.69 (dd, CH2, 1H), 2.90 (m, CH2, 2H), 3.31 (dd, CH2, 1H), 3.87 (dd, CH, 1H), 7.27−7.50 (m, C6H5, 5H). BT/MCP/PhMI-1,4-Adduct (Isomer I): Rf = 0.82 (hexane/ethyl acetate =1/1). 1H NMR (300 MHz, CDCl3) δ 0.90 (t, CH3, 3H), 1.38 (ddd, CH2, 2H), 1.53 (m, CH2, 2H), 1.69 (m, CH2, 1H), 2.31 (m, CH2, 1H), 2.41 (t, CH2, 2H), 2.48 (t, CH2, 2H), 2.59 (dd, CH2, 1H), 2.87 (dd, CH2, 1H), 3.08 (ddd, CH, 1H), 3.21 (d, CH2, 2H), 3.46− 3.58 (m, CH, 1H), 5.27−5.33 (d, =CH, 1H), 7.10−7.76 (m, C6H5, 5H). 13C NMR (75 MHz, CDCl3): δ 13.9 (CH3), 22.1, 28.3, 31.3, 31.4, 31.5, 32.4, and 33.8 (CH2), 44.3 and 46.7 (CH), 125.2 (=CH),
d[M1] [M1](r1[M1] + [M 2]) = d[M 2] [M 2](r2[M 2] + [M1])
(2)
r1 = k11/k12
(3)
r2 = k 22/k 21
(4)
where M1 and M2 are MCP or IP and PhMI, respectively. The monomer reactivity ratios, r1 and r2, which are defined by eqs 3 and 4, respectively, were successfully determined using a terminal unit model and the nonlinear least-squares curve fitting,59 Fineman−Ross,60 and Kelen−Tüdõs61 methods.
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RESULTS AND DISCUSSION Copolymerization of MCP with RMIs. The radical copolymerization of MCP with the RMIs was carried out in 1,2-dichloroethane at 60 °C in the presence of AIBN. The highmolecular-weight copolymers (Mn > 105) were produced in a high yield, independent of the N-alkyl substituent structures of the RMIs, as shown in Table 1 and Figure 1. The copolymerization of MCP with PhMI gave a copolymer with higher yields than those with the other N-alkyl-substituted RMIs. The eminent reactivity of MCP during the copolymerization with the RMIs was quite different from the poor homopolymerization ability55 leading to the formation of oligomers with an Mn value less than 103. The 1H NMR spectroscopy revealed the formation of the 1:1 composition copolymers over the entire range of the comonomer composition, i.e., the poly(MCP-alt-RMI)s, as will be shown later. In general, an electron-deficient ene compound tends to react with an electron-donating 1,3-diene to produce a Diels− 9528
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Figure 1. Time-copolymer yield relationships during the radical copolymerization of MCP with the RMIs in 1,2-dichloroethane at 60 °C; [MCP] = [RMI] = 0.50 mol/L; [AIBN] = 1.0 mmol/L. Key: (○) MMI, (Δ) BMI, (◊) CHMI, and (■) PhMI.
Alder adduct. However, no Diels−Alder adduct was produced during the reaction of MCP with the RMIs. This is due to the structure of the diene moiety of MCP as the fixed s-trans conformation. In contrast to the exclusive formation of the copolymers of MCP with the RMIs, cyclic diene compounds with fixed s-cis conformations, such as cyclopentadiene and furan, readily undergo the Diels−Alder reaction with the RMIs. Noncyclic dienes with a flexible and nonfixed conformation provide both a Diels−Alder adduct and a copolymer. The corresponding Diels−Alder adduct was actually produced as the major product during the reaction of isoprene (IP) with PhMI. Figure 2a shows the time−conversion curves for the reaction of IP and PhMI in CDCl3 at 60 °C in the presence of AIBN. The rate of Diels−Alder adduct formation was much greater than that of the copolymer formation. The thermal properties of the resulting poly(MCP-alt-RMI)s were investigated by TG and DSC measurements in a nitrogen stream at the heating rate of 10 °C. The results are summarized in Table 1. The Td5 values were over 345 °C for all the copolymers. The Tg values depended on the N-substituents, and increased by the introduction of bulky cyclohexyl and phenyl groups. All the poly(MCP-alt-RMI)s were soluble in chloroform, dichloromethane, 1,2-dichloroethane, and tetrahydrofuran, and insoluble in methanol, ethanol, acetone, acetonitrile, and n-hexane. The solubility in the other solvents was dependent on the N-substituent structure of the copolymers (see Table S3). The copolymer containing a large cyclohexyl substituent was soluble in toluene, while the methyl- and phenyl-substituted derivatives were soluble in polar solvents, such as N,N-dimethylformamide and dimethyl sulfoxide. The casting of the chloroform solution of the copolymers provided transparent and flexible films, of which the optical properties were investigated. The visible light transmittance was greater than 95% and the nD and νD values were 1.54−1.58 and 42−45, respectively. The poly(MCP-altPhMI) exhibited a slightly higher nD value and a lower νD value than those of the other N-alkyl-substituted poly(MCP-altRMI)s. A double bond in the MCP repeating unit was hydrogenated by a chemical reduction using p-toluenesulfonyl hydrazide at 120 °C for 1 h.58 No change in the molecular weight was observed during the reaction. The hydrogenation efficiency reached only a 61% conversion for the reaction of poly(MCP-alt-PhMI) under the conditions used in this study. Nevertheless, the polymer after transformation exhibited the
Figure 2. (a) Competition of alternating copolymerization and Diels− Alder addition during the reaction of IP with PhMI in CDCl3 at 60 °C. [IP] = [PhMI] = 0.50 mol/L, [AIBN] = 10 mmol/L. The PhMI conversion and Diels−Alder adduct yield were determined by 1H NMR spectroscopy. The copolymer yield was determined based on the weight of the isolated copolymer. (b) 1H NMR spectrum of a mixture of PhMI and the Diels−Alder adduct.
Td5 of 375 °C, which was 10 °C higher than the original one (364 °C). The Tg value decreased from 159 to 149 °C due to an increase in the chain flexibility. Alternating Copolymerization Mechanism. In order to evaluate the copolymerization reactivity of MCP and IP with PhMI, the copolymerizations were carried out under the conditions of various comonomer compositions. The monomer reactivity ratios, r1 and r2, were determined based on the comonomer−copolymer composition curves (Figure 3), which were drawn using the results obtained by the curve fitting method. It is noted that the MCP copolymerization precisely produced alternating copolymers for all the comonomer compositions. The r1 and r2 values were 0.010 and 0.0080, respectively, for the MCP (M1)−PhMI (M2) system (Table 2). The both reactivity ratios were much lower than those for the IP (M1)−PhMI (M2) copolymerization, i.e., r1 = 0.12 and r2 = 0.036. The Fineman−Ross and Kelen−Tüdõs methods gave similar r1 and r2 values, as summarized in Table 2. The 1/r2 values indicated that the relative reactivity of MCP toward the PhMI radical was 5−20 times higher than that of IP due to the enhanced monomer reactivity of MCP with an exomethylene group. The 1H and 13C NMR spectra of the copolymers of MCP with the RMIs (Figures 4 and 5) suggested the exclusive formation of the 1,4-repeating structure of the MCP units and 9529
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Figure 3. Comonomer-copolymer composition curves for the radical copolymerization of (a) MCP and (b) IP (M1) with PhMI (M2). Copolymerization conditions: ([diene]+[PhMI]) = 1.0 mol/L, [AIBN] = 10 mmol/L in 1,2-dichloroethane at 60 °C. The curves were drawn using the monomer reactivity ratios determined by the nonlinear least-squares curve fitting method.
Figure 4. 1H NMR spectrum of the copolymer produced during the copolymerization of MCP with MMI in 1,2-dichloroethane at 60 °C. See Table 1 for the copolymerization conditions.
no formation of any other structures, i.e., 1,2- and 3,4-forms. The peaks due to the protons and carbons of an exomethylene moiety in the 3,4-repeating unit, which should be observed in the region of 4.7−4.9 ppm,55 were not found in the spectra of the poly(MCP-alt-RMI)s. The results of the 13C NMR analysis of the copolymers also determined the regiospecific formation of the 1,4-repeating MCP unit, although the 1,4- and 4,1propagations could not be differentiated based on the NMR information because of the symmetrical RMI repeating unit. In contrast, the weak signals due to the 1,2- and 3,4structures were observed in the 1H NMR spectrum of poly(IPalt-PhMI) in the addition to the major peaks due to the 1,4repeating unit (Figure 6), when the IP content in the copolymer was as high as 68 mol %. The high regulation of the regioselectivity of MCP was due to the high alternating propagation during the copolymerization, while the addition of the IP propagating radical to the IP monomer occurred in a less-controlled fashion. The comonomer-copolymer composition curve for the copolymerization of IP and PhMI in Figure 3b also indicated the presence of the successive IP units in the copolymer chains. Thus, the NMR spectroscopy indicated the predominant formation of 1,4-repeating structure of MCP. DFT Calculations for Regiospecific Propagation Mechanism Analysis. The regiospecific propagation of MCP was further investigated by the DFT calculations using the model reactions shown in Figure 7. The energy difference (ΔE) between the reactant and the product and the difference in the competitive reactions for each step [Δ(ΔE)] are summarized in Table 3. The DFT calculation results indicated the plausible addition of the RMI radical to the 1- and 4positions of the MCP monomer with the ΔE values of −27.4 and −23.9 kcal/mol, respectively. They were thermodynamically favored compared to the 2- and 3-additions (ΔE = −3.0 and −8.9 kcal/mol, respectively). The reaction selectivity should strictly be discussed based on the activation energies for each propagation, but not the ΔE values. In a previous paper,
Figure 5. 13C NMR spectra of the copolymers produced during the copolymerization of MCP with (a) MMI, (b) CHMI, and (c) PhMI in 1,2-dichloroethane at 60 °C. See Table 1 for the copolymerization conditions. Asterisk denotes the solvent.
however, we already confirmed that the ΔE values were closely related to the absolute activation energies for the RMI copolymerization with styrene derivatives;24 i.e., the ΔE values negatively higher, and the activation energies lower. It was expected that the 1-addition favorably occurred due to less steric repulsion between the radical and the diene monomer during the propagation. The Mulliken atomic charges on the carbon atoms of the diene moiety of MCP were also calculated by the DFT method. The results are summarized in Table 4, in which the observed 13C NMR chemical shift values are also shown. It was confirmed that an electron density on the C1 carbon was higher than that on C4. This result agreed well with the NMR chemical shift values. The highest density and the
Table 2. Monomer Reactivity Ratios Determined for Copolymerizations of MCP and IP (M1) with PhMI (M2) in 1,2Dichloroethane at 60 °C curve fitting
Fineman−Ross
Kelen−Tüdõs
comonomers
r1
r2
r1
r2
r1
r2
MCP (M1)−PhMI (M2) IP (M1)−PhMI (M2)
0.010 0.12
0.0080 0.036
0.0099 0.12
∼0 0.046
0.012 0.18
0.0026 0.059
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the structures of the products were investigated by 1H NMR spectroscopy (Figure 9). Fraction I consisted of almost a single component and assigned as the 1:1 adduct of BT and PhMI (BT/PhMI-adduct). The NMR spectrum of fraction II included a large number of sharp and well-split peaks (Figure 9b), suggesting the formation of the other oligomers, such as the BT/MCP-adducts, BT/MCP/PhMI-adducts, BT/PhMI/MCPadducts, and others. The entire spectral pattern of fraction III was partially similar to that of fraction II, but much broader. This suggested that Fraction III may include higher oligomers, such as the BT/MCP/PhMI/MCP-adducts, BT/MCP/PhMI/ MCP/PhMI-adducts, or larger oligomers. In the NMR spectrum of Fraction II, no peak was observed due to the methine proton of the maleimide ring adjacent to the butylthiyl substituent, while the corresponding doublet peaks were observed at 3.9 ppm in the spectrum of the BT/PhMI-adduct (Figure 9a). This indicated no formation of BT/PhMI/MCPadduct. Using silica gel column chromatography with a hexane/ ethyl acetate mixture (7/3 in volume) as the eluent, two major products were successfully isolated from fraction II. They were assigned as the BT/MCP/PhMI-1,4-adducts (Scheme 3) according to an analytical process for the precise structure determination of the products as follows. The 1H and 13C NMR spectra of one of the isolated adducts (isomer I with Rf = 0.82) are shown in Figure 10. The 2D NMR spectra (HSQC and HHCOSY) used for the structure determination are shown in Figure 11. It was revealed that the compound included two kinds of CO, single CC, single phenyl, two kinds of methyls, seven kinds of methylenes, and single methyl groups, based on the number, chemical shifts, and the other correlation information on the peaks detected in the NMR spectra. We considered the BT/MCP/PhMI-adducts as the candidates for the isolated oligomers. In the HSQC spectrum, the methyl carbon at 13.9 ppm, the five methylene carbons at 22.1, 31.4, 31.5, 32.4, and 33.8 ppm, and two methine carbons at 44.3 and 46.7 ppm were related to the corresponding protons, as summarized in Table 5. Two sets of nonequivalent methylene protons were also observed at 1.69 and 2.31 ppm as the multiplets, and at 2.59 and 2.87 ppm as the double doublet, being correlated to the methylene carbons at 28.3 and 31.3 ppm, respectively. The HHCOSY spectrum indicated that one set of nonequivalent methylene protons (HA and HB, dd at 2.59 and 2.87 ppm) are correlated to one of the
Figure 6. 1H NMR spectra of the copolymers produced during copolymerization of IP with PhMI in 1,2-dichloroethane at 60 °C. The IP contents in the copolymers: (a) 50.4 mol %; (b) 67.8 mol %.
lowest chemical shift value on the C1 atom supported the occurrence of the radical addition at the C1 position of MCP rather than that at the C4 position. This result was further confirmed by the isolation of the 1:1 adduct of the MCP and RMI and the structure analysis of the product formed during telomerization (see next section). The second step of the regiospecific propagation is the RMI addition to the allyl radical. The 1,4-propagation was estimated to be energetically favored. The calculations for the larger oligomers provided similar results, as shown in Table 3. Telomerization. In order to discuss the preferred 1,4propagation of MCP, the telomerization of MCP and PhMI was carried out in the presence of 1-butanethiol (BT) as the chain transfer agent and the resulting oligomers were isolated and characterized (Scheme 3). The telomerization was carried out under the conditions at a low monomer concentration ([MCP] = [PhMI] = 0.1 mol/L) and a high BT concentration ([BT] = 2.0 mol/L) with AIBN as the radical initiator in 1,2dichloroethane at 60 °C for 1 h. After the reaction, a methanolsoluble fraction was further divided into some fractions using preparative SEC. Fractions I to III in Figure 8 were isolated and
Figure 7. DFT calculation results for the model reactions related to the preferred regiospecific propagation during the radical copolymerization of MCP with the RMIs. See also Table 3 and Figure S11 for the results of the larger oligomers. 9531
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Table 3. DFT Calculation Results for Possible Propagation Reactions During Alternating Copolymerization of MCP with MMIa monomer
reaction
ΔE (kcal/mol)
Δ(ΔE) (kcal/mol)
CH3−MMI•
MCP
CH3−MMI−MCP•
MMI
CH3−MMI−MCP−MMI• CH3−MMI−MCP−MMI−MCP•
MCP MMI
addition to 1-position of MCP addition to 2-position of MCP addition to 3-position of MCP addition to 4-position of MCP 1,4-propagation of MCP unit 1,2-propagation of MCP unit addition to 1-position of MCP 1,4-propagation of MCP unit 1,2-propagation of MCP unit
−27.4 −3.0 −8.9 −23.9 −11.5 −3.3 −26.6 −12.6 −2.2
0 24.4 18.5 3.5 0 8.2 − 0 10.4
radical
a
See Figure 7 and Figure S12 for the detailed repeating structures of the radicals.
Table 4. Mulliken Atomic Charges on the Diene Moiety of MCP Determined by DFT Calculations and Observed 13C NMR Chemical Shifts carbona
Mulliken atomic charge
C1 C2 C3 C4
−0.467 0.080 −0.180 −0.179
C NMR chemical shiftb (δ in ppm)
13
102.2 155.1 134.6 139.5
a
See Figure 7 for the numbering of the carbon atoms C1 to C4 of MCP. bObserved in CDCl3 at room temperature.
Scheme 3
Figure 8. Elution curve for the preparative SEC of the telomerization mixture.
Figure 9. 1H NMR spectra of fractions I−III isolated from the telomerization mixture by preparative SEC. Measurement solvent, CDCl3.
telomerization provided evidence for the 1,4-propagation of the MCP repeating unit. Another oligomer isolated from fraction II (isomer II with Rf = 0.92) was also characterized and revealed to have a chemical structure similar to that of isomer I. The NMR data for both isomers are summarized in Table 5 (see also Figures S15 and S16). As a result, isomer II was also assigned the BT/MCP/ PhMI-1,4-adduct. The BT/MCP/PhMI-1,4-adduct includes two asymmetric carbons and exists as a set of diastereomers, i.e., the (R*,R*)- and (R*,S*)-form isomers. The product ratio of isomers I and II was determined to be 64/36, based on the intensity ratio of the peaks due to the CCH bond. In this
methine protons (HC, ddd at 3.08 ppm). The observed coupling constants were as follows: J(HA-HB) = 18.2 Hz, J(HAHC) = 9.3 Hz, and J(HB-HC) = 4.8 Hz (Figure 12, see also Figure S16). The nonequivalent HA and HB exhibited no further correlation to any other protons in the HHCOSY spectrum. The methine proton HC was weakly correlated to another methine proton, HD. Consequently, a partial structure for the skeleton of the isolated oligomer was proposed as shown in Figure 12. The BT/MCP/PhMI-1,4-adduct uniquely satisfied these conditions and the other correlations between all the carbons and protons were rationally explained. The precise NMR analysis of the oligomeric products formed during 9532
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were calculated to be 6.5 × 107 and 2.1 × 105 mol/(L s), respectively,63 suggesting the predominant formation of the BT/MCP-adducts, but not the BT/PhMI-adduct. However, the BT/PhMI-adduct was actually isolated as fraction I among the possible oligomers produced during the telomerization reactions (Figure 9a). The reason why the BT/MCP-adduct could not be isolated in this study was possibly due to the preferred addition of PhMI to the MCP radical rather than the chain transfer to BT (Scheme 4). On the other hand, the formation of the PhMI-terminated oligomers is quite rationally explained by the larger chain transfer constants of the maleimide radical to thiols; the kct values to a thiol were reported to be 1.4 mol/(L s) and 8.2 × 103 mol/(L s) for the polymerizations of butadiene and maleimide, respectively.63 This well accounts for the PhMI-terminated oligomer formation as the experimental results in this study. We assumed that the additions of the thiyl radical to MCP and PhMI competitively occurred during the initial step during the reactions for the oligomer formation. The produced BT/PhMI radical was rapidly terminated by hydrogen abstraction from BT, while another BT/MCP radical further propagated by the reaction with PhMI and the resulting BT/MCP/PhMI radical readily underwent chain transfer to BT to produce the BT/ MCP/PhMI-adduct, which was the main component of fraction II. These kinetic analyses well explained the isolation of three kinds of oligomers, i.e., the BT/PhMI-adduct and BT/MCP/ PhMI-adducts as two diastereomers, as the major products during the telomerization.
Figure 10. 1H and 13C NMR spectra of BT/MCP/PhMI-1,4-adduct (isomer I) separated from the telomerization mixture by preparative SEC, followed by silica gel column chromatography.
■
CONCLUSION The N-substituted maleimide derivatives, the RMIs, have been used as the comonomers during the radical alternating copolymerization with electron-donating monomers, such as olefins and styrenes, to produce thermally stable alternating copolymers. The RMIs were also used for the random copolymerization with (meth)acrylates and vinyl chloride in order to modify the thermal stability of the commodity polymers. Otherwise, the RMI groups were used as a highly reactive moiety for the Diels−Alder and thiol−ene reactions, which are valid for the structural and functional designs of polymer materials. At the same time, however, the nature of the
study, we did not assign which of these two isomers is the diastereomer. The other minor products were also included in fraction II, but they could not be isolated as the pure compounds because of their too small amount contained in the fraction. It is well-known that the rate of the addition of an electrophilic thiyl radical to electron-donating alkene and alkynes is much higher than that to electron-deficient alkenes and alkynes.14,62 The rate constants (kct) for the addition of the methylthiyl radical (CH3S•) to 1,3-butadiene and maleimide
Figure 11. HSQC and HHCOSY spectra of BT/MCP/PhMI-1,4-adduct (isomer I) separated from the telomerization mixture by preparative SEC, followed by silica gel column chromatography. For isomer II, see Figure S16. 9533
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Table 5. 1H and 13C NMR Spectral Data for the Two Isomers of BT/MCP/PhMI-1,4-Adducts (Measured in CDCl3 at Room Temperature, Unit Is δ in ppm) isomer I
a
position
structure
a b c g, g′ d h l, l′ e k f i, j o, p, q, r m, n
CH3 CH2 CH2 CH2a CH2 CH2 CH2a CH2 CH CH CCH C6H5 CO
1
H NMR
0.90 (t) 1.38 (ddd) 1.53 (m) 1.69 (m), 2.31 (m) 2.41 (t) 2.48 (t) 2.59 (dd), 2.87 (dd) 3.21 (d) 3.08 (ddd) 3.46−3.58 (m) 5.27−5.33 (m) 7.10−7.76 (m)
isomer II 13
C NMR
13.9 22.1 31.5 28.3 31.4 33.8 31.3 32.4 44.3 46.7 125.2, 146.0 126.5, 128.8, 129.3, 132.0 175.8, 178.3
1
H NMR
0.91 (t) 1.40 (ddd) 1.53 (m) 1.60 (m), 2.19 (m) 2.43 (t) 2.48 (m) 2.57 (dd), 2.88 (dd) 3.22 (d) 3.12 (ddd) 3.35−3.50 (m) 5.53−5.58(m) 7.10−7.76 (m)
13
C NMR
13.9 22.2 32.0 25.9 31.5 33.6 31.4 32.3 43.9 47.0 126.6, 144.1 127.8, 128.8, 129.3, 132.0 176.0, 178.2
Nonequivalent methylene protons.
with the 1,3-diene monomers. The MCP monomer is a 1,3diene monomer derived from one of the monoterpenoids as the renewable resources and its cyclic and exomethylene structure totally prevents the Diels−Alder reaction with the RMIs. In this study, we demonstrated the synthesis of the high-molecularweight and alternating copolymers of MCP and the RMIs and their excellent thermal stability and optical properties, similar to those for alternating copolymers of olefins and styrenes with the RMIs. A mechanism for the regiospecific 1,4-propagation of the MCP monomers via a radical reaction process was discussed based on the results of DFT calculations as well as the precise structure determination of the model oligomers.
Figure 12. Possible partial structure and coupling constants between the methylene and methine protons for isomer I.
RMIs as strong dienophiles for Diels−Alder reactions has been refused for use as the comonomer for the copolymerization Scheme 4
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Recently, the regiospecific radical polymerizations were reported for the copolymerization of diene monomers with nonvinyl monomers, such as molecular oxygen64,65 and sulfur dioxide.66,67 The regiospecific radical polymerization is still one of the most important and challenging topics as well as the sequence- and stereocontrol of polymers using a radical polymerization process.68,69 The control of the fine structures of vinyl and diene polymers by radical chain polymerizations will become more important for the fabrication of highperformance transparent and high-temperature polymer materials used in the electronics and optoelectronics application fields in the future.
■
ASSOCIATED CONTENT
S Supporting Information *
Radical copolymerization results, Diels−Alder reaction results, solubility of polymers, monomer reactivity ratio determination, NMR spectra, DSC curves, refractive index data, TG curves, DFT calculation results, Fineman−Ross and Kelen−Tüdõs plots, and HHCOSY spectrum. This material is available free of charge via the Internet at http://pubs.acs.org/.
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AUTHOR INFORMATION
Corresponding Author
*(A.M.) Fax: +81-72-254-9292. E-mail: matsumoto@chem. osakafu-u.ac.jp. Notes
The authors declare no competing financial interest.
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