Article pubs.acs.org/Macromolecules
Sequence-Controlled Radical Copolymerization of N‑Substituted Maleimides with Olefins and Polyisobutene Macromonomers To Fabricate Thermally Stable and Transparent Maleimide Copolymers with Tunable Glass Transition Temperatures and Viscoelastic Properties Miki Hisano,† Kyota Takeda,† Tsutomu Takashima,‡ Zhengzhe Jin,‡ Akira Shiibashi,‡ 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 ‡ Central Technical Research Laboratory, Research & Development Division, JX Nippon Oil & Energy Corporation, 8 Chidori-cho, Naka-ku, Yokohama 231-0815, 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: Thermally stable and transparent polymers were synthesized by the sequence-controlled radical copolymerization of N-substituted maleimides (RMIs) with various olefins as well as polyisobutene (PIB) macromonomers. The copolymerization behavior significantly depended on the olefin structures, leading to the formation of AB-alternating and AAB-periodic (2:1 sequence-controlled) copolymers. The sequence-controlled radical copolymerization mechanism was discussed based on the monomer reactivity ratios, which were determined using terminal and penultimate unit models for the copolymerization systems investigated in this study as well as in the literature. The olefin comonomers were classified into several groups according to the copolymerization fashions under the terminal and penultimate unit controls and their conjugated structure and the steric bulkiness of the substituents. The copolymers exhibited excellent thermal properties; the onset temperatures of the resulting copolymers were higher than 300 °C, and the glass transition temperature (Tg) values varied over the temperature range of −68 to 210 °C, depending on the structure of the N- and α-substituents of the comonomer repeating units. The introduction of sterically hindered substituents increased the Tg values, while the introduction of the PIB segments as the side chain of the copolymers resulted in a significant decrease in the Tg value and unique fluidity. The optical and viscoelastic properties of the high-Tg copolymers were investigated.
■
INTRODUCTION
poly(RMI)s simultaneously imply a disadvantage in terms of undeniable brittleness. On the other hand, the RMIs are valuable as comonomers for radical copolymerization in order to improve the properties of commodity polymers.32,33 In particular, the alternating copolymer of the RMIs with isobutene (IB) exhibited excellent thermal, optical, and mechanical properties.34−36 Recently, the introduction of polar groups37 and cyclic structures38−40 into the olefin repeating units led to further increases in the thermally stability and the glass transition temperatures (Tg) of the copolymers, but it was still difficult to overcome a mechanical issue. Polyisobutene (PIB),
Sequence-controlled polymerization is one of the most challenging topics for polymer synthesis during recent years because naturally occurring polymers with complicated and well-defined sequences, such as DNA and proteins, exhibit smart performances.1−5 Many attempts to fabricate artificial polymers with highly controlled sequence structures have been made using solid-state synthesis,6 genetic engineering,7,8 template polymerization,9−12 topochemical polymerization,13−15 regiospecific polymerization,16−21 alternating copolymerization,22,23 preorganized oligomers,4,24,25 and postpolymerization approaches.26,27 The N-substituted maleimides (RMIs) readily polymerize in the presence of a radical initiator to give highmolecular-weight and thermally stable polymers28,29 with a substituted methylene repeating structure,30,31 but the resulting © XXXX American Chemical Society
Received: July 17, 2013 Revised: August 20, 2013
A
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290 g/mol, α-content 88.2%) was obtained as the residue. The contents of the dimer, trimer, tetramer, pentamer, and the others were 35.4, 24.2, 17.7, 10.1, and 12.6 wt %, respectively. The well-defined IB oligomers (B3, B4, and B5) were separated from the oligomer mixture by preparative size exclusion chromatography (SEC). The PIB with a high molecular weight (Mn = 1.5 × 103 g/mol and Mw/Mn = 1.8 by SEC, Mn = 1.23 × 103 g/mol and α-content = 88.5% by NMR spectroscopy) was similarly synthesized by the cationic polymerization of IB using BF3. B3: MW 168.4 g/mol, α-content 91.7%. 1H NMR (300 MHz, CDCl3) δ: 5.17−5.08 (m, CH2C (β-olefin), 1H), 4.85 (m, CH2C (α-olefin), 1H), 4.63 (m, CH2C (α-olefin), 1H), 2.00 (s, CH2 CCH2, 2H), 1.78 (m, CH2CCH3, 3H), 1.29 (s, CH2CCH2CCH2, 2H), and 1.00 (s, CH2CCH2C(CH3)2CH2, 6H), 0.99 (s, C(CH3)3, 9H). 13C NMR (75 MHz, CDCl3) δ: 144.4 (CH2C), 114.6 (CH2 C), 56.1 (CH2C(CH3) 3), 53.4 (CH 2CCH2), 36.3 (CH2 CCH2CCH2), 32.5 (C(CH3)3), 30.3 (C(CH3)3) 29.3 (other CH3), and 26.1 (CH2CCH3). B4: MW 224.4 g/mol, α-content 90.9%. 1H NMR (300 MHz, CDCl3) δ: 5.17−5.08 (m, CH2C (β-olefin), 1H), 4.85 (m, CH2C, 1H), 4.64 (m, CH2C, 1H), 2.00 (s, CH2CCH2, 2H), 1.78 (m, CH2CCH3, 3H), 1.37 (s, CH2(CH3)3, 2H), 1.33 (s, CH2 CCH2CCH2, 2H), 1.08 (s, C(CH3)2CH2C(CH3)3, 6H), 1.02 (s, CH2CCH2C(CH3)2CH2, 6H), 0.98 (s, C(CH3)3, 9H). 13C NMR (75 MHz, CDCl3) δ: 144.0 (CH2C), 114.3 (CH2C), 58.1 (CH2C(CH3)3), 56.3 (CH2CH2C(CH3)3), 53.7 (CH2CCH2), 37.5 (CCH2C(CH3)3), 36.1 (CH2CCH2C), 32.1 (C(CH3)3), 30.6 (C(CH3)2CH2C(CH3)3), 29.9 (C(CH3)3), 29.3 (CH2CCH2C(CH3)3), and 25.7 (CH2CCH3). B5: MW 280.6 g/mol, α-content 90.1%. 1H NMR (300 MHz, CDCl3) δ: 5.17−5.08 (m, CH2C (β-olefin), 1H), 4.85 (m, CH2C, 1H), 4.64 (m, CH2C, 1H), 1.99 (s, CH2CCH2, 2H), 1.78 (m, CH 2 CCH 3 , 3H), 1.40 (s, CH 2 (CH 3 ) 3 , 2H), 1.37 (s, CH2CCH2(CH3)3, 2H), 1.33 (s, CH2CCH2CCH2, 2H), 1.10 (s, C(CH3)2CH2(CH3)3, 6H), 1.08 (s, CH2CCH2CCH2(CH3)2CH2, 6H), 1.02 (s, CH2CCH2C(CH3)2CH2, 6H), 0.99 (s, C(CH3)3, 9H). 13 C NMR (75 MHz, CDCl3) δ: 144.0 (CH2C), 114.3 (CH2C), 58.7 (CH2C(CH3)3), 58.1 (CH2CCH2C(CH3)3), 56.8 (CH2 CCH2CCH2), and 53.8 (CH2CCH2), 37.8 (CCH2C(CH3)3), 37.7 (CCH2CCH2C(CH3)3), 36.2 (CH2CCH2C), 32.2 (C(CH3)3), 31.0 (C(CH3)2CH2C(CH3)3), 30.7 (C(CH3)2CH2C(CH3)2CH2C(CH3)3), 29.9 (C(CH3)3), 29.3 (CH2CCH2C(CH3)3), and 25.7 (CH2 CCH3). PIB22: Mn (SEC) = 1.5 × 103 g/mol and Mw/Mn (SEC) = 1.8, Mn (NMR) = 1.23 × 103 g/mol, α-content 88.5%. 1H NMR (300 MHz, CDCl3) δ: 5.18−5.08 (m, CH2C (β-olefin), 1H), 4.85 (m, CH2C (α-olefin), 1H), 4.64 (m, CH2C (α-olefin), 1H), 2.00 (s, CH2 CCH2, 2H), 1.78 (s, CH2CCH3, 3H), 1.48−1.35 (m, other CH2 40H), and 1.27−0.85 (m, CH3, 129H). 13C NMR (75 MHz, CDCl3) δ: 144.0 (CH2C), 114.4 (CH2C), 59.5 (other CH2), 58.8 (CH2 CCH2CCH2CCH2CCH2), 58.2 (CH2CCH2CCH2CCH2), 56.8 (CH2CCH2CCH2), 53.7 (CH2CCH2), 38.1 (other quaternary carbons), 37.8 (CH2CCH2CCH2C), 36.2 (CH2CCH2C), 32.4 (C(CH3)3), 31.2 (other CH3), 30.8 (CH2CCH2CCH2C(CH3)3), 31.2−30.8 (other CH3 and C(CH3)3), 29.4 (CH2CCH2C(CH3)3), and 25.8 (CH2CCH3). Polymerization Procedures. The RMIs, the olefins, a radical initiator, and 1,2-dichloroethane (for solution polymerization) 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, and then the polymerization mixture was poured into a large amount of methanol or methanol/water mixture (4/1 in volume) to precipitate the copolymers. The copolymers were filtered out, washed, and then dried in vacuo. The yield of the copolymers was gravimetrically determined. The copolymers were purified by repeated precipitation procedures using chloroform as the solvent and the appropriate precipitants. The composition of the obtained copolymers was determined by 1H NMR spectroscopy. The copolymerization of EHMI with the IB oligomers was carried out using a similar procedure. The RMIs, the oligomers, and an initiator were placed in a glass ampule. After the freeze−thaw cycles, the ampule
which is produced by cationic polymerization on an industrial scale, is one of the typical amorphous polymers with a low-Tg value below −70 °C. PIB is used as a viscosity modifier, fuel additive, lubricating oil additive, and tack improver in adhesive formulations because of its unique properties, such as very low permeability, excellent thermal and oxidative stabilities, and ozone resistance.41,42 Transparent and thermally stable polymers with precisely controlled chain structures have recently attracted attention because high-performance transparent materials are one of the key materials for electronic and photonic applications, and various types of polymers and their composites with excellent optical and mechanical properties have been developed.43−47 In this study, the radical copolymerization of the RMIs was carried out using low-molecular-weight olefins as the αsubstituted propylene derivatives (O1 to O4) and the IB oligomers (B2 to B5) with a well-defined structure as the comonomers (Scheme 1) in order to investigate the effects of Scheme 1
the structures of the used olefins and oligomers on the copolymerization reactivity as well as the thermal, optical, and viscoelastic properties of the resulting copolymers. We also synthesized a new type of maleimide copolymer with a low-Tg PIB segment in the side chain using a PIB macromonomer (PIB22) during the copolymerization.
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EXPERIMENTAL SECTION
Materials. 2,2′-Azobis(isobutyronitrile) (AIBN) and 2,2′-azobis(4methoxy-2,4-dimethylvaleronitrile) (AMVN) were purchased from Wako Pure Chemical Industries, Ltd., Osaka, and recrystallized from methanol. Dimethyl 2,2′-azobisisobutyrate (MAIB, Wako Pure Chemical Industries, Ltd., Osaka) was recrystallized from n-hexane. N-Phenylmaleimide (PhMI, mp 90 °C) (Tokyo Chemical Industry Co., Ltd., Tokyo) was sublimated under reduced pressure. NEthylmaleimide (EMI, mp 46 °C) (Tokyo Chemical Industry Co., Ltd.) was used as received without further purification. NMethylmaleimide (MMI, mp 97 °C), N-n-butylmaleimide (BMI), and N-(2-ethylhexyl)maleimide (EHMI) were synthesized according to the method reported in a previous paper.48 2-Methyl-1-butene (O1), 2methyl-1-pentene (O2), 2,3-dimethyl-1-pentene (O3), 2,4-dimethyl-1pentene (O4), and 2,4,4-trimethyl-1-pentene (B2) (Tokyo Chemical Industry Co., Ltd., Tokyo) and the solvents were distilled before use. The PIB macromonomer was synthesized by the cationic polymerization of IB (40.0 g, 713 mmol) with the initiator system consisting of BF3-1.5CH3OH (1.5 g, 12.9 mmol) and methanol (0.4 g, 12.9 mmol) in isooctane at −10 °C for 0.5 h. After the polymerization, a large amount of methanol (10 g) was added, the solution was washed with aqueous NaOH, and then the unreacted IB and the solvent were distilled under reduced pressure. A mixture of the IB oligomers (Mn = B
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was sealed. The solution was heated at 60 °C for 5 h, and then the polymerization mixture was poured into a large amount of methanol to precipitate the copolymers. The copolymers were filtered out or separated by decantation, washed, and then dried in vacuo. The yield of the copolymers was gravimetrically determined. The copolymers were purified by repeated precipitation procedures using chloroform as the solvent and methanol. The composition of the obtained copolymers was determined by 1H NMR spectroscopy. For the copolymerization of the RMIs with PIB22, the PIB22 was first put in a glass ampule and added a 1,2-dichloroethane or toluene solution containing the RMIs and an initiator. The solution was heated at 60 or 80 °C for 24 h, and then the polymerization mixture was poured into a large amount of methanol to remove the monomer and initiator. The copolymers were separated by decantation, washed, and then dried in vacuo. The copolymers were isolated by preparative SEC because it was difficult to separate the copolymers and PIB22 by precipitation. Poly(MMI-co-O1). 1H NMR (300 MHz, CDCl3) δ: 4.43−2.52 (m, CHCH and NCH3, 5H), 2.52−0.31 (m, CH2 and CH3, 10H). IR (KBr): νCO (cm−1) 1697. Poly(MMI-co-O2). 1H NMR (300 MHz, CDCl3) δ: 4.25−2.44 (m, CHCH and NCH3, 5H), 2.44−0.47 (m, CH2 and CH3, 12H). IR (KBr): νCO (cm−1) 1695. Poly(MMI-co-O3). 1H NMR (300 MHz, CDCl3) δ: 4.65−2.40 (m, CHCH and NCH3, 5H), 2.40−0.28 (m, CH, CH2 and CH3, 12H). Poly(MMI-co-O4). 1H NMR (300 MHz, CDCl3) δ: 4.26−2.39 (m, CHCH and NCH3, 5H), 2.39−0.42 (m, CH, CH2 and CH3, 14H). IR (KBr): νCO (cm−1) 1696. Poly(MMI-co-B2). 1H NMR (300 MHz, CDCl3) δ 4.66−2.51 (m, CHCH and NCH3, 5H), 2.51−0.47 (m, CH2 and CH3, 16H). IR (KBr): νCO (cm−1) 1698. Poly(BMI-co-O1). 1H NMR (300 MHz, CDCl3) δ: 4.28−2.51 (m, CHCH and NCH2, 4H), 2.51−0.20 (m, CH2 and CH3, 17H). 13C NMR (75 MHz, CDCl3) δ: 182.5−174.8 (CO), 56.2−35.5 (CH2 main chain, CCH3, CH, and NCH2), 29.5 (NCH2CH2), 23.1−19.2 (CCH3, CCH2CH3, and NCH2CH2CH2CH3), 13.5 (NCH2CH2CH2CH3), 7.9 (CCH2CH3). IR (KBr): νCO (cm−1) 1696. Poly(BMI-co-O2). 1H NMR (300 MHz, CDCl3) δ: 4.21−2.16 (m, CHCH and NCH2, 4H), 2.16−0.10 (m, CH2 and CH3, 19H). 13C NMR (75 MHz, CDCl3) δ: 182.5−172.4 (CO), 59.2−38.2 (CH2 main chain, CCH3, CH, and NCH2), 29.5 (NCH2CH2), 22.3-21.2 (CCH3 and CCH2CH2CH3), 20.0 (NCH2CH2CH2CH3), 16.6 (CCH2CH2CH3), 14.5 (CCH2CH2CH3), 13.5 (NCH2CH2CH2CH3). IR (KBr): νCO (cm−1) 1696. Poly(BMI-co-O3). 1H NMR (300 MHz, CDCl3) δ: 4.48−2.43 (m, CHCH and NCH2, 4H), 2.43−0.09 (m, CH, CH2, and CH3, 19H). 13C NMR (75 MHz, CDCl3) δ: 181.6−174.2 (CO), 58.2−33.8 (CH2 main chain, CCH3, CHCH3, CH, and NCH2), 29.5 (NCH2CH2), 23.4−20.5 (CCH3), 20.0 (NCH2CH2CH2CH3), 13.5 (NCH2CH2CH2CH3), 7.9 (CHCH3). IR (KBr): νCO (cm−1) 1697. Poly(BMI-co-O4). 1H NMR (300 MHz, CDCl3) δ: 4.33−2.36 (m, CHCH and NCH2, 4H), 2.36−0.16 (m, CH, CH2, and CH3, 21H). 13C NMR (75 MHz, CDCl3) δ: 182.2−174.1 (CO), 58.8−34.5 (CH2 main chain, CCH3, CCH2CH, CH, and NCH2), 29.5 (NCH2CH2), 28.2−21.0 (CCH3, CHCH3, and CHCH3), 20.0 (NCH2CH2CH2CH3), 13.5 (NCH2CH2CH2CH3). IR (KBr): νCO (cm−1) 1697. Poly(BMI-co-B2). 1H NMR (300 MHz, CDCl3) δ: 4.44−2.52 (m, CHCH and NCH2, 4H), 2.52−0.13 (m, CH2 and CH3, 23H). IR (KBr): νCO (cm−1) 1696. Poly(EHMI-co-B2). 1H NMR (300 MHz, CDCl3) δ: 4.40−2.57 (m, CHCH and NCH2, 4H), 2.57−0.12 (m, CH, CH2, and CH3, 31H). IR (KBr): νCO (cm−1) = 1694. Poly(EHMI-co-B3). 1H NMR (300 MHz, CDCl3) δ: 4.24−2.44 (m, CHCH and NCH2, 4H), 2.44−0.23 (m, others, 39H). Poly(EHMI-co-B4). 1H NMR (300 MHz, CDCl3) δ: 4.47−2.43 (m, CHCH and NCH2, 4H), 2.43−0.18 (m, others, 47H). Poly(EHMI-co-B5). 1H NMR (300 MHz, CDCl3) δ: 4.64−2.46 (m, CHCH and NCH2, 4H), 2.46−0.18 (m, others, 55H).
Poly(EMI-co-PIB22). 1H NMR (300 MHz, CDCl3) δ: 4.08−2.44 (m, CHCH and NCH2, 4H), 2.44−0.27 (m, others, 179H). Poly(EHMI-co-PIB22). 1H NMR (300 MHz, CDCl3) δ: 4.24−2.47 (m, CHCH and NCH2, 4H), 2.47−0.24 (m, others, 191H). Poly(BMI). 1H NMR (300 MHz, CDCl3) δ: 4.85−2.41 (m, CHCH and NCH2, 4H), 2.41−1.09 (m, CH2, 2H), 1.09−0.46 (s, CH3, 3H). Poly(EHMI). 1H NMR (300 MHz, CDCl3) δ: 4.89−2.42 (m, CHCH and NCH2, 4H), 2.12−1.52 (s, CH, 1H), 1.52−0.99 (s, CH2, 8H), 0.99−0.23 (s, CH3, 6H). 13C NMR (75 MHz, CDCl3) δ: 178.3−175.0 (CO), 46.8−40.5 (CHCH and NCH 2 ), 37.2 (CH), 30.2 (NCH2CHCH2CH3), 28.5 (NCH2CHCH2CH2CH2CH3), 23.4 and 23.0 (NCH2CHCH2CH2CH2CH3 and NCH2CHCH2CH2CH2CH3), 14.1 (NCH2CHCH2CH2CH2CH3), 10.2 (NCH2CHCH2CH3). IR (KBr): νCO (cm−1) 1693. Determination of Monomer Reactivity Ratios. a. Terminal Model. The terminal model involves the use of four propagation reactions (Scheme 2).
Scheme 2
For the Fineman−Ross plot in the terminal model49
F
f−1 F2 = r1 − r2 f f
(1) 50
Equation 1 is further reformulated for the Kelen−Tüdõs plot.
r ⎞ r ⎛ η = ⎜r1 + 2 ⎟ξ − 2 ⎝ ⎠ α α
(2)
where η and ξ are represented as follows.
η=
F(1 − f )/f α + F 2/f
(3)
ξ=
F 2/f α + F 2/f
(4)
The α value was determined by the equation
⎛ F2 ⎞ ⎛ F2 ⎞ ⎜ ⎟ ⎜ ⎟ ⎝ f ⎠max ⎝ f ⎠min
α= 2
2
(5) 2
where (F /f)max and (F /f)min are the highest and lowest F /f values, respectively, among each set of data. b. Penultimate Unit Model. The penultimate unit model involves the use of eight propagation reactions (Scheme 3) with four monomer reactivity ratios (eqs 6−9).
r11 = k111/k112
(6)
r12 = k122/k121
(7)
r21 = k 211/k 212
(8)
r22 = k 222/k 221
(9)
With these four monomer reactivity ratios, the copolymer composition is represented by eq 10.
( ) ( )
1 + r21F f= 1+ C
r12 F
r11F + 1 r21F + 1
r22 + F r12 + F
(10)
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Mn) were determined by analytical SEC with tetrahydrofuran (THF) as the eluent using a Tosoh CCPD RE-8020 system and calibration with standard polystyrenes. The preparative SEC was carried out using a JAI LC-9201 equipped with JAIGEL-1H, -2H, -2.5H, and -3H columns (exclusion limits are 1 × 103, 5 × 103, 2 × 104, and 7 × 104, respectively) using chloroform as the eluent. The thermogravimetric and differential thermal analyses (TG/DTA) were carried out using a Seiko TG/DTA 6200 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 JASCO V-550 spectrophotometer. The refractive index and Abbe number (νD) were determined using an Abbe-type refractometer (Atago Co., Ltd., Model DR-M2). The νD is represented by the equation
Scheme 3
where F = [M1]/[M2] and f = d[M1]/d[M2]. In this case, the r11 and r21 values can be regarded as zero due to the lack of the homopolymerization ability of the olefins. Therefore, eq 10 is reformulated as the Fineman−Ross plot (eq 11).
1 − 2f f 1 = 2 r22 − F(1 − f ) r12 F (1 − f )
vD = (nD − 1)/nF − nC
where nF, nD, and nC represent the refractive indices at 486 nm (F line), 589 nm (D line), and 656 nm (C line), respectively. The dynamic mechanical analysis (DMA) was carried out using a Seiko DMS-6100 dynamic mechanical analyzer with a flexural mode at different frequencies (0.5, 1, 2, 5, and 10 Hz) and the heating rate of 2 °C/ min under atmospheric conditions. Test plates were prepared by heating to a temperature at 40−50 °C over each Tg value and the compression of the polymer powders (3−4 g) under a load of 10−20 MPa using a metal mold, AH-K1, and a heat-pressing machine, AsOne AH-2003. A silicone spray (KF965SP, Shin-Etsu Chemical Co., Ltd.) was used as the mold lubricant. The plate was cut into the sample size of 50 × 10 × 2 mm3.
(11)
Equation 11 is further reformulated for the Kelen−Tüdõs plot.
r ⎞ r ⎛ η = ⎜r1 + 2 ⎟ξ − 2 ⎝ α⎠ α
(12)
where η and ξ are represented as follows:
η=
1 − 2f F(1 − f ) f
α+ ξ=
F 2(1 − f )
f /F 2(1 − f ) α + f /F 2(1 − f )
■
(13)
⎧ ⎫ ⎧ ⎫ f f ⎨ 2 ⎬ ⎨ 2 ⎬ ⎩ F (1 − f ) ⎭max ⎩ F (1 − f ) ⎭min
RESULTS AND DISCUSSION
Determination of Monomer Reactivity Ratios. In order to determine the monomer reactivity ratios, the copolymerization of the RMIs with the olefins was carried out. The copolymerization results are shown in Table S1. The copolymer compositions were determined on the basis of the relative intensity of the characteristic peaks by 1H NMR spectroscopy, and then the monomer reactivity ratios were calculated based on the analysis of the obtained comonomer−copolymer composition curves. All the copolymers were recovered at a low conversion to allow the analysis of the copolymerization parameters using the Mayo−Lewis equation (eq 17).
(14)
The α value was determined by the equation
α=
(16)
(15)
where {f /F2(1 − f)}max and {f /F2(1 − f)}min are the highest and lowest f /F2(1 − f) values, respectively, among each set of data. The Fineman− Ross and Kelen−Tüdõs plots are shown in Figures S7−S9. The determined monomer reactivity ratios are summarized in Table S2. Measurements. The NMR spectra were recorded in CDCl3 using a Bruker AV300N spectrometer. The number-average molecular weight (Mn), weight-average molecular weight (Mw), and polydispersity (Mw/
d[M1] [M1](r1[M1] + [M 2]) = d[M 2] [M 2](r2[M 2] + [M1])
(17)
Figure 1. Comonomer−copolymer composition curves for copolymerizations of olefins (M1) with RMIs (M2). (a) O1−PhMI (●) and B2−PhMI (○) as alternating copolymerization under terminal control, (b) O4−PhMI (■) and O3−PhMI (□) as 2:1 sequence-controlled and alternating copolymerizations under the penultimate unit control, and (c) B2−EMI (◆) and B2−EHMI (◇) as 2:1 sequence-controlled copolymerization under the penultimate unit control. See Table 1 for the determined monomer reactivity ratios. D
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The r1 and r2 values, which are defined by eqs 18 and 19, respectively, were determined by the nonlinear least-squares curve fitting, Fineman−Ross, and Kelen−Tüdõs methods. The results obtained by the nonlinear least-squares curve fitting method are shown in Figure 1 and Table 1. The results using the
RMI contents during the other copolymerizations. This is due to the effect of the steric bulkiness of the N-substituent at the longrange distance for the interactions of the polymer chains and the monomer including the penultimate unit effects. There are many reports on the copolymerization of the RMIs with various olefins in the literature.53 The olefin structures and the monomer reactivity ratios for the alternating copolymerizations under the terminal control and the sequence-controlled copolymerizations under the penultimate unit control are summarized in Tables 2 and 3, respectively. The olefin
Table 1. Determination of Monomer Reactivity Ratios for Radical Copolymerization of Olefins (M1) with RMI (M2) in 1,2-Dichloroethane at 60 °C olefin (M1)
RMI (M2)
r1
r2
O1 O3 O4 B2 B2 B2
PhMI PhMI PhMI PhMI EMI EHMI
0.013
0.17
0.0076
r11
r12
r21
r22
0 0
0.49 3.94
0 0
0.22 0.17
0 0
1.33 1.95
0 0
0.26 0.15
Table 2. Monomer Reactivity Ratios for Radical Alternating Copolymerization of Olefins (M1) and RMIs (M2) Using the Terminal Model in the Literature
0.16
Fineman−Ross and Kelen−Tüdõs methods are shown in Figures S7−S9 and Table S2.
r1 = k11/k12
(18)
r2 = k 22/k 21
(19)
olefin (M1)
RMI (M2)
solvent
r1
r2
ref
styrene IB
PhMI MMI
BC5 BC6
BMI BMI
benzene chloroform dioxane 1,2-dichloroethane 1,2-dichloroethane
0.07 0 0 0.046 0.019
0.01 0.14 0.47 0.0043 0.047
30 34 34 39 39
monomers can be classified into three types according to their conjugated structure and the bulkiness of the substituents on the vinyl moiety (Figure 2). The 1:1 alternating copolymers are produced during the copolymerization with the A-type olefins as planar and conjugated olefins as well as the copolymerization with less-hindered and nonconjugated olefins such as IB (type B). The olefin O1 also served as the type B. On the other hand, hindered and nonconjugated olefins with the saturated or unsaturated substituents are classified as the C-type and exhibited the 2:1 sequence-controlled copolymerizations by the penultimate unit effect. It was previously revealed that the sequence-controlled copolymerization of the RMIs with the BCms (m = 5−7) occurred in the 1:1 alternating and 2:1 sequence-controlled fashions according to the BCm monomer reactivity, which significantly depended on the coplanarity of the exomethylene moiety and the benzene ring. The BC5 and BC6 had a planar molecular structure, while BC7 showed a strained and nonplanar structure.39 The production of the copolymers with a 2:1 composition by deviation from the planar structure of the monomer is considered to be related to a decrease in the monomer reactivity. For the olefins with a noncyclic structure investigated in the previous and present studies, IB and O1 are classified as the Btype and the others as the C-type olefins. In a more strict sense, several olefins should be recognized on the border between the B- and C-types. The alternating copolymer was obtained during the copolymerization of B2 with PhMI due to the fact that the neopentyl group at the α-position is not important as the bulky substituent during the cross-propagation because of the steric strain relief.59,60 Namely, a tert-butyl group generally brings highly strained condition, but the insertion of a methylene spacer can avoid such strain. This readily agrees with the copolymerization results in Figure 1a, in which the magnitude of the steric effect of B2 is the same level of that of O1 during the copolymerization with PhMI; the comonomer−copolymer composition curves completely overlapped those of the O1− PhMI and B2−PhMI systems. Previously, Matsumoto et al. reported that an alternating copolymer was produced during the copolymerization of B2 (M1) with N-(2,6-dimethylphenyl)maleimide (M2); r1 = 0 and r2 = 0.086 in chloroform at 60 °C.61 In this case, the monomer reactivity ratios were successfully
where M1 and M2 are the olefins and the RMIs, respectively. The curve fitting method resulted in good results over the entire range of compositions, while the monomer reactivity ratios estimated by the Fineman−Ross and Kelen−Tüdõs methods gave scattered plots and the comonomer−copolymer composition curves based on the calculated values partially failed to fit the experimental data (see Supporting Information). The monomer reactivity ratios, r 1 and r 2 , for the copolymerization systems of PhMI with O1 and B2 were successfully determined using the terminal model and confirmed to be close to zero; r1 = 0.013 and r2 = 0.17 for the O1 (M1)− PhMI (M2) and r1 = 0.0076 and r2 = 0.16 for B2 (M1)−PhMI (M2), as shown in Figure 1a. This indicated the production of the copolymers with a highly controlled alternating repeating structure during the copolymerization of these comonomer combinations. On the other hand, for the copolymerization of PhMI with O3 and O4 containing bulkier α-substituents, the copolymerization behavior was entirely different from the alternating copolymer formation under the terminal control, and the monomer reactivity ratios were uniquely determined using the penultimate unit model; r12 = 0.49 and r22 = 0.22 for the O3 (M1)−PhMI (M2) and r12 = 3.94 and r22 = 0.17 for O4 (M1)−PhMI (M2). Only inappropriate values were obtained using the terminal model. Thus, the bulkier olefins provided 1:1 alternating and 2:1 sequence-controlled copolymers under the penultimate unit control, depending on the structure of the substituent groups, i.e., isopropyl and isobutyl substituents. The copolymerization of EMI and EHMI with B2 gave the r12 and r22 values, which can be determined using the penultimate unit model;51,52 r12 = 1.33 and r22 = 0.26 for the B2 (M1)−EMI (M2) system and r12 = 1.95 and r22 = 0.15 for the B2 (M1)−EHMI (M2) system. Interestingly, the fashion of the copolymerization of B2 changed depending on the maleimide structure, i.e., the alternating copolymerization with PhMI and the 2:1 sequence controlled copolymerization with EMI and EHMI including Nalkyl groups. In general, the monomer reactivity of PhMI is higher than those of the RMI with N-alkyl substituents because of its more conjugated monomer structure, but the PhMI content incorporated into the copolymer was lower than the E
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Table 3. Monomer Reactivity Ratios for Radical Copolymerization of Olefins (M1) and RMIs (M2) Using the Penultimate Unit Model in the Literature RMI (M2)
solvent
r11
r12
r21
r22
ref
limonene
EMI HEMIa CHMIb PhMI
β-pinene
MMI EMI PhMI
O2 O3 isopropenylcyclohexane 4-isopropenylcyclohexene 1 hexene vinylcyclohexane
PhMI PhMI PhMI PhMI PhMI PhMI
4-vinylcyclohexene BC7
PhMI BMI
PhC(CF3)2OH PhC(CF3)2OH PhC(CF3)2OH PhC(CF3)2OH dichloromethane 1,2-dichloroethane 1,2-dichloroethane 1,2-dichloroethane PhC(CF3)2OH tetrahydrofuran 1,2-dichloroethane 2,2,2-trifluoroethanol perfluoro-tert-butanol PhC(CF3)2OH PhC(CF3)2OH PhC(CF3)2OH PhC(CF3)2OH PhC(CF3)2OH PhC(CF3)2OH dichloromethane PhC(CF3)2OH none (in bulk)
0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
17 15 21 18.7 600 7.1 6.5 8.3 7.6 15 10 2.5 3.2 2.7 3.0 2.8 30 2.1 1.9 90 56 2.9
0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
0.029 0.0060 0.0059 0.0042 0.83 0.20 0.16 0.10 0.0011 0.53 0.24 0.032 0.0058 0.0010 0.067 0.030 0.25 0.11 0.21 1.3 0.47 0.25
55 56 55 54 55 57 57 57 55 58 58 58 58 55 55 55 55 55 55 55 55 39
olefin (M1)
a
HEMI: N-(2-hydroxyethyl)maleimide. bCHMI: N-cyclohexylmaleimide.
Figure 2. Classification of the olefins used for copolymerization of RMIs with various olefins in this study and the literature. The O3−PhMI copolymerization exceptionally produced the alternating copolymers under the penultimate unit control. The B2−PhMI copolymerization produced the alternating copolymers under the terminal control, while B2−EMI and B2−EHMI produced the 2:1 sequence-controlled copolymers under the penultimate unit control (see Table 1).
Being different from the copolymerization systems with O1 and B2, penultimate unit effects were observed during the copolymerization with O3 and O4. The inverse of the r12 (1/r12) values, which represents the reactivity of the PhMI radical to the olefin monomers, were 2.0 and 0.25 for the O3 and O4, respectively. Recently, Kamigaito et al. reported the 1/r12 value of 0.33 in the fluorinated cumyl alcohol, PhC(CF3)2OH, for the same copolymerization system of O3 (M1)−PhMI (M2).55 The 1/r12 value observed in this study was higher than the reported one, being ascribed to the solvent effects.58,62 It was reported that the fluoroalcohol diminished the homopropagation of PhMI via a hydrogen bonding interaction with the carbonyl
determined using the terminal model, but the absolute propagation rate constants could not be well explained by the terminal model and the participation of the penultimate unit groups was suggested for the determination of the propagation rate. In contrast, the copolymerization of B2 with the RMIs including the N-alkyl substituents, for example, MMI, EMI, and EHMI, provided RMI-rich and 2:1 sequence controlled copolymers, but not alternating copolymers under the penultimate unit control. These results suggested the longdistance interactions concerning the penultimate units to determine the copolymerization fashion. F
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Table 4. Synthesis and Thermal Properties of a High-Tg Poly(MMI-co-olefin)sa olefin
Es
Esc
Es(CH2R)
σpara
σ*
yield (%)
Mn/103 (g/mol)
O1 O2 O3 O4 B2 B2b
−0.07 −0.36 −0.47 −0.93 −1.74
−0.38 −0.67 −1.08 −1.24 −2.05
−0.36 −0.39 −0.93 −0.35 −0.34
−0.15 −0.13 −0.15 −0.12 −0.15
−0.10 −0.12 −0.19 −0.13 −0.17
50.7 33.2 8.3 15.3 21.7 79.1
15.8 13.6 1.5 5.3 6.6 46.0
Mw/Mn MMI mol % in copolymer 1.5 1.4 1.6 1.3 1.5 2.1
60.0 68.0 68.0 73.6 60.8 61.5
Td5 (°C)
Tmax (°C)
Tg (°C)
342 323 335 350 358 365
422 405 385 408 407 422
182 181 161 201 210 190
Copolymerization conditions: [MMI] = [olefin] = 0.2 mol/L, [AIBN] = 10 mmol/L in 1,2-dichloroethane at 60 °C for 5 h. The Es and Esc are the Taft’s steric constant for the R substituent of CH2C(CH3)R and the modified one by Hancock (refs 63 and 64). The Es(CH2R) is the constant for the R substituent of CH2C(CH3)CH2R (ref 65). The σpara and σ* are the Hammett’s polar constants. bEMI was used instead of MMI. [EMI] = [B2] = 2.0 mol/L, [AIBN] = 40 mmol/L in 1,2-dichloroethane at 60 °C for 15 h. a
tried to analyze using the Taft’s steric and the Hammett’s polar substituent constants.63 The Es and Esc are the Taft’s steric constant as the most typical parameter and its modified one by Hancock, respectively.63,64 The latter parameters imply that the steric effect is modified by the substituents on the β-carbons of the reaction center. The σpara and σ* are typical polar constants for conjugated and nonconjugated substrates, respectively. As a result, neither the steric nor polar parameters provided a clear relationship for the copolymer yields and the Mn values. We found that another steric constant, Es(CH2R),65 which reflects the steric effect at a long-range distance, showed a better correlation with the copolymerization reactivity results, although we did not obtain a linear relationship between the copolymerization reactivity and the steric parameters of the substituents. The thermal stability of the poly(MMI-co-olefin)s was examined by TG analysis in a nitrogen stream at the heating rate of 10 °C/min. The results are shown in Figure 3a. The decomposition of the copolymers proceeded via a one-step reaction; the onset temperatures of the decomposition of the copolymers were very high (Td5 > 320 °C), and the Tmax values were over 385 °C. The weight residue at 450 °C was less than
groups in the PhMI unit. In this study, 1,2-dichloroethane was used as the solvent with a Lewis acidity weaker than those of the fluoroalcohols. It should be noted that not only the olefin structure but also the solvent effects affected the copolymerization fashions. Furthermore, it was revealed that the O4 structure played a greater role as the penultimate unit than that of O3 based on the 1/r12 values. As described above, the steric effect of B2 can be ignored due to the relief of its steric strain. Considering the fact that the O4 structure includes a bulky structure despite the presence of the methylene spacer, it suggests that the relief effect of steric strain for the tert-butyl group in the β-position may be stronger than that of the isopropyl group. The bulkier and stiff substituents tend to induce more significant penultimate unit effects as shown in the results of the copolymerization of the olefins with a cyclohexene group, resulting in the greater r12 values (see Table 3). High-Tg Copolymer Synthesis Using MMI. The radical copolymerization of MMI with the olefins was carried out in 1,2dichloroethane ([MMI] = [olefin] = 0.2 mol/L) at 60 °C for 5 h in the presence of AIBN (10 mmol/L) as the radical initiator. The copolymerization homogeneously occurred, and the obtained copolymers were isolated as colorless powders after reprecipitation. The results of the copolymerization and the thermal properties of the obtained copolymers are summarized in Table 4. The 1H NMR spectroscopy revealed the formation of copolymers with MMI-rich compositions (MMI mol % in copolymer = 60.0−73.6 mol %) during the copolymerization of MMI with all the olefins. The 2:1 sequence-controlled copolymerization of B2 with EMI and EHMI has already been described. The copolymerization of MMI with various olefins also probably proceeded in a similar fashion, being different from the formation of the alternating copolymers of the MMI with IB, styrene, and other electron-donating vinyl monomers. With the introduction of the bulky alkyl groups into the olefins, the obtained copolymer yields and the Mn values decreased in the following order: O1 > O2 > O3. Thus, the introduction of a branched structure at the position adjacent to the reacting CC bond of the olefins led to significant lowering of the reactivity. The introduction of a methylene spacer relieved the steric repulsion, as was already shown in the results of the copolymerizations with O4 and B2. Steric hindrance plays an important role during the cross-propagation of a sterically hindered olefin radical to an RMI monomer with a 1,2disubstituted ethylene structure. On the other hand, the ratedetermining step is the addition of an olefin monomer to an RMI propagating radical during the copolymerization of RMI with an olefin.39 Both cross-propagations can be accelerated by the introduction of an electron-donating substituent into the olefin structure. The copolymerization results in Table 4 were
Figure 3. TG curves of (a) poly(MMI-co-olefins)s and (b) poly(BMIco-olefins)s in a nitrogen stream at the heating rate of 10 °C/min. G
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Table 5. Synthesis and Thermal Properties of the High-Molecular-Weight Poly(BMI-co-olefin)s and Poly(EHMI-co-olefin)sa RMI BMI
EHMI
olefin (mol/L)
yield (%)
Mn/104 (g/mol)
Mw/104 (g/mol)
Mw/Mn
RMI mol % in copolymer
Td5 (°C)
Tmax (°C)
Tg (°C)
O1 (2.0) O2 (2.0) O3 (bulk) O4 (2.0) O4 (bulk) B2 (2.0) B2 (bulk) IB (2.0)b B2 (bulk) B3 (bulk) B4 (bulk) B5 (bulk)
78.1 76.6 50.2 51.0 71.7 47.3 62.5 90.4 50.6 21.7 17.6 18.0
4.1 6.5 2.9 3.9 9.5 4.5 9.2 18.0 10.2 3.4 2.6 2.5
8.0 14.4 9.3 7.2 20.2 8.6 18.9 48.5 20.6 6.1 4.4 4.1
2.0 2.2 3.2 1.9 2.1 1.9 2.1 2.7 2.0 1.8 1.7 1.7
60.3 67.9 71.4 71.3 71.6 62.4 64.4 53.7 64.1 68.8 62.8 64.5
371 357 378 352 345 381 377 323 377 361 368 376
425 420 426 411 420 423 422 428 428 427 430 426
105 108 116 119 123 145 148 40 95 62 38 −13
Copolymerization conditions: [RMI]/[olefin]/[AIBN] = 50/50/1 in bulk or 1,2-dichloroethane at 60 °C for 5 h. b[EHMI] = 0.2 mol/L, [IB] = 2.0 mol/L, [AMVN] = 10 mmol/L in 1,2-dichloroethane at 30 °C for 5 h.
a
2%. The Tg values determined by the DSC measurement were also high in the range of 161−210 °C. Especially, the Tg values of poly(MMI-co-O4) and poly(MMI-co-B2) with sterically hindered α-substituents, such as isobutyl and neopentyl groups, were higher than those for the other poly(MMI-co-olefin)s and the previously reported poly(MMI-alt-IB).34,35 This indicated that the introduction of bulkier substituents into the α-position of the olefin repeating units leads to an increase in the Tg value of the copolymers. The copolymerization with EMI as the liquid monomer under the conditions at a high monomer concentration ([EMI] = [IB] = 2.0 mol/L) produced a high-Tg (Tg = 190 °C) and high-molecular-weight copolymer (Mn = 4.6 × 104 g/mol) in a high yield. High-Molecular-Weight Copolymer Synthesis Using BMI and EHMI. The polymerizations at a high monomer concentration were carried out in bulk or solution in order to obtain high-molecular-weight copolymers using BMI and EHMI as the liquid monomers. The copolymers with the Mn values of (3−10) × 104 g/mol were produced, and a high content of the RMI repeating units in the copolymer (60.3−71.6 mol % of the BMI and EHMI units) was confirmed by 1H NMR spectroscopy (Table 5). The TG curves for the high-molecular-weight poly(BMI-co-olefin)s are shown in Figure 3b. The Td5 and Tmax values of the poly(RMI-co-olefins)s in Table 5 were 345− 381 and 411−430 °C, respectively, independent of the structures of the α-substituents of the olefins and the Nsubstituents of the RMIs. The poly(BMI-co-olefin)s exhibited Tg values over the wide temperature range of 105−148 °C, and they increased with an increase in the steric bulkiness of the substituents of the olefin comonomers. The Tg values for the poly(BMI-co-olefin)s were ca. 80 °C lower than those for the corresponding poly(MMI-co-olefin)s shown in Table 4. In this study, we also used other types of olefin comonomers, B3, B4, and B5 (Scheme 1), as the IB oligomers with a welldefined structure. They were produced by the cationic polymerization of IB and subsequently separated from the mixture of the IB oligomers with different degrees of polymerization using preparative SEC (Figure 4). The isolated oligomers were characterized by 1H and 13C NMR spectroscopies (Figure 5). The oligomers contained two types of unsaturated terminal groups, as shown in the 1H NMR spectra. The α-olefin contents were estimated to be 90.1−91.7% on the basis of the relative intensity of the NMR signals observed at 4.6 and 5.1 ppm due to the α- and β-olefins, respectively. The peak observed at 4.8 ppm was assigned as the sterically hindered 1,1-
Figure 4. RI and UV responses for the preparative SEC trace of the mixture of the IB oligomers produced by the cationic polymerization of IB.
disubstituted ethylene structure of (α′-olefin in Figure S12) as the minor product. The α′-olefin was produced by the addition of the propagating carbocation to the exomethylene group of the resulting IB oligomers during cationic polymerization, followed by the β-elimination of a proton. The β- and α′-olefins were inert during the radical copolymerization because of their significant steric hindrance. The radical copolymerizations of EHMI with B2 to B4 produced powdery copolymers with a Tg value higher than room temperature, while the copolymerization using the large oligomer B5 provided a viscous solid copolymer. The Tg values of the copolymers decreased from 95 to −13 °C based on the degree of polymerization of the oligomers. On the basis of the results in Tables 4 and 5, it is noteworthy that the Tg values are variable depending on the N- and α-substituents of the RMIs and olefins. The poly(RMI-co-B2)s exhibited the highest Tg values for each series of copolymers using MMI, BMI, and EHMI due to the suppressed dynamic chain motion by the introduction of a bulky neopentyl group neighboring the main chain. The poly(MMI-co-B2) and poly(EHMI-co-B5) exhibited the highest and lowest Tg values, i.e., 210 and −13 °C, respectively, in this study. The copolymerization reactivity decreased with the increasing size of the substituents of the oligomers in the order of IB > B2 > H
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Figure 5. 1H and 13C NMR spectra of the isolated IB oligomers (B3−B5) with well-defined structures in CDCl3 at room temperature. Asterisk denotes the α′-olefin structure (see Figure S12), which was produced during the cationic polymerization of IB as the side product. The yield of the α′-olefin product was less than 10%.
Table 6. Synthesis of the Poly(RMI-co-PIB22)sa LMW peakb
HMW peakc
comonomers
Mw/103 (g/mol)
Mw/Mn
Mw/103 (g/mol)
Mw/Mn
yieldd (wt %)
RMI mol % in copolymer
Td5 (°C)
Tmax (°C)
Tg (°C)
EMI/PIB22 EHMI/PIB22 PIB22
2.9 2.7
1.9 2.3
29.0 33.3
1.5 1.7
44.4 55.5
63.7e 61.3e 0
320e 312e 221
402e 404e 367
−65e −68e −84
a Copolymerization conditions: [RMI]/[PIB22]/[MAIB] = 20/20/1 in 1,2-dichloroethane (100 wt %) at 60 °C for 24 h. bLower-molecular-weight (LMW) peak due to PIB22 in the SEC curve in Figure 6. cHigher-molecular-weight (HMW) peak due to the resulting copolymers in the SEC curve in Figure 6. dEstimated from the intensity ratio (RI detector) of the HMW and LMW peaks. eEvaluated for the isolated poly(RMI-co-PIB22)s.
B3 ∼ B4 ∼ B5. The reactivity of the higher IB oligomers was insensitive to the degree of polymerization; i.e., B3 to B5 exhibited similar copolymerization reactivities. This result agrees quite well with the results of the polymerization reactivity of macromonomers in the literature.66 It is assumed that the copolymerization of EHMI with B2 to B5 produced the copolymers with a 2:1 composition (the EHMI content = 62.8− 68.8 mol % in the copolymers) by the penultimate unit effect, while a copolymer with an equimolar composition (EHMI 53.7 mol %) was produced during the copolymerization with IB under the terminal control. Copolymerization of RMIs with PIB22. The radical copolymerization of the RMIs with PIB22 was carried out in
order to investigate the properties of the copolymers including the PIB segment in the side group. The EMI and EHMI monomers were immiscible with PIB22 in the bulk, and a phase separation occurred even at a 60 °C polymerization temperature. The copolymerization of the RMIs with PIB22 were performed in the presence of the same amount of 1,2dichloroethane as the cosolvent. The effects of the kind and amount of the used solvents (1,2-dichloroethane and toluene, 25−100 wt % to the comonomers) and the temperature (60 and 80 °C) were investigated (Figure S13), and the results obtained under the most appropriate conditions are shown in Table 6. Because the resulting copolymers could not be isolated from the remaining PIB22, the copolymer yield was estimated by the peak I
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intensity ratio (RI detector) of the peaks observed in the SEC elution curves (Figure 6).
Figure 7. Fluidity property of (a) PIB22, (b) poly(EMI-co-PIB22), and (c) poly(2EHMI-co-PIB22). Figure 6. SEC curves of the mixture of poly(RMI-co-PIB22) and PIB22 after the copolymerization (dotted line) and poly(RMI-co-PIB22) isolated by the preparative SEC (solid line): (a) poly(EMI-co-PIB22); (b) poly(EHMI-co-PIB22).
Table 7. Optical Properties of the Poly(RMI-co-olefin)sa
The copolymers were produced in 44−56% yields and separated from the copolymerization mixture by preparative SEC, as shown in the elution curves by the solid lines in Figure 6. The results for the characterization of the isolated poly(RMIco-PIB22)s are shown in Table 6. The Td5 and Tmax values were over 310 and 400 °C, respectively, which were higher than the values for the original PIB22 (Td5 = 221 °C and Tmax = 367 °C). The Tg values were −65 and −68 °C for poly(EMI-co-PIB22) and poly(EHMI-co-PIB22), respectively, being relatively higher than that for PIB22 (Tg = −84 °C). This is due to the rigid chain structure of the copolymers containing the maleimide repeating units. The 1H NMR spectra of the isolated poly(RMI-co-B22)s confirmed the formation of the copolymers with a 2:1 composition; the RMI mol % in the copolymer was 63.7 and 61.3% for poly(EMI-co-PIB22) and poly(EHMI-co-PIB22), respectively. The copolymers with PIB22 exhibited a characteristic fluidity. The flowing rate was significantly reduced by the introduction of the RMI repeating unit in the main chain, although both Tg values of the PIB macromonomers and the poly(ERMI-co-PIB22)s as the grafted copolymers were much lower than room temperature. The poly(RMI-co-PIB22)s were frozen without any fluidity even after 12 h as shown in Figure 7, while the PIB22 readily began to flow as soon as its container was turned upside down. This is due to the highly branched structure and the intermolecular interaction of the polar RMI repeating units of the copolymers. Optical and Viscoelastic Properties of the High-Tg Copolymers. The poly(RMI-co-olefin)s were soluble in many organic solvents, such as chloroform, 1,2-dichloroethane, THF, toluene, and acetone. Poly(EHMI-co-B2) was also soluble in nhexane due to a large volume of the hydrocarbon substituents of both the EHMI and B2 repeating units. Transparent films were obtained by casting the solution and drying. In the UV−vis spectra of the poly(RMI-co-olefin) films (60−70 μm thickness), no absorption was observed, and the transparency was greater than 90% in the visible light region. The refractive index values and the Abbe numbers (νD) of the poly(RMI-co-olefin)s are summarized in Table 7. The nD and νD values were 1.50−1.51 and 46−52, respectively. These values were comparable to those
polymer
RMI mol % in copolymer
nD
νDa
n∞b
Db
poly(BMI-co-O1) poly(BMI-co-O2) poly(BMI-co-O4) poly(BMI-co-B2) poly(EHMI-co-B2)
60.3 67.9 71.3 62.4 64.1
1.514 1.510 1.511 1.503 1.498
48.4 47.9 46.3 47.8 51.6
1.498 1.493 1.493 1.487 1.483
5662 5646 5917 5637 5186
Abbe number: νD = (nD − 1)/(nF − nC). bEstimated from curve fitting using the simplified Cauchy formula. The unit of D is nm2.
a
for a commodity transparent polymer, e.g., nD = 1.49−1.51 and νD = 42−53 for poly(methyl methacrylate) and other polymethacrylates.47 The poly(RMI-co-olefin)s were also revealed to have an optical property superior to that for the previously reported poly(RMI-alt-styrene)s and poly(RMI-altBCm)s (nD = 1.53−1.56 and νD = 38−44).39,40 The wavelength dispersion of the refractive indicies (nλ) is shown in Figure 8 with a fitted curve using the simplified Cauchy’s formula (eq 20).21,47 The estimated refractive index at the infinite wavelength (n∞) and the dispersion coefficient (D) are listed in Table 6. The larger the N-substituents of the RMIs and the αsubstituents of the olefins, the lower the nD values of the copolymers.
Figure 8. Wavelength dispersion of the poly(RMI-co-olefin)s: poly(BMI-co-O1), poly(BMI-co-O2), poly(BMI-co-B2), and poly(EHMIco-B2) . J
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nλ = n∞ + D/λ 2
(0.5−10 Hz), the apparent activation energies (Ea) were calculated using the following Arrhenius equation (Figure S16).
(20)
The viscoelastic properties of poly(RMI-co-B2)s were investigated at the frequencies of 0.5−10 Hz in the range of −150 °C to a temperature over each Tg. The results are summarized in Table 8, in which the results for the dynamic
Ea = 2.303R
tan δ
Tg (°C)
a
polymer poly(BMI-co-B2) poly(EHMI-co-B2) poly(BMI) poly(EHMI)
920 500 540 250
DSC
E″ at 1 Hz
temp (°C)
intensity
Eα (kJ/mol)
148 94 182 107
155 103 190 125
180 143 227 158
124 85 93 105
349 190 341 259
(21)
where R, f, and T are represented as the gas constant, frequency, and absolute temperature, respectively. The Ea values increased as the enlarged chain rigidity in the order of poly(EHMI-co-B2) < poly(EHMI) < poly(BMI-co-B2) and poly(BMI). This order agreed with the orders observed for the Tg values determined by DSC and the peak temperatures of viscoelastic measurements (E″ and tan δ data). All the polymers exhibited broad and weak β-dispersions due to the side-chain dynamics over the temperature range of −100 to 0 °C. This indicates that these high-Tg polymers have frozen main chain surrounded by the dynamically moving side groups at room temperature. The coexistence of a rigid main-chain and the flexible side groups is important for the design of the maleimide polymer materials with a high-Tg value and high toughness. Actually, transparent and tough films were obtained by casting the solutions of the poly(RMI)s and the copolymers containing the n-butyl and 2ethylhexyl substituents (see Figure 8 and Table 6).
Table 8. Viscoelastic Properties of the Poly(RMI-co-B2)s and the Poly(RMI)s at 1 Hz in the Flexural Mode E′a (MPa)
d(log f ) d(1/T )
Determined at 30 °C and 1 Hz frequency.
mechanical measurements of poly(BMI) and poly(EHMI) are also shown as a comparison. The poly(RMI)s were synthesized under the following conditions: [RMI] = 2.0 mol/L, [AIBN] = 20 mmol/L in 1,2-dichloroethane at 60 °C for 15 h; Mn = 6.8 × 104 g/mol, Mw = 24.8 × 104 g/mol, and Mw/Mn = 3.6 for poly(BMI), and Mn = 5.3 × 104 g/mol, Mw = 16.6 × 104 g/mol, and Mw/Mn = 3.1 for poly(EHMI). Figure 9 shows the
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CONCLUSIONS
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ASSOCIATED CONTENT
The copolymers of the RMIs with IB were previously reported to proceed in a highly alternating fashion to produce highperformance transparent maleimide copolymers. The other types of electron-donating olefins, such as styrenes and alkyl vinyl ethers, also undergo an alternating copolymerization with the RMIs. In this study, however, we found that the copolymerization of bulkier olefin monomers led to the formation of copolymers with a 2:1 composition of the RMIs and the olefins under penultimate unit control during the copolymerization. Recently, the AB-alternating and 2:1 sequence-controlled (AAB-periodic) copolymerizations of the RMIs with styrene derivatives and olefins have attracted growing interest because of their significant potential for manufacturing sophisticated synthetic polymers with a complicated and welldefined sequence structure. In this study, we revealed the effects of the olefin and RMI structures on the copolymerization reactivity and the properties of the resulting copolymers with the controlled sequence structures. The copolymers with a 2:1 composition of MMI and the olefins were revealed to be high-Tg copolymers, while the copolymerization of EHMI with the isobutene oligomers and macromonomers produced low-Tg copolymers. We demonstrated the rational design of the thermally stable and transparent maleimide copolymers with tunable Tg values varying over the wide temperature range of −68 to 210 °C. The sequence-controlled radical copolymerization of the RMIs with various olefins is valuable for the synthesis of high-performance transparent polymer materials.
Figure 9. Viscoelastic properties of (a) poly(EHMI-co-B2), (b) poly(EHMI), (c) poly(BMI-co-B2), and (d) poly(BMI) determined at 1 Hz and the heating rate of 2 °C/min.
comparison of the flexural results at 1 Hz upon heating from room temperature to the Tg values. The storage modulus (E′) values were 920, 500, 540, and 250 MPa at 30 °C for poly(BMIco-B2), poly(EHMI-co-B2), poly(BMI), and poly(EHMI), respectively. The flexural moduli of both copolymers were higher than those of the corresponding homopolymers, and the poly(BMI-co-B2) showed the highest E′ value. As the temperature increased, the E′ values decreased and a typical thermoplastic behavior was observed in the temperature region over the Tg. The loss modulus (E″) values of the copolymers showed a peak at 155 and 103 °C for poly(BMI-coB2) and poly(EHMI-co-B2), respectively. Similarly, the peak temperatures of tan δ were also determined as 180 and 143 °C, respectively. These results agreed with the Tg results determined by the DSC measurement; the Tg values were 148 and 94 °C for poly(BMI-co-B2) and poly(EHMI-co-B2), respectively. Based on the plots of the tan δ values as a function of the temperature determined by the flexural experiments at various frequencies
S Supporting Information *
NMR spectra, monomer reactivity ratio determination, TG curves, IB oligomer data, SEC curves, UV−vis data, and viscoelastic properties. This material is available free of charge via the Internet at http://pubs.acs.org. K
dx.doi.org/10.1021/ma401499v | Macromolecules XXXX, XXX, XXX−XXX
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AUTHOR INFORMATION
Corresponding Author
*Fax +81-72-254-9292; e-mail
[email protected]. jp (A.M.). Notes
The authors declare no competing financial interest.
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