Periodically Functionalized and Grafted Copolymers via 1:2-Sequence

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Periodically Functionalized and Grafted Copolymers via 1:2Sequence-Regulated Radical Copolymerization of Naturally Occurring Functional Limonene and Maleimide Derivatives Masaru Matsuda, Kotaro Satoh, and Masami Kamigaito* Department of Applied Chemistry, Graduate School of Engineering, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8603, Japan ABSTRACT: Naturally occurring hydroxy-functionalized limonene analogues, i.e., monoterpene alcohols such as perillyl alcohol and carveol, were radically copolymerized with cyclohexylmaleimide (CyMI) in PhC(CF3)2OH via 1:2sequence-regulated propagation to obtain periodically functionalized bio-based copolymers possessing one hydroxyl group in every three-monomer unit. Alternatively, a combination of hydroxy-functionalized maleimide (N-2-hydroxyethylmaleimide: HEMI) and limonene resulted in another periodically functionalized copolymer possessing two hydroxyl groups for every three-monomer unit. These copolymerizations were fitted well by the penultimate model, where the hydroxyl functions did not have a significant effect on the selective propagation, as has been reported for a combination of nonfunctionalized limonene and CyMI. The periodic hydroxyl groups can be quantitatively converted into carbamate moieties by a polymer reaction with isocyanate to result in another series of 1:2 and 2:1 periodically functionalized copolymers. Periodically grafted copolymers possessing one or two graft chains repeating in threemonomer units were prepared by radical copolymerization of chlorine-functionalized limonene or maleimide derivatives, which were synthesized from hydroxy-functionalized monomers, followed by ruthenium-catalyzed living radical polymerization of methyl methacrylate initiated from periodically introduced C−Cl bonds in the backbone copolymers.

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INTRODUCTION

ordered monomer sequence, the controlled sequence is limited to diads, i.e., an AB alternating fashion.37,38 Recently, we found that an unprecedented selective 1:2sequence-regulated radical copolymerization proceeds between naturally occurring terpenes, such as limonene (1) and βpinene, and maleimide derivatives in fluorinated alcohol [PhC(CF3)2OH] to give a highly controlled triad sequence in an ABB fashion, where A and B denote terpenes and maleimide derivatives, respectively.13 Furthermore, detailed mechanistic studies revealed that the key to this higher-order monomer sequence control can be attributed to the bulky and characteristic structures of naturally occurring terpenes, which generate relatively electron-rich tertiary carbon radical species with adjacent cyclohexenyl substituents, and the hydrogenbonding interaction between the fluorinated alcohol as a solvent and the maleimide unit as a comonomer.14,39 Another important point of this study, particularly with respect to the sustainable development of polymer materials, is the use of relatively abundant naturally occurring compounds as renewable resources for novel bio-based copolymers40−47 possessing controlled monomer sequences as well as high glass transition

Sequence regulation in synthetic polymers is one of the most challenging topics in polymer chemistry because polymer structures and properties depend on not only the monomer compositions but also their arrangements.1−34 In particular, arrangements of functional groups in polymer chains are critical to determining higher-order polymer structures as well as polymer properties and functions, as is obvious in not only many synthetic polymers but also natural polymers such as proteins.35 In polymer synthesis, polar functional groups, however, often disturb the propagation reaction of growing chains, particularly in ionic and coordination polymerization, and are usually protected during the polymerization reaction for their introduction into polymer chains. In contrast, radical polymerization is highly tolerant to these functional groups due to the neutrality of the growing species and can be widely used for direct polymerization of various functional monomers.36 However, in general, none of the chain-growth polymerization reactions, including radical polymerization, can achieve control of the monomer sequence; monomers react statistically, in most cases, according to their monomer reactivity ratios to result in statistical or “random” incorporation of the functional groups into the copolymers. Although alternating radical copolymerizations are typical exceptions that enable some © 2013 American Chemical Society

Received: May 16, 2013 Revised: June 20, 2013 Published: July 3, 2013 5473

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Scheme 1. Periodically Functionalized and Grafted Copolymers via 1:2-Sequence-Regulated Radical Copolymerization of Naturally Occurring Functional Limonene and Maleimide Derivatives

mers.36,50,51 Although 1:1 alternating copolymerization of styrene and maleimide derivatives has already been employed for the synthesis of various alternating graft or heterograft copolymers,52−60 there are no reports on the synthesis of such periodically grafted copolymers.

temperatures and optical activities originating from the bulky and chiral characteristics of naturally occurring monomers. This study is focused on the periodic incorporation of functional moieties into bio-based sequence-regulated copolymers via direct radical copolymerization of functionalized limonene or maleimide derivatives in fluorinated alcohol (Scheme 1). More specifically, we first focused on monoterpene alcohol, such as perillyl or perilla alcohol (2; p-mentha1,8-dien-7-ol) and carveol (3; p-mentha-1,8-dien-6-ol), which can be obtained from a variety of essential oils, e.g., lavender, spearmint, and orange,48,49 as hydroxy-functionalized natural terpene monomers for periodically functionalized copolymers possessing one hydroxyl group for every three-monomer unit via radical copolymerization with N-cyclohexylmaleimide (CyMI) in PhC(CF3)2OH. In addition, nonfunctionalized limonene (1) was copolymerized with N-2-hydroxyethylmaleimide (HEMI) as a hydroxy-functionalized maleimide for periodically functionalized copolymers possessing two hydroxyl groups for every three-monomer unit. These incorporated hydroxyl groups in the obtained periodic copolymers were further converted into carbamate groups via a polymer reaction with isocyanate to generate different periodically functionalized copolymers. As another application of selective 1:2-sequence-regulated radical copolymerization, periodically grafted copolymers were targeted by the copolymerization of functionalized monomers possessing a reactive C−Cl bond, which were synthesized from hydroxy-functionalized monomers, followed by the metalcatalyzed living radical polymerization of vinyl mono-

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EXPERIMENTAL SECTION

Materials. (+)-D-Limonene (1) (Aldrich, 97%), (−)-perillyl alcohol (2) (Aldrich, 96%), (−)-carveol (3) (Aldrich, 97%), methyl methacrylate (MMA; Tokyo Kasei; >98%), and PhC(CF3)2OH (Wako, >99%) were distilled from calcium hydride under reduced pressure before use. N-Cyclohexylmaleimide (CyMI) (Aldrich, 97%) and 2,2′-azobis(isobutyronitrile) (AIBN) (Kishida, >99%) were purified by recrystallization from toluene and methanol, respectively. N-2-Hydroxyethylmaleimide (HEMI) was synthesized according to the literature.61 Perillyl 2-chloro-2-phenylacetate (2-Cl) and 2maleimidoethyl 2-chloro-2-phenylacetate (HEMI-Cl) were synthesized as below. α-Chlorophenylacetyl chloride (Merck, 98%), Ru(Ind)Cl(PPh3)2, (provided from Wako), and Al(acac)3 (acac: acetylacetonate; Wako, >98%) were used as received. Toluene (Kanto, >99.5%; H2O 99%). 1H NMR (CDCl3, rt): δ 1.33−2.15 (m, 10H), 4.54 (m, 2H, CH2−O), 5474

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4.67−4.71 (m, 2H, CH2C), 5.35 (s, 1H, CH−Cl), 5.70 (m, 1H, CHC), 7.45−7.49 (m, 3H, m, p-ArH), 7.30−7.40 (m, 2H, o-ArH). 2-Maleimidoethyl 2-Chloro-2-phenylacetate (HEMI-Cl). αChlorophenylacetyl chloride (15.0 mL, 0.103 mol) was added dropwise to a solution of HEMI (14.033 g, 0.0994 mol) and triethylamine (14.3 mL, 0.103 mol) in dried CHCl3 (100 mL) at 0 °C. After stirring at ambient temperature, the solution was washed with water, and then all organic phase was collected. The obtained crude product was purified by recrystallization from acetone. HEMI-Cl was obtained as white solid (23.3 g, yield = 80%, purity >99%). 1H NMR (CDCl3, rt): δ 3.73−3.85 (t, 2H, N−CH2), 4.23−4.38 (m, 2H, CH2− O), 5.29 (s, 1H, CH−Cl), 6.63 (s, 2H, CHCH), 7.38−7.44 (m, 3H, m, p-ArH), 7.32−7.36 (m, 2H, o-ArH). Radical Copolymerization in PhC(CF3)2OH. Polymerization was carried out by the syringe technique under dry nitrogen in oven-dried and sealed glass tubes. A typical example for 2 and CyMI copolymerization with AIBN in PhC(CF3)2OH is given below. In a 50 mL round-bottomed flask were placed PhC(CF3)2OH (0.87 mL), PhC(CF3)2OH solution of CyMI (3.0 mL of 1200 mM, 3.6 mmol), 2 (0.27 mL, 1.7 mmol), and PhC(CF3)2OH solution of AIBN (0.36 mL, 0.036 mmol) at room temperature. The total volume of the reaction mixture was 4.5 mL. Immediately after mixing, aliquots (0.6 mL each) of the solution were distributed via a syringe into baked glass tubes, which were then sealed by flame under nitrogen atmosphere. The tubes were immersed in thermostatic oil bath at 60 °C. In predetermined intervals, the polymerization was terminated by cooling the reaction mixture to −78 °C. Monomer conversion was determined from the concentration of residual monomer measured by 1H NMR spectroscopy. The quenched reaction solutions were evaporated to dryness to give poly(2-co-CyMI). Polymer Reaction between Hydroxy-Functionalized Copolymers and Isocyanate. The polymer reaction between the hydroxyfunctionalized copolymers and isocyanate was carried out under a dry nitrogen atmosphere in a 50 mL round-bottom flask. A typical example for poly(2-co-CyMI) is given below. Phenyl isocyanate (0.21 mL, 1.9 mmol) and dibutyltin dilaurate (1.17 mL, 1.9 mmol) as a catalyst were added to a solution of poly(2-co-CyMI) (0.20 g) in anhydrous THF (10 mL), and then the mixture was stirred for 24 h at 40 °C to give a turbid solution. After the removal of the solvent by evaporation, the product was purified by precipitation (0.19 g, yield 78%). Ruthenium-Catalyzed Living Radical Graft Polymerization of MMA. A typical example for the graft polymerization of MMA from poly(2-Cl-co-CyMI) with Ru(Ind)Cl(PPh3)2/Al(acac)3 is given below. In a 50 mL round-bottomed flask was placed poly(2-Cl-co-CyMI) (87.6 mg, 0.13 mmol C−Cl bonds), Ru(Ind)Cl(PPh3)2 (10.1 mg, 0.013 mmol), Al(acac)3 (0.1686 g, 0.52 mmol), toluene (9.98 mL), and MMA (2.78 mL, 26 mmol) at room temperature. The total volume of reaction mixture was 13 mL. Immediately after mixing, aliquots (1.0 mL each) of the solution were distributed via a syringe into baked glass tubes, which were then sealed by flame under nitrogen atmosphere. The tubes were immersed in thermostatic oil bath at 80 °C. In predetermined intervals, the polymerization was terminated by cooling the reaction mixture to −78 °C. Monomer conversion was determined from the concentration of residual monomer measured by 1 H NMR spectroscopy. The quenched reaction solutions were precipitated into n-hexane and isolated by centrifugation. After two times of the precipitation, the precipitate was evaporated to dryness to yield the product, which was subsequently dried overnight in vacuo at room temperature. Measurements. Monomer conversion was determined from the concentration of residual monomer measured by 1H NMR spectroscopy with reaction solvent as an internal standard. 1H NMR spectra for monomer conversion were recorded in CDCl3 at 25 °C on a Varian Mercury 300 spectrometer, operating at 300 MHz, and 1H NMR spectra for the product copolymer were recorded in CDCl3 at a 55 °C on a JEOL ECS-400 spectrometer, operating at 400 MHz. MALDI-TOF-MS spectra were measured on a Shimazu AXIMA-CFR Plus mass spectrometer (linear mode) with trans-2-[3-(4-tertbutylphenyl)-2-methyl-2-propenylidene]malononitrile (DCTB) as the ionizing matrix and sodium trifluoroacetate as the ion source.

The number-average molecular weight (Mn) and the molecular weight distribution (Mw/Mn) of the copolymers were determined by sizeexclusion chromatography (SEC) in THF at 40 °C on two polystyrene gel columns [Shodex KF-805 L (pore size: 20−1000 Å; 8.0 mm i.d. × 30 cm) × 2; flow rate 1.0 mL/min] for 1-co-CyMI, 2-co-CyMI, 3-coCyMI, 2-Cl-co-CyMI, 1-co-HEMI-Cl, and the graft copolymers or in DMF containing 100 mM LiCl at 40 °C on two polystyrene gel columns [TSKgel α-M (pore size: 13 μm; 7.8 mm i.d. × 30 cm) and TSKgel α-3000 (pore size: 7 μm; 7.8 mm i.d. × 30 cm); flow rate 1.0 mL/min] for 1-co-HEMI and its carbamate copolymers, each connected to a JASCO PU-2080 precision pump and a JASCO RI2031 detector. The columns were calibrated against eight standard poly(MMA) samples (Shodex; Mp = 202−1 950 000; Mw/Mn = 1.02− 1.09). The MALLS analysis was performed in THF on a Dawn HELEOS photometer (Wyatt Technology; λ = 633 nm). The refractive index increments (dn/dc = 0.1305 [poly(2-Cl-co-CyMI)], 0.1340 [poly(1-co-HEMI-Cl)], 0.0928 [graft copolymers obtained from poly(2-Cl-co-CyMI)], and 0.101 [graft copolymers obtained from poly(1-co-HEMI-Cl) mLg−1] were measured in THF at 40 °C on Optilab rEX (Wyatt Technology) (λ = 633 nm).

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RESULTS AND DISCUSSION Periodically Hydroxy-Functionalized Copolymers. Naturally occurring hydroxy-functionalized limonene analogues, monoeterpene alcohols (2 and 3), were copolymerized with a nonfunctionalized maleimide, CyMI, in PhC(CF3)2OH at 60 °C, where the initial monomer feed ratio of the two monomers was [Lim]0/[CyMI]0 = 1/2 (Figure 1). Alter-

Figure 1. Radical copolymerization of limonene analogues (Lim) and maleimide derivatives (MI) in PhC(CF3)2OH at 60 °C; [Lim]0/[MI]0 = 400/800 mM, [AIBN]0 = 8.0 mM.

natively, a hydroxy-functionalized maleimide, HEMI, was similarly copolymerized with a nonfunctionalized limonene (1) under the same conditions. In all cases, both terpene and maleimide derivatives were consumed at the same rate in a manner similar to that for the copolymerization of 1 and CyMI (red symbols in Figure 1),13,14 suggesting that similar 1:2 copolymerizations occur irrespective of the presence of hydroxyl groups in the monomers. In particular, the consumption rates of 1 and HEMI were almost the same as those of 1 and CyMI, whereas the rates were much smaller for 2 and 3. The molecular weights of the products decreased with decreasing polymerization rate; i.e., 1 > 2 > 3, most likely because the chain-transfer reactions via the allylic hydrogen abstraction became more pronounced in the order of methyl or primary (1) < primary alkoxy (2) < secondary alkoxy (3) via the formation of more stable allylic radical species. The structures of the resultant copolymers were analyzed by 1 H NMR spectroscopy (Figure 2). Figure 2 shows the 1H NMR 5475

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Figure 2. 1H NMR spectra [(A−C) in CDCl3 at 55 °C, (D) in DMSO-d6 at 80 °C] of the copolymers obtained in PhC(CF3)2OH at 60 °C; [Lim]0/ [MI]0 = 400/800 mM, [AIBN]0 = 8.0 mM.

Figure 3. Copolymer composition curves for the copolymerization of limonene analogues (M1) and maleimide derivatives (M2) in PhC(CF3)2OH at 60 °C; [M1]0 + [M2]0 = 1200 mM, [M1]0/[M2]0 = 1/7, 1/3, 1/1, 3/1, 7/1, [AIBN]0 = 8.0 mM. M1/M2: 1/CyMI (A), 2/CyMI (B), 3/CyMI (C), 1/ HEMI (D). The dotted lines were fitted by the modified Kelen−Tüdõs method for (A−C) or curve-fitting method for (D), assuming that the values of r11 and r21 are 0.

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Figure 4. MALDI-TOF-MS spectra of poly(1-co-CyMI) (Mn = 2100, Mw/Mn = 1.55) (A), poly(2-co-CyMI) (Mn = 1700, Mw/Mn = 1.79) (B), and poly(1-co-HEMI) (Mn = 1900, Mw/Mn = 1.67) (C) obtained with CBTC in PhC(CF3)2OH at 60 °C; [Lim]0/[MI]0 = 400/800 mM, [CBTC]0 = 10 mM, [AIBN]0 = 5.0 mM. The calculated molar masses are those of the expected copolymer structures with sodium ion from the salt (CF3CO2Na) for the MALDI-TOF-MS analysis.

ratios. All composition ratios are nearly constant at 0.33, irrespective of the presence of hydroxyl groups in the monomers (2, 3, and HEMI). Similar to reports for nonfunctionalized monomer pairs (1 and CyMI),13,14 the monomer reactivity ratios were best fitted by a penultimate model, which was used to calculate r12 and r22 based on the modified Kelen−Tüdõs or curve-fitting method, assuming that radical homopropagation of unconjugated limonene analogues does not occur (r11 = r21 = 0). The r12 and r22 values were calculated to be 14 and 0.017 for 2 and CyMI, 12 and 0.017 for 3 and CyMI, and 14.5 and 0.0060 for 1 and HEMI, respectively. Thus, irrespective of the hydroxyl functions, the ∼∼M1M2• radical favors M2 (r12 > 10) to result in ∼∼M2M2•, which exclusively reacts with M1 (r22 < 0.02) to give ∼∼M2M1•, and the resulting ∼∼M2M1• selectively adds to M2 (r21 = 0) to generate the ∼∼M1M2• radical again. These results indicate that these three consecutive selective radical propagations also occur for limonene analogues with similar skeletons or maleimide derivatives irrespective of additional functional groups. Thus, specific structures of limonene with isopropenylcyclohexene units, maleimide comonomers, and fluoroalcohol are crucial for 1:2-sequence-regulated radical copolymerization.13,14 The RAFT copolymerization of 2 and CyMI or 1 and HEMI was also investigated using S-cumyl S′-butyl trithiocarbonate (CBTC) as a RAFT agent to obtain chain-end-defined copolymers for the analysis of monomer sequences in the hydroxy-functionalized copolymers by MALDI-TOF-MS (Figure 4).13,14,36 The main series of peaks in the hydroxyfunctionalized copolymers (Figure 4B,C) are separated by the total molar masses for one limonene analogue (L or P) and two

spectra of poly(1-co-CyMI) (A), poly(2-co-CyMI) (B), poly(3co-CyMI) (C), and poly(1-co-HEMI) (D). In all of the spectra, the strong signals near 1−3 ppm can be assigned to aliphatic protons of the limonene analogue and cyclohexyl maleimide units including the pendent and main-chain groups. In addition to these signals, several characteristic protons, i.e., olefinic protons (e) in the limonene analogue units and methine protons (l) adjacent to the nitrogen in CyMI units, appeared near 5−6 and 4 ppm, respectively. For the copolymers with HEMI (Figure 2D), two methylene groups (q and r) both adjacent to the heteroatoms appeared at 3.3−3.6 ppm along with the emergence of hydroxyl protons (s) at 4.3−4.5 ppm. The copolymer compositions, i.e., limonene analogues/ maleimide derivatives [Lim/MI(NMR)], were then calculated from the peak intensity ratios of these characteristic protons (e for 1, 2, and 3; l for CyMI; q and r for HEMI). The obtained compositions agreed well with the values calculated from the initial charge ratio and conversions of the two monomers, indicating that all of the consumed monomers were incorporated into the copolymers. Furthermore, all of these values were very close to 1:2. These results indicate that 1:2 copolymers possessing hydroxy groups were similarly obtained in the radical copolymerization of limonene analogues and maleimide derivatives at 1:2 feed ratios in PhC(CF3)2OH. To obtain monomer sequence information, the monomer reactivity ratio of limonene analogues (M1) and maleimide derivatives (M2) was determined by copolymerization at various monomer feed ratios in PhC(CF3)2OH at 60 °C. Figure 3 shows the resultant copolymer composition curves, in which the incorporated ratios of limonene derivatives in the resulting copolymers are plotted against the monomer feed 5477

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Figure 5. 1H NMR spectra (DMSO-d6, 80 °C) of the carbamate-functioned copolymers obtained by the polymer reaction of poly(2-co-CyMI) (A) or poly(1-co-HEMI) (B) with phenyl isocyanate.

and r′) originally adjacent to the hydroxyl moieties. The polymer reaction proceeded almost quantitatively, and the ratios of limonene analogues and maleimide units were nearly 1:2, as calculated from the peak intensity ratios based on the phenyl groups, indicating the formation of another series of periodically functionalized copolymers possessing one or two carbamate moieties for each three-monomer unit. Periodically Grafted Copolymers. As another series of functional monomers for synthesizing periodically grafted copolymers, limonene and maleimide derivatives, 2-Cl and HEMI-Cl, possessing reactive C−Cl bonds as initiating moieties for living radical polymerization, were prepared from 2 and HEMI, respectively, via esterification of their hydroxyl groups with α-chlorophenylacetyl chloride.62,63 Free radical copolymerizations of 1:2 mixtures of 2-Cl and CyMI or 1 and HEMI-Cl were then conducted in PhC(CF3)2OH at 60 °C (Figure 6). In both cases, the comonomers were consumed at the same rate to give copolymers similar to those obtained for 1 and CyMI (red circles in Figure 6), whereas the consumption rates of 2-Cl and CyMI were low (blue circles), similar to the result for 2 and CyMI (see above). These results suggest that the presence of C−Cl bonds does not have a significant effect on copolymerization in fluorinated alcohol. To obtain more detailed information regarding the monomer sequence in radical copolymerization of C−Cl-containing monomers, monomer reactivity ratios were analyzed by varying the comonomer feed ratios ([Lim (M1)]0/[MI (M2)]0 = 1/7, 1/3, 1/1, 3/1, 7/1) in PhC(CF3)2OH at 60 °C. As shown in Figure 7, the copolymer composition ratios are almost constant at 0.33, irrespective of the comonomer feed ratios, as was

maleimide derivative (M or H) units, similar to those reported for limonene (L) and CyMI (M) (Figure 4A), and can be assigned to a series of end-to-end sequenced copolymers [C(M-M-P)n-M-S or C-(H-H-L)n-H-S] (C: cumyl group; S: trithiocarbonyl group) with well-defined initiating and capping terminals along with the M-M-P or H-H-L repeating sequence. The small series of peaks, of which the molar mass differences compared with the main series are close to those of the monomer units, can be assigned to other copolymer series with one additional maleimide and one additional limonene analogue unit at the capping terminals, as has been reported for copolymers of 1 and CyMI.13 However, much smaller peaks, which are observed for the copolymers of 2, might be due to irreversible chain-transfer reactions originating from the primary alkoxy allyl hydrogen in 2 as mentioned above. These results again indicate that the copolymerization of these hydroxy-functionalized monomers proceeds in the ABBsequence fashion irrespective of the functional groups to result in periodically functionalized copolymers with one or two hydroxyl groups repeating in three-monomer units. To synthesize other periodically functionalized copolymers by utilizing the resulting hydroxy-functionalized copolymers, the polymer reaction between poly(2-co-CyMI) or poly(1-coHEMI) and phenyl isocynate was investigated in THF in the presence of a tin catalyst at 40 °C. The reaction between alcohol and isocyanate generally proceeds at a high yield and is often effectively used for polymer reactions. Figure 5 shows the 1 H NMR spectra of the copolymers obtained after the polymer reaction, in which phenyl (u) and carbamate protons (t) clearly appeared along with downfield shifts of methylene protons (i′ 5478

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obtained from their parent hydroxy-functionalized monomers, respectively, whereas several peaks (i″ in Figure 9A and r″ in Figure 9B) shifted downfield, along with the emergence of new peaks (v and w) due to esterification by the α-chlorophenylacetyl moiety. The copolymer compositions, 2-Cl/CyMI(NMR) or 1/HEMI-Cl(NMR), were then calculated from the peak intensity ratios of phenyl (w for 2-Cl) to methine protons adjacent to nitrogen (j for CyMI) or olefin (e for 1) to phenyl (w for HEMI-Cl) protons, respectively, and were observed to be nearly 1/2, which agrees well with the values calculated from the initial charge ratio and conversion of the two monomers. Thus, the obtained copolymers, poly(2-Cl-coCyMI) and poly(1-co-HEMI-Cl), possess one and two C−Cl bonds in each repeating three-monomer unit, respectively. Furthermore, the number-average initiating sites [Fn(C−Cl)] per backbone polymer chain were calculated by absolute Mn values determined by MALLS [Mn(MALLS)] and their copolymer compositions [M1/M2(NMR)] and were found to be 23.6 for poly(2-Cl-co-CyMI) and 40.4 for poly(1-co-HEMICl) (Table 1). The metal-catalyzed living radical graft polymerization of MMA was then examined using the periodically chlorinefunctionalized copolymers as macroinitiators in conjunction with Ru(Ind)Cl(PPh3)2 as a catalyst in the presence of Al(acac)3, which was used as a cocatalyst.64−68 As shown in Figure 8C,D, the SEC curves shifted to higher molecular weights while retaining unimodal shapes as the MMA was consumed. Although the Mn(SEC) values measured by the RI detector were lower than the calculated values, assuming that the consumed MMA was polymerized from the C−Cl bonds, due to the difference in the hydrodynamic volumes between MMA and the PMMA standard as well as the branched structures of the resulting graft copolymers, the absolute Mn values determined by MALLS were close to the calculations (Table 1). The 1H NMR spectra of the graft copolymers primarily exhibited the characteristic signals of the methyl (c), methylene (d), and methoxy (b) of the PMMA graft chains in addition to the weaker signals of phenyl (w) protons originating from the initiating moieties of the backbone (Figure 9C,D). The number-average degrees of polymerization of MMA [m(NMR)], as calculated from the initiating moiety (w/5) to the main chain MMA (b/3) unit, were found to be m(NMR) = 96 (Figure 9C) and 40 (Figure 9D), which are comparable to those calculated from the MMA conversion and initial

Figure 6. Copolymerization of limonene analogues (Lim) and maleimide derivatives (MI) possessing a reactive C−Cl bond for the metal-catalyzed living radical polymerization in PhC(CF3)2OH at 60 °C; [Lim]0/[MI]0 = 400/800 mM, [AIBN]0 = 8.0 mM.

observed for their precursors; i.e., hydroxy-functionalized monomers, 2, or HEMI (see Figure 3B,D). The monomer reactivity ratios were calculated by the modified Kelen−Tüdõs method for the penultimate model, assuming that homopropagation of unconjugated olefin does not occur. These ratios are r12 = 17 and r22 = 0.036 for 2-Cl/CyMI and r12 = 15 and r22 = 0.028 for 1/HEMI-Cl and are similar to those for the hydroxy-functionalized monomers. Thus, a similar 1:2 monomer sequence also forms for the radical copolymerization of C−Cl-containing monomers. The periodically chlorine-functionalized copolymers were then used as a backbone or as stem polymers, from which subsequent metal-catalyzed living radical graft polymerization of MMA was initiated for the synthesis of periodically grafted copolymers based on the “grafting-from” method.36,50,51 Prior to graft polymerization, a relatively large amount of periodically chlorine-functionalized copolymers was synthesized by radical copolymerization of 2-Cl/CyMI or 1/HEMI-Cl in PhC(CF3)2OH and was characterized by SEC (Figure 8A,B) and 1 H NMR (Figure 9A,B). The obtained copolymers show molecular weights similar to those presented in Figure 8, where the Mn values measured by MALLS are slightly higher than those obtained by an RI detector equipped with SEC, most likely due to difference between the hydrodynamic volumes of the copolymers and those of standard PMMA. The resultant poly(2-Cl-co-CyMI) and poly(1-co-HEMI-Cl) show 1H NMR spectra that are similar to those of poly(2-coCyMI) (Figure 2B) and poly(1-co-HEMI) (Figure 2D)

Figure 7. Copolymer composition curves for the copolymerization of Lim and MI in PhC(CF3)2OH at 60 °C; [Lim]0 + [MI]0 = 1200 mM, [Lim]0/ [maleimide]0 = 1/7, 1/3, 1/1, 3/1, 7/1, [AIBN]0 = 8.0 mM. The dotted lines were fitted by the modified Kelen−Tüdõs method assuming that the values of r11 and r21 are 0. 5479

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Figure 8. SEC curves of periodically chlorine-functionalized copolymers obtained by radical copolymerization of 2-Cl/CyMI (A) or 1/HEMI-Cl (B) and periodically grafted copolymers obtained by ruthenium-catalyzed living radical polymerization of MMA initiated from poly(2-Cl-co-CyMI) (C) and poly(1-co-HEMI-Cl) (D). Polymerization condition: [M1]0/[M2]0/[AIBN]0 = 400/800/8.0 mM in PhC(CF3)2OH at 60 °C for (A) and (B); [MMA]0/[C−Cl]0/[Ru(Ind)Cl(PPh3)2]/[Al(acac)3]0 = 2000/10/1.0/40 mM in toluene at 80 °C for (C); [MMA]0/[C−Cl]0/[Ru(Ind)Cl(PPh3)2]/[Al(acac)3]0 = 2000/10/0.50/20 mM in toluene at 40 °C for (D).

Figure 9. 1H NMR spectra of periodically chlorine-functionalized copolymers (CDCl3, 55 °C) obtained by radical copolymerization of 2-Cl/CyMI (A) or 1/HEMI-Cl (B) and periodically grafted copolymers (acetone-d6, 50 °C) obtained by ruthenium-catalyzed living radical polymerization of MMA initiated from poly(2-Cl-co-CyMI) (C) and poly(1-co-HEMI-Cl) (D). Polymerization condition: [M1]0/[M2]0/[AIBN]0 = 400/800/8.0 mM in PhC(CF3)2OH at 60 °C for (A) and (B); [MMA]0/[C−Cl]0/[Ru(Ind)Cl(PPh3)2]/[Al(acac)3]0 = 2000/10/1.0/40 mM in toluene at 80 °C for (C); [MMA]0/[C−Cl]0/[Ru(Ind)Cl(PPh3)2]/[Al(acac)3]0 = 2000/10/0.50/20 mM in toluene at 40 °C for (D).

Table 1. Periodically Chlorine-Functionalized Copolymers by Radical Copolymerization of Limonene Analogues (M1) and Maleimide Derivatives (M2) and Periodically Grafted Copolymers by Ruthenium-Catalyzed Living Radical Graft Polymerization of MMA Mn entry Lim (M1) a

1 2b 3a 4c

2-Cl n/a 1 n/a

m

MI (M2)

conv (%)d M1/M2 or MMA

M1/M2 (NMR)e

Mw (MALLS)f

SECg

MALLSf

calcdh

Mw/Mn (SEC)g

CyMI n/a HEMI-Cl n/a

49/48 44 91/88 20

33/67 n/a 35/65 n/a

24 200 410 500 37 200 331 400

9 000 96 100 7 300 86 000

15 900 246 300 14 800 167 100

n/a 247 500 n/a 176 600

1.74 1.71 2.47 1.74

Fn(C−Cl)i calcdj 23.6 n/a 40.4 n/a

n/a 88 n/a 40

NMRk n/a 96 n/a 40

a Polymerization condition: [M1]0/[M2]0/[AIBN]0 = 400/800/8.0 mM in PhC(CF3)2OH at 60 °C. bPolymerization condition: [MMA]0/[C−Cl]0/ [Ru(Ind)Cl(PPh3)2]0/[Al(acac)3]0 = 2000/10/1.0/40 mM in toluene at 80 °C. cPolymerization condition: [MMA]0/[C−Cl]0/[Ru(Ind)Cl(PPh3)2]0/[Al(acac)3]0 = 2000/10/0.50/20 mM in toluene at 40 °C. dDetermined by 1H NMR analysis of residual monomers in the reaction mixture. eDetermined by 1H NMR analysis of the isolated copolymers. fMeasured by multiangle laser light scattering (MALLS) detector equipped with size-exclusion chromatography (SEC). gDetermined by SEC. hMn(calcd) = Mn(backbone polymer, MALLS) + m × Fn × MW(MMA). i Determined by Mn(MALLS) and M1/M2(NMR). j([MMA]0/[C−Cl]0) × conv(MMA). kDetermined by 1H NMR.

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monomer feed ratios, m(calcd) = 88 and 40, respectively. These results suggest that the graft polymerization successfully proceeded to afford periodically grafted copolymers with one or two PMMA grafting chains for every repeating threemonomer unit.

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CONCLUSION In conclusion, functionalized limonene or maleimide derivatives possessing hydroxyl and other related groups were employed in 1:2- or 2:1-sequence-regulated radical copolymerization with nonfunctionalized maleimide or limonene in fluorinated alcohol to achieve periodically functionalized copolymers with one or two functional moieties in repeating three-monomer units, respectively. The periodic hydroxyl groups can be quantitatively converted into carbamate groups by a polymer reaction with isocyanate to result in another series of periodically functionalized copolymers. Periodically grafted copolymers with one or two graft chains in repeating three-monomer units can also be prepared via the 1:2- or 2:1-sequence-regulated radical copolymerization of chlorine-functionalized monomers followed by metal-catalyzed living radical polymerization. Thus, naturally occurring functional limonene analogues are useful building blocks for sequence-regulated functional bio-based copolymers, which can exhibit novel functions and properties originating from the regulated sequence distribution of functional groups and other structural features such as rigidity and chirality.13,14

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (M.K.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported in part by the Funding Program (Green Innovation GR051; Precision Polymerization of PlantDerived Vinyl Monomers for Novel Bio-Based Polymers) for Next-Generation World-Leading Researchers from the Cabinet Office, Government of Japan and Program for Leading Graduate Schools “Integrative Graduate Education and Research Program in Green Natural Sciences”.

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