Biobased Polymers via Radical Homopolymerization and Copolymerization of a Series of Terpenoid-Derived Conjugated Dienes with exo-Methylene and 6-Membered Ring

A series of exo-methylene 6-membered ring conjugated dienes, which are directly or indirectly obtained from terpenoids, such as β-phellandrene, carvone, piperitone, and verbenone, were radically polymerized. Although their radical homopolymerizations were very slow, radical copolymerizations proceeded well with various common vinyl monomers, such as methyl acrylate (MA), acrylonitrile (AN), methyl methacrylate (MMA), and styrene (St), resulting in copolymers with comparable incorporation ratios of bio-based cyclic conjugated monomer units ranging from 40 to 60 mol% at a 1:1 feed ratio. The monomer reactivity ratios when using AN as a comonomer were close to 0, whereas those with St were approximately 0.5 to 1, indicating that these diene monomers can be considered electron-rich monomers. Reversible addition fragmentation chain-transfer (RAFT) copolymerizations with MA, AN, MMA, and St were all successful when using S-cumyl-S’-butyl trithiocarbonate (CBTC) as the RAFT agent resulting in copolymers with controlled molecular weights. The copolymers obtained with AN, MMA, or St showed glass transition temperatures (Tg) similar to those of common vinyl polymers (Tg ~ 100 °C), indicating that biobased cyclic structures were successfully incorporated into commodity polymers without losing good thermal properties.


Introduction
Polymeric materials, such as plastics, rubbers, and fibers, are indispensable products in our daily life and are produced from a variety of low-molecular-weight molecules as building blocks. Radical polymerization is one of the most effective and widely used techniques to produce a wide variety of polymeric materials from various vinyl monomers due to the high reactivity of radical species and its robustness toward polar functional groups in the monomer substituents as well as aqueous compounds in the reaction mixture [1]. In addition, recent progress in controlled/living or reversible deactivation radical polymerization (RDRP) has dramatically expanded the scope of radical polymerization to synthesize precision polymers with various well-defined structures that can be used as high-performance materials [1][2][3][4][5][6][7][8][9][10].
The exo-methylene-conjugated dienes can be radically homo-and copolymerized with roleum-derived common vinyl monomers. However, there have been no comprehensive studie radical homo-and copolymerization of these terpenoid-derived exo-methylene 6-membered ring jugated dienes except for a few results on the homo-and copolymerizations with some specifi nomers, such as styrene, acrylonitrile, and N-phenylmaleimide [65][66][67]. These results suggest tha e of the exo-methylene 6-membered ring conjugated dienes do not seem highly reactive in radica opolymerization and that they can be copolymerized with electron-deficient monomers. Simila ults were reported for 3-methylenecyclopentene, which is an exo-methylene 5-membered ring jugated diene obtained from the ring-closing metathesis of myrcene [68,69].
In this work, we fully examined radical homo-and copolymerizations of a series of terpenoid ived exo-methylene 6-membered ring conjugated dienes (β-Phe, HCvD, PtD, and VnD) usin ious common vinyl monomers, including methyl acrylate (MA), acrylonitrile (AN), methy thacrylate (MMA), and styrene (St), as comonomers to evaluate their reactivity in racia ymerization and the thermal properties of the resulting biobased polymers as heat-resistan terials with unique alicyclic skeletons originating from naturally occurring terpenoid molecules.

esults and Discussion
Scheme 1. Radical homo-and copolymerization of a series of terpenoid-derived exo-methylene 6-membered-ring conjugated dienes.
In this work, we fully examined radical homo-and copolymerizations of a series of terpenoid-derived exo-methylene 6-membered ring conjugated dienes (β-Phe, HCvD, PtD, and VnD) using various common vinyl monomers, including methyl acrylate (MA), acrylonitrile (AN), methyl methacrylate (MMA), and styrene (St), as comonomers to evaluate their reactivity in racial polymerization and the thermal properties of the resulting biobased polymers as heat-resistant materials with unique alicyclic skeletons originating from naturally occurring terpenoid molecules.
The stereoregularity is another interesting issue of the obtained polymers. The poly((−)-HCvD) obtained via radical polymerization of the chiral monomer was easily soluble in common organic solvents, such as hexane and tetrahydrofuran, whereas an insoluble polymer in these solvents was obtained via the regioselective and stereospecific cationic polymerization of the same chiral monomer [64]. The 13 C NMR spectrum of poly((−)-HCvD) obtained via the radical process showed a larger number of peaks than that obtained in the cationic process, in which the chiral center attached to the isopropyl group can dictate the stereospecificity at low temperature ( Figure S2).
Thus, both regioselectivity and stereospecificity in the polymerizations of a series of terpenoid-derived exo-methylene conjugated dienes were influenced by the propagating species, among which the radical species resulted in lower selectivities due to the free radical propagating species as well as the higher reaction temperature.
The thermal properties of poly(β-Phe), poly((−)-HCvD), and poly(PtD) obtained in radical polymerization at 100 • C were evaluated by differential scanning calorimetry (DSC) ( Figure S3). The glass transition temperatures (T g ) were observed for all polymers. The T g s of poly((−)-HCvD) and poly(PtD) were approximately 110 • C due to the cyclic structures incorporated into the main chain via regioselective 1,4-conjugation, whereas the T g of poly(β-Phe) was 66 • C, which was lower than that of poly(β-Phe) (T g = 87 • C) obtained in cationic polymerization [63] due to the decreased 1,4-regioselectivity in radical polymerization. In addition, the poly((−)-HCvD) obtained in radical polymerization showed a slightly lower T g than that obtained in the cationic polymerization of (−)-HCvD (T g = 105 vs. 114 • C) and showed no melting peak due to the lack of stereospecificity in radical polymerization [64].
These results show that a series of the exo-methylene 6-membered ring conjugated dienes are slowly homopolymerized by radical species and that the regioselectivity and stereospecificity are lower for some monomers than those in cationic polymerization. The stereoregularity is another interesting issue of the obtained polymers. The poly((−)-HCvD) obtained via radical polymerization of the chiral monomer was easily soluble in common organic solvents, such as hexane and tetrahydrofuran, whereas an insoluble polymer in these solvents was obtained via the regioselective and stereospecific cationic polymerization of the same chiral monomer [64]. The 13 C NMR spectrum of poly((−)-HCvD) obtained via the radical process showed a larger number of peaks than that obtained in the cationic process, in which the chiral center attached to the isopropyl group can dictate the stereospecificity at low temperature ( Figure S2).
Thus, both regioselectivity and stereospecificity in the polymerizations of a series of terpenoidderived exo-methylene conjugated dienes were influenced by the propagating species, among which the radical species resulted in lower selectivities due to the free radical propagating species as well as the higher reaction temperature.
The thermal properties of poly(β-Phe), poly((−)-HCvD), and poly(PtD) obtained in radical polymerization at 100 °C were evaluated by differential scanning calorimetry (DSC) ( Figure S3). The glass transition temperatures (Tg) were observed for all polymers. The Tgs of poly((−)-HCvD) and poly(PtD) were approximately 110 °C due to the cyclic structures incorporated into the main chain via regioselective 1,4-conjugation, whereas the Tg of poly(β-Phe) was 66 °C, which was lower than that of poly(β-Phe) (Tg = 87 °C) obtained in cationic polymerization [63] due to the decreased 1,4regioselectivity in radical polymerization. In addition, the poly((−)-HCvD) obtained in radical polymerization showed a slightly lower Tg than that obtained in the cationic polymerization of (−)-HCvD (Tg = 105 vs. 114 °C) and showed no melting peak due to the lack of stereospecificity in radical polymerization [64].

Radical Copolymerization with Various Common Vinyl Monomers
A series of terpenoid-derived exo-methylene 6-membered ring conjugated dienes were then copolymerized with various common vinyl monomers, such as MA, AN, MMA, and St, at a 1:1 feed ratio in toluene at 60 • C using AIBN ( Figure S4 and Table S1). Radical copolymerizations occurred in almost all cases except for copolymerization with (−)-VnD, in which conversions of both (−)-VnD and comonomers were very low and the obtained products were only oligomers (M n ≤ 1 × 10 3 ). Therefore, (−)-VnD is only slightly reactive in radical homo-and copolymerization, probably because the bulky structure with a bicyclic structure and a methyl group at the 4-position makes propagation very difficult in the radical process. Although the other exo-methylene-conjugated dienes were all copolymerized, the reactions at 60 • C were generally slow and resulted in polymers with relatively low molecular weights (M n = 1 × 10 3 -2 × 10 4 ).
Therefore, the radical copolymerizations of β-Phe, (-)-HCvD, and PtD with MA, AN, MMA, and St at a 1:1 feed ratio were investigated at 100 • C in toluene using VAm-110 as the radical initiator ( Table 2 and Figure 2 and Figure S5). The polymerization rates, monomer conversion, and molecular weight of the resulting copolymers (M n = 6 × 10 3 -4 × 10 4 ) were drastically improved, indicating that a higher reaction temperature is favorable for not only homo-but also copolymerization of a series of the exo-methylene 6-membered ring conjugated dienes. Table 2. Radical copolymerization of terpenoid-derived exo-methylene 6-membered ring conjugated dienes (M 1 ) and various common vinyl monomers (M 2 ) in toluene at 100 • C a .  In general, electron-deficient comonomers were suitable for copolymerization, where the reaction rates, conversions, and molecular weights increased in the following order: AN > MA > MMA > St, suggesting that these exo-methylene cyclic conjugated dienes are electron-rich monomers. Among the cyclic conjugated dienes, (−)-HCvD and PtD showed similar reactivities, resulting in polymers with similar molecular weights in all cases, while β-Phe resulted in slower polymerizations and lower molecular weights. However, the incorporation ratio of β-Phe was generally greater than that of (−)-HCvD and PtD for any comonomer (Table 2). These results suggest that relatively high incorporation of β-Phe leads to a high probability of the β-Phe-derived propagating radical species, by which the propagation is slow to result in copolymers with relatively low molecular weights. More details on the monomer reactivity ratio will be studied and discussed in the next section.
All the obtained copolymers were analyzed by 1 H NMR to clarify the copolymer structures, particularly regarding the incorporation ratio of comonomers and the regiospecificity of the diene In general, electron-deficient comonomers were suitable for copolymerization, where the reaction rates, conversions, and molecular weights increased in the following order: AN > MA > MMA > St, suggesting that these exo-methylene cyclic conjugated dienes are electron-rich monomers. Among the cyclic conjugated dienes, (−)-HCvD and PtD showed similar reactivities, resulting in polymers with similar molecular weights in all cases, while β-Phe resulted in slower polymerizations and lower molecular weights. However, the incorporation ratio of β-Phe was generally greater than that of (−)-HCvD and PtD for any comonomer (Table 2). These results suggest that relatively high incorporation of β-Phe leads to a high probability of the β-Phe-derived propagating radical species, by which the propagation is slow to result in copolymers with relatively low molecular weights. More details on the monomer reactivity ratio will be studied and discussed in the next section.
All the obtained copolymers were analyzed by 1 H NMR to clarify the copolymer structures, particularly regarding the incorporation ratio of comonomers and the regiospecificity of the diene monomers during the copolymerizations (Figure 3, Figures S6-S8). For example, Figure 3 shows spectra of a series of copolymers obtained with β-Phe, (−)-HCvD, or PtD and MA. In addition to the characteristic signals of the conjugated diene units, all the spectra showed additional signals of MA units, including not only main-chain methylene and methine protons but also methyl ester protons (k). In particular, olefinic protons (b) were observed at approximately 5-6 ppm for the copolymers of β-Phe or PtD ( Figure 3A,C), although the peaks were slightly broader than those of the homopolymers of β-Phe or PtD ( Figure 1A,C). In addition, almost no olefinic protons were observed for the copolymers of (−)-HCvD ( Figure 3B), indicating that the high 1,4-conjugated addition polymerization also proceeded in the copolymerization. For the other various copolymers with AN, MMA, or St ( Figures S6-S8), β-Phe or PtD olefinic protons were similarly observed at approximately 4-6 ppm, although the chemical shifts were dependent on the comonomers. Furthermore, for the copolymers of (−)-HCvD with all comonomers, almost no olefinic protons were observed.
Molecules 2020, 25, x FOR PEER REVIEW 8 of 17 Furthermore, for the copolymers of (−)-HCvD with all comonomers, almost no olefinic protons were observed. The incorporation ratios of the conjugated dienes (M1) and common vinyl monomers (M2) were calculated by the peak intensity ratios of the diene and vinyl monomer units in the 1 H NMR spectra. The incorporation ratios (M1/M2(NMR)) obtained by NMR of the resulting copolymers were close to the calculated values (M1/M2(Calcd)) based on the feed ratio and monomer conversions, indicating The incorporation ratios of the conjugated dienes (M 1 ) and common vinyl monomers (M 2 ) were calculated by the peak intensity ratios of the diene and vinyl monomer units in the 1 H NMR spectra. The incorporation ratios (M 1 /M 2 (NMR)) obtained by NMR of the resulting copolymers were close to the calculated values (M 1 /M 2 (Calcd)) based on the feed ratio and monomer conversions, indicating that the consumed monomers were efficiently incorporated into the copolymers. Furthermore, the regioselectivities of the diene units were calculated by the olefinic protons and other protons for all copolymers. The regioselectivities in the copolymers were basically the same as those for the homopolymers. Namely, high 1,4-selectivity was attained for (−)-HCvD and PtD, whereas the 1,4-selectivity for β-Phe was approximately 80%.
Thus, a series of terpenoid-derived exo-methylene 6-membered ring conjugated dienes are efficiently copolymerized with various common vinyl monomers, including MA, AN, MMA, and St, via mainly 1,4-conjugated additions resulting in copolymers with biobased monomer incorporation ratios ranging from 40 to 60 mol% at a 1:1 feed ratio.

Monomer Reactivity Ratio
To further clarify the copolymerizability of a series of the exo-methylene 6-membered ring conjugated dienes in radical copolymerizations, the monomer reactivity ratios were determined by analyzing the comonomer compositions of the copolymers obtained in the initial stages (total conversion < 10%) of the copolymerizations at various monomer feed ratios ([M 1 ] 0 + [M 2 ] 0 = 5000 mM, [VAm-110] 0 = 30 mM in toluene at 100 • C). Figure 4 shows the copolymer composition curves, in which the diene unit content in the resulting copolymers was plotted against the diene content in the monomer feeds. The data were analyzed using the terminal model and were fitted well with the solid lines obtained using the Kelen-Tüdõs method. The obtained r 1 and r 2 values are summarized in Table 2.
In general, all the plots and fitted curves for HCvD and PtD were close to each other, whereas those for β-Phe were located above the others, indicating that the incorporation of β-Phe units in the resulting copolymers was consistently greater than that of HCvD and PtD at the same feeds. Correspondingly, the r 1 values of β-Phe were consistently larger than those of HCvD and PtD. In addition, the r 1 values of HCvD and PtD were relatively close. These results indicate that HCvD and PtD, which both undergo selective 1,4-regioselective propagation, possess similar reactivities, while β-Phe with a decreased 1,4-regioselectivity shows a higher copolymerizability than HCvD and PtD.
When focusing on the copolymerizations with electron-deficient comonomers (MA, AN, and MMA), both r 1 and r 2 were less than 1, and in particular, those for HCvD and PtD with AN were less than 0.1, indicating that alternating copolymerization predominantly occurs, as also suggested by the almost constant plot for the various monomer feed ratios ( Figure 4B). This is because the exo-methylene-conjugated diene can be considered an electron-rich monomer, whereas AN is the most electron-deficient monomer among the comonomers. For the copolymerizations with St, the sequence of the copolymer is more random because both of the r values are relatively close to 1, although both monomers are slightly more reactive than the other monomers.
When the r values of the exo-methylene cyclic conjugated dienes are compared with those of a linear conjugated diene, such as butadiene (Bd), they are partially different. The , which are similar to those obtained in copolymerizations with the exo-methylene cyclic conjugated dienes, indicating that the propagating radical species derived from common comonomers can react with cyclic conjugated dienes with reactivity similar to that of the Bd monomer.
Thus, a series of terpenoid-derived exo-methylene 6-membered ring conjugated dienes possess sufficiently high copolymerizability with various common vinyl monomers to be efficiently incorporated into the polymer main chains via 1,4-conjugated additions, which results in cyclic structures in the main chain of the copolymers.   Table 2).
In general, all the plots and fitted curves for HCvD and PtD were close to each other, where ose for β-Phe were located above the others, indicating that the incorporation of β-Phe units in t sulting copolymers was consistently greater than that of HCvD and PtD at the same feed orrespondingly, the r1 values of β-Phe were consistently larger than those of HCvD and PtD. dition, the r1 values of HCvD and PtD were relatively close. These results indicate that HCvD an D, which both undergo selective 1,4-regioselective propagation, possess similar reactivities, whi Phe with a decreased 1,4-regioselectivity shows a higher copolymerizability than HCvD and PtD When focusing on the copolymerizations with electron-deficient comonomers (MA, AN, an  Table 2).

Thermal Properties of Copolymers
The thermal properties of the various obtained copolymers were evaluated by DSC ( Figure 5). All the obtained copolymers showed their characteristic T g s. For the copolymers of MA, the T g values were much higher than that of the homopolymer of MA (T g~1 0 • C) [70] due to the incorporated terpenoid-derived alicyclic structure, and the T g values ranged from 50 to 80 • C depending on the structure of the cyclic conjugated dienes. Furthermore, the T g values of the copolymers of MMA, AN, and St with (−)-HCvD and PtD were approximately 100 • C, which is similar to the T g values of the homopolymers, indicating that the biobased units can be introduced into common vinyl polymers without losing their good thermal properties. This could be a unique benefit of the biobased exo-methylene 6-membered ring conjugated dienes as comonomers for MMA, AN, and St, in contrast to linear olefins and dienes, which generally have lower T g s due to the soft hydrocarbon units. Thus, terpenoid-derived alicyclic exo-methylene conjugated dienes are promising monomers as novel rigid hydrocarbon units that can be introduced at a high incorporation ratio in radical copolymerization.
All the obtained copolymers showed their characteristic Tgs. For the copolymers of MA, the Tg values were much higher than that of the homopolymer of MA (Tg ~ 10 °C) [70] due to the incorporated terpenoid-derived alicyclic structure, and the Tg values ranged from 50 to 80 °C depending on the structure of the cyclic conjugated dienes. Furthermore, the Tg values of the copolymers of MMA, AN, and St with (−)-HCvD and PtD were approximately 100 °C, which is similar to the Tg values of the homopolymers, indicating that the biobased units can be introduced into common vinyl polymers without losing their good thermal properties. This could be a unique benefit of the biobased exomethylene 6-membered ring conjugated dienes as comonomers for MMA, AN, and St, in contrast to linear olefins and dienes, which generally have lower Tgs due to the soft hydrocarbon units. Thus, terpenoid-derived alicyclic exo-methylene conjugated dienes are promising monomers as novel rigid hydrocarbon units that can be introduced at a high incorporation ratio in radical copolymerization.

Reversible Addition Fragmentation Chain-Transfer (RAFT) Copolymerization of (-)-HCvD and Various Common Vinyl Monomers
To control the copolymerization for precision synthesis of the novel biobased polymers, the RAFT copolymerization of (−)-HCvD and various common vinyl monomers (MA, MMA, AN, and St) was investigated at a 1:1 feed ratio using S-cumyl-S'-butyl trithiocarbonate (CBTC) as the RAFT agent and VAm-110 as the radical initiator in toluene at 100 • C ( Figure 6 and Figure S9). In all cases, the copolymerizations proceeded smoothly even in the presence of the RAFT agent at conversion ratios similar to those in radical copolymerization without the RAFT agent ( Figure S9).
To control the copolymerization for precision synthesis of the novel biobased polymers, the RAFT copolymerization of (−)-HCvD and various common vinyl monomers (MA, MMA, AN, and St) was investigated at a 1:1 feed ratio using S-cumyl-S'-butyl trithiocarbonate (CBTC) as the RAFT agent and VAm-110 as the radical initiator in toluene at 100 °C ( Figure 6 and Figure S9). In all cases, the copolymerizations proceeded smoothly even in the presence of the RAFT agent at conversion ratios similar to those in radical copolymerization without the RAFT agent ( Figure S9). The Mn values of the resulting copolymers increased in direct proportion to total monomer conversion in all cases and were close to the calculated values assuming that one molecule of CBTC generates one polymer chain ( Figure 6A). In addition, as the polymerization proceeded, the SEC curves shifted to higher molecular weights while maintaining relatively narrow molecular weight distributions (Mw/Mn = 1.2-1.5) ( Figure 6B).
The 1 H NMR spectra ( Figure S10) of the copolymers obtained in the RAFT copolymerization showed similar large absorptions originating from the terpenoid-based cyclic conjugated dienes and common vinyl monomer units and small signals of both the α-and ω-terminals derived from the RAFT agent, for which signals at 7.3-7.4 and 3.3-3.4 ppm can be attributed to the aromatic and methylene protons originating from CBTC, respectively. The incorporations of (−)-HCvD units into the copolymers were similarly calculated by the peak intensity ratios and were almost the same as those for the copolymers obtained in radical copolymerization without the RAFT agent. The Mn values calculated from the peak intensity ratios of the α-and ω-RAFT terminals to the repeating units were close to both Mn(SEC) and Mn(Calcd), suggesting that one CBTC molecule generates one polymer chain. These results indicate that CBTC is suitable for controlling the RAFT copolymerization of (−)-HCvD and various common vinyl monomers to produce copolymers with controlled molecular weights and chain-end groups, which can be used for synthesis of the novel biobased functional polymers. The M n values of the resulting copolymers increased in direct proportion to total monomer conversion in all cases and were close to the calculated values assuming that one molecule of CBTC generates one polymer chain ( Figure 6A). In addition, as the polymerization proceeded, the SEC curves shifted to higher molecular weights while maintaining relatively narrow molecular weight distributions (M w /M n = 1.2-1.5) ( Figure 6B).

Materials and Methods
The 1 H NMR spectra ( Figure S10) of the copolymers obtained in the RAFT copolymerization showed similar large absorptions originating from the terpenoid-based cyclic conjugated dienes and common vinyl monomer units and small signals of both the αand ω-terminals derived from the RAFT agent, for which signals at 7.3-7.4 and 3.3-3.4 ppm can be attributed to the aromatic and methylene protons originating from CBTC, respectively. The incorporations of (−)-HCvD units into the copolymers were similarly calculated by the peak intensity ratios and were almost the same as those for the copolymers obtained in radical copolymerization without the RAFT agent. The M n values calculated from the peak intensity ratios of the αand ω-RAFT terminals to the repeating units were close to both M n (SEC) and M n (Calcd), suggesting that one CBTC molecule generates one polymer chain. These results indicate that CBTC is suitable for controlling the RAFT copolymerization of (−)-HCvD and various common vinyl monomers to produce copolymers with controlled molecular weights and chain-end groups, which can be used for synthesis of the novel biobased functional polymers.

Synthesis of (-)-VnD
(−)-VnD was prepared by the Wittig reaction of purified (−)-verbenone. (Methyl)triphenylphosphonium bromide (140.2 g, 0.392 mol) was suspended in dry THF (700 mL) at 0 • C in a three-necked 2 L round-bottom flask equipped with three-way stopcocks. Potassium tert-butoxide (44.1 g, 0.392 mol) was added resulting in a yellow suspension. After 30 min, (−)-verbenone (40 mL, 0.261 mol) was added to the yellow suspension resulting in an orange product, and the reaction mixture was stirred at room temperature. After 16 h, the conversion of verbenone reached >99% by 1 H NMR. Then, water (400 mL) was added to quench the reaction. The organic layer was concentrated by rotary evaporation. Hexane (300 mL) was added to the obtained residue. After the mixture was filtered to remove the precipitated triphenylphosphine oxide, the obtained solution was concentrated. The obtained residue was similarly treated again to yield the crude (−)-VnD product (32.1 g, 82% yield). Further purification was conducted by distillation to produce a colorless oil of pure (−)-VnD (28.9 g, 74% yield, >99% purity, bp 55

Synthesis of PtD
PtD was prepared by the Wittig reaction of distilled piperitone. (Methyl)triphenylphosphonium bromide (131.6 g, 0.369 mol) was suspended in dry THF (700 mL) at 0 • C in a three-necked 2 L round-bottom flask equipped with three-way stopcocks. Potassium tert-butoxide (41.3 g, 0.369 mol) was added to the suspension resulting in a yellow suspension. After 30 min, piperitone (40 mL, 0.246 mol) was added to the yellow suspension to produce an orange product, and the reaction mixture was stirred at room temperature. After 16 h, the conversion of piperitone reached >99% by 1 H NMR. Then, water (400 mL) was added to quench the reaction. The organic layer was concentrated by rotary evaporation. Hexane (300 mL) was added to the obtained residue. After the mixture was filtered to remove the precipitated triphenylphosphine oxide, the obtained solution was concentrated. The crude product after the Wittig reaction (30.3 g, 82% yield, 81% purity) was purified by column chromatography on silica gel with n-hexane as the eluent to yield the product with a higher purity (19.9 g, 36% yield, 94% purity). Further purification was conducted by distillation to produce the colorless PtD oil ( Figures S11, S14, and S15.

RAFT Copolymerization
RAFT copolymerization was carried out using the syringe technique under dry nitrogen in sealed glass tubes. A typical example for (-)-HCvD and MA copolymerization with CBTC and VAm-110 in toluene is given below. (−)-HCvD (0.60 mL, 3.3 mmol), MA (0.30 mL, 3.3 mmol), CBTC (0.13 mL, 508 mM solution in toluene, 0.066 mmol), and VAm-110 (1.19 mL of 18.6 mM solution in toluene, 0.022 mmol) were placed in a baked 25 mL graduated Schlenk flask equipped with a three-way stopcock at room temperature. The total volume of the reaction mixture was 2.2 mL. Immediately after mixing, aliquots (0.4 mL each) of the solution were distributed via syringe into baked glass tubes, which were then sealed by flame in a nitrogen atmosphere. The tubes were immersed in a thermostatic oil bath at 100 • C. At predetermined intervals, the polymerization was terminated by cooling the reaction mixtures to −78 • C. The monomer conversion was determined from the concentration of residual monomer measured by 1 H NMR with toluene as an internal standard (20 h, 32% for (−)-HCvD and 26% for MA). The quenched reaction solutions were evaporated to dryness to give the product copolymer (M n = 3100, M w /M n = 1.24).

Measurements
The 1 H NMR spectra for the monomer conversion and product polymer were recorded on a JEOL ECS-400 spectrometer operating at 400 MHz. The number-average molecular weight (M n ) and molecular weight distribution (M w /M n ) of the product polymer were determined by SEC in THF at 40 • C on two polystyrene gel columns (Tosoh Multipore H xL -M (7.8 mm i.d. × 30 cm) × 2; flow rate: 1.0 mL/min) connected to a JASCO PU-2080 precision pump and a JASCO RI-2031 detector. The columns were calibrated with standard polystyrene samples (Agilent Technologies; M p = 580-3,242,000, M w /M n = 1.02-1.23). The glass transition temperature (T g ) of the polymers was recorded on a Q200 differential scanning calorimeter (TA Instruments, Inc.). Samples were first heated to 150 • C at 10 • C/min, equilibrated at this temperature for 10 min, and cooled to 0 • C at 5 • C/min. After being held at this temperature for 5 min, the samples were then reheated to 200 • C at 10 • C/min. All T g values were obtained from the second scan after removing the thermal history.

Conclusions
In conclusion, radical polymerizations of a series of terpenoid-derived exo-methylene 6-membered ring conjugated dienes were investigated with the aim of developing novel biobased polymeric materials. Although their reactivities for homopolymerization in radical processes were lower than those in cationic processes, copolymerizations with various common vinyl monomers efficiently occurred, resulting in unique biobased copolymers. The substituents on the 6-membered ring of exo-methylene conjugated dienes affected the regiospecificity and monomer reactivity. The obtained copolymers contained relatively high biobased units, ranging from 40 to 60 mol% even at a 1:1 feed ratio, and the glass transition temperatures were comparable to those of the homopolymers obtained from common vinyl monomers. In addition, biobased units can be incorporated into copolymers in a controlled fashion via the RAFT process. We believe that these exo-methylene-conjugated dienes can be new building blocks to synthesize copolymers with terpenoid-derived alicyclic structures, which serve as hydrocarbon units with good thermal properties.  Table S1. Radical copolymerization of terpenoid-derived exo-methylene 6-membered ring conjugated dienes (M 1 ) and various common vinyl monomers (M 2 ) in toluene at 60 • C.

Conflicts of Interest:
The authors declare no conflict of interest.