Introduction

Bio-based plastics can be sustainably produced from biomass, a renewable resource, without using fossil fuels. As such, these environmentally friendly polymer materials are carbon neutral and their production will lead to a reduction in CO2 emissions. Therefore, the development of high-performance bio-based plastics derived from renewable resources has received considerable interest in recent years1,2,3,4,5,6. For example, the high-performance bio-based polyimides recently developed by Suvannasara et al. using 4-aminocinnamic acid derived from genetically manipulated Escherichia coli show ultrahigh thermal resistance with Td10 values higher than 425 °C6.

Cinnamic monomers are aromatic compounds that can be extracted from certain plants, such as Cinnamomum cassia, and derived by thermal decomposition of lignin, a complex phenolic biopolymer comprising wood or straw fiber cell walls along with cellulose7,8,9. Characteristically, these compounds have an aromatic substituent at the β-position of an α,β-unsaturated carbonyl group. Notably, successful polymerization of these compounds at only the carbon–carbon double bond moiety would produce polymers with a linear saturated carbon–carbon skeleton as the polymer backbone and alternating aromatic rings and carboxylate (or ester) groups as side chains. Owing to this structure, such polymers are expected to be applicable as high-performance and multifunctional bio-based polymeric materials that combine the features of both polystyrene (PSt) and acrylic resins. Further, they may possess high heat resistance and a rigid polymer backbone derived from the interactions between aromatic side chains and the ditacticity attributed to α- and β- substituents.

However, convenient addition polymerization of cinnamic monomers has rarely been reported owing to steric hindrance of the aromatic substituent located at the β-position and delocalization of the electrons throughout the molecule via unsaturated π-bonding10,11,12,13,14,15,16,17,18. Sapiro et al. reported the polymerization of ethyl cinnamate under harsh high-temperature and high-pressure conditions (125 °C, 4,000 atm)10. A white powdery product was obtained in moderate yield (58.5%), but the molecular weight of the product was not provided. Marvel et al. reported that the radical polymerization of cinnamates using 2,2′-azobisisobutyronitrile (AIBN) as an initiator for a long period of 1 month gave a white powdery product in low yield (10%)11. In addition to the inefficiency of these methods, polymerization at only the carbon–carbon double bonds was not clearly demonstrated. Moreover, a comprehensive evaluation of the physical properties of these polymers is required.

As an alternative to these methods, Terao et al. reported the copolymerization of cinnamic monomers with vinyl monomers, such as methyl acrylate or styrene, via living radical methods including metal-catalyzed living radical polymerization, atom transfer radical polymerization, reversible addition-fragmentation chain transfer polymerization, and nitroxide-mediated polymerization19. However, as the incorporation of cinnamic monomers into the copolymers ranged from 15 to 40 mol%, the obtained materials are unlikely to exhibit the properties expected for homopolymers of cinnamic compounds. Therefore, the development of a convenient homopolymerization system for cinnamic monomers is highly desirable.

Alkyl crotonates are α,β-unsaturated carbonyl compounds with a methyl group at the β-position. Similar to cinnamic monomers, there are well-known difficulties in using alkyl crotonates (or crotonic acid) as a monomer to produce polymers with sufficient molecular weights by classical radical polymerization methods using AIBN or benzoyl peroxide and by typical anionic polymerization methods using an alkyl lithium or Grignard reagent17,18,19,20,21,22. Group-transfer polymerization (GTP) systems for various alkyl crotonates have been developed using a Lewis acid (HgI2 or CdI2) as a catalyst and iodotrialkylsilane as a cocatalyst23,24,25,26,27. Chen and co-workers reported an efficient catalytic Lewis pair polymerization system for methyl crotonate using N-heterocyclic carbene as a Lewis base and an aluminum or a boron compound as a Lewis acid28,29. Additionally, organic-acid-catalyzed GTP systems were developed by Chen and co-workers and Kakuchi et al. for the polymerization of (meth)acrylates30,31,32. In the last year, we applied such an organic-acid-catalyzed GTP system to the polymerization of various alkyl crotonates33,34. The GTP system in our study used an organic acid such as N-(trimethylsilyl)bis(trifluoromethanesulfonyl)imide (Tf2NTMS) as a catalyst and a silyl ketene acetal such as 1-methoxy-1-(trimethylsiloxy)-2-methyl-1-propene (MTS) as an initiator (Supplementary Fig. 1). Owing to the success of this approach for polymerizing alkyl crotonates, it is possible that it could be modified to realize the convenient homopolymerization of cinnamic monomers.

Herein, we successfully apply an organic-acid-catalyzed GTP system to the polymerization of alkyl cinnamates and their derivatives to produce homopolymers with molecular weights (Mn) of ~18,100 g mol−1. Structural characterization of the obtained polymers using NMR spectroscopy and matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF-MS), as well as determination of the molecular structures of their low-molecular-weight homologs by single-crystal X-ray structural analysis, confirms that polymerization proceeds at only the carbon–carbon double bond moiety. In addition, evaluation of the thermal properties (thermal decomposition temperature (Td) and glass transition temperature (Tg)) of the obtained polymers using thermogravimetric analysis (TGA) and dynamic mechanical analysis (DMA) reveals these materials to have high thermal stabilities.

Results

Polymerization of cinnamates and their derivatives by GTP

The polymerization of alkyl cinnamates and their derivatives was performed by the GTP technique in the presence of MTS as an initiator and various organic acid catalysts. Table 1 summarizes the yields and molecular weights of the obtained polymers. When GTP was performed for 7 days with methyl cinnamate as a monomer in the presence of Tf2NTMS at temperatures ranging from room temperature to −35 °C, corresponding poly(methyl cinnamate)s were obtained with number-average molecular weights (Mn) ranging from 7,300 to 13,000 g mol−1, yields ranging from 23 to 46%, and dispersities (Đs) ranging from 1.47 to 1.84 (Entries 1–4, Table 1). Notably, poly(methyl cinnamate) with a relatively high number-average molecular weight (Mn = 13,000 g mol−1) and a narrow Đ (1.47) was obtained when polymerization was allowed to proceed for 7 days at −35 °C (Entry 4, Table 1), whereas a Mn of only 7,300 g mol−1 was achieved for the reaction carried out at room temperature (Entry 1, Table 1). This result suggests that side reactions including intramolecular termination reactions, such as cyclization and/or isomerization of the propagating chain-end groups, as proposed for the GTP of methacrylate, were suppressed at low temperatures (−35 °C), with the propagation reaction proceeding preferentially35,36,37 (Supplementary Fig. 2). However, when the polymerization was performed at lower temperatures such as −80 °C, the unreacted monomers and the reaction products precipitated in the reaction solution owing to their low solubilities.

Table 1 GTP of cinnamates and their derivatives with MTS in the presence of various organic acid catalystsa
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Next, we investigated various conditions for GTP at −35 °C. When the feed ratio of monomer to initiator was varied from 50 to 25 (mol mol−1) for GTP performed for 14 days at −35 °C, the value of the molecular weight divided by the yield of the obtained polymer was proportional to this feed ratio (Entries 5–9, Table 1) (Supplementary Fig. 3). This result indicates that this polymerization proceeds through the general GTP mechanism. Using the reaction conditions of Entry 4 but extending the reaction time to 28 days increased the yield and Mn of the obtained polymer from 43 to 81% and from 13,000 to 18,100 g mol−1, respectively (Entry 10, Table 1). This result is a remarkable example of the convenient homopolymerization of cinnamates under mild conditions as compared to the literature data10,11,12,13,14,15,16,17,18.

Furthermore, we investigated the effect of the catalyst in the GTP system. Various organic acid catalysts, including 1-[bis(trifluoromethanesulfonyl)methyl]-2,3,4,5,6-pentafluorobenzene (C6F5CHTf2), bis(trifluoromethanesulfonyl)imide (Tf2NH), and triphenylmethylium tetrakis(pentafluorophenyl)borate (Ph3C[B(C6F5)4]), all gave the corresponding methyl cinnamate polymer. However, these catalysts were less active than Tf2NTMS for the GTP of methyl cinnamate (Entries 12–15, Table 1). The convenient polymerization of methyl cinnamate prompted us to investigate the polymerization activities of various monomers in the GTP system using Tf2NTMS.

Therefore, we examined the GTP of various monomers using Tf2NTMS (Entries 16–22, Table 1). The effect of an ester substituent was studied by comparing the polymerizations of ethyl cinnamate and iso-propyl cinnamate. As expected, the polymer yield decreased when the bulkiness of the ester substituent of the alkyl cinnamates increased (iso-propyl > ethyl > methyl) because the steric hindrance of the ester substituent inhibited polymerization. Unexpectedly, as the bulkiness of the ester substituent of the alkyl cinnamates increased, the Đ of the obtained polymer narrowed from 1.27 (methyl cinnamate) to 1.06 (iso-propyl cinnamate).

Polymerizations of methyl cinnamate derivatives (methyl 4-methyl cinnamate, methyl 4-methoxy cinnamate, methyl 3,4-dimethoxy cinnamate, and methyl 3,4,5-trimethoxy cinnamate) were performed to investigate the effect of substituents in the aromatic ring on the polymerization activity and properties of the obtained polymer. The molecular weight of the obtained polymer was slightly higher with methyl 4-methyl cinnamate (Mn = 8,300 g mol−1) than with methyl cinnamate (Mn = 7,300 g mol−1), although the polymer yields were similar (35%) (Entries 16 and 19, Table 1). In the case of methyl 4-methoxy cinnamate, the molecular weight of the obtained polymer was lower (Mn = 3,000 g mol−1) but the polymer yield was higher than with methyl cinnamate (66% and 35%, respectively) (Entries 16 and 20, Table 1). Unfortunately, polymerization did not proceed when methyl 3,4-dimethoxy cinnamate and methyl 3,4,5-trimethoxy cinnamate were used as monomers.

Structural analysis of poly(methyl cinnamate)

1H and 13C NMR analyses of poly(methyl cinnamate) were performed to determine the molecular structure of the polymer. Figure 1 shows the 1H and 13C NMR spectra of the obtained poly(methyl cinnamate) (Entry 5, Table 1) and the methyl cinnamate monomer. The α-alkene proton peak (Hf, δ = 6.43 ppm) and the β-alkene proton peak (He, δ = 7.69 ppm) of the monomer (Fig. 1a) disappeared in the 1H NMR spectrum of the obtained polymer (Fig. 1b). Further, signals (CE, CF) derived from the newly formed methylene skeleton were observed in the range of 40–60 ppm in the 13C NMR spectrum of the obtained polymer38 (Fig. 1d). These observations suggested that the carbon–carbon double bond moiety of the monomer was enchained and propagation proceeded.

Fig. 1
figure 1

1H and 13C NMR spectra of methyl cinnamate monomer and poly(methyl cinnamate). a 1H NMR spectrum of methyl cinnamate monomer. b 1H NMR spectrum of poly(methyl cinnamate) (Entry 5, Table 1). c 13C NMR spectrum of methyl cinnamate monomer. d 13C NMR spectrum of poly(methyl cinnamate) (Entry 5, Table 1). 1H NMR spectra were measured in CDCl3 at room temperature and 500 MHz. 13C NMR spectra were measured in CDCl3 at room temperature and 125 MHz

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Additionally, in the 1H NMR spectrum of the obtained polymer, resonance peaks were observed for the methyl protons (HL, HM) at the initiation chain-end derived from MTS near 0.5 and 1.0 ppm; the methoxy protons (HH, HI) in the side chains and at the initiation chain-end, the methine protons (HE, HF) in the main chain, and the methylene proton (HN) at the termination chain-end in the region of 1.7–4.0 ppm; and the phenyl protons (HA, HB, HC) in the side chains in the region of 5.5–7.5 ppm19,23. However, all the peaks were broad.

The molecular structure of the polymer was also determined by MALDI-TOF-MS analysis of the obtained poly(methyl cinnamate) (Entry 12, Table 1). As shown in Fig. 2a, the mass spectrum mainly consists of one major series (A) and one minor series (B) of mass ions. For major series A, a plot of the m/z values against the repeat unit number (n) of methyl cinnamate monomers gave a straight line with a slope of m/z = 162 and an intercept of m/z = 125 (Fig. 2b). This slope corresponds to the mass of the methyl cinnamate monomer, and the intercept corresponds to the sum of the masses for a polymer chain-end with a formula of C5H10O2 and Na+ (derived from CF3COONa added as an ionizing agent in the MALDI-TOF-MS measurement). This result clearly shows that the polymer of main series A has the structural formula (CH3)2[CH3OC( = O)]C–(methyl cinnamate)n–H. Herein, (CH3)2[CH3OC( = O)]C at the initiation chain-end is derived from MTS, whereas the methylene group at the termination chain-end is generated by acid treatment of the terminal silyl ether group in the living polymer (Supplementary Fig. 1).

Fig. 2
figure 2

MALDI-TOF-MS spectrum of poly(methyl cinnamate). a MALDI-TOF-MS spectrum of poly(methyl cinnamate) (Entry 12, Table 1). b Plot of m/z values of major series A vs the number of methyl cinnamate repeat units (n). c Plot of m/z values of minor series B vs the number of methyl cinnamate repeat units (n). MALDI-TOF-MS was measured using dithranol as a matrix and CF3COONa as an ionizing agent

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By contrast, the polymer corresponding to minor series B is not considered to have the same molecular structure, even though series B also has mass ion intervals corresponding to a repeat unit of m/z = 162. A detailed analysis of the mass spectrum revealed that the polymer of minor series B corresponded to the loss of one methanol molecule from the polymer of main series A. Thus, the polymer of minor series B is considered to correspond to poly(methyl cinnamate) with a terminal cyclic structure, generated via the cyclization of the living chain-end (Supplementary Fig. 2). These results revealed that the polymers synthesized using the GTP system consist of mainly linear forms but may include terminal cyclic structures. Additionally, the presence of cyclic structures could be confirmed by the ketone carbons observed in the 13C NMR analysis of the oligomer sample synthesized by the GTP of methyl cinnamate with MTS and Tf2NTMS in CH2Cl2 at room temperature, as the cyclization reaction was promoted at this temperature (Supplementary Fig. 4).

Structural analysis of oligo(methyl cinnamate)

We gained more detailed information about the structure of the obtained poly(methyl cinnamate) by analysing the structures of low-molecular-weight homologs.

Figure 3a–d shows the size-exclusion chromatography (SEC) curves for the synthesized mixture of low-molecular-weight homologs and for the samples separated by SEC fractionation from the mixture. Each peak in Fig. 3a was characterized as a linear oligomer (trimer to hexamer) by 1H NMR and MALDI-TOF-MS analyses (Supplementary Figs. 5–8). Figure 3e–g shows the 1H NMR spectra of the separated oligomer samples (tetramer to hexamer). The proton resonance peaks of the oligomers substantially agreed with those of poly(methyl cinnamate) (Figs. 1b and 3e–g). When a linear oligomer has a polymerization degree of N, one molecule should contain (N + 1) methyl ester groups. By focusing on the region derived from the methyl ester groups, it could be inferred that each oligomer included multiple kinds of diastereomers because multiple sets of (N + 1) peaks were observed in each 1H NMR spectrum (Fig. 3e–g). We separated the diastereomers by normal-phase high-performance liquid chromatography (HPLC) fractionation to acquire the isomers that are observed as a major component in the 1H NMR spectrum.

Fig. 3
figure 3

Fractionation of low-molecular-weight homologs of poly(methyl cinnamate). a SEC curve of the mixture of low-molecular-weight homologs of poly(methyl cinnamate) synthesized by GTP in the presence of MTS and Tf2NTMS. b SEC curve of the hexamer separated from the mixture. c SEC curve of the pentamer separated from the mixture. d SEC curve of the tetramer separated from the mixture. e 1H NMR spectrum of the hexamer separated from the mixture. f 1H NMR spectrum of the pentamer separated from the mixture. g 1H NMR spectrum of the tetramer separated from the mixture

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Figure 4 shows the ORTEP representations of each isolated isomer. These molecular structures clearly show that each oligomer was composed of a linear saturated carbon–carbon skeleton with alternating phenyl groups and methyl ester groups in the side chains in a head-to-tail direction. Additionally, the relative conformations of the asymmetric carbon atoms in the main chains of the representative isomers exhibited (et)nee sequences (e: erythro, t: threo) in the head-to-tail direction, corresponding to a disyndiotactic structure. The disyndiotactic selectivities of the oligomers, as calculated from the integral ratio of the proton resonance peaks derived from the terminal methyl groups in the 1H NMR spectra, were 45% (tetramer), 30% (pentamer), and 22% (hexamer).

Fig. 4
figure 4

ORTEP drawing of the X-ray crystal structure of disyndiotactic oligomers. a Hexamer of methyl cinnamate. b Pentamer of methyl cinnamate. c Tetramer of methyl cinnamate. All hydrogen atoms are omitted for clarity. Carbon atoms are shown in white and oxygen atoms are shown in red (thermal ellipsoids at the 50% probability level). The crystallographic data are summarized in Supplementary Table 1. Single crystals of methyl cinnamate oligomers suitable for X-ray crystallography were grown in a mixed solvent of CHCl3/n-hexane

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Thermal properties of poly(alkyl cinnamate) derivatives

We investigated the thermal properties (Td5, Td50, and Tg) of the obtained polymers by TGA and DMA, as summarized in Table 2. Two representative poly(methyl cinnamate) samples (Entry 5, Table 1, Mn = 15,000 g mol−1; Entry 13, Table 1, Mn = 4,600 g mol−1) were evaluated. Despite the difference in molecular weight, the Td5 (340 °C and 341 °C) and Td50 values (377 °C and 374 °C) of these polymers were similar. Thus, within this molecular weight range, the molecular weight did not influence the Td of poly(methyl cinnamate). In the case of poly(ethyl cinnamate) (Entry 17, Table 1), which has a bulkier ester substituent than poly(methyl cinnamate), the Td5 and Td50 values were lower than those of poly(methyl cinnamate). By contrast, the Td5 and Td50 values of poly(methyl 4-methoxy cinnamate) (Entry 20, Table 1) were the highest among the obtained polymers (358 °C and 407 °C, respectively).

Table 2 Thermal properties (Td5, Td50, and Tg) of the polymers synthesized from various cinnamates and their derivatives
Full size table

For the two examined poly(methyl cinnamate) samples, the Tg values were similar (165 °C and 163 °C, respectively). In the case of poly(ethyl cinnamate), the Tg value was lower than that of poly(methyl cinnamate) (122 °C), as observed for the Td results. The Tg value of poly(methyl 4-methyl cinnamate) (Entry 19, Table 1) was the highest among the obtained polymers (194 °C) (Supplementary Fig. 9). Previously, it has been reported that the Tg of atactic-poly(methyl methacrylate) (PMMA) is ~105–122 °C, the Tg of syndiotactic-PMMA is ~105–120 °C, the Tg of isotactic-PMMA is ~51–107 °C, and the Tg of PSt is ~85–107 °C39. Thus, the poly(alkyl cinnamate)s and derivatives synthesized using the GTP system have very high thermal stabilities, which are comparable to and even exceed that of polycarbonate (Tg = 137–154 °C), a well-known engineering plastic39.

Discussion

Cinnamic monomers, which are known to be difficult to polymerize by classical polymerization methods, were successfully homopolymerized by the GTP technique through the general GTP mechanism. Compared with literature examples10,11,12,13,14,15,16,17,18, our approach realized convenient homopolymerization of cinnamates under relatively mild conditions. This behavior is likely due to the easy repetition of the Michael reaction in the GTP system. The low temperature used for this reaction could suppress side reactions, allowing propagation to proceed preferentially. Notably, the bulkiness of the ester substituent was found to be an effective method of controlling the structure of the polymer main chain. For instance, the bulky ester of iso-propyl cinnamate suppressed the termination reactions, as confirmed by the low amount of terminal cyclic structures observed in the MALDI-TOF-MS analysis of poly(iso-propyl cinnamate) (Entry 18, Table 1) (Supplementary Fig. 10). Furthermore, the polymerization results for various methyl cinnamate derivatives (Entries 19–22, Table 1) suggested that the polymerization activity of the monomer is influenced considerably by the steric hindrance and/or the electronic contribution of the substituents on the aromatic ring.

Using this GTP technique, polymers with a linear saturated carbon–carbon skeleton and alternating side chains of aromatic rings and ester groups were generated, as confirmed by NMR and MALDI-TOF-MS analyses. Importantly, polymerization in this system proceeded only at the carbon–carbon double bond moiety of the monomer without the involvement of the conjugated aromatic ring, as indicated by NMR and X-ray crystallography analyses of the low-molecular-weight homologs (Supplementary Fig. 11). However, the stereoregularity of the obtained polymer was likely not very high, as broad peaks were observed in the 1H NMR spectrum (Fig. 1b) and some terminal cyclic structures were detected by MALDI-TOF-MS analysis. By contrast, the low-molecular-weight homologs revealed some preference for disyndiotactic stereoregularity (Fig. 4). Thus, it is suggested that a threo addition and trance opening mechanism proceeded with some priority in our cinnamate polymerization system, as Ute and co-workers previously reported that the alkyl crotonate polymerization system forms a disyndiotactic polymer chain by a threo addition and trance opening mechanism23,24 (Supplementary Fig. 12). However, in the case of the current GTP system, as the propagation reaction proceeds, the stereoregularity of the polymer decreases.

Surprisingly, the obtained polymers exhibited high thermal stabilities comparable to and even exceeding those of engineering plastics. The structural analyses clearly revealed that the poly(alkyl cinnamate)s and derivatives synthesized using the GTP system have substituents at both the α-position and the β-position in the main chain, which is expected to decrease the flexibility of the main chain. The increase in stiffness derived from the decrease in flexibility might have a considerable effect on the high thermal stability. Moreover, polymer stereoregularity, which is somewhat low at present, would affect the thermal stability to some extent, as a strong correlation between the tacticity and the thermal stability of PMMA is already known40,41. In the future, it is expected that high-performance polymer materials, such as super engineering plastics, can be produced by controlling the stereoregularity of the polymer. Therefore, we will study the efficiency of various monomers, initiators, and catalysts for the GTP system toward achieving the synthesis of highly stereoregular polymers.

Methods

Materials

Methyl cinnamate, ethyl cinnamate, and iso-propyl cinnamate were obtained from Tokyo Chemical Industry Co. (TCI) and were used without further purification. Methyl 4-methyl cinnamate, methyl 4-methoxy cinnamate, methyl 3,4-dimethoxy cinnamate, and methyl 3,4,5-trimethoxy cinnamate were prepared by esterification of the corresponding cinnamic acids (4-methylcinnamic acid, 4-methoxycinnamic acid, 3,4-dimethoxycinnamic acid, and 3,4,5-trimethoxycinnamic acid, which were obtained from TCI) with methanol in the presence of sulfuric acid according to the literature42. MTS (obtained from TCI) was distilled over CaH2 and was stored at −35 °C in a glovebox. C6H5CHTf2, Tf2NH, Tf2NTMS, Ph3C[B(C6F5)4], and CHCl3 were obtained from TCI and Wako Pure Chemical Industries and were used without further purification. Argon and nitrogen were used after purification by passing through a dry clean column (4A molecular sieves) and a Gas clean CC-XR column purchased from Nikka Seiko. CH2Cl2 (Kanto Chemical Co., super dehydrated grade) was purified via a SPS 800 system (MBraun).

Monomer synthesis

Methyl 4-methyl cinnamate was synthesized by the reaction of 4-methylcinnamic acid and methanol. 4-Methylcinnamic acid (24.9 g, 0.15 mol) and methanol (300 mL) were charged into a flask, followed by the dropwise addition of sulfuric acid (2.5 mL) with vigorous stirring at 60 °C. The mixture was refluxed for 5 h at 80 °C. After concentrating, the mixture was poured into saturated NaHCO3 aq. and extracted with CHCl3. Then, the organic layer was washed with water and brine, and dried over Na2SO4. After filtration, the organic layer was concentrated. The residue was dissolved in hexane, which was then evaporated until a white solid was separated. Cooling the solution yielded methyl 4-methyl cinnamate as white crystals (24.3 g, 89.6%). The syntheses of methyl 4-methoxy cinnamate (85.9% yield), methyl 3,4-dimethoxy cinnamate (78.8% yield), and methyl 3,4,5-trimethoxy cinnamate (93.6% yield) were carried out in a similar manner.

Polymerization

All manipulations were performed under a dry and oxygen-free argon atmosphere using standard high-vacuum Schlenk techniques or a glovebox. A typical polymerization procedure (Entry 10, Table 1) is as follows: methyl cinnamate (2.10 g, 13 mmol), MTS (45.1 mg, 0.26 mmol), and CH2Cl2 (5 mL) were charged into a Schlenk tube in a glovebox. A CH2Cl2 solution (2.5 mL) of Tf2NTMS (91.5 mg, 0.26 mmol) was added to initiate the polymerization reaction at −35 °C. After a predetermined reaction period, methanol (10 mL) was poured into the reaction mixture, and then the solvents were removed by evaporation under reduced pressure. The residue was dissolved in CHCl3, and the solution was poured into a large amount of hexane. The precipitated polymer was collected by filtration, washed several times with hexane, and dried under vacuum for 24 h. The polymerizations of various cinnamates and their derivatives using various organic acid catalysts were carried out in a similar manner.

Oligo(methyl cinnamate) synthesis for X-ray analysis

Methyl cinnamate (4.05 g, 25 mmol), MTS (1.45 g, 8.3 mmol), and CH2Cl2 (2.5 mL) were charged into a Schlenk tube in a glovebox. A CH2Cl2 solution (6.5 mL) of Tf2NTMS (88.3 mg, 0.25 mmol) was added to initiate the reaction at −35 °C. After 5 h, the reaction was terminated by adding 10 mL of methanol. The mixture was evaporated under reduced pressure to remove the volatile components. Then, 40 mL of CHCl3 was added to the residue. The CHCl3-insoluble part was removed by filtration, and the sample of mixed oligomers was obtained as the CHCl3-soluble part.

Fractionation of oligomers

Previously, Ute and co-workers reported the single-crystal X-ray structural analysis of alkyl crotonate oligomers separated by SEC and HPLC to determine the configuration of the disyndiotactic stereospecific poly(alkyl crotonate) synthesized by GTP using HgI2 or CdI2 as a catalyst and iodotrialkylsilane as a cocatalyst23,24,43. Therefore, we used a similar strategy to determine the structures of the obtained poly(alkyl cinnamate)s.

Fractionation by SEC was performed at room temperature using a preparative HPLC system (Shimadzu LC-20AR Semi-Preparative Solvent Delivery Unit, Shimadzu RID-10A Refractive Index Detector, and Shimadzu DGU-20A 3 R Degassing Unit) equipped with a Shodex K-2001 column (20.0 mm ID × 300 mm; pore size, 6 μm; plate number, >18,000; exclusion limit, 1500) and three Shodex K-802 columns (8.0 mm ID × 300 mm; pore size, 6 μm; plate number, >18,000; exclusion limit, 5000) using CHCl3 as the mobile phase at a flow rate of 1.0 mL min−1. The injection volume for each fractionation step was 300 μL. The weight yield of each oligomer was as follows: 15.2 wt% (larger than hexamer), 8.6 wt% (hexamer), 12.2 wt% (pentamer), 13.8 wt% (tetramer), 21.5 wt% (trimer), and 28.6 wt% (smaller than trimer).

Fractionation by normal-phase HPLC was performed at room temperature using the same system as for SEC fractionation, with the exception of the stationary and mobile phases. Two Develosil 100–5 columns (10.0 mm ID × 250 mm) were used as the stationary phase and a mixture of CH3CN and n-butyl chloride (9:91, v/v) was used as the mobile phase at a flow rate of 2.0 mL min−1. The injection volume for each fractionation step was 100 μL.

Measurements

All molecular weight data of polymer samples were obtained by SEC at 40 °C, using a Shimadzu GPC system equipped with an RID-10A refractive index detector, a Shodex GPC K-802 column, and a Shodex GPC K-806M column. CHCl3 was used as the eluent at a flow rate of 0.8 mL min−1, and a sample concentration of 1.0 mg mL−1 was applied. PSt standards with a low polydispersity were used to make a calibration curve.

1H NMR and 13C NMR measurements of the polymer and low-molecular-weight homolog samples were carried out on a Varian NMR system. The samples were dissolved in CDCl3 and the 500 MHz 1H NMR spectra were recorded at room temperature. The data was calibrated against tetramethylsilane (TMS, δ 0.00). The 125 MHz 13C NMR spectra recorded at room temperature in a CDCl3 solution. The data was calibrated against TMS (δ 0.00). The data were analyzed using the JEOL ALICE 2 and Mnova NMR software.

MALDI-TOF-MS analyses were conducted using an ultrafleXtreme MALDI-TOF spectrophotometer (Bruker Daltonics) operating in the linear or reflection mode at an accelerating voltage of 20 kV. The samples for MALDI-TOF-MS were prepared by mixing the obtained polymer or oligomer (30 mg mL−1, 2 μL), the matrix (1,8-dihydroxy-9(10 H)-anthracenone (dithranol), 10 mg mL−1, 2 μL), and the ionizing agent (sodium trifluoroacetate, 10 mg mL−1, 8 μL) in THF, and then depositing the mixture on an MTP 384 target plate ground steel T F. External calibration was conducted prior to data acquisition. The detected mass list was extracted using the FlexAnalysis software (Bruker Daltonics).

X-ray diffraction data collection was performed on a Rigaku XtaLAB P200 diffractometer equipped with a DECTRIS PILATUS 200 K hybrid pixel array detector (HPAD), using a 3 kW sealed tube of Mo Kα radiation (λ = 0.71073 Å) at ‒100 °C. The Bravais lattice and the unit cell parameters were determined by the CrysAlisPro version 171.38.46 software package. The raw frame data were processed and absorption corrections were done using CrysAlisPro. The structure was solved using SIR-201444 and SHELXL-201845. Structural refinement was performed using the WINGX-Version 2014.1 system46, on F2 anisotropically for all of the non-hydrogen atoms by the full-matrix least-squares method. The hydrogen atoms were placed at the calculated positions and were included in the structure calculation without further refinement of the parameters. The residual electron densities were of no chemical significance. The methyl ester groups (C(1), C(2), C(57), C(58), O(1), O(2), O(13), O(14)) of the pentamer were modeled as disordered at 50% occupancy. The methyl carbons (C(13), C(56)) of the pentamer were modeled as disordered over two positions at 70% and 30% occupancies, respectively.

CCDC numbers 1898516 (hexamer), 1898517 (pentamer), and 1898515 (tetramer) contain the supplementary crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif. The CIFs are also provided as Supplementary Data 1–3.

TGA was performed on a SII Nano Technology TG/DTA7200 thermal analyser from room temperature to 500 °C under a nitrogen atmosphere at a heating rate of 10 °C min‒1. Td5 is defined as the temperature at which the weight loss reaches 5% of the total weight. Td50 is defined as the temperature at which the weight loss reaches 50% of the total weight.

The Tg of each polymer was measured by DMA (Perkin Elmer DMA800) under a nitrogen atmosphere, by cooling the sample to −150 °C at 10 °C min−1, and then recording the DMA scan from −150 to + 300 °C at a heating rate of 5 °C min−1 and a frequency of 1 Hz.