Poly(bridged bicycle)s synthesized via cyclopolymerization of cyclic diacrylates

Abstract

Cyclopolymerization of cyclic diacrylate monomers prepared via a reversible conjugate substitution reaction provided polymers with bridged bicyclic skeletons. The monomer derived from salicylic acid formed a stable conformation with two vinyl groups close together, resulting in selective cyclopolymerization to afford polymers with high glass transition temperatures.

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Bridged bicyclic compounds, such as camphor and β-pinene, are organic molecules that occur widely in nature. Because of their rigid structure with fixed bond angles, bridged bicyclic compounds have attracted the attention of chemists for a long time.¹ Nowadays, bridged bicycles have been recognized as effective motifs in catalysts,^2–5 ligands,^6,7 molecular machines,^8,9 and chemical sensors.^10,11 In polymer chemistry, bridged bicycles have been applied in designing polymers with high thermal stability because their restricted bond rotation provides rigidity to the polymer chains.^12,13 Since bridged bicyclic skeletons reduce the interchain interaction due to the bulky structure, this unit is also effective for preparing porous materials.¹⁴ Recently, Ishiwari et al. applied the removal of the bridged structure in switching the backbone rigidness.¹⁵

There are several approaches to access polymers containing bridged bicycles. The simplest but most reliable method is the polymerization of monomers consisting of bridged bicycles.^12,13,16 McKeown et al. reported an elegant approach via polymerization accompanying bridged bicycle formation.¹⁷ These examples are based on polycondensation reactions, whereas herein, we report the synthesis of poly(bridged bicycle)s via chain polymerization. Cyclopolymerization of divinyl monomers is a typical procedure for incorporating cyclic moieties into the main chain of vinyl polymers.^18–23 Thus, cyclopolymerization of cyclic divinyl monomers is expected to yield poly(bridged bicycle)s (Scheme 1A).

Scheme 1 Cyclopolymerization of the cyclic divinyl monomer to poly(bridged bicycle) (A). Reversible covalent bond formation via conjugate substitution reaction (B). Synthesis of cyclic divinyl monomers by dynamic covalent chemistry (C).

Dynamic covalent chemistry, a molecular strategy symbolized by reversible covalent bond formation, is an efficient toolbox for the preparation of cyclic compounds.²⁴ Xu et al. reported the dynamic property of α-(acyloxymethyl)acrylates in the presence of triethylamine (Et₃N) via a transesterification mechanism.²⁵ Recently, we explained this dynamic property of α-(acyloxymethyl)acrylates by a reversible carboxylic acid exchange mechanism by conjugate substitution reaction (Scheme 1B).²⁶ That is, the nucleophilic attack of the Et₃N catalyst results in the formation of an ammonium intermediate, and the reverse reaction recovers the α-(acyloxymethyl)acrylates. In our previous study, we applied this dynamic covalent bond in vitrimers, a class of network polymers with dynamic cross-linking.²⁶ In this study, we found that the dynamic properties of α-(acyloxymethyl)acrylates are advantageous for preparing cyclic divinyl monomers, as bond formation occurs on the methacrylic skeletons.

An equimolar mixture of α-(chloromethyl)acryloyl chloride (1)^27–29 and hydroxycarboxylic acid (2) was reacted with Et₃N (Scheme 1C). For example, the reaction of 1 and 2b was monitored using high-performance liquid chromatography (HPLC). The chromatogram obtained after 30 min suggested a complicated composition of the reaction mixture (Fig. 1A), probably including macrocycles and polymers as well as unreacted reagents. In contrast, the chromatogram changed to two main peaks after 3 days. The addition of diethyl ether (Et₂O) induced crystallization of the product, which afforded a chromatogram with a single peak corresponding to the main product. Single-crystal X-ray diffraction analysis after recrystallization suggested the isolation of dimer 4b (Fig. 1B, yield: 50%). For the structure with C_i symmetry, dimer 4b appeared to be thermodynamically stable and formed preferentially over the other possible products. 4e was obtained from 2e (Fig. 1C); however, the yield was low (7.0%). In contrast, salicylic acid (2f) afforded cyclic dimer 4f in 76% yield (Fig. 1D). NMR and FTIR spectroscopy also confirmed the structures of 4b, 4e, and 4f (Fig. S1–S6). On the other hand, the other examined hydroxycarboxylic acids, 2a, 2c, and 2d, resulted in products with poor solubility in common organic solvents such as Et₂O, THF, dimethyl sulfoxide (DMSO), and chloroform (CHCl₃), preventing further analysis.

Fig. 1 HPLC charts during and after the reaction between 1 and 2b (A). Single-crystal structures of 4b (B), 4e (C), and 4f (D). Chemical structure (E) and color-coded single-crystal structure (F) of 4f.

4b was not completely soluble in N,N-dimethylformamide (DMF) but was soluble in 1,2-dichloroethane (DCE). Thus, radical polymerization of 4b was conducted in DCE using 2,2′-azobisisobutylonitrile (AIBN) as the initiator at 65 °C for 18 h (Scheme 2 and Table 1, entry 1). The resulting polymer was poorly soluble in chloroform and DMF, and the size-exclusion chromatography (SEC) curve of the chloroform-soluble fraction exhibited a unimodal peak (Fig. S16). Although the ¹H NMR spectrum exhibited broad peaks, vinylidene signals were observed over 6.5–5.4 ppm (Fig. S7). The composition of the units with the remaining vinylidene pendants was roughly estimated to be 54%, based on the intensity ratio of the vinylidene signals to the others, except for the solvent signals. The poor solubility and residual vinylidene groups suggest that 4b did not undergo selective cyclopolymerization rather than branching and cross-linking. In fact, the copolymerization of 4b with methyl methacrylate (MMA) resulted in gelation (entry 2). Polymerization of 4e also resulted in gelation (entry 3). These results implied that a specific strategy is required to induce selective cyclopolymerization.

Scheme 2 Radical (co)polymerization of cyclic diacrylate 4 and MMA.

Table 1 Radical (co)polymerization of cyclic diacrylate 4 and MMA

Entry^a	M1	M2	p 1 ^b (%)	Yield (%)	M n ^c (g mol^−1)	Ð	P 1 ^d^,^e (%)	T d5 (°C)	T g (°C)
a Monomer (10 mmol) was polymerized using AIBN (4 mol%) as an initiator in DMF (74 mL) at 65 °C for 18 h. b Mole fraction of M1 in monomer mixtures. c Determined using SEC (DMF, polystyrene standards, 40 °C). d Mole fraction of the M1 units in the resulting polymers. e Determined using ^1H NMR spectrometry (400 MHz, CDCl3, 25 °C). f Monomer (1.0 mmol) was polymerized in DCE (2.0 mL). g Not observed below 200 °C. h Estimated at the onset of the peak. i 4f (0.74 mmol) was suspended in DMF (1.5 mL). j Monomer (11 mmol) was dissolved in 1,4-dioxane (11 mL).
1^f	4b		100	50	5300	1.48	100	212	>200^g
2^f	4b	MMA	50	Gelation
3^f	4e		100	Gelation
4^i	4f		100	94	10 300	1.28	100	315	223^h
5	4f	MMA	15	68	9370	1.55	16	190	149
6	4f	MMA	10	54	10 100	1.51	12	196	128
7	4f	MMA	5	50	9820	1.34	5.0	192	95
8	4f	MA	5	46	10 900	1.42	9.6	292	64
9	4f	EA	5	62	15 100	1.87	11	326	30
10^j	BnMA	EA	5	82	9580	2.82	5.6	295	−14

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In sharp contrast, polymerization of 4f resulted in a soluble polymer with a narrow molar mass distribution (entry 4), and the ¹H, ¹³C, HQSC, and HMBC NMR spectra exhibited no vinylidene signals (Fig. S8 and S9). These results suggested that selective cyclopolymerization affords a poly(bridged bicycle). The crystal structure of 4f (Fig. 1D) has a cis-conformation in which the two acryl moieties are located closely and align in the same direction. 4f was composed of four planar skeletons: two acrylic moieties and two salicylic moieties (Fig. 1E). The space-filling model (Fig. 1F) suggests that these four planar skeletons restrict bond rotation and ring flip, leading to the cis-conformation being fixed, even in solution. This restricted conformation is considered to be the reason for the selective cyclopolymerization to form a poly(bridged bicycle). In other words, because 4b (Fig. 1B) and 4e (Fig. 1C) had trans-conformers, these monomers did not undergo selective cyclopolymerization. The copolymerization of 4f with MMA was also investigated (entries 5–7). In all entries, polymers soluble in DMF and chloroform were obtained, and the ¹H NMR spectra suggested the occurrence of cyclopolymerization (Fig. S10–S12). The conversion growth over reaction time shown in entry 6 (Fig. S19A) suggested that 4f had higher reactivity than MMA. In fact, the copolymer compositions estimated from the ¹H NMR spectra suggested that the mole fractions of 4f were higher than the feed ratios. The thermogravimetric (TG) analysis (Fig. S17) revealed that the weight remaining after heating to 500 °C closely matched the weight percentage of aromatic rings in the copolymers, as calculated from the mole fraction obtained through ¹H NMR spectroscopy (Table S1). This consistency confirmed the reliability of the copolymer composition determined by ¹H NMR spectroscopy. The T_g values of the copolymers increased as the weight fraction of 4f units increased (Fig. S19B and C). The copolymerization of 4f with methyl acrylate (MA) and ethyl acrylate (EA) was also investigated (entries 8 and 9), resulting in high-T_g copolymers. For example, the copolymer with EA exhibited a T_g of 30 °C, which was higher than that of the copolymer prepared from benzyl methacrylate (BnMA) and EA at a similar feed ratio (entry 10; T_g = −14 °C). The high T_g reflects the rigidity of the bridged bicycle skeletons; however, because the bulkiness of the bridged bicycle skeletons prevents intermolecular interactions despite the presence of two aromatic rings in the 4f unit, the obtained polymers, including the homopolymer of 4f, exhibited good solubility in common organic solvents, CHCl₃ and DMF. The copolymer of 4f and EA obtained in entry 9 afforded a self-standing colorless cast film (Fig. 2). The UV–vis spectrum of the film with a thickness of 151 μm indicated a transmittance of 55–68%.

Fig. 2 Photograph and UV-vis spectrum of the cast film of the copolymer of 4f and EA (entry 9).

In conclusion, the cis-conformer of 4f, prepared in moderate yield (76%) using the dynamic covalent chemistry of α-(acyloxymethyl)acrylates, has an appropriate structure to enhance cyclopolymerization. The resulting bridged bicycle skeletons were effective in improving the T_g of the acrylic resins while guaranteeing their solubility. The construction of bridged bicycle skeletons via cyclopolymerization is interesting from the perspective of basic polymer chemistry. In addition, we believe that it can contribute to polymer engineering because it can be applied to the copolymerization of various vinyl monomers without the need for special polymerization techniques.

Author contributions

Y. K.: conceptualization, funding acquisition, project administration, resources, supervision, visualization, writing – original draft; T. Y.: data curation, formal analysis, investigation, methodology, visualization, writing – review & editing, validation; N. N.: investigation, methodology, writing – review & editing.

Conflicts of interest

There are no conflicts to declare.

Data availability

The data supporting this article have been included as part of the SI. SI includes the following details: Experiments, ¹H and ¹³C NMR spectra, IR spectrum, SEC curves, thermogravimetric analysis (TGA) curves, differential scanning calorimetry (DSC) curves, thermal properties of the copolymers of 4f and MMA (Table S1), the plots of monomer conversions during copolymerization of 4f and MMA (entry 6), and the plots of weight fraction of 4f against T_g. See DOI: https://doi.org/10.1039/d5py00631g.

CCDC 2467318, 2467319 and 2467322 contains the supplementary crystallographic data for this paper.^30a–c

Acknowledgements

The authors appreciate Dr Takumi Noda, Dr Yu Kitazawa, and Prof Mutsumi Kimura for single-crystal X-ray diffraction. This study was financially supported by JSPS KAKENHI Grant Number JP 22H02129 and 23K23397.

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