Sugar-Derived Cyclic Acetals as Comonomers for Cationic Copolymerization with Vinyl Ethers

Sugar-derived cyclic acetals were demonstrated to undergo cationic copolymerization with a vinyl ether (VE) via the vinyl-addition and ring-opening mechanisms. Cyclic acetals were synthesized by the condensation reactions of sugar alcohols (mannitol, xylose, erythritol, and sorbitol) with aldehydes or ketones. These cyclic acetals differ in the number of cyclic acetal moieties, the number of ring members, substituents, and stereo configuration. Such structural features greatly affected the copolymerization behavior. For example, erythritol-derived cyclic acetals consisting of two isolated five-membered cyclic acetal moieties undergo copolymerization with 2-chloroethyl VE, while mannitol-derived cyclic acetals consisting of three fused seven- and six-membered cyclic acetal moieties were not incorporated into polymer chains. Moreover, a xylose-derived cyclic acetal that has a p-methoxyphenyl substituent at the carbon atom adjacent to two oxygen atoms underwent copolymerization most effectively among the cyclic acetals examined in this study, which is likely due to the efficient generation of a carbocation via ring-opening. In addition, cationic initiators or Lewis acid catalysts affected the frequency of crossover reactions, resulting in a difference in incorporated ratios of sugar-derived cyclic acetals in polymer chains. The copolymers synthesized from VEs and sugar-derived cyclic acetals were degraded by acid via cleavage of the VE-to-cyclic acetal crossover reaction-derived acetal moieties in the main chain. A gluconic acid-derived cyclic acetal that has both cyclic acetal and cyclic hemiacetal ester moieties was ineffective for copolymerization with a VE, whereas an oxirane was successfully copolymerized with the gluconic acid-derived monomer via the ring-opening of the cyclic hemiacetal ester moiety.