Alternating Cationic Copolymerization of Vinyl Ethers and Aryl-Substituted Cyclic Acetals: Structural Investigation of Effects of Cyclic Acetals on Copolymerizability

Abstract

The effects of the structural difference of cyclic acetals were investigated in the cationic copolymerization with vinyl monomers via the concurrent vinyl-addition and ring-opening mechanisms. A series of alkyl- and aryl-substituted cyclic acetals were successfully copolymerized with 2-chloroethyl vinyl ether (CEVE) under appropriate conditions. In particular, copolymerization of an aryl-substituted 2-(4-methoxyphenyl)-1,3-dioxolane (PMPDOL) with CEVE involved exclusive crossover reactions between PMPDOL and CEVE, resulting in alternating copolymers. Copolymerization of PMPDOL and other vinyl ethers and styrene derivatives also proceeded via the frequent crossover reactions, while the copolymerization of 2-methyl-1,3-dioxolane, a methyl-substituted counterpart of PMPDOL, with vinyl monomers except for CEVE proceeded negligibly. The difference in the substituents of cyclic acetals significantly affected the electronic and steric environments around the carbocation generated in the propagation reaction, which is related to the frequency of the crossover reaction. Acid hydrolysis of alternating copolymers resulted in complete degradation and selective generation of a single compound due to the periodic incorporation of acetal structures in the main chains, which supported the well-defined structure of copolymers. The monomer reactivity ratios were also consistent with the copolymerizability difference between the aryl- and alkyl-substituted cyclic acetals. The structure–polymerizability relationship of cyclic acetals in the copolymerization was discussed based on the reaction mechanism during the propagating reaction