Controlled cationic copolymerization of vinyl monomers and cyclic acetals via concurrent vinyl-addition and ring-opening mechanisms: the systematic study of structural effects on the copolymerization behavior

The effects of the structure of cyclic acetals on their copolymerization with vinyl ethers (VEs) via concurrent vinyl-addition and ring-opening mechanisms were revealed through a systematic study using five-, six-, and seven-membered cyclic acetals with no substituents or one or two methyl substituents. The controlled cationic copolymerization of 2-chloroethyl vinyl ether (CEVE) and various cyclic acetals successfully proceeded in a living manner. Importantly, the copolymerization times (10 s–70 h) and the sequences of the copolymers (multiblock, random, or approximately alternating) significantly differed depending on the cyclic acetal used. An increase in the number of methyl substituents at the 2-position of the cyclic acetal improved the stability of the generated carbocation, which led to copolymerization with both frequent crossover reactions and at high polymerization rates. The ring member was responsible for the amount of ring strain and its Lewis basicity, which were also related to the frequency of the crossover reactions and the polymerization rates. The controlled copolymerization of cyclic acetals with isobutyl vinyl ether, which is a more reactive VE than CEVE, also proceeded, and the frequencies of the crossover reactions were lower. The key factors affecting the crossover reactions are discussed on the basis of the reactivities of the vinyl monomers and cyclic acetals.