Cationic ring‐opening polymerization of 2‐isopropenyl‐4‐methylene‐1,3‐dioxolane

Preparation and cationic ring-opening polymerization of 2-isopropenyl-4-methylene-1,3-dioxolane (VI) was performed. Unsaturated cyclic acetal VI was prepared by dehydrochlorination of 2-isopropenyl-4-chloromethyl-1,3-dioxolane, which was easily obtained from methacrolein and epichlorohydrin, with sodium methoxide at ambient temperature. The cationic polymerization of VI with BF3OEt2 or CF3SO3H at −78°C afforded only crosslinked polymers, whereas the polymerization by CH3SO3H gave soluble poly(keto-ether) which consisted of units VII containing an isopropenyl group in the side chain and units VIII containing a carbon-carbon double bond in the main chain. The reaction of VI with ethanethiol in the presence of protic acid was also carried out as a model reaction of the polymerization. The reaction initiated by the addition of proton to the 4-methylene group of VI, and quantitative ring-opening isomerization followed by the addition of ethanethiol afforded acyclic ketone IX and X. On the basis of the model reaction, the polymerization mechanism is also discussed. © 1993 John Wiley & Sons, Inc.