Rational Design of Oxirane Monomers for Efficient Crossover Reactions in Concurrent Cationic Vinyl-Addition and Ring-Opening Copolymerization with Vinyl Ethers

Abstract

Rational structural design of oxirane monomers is demonstrated here to be highly conducive to crossover reactions in the concurrent cationic vinyl-addition and ring-opening copolymerization of alkyl vinyl ethers and oxiranes. The key to the efficient crossover reactions was the smooth transformation of the once-formed oxirane-derived oxonium ion into the ring-opened carbocation, which then reacted with a vinyl monomer. For example, oxiranes that form resonance-stabilized, allyl-type carbocations (i.e., isoprene monoxide and butadiene monoxide) were polymerized through efficient crossover reactions in copolymerization with isopropyl vinyl ether, yielding copolymers composed of relatively short monomer sequences. In particular, the copolymerization of isoprene monoxide and isopropyl vinyl ether generated random or alternating-rich copolymers. The reaction using an oxirane that forms tertiary carbocations (isobutylene oxide) also proceeded via crossover reactions; however, the crossover frequency was lower, which resulted in a multiblock-like copolymer. In contrast to the three oxiranes that yielded copolymers, oxiranes that possibly result in secondary carbocations from their ring-opening (3,3-dimethyl-1,2-butylene oxide and 1,2-butylene oxide) hardly induced crossover reactions. In addition to the ease of carbocation generation via ring-opening, the nucleophilicity of the monomer and the reactivity of the oxirane-derived carbocations were shown to substantially affect the copolymerization behavior. These factors are discussed on the basis of the monomer reactivity ratios that were determined for the copolymerization systems using vinyl ethers with different reactivities, i.e., isopropyl vinyl ether or ethyl vinyl ether, toward oxiranes.