Styrene derivatives were demonstrated to cationically copolymerize with 2,2-disubstituted oxiranes, which can generate a tertiary carbocation by the ring opening of the oxonium ion, through crossover reactions. The copolymerization proceeded with comparable consumptions of both the vinyl and cyclic monomers when appropriate combinations, such as p-methoxystyrene and 2-ethylhexyl beta-methylglycidyl ether or p-methylstyrene and beta-methylepichlorohydrin, in terms of reactivity were used. The homopropagation reactions of both monomers occur preferentially compared to the crossover reactions, which resulted in multiblock copolymers with relatively long blocks. The oxirane-derived carbocation caused both Friedel-Crafts reactions with a styrene derivative and beta-proton elimination as chain transfer reactions. For the analysis of the copolymer structures, unlike the acetal structure resulting from the crossover from a vinyl ether to an oxirane, the sec-benzylic ether structure derived from the crossover from a styrene derivative to an oxirane was difficult to detect by H-1 NMR spectroscopy due to overlap with the peaks of the oxirane units in the main chain. However, the occurrence of crossover reactions was confirmed by C-13 NMR analysis by comparison with the spectra of model polymers. Acidolysis of the sec-benzylic ether structure by trifluoroacetic acid also facilitated the determination of the copolymer structure.