Cationic copolymerizations of styrene derivatives and benzaldehyde (BzA) were systematically investigated for the purpose of both achieving living alternating copolymerization and creating "degradable polystyrene derivatives". As a result of the optimization of reaction conditions, the copolymerization of pmethylstyrene (pMeSt) and BzA was demonstrated to proceed in a living manner with GaCl 3 as a catalyst in the presence of tetrahydropyran, yielding a copolymer with a nearly alternating sequence and a controllable molecular weight. sec-Benzylic ether structures were generated in the main chain of the copolymer due to the crossover reactions between the monomers; hence, the copolymer was degraded into low-molecular-weight compounds under acidic conditions. In addition, the copolymer exhibited a glass transition temperature and degradation temperature that were comparable to those of the pMeSt homopolymer.