Selective acetal-initiated living cationic polymerization was successfully employed for the precision synthesis of block and graft copolymers containing poly(p-methylstyrene) and poly(alkyl vinyl ether) segments. The synthesis involved the living cationic polymerization of p-methylstyrene from the acetal moieties at the chain ends or in the side chains of poly(alkyl vinyl ether). Quantitative initiation of the reaction from the acetal moieties and efficient propagation were achieved via the use of a combined initiating system, TiCl₄ and SnCl₄, in the presence of ethyl acetate as an added base and 2,6-di-tert-butylpyridine as a proton trap reagent in CH₂Cl₂ at 0 °C. Appropriate ratios of acetal units, TiCl₄, and SnCl₄ were indispensable for the quantitative reaction. Solubility and thermal analyses indicated that the properties of the block and graft copolymers were mainly governed by the poly(p-methylstyrene) segments. The synthesis of block and graft copolymers using a more reactive alkoxystyrene was also achieved via the initiation reaction from acetal moieties.