Cationic terpolymerization of an alkyl vinyl ether (VE) with oxetane and methyl ethyl ketone (MEK) proceeded via concurrent vinyl-addition, ring-opening, and carbonyl-addition mechanisms Highly selective crossover reactions occurred in a one-way cycle, resulting in the generation of rnultiblock polymers with constitutional repeating units comprising a poly(VE) block, a polyoxetane block, and a single MEK unit. The ketone efficiently functioned as a non-homopolymerizable monomer, which reacted with the oxetane-derived oxonium ion to generate an alkoxy group adjacent carbocation or an oxocarbenium ion. Furthermore, the resulting carbocation allowed to react with only VEs. The polymerization, which was induced with an initiating system of Ph3C+PF6-, 2,6-di-tert-butylpyridine, and 1,4-dioxane, was partly mediated by long-lived propagating species. NMR analysis of the chain end structures revealed that mono- and difluorophosphate moieties generated via the hydrolysis of PF6(-) were partially responsible for the generation of these long-lived species.