Quasiliving carbocationic polymerization of methyl vinyl ether (MVE) was achieved with the p-dicumyl chloride (p-DCC)/AgSbF6initiator system by the slow and continuous monomer-addition (quasiliving) technique. A polar solvent (CH2Cl2) and a low reaction temperature (-70°C) were optimum for the quasiliving MVE polymerization. Under these conditions, the number-average molecular weight (Mn) of poly(MVE) increased linearly with the cumulative weight of added monomer (WMVE), and linearMnversus WMVEplots passed through the origin.Mn's were inversely proportional to the initial initiator (p-DCC) concentration. Reactions in a nonpolar solvent (toluene) at -70°C or in a polar solvent (CH2Cl2) at −30°C resulted in deviations from these quasiliving characteristics. Block polymerization of MVE from quasiliving poly(isobutyl vinyl ether) dications by the quasiliving technique (p-DCC/AgSbF6initiator, CH2Cl2solvent,(-70°C) led to novel isobutyl vinyl ether (IBVE)-MVE block polymers in high yield (>93 wt%) and at high blocking efficiency. The block polymers, most likely poly(MVE-b-IBVE-b-MVE), havingMn= 10,900–14,000 [Mn(center block) = 6,200–9,0001, were soluble in n-heptane and insoluble in water, and gave hazy homogeneous solutions when dissolved in methanol at room temperature.