Quasiliving carbocationic polymerization of isobutyl vinyl ether (IBVE) was achieved with the p-dicumyl chloride (p-DCC)/AgSbF6initiating system in methylene chloride solvent at -70 and -90°C. The monomer was introduced as a steady, continuous stream to a premixed, well-stirred initiator solution (quasiliving technique). The number-average molecular weights (Mn) of poly(IBVE) increased linearly with thecumulative weight of added monomer (WIBVE), and linearMnversus WIBVEplots passing through the origin were obtained.Mn's were inversely proportional to the initial initiator concentration ([p-DCC]0) and were in agreement with theoretical values (WIBVE/[p-DCC]0) expected for quasiliving systems, The number of polymer chains produced per unit initiator (p-DCC) was close to unity and remainec unchanged during the early stages of the reactions. The poly-(IBVE) exhibited narrow molecular weight distributions withMw/Mnranging from 1.4 to 1.7. Polymerizations using methylene chloride solvent at highertemperatures (-30 and -50°C) also yielded linear ascendingMnversus WIBVEplots; however, they showed intercepts. Using a nonpolar solvent (n-heptane),Mndid not increase with WIBVE(-50°C) orMnversus WIBVEplots were strongly curved (-70°C), indicating slow initiation and the absence of quasiliving conditions. Evidently optimum conditions for quasiliving IBVE polymerization prevail only in polar media at lower temperatures.