The synthesis of well-defined living polymers of isobutyl vinyl ether (IBVE) has been accomplished with trifluoromethanesulfonic (triflic) acid or Lewis acids plus proton sources as initiators. Excess alkyl sulfides were used to stabilize the carbocationic chain ends. The number-average molecular weight (Mn) of the polymers obtained was directly proportional to monomer conversion. The molecular weight distribution (MWD) was narrow. As the sulfide concentration was increased, the rate of polymerization decreased and a nearly monodisperse polymer was obtained. With triflic acid initiator living conditions were observed at -40 and -15 °C. With EtAlCl2 initiator the living polymer is obtained even at 0 °C. When the bulkiness of the sulfide was increased, the rate of polymerization increased but the system deviated from living behavior. Mechanistic studies indicate that alkyl sulfides reduce the concentration of active propagating species via reversible sulfonium ion formation.