Cationic polymerization of isobutyl vinyl ether (IBVE) with acetic acid (CH3COOH)/tin tetrahalide (SnX4 : X = Cl, Br, I) initiating systems in toluene solvent at 0 degrees C was investigated, and the reaction conditions for Living polymerization of IBVE with the new initiating systems were established. Among these tin tetrahalides, SnBr4 was found to be the most suitable Lewis acid to obtain Living poly(IBVE) with a narrow molecular weight distribution (MWD). The polymerization with the CH3COOH/SnBr4 system, however, was accompanied with the formation of a small amount of another polymer fraction of very broad MWD, probably due to the occurrence of an uncontrolled initiation by SnBr4 coupled with protonic impurity. Addition of 1,4-dioxane (1-1.25 vol %) or 2,6-di-tert-butylpyridine (0.1-0.6mM) to the polymerization mixture completely eliminated the uncontrolled polymer to give only the living polymer with very narrow MWD ((M) over bar(w)/(M) over bar(n) less than or equal to 1.1; (M) over bar(w), weight-average molecular weight; (M) over bar(n), number-average molecular weight). The (M) over bar(n) of the polymers increased in direct proportion to monomer conversion, continued to increase upon sequential addition of a fresh monomer feed, and was in good agreement with the calculated values assuming that one CH3COOH molecule formed one polymer chain. Along with these results, kinetic study and direct H-1-NMR observation of the living polymerization indicated that CH3COOH and SnBr4 act as so-called "initiator" and "activator", respectively, and the living polymerization proceeds via an activation of the acetate dormant species. The basic additives such as 1,4-dioxane and 2,6-di-tert-butylpyridine would serve mainly as a "suppressor" of the uncontrolled initiation by SnBr4. The polymers produced after quenching the living polymerization with methanol possessed the acetate dormant terminal and they induced Living polymerization of IBVE in conjunction with SnBr4 in the presence of 1,4-dioxane.