Effects of steric crowding of the substituent of carboxylate counteranions on living cationic polymerization of isobutyl vinyl ether (IBVE) were investigated with the use of two series of carboxylic acids with various carbonyl substituents [RCOOH; R = (aliphatic series) CH,CH,, (CH,),CH, (CH,),CI(aromatic series) C6H5CH2, (C6H5)(2)CH, (C6H5)(3)C] in conjunction with tin tetrabromide (SnBr4) and 1,4-dioxane (DO) in toluene at 0 degrees C. The overall polymerization rate increased with increasing the bulkiness of the substituents R in both the series: R = CH, (1) = CH,CH, (1) < (CH3)(2)CH (1.76) < (CH3)(3)C (2.31); C6H5CH2 (0.84) < (C6H5)(2)CH (0.98) < (C6H5)(3)C (1.74); the values in the parentheses show the relative polymerization rate. In all the polymerizations, the number-average molecular weight (M-n) of the polymers was directly proportional to monomer conversion and in good agreement with the calculated values, assuming that one RCOOH molecule forms one polymer chain. The living nature of these polymerizations was further confirmed by a linear increase in M, of the polymers upon sequential addition of a fresh monomer feed to the almost completely polymerized reaction mixtures. In the polymerizations with sterically less hindered carboxylic acids [R = CH3CH2, (CH3)(2)CH, C6H5CH2, (C6H5)(2)CH], the molecular weight distribution (MWD) of the polymers was very narrow (M,IM, < 1.1) throughout the polymerizations. In contrast, with bulkier substituent-containing counterparts [R = (CH3)(3)C, (C6H5)(3)C], the polymerizations led to the polymers of relatively broad MWD (M-w/M-n congruent to 1.5 at ca. 100% monomer conversion). The bulky substituents such as (CH3)(3)C and (C6H5)(3)C may decrease the interconversion rate between a dormant and an active species and increase the time-average concentration of the active growing species. The stereoregularity of the obtained polymers was not changed much with the steric environment of the counteranion (meso: 66-69%).