Model reactions of the living cationic polymerization of isobutyl vinyl ether (IBVE) with HCl-IBVE adduct [1 : CH3CH(OiBu)Cl]/SnCl4 initiating system were directly analyzed by C-13 and H-1 NMR spectroscopy in CD2Cl2 at -78 degrees C to demonstrate the formation of carbocation from 1. Under the polymerization condition, the 1/SnCl4 initiating system gave nonliving polymers, but in the presence of nBu(4)NCl, it induced living polymerization, and the spectral data were closely correlated with the salt-assisted transition from nonliving to living polymerization. For example, upon mixing SnCl4 with 1, the a-methine and the pendant methylene absorptions of 1 shifted remarkably downfield (e.g., up to 227 ppm in C-13 NMR for the former), and the extent of the downfield shifts increased with increasing SnCl4 concentration. On further addition of the salt, the downfield signals returned to the original upfield positions close to those for the:covalent adduct 1. Thus, these changes and coupling constant ((1)J(CH)) analysis demonstrated that 1/SnCl4 generates an sp(2)-type carbocationic species that is in a rapid and dynamic exchange equilibrium with the covalent precursor 1, and adding nBu(4)NCl suppresses the growing carbocation, thereby leading to living cationic polymerization.