Living cationic polymerization of styrene has been achieved with TiCl3(OiPr) as an activator, where its Lewis acidity was modulated by introduction of an electron-donating isopropoxy group onto highly electron-deficient titanium(IV) chloride, in conjunction with HCl-styrene adduct (1) and nBu(4)-NCl in CH2Cl2 at -40 and -78 degrees C. The number-average molecular weights (M(n)) of the polymers increased in direct proportion to monomer conversion and agreed well with the calculated values, assuming that one polymer chain is generated per molecule of adduct 1. The molecular weight distributions were narrow throughout-the reactions ((M) over bar(w)/M(n) similar to 1.1). In contrast, a stronger Lewis acid, TiCl4, induced ill-controlled polymerizations, and a weaker titanium(IV) acid, TiCl2(OiPr)(2), was not effective in the styrene polymerizations. Thus, the selection of Lewis acids is crucial for the living cationic polymerizations so that their Lewis acidity is fit for the monomer reactivity.