Highly isotactic-specific cationic polymerization has been achieved for isobutyl vinyl ether (IBVE) with bis[(2,6-diisopropyl)phenoxy]titanium dichloride [TiCl2{OC(6)H(4)i-Pr-2(2,6-)}(2) (2)] as a Lewis acid catalyst (activator) to be coupled with the IBVE-HCl adduct (initiator). The polymerization, homogeneous and quantitative, was carried out in n-hexane at -78 degrees C in the presence of a bulky pyridine (2,(2,6-di-tert-butyl-4-methylpyridine) to give isotactic-rich polymers with a meso dyad (m) = 90-92% (M-n, similar to 20 000). Under similar conditions unsubstituted counterparts (TiCl4, SnCl4, etc.) gave nearly atactic products (m = 68%). With catalyst 2, the isotactic content m increased at lower temperature, in less polar solvents, and with the added pyridine (m = 64-92%). To design Lewis acids for stereoregulation, a series of titanium dichlorides [TiCl2(OAr)(2)] were also employed where the metal center carries two substituted phenoxy groups, and a pronounced dependence of isotacticity was observed on the bulkiness, positions, and structure of ring substituents on OAr. Overall, isopropyl groups at 2,6-positions led to the highest isotacticity, whereas bulkier or planarlike groups such as tert-butyl or phenyl were less effective, suggesting that not only bulkiness but also three-dimensional shapes of Lewis acids are critical in stereoregulation in cationic polymerization.