Stereoregulation in the cationic polymerization of various alkyl vinyl ethers was investigated with bis[(2,6-diisopropyl)phenoxy] titanium dichloride (1; catalyst;) in conjunction with the HCl adduct of isobutyl vinyl ether as an initiator in n-hexane at -78 degreesC. The tacticities depended on the substituents of the monomers. Isobutyl and isopropyl vinyl ethers gave highly isotactic polymers (mm = 83%), whereas tert-butyl and n-butyl vinyl ethers resulted in lower isotactic contents (mm similar to 50%) similar to those for TiCl4, a conventional Lewis acid, thus indicating that the steric bulkiness of the substituents was not the critical factor in stereoregulation. A statistical analysis revealed that the high isospecificity was achieved not by the chain end but by the catalyst 1 or the counteranion derived there from. (C) 2001 John Wiley & Sons, Inc.