Stereoregulation in cationic polymerization by designed Lewis acids. II. Effects of alkyl vinyl ether structure

Stereoregulation in the cationic polymerization of various alkyl vinyl ethe rs was investigated with bis[(2,6-diisopropyl)phenoxy] titanium dichloride (1; catalyst;) in conjunction with the HCl adduct of isobutyl vinyl ether a s an initiator in n-hexane at -78 degreesC. The tacticities depended on the substituents of the monomers. Isobutyl and isopropyl vinyl ethers gave hig hly isotactic polymers (mm = 83%), whereas tert-butyl and n-butyl vinyl eth ers resulted in lower isotactic contents (mm similar to 50%) similar to tho se for TiCl4, a conventional Lewis acid, thus indicating that the steric bu lkiness 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 the re from. (C) 2001 John Wiley & Sons, Inc.