Stereoregulation in Cationic Polymerization by Designed Lewis Acids. 1. Highly Isotactic Poly(isobutyl vinyl ether) with Titanium-Based Lewis Acids

Highly isotactic-specific cationic polymerization has been achieved for isobutyl vinyl ether (IBVE) with bis[(2,6-diisopropyl)phenoxy]titanium dichloride [TiCl 2 {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 °C in the presence of a bulky pyridine (2,6-di- tert -butyl-4-methylpyridine) to give isotactic-rich polymers with a meso dyad ( m ) = 90−92% ( M̄ n ∼ 20 000). Under similar conditions unsubstituted counterparts (TiCl 4, SnCl 4, 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 [TiCl 2 (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.