Stereospecific polymerization of 1-hexene catalyzed by ansa-metallocene/methylaluminoxane systems under high pressures

Stereospecific polymerization of 1-hexene under high pressures (up to 1,000 MPa = ca. 10,000 atm) using metallocene/methylaluminoxane (MAO) catalysts was investigated. Several C-2-symmetric ansa-metallocenes, their meso-isome rs, and two C-s-symmetric ansa-metallocenes were employed as catalyst precu rsors. In the course of this study, novel C-2-symmetric germylene-bridged a nsa-metallocenes, (rac-[Me2Ge(eta(5)-C5H-2,3,5-Me-3)(2)MCl2] (M = Zr, rac-4 a; M = Hf, rac-4b), have been prepared. High pressures induced enhancement of the catalytic activity and the molecular weight of the polymers in most of the catalysts. The maximum of both the catalytic activity and the molecu lar weight of the polymers was mostly observed at 100-500 MPa in each catal yst, although the enhanced ratio was smaller than that observed for nonbrid ged metallocenes. Isospecificity of the C-2-symmetric ansa-metallocene cata lysts was essentially maintained even under high pressure. Highly isotactic polyhexene ([mmmm] = 91.6%) with very high molecular weight (M-w = 2,360,0 00) was achieved by rac-4b under 250 MPa. High pressures slightly decreased syndiotacticity when the C-s-symmetric ansa-metallocene, isopropylidene(1- eta(5)-cyclopentadienyl)(9-eta(5)-fluorenyl)zirconium dichloride 5, was emp loyed. (C) 1999 John Wiley & Sons, Inc.