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 C₂‐symmetric ansa‐metallocenes, their meso‐isomers, and two C_s‐symmetric ansa‐metallocenes were employed as catalyst precursors. In the course of this study, novel C₂‐symmetric germylene‐bridged ansa‐metallocenes, (rac‐Me₂Ge(η⁵‐C₅H‐2,3,5‐Me₃)₂MCl₂] (M = Zr, rac‐4a; 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 molecular weight of the polymers was mostly observed at 100–500 MPa in each catalyst, although the enhanced ratio was smaller than that observed for nonbridged metallocenes. Isospecificity of the C₂‐symmetric ansa‐metallocene catalysts was essentially maintained even under high pressure. Highly isotactic polyhexene (mmmm] = 91.6%) with very high molecular weight (M_w = 2,360,000) was achieved by rac‐4b under 250 MPa. High pressures slightly decreased syndiotacticity when the C_s‐symmetric ansa‐metallocene, isopropylidene(1‐η⁵‐cyclopentadienyl)(9‐η⁵‐fluorenyl)zirconium dichloride 5, was employed. © 1999 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 37: 283–292, 1999