Olefin Polymerization Using Highly Congested ansa-Metallocenes under High Pressure:  Formation of Superhigh Molecular Weight Polyolefins

Polymerization of 1-hexene or 1-octene under high pressures (100−1000 MPa) was investigated using permethylated ansa-metallocenes/methylaluminoxane (MAO) as catalyst systems. Besides the known zirconocene complex (1, Me2Si(η5-C5Me4)2ZrCl2), dialkylsilylene- and dimethylgermylene-bridged hafnocene dichlorides, R2E(η5-C5Me4)2HfCl2 (2, R = Me, E = Si; 3, R = Et, E = Si; 4, R = vinyl, E = Si; 5, R = Me, E = Ge), were synthesized and structurally characterized. The catalytic activity of the ansa-metallocenes was remarkably enhanced under high pressures despite their very congested structures. Poly(1-hexene) with unprecedented high molecular weight such as M w = 1.02 × 107 (M w/M n = 3.79, by GPC) was obtained under 750 MPa with 2. Studies on termination reactions revealed that the bimolecular chain-transfer process (β-hydrogen transfer to olefin) is much accelerated under 500 MPa in 1, while β-hydrogen elimination to metal is still major termination process in 2. This indicated that two complexes that have same structure but central metals showed quite different high-pressure effects.