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(eta(5)-C5Me4)(2)ZrCl2), dialkylsilylene- and dimethyl germylene-bridged hafnocene dichlorides, R2E(eta(5)-C5Me4)(2)HfCl2 (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 x 10(7) (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 (beta-hydrogen transfer to olefin) is much accelerated under 500 MPa in 1, while beta-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.