REACTION-MECHANISMS IN METALLOCENE-CATALYZED OLEFIN POLYMERIZATION

1. Studies by H-1 NMR on equilibria between contact ion pairs such as Cp(2)ZrCH(3)(delta+)... H3C-B(C6F5)(3)(delta-) and binuclear alkyl zirconocene cations of the type (Cp(2)ZrCH(3))(2)(mu-CH3)(+) lead to the conclusion that these binuclear species must generally be considered as participants in all homogeneous Ziegler-Natta systems. 2. Different polypropene chain lengths, which are obtained from cis- and trans-1D - propene with the catalyst en(thind)(2)ZrCl2/MAO, show that exchange of alpha-H with alpha-D atoms affect the rate of chain growth by a large kinetic isotope effect; this supports the notion that an alpha-agostic interaction facilitates the olefin insertion step. 3. A strong increase in polymer chain lengths, which is caused by the presence of alpha-methyl groups in ansa-zirconocene catalysts, is shown, by the effects of propene pressure on M(w), to be due to the suppression of the otherwise predominant direct beta-H-transfer to a coordinated olefin molecule by these alpha-substituents.