Possible effects on the polyethene chain structure of trimethylaluminum coordination to zirconocene catalysts

Abstract

Ethene was polymerized with the catalytic systems L2ZrCl2/MAO/TMA (where L = Cp, Me5Cp, or Me4Cp; Cp = η5-cyclopentadienyl; MAO = methylaluminoxane; and TMA = trimethylaluminum) at 60°C, 2 bar, and AlTMA/Zr ratios of 0-2700. The polymerization activity was reduced with the addition of TMA for L = Cp but was almost unaffected for the methyl-substituted catalysts. Increasing the TMA concentration resulted in a lower molecular weight of the polymer, with the largest effect for L = Me5Cp. A gel permeation chromatography analysis of the polymers revealed a high molecular weight shoulder and a nearly bimodal distribution for L = Me5Cp at high TMA concentrations. A possible explanation of such a shoulder in terms of long-chain branching was ruled out by dynamic viscosity measurements. The origin of this effect more likely stemmed from competition between chain transfer to aluminum and β-hydrogen transfer reactions at two different sites, one TMA-sensitive and one TMA-insensitive. Polymerizations at various pressures and temperatures substantiated this assumption. A clue to the underlying mechanism came from investigations of chain transfer to TMA studied with density functional calculations. Complexation of Me3Al to Zr was much stronger for L = Cp than for L = Me5Cp. However, the overall chain-transfer barrier was much higher for L = Cp. These results agreed both with the reduced activity for L = Cp and with the strongly reduced molecular weight for L = Me5Cp observed with the addition of TMA.