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

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 Al_TMA/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. © 2001 John Wiley & Sons, Inc. J Polym Sci Part A: Polym Chem 39: 3566–3577, 2001