Unexpected kinetics in the polymerization of ethene by Cp* ₂ZrCl ₂/MAO

A sterically hindered metallocene catalyst, Cp* ₂ZrCl ₂ activated with methylaluminoxane (MAO), is found to polymerize ethene at temperatures up to 60° with a good propagation rate constant but low number of active sites, and with negligible β-hydride elimination or β-hydride transfer to monomer. Moreover, transmetalation to Al is found to be effectively irreversible for alkyl groups larger than Me. With the major mechanisms for chain transfer and termination suppressed, one might expect a living polymerization. The bulk polymerization of ethene was indeed found to be quasi-living even when performed at well above room temperature, and furthermore provided rate constants which agreed remarkably well with those from the mass-spectrometric study.