Interaction of zirconocenes with polymethylalumoxane. Kinetics of methane liberation

The kinetics of methane liberation during polymethylalumoxane (MAO) interac tion with zirconocenes Cp2ZrCl2, Ind(2)ZrCl(2), Me2SiCp2ZrX2, Me(2)SiInd(2) ZrX(2), and Me2SiCp*N'BuZrX2 (X = Cl, Me), were studied in a wide range of metallocene concentration (1 x 10(-3) - 1 x 10(-5) mol/l) and Al/Zr ratio ( 100-24 000) at the narrow range of MAO concentrations (0.1-0.3 mol/l). The reaction of methane liberation follows apparent first order on zirconium. T he total methane yield is suggested to be determined by active aluminum spe cies present in MAO, so that [CH4](infinity) being similar to 0.01 [Al] bot h in reactions of MAO self-condensation and those catalyzed by dissolved zi rconocenes. Comparison of the kinetic parameters of the reaction derived in the presence of zirconocenes shows considerable increase in both specific initial rates of methane liberation and the specific methane yield (per mol e of zirconium) at metallocene concentration lower than 1 x 10(-4) mol/l. M ultiple regeneration of metal-carbon (M-C) bonds was revealed. According to the data of analysis of the effect in terms of dissociative equilibrium, t he active species that catalyze the reaction of condensation of MAO are cat ionic particles. It has been demonstrated that the kinetic parameters of th e reaction were also sensitive to the nature of zirconocene. (C) 2000 Elsev ier Science B.V. All rights reserved.