Termination, Isomerization, and Propagation Reactions during Ethene Polymerization Catalyzed by Cp2Zr−R+ and Cp*2Zr−R+. An Experimental and Theoretical Investigation

Abstract

Ethene polymerization in toluene has been studied in the temperature range −7 to +97 °C and pressure range 0.28 to 9 bar, using two different L2ZrCl2/methylaluminoxane (MAO) catalyst systems. With bis(cyclopentadienyl)zirconium dichloride (Cp2ZrCl2, L = Cp), the average activity over 1 h increases with temperature between 10 and 97 °C. With bis(pentamethylcyclopentadienyl)zirconium dichloride (Cp*2ZrCl2, L = Cp*), a maximum average activity over 1 h is observed at 45 °C. If propagation and deactivation effects are separated through kinetic modeling, the activity corresponding to chain propagation is found to increase in the whole temperature range for both catalysts. The molecular weight is higher with L = Cp* than with L = Cp below 80 °C. Above 80 °C, the opposite is observed. With L = Cp*, the molecular weight increases with increasing ethene pressure up to about 2 bar, where it levels off. With L = Cp, the molecular weight is independent of pressure between 0.28 and 9 bar. The ratio between vinyl and trans-vinylene unsaturation is approximately 6:1 with L = Cp and 1:1 with L = Cp*, both slightly increasing with increasing ethene pressure. As the temperature is increased, the relative vinyl content decreases with L = Cp and increases with L = Cp*. On the basis of density-functional calculations, we present a reaction scheme consistent with most of the experimental results. This reaction scheme, in which different agostic interactions play a crucial role, assumes a Cossee-like mechanism for chain propagation, chain termination via hydrogen transfer to a coordinated monomer (for both catalysts) or to the metal (for L = Cp*), and chain isomerization via partial hydrogen transfer to the metal, relative rotation of the olefin and the hydride, and reinsertion of the coordinated olefin. The calculated activation energy for propagation is 25−35 kJ/mol for L = Cp*, in fair agreement with the experimental value of 17 kJ/mol. For L = Cp, we calculate an activation energy of 10−20 kJ/mol, whereas the experimentally derived value is 61 kJ/mol. The poor agreement for L = Cp may indicate that the polymerization is influenced by the surrounding solvent and MAO. The calculated difference in activation energy between chain propagation and termination is larger for L = Cp* than for L = Cp, in qualitative agreement with the stronger temperature dependence of the molecular weight observed with L = Cp*. Chain isomerization is found to be easier, relative to termination, with L = Cp* than with L = Cp. This may account for the large amount of trans-vinylene unsaturation observed when Cp*2ZrCl2 is used as catalyst.