TERMINATION, ISOMERIZATION, AND PROPAGATION REACTIONS DURING ETHENE POLYMERIZATION CATALYZED BY CP2ZR-R-ASTERISK-ZR-2-R+ - AN EXPERIMENTAL AND THEORETICAL INVESTIGATION( AND CP)

Ethene polymerization in toluene has been studied in the temperature r ange -7 to +97 degrees C and pressure range 0.28 to 9 bar, using two d ifferent L2ZrCl2/methylaluminoxane (MAO) catalyst systems. With bis(cy clopentadienyl)zirconium dichloride (Cp2ZrCl2, L = Cp), the average ac tivity over 1 h increases with temperature between 10 and 97 degrees C . With bis(pentamethylcyclopentadienyl)zirconium dichloride (Cp2ZrCl2 , L = Cp), a maximum average activity over 1 h is observed at 45 degr ees C. If propagation and deactivation effects are separated through k inetic modeling, the activity corresponding to chain propagation is fo und to increase in the whole temperature range for both catalysts. The molecular weight is higher with L = Cp than with L = Cp below 80 deg rees C. Above 80 degrees C, the opposite is observed. With L = Cp, th e molecular weight increases with increasing ethene pressure up to abo ut 2 bar, where it levels off. With L = Cp, the molecular weight is in dependent 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 press ure. As the temperature is increased, the relative vinyl content decre ases with L = Cp and increases with L = Cp. On the basis of density-f unctional calculations, we present a reaction scheme consistent with m ost of the experimental results. This reaction scheme, in which differ ent agostic interactions play a crucial role, assumes a Cossee-like me chanism 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 m etal, relative rotation of the olefin and the hydride, and reinsertion of the coordinated olefin. The calculated activation energy for propa gation is 25-35 kJ/mol for L = Cp, in fair agreement with the experim ental value of 17 kJ/mol. For L = Cp, we calculate an activation energ y of 10-20 kJ/mol, whereas the experimentally derived value is 61 kJ/m ol. The poor agreement for L = Cp may indicate that the polymerization is influenced by the surrounding solvent and MAO. The calculated diff erence in activation energy between chain propagation and termination is larger for L = Cp than for L = Cp, in qualitative agreement with t he stronger temperature dependence of the molecular weight observed wi th L = Cp. Chain isomerization is found to be easier, relative to ter mination, with L = Cp than with L = Cp. This may account for the larg e amount of trans-vinylene unsaturation observed when Cp2ZrCl2 is use d as catalyst.