Termination, isomerization, and propagation reactions during ethene polymerization catalyzed by Cp sub 2 Zr-R exp + and Cp exp sub 2 Zr-R exp + . An experimental and theoretical investigation

• Published in: Macromolecules Vol. 31; no. 21; pp. 7149 - 7165
• Main Authors: Rytter, E, Thorshaug, K, Stovneng, J A, Ystenes, M
• Format: Journal Article
• Language: English
• Published: 20.10.1998
• ISSN: 0024-9297

Ethene polymerization in toluene has been studied in the temperature range -7 to +97 deg C and pressure range 0.28-9 bar, using two different L sub 2 ZrCl sub 2 /methylaluminoxane (MAO) catalyst systems. 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 exp * ), 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 exp * , 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 exp * than for L=Cp, in qualitative agreement with the stronger temperature dependence of the molecular weight observed with L=Cp exp * . Chain isomerization is found to be easier, relative to termination, with L=Cp exp * than with L=Cp. This may account for the large amount of trans-vinylene unsaturation observed when Cp sub 2 exp * ZrCl sub 2 is used as catalyst.