Ab initio study of hydrogenolysis as a chain transfer mechanism in olefin polymerization catalyzed by metallocenes

Abstract

Hydrogenolysis is a commonly used method to control the molecular weight or size of polymers obtained by Ziegler–Natta and related metallocene catalysts. However, the precise mechanism governing these controlling processes is still unknown. It is most accepted that the insertion of hydrogen molecule into the metal–alkyl bond of the catalyst active species competes favorably against the ethylene insertion. Thus, the newly formed hydride complex would react with an incoming ethylene starting a new polymer chain. Ab initio calculations at the B3LYP/LANL2DZ level of theory have been performed to calculate the reaction profile of both ethylene and hydrogen insertions into the metal–carbon bond of a zirconocene catalyst system. It has been observed that the activation barrier for the hydrogen molecule insertion is lower than that in case of the ethylene insertion (4.58 vs 7.48kcal/mol), supporting the idea of a favored hydrogenolysis process controlling the molecular weight of the polymers. In addition, the ethylene insertion into the hydride complex formed after hydrogenolysis was also studied. No barrier for this reaction has been found, indicating that the initiation of a new chain is an easy step from both thermodynamic and kinetic points of view. The energetic data obtained in the present work provide a reasonable explanation for some experimental facts such as broadening of polymer molecular weight distributions and the early consumption of hydrogen present in the ethylene polymerization.