Theoretical study of hydrogenolysis termination processes in ethylene polymerization

The reaction of molecular hydrogen with the active species of a model metallocene catalyst is investigated through quantum mechanical calculations. The results are compared to ethylene insertion in the corresponding polymerization reaction. Hydrogenolysis and ethylene insertion exhibit both frontside and backside mechanisms. However, while ethylene insertion proceeds preferably through frontside mechanism, hydrogenolysis occurs mainly through the backside one. The activation energies are close but hydrogenolysis is slightly more favorable than ethylene insertion, in agreement with experimental findings. Agostic interactions have been found to play an important role, like in the chain propagation process.