Polyethylene Chain Growth on Zinc Catalyzed by Olefin Polymerization Catalysts:  A Comparative Investigation of Highly Active Catalyst Systems across the Transition Series

Highly active transition metal ethylene polymerization catalysts across the transition series have been investigated for their ability to catalyze chain growth on zinc. In reactions of various catalysts with ZnEt2, product distributions range from Schulz−Flory to Poisson, with several catalysts showing intermediate behavior. A statistical modeling program is introduced to correlate product distributions with the relative rates of propagation, chain transfer to zinc, and β-H transfer. Six regimes have been identified, ranging from Schulz−Flory to pure Poisson where chain transfer to metal is the only termination process, through to combined alkane/alkene distributions where β-H transfer is competitive with chain transfer to metal. It is concluded that, while catalyzed chain growth (CCG) is favored by a reasonable match between the bond dissociation energies of both the main group and transition metal alkyl species, the M−C bond energies of the bridging alkyl species, and hence the stabilities of any hetero-bimetallic intermediates or transition states, are key. The latter are strongly influenced by the steric environment around the participating metal centers, more bulky ligands leading to a weakening of the bonds to the bridging alkyl groups; CCG is thus usually more favored for sterically hindered catalysts.