Role of the Metal Center in the Ethylene Polymerization Promoted by Group 4 Complexes Supported by a Tetradentate [OSSO]-Type Bis(phenolato) Ligand

The mechanism responsible for the production of branched polyethylene from ethylene feed during its polymerization promoted by dichloro{1,4-dithiabutanediyl-2,2'-bis(4,6-di-tert-butyl-phenoxy)} titanium complex 1 activated by methylalumoxane (MAO) was investigated by using a density functional theory (DFT) approach. The following processes, chain propagation (CP) reaction, beta hydrogen elimination (beta HE, hydrogen elimination from an alkyl chain to the metal in the absence of a coordinated alkene) and transfer (beta HT, hydrogen transfer from the alkyl chain to a coordinated alkene) were considered to individuate the possible competing reactions responsible for the production of ethylene oligomers. The latter two processes are generally ascribed as termination reactions for the chain propagation. We found that beta HT is the more plausible termination pathway leading to the formation of oligomers that can be reinserted in the growing chain, and thus giving the branched polyethylene. Furthermore, by comparing the energetic profile of CP and beta HT for the analogue zirconium compound 2 we found that beta HT is 50 times more competitive with CP than for the Ti-based compound. As a matter of fact, the MAO activated zirconium complex exclusively produces oligomers with even number of carbon atoms, thus confirming fairly well the calculations that indicated the possibility of tuning the catalytic activity and selectivity of this class of compounds simply by changing the nature of the metal center.