Selective Degenerative Benzyl Group Transfer in Olefin Polymerization

Abstract

The kinetics of 1-hexene polymerization was investigated using a previously studied zirconium amine bis-phenolate catalyst, Zr[tBu-ONTHFO]Bn2, where the effect of substoichiometric amounts of activator on the polymerization was studied to more clearly elucidate the mechanism of degenerative benzyl-group transfer. Comprehensive kinetic analysis was performed for a diverse set of data including monomer consumption, evolution of molecular weight, and end-group counts over a range of activator to precatalyst ratios, where the analysis determined the rates of association and dissociation of a binuclear complex (BNC) intermediate through which degenerative transfer proceeds. Kinetic modeling indicates that the benzyl-group transfer inside the BNC is rapid, as supported by 1H NMR. Rapid association and dissociation of the BNC enable complete activation of all precatalysts even under the condition of substoichiometric amounts of activator through a degenerative benzyl-group transfer. Through the use of a novel experimental technique wherein a labeled catalyst is introduced during a normal polymerization reaction, this process has been observed to instantaneously activate all incoming precatalyst and effectively shut down the misinsertion pathway. The kinetic analysis shows that BNC has a faster initiation rate than a typical catalyst–ion pair, which may be due to the anion being previously displaced by the incoming unactivated precatalyst.