Kinetics of Ethylene/1-Hexene Copolymerization over a Single-Site Hafnium Bis(phenolate) Catalyst: Insights into Insertion Complexity and Deactivation Pathways

Abstract

Ethylene and 1-hexene were copolymerized by a hafnium salan-type catalyst activated by B(C₆F₅)₃ producing copolymers over a wide range of 1-hexene incorporation. GPC analysis of the resulting copolymers showed a low dispersity index, suggesting that the catalyst behaves as a single-site catalyst. Deuterium labeling experiments revealed that primary active sites decreased as secondary sites increased throughout the reaction. Deactivation of the Hf catalyst during copolymerization was observed; however, the deactivation was suppressed at a higher concentration of 1-hexene. Analysis of multiresponse kinetic data including 1-hexene consumption, active-site quantification, end-group analysis, and triad composition analysis led to determination of the polymerization mechanism and the associated rate constants. Major features of the kinetic model of this catalyst system are (1) the rate constants for ethylene insertion are faster than for 1-hexene insertion by a factor of 2; however, (2) the rate constants are significantly affected by the last monomer inserted. The rate constants for inserting into an ethylene group are slower than for inserting into 1-hexene group by a factor of 10. The data support a mechanism where catalyst sites that misinsert 1-hexene following an ethylene insertion become inactive.