Optimizing the Cation Binding Pocket in Nickel Phenoxyimine Catalysts Improves Ethylene Polymerization Efficiency

Abstract

Cation tuning is a simple yet powerful strategy to modulate the reactivity of polymerization catalysts, but the design rules to achieve maximum cation effects are not well understood. In the present work, it was demonstrated that inserting a methylene spacer between a nickel phenoxyimine complex and an M-polyethylene glycol (PEG) (where M = Li⁺, Na⁺, K⁺, or Cs⁺) unit led up to >70-fold increase in ethylene polymerization activity and 6-fold higher polymer molecular weight relative to that of the first-generation catalysts. It is hypothesized that these effects are due to the exclusive formation of 1:1 over 2:1 nickel:alkali species and closer proximity of the M-PEG moiety to the nickel center. These results suggest that the successful creation of cation-responsive catalysts requires an understanding of the cation binding stoichiometry as well as the structural and electronic changes associated with its host–guest interactions.