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

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.