Accelerating ethylene polymerization using secondary metal ions in tetrahydrofuran

We have prepared a new series of nickel phosphine phosphonate ester complexes that feature two metal-chelating polyethylene glycol (PEG) side arms. Metal binding and reactivity studies in polar solvents showed that they readily coordinate external cations, including alkali (Li⁺, Na⁺, K⁺), alkaline (Mg²⁺, Ca²⁺), transition (Sc³⁺, Co²⁺, Zn²⁺), post-transition (Ga³⁺), and lanthanide (La³⁺) metals. Although olefin polymerization reactions are typically performed in non-polar solvents, which cannot solubilize +2 and +3 metal cations, we discovered that our nickel catalysts could promote ethylene polymerization in neat tetrahydrofuran. This advance allowed us, for the first time, to systematically investigate the effects of a wide range of M⁺, M²⁺, and M³⁺ ions on the reactivity of nickel olefin polymerization catalysts. In ethylene homopolymerization, the addition of Co(OTf)₂ to our nickel-PEG complexes provided the largest boost in activity (up to 11-fold, 2.7 × 10⁶ g mol^-1 h^-1) compared to that in the absence of external salts. The catalyst enhancing effects of secondary metals were also observed in studies of ethylene and polar olefin (e.g., propyl vinyl ether, allyl butyl ether, methyl-10-undecenoate, and 5-acetoxy-1-pentene) copolymerization. Notably, combining either Co²⁺ or Zn²⁺ with our nickel complexes increased the rates of polymerization in the presence of propyl vinyl ether by about 5.0- and 2.4-fold, respectively. Although further studies are needed to elucidate the structural and mechanistic roles of the secondary metals, this work is an important advance toward the development of cation-switchable polymerization catalysts.