Ethylene polymerization using heterogeneous multinuclear nickel catalysts supported by a crosslinked alpha diimine ligand network

Abstract

Nickel(II) alpha diimine catalysts typically produce high molecular weight low crystallinity amorphous polyethylenes via a chain-walking propagation mechanism consisting of consecutive beta hydrogen elimination and reinsertion reactions. In this report the synthesis of a crosslinked alpha diimine ligand and metalation with NiBr₂(dme) affords a multinuclear heterogeneous pre-catalyst which exhibits significant differences from the homogeneous analogue: reduced chain-walking, slower termination, and higher activities. Whereas the homogeneous analogues are known to afford polyethylenes with low crystallinity, high molecular weights, and high activity (<1% crystallinity, 163 kDa, and 520 kg PE mol Ni⁻¹ h⁻¹, respectively), the heterogeneous system under identical conditions yielded polyethylene with increases in all three parameters (20% crystallinity, 217 kDa, and 1377 kg PE mol Ni⁻¹ h⁻¹, respectively). The branching content was further characterized by quantitative ¹³C NMR to reveal that not only did the alkyl branches decrease in number, but also in average branch length, indicative of a decreased propensity for the multinuclear catalyst to chain-walk. Several advantages of heterogeneous catalysis were also observed including the removal of hazardous metal residue, gas-phase polymerization, co-catalyst recovery, and less reactor fouling.