Attractive Ni^…O Interactions Enable Non-Alternating Ethylene-Carbon Monoxide Copolymerization

In-chain functional groups can reduce the environmental impact of polyethylene waste and enhance its recyclability. A significant advance was the realization of the long-sought catalytic copolymerization of carbon monoxide with ethylene in a non-alternating manner, providing keto-polyethylene materials with low densities of photodegradable keto groups in the chain. Despite this breakthrough, the state-of-the-art catalysts' limited carbon monoxide tolerance is a hurdle in further developing more sustainable polyethylenes. Here we show how these fundamental issues can be addressed by implementation of attractive Ni···O interactions in novel as well as state-of-the-art neutral nickel catalyst motifs. Incorporation of P-bound 2,6-diphenoxyphenyl moieties into both phosphine-imidate and phosphine-phenolate ligand frameworks provides highly active and robust catalysts that generate keto-polyethylenes not accessible to date. Theoretical calculations reveal that Ni···O interactions lower cis/trans isomerization barriers of coordinated ethylene, thereby driving ethylene insertion along the desired non-alternating pathway. At the same time, steric constraint raises the energy barriers for carbon monoxide insertion and reductive elimination, effectively suppressing undesired extensive carbon monoxide insertion and catalyst deactivation. The concept uncovered enables operating conditions, productivities and in-chain functional group concentrations not possible with existing catalysts, and provides perspectives for putting much needed environmentally benign polyolefins into practice.