Manipulation of polymer branching density in phosphine-sulfonate palladium and nickel catalyzed ethylene polymerization

Abstract

Phosphine-sulfonate-based Pd and Ni complexes are widely recognized as potent catalysts for ethylene polymerization and copolymerizations with polar functionalized comonomers. Significant efforts have been directed towards modifying ligand structures and improving catalyst properties. In this contribution, we install a heterocyclic unit in the phosphine-sulfonate ligand framework and study the properties of their corresponding Pd and Ni catalysts. The furyl- and benzofuryl-derived phosphine-sulfonate palladium catalysts show high activities for ethylene polymerization and copolymerization with polar monomers (methyl acrylate, butyl acrylate, 6-chloro-1-hexene, 10-undecenol, and 10-hendecenoic acid). The furyl- and benzofuryl-derived phosphine-sulfonate nickel catalysts are also highly active for ethylene polymerization. The N-methylpyrrolyl-, thienyl-, and benzothienyl-derived palladium and nickel catalysts demonstrate very low or no activity in ethylene polymerization. The microstructures of the resulting polyethylene (highly linear vs. highly branched) are significantly affected by the nature of the heterocyclic substituent. Interactions between the heteroatoms and the metal centers, as well as the π–π stacking between the heterocyclic unit and the benzo backbone in the ligand are hypothesized to play important roles in determining the properties of these metal catalysts.