Methylene-Bridged Bisphosphine Monoxide Ligands for Palladium-Catalyzed Copolymerization of Ethylene and Polar Monomers

Abstract

A series of palladium complexes bearing a bisphosphine monoxide with a methylene linker, that is, [κ²-<i>P</i>,<i>O</i>-(R¹₂P)CH₂P(O)R²₂]PdMe(2,6-lutidine)][BAr^F₄] (Pd/BPMO), were synthesized and evaluated as catalysts for the homopolymerization of ethylene and the copolymerization of ethylene and polar monomers. X-ray crystallographic analyses revealed that these Pd/BPMO complexes exhibit significantly narrower bite angles and longer Pd-O bonds than Pd/BPMO complexes bearing a phenylene linker, while maintaining almost constant Pd-P bond lengths. Among the complexes synthesized, menthyl-substituted complex <b>3f</b> (R¹ = (1<i>R</i>,2<i>S</i>,5<i>R</i>)-2-isopropyl-5-methylcyclohexan-1-yl; R² = Me) showed the best catalytic performance in the homo- and copolymerization in terms of molecular weight and polymerization activity. Meanwhile, complex <b>3e</b> (R¹ = <i>t</i>-Bu; R² = Me) exhibited a markedly higher incorporation of comonomers in the copolymerization of ethylene and allyl acetate (≤12.0 mol %) or methyl methacrylate (≤0.6 mol %). The catalytic system represents one of the first examples of late-transition-metal complexes bearing an alkylene-bridged bidentate ligand that afford high-molecular-weight copolymers from the copolymerization of ethylene and polar monomers.