Palladium-Catalyzed Nonalternating Copolymerization of Ethene and Carbon Monoxide: Scope and Mechanism

Abstract

A series of nonalternating ethene/carbon monoxide copolymers with very low carbon monoxide content were synthesized using a neutral palladium catalyst bearing a phosphine-sulfonate ligand. The ratio of the binding affinities of carbon monoxide and ethene to the palladium chelate complex (P∩SO3)PdCH2CH2C(O)CH3 has been determined by NMR spectroscopy to be ∼50:1 at 25 °C. The migratory insertions of ethene and carbon monoxide into the palladium methyl complex (P∩SO3)PdCH3(py) have also been examined by NMR spectroscopy, giving the following activation parameters: ΔG⧧(25 °C) = 21.7 kcal mol−1 (ΔH⧧ = 17.7(1) kcal mol−1 and ΔS⧧ = −13.6(4) eu) for ethene insertion; ΔG⧧(25 °C) = 13.0 kcal mol−1 (ΔH⧧ = 16.6(8) kcal mol−1 and ΔS⧧ = 20(434) eu) for carbon monoxide insertion. Our study suggests that the unusually small difference in ethene and carbon monoxide binding affinities plays an important role in determining the copolymer composition.