Palladium-catalyzed nonalternating copolymerization of ethene and carbon monoxide: Scope and mechanism

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)CH ₃ 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) = 21.7kcal 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.