Mechanistic Studies of the Palladium-Catalyzed Copolymerization of Ethylene and α-Olefins with Methyl Acrylate

Abstract

Mechanistic aspects of palladium-catalyzed insertion copolymerizations of ethylene and α-olefins with methyl acrylate to give high molar mass polymers are described. Complexes [(N∧N)Pd(CH2)3C(O)OMe]BAr‘4 (2) or [(N∧N)Pd(CH3)(L)]BAr‘4 (1: L = OEt2; 3: L ⋮ NCMe; 4: L ⋮ NCAr‘) (N∧N ≡ ArNC(R)−C(R)NAr, e.g., Ar ⋮ 2,6-C6H3(i-Pr)2, R ⋮ H (a), Me (b); Ar‘ ⋮ 3,5-C6H3(CF3)2) with bulky substituted α-diimine ligands were used as catalyst precursors. The copolymers are highly branched, the acrylate comonomer being incorporated predominantly at the ends of branches as −CH2CH2C(O)OMe groups. The effects of reaction conditions and catalyst structure on the copolymerization reaction are rationalized. Low-temperature NMR studies show that migratory insertion in the η2-methyl acrylate (MA) complex [(N∧N)PdMe{H2CCHC(O)OMe}]+ (5) occurs to give initially the 2,1-insertion product [(N∧N)PdCH(CH2CH3)C(O)OMe]+ (6), which rearranges stepwise to yield 2 as the final product upon warming to −20 °C. Activation parameters (ΔH⧧ = 12.1 ± 1.4 kcal/mol and ΔS⧧ = −14.1 ± 7.0 eu) were determined for the conversion of 5a to 6a. Rates of ethylene homopolymerization observed in preparative-scale polymerizations (1.2 s-1 at 25 °C, ΔG⧧ = 17.4 kcal/mol for 2b) correspond well with low-temperature NMR kinetic data for migratory insertion of ethylene in [(N∧N)Pd{(CH2)2nMe}(H2CCH2)]+. Relative binding affinities of olefins to the metal center were also studied. For [(N∧N)PdMe(H2CCH2)]+ + MA ⇌ 5a + H2CCH2, Keq(−95 °C) = (1.0 ± 0.3) × 10-6 was determined. Combination of the above studies provides a mechanistic model that agrees well with acrylate incorporations observed in copolymerization experiments. Data obtained for equilibria 2 + H2CCHR‘‘ ⇌ [(N∧N)Pd{(CH2)3C(O)OMe}(H2CCHR‘‘)]+ (R‘‘ ⋮ H, Me, nC4H9) shows that chelating coordination of the carbonyl group is favored over olefin coordination at room temperature. Formation of chelates analogous to 2 during the copolymerization is assumed to render the subsequent monomer insertion a turnover-limiting step.