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

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⧧ = ...