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(CH 2 ) 3 C(O)OMe]BAr‘ 4 ( 2 ) or [(N ∧ N)Pd(CH 3 )(L)]BAr‘ 4 ( 1: L = OEt 2; 3: L ⋮ NCMe; 4: L ⋮ NCAr‘) (N ∧ N ≡ ArN C(R)−C(R) NAr, e.g., Ar ⋮ 2,6-C 6 H 3 ( i -Pr) 2, R ⋮ H ( a ), Me ( b ); Ar‘ ⋮ 3,5-C 6 H 3 (CF 3 ) 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 −CH 2 CH 2 C(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 η 2 -methyl acrylate (MA) complex [(N ∧ N)PdMe{H 2 C CHC(O)OMe}] + ( 5 ) occurs to give initially the 2,1-insertion product [(N ∧ N)PdCH(CH 2 CH 3 )C(O)OMe] + ( 6 ), which rearranges stepwise to yield 2 as the final product upon warming to −20 °C. Activation parameters (Δ H ⧧ = 12.1 ± 1.4 kcal/mol and Δ S ⧧ = −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, Δ G ⧧ = 17.4 kcal/mol for 2b ) correspond well with low-temperature NMR kinetic data for migratory insertion of ethylene in [(N ∧ N)Pd{(CH 2 ) 2 n Me}(H 2 C CH 2 )] + . Relative binding affinities of olefins to the metal center were also studied. For [(N ∧ N)PdMe(H 2 C CH 2 )] + + MA ⇌ 5a + H 2 C CH 2, K eq (−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 + H 2 C CHR‘‘ ⇌ [(N ∧ N)Pd{(CH 2 ) 3 C(O)OMe}(H 2 C CHR‘‘)] + (R‘‘ ⋮ H, Me, n C 4 H 9 ) 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.