201007-19-4Relevant articles and documents
Mechanistic studies of the palladium-catalyzed copolymerization of ethylene and α-olefins with methyl acrylate
Mecking, Stefan
, p. 888 - 899 (2007/10/03)
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 @? ArN = C(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{H2C = CHC(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)(2n)Me}(H2C = CH2)]+. Relative binding affinities of olefins to the metal center were also studied. For [(N@?N)PdMe(H2C = CH2)]+ + MA @? 5a + H2C = CH2, K(eq)(-95°C = (1.0 ± 0.3) x 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 + H2C = CHR' @? [(N@?N)Pd{(CH2)3C(O)OMe}(H2C = CHR')]+ (R' @? H, Me, (n)C4H9) 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.
