MECHANISTIC STUDIES OF THE PALLADIUM-CATALYZED COPOLYMERIZATION OF ETHYLENE AND ALPHA-OLEFINS WITH METHYL ACRYLATE

Mechanistic aspects of palladium-catalyzed insertion copolymerizations of ethylene and alpha-olefins with methyl acrylate to give high molar mass polymers are described. Complexes [(N boolean AND N)Pd(CH2)(3)C( O)OMe]BAr4' (2) or [(N boolean AND N)Pd(CH3)(L)]BAr4' (1: L = OEt2; 3: L = NCMe; 4: L = NCAr') (N boolean AND 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)) wi th bulky substituted alpha-diimine Ligands were used as catalyst precu rsors. The copolymers are highly branched, the acrylate comonomer bein g 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 st udies show that migratory insertion in the eta(2)-methyl acrylate (MA) complex [(N boolean AND N)PdMe{H2C=CHC(O)OMe}](+) (5) occurs to give initially the 2,1-insertion product [(N boolean AND N)PdCH(CH2CH3)C(O) OMe](+) (6), which rearranges stepwise to yield 2 as the final product upon warming to -20 degrees C. Activation parameters (Delta H double dagger = 12.1 +/- 1.4 kcal/mol and Delta S double dagger = -14.1 +/- 7 .0 eu) were determined for the conversion of 5a to 6a. Rates of ethyle ne homopolymerization observed in preparative-scale polymerizations (1 .2 s(-1) at 25 degrees C, Delta G double dagger = 17.4 kcal/mol for 2b ) correspond well with low-temperature NMR kinetic data for migratory insertion of ethylene in [(N boolean AND N)Pd{(CH2)(2n)Me}(H2C=CH2)](). Relative binding affinities of olefins to the metal center were als o studied. For [(N boolean AND N)PdMe(H2C=CH2)](+) + MA <----> 5a H2C- CH2, K-eq(-95 degrees C) = (1.0 +/- 0.3) x 10(-6) was determined. Comb ination of the above studies provides a mechanistic model that agrees well with acrylate incorporations observed in copolymerization experim ents. Data obtained for equilibria 2 + H2C-CHR '' <----> [(N boolean A ND N)Pd{(CH2)(3)C(O)OMe}(H2C = CHR '')](+) (R '' = H, Me, (C4H9)-C-n) shows that chelating coordination of the carbonyl group is favored ove r olefin coordination at room temperature. Formation of chelates analo gous to 2 during the copolymerization is assumed to render the subsequ ent monomer insertion a turnover-limiting step.