DFT studies on isomerization reactions in the copolymerization of ethylene and methyl acrylate catalyzed by Ni-diimine and Pd-diimine complexes.

Gradient corrected density functional theory (DFT) has been used to investigate the isomerization reactions in the process of the ethylene/methyl acrylate copolymerization catalyzed by Pd-dimine and Ni-dimine complexes, modeled by a generic system NwedgeN-M-(CH3)+; NwedgeN=-N(H)-C(H)-C(H)-N(H)-. The influence of the polar group and of the metal on the isomerization mechanism was studied. The results show that for the Pd-catalyst the isomerization follows the standard mechanism observed in homopolymerization processes, with the beta-hydrogen-transfer to the metal and formation of a pi-olefin-hydride complex. Electron withdrawing character of the polar group results in an increase of the hydride energy and the isomerization barrier. For the Ni-catalyst the overall isomerization picture is modified by the formation of a sigma-olefin-hydride complex, in which the olefin is coordinated to the metal by the oxygen atom of the polar group. Such a sigma-olefin-hydride is lower in energy for the Ni catalyst than the pi-olefin-hydride complex by 9.6 kcal mol(-1) . The latter is preferred by 2.6 kcal mol(-1) for the Pd-based system. The calculated isomerization barriers are 20.9 and 24.0 kcal mol(-1) (with respect to the initial 4-member chelate) for the Pd-catalyst and Ni-catalyst, respectively. This can result in a larger fraction of ester group directly connected to the copolymer backbone observed experimentally for the Ni-catalyst. [Figure: see text]. Structure of the four-membered and five-membered chelates formed after 2,1-insertion of methyl acrylate into the metal-alkyl bond of the catalyst.