A Comparison of Ni- and Pd-Diimine Complexes as Catalysts for Ethylene/Methyl Acrylate Copolymerization. A Static and Dynamic Density Functional Theory Study

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Abstract

Gradient-corrected density functional theory (DFT) has been used to study the elementary reactions for the mechanism of ethylene/methyl acrylate copolymerization catalyzed by Pd- and Ni-diimine complexes, N∧N-M-(n-C3H7)+; N∧N = −N(Ar)-C(R)-C(R)-N(Ar)−. The main goal was to understand the differences between the Pd- (active copolymerization catalyst) and Ni-systems (inactive under the same conditions) and, thus, the factors that determine the catalyst activity in these processes. The acrylate insertion into the metal−alkyl bond, the stability of the insertion products, and the complexation of the next monomer (ethylene and acrylate) have been studied by static calculation, and the molecular dynamics approach has been applied to study the ethylene insertion following the acrylate insertion. To account for the steric influence on the acrylate insertion barriers and the stability of isomeric chelate-ethylene complexes, calculations have been carried out for both the model [Ar = H, R = H] and the real catalyst [Ar = 2,6-C6H3(i-Pr)2; R = CH3]. It has been found that acrylate insertion follows the same mechanism for the Ni- and Pd-complexes. The 2,1-insertion is a preferred pathway for the acrylate incorporation. For both catalysts the acrylate 2,1-insertion barriers (12.4 and 13.5 for Pd and Ni with the real catalyst, respectively) are lower than the barriers for the insertion of ethylene (16.8 and 14.2 for Pd and Ni). The chelates formed after the acrylate insertion are slightly more stable for Ni than for Pd, with the five-membered system having the lowest energy. The MD results show that after the acrylate incorporation the activation barriers for the ethylene insertions starting from the isomers without the chelating bond are substantially lower than those starting from the chelated complexes. The ethylene insertion barriers are lower for Ni than for Pd in any case. The barriers for the opening of the chelate prior to insertion have been found to be lower for Pd (ΔG⧧ = 11.3 kcal/mol) than for Ni (14.4 kcal/mol) for the generic catalyst. The opening of the chelates is facilitated by the presence of the steric bulk on the catalyst; the effect is stronger for Ni than for Pd. Therefore, the present results suggest that the most important difference in the mechanisms of the copolymerization of the methyl acrylate with ethylene between the Pd- and Ni-diimine complexes is an initial poisoning of the catalyst by the O-binding mode in the latter case.

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