Diphosphazane-monoxide and Phosphine-sulfonate Palladium Catalyzed Ethylene Copolymerization with Polar Monomers: A Computational Study

Density functional theory (DFT) calculations have been comparatively carried out to disclose the origin of different catalytic performance of diphosphazane-monoxide and phosphine-sulfonate palladium complexes toward copolymerization of ethylene and vinyl polar monomers. A theoretical comparison of the two catalytic systems indicates that the rigid five-membered backbone and cationic nature of the diphosphazane-monoxide palladium complex are beneficial for its copolymerization activity. Having achieved agreement between theory and experiment, it is found that the favorable 2,1-selective insertion of methyl methacrylate (MMA) into the diphosphazane-monoxide palladium complex originates from less geometrical deformation and the MeO···H interaction between the ancillary ligand and the methyl of MMA. In this catalyst system, it is kinetically difficult for the MMA pre-enchained species to undergo subsequent ethylene insertion due to the steric repulsion between the methyl of MMA and the inserting ethylene moiety, but it relatively favors β-H elimination, yielding the MMA-terminated copolymer observed experimentally. In contrast, comparable insertion energy barriers were observed for phosphine-sulfonate palladium catalyzed MMA 1,2- and 2,1-insertions, which may account for the lower regioselectivity of this catalyst