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 diphospha- zane-monoxide palladium complex originates from less geometrical deformation and the MeO center dot center dot center dot 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 beta-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.