DFT studies on the copolymerization of α-olefins with polar monomers:: Comonomer binding by nickel- and palladium-based catalysts with Brookhart and Grubbs ligands

Gradient-corrected density functional theory has been used to study the polar monomer binding mode in complexes with Ni- and Pd-based Brookhart-diimine (cationic) and Grubbs-salicylaldiminato (neutral) catalysts. Methyl acrylate and vinyl acrylate, as well. as their fluorinated derivatives, have been studied as the comonomers in the copolymerization with cr-olefins. Two binding modes have been considered: the pi -complexes in which a polar monomer is bound by its olefinic functionality, and the O-complexes with a monomer bound by its carbonyl oxygen. The role of the electronic and the steric effects has been investigated, by considering the simplified (generic) models and the examples of the real catalysts. An energy decomposition of the contributions to the binding energies has been performed. The results show that for the Pd-based Brookhart system (active copolymerization catalyst) the pi -complex is preferred, while for its Ni analogue (inactive) the O-complex is more stable. Further, the difference between Ni- and Pd-systems has mainly a steric (electrostatic + Pauli repulsion) origin: there is practically no difference in the orbital-interaction contribution to the binding energy between Ni- and Pd-based systems, as far as a comparison between the two binding modes is concerned. For the fluorinated monomers, the preference of the O-complex is decreased or reversed. However, the binding energies of both the pi- and O-complexes are affected. In the complexes with Grubbs catalyst the rr-complexes are strongly preferred for both the Ni- and Pd-based systems. The presence of bulky substituents on the catalysts results in a decrease in the binding energies of both the pi- and O-complexes; the preference of the binding mode is not affected. Finally, the relative binding constants of the ethylene, propylene, and acrylate in the complexes involving the real Pd-Brookhart system agree with the experimental data for a similar catalyst: the ethylene complexes are most stable, followed by those of propylene and acrylate.