Is the polymerization of linear α-olefins by transition-metal carbene complexes a viable process?: A theoretical study based on density functional theory

Gradient-corrected density functional theory has been used to study the mechanism of alpha-olefin polymerization catalyzed by transition-metal carbene complexes. Taking the [CH2=Re(NO)(2)(PMe3)](+) complex as an example, we investigated the possible elementary steps of the polyethylene formation. This "carbene-to-metallacycle" style mechanism based on a pure carbene intermediate starts with the coordination of the ethylene. Then ethylene reacts with the carbene complex by a [2 + 2] addition. Metallacyclobutane decomposition to a new carbene complex takes place by an alpha-hydrogen transfer reaction. We have also investigated the possible side reactions for the metallacyclobutane decomposition by metathesis reaction, cyclization, and beta-hydride transfer reactions. Calculations have been performed on the monomer and carbene complexes, on the possible intermediates, ethylene pi-complexes, metallacyclobutanes, cyclopropyl complexes, cyclopropyl hydride complexes, propylidene complexes, propene :pi-complexes, allyl-hydride intermediates, and transition states. The results show that when the [CH2=Re(NO)(2)(PMe3)(+) complex is used as a catalyst, the beta-hydride transfer is both kinetically and thermodynamically favored over the alpha-hydride transfer reaction; therefore, ethylene polymerization is not viable. Modifying the catalyst by changing the metal center does not change the preference for the beta-elimination products. However, the application of strongly electron donating ligands can make the metal center sufficiently electron rich to stabilize the new propylidene ligand over the formation of an olefin pi-complex.