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 α-olefin polymerization catalyzed by transition-metal carbene complexes. Taking the [CH 2 Re(NO) 2 (PMe 3 )] + 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 α-hydrogen transfer reaction. We have also investigated the possible side reactions for the metallacyclobutane decomposition by metathesis reaction, cyclization, and β-hydride transfer reactions. Calculations have been performed on the monomer and carbene complexes, on the possible intermediates, ethylene π-complexes, metallacyclobutanes, cyclopropyl complexes, cyclopropyl hydride complexes, propylidene complexes, propene π-complexes, allyl−hydride intermediates, and transition states. The results show that when the [CH 2 Re(NO) 2 (PMe 3 )] + complex is used as a catalyst, the β-hydride transfer is both kinetically and thermodynamically favored over the α-hydride transfer reaction; therefore, ethylene polymerization is not viable. Modifying the catalyst by changing the metal center does not change the preference for the β-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 π-complex.