Metal-carbon bond strengths under polymerization conditions: 2,1-insertion as a catalyst stress test

Quantitative agreement between experimentally determined M–C bond dissociation energies (BDE) and DFT predictions (M06-2X/TZ//TPSSTPSS/DZ) can be reached by choosing the correct anchor for experimentally derived BDE. For the example of the archetypical metallocene catalyst Cp2TiCl2, it is shown that titanium–carbon bonds are very weak under polymerization conditions and fluctuate; steric strain is introduced after 2,1 insertion and via olefin capture. Thus, homolysis can become competitive with chain propagation. Depending on the catalyst and temperature, 2,1 insertion can be only a temporary inconvenience (dormancy) or a definitive decay event. It is then shown for a set of nine common Ti and Zr polymerization catalysts how ligand variation affects the metal–carbon BDE. Predicted stabilities of the M(IV) oxidation state with respect to homolysis are in nice agreement with the experimentally observed temperature tolerance of the various catalysts: homolysis is easier for Ti than for Zr, and cyclopentadienyl groups in particular facilitate homolysis, especially in bis-cyclopentadienyl systems.