Explanation of the Different Reaction Behaviors of Bridged and Unbridged Cationic Single Component Zirconocene Catalysts in MMA Polymerizations:  a Density Functional Study

Density functional (DFT) calculations were carried out on the B3LYP level of theory on bridged and unbridged cationic zirconocene complexes to clarify why unbridged [Zr(Cp)2(Me)(thf)][BPh4] is not active in the polymerization of methyl methacrylate (MMA), whereas [Zr{(Cp)2CMe2}(Me)(thf)][BPh4] polymerizes MMA readily, as we showed experimentally. It is shown, that the initial step of the polymerizationthe transfer of the metallocene bound methyl group to the first MMA moleculeis endothermic for [Zr(Cp)2(Me)(MMA)]+ but exothermic for [Zr{(Cp)2CMe2}(Me)(MMA)]+. Furthermoreand more importantthe activation energy is higher for the reaction of the unbridged compound compared to the activation energy of the bridged compound. From these results and an estimation of the rate constants it follows that in the temperature range investigated experimentally (−30 to +30 °C) either the endothermic product is not formed at all and/or the reversed reaction to the reactant is much faster than the reaction with the next MMA molecule. These results are confirmed by the inclusion of solvent effects by the CPCM solvation model (also known as COSMO). Alternative reaction paths for both cations were also investigated and found to be energetically much less favorable.