A Combined Density Functional and Molecular Mechanics Study on Olefin Polymerization by Metallocene Catalysts

Abstract

A study based on density functional theory (DFT) and molecular mechanics is presented on processes related to ethylene polymerization by metallocenes. The study makes use of Cp2Zr-CH 3 + as the model catalysts and all calculations are based on the AMOL program system. Calculations have been carried out on the energy profile for the insertion reaction: Cp2M-CH3 1a + H2C = CH2 → Cp2M-CH2CH2CH3 1d. The computational study has shown that a π-ethylene complex with a very shallow potential well is involved as an intermediate in the insertion process. Also the barrier to insertion is modest and amounts to less than 1 kcal/mol. The propyl complex, 1d, can adopt several conformations including structures in which a C-H bond is involved in a β-agostic interaction, or a γ-agostic interaction with the metal center. The structure with a β-agostic interaction is the most stable. Our study does support the view that α-agostic interactions are important for the stability of 1a. It is suggested that the experimental barrier of activation for the propagation in olefin polymerization is due to the rearrangement of 1d in order to free the coordination site for the next insertion, rather than the actual insertion. Two chain terminating steps have been studied, involving β-hydrogen elimination: Cp2Zr+-CH2CH3 → Cp2Zr+-H + H2C = CH2, and C-H activation: Cp2Zr+-CH3 + H2C = CH2→Cp2Zr+-CH = CH2 + CH4. The β-hydrogen elimination reaction is endothermic by 42 kcal/mol whereas the C-H activation reaction is exothermic by 14.7 kcal/mol. It is concluded that β-hydride elimination is unlikely a dominant chain termination step based on thermodynamic and kinetic considerations.