(628d) A Dft Study of Olefin Polymerization by Ti and Zr Single-Site Catalysts Containing Mixed Cyclopentadienyl/Aryloxide Ligation
Single-site olefin polymerization catalysts are organometallic complexes composed of a transition metal that is positively charged, several organic ligands and a weakly coordinating counter-anion. The structure of the catalytic complex and the reaction conditions affect not only the catalyst's activity, but also the molecular weight distribution and other properties of the polymer produced. We have used density functional theory (DFT) to investigate olefin polymerization by Ti and Zr single-site catalysts containing mixed cyclopentadienyl/aryloxide ligation. Transition state geometries and energies were determined for each of the following reaction steps: (1) catalyst activation, (2) chain initiation, (3) olefin coordination, (4) olefin insertion, (5) chain transfer, (6) chain termination and (7) catalyst deactivation. Both frontside and backside chain propagation pathways were investigated. Changes in the transition state structures and energies were studied as the composition of the catalyst and solvent were changed. Computations for a series of catalyst structures with different degrees of steric congestion at the metal center were used to study steric effects on the transition state energies. Electron withdrawing and donating groups added to the aryloxide ring were used to study electronic effects on the transition state energies.
Increasing the solvent dielectric constant was found to lower barriers for chain propagation, consistent with the lowering of the energy required to separate the catalyst ion pair. In addition it was observed that partial coordination between ligand substituents and the metal center can also lower the catalyst ion pair separation energy, leading to an increase in catalyst activity that is consistent with experimental observations. We will discuss progress that is being made towards understanding the relationships between catalyst structure and transition state energies for each of the reaction steps. The predictions of all DFT simulations will be compared to experimentally determined kinetic constants as determined by other members of the Purdue team.