Quantitative structure activity relationships for olefin polymerization catalyzed by titanium and zirconium complexes with mixed cyclopentadienyl/aryloxide ligation

Thomas Alan Manz, Purdue University

Abstract

This project developed quantitative models for predicting rate constants of single-site olefin polymerization catalysts. These models were used to design new catalysts with improved performance. Theoretical modeling of Ti and Zr complexes containing mixed cyclopentadienyl/aryloxide ligation was performed in order to correlate changes in catalyst structure to changes in reactivity for different steps in the polymerization process. Variations in metal, ligands, counterion, solvent, and initiating group were considered. Catalyst descriptors were computed from density functional theory (DFT) and used to build structure-activity correlations fit to available experimental data. Ion pairing and steric congestion at the metal were found to be two controlling factors of catalyst reactivity. Ion pairing was quantified by the electronic ion pair separation energy (EIPS). Steric congestion was quantified by ligand cone angles and free solid angles. Depending on the catalyst structure and reaction conditions, different types of ion pairs (inner sphere, outer sphere, or solvent-separated) were found to be thermodynamically and/or kinetically favored. Transition state geometries were computed for activation, initiation, propagation, chain release, and catalyst deactivation pathways. A theory was developed that explains the underlying reasons why chain initiation is facile for some single-site olefin polymerization catalysts but slow for other catalysts, and this theory correctly predicts whether chain initiation is facile or slow for each catalyst investigated. In the absence of monomer, the two important deactivation pathways for dimethyl complexes activated with tris(pentafluorophenyl)borane were H transfer from counterion to methyl initiating group and pentafluorophenyl transfer from counterion to metal. The dominant chain release mechanism is chain transfer to monomer. Quantitative structure-activity relationships (QSARs) were developed for the rate constants of these various reaction steps. Projections are made for how monomer variations could be incorporated into the structure-activity correlations. Molecular orbital and bond order analysis were used to study the nature of metal-ligand bonding in half-metallocene aryloxide and arylsulfide complexes. Catalysts containing an ortho phenyl or halogen substituent on the aryloxide ligand were found to exhibit opportunistic ligand coordination upon ion pair separation that reduced EIPS and increased catalyst reactivity. New catalysts are proposed that are expected to have higher reactivity.

Degree

Ph.D.

Advisors

Thomson, Purdue University.

Subject Area

Inorganic chemistry|Polymer chemistry|Chemical engineering

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