DFT Analysis of the Quantitative Effects of Ion Pairing and Sterics for 1-Hexene Polymerization Catalyzed by Mixed Cp′/ArO Complexes
Kendall T. Thomson1, Thomas A. Manz1, James M. Caruthers1, W. Nicholas Delgass1, Khamphee Phomphrai2, Mahdi M. Abu-Omar3, Shalini Sharma2, Grigori A. Medvedev1, Krista A. Novstrup1, and Andrew E. Fenwick2. (1) School of Chemical Engineering, Purdue University, 480 Stadium Mall Drive, West Lafayette, IN 47907-2100, (2) Chemistry Department, Purdue University, West Lafayette, IN 47907, (3) Department of Chemistry, Purdue University, 560 Oval Drive, West Lafayette, IN 47907-2084
A series of Ti and Zr single-site catalysts with mixed cyclopentadienyl/aryloxide ligation were synthesized and used to polymerize 1-hexene. Effects of solvent, metal, counter-ion, and ligand structure were investigated experimentally and by DFT calculations. A solvent with a high dielectric constant led not only to an increase in the chain propagation rate but also to a change in the reaction order. Some of the catalysts displayed opportunistic bonding of the ligand to the metal center upon partial ion pair separation, and this led to a dramatic increase in catalyst reactivity. Natural bond orbital analysis was used to quantify the order of this opportunistic bond. Structure-activity correlations were developed that use DFT-computed catalyst descriptors to explain experimental reactivity trends for chain propagation, initiation, and transfer steps. Chain transfer was facile for uncongested catalysts and slow for congested catalysts. Chain initiation was facile for uncongested catalysts and congested catalysts with opportunistic ligand bonding but slow for congested catalysts without opportunistic ligand bonding. DFT calculations were performed to explain these results.