Quantitative Effects of Ion Pairing and Sterics on Chain Propagation Kinetics for 1-Hexene Polymerization Catalyzed by Mixed Cp′/ArO Complexes

A series of single-site catalysts with mixed cyclopentadienyl/aryl oxide ligation were synthesized and used to polymerize 1-hexene. The effects of solvent, metal, counterion, and ligand structure were investigated by experiments 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. Catalyst reactivity was found to be controlled by the difficulty of ion pair separation and steric congestion at the metal center, which were quantified from DFT simulation by SCF ion pair separation energies and ligand cone angles. A Cp*Ti(OC6H4-2-Br)Me-2/B(C6F5)(3) catalyst exhibits unusually high reactivity, which was correlated to the formation of a partial bond between the aryloxide ortho substituent bromide and Ti upon ion pair separation. Natural bond orbital analysis was used to quantify the order of this opportunistic ligation for a series of catalysts. Kinetic analysis and a structure-activity correlation were applied to interpret the experimental results.