Density Functional Study on Activation and Ion-Pair Formation in Group IV Metallocene and Related Olefin Polymerization Catalysts
journal contributionposted on 25.10.1999, 00:00 by Mary S. W. Chan, Kumar Vanka, Cory C. Pye, Tom Ziegler
The enthalpy of activation by B(C6F5)3 and subsequent ion pair formation for the mono(cyclopentadienyl), constrained-geometry, and bis(cyclopentadienyl) titanium and zirconium precatalysts were investigated by DFT methods. Solvation effects were incorporated by single-point calculations using the conductor-like screening model, and where appropriate, a single molecule of the solvent was included to model the short-range solvent−solute interactions. The enthalpy of methide abstraction to form a contact ion pair was exothermic for all systems investigated, and the electron-donating ability of the ligands around the metal center has the most predominant effect on its magnitude. Subsequent studies focused on the reactions of this contact ion pair with the olefin and the solvent (toluene). The insertion of ethylene between the cationic and the anionic moieties was found to be an endothermic process for all six catalyst precursors investigated. The bis(cyclopentadienyl) systems showed the least endothermic ethylene complexation energy of 6.1 and 8.2 kcal/mol for the titanium and zirconium precatalysts, respectively. The insertion of toluene between the contact ion pair was found to be exothermic for both the mono(cyclopentadienyl) and the zirconium constrained-geometry catalysts but endothermic for the titanium constrained-geometry catalyst and both of the bis(cyclopentadienyl) catalysts. The bulky ligands hinder the approach of the toluene in the bis(cyclopentadienyl) systems, making olefin complexation the more competitive route. The strong tendency for the mono(cyclopentadienyl) and constrained-geometry systems to coordinate with toluene may be an obstacle to olefin complexation.