Fluxional bis(phenoxy-imine) Zr and Ti catalysts for polymerization

Abstract

Bis(phenoxy-imine) complexes of zirconium or titanium, a type of Fenokishi-Imin catalysts, allow the production of polyethylenes with well-defined bimodal molecular weight distributions. Interestingly, by substitution of phenyl rings in the bis(phenoxy-imine) ligands by perfluorinated phenyl rings, the polymerization reaches a desired unimodal behavior. These catalysts have three isomers of similar energy that can be easily interconverted. It is likely that the bimodal behavior is due to the coexistence of more than one isomer in the reaction vessel. Here, we perform static and dynamic DFT calculations to understand the isomerization of the catalytic active species. We analyze the relative Gibbs energies of the different isomers and the barriers for the isomerization processes. Further characterization of the isomers is obtained through stereo maps, aromaticity measures, and NCI plots. Our results show that by changing the phenyl group by a perfluorinated phenyl ring, one of the isomers is particularly stabilized, thus explaining the unimodal behavior of the polyethylene production process.