Chiang Mai Journal of Science

 Group 4 metallocene has gained more attention recently as a potential catalyst for ring-opening polymerization of lactones. Introduction of a silicon-bridge and/or modifications of biscyclopentadiene (Cp₂) ligand with different π-ligands such as bisindene (Ind₂), mixed indene/cyclopentadiene (CpInd) or mixed fluorene/cyclopentadiene (CpFlu) have been found to significantly help improve catalyst activities, polymer yields and molecular weight. In order to relate the silicon bridge (B) and π-ligand (L) effects with the catalyst activities, molecular structures and properties of those bridged dimethylzirconocene complexes as well as their insertion behaviors with ε-caprolactone were investigated by using the density functional theory method. Calculations showed that different π-ligands had a profound effect on the electronic geometries and related properties of the title complexes. All bridged catalysts were geometric constraints compared to the unbridged catalysts. The HOMO-LUMO gap and dipole moments were decreased as the large π-ligands were introduced (Cp₂ > CpInd > Ind₂ > CpFlu). Only Si and Zr atoms were observed to carry a positive charge with Si having the most. For the monomer-inserted complexes, most catalyst structures shared the same orientation of ε-caprolactone with its ester group pointing away from a methyl group of Zr atom, except the catalyst with Ind₂ ligand due to steric hindrance. A planar conformation of the monomer was observed in all catalysts and, hence, is required for each insertion. The data provide a structural basis for both inactivated and activated bridged-dimethylzirconocene complex catalysts with different π-ligands, which will be helpful for further studies on the lactone polymerization initiated by group 4 metallocene.