Role of titanium oxidation states in polymerization activity of Ziegler–Natta catalyst: A density functional study

The role of titanium oxidation states in olefin polymerization activity for Ziegler–Natta (ZN) catalyst has been investigated using density functional calculations at B3LYP/LANL2DZ as well as extended LANL2DZ basis that includes diffuse and polarization functions for C, H and Cl. Using the simple [TiCl2CH3]n (n = +1, 0, −1) model catalyst systems, we could rationalize some of the well-known experimental facts with varying Ti oxidation states (+4, +3, +2) in the real ZN systems. Firstly, irrespective of Ti oxidation states, the activation barriers (Eact) for ethylene and syn propylene insertion in Ti–CH3 bond are comparable in accordance with experimental and modeling studies. Secondly, it was observed that Ti(IV) catalyst has the lowest Eact which progressively increase in the order Ti(IV) < Ti(III) < Ti(II) high spin < Ti(II) low spin catalysts in line with experimental and several modeling results. The effect of solvation on olefin insertion barriers are seen more prominent in case of Ti(IV) systems compared to other oxidation states.

Graphical abstractThe role of titanium oxidation states (+4, +3, +2) in ethylene and propylene polymerization activity of Ziegler–Natta catalyst has been investigated using density functional calculations at B3LYP/LANL2DZ and higher basis set on the simple [TiCl2CH3]n (n = +1, 0, −1) model catalysts.Figure optionsDownload full-size imageDownload as PowerPoint slide