New Insights into the Nature of Ti(II) and Ti(III) Active Sites in the Heterogeneous Ziegler–Natta Catalyst

This work presents new insights into the nature of the rarely studied Ti(II) active sites in the heterogeneous Ziegler–Natta catalyst. The effect of diethylaluminum chloride (DEAC) on the Ti(II) and Ti(III) active sites as well as their olefin insertion behavior has been systematically investigated by density functional theory (DFT) calculations. The adsorption behavior of AlRnCl3-n species (R = Me, Et, and iBu; n = 1,2,3) on the bare Ti(II) active site indicates a more stable bridge mode than the monodentate mode due to the appreciable attraction between the coordinated atom and the Ti center. The monodentate and bridge coordination modes of DEAC on the bare Ti(II) active site significantly influence the geometries of the Ti(II) active sites and the electronic properties of the Ti center. The lower apparent activation energies of propylene insertion than ethylene insertion for the Ti(III) active sites indicate that the Ti(III) active sites are considered to produce copolymers with a high α-olefin content in ethylene/α-olefin copolymerization. The opposite trend in the DEAC-coordinated Ti(II) active sites suggests that they can produce the copolymer with a low α-olefin content in ethylene/α-olefin copolymerization. In addition, the DEAC monodentate-coordinated Ti(II) active site is considered to be the active site for producing oligomers, owing to its lower ΔGAT value (corresponding to termination) than ΔGAI value (corresponding to insertion), which leads to a high tendency of occurrence of termination.