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 behaviors were systematically investigated by density functional theory (DFT) calculations. The adsorption behaviors of AlR n Cl 3-n species (R=Me, Et, and i Bu; n=1,2,3) on the bare Ti(II) active site indicate a more stable bridge mode than the monodentate mode due to the appreciable attraction between the coordinated atom and the Ti center. The two adsorption modes of DEAC on the bare Ti(II) active site (Model A), Model B for the monodentate adsorption and Model C for bridge adsorption, have a significant influence on both 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 (Model D and Model E) indicate that the Ti(III) active sites are considered to produce copolymer with a high α -olefin content in ethylene/ α -olefin copolymerization. The opposite trend in the Ti(II) active sites (Model B and Model C) suggests they may produce the copolymer with a low α -olefin content in ethylene/ α -olefin copolymerization. In addition, Model B is considered to be the active site to produce oligomers, owing to its lower Δ G AT value (corresponding to termination) than Δ G AI value (corresponding to insertion) that leads to a high tendency for occurrence of termination.