Active centers in Ziegler–Natta catalysts: Formation kinetics and structure

The article presents experimental data on the formation rates of active centers in polymerization reactions of ethylene with solid Ti-based Ziegler–Natta catalysts (γ and δ forms of TiCl3, TiCl2) and five types of supported catalysts, TiCl4/MgCl2/SiO2, TiCl3/MgCl2/SiO2 and TiCl4/MgCl2/diisobutyl phthalate. Effects of the following factors on the center formation are discussed: reaction temperature, monomer concentration, the amount of cocatalyst, the initial valence state of Ti atoms in the catalyst, pre-reactions of catalysts with cocatalysts, the presence of molecular hydrogen, effects of 1-alkenes in ethylene polymerization reactions. A combination of the results produces the basis for the formation mechanism of active centers in Ziegler–Natta catalysts, which involves reduction of numerous Ti species on the catalyst surface to IITi and oxidation addition of alkenes to IITi with the generation of IVTi-based polymerization centers.