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 (gamma and delta 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 Ti-II and oxidation addition of alkenes to Ti-II with the generation of Ti-IV-based polymerization centers. (C) 2012 Elsevier Inc. All rights reserved.