Distribution of Active Sites According to Their Stereospecificity in Propylene Polymerization over Ziegler-Natta Catalysts of Different Composition
The catalysts with a more homogeneous distribution of titanium compounds and higher portion of active sites can be used as efficient model systems to study the composition and structure of active sites and the polymerization mechanism. The ESR study showed that after the interaction with AOC only the isolated Ti(III) compounds are observed in the TiCl4/MgCl2 catalysts with a low (≤ 0.1 wt%) titanium content, as opposed to the catalysts with high titanium content, which include both the isolated and associated titanium compounds in different oxidation states. The mononuclear Ti3+ compounds can be regarded as precursors of active sites in these catalysts. Adsorption of small amounts of TiCl4 on magnesium chloride presumably results in binding of TiCl4 to fourfold-coordinated magnesium ions on the (110) face.
The TREF method can be used in different temperature ranges to divide each PP sample into individual fractions, which are believed to be produced by active sites with different stereospecificities. In this work, we studied the distribution of active sites during propylene polymerization with respect to their stereospecificity using the TiCl4/MgCl2 catalyst with a low titanium content (0.1 wt%, TMC-0.1) as compared to the usual TiCl4/MgCl2 catalyst with titanium content of 3.5 wt% (TMC-3.5) and Solvay type TiCl3 catalyst which does not contain any support. The effect of introduction of an external donor during propylene polymerization on formation of active sites with different stereospecificities was also investigated for these catalysts.