Supported titanium-magnesium catalysts for propylene polymerization

Abstract

The results of studies of the synthesis and properties of supported titanium-magnesium catalysts for propylene polymerization performed at the Boreskov Institute of Catalysis, Siberian Branch, Russian Academy of Sciences, are considered. The composition of the catalysts is TiCl4/D1/MgCl2-AlEt3/D2, where D1 and D2 are stereoregulating donors. With the use of the procedure proposed for the synthesis of titanium-magnesium catalysts, the morphology of catalyst particles depends on the stage of the preparation of a Mg-containing support. The titanium-magnesium catalysts developed afforded polypropylene (PP) in a high yield; this PP was characterized by high isotacticity and excellent morphology. The controllable fragmentation of the catalyst by the polymer is of crucial importance for the retention of the morphology of titanium-magnesium catalyst particles in PP. The fragmentation of catalyst particles to microparticles occurred in the formation of more than 100 g of PP per gram of the catalyst. The surface complexes were studied by DRIFT and MAS NMR spectroscopy and chemical analysis. It was shown that the role of internal donors is to regulate the distribution of TiCl4 on different MgCl2 faces and, thereby, to influence the properties of PP. It was found that chlorine-containing complexes of aluminum compounds were formed on the catalyst surface by the interaction of the catalyst with AlEt3; these complexes can block the major portion of titanium chloride. Data on the number of active sites and the rate constants of polymer chain propagation (kp) at various sites indicate that donor D1 increases the stereospecificity of the catalyst because of an increase in the fraction of highly stereospecific active sites, at which kp is much higher than that at low-stereospecificity active sites. Donor D2 enhances the role of D1. Similar values of kp at sites with the same stereospecificity in titanium-magnesium catalysts and TiCl3 suggest that the role of the support mainly consists in an increase in the dispersity of titanium chloride.