Ziegler-Natta catalysts for propylene polymerization: Chemistry of reactions leading to the formation of active centers

Abstract

The article describes chemical reactions between a fourth-generation solid Ziegler-Natta catalyst for propylene polymerization, TiCl 4/MgCl 2/diisobutyl phthalate (DIBP), and cocatalyst mixtures containing AlEt 3 and an external donor compound, (Cy)(Me)Si(OMe) 2. The solid catalyst component contains several surface complexes of diisobutyl phthalate with MgCl 2 and TiCl 4, as well as complexes of MgCl 2 and o-phthaloyl chloride, which is formed in a reaction between DIBP and TiCl 4. When the solid catalyst is contacted with the cocatalyst mixture, all these adsorbed carbonyl species react with AlEt 3. The reactions reduce the carbonyl groups of DIBP and lead to the formation of various dialkylaluminum alkoxides. The order of reactivity of the complexes in the reactions with AlEt 3 is: phthaloyl chloride/MgCl 2 > DIBP/MgCl 2 ≈ DIBP/TiCl 4. These reactions result in a complete removal of phthaloyl chloride from the catalyst surface, in a significant reduction of the total content of the MgCl 2/DIBP and TiCl 4/DIBP complexes, and in the reduction of surface Ti IV species to Ti III. The second component of the cocatalyst mixture, complexes of (Cy)(Me)Si(OMe) 2 with AlEt 3, are strongly adsorbed on the surface of the solid catalyst and on the surface of the products of its reactions with excess AlEt 3. The most probable coordination site for the silane species is surface Ti atoms, including the active centers in olefin polymerization reactions.