Precise arguments on the distribution of stereospecific active sites on MgCl₂‐supported ziegler‐natta catalysts

Abstract

Abstract The stereospecific nature of active sites on various MgCl 2 ‐supported Ziegler‐Natta catalysts was investigated by stopped‐flow technique combined with temperature rising elution fractionation (TREF) method. A modified three‐sites model with precise description of the stereospecific nature of various types of active sites stemmed from surface titanium species, Al‐alkyl compounds, Mg‐compounds and electron donors has been proposed. It was demonstrated that the isospecificity of active sites strongly depends on the bulkiness of the ligands situated at the two most important ligand positions for construction of asymmetry and chirality of the active sites with steric hindrance. In general, there may exist both monometallic and bimetallic sites in heterogeneous Ziegler‐Natta catalyst system. The kinds of active titanium species with different chemical structures on this catalyst system should be limited, whereas, the non‐discrete distribution of isospecificity of active sites could be considered to generate from the numerous types of steric and electronic effects from the surroundings of active titanium species as well as large number of reversible and dynamic transformation reactions simultaneously occurred on the heterogeneous catalyst surface.