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

Abstract

The stereospecific nature of active sites on various MgCl2‐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.

Experimental and Computational Approaches on the Isospecific Role of Monoester‐Type Internal Electron Donor for TiCl4/MgCl2 Ziegler‐Natta Catalysts

“…[1] Whereas, the mechanism regarding the origin of isospecificity of active sites particularly the isospecific role of ED in heterogeneous Ziegler-Natta catalysis are still open for discussion even after more than 50 years of exploration. [2][3][4][5][6][7][8][9] Further experimental investigation for a complete mechanistic understanding was hindered by the complexity of the heterogeneous catalyst system. In recent decades, computational molecular modeling of heterogeneous polyolefin catalysis are playing more and more important role for further development in this field.…”

Study on the microstructures of polypropylene synthesized by a novel MgCl2-supported and low Ti-loading Ziegler–Natta catalyst

“…However, it was reasonable to suggest that there were six types of growing ways of polymer chains in the novel catalytic system. We believed that Ti atoms in the catalytic system were uniformly dispersed on MgCl 2 supports, forming active Ti-species (a, b, c, d), which could react with AlEt 3 at equilibrium [12] as shown in Fig. 2 [33 -35].…”

“…Here, β a (or β b ) represents the probability that the configuration is the same between the inserted monomer and the former one. The polymer chain propagation on the surface of Ti-species c (or d) would follow the enantiomorphicsite model [12], forming a certain pentad [xxxx] in polymer chain with a probability of g i (β c ) (or g i (β d )) [37]. Here, β c (or β d ) represents the probability of a inserted monomer with d-configuration.…”

A new method of active center determination for olefin polymerization with supported Ziegler-Natta catalysts

“…It causes poorly controllable broad molecular-weight distribution (MWD) and copolymer composition distribution of the polymerization products, [1][2][3][4][5][6][7][8][9] which are direct reasons for the poorer mechanical properties of Ziegler-Natta polyolefins in comparison to polyolefins synthesized with single-site catalysts, such as metallocenes. By determining the chemical structure and catalytic properties of each member in the family of 4-7 types of active centers, it will be possible to find suitable ways of reducing the active center multiplicity in heterogeneous Ziegler-Natta catalyst.…”