Based on our previous study on the donor‐free TiCl4/MgCl2 catalyst, the stereospecific nature of active sites on this catalyst after the addition of an internal donor, ethylbenzoate (EB), was investigated by stopped‐flow propylene polymerization combined with temperature rising elution fractionation (TREF) and gel permeation chromatography (GPC) methods. The addition of donor EB was observed to significantly restrain the formation of aspecific sites (AS sites), to drastically promote formation of sites with the second highest isospecificity (IS2 sites) and to construct some new active sites with the highest isospecificity (IS3 sites). It was first observed that the formation of IS3 sites still strongly depends on the interaction between the catalyst and cocatalyst (up to 10 s of pretreatment) even in the presence of internal donor. Based on Busico and co‐workers' three sites model and our previous report, a plausible mechanism for the transformation of stereospecific active sites through some secondary bimetallic complexation reactions was proposed. The extraction of EB from the catalyst by the cocatalyst was found to initiate after 10 s of pretreatment resulting in partial transformation of IS3 sites into AS sites and IS1 sites.

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.

The formation, deactivation and transformation of stereospecific active sites on the donor‐free TiCl4/MgCl2 catalyst induced by short time reaction with triethylaluminium (TEA) cocatalyst (pretreatment time: 0 ∼ 60 s) were investigated by stopped‐flow propylene polymerization combined with temperature rising elution fractionation (TREF) and gel permeation chromatography (GPC) methods. We got the first report that the origin of isospecificity of active sites derived from the catalyst substrate and cocatalyst were individually observed without and with TEA pretreatment, respectively. Based on the three‐sites model of Corradini and Busico et al., a plausible mechanism of the formation and transformation of stereospecific active sites was proposed using active sites models in terms of specified roles of Al‐alkyl cocatalyst. It was demonstrated that the introduction of bulky alkyl groups instead of chlorine atoms into the neighborhood of active sites seems to be crucial for the generation of active sites with the highest isospecificity. magnified image

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The stopped‐flow polymerization of propylene was carried out using an internal donor‐free MgCl2‐supported Ziegler catalyst in the absence or presence of external electron donors. The variation in the isospecific active sites was investigated based on the isotacticity distribution of the poly(propylene) analyzed by the TREF method. Highly isospecific active sites derived from the highest isotactic fraction (elution temperature by TREF: >112 °C) exist in the electron donor‐free catalyst system. The addition of external electron donors converted parts of the aspecific into isospecific active sites, but showed no effects on the highest isospecific active sites. The external electron donor sterically affects a coordination vacancy of each aspecific titanium species and, consequently, transfers it into an isospecific active site of high, but not highest isospecificity.