Abstract
Based on our previous study on the donor‐free TiCl~4~/MgCl~2~ 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 (IS~2~ sites) and to construct some new active sites with the highest isospecificity (IS~3~ sites). It was first observed that the formation of IS~3~ 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 IS~3~ sites into AS sites and IS~1~ sites.
Plausible mechanism for the transformation of active sites with lower isospecificity (AS sites) into the highest isospecific active site (IS~3~ sites) on the TiCl~4~/MgCl~2~ catalyst in the presence of internal donor EB during the pretreatment, X: Cl or Et; M: Mg or Ti. M and Mg are bound to the catalyst substrate through chlorine bridges; □: coordination vacancy.
imagePlausible mechanism for the transformation of active sites with lower isospecificity (AS sites) into the highest isospecific active site (IS~3~ sites) on the TiCl~4~/MgCl~2~ catalyst in the presence of internal donor EB during the pretreatment, X: Cl or Et; M: Mg or Ti. M and Mg are bound to the catalyst substrate through chlorine bridges; □: coordination vacancy.