Variation in the Isospecific Active Sites of Internal Donor‐Free MgCl2‐Supported Ziegler Catalysts: Effect of External Electron Donors

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.     

Kinetic evaluation of various isospecific active sites on MgCl2-supported Ziegler catalysts

The formation, variation and conversion of isospecific active sites were investigated, based on the isotacticity distribution of the polypropenes analyzed by the temperature rising elution fractionation (TREF) method. Stopped-flow polymerization of propene was carried out with a MgCl₂-supported Ziegler catalyst in the absence or presence of an internal or external electron donor so that the effects and roles of the electron donors could be clarified. The results showed that various kinds of active sites with different isospecificities, including the highest isospecific active sites responsible for producing the highest isotactic fraction (elution temperature: > 112°C) existed, even in the electron donor-free catalyst system. The isospecificity of the active sites in the donor-free catalyst might have originated from a surface monolayer multinuclear titanium species, namely an “island” of titanium species. The addition of the external electron donor converted a part of the aspecific and/or low isospecific active sites into the second highest isospecific active sites, but showed no effect on the formation of the highest isospecific active sites, whereas the addition of an internal donor played an important role in creating the highest isospecific active sites as well as suppressing the formation of the aspecific active sites.