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 inter nal donor-free MgCl2-supported Ziegler catalyst in the absence or presence of external electron donors. The variation in the iso-specific active sites was investigated based on the isotacticity distribution of the poly(propyl ene) analyzed by the TREF method. Highly isospecific active sites derived f rom the highest isotactic fraction (elution temperature by TREF >112 degree sC) exist in the electron donor-free catalyst system. The addition of exter nal electron donors converted parts of the aspecific into isospecific activ e 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 isospeci fic active site of high, but not highest isospecificity.