MECHANISM OF DEACTIVATION AND REACTIVATION OF ZIEGLER-NATTA CATALYSTS FOR PROPENE POLYMERIZATION

Propene polymerization experiments have been carried out in the temperature range 50-95-degrees-C using stereospecific MgCl2 supported Ziegler-Natta catalysts. Even in the absence of hydrogen, the terminal double bonds expected from the classical beta-elimination mechanism cannot be observed in the FT-IR spectra of the polymers, which are fully saturated. At high temperature, the residual stable activity is proportional to the alkylaluminium concentration. Reactivation of partially deactivated catalysts is possible upon addition of hydrogen: the greatest effect is observed for the highest temperature. A deactivation mechanism is proposed which produces a saturated polymer and a pi-allyl titanium species. Mechanisms for the reactivation of this species by alkylaluminium or hydrogen are also proposed. They are discussed and compared with other activation mechanisms suggested in the literature for both heterogeneous and homogeneous catalysis.