MECHANISM OF DEACTIVATION AND REACTIVATION OF ZIEGLER-NATTA CATALYSTSFOR PROPENE POLYMERIZATION

Propene polymerization experiments have been carried out in the temper ature range 50-95-degrees-C using stereospecific MgCl2 supported Ziegl er-Natta catalysts. Even in the absence of hydrogen, the terminal doub le bonds expected from the classical beta-elimination mechanism cannot be observed in the FT-IR spectra of the polymers, which are fully sat urated. At high temperature, the residual stable activity is proportio nal to the alkylaluminium concentration. Reactivation of partially dea ctivated catalysts is possible upon addition of hydrogen: the greatest effect is observed for the highest temperature. A deactivation mechan ism is proposed which produces a saturated polymer and a pi-allyl tita nium species. Mechanisms for the reactivation of this species by alkyl aluminium or hydrogen are also proposed. They are discussed and compar ed with other activation mechanisms suggested in the literature for bo th heterogeneous and homogeneous catalysis.