Strong Activation of MgCl2-Supported Ziegler-Natta Catalysts by Treatments with BCl3: Evidence and Application of the 'Cluster' Model of Active Sites

摘要

Heterogeneous Ziegler-Natta precatalysts (with phthalate as internal donor) were modified by treatments with BCl3 (2 h in heptane; T = 20-90 degrees C B/Ti = 0.1-5) before their use in the polymerization of propylene to modify the active sites distribution. If performed on previously "detitanated" precatalysts, the treatment leads to a strong increase of productivity (up to one order of magnitude) without drastic modifications of polypropylenes properties (tacticity, molecular weight distribution). In addition, these findings are in good agreement with the hypothesis of a "cluster" organization of active sites allowing to rationalize activation by BCl3 by formation of heteronuclear B-Ti clusters. The activation method was also applied to unmodified precatalysts and gave a significant gain of productivity. The simple and versatile activation process can also be performed under mild conditions (low T and low [BCl3]).