MgCl2-Supported Ziegler-Natta Catalysts for Propene Polymerization: Before Activation

Modern "high-yield" Ziegler-Natta (HY-ZN) catalysts for the isotactic polymerization of propene consist of MgCl2/TiCl4/electron donor/AlEt3 formulations which are much more complex and far less understood than the early TiCl3-based versions. Many important aspects, such as the structure and formation of the active sites and the details of their modification by means of organic electron donors, are largely unknown, and the resulting hybrid (organic-inorganic) catalysts therefore can be regarded as black boxes. For a long time, the similarity between the layered crystal lattices of "violet" TiCl3 and MgCl2 and the concept of epitaxial TiCl4 chemisorption on the latter have been set at the foundation of the mechanistic hypothesis on the inner functioning of HY-ZN systems. Here we provide experimental and computational evidence that metastable nonepitaxial MgCl2/TiCl4 adducts with dynamic character are involved in the activation of the precatalyst with AlEt3 and its deactivation with alkoxysilanes (competent surface modifiers in the catalytic state). These findings can have important consequences in the modeling of catalytic Ti species.