Ziegler-Natta catalysts for polypropylene: breaking the paradigm of epitaxy

• State-of-the-art Density Functional Theory (D-DFT) calculations and topological analyses of the active site models of Zigler-Natta catalysts. • Newly proposed hypothesis of nonepitaxial TiCl 4 -ID adducts with ID molecules directly bonded to the active Ti is in much better agreement with the experimental facts. • This paradigm shift re-defines structure-properties relations for these important catalysts and can hamper rational ID design. Ziegler-Natta (ZN) catalysts for stereoselective propene polymerization originated as relatively simple mixtures of crystalline TiCl 3 in a layered modification and an Al-alkyl activator. At a later stage, to enhance productivity with respect to Ti and decrease the amount of acidic Ti–Cl residues in the polymer, supported versions were introduced in which TiCl 4 is adsorbed on a nanocrystalline MgCl 2 matrix and subsequently alkylated and reduced by AlEt 3 . The addition of certain organic electron donors to the solid precatalyst (‘Internal Donor’, ID) and, in most cases, to AlEt 3 (‘External Donor’, ED) is mandatory to achieve a high stereoselectivity, but how donor modification works in detail is not well understood. For several decades now the models of catalytic species for Mg–Ti systems have been inspired by the structural similarity between the layer lattices of MgCl 2 and TiCl 3 , which led many to propose that the active site precursors are epitaxial TiCl 4 adsorbates with coordinatively saturated octahedral Ti, and hence no room left for direct Ti-Donor bonding interactions. In the present communication, based on state-of-the-art Density Functional Theory calculations and topological analyses of the surface, we show that the hypothesis of non-epitaxial TiCl 4 -ID adducts with ID molecules directly bonded to the active Ti is in much better agreement with the experimental facts. Breaking the paradigm of epitaxy leads to redefine structure-properties relations for these important catalysts, and can change the strategy of donor design.