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revised on 24.08.2020 and posted on 25.08.2020by Anton Ashuiev, Matthieu Humbert, David Gajan, Sebastien Norsic, Jan Blahut, Keith Searles, Daniel Klose, Anne Lesage, Guido Pintacuda, Jean Raynaud, Vincent Monteil, Christophe Copéret, Gunnar Jeschke
Despite decades of extensive studies, the atomic-scale structure of active sites in heterogeneous Ziegler-Natta (ZN) catalysts remains elusive and a matter of debate. Here, the structure of polymerization ZN catalysts is elucidated from magnetic resonance experiments carried out on samples reacted with increasing amounts of BCl3 so as to enhance the concentration of active sites and observe clear spectroscopic signatures. Notably, we show that EPR and NMR spectroscopy of the activated ZN catalysts enables to observe paramagnetic species whose amount increases in conjunction with the catalytic activity. The joint application of 2D HYSCORE experiments and DFT calculations reveals the presence of bimetallic alkyl-Ti(III),Al complexes that are assigned to the catalytic centers of MgCl2-supported Ziegler-Natta catalysts