Spectroscopic Signature and Structure of the Active Sites in Ziegler-Natta Polymerization Catalysts Revealed by Electron Paramagnetic Resonance

Spectroscopic Signature and Structure of the Active Sites in Ziegler-Natta Polymerization Catalysts Revealed by Electron Paramagnetic Resonance Publication of CRMN in J. Am. Chem. Soc. on June 25, 2021. Despite decades of extensive studies, the atomic-scale structure of the active sites in heterogeneous Ziegler-Natta (ZN) catalysts, one of the most important processes of the chemical industry, remains elusive and a matter of debate. In the present work, the structure of active sites of ZN catalysts in the absence of ethylene, referred to as dormant active sites, 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. Using electron paramagnetic resonance (EPR) and NMR spectroscopies, in particular 2D HYSCORE experiments complemented by density functional theory (DFT) calculations, we show that the activated ZN catalysts contain bimetallic alkyl-Ti(III),Al species whose amount is directly linked to the polymerization activity of MgCl₂-supported Ziegler-Natta catalysts.