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

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. This
connects those spectroscopic signatures to the active species formed
in the presence of ethylene and enables us to propose an ethylene
polymerization mechanism on the observed bimetallic alkyl-Ti­(III),Al
species based on DFT computations.