Preactive Site in Ziegler–Natta Catalysts

Ziegler–Natta catalysts are one of the most important industrial catalysts for the production of isotactic polypropylene, yet they are still not properly understood at the molecular level. We explore the potential of 35,37Cl and 47,49Ti solid-state NMR to study the formation of catalytically relevant titanium sites. First, a systematic study of titanium chlorides (TiCl2, TiCl3, and TiCl4) is undertaken to gain insight into the spectral characteristics for different titanium coordinations. For these materials, chlorine spectra can be relatively straightforwardly obtained, despite their strongly broadened quadrupolar line shapes. The sensitivity of titanium NMR to its local environment is exemplified by the TiCl4 spectrum, for which a small quadrupolar interaction is found despite the nearly symmetric Ti coordination. Upon wet impregnation of MgCl2 with TiCl4, TiCl4 is immobilized at the surface, retaining its tetrahedral coordination. For a ball-milled binary MgCl2/TiCl2 adduct, a ternary system where dono...