Probing the Coordinative Unsaturation and Local Environment of Ti³⁺ Sites in an Activated High-Yield Ziegler–Natta Catalyst

The typical activation of a fourth generation Ziegler–Natta catalyst TiCl₄/MgCl₂/phthalate with triethyl aluminum generates Ti³⁺ centers that are investigated by multi-frequency continuous wave and pulse EPR methods. Two families of isolated, molecule-like Ti³⁺ species have been identified. A comparison of the experimentally derived g tensors and ^35,37Cl hyperfine and nuclear-quadrupole tensors with DFT-computed values suggests that the dominant EPR-active Ti³⁺ species is located on MgCl₂(110) surfaces (or equivalent MgCl₂ terminations with tetra-coordinated Mg). O₂ reactivity tests show that a fraction of these Ti sites is chemically accessible, an important result in view of the search for the true catalyst active site in olefin polymerization.