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. Twofamilies of isolated, molecule-like Ti3+ species have been identified. Acomparison of the experimentally derived gten sors 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.