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 Ti(3+)centers that are investigated by multi-frequency continuous wave and pulse EPR methods. Two families of isolated, molecule-like Ti(3+)species have been identified. A comparison of the experimentally derived gtensors and Cl-35,Cl-37 hyperfine and nuclear-quadrupole tensors with DFT-computed values suggests that the dominant EPR-active Ti(3+)species is located on MgCl2(110) surfaces (or equivalent MgCl2 terminations with tetra-coordinated Mg). O-2 reactivity tests show that a fraction of these Tisites is chemically accessible, an important result in view of the search for the true catalyst active site in olefin polymerization.