Direct Observation of Radicals in the Activation of Ziegler-Natta Catalysts

Are radicals involved in the activation of a heterogeneous Ziegler–Natta catalyst? The activation of titanium centers in an active Ziegler–Natta model system initiated by alkyl aluminum compounds has been shown by ESR spectroscopy under ultrahigh-vacuum conditions to proceed, in the case of trimethylaluminum, by a radical mechanism (see picture). In contrast, no indication of radical production is found when the ethyl derivative is used for activation.