UV/VIS Studies on the Activation of Zirconocene-Based Olefin-Polymerization Catalysts

Equilibria leading to the formation of active zirconocene catalysts were studied by observing changes in the positions of ligand-to-metal Charge transfer bands of these complexes. UV/VIS-spectra of Me2Si(Ind)2ZrX2 (X = Cl, Me) and Me2Si(2-Me-Benzind)2ZrX2 (X = Cl, Me) treated with MAO or, in the case of X = Me, with PhNMe2H+ (F5C6)4B- indicate that binuclear species of the type (Cpx 2ZrMe)2(μ-Me)+ are not formed in the MAO-activated reaction systems. Spectra obtained for MAO-activated catalysts are identical irrespective of whether they are derived from dichloride, dimethyl or biphenolate zirconocene derivatives. The abstracted ligands thus appear to be without coordinative contact to the cationic Zr center. Spectra of the species generated with large MAO excess (>1000:1) indicate that cationic trimethylaluminium adducts are formed, as judged by their similarity to spectra of heterometallic dinuclear cations of the type Cpx 2Zr(μ-Me)2AlMe2 + (F5C6)4B-. These TMA adducts thus appear to be the catalytically most active entities in MAO-activated reaction systems.