Activation of iPr(CpFluo)ZrCl2 by methylaluminoxane, 4 UV visible spectroscopic study in hydrocarbon and chlorinated media

The olefin polymerization catalytic system iPr(CpFluo)ZrCl2/MAO was investi gated by UV/visible absorption spectroscopy in solvents of various polarity at 20 degrees C. It was shown that the UV/visible main absorption band of the zirconocene, which can be related to the Ligand to Metal Charge Transfe r bands (LMCT), varies greatly upon incremental addition of MAO or AlMe3. F or low [AlMe3]/[Zr]or [MAO]/[Zr] ratios (Al:Zr < 20), a hypsochromic shift of the initial iPr(CpFluo)ZrCl2 absorption band, corresponding to the monom ethylation of the metallocene dichloride, is observed. On the contrary, for higher Al/Zr ratios (with MAO uniquely), a bathochromic shift of the metal locene absorption band proceeding in two distinct steps is observed. These two steps are interpreted by the successive formation of associated (or com plexed) and dissociated active zirconocenium species. The changes in the ab sorption spectra of iPr(CpFluo)ZrCl2/MAO are correlated with the initial ac tivity observed for hex-l-ene polymerization in the various solvents examin ed. In the same way, the catalytic activity loss with polymerization time o bserved at elevated Al/Zr ratios (>1000) with this system is attributed to a reduction of active species concentration, as indicated by the intensity decrease of their UV/visible absorption band. This reveals the low stabilit y of active species formed from iPr(CpFluo)ZrCl2 compared to those from rac -Et(Ind)(2)ZrCl2.