SUMMARY The U.V./visible spectrum of the ruc-Et(Ind)2ZrC12/methylaluminoxane (MAO) catalytic system in toluene at 20°C, which is related to the 'ligand to metal charge transfer' bands (LMCT), varies greatly with the MAO/Zr ratio (mole ratio AVZr). Indeed, for MAO/Zr ratios <30, a hypsochromic: shift of the absorption band is observed and is explained by monomethylation of the metallocene dichloride. Conversely, a progressive bathochromic shift of the absorption band is observed for MAO/Zr ratios ranging from 30 to 2000. This behavior is interpreted by the formation of an increasing proportion of dissociated to associated zirconocenium species [L2ZrX]+ [MAOClI-. The A,,,= changes of the U.V./visible spectrum of the rucEt(Ind)2ZrC12/MA0 system were correlated with its catalytic activity for the hex-1 -ene polymerization. Moreover, a conductimetric study performed on the ruc-Et(Ind)2ZrC12/MA0 system for different MAO/Zr ratios is also in agreement with the formation of ionic species with different nature and ionization degrees.

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The homopolymerization of hex-1-ene catalyzed by the system rac-EtInd,ZrCI,/MAO was investigated in solvents of various polarity (toluene, methylene dichloride and heptane). Polymerization kinetics were monitored by dilatometry. It is shown that reaction proceeds by long-lived active species rapidly formed and remaining at constant concentration during the whole polymerization. However, the molar masses of the polymer remain controlled by a transfer process. A strong enhancement of the activity is observed in pure CH2C12 compared to the one measured in toluene. Besides, the use of CH,CI, allows to reduce the amount of methylaluminoxane (MAO) by a factor 20 to reach the plateau of maximum activity, without affecting the isotacticity of the polymer. The increase of activity in CH,CI,, even for low MAO/Zr ratios, is explained by an easier ionic dissociation of the Zr-X bond (X = C1 or methyl), yielding the formation of cation-like active species. A preformation of the catalytic system in minute amount of CH,Cl, followed by hex-1-ene polymerization in heptane or toluene was found to yield also high catalytic activity at a low MAO/Zr ratio. These results suggest that the beneficial effect of methylene dichloride on the formation of the active species is preserved. A tentative explanation is proposed. A first extension of the results obtained with the a-olefin to ethylene polymerization is also presented.