Activation of rac-ethylenebis(indenyl)zirconium dichloride with a low amount of methylaluminoxane (MAO) for olefin polymerizations

The homopolymerization of hex-1-ene catalyzed by the system rac-EtInd(2)ZrCl(2)/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 CH2Cl2 compared to the one measured in toluene. Besides, the use of CH2Cl2 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 CH2Cl2, even for low MAO/Zr ratios, is explained by an easier ionic dissociation of the Zr-X bond (X = Cl or methyl), yielding the formation of cation-like active species. A preformation of the catalytic system in minute amount of CH2Cl2 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 alpha-olefin to ethylene polymerization is also presented.