ACTIVATION OF RAC-ETHYLENEBIS(INDENYL)ZIRCONIUM DICHLORIDE WITH A LOWAMOUNT 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 (toluen e, methylene dichloride and heptane). Polymerization kinetics were mon itored by dilatometry. It is shown that reaction proceeds by long-live d active species rapidly formed and remaining at constant concentratio n during the whole polymerization. However, the molar masses of the po lymer 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 incr ease of activity in CH2Cl2, even for low MAO/Zr ratios, is explained b y an easier ionic dissociation of the Zr-X bond (X = Cl or methyl), yi elding 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 cata lytic activity at a low MAO/Zr ratio. These results suggest that the b eneficial effect of methylene dichloride on the formation of the activ e species is preserved. A tentative explanation is proposed. A first e xtension of the results obtained with the alpha-olefin to ethylene pol ymerization is also presented.