Ethene/propene copolymerisations with rac-EBTHIZrR2/alumoxane: σ-Ligands effect

We report here about the role played by the sigma-ligands of a metallocene in ethene/propene (E/P) copolymerisations promoted by rac-[ethylenebis(4,5,6,7-tetrahydro-1-indenyl)]zirconium dichloride (1), and its corresponding dimethyl derivative rac-EBTHIZrMe2 (2) and binaphtholate rac-EBTHIZr(BNP) (3) derivative, in the presence of several alumoxanes. Alumoxanes alternative to the traditional polymethylalumoxane (MAO) were synthesised and characterised. They are based on Al(2,4,4-trimethylpentyl)3 (TIOA) (4) and were prepared by reaction of (4) with water to give either tetraisooctylalumoxane (TIOAO) (5) or polyisooctylalumoxane (TAO) (6) depending on the stoichiometry of the reaction. H-1 NMR spectra of these products are characterised by the presence of broad bands and of resolved multiplets. Their relative amount in the region between 1.9 and 2.5 ppm was identified as the "fingerprint" of the polymerisation activity. A comparison of the results obtained in polymerisation with metallocenes having different sigma-ligands allowed us to shed more light on the role played by those species. In conclusion, the nature of the sigma-ligands of a metallocene affects its catalytic activity in polymerisation, as a function of the alumoxane employed and of the relative ratio with the metallocene, and this effect is enhanced in "critical conditions," i.e. with a weakly activating cocatalyst or at a low Al/Zr ratio.