Characterization of supported bimetallic catalysts for ethylene polymerization: The effect of external Lewis bases

Abstract

The homopolymerization of ethylene was investigated in the presence of a hafnium/titanium (1:1) catalyst supported on magnesium chloride and activated with triisobutylaluminum to which different Lewis bases (LB) were added in various molar ratios. All the investigated LB, i.e. dibutyl ether, butyl acetate, dibutyl phthalate, tetramethylpiperidine, tetraethoxysilane, and phenyltriethoxysilane, show a similar behaviour: polymer yield, molecular weight and molecular weight distribution decrease with increasing the LB:transition metal molar ratio. However, distinct features are observed when changing the nature of the LB, as well as its amount. As far as the polymer yield is concerned, dibutyl ether and tetramethylpiperidine show the lowest and the highest decreasing effect, respectively. Dibutyl ether has very little effect on molecular weight and molecular weight distribution. By contrast, silanes cause a remarkable decrease of molecular weight when molar ratios LB/(Ti + Hf) greater than 0.5 are used. Finally, tetramethylpiperidine gives strongly reduced molecular weight and molecular weight distribution when used at molar ratios appreciably lower than 0.5, while additional amounts do not have any further effect. These results are discussed in terms of possible selective interactions of different catalytic sites with the investigated LB.