Francesco Menconi

Comparative behaviour of hafnium and titanium catalysts in copolymerization of α-olefins

Abstract Supported hafnium and titanium catalysts were prepared under the same conditions starting from the corresponding tetrakisbutanolates, which were supported on MgCl2 by reaction of a suspension of Hf/Mg catalyst with aluminum alkyl chlorides. The systems were employed in the copolymerization of 1-hexene with 4-methyl-1-pentene. The extent of polymerization, as well as the copolymer composition, sequence distribution, stereoregularity, and molecular weight were evaluated. The hafnium catalysts gave higher molecular weights and better isotactic stereoregularity but displayed lower activity than the titanium catalysts, but the co-monomer distributions were similar for the two systems. These results are discussed in terms of the number and type of active sites.

Effect of protic compounds on ethylene polymerization by HfTi supported catalysts

Abstract The effect was investigated of the addition of different protic compounds, such as water, n-butanol, butanoic acid and methyl ethyl ketone, to triisobutylaluminium used as the cocatalyst for an HfTi bimetallic catalyst supported on magnesium dichloride (Ti1.0Hf1.3Mg2.0Al0.85Cl12.45) in the polymerization of ethylene. All the above compounds show a similar behaviour by decreasing catalyst activity, increasing polymer molecular weight and narrowing polymer molecular weight distribution. Minor differences are observed in the end-group unsaturations. An attempt has been made to correlate these effects to the interaction of hafnium and titanium sites with the reaction products from AlR3 and the different protic compounds.

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.

Distinct behaviour of Ti and Hf supported catalysts in the polymerization of different monoalkenes

Abstract MgCl2 supported Ti and Hf catalysts were comparatively tested with respect to the polymerization of differently shaped monoalkenes. The ratio between the average polymerization rates of Ti and Hf catalysts during the first 120 min ( V Ti p V Hf p is 10 for ethylene, increases to 63 for propylene and returns to ∼ 12–20 for higher or bulkier monoalkenes, suggesting a distinct shape selectivity of active sites.