Stefano Malquori

Cobalt-catalyzed cyclo-cotrimerization of alkynes and heterocumulenes

Abstract The cyclo-cotrimerization of alkynes and heterocumulenes (carbodiimides, isocyanates and carbon dioxide) in the presence of [Co(η5-C5H5)-(C2H4)2], (I), has been studied. This complex exhibits excellent catalytic properties in the cocyclization of alkynes with diimides and isocyanates. Variable amounts of benzene derivatives are also formed in all cases. The yields of both benzene compounds and co-trimers are particularly good in the case of terminal alkynes. (I) is also active in the cocyclization of 1-hexyne with CO2 to give 4,6-dibutyl-2-pyrone in poor yields. The regioselectivity of the reactions involving RNCNR or RNCO is markedly dependent on the nature of the R group. Two isomeric cotrimers, i.e. the 4,6- and the 3,6-disubstituted derivatives, are formed when R is the cyclohexyl group. When R is the phenyl or the p-tolyl group, only one isomer is formed: the 4,6-disubstituted derivative in the case of carbodiimides, and the 3,6-disubstituted derivative in the case of isocyanates.

Supported hafnium catalysts for ethylene polymerization

Abstract Hafnium polymerization catalysts supported on spray-dried magnesium chloride have been prepared by precipitation with alkylaluminium chlorides from paraffin solutions of hafnium(tetra-n-butanolate). These catalysts have been used for ethylene polymerization after activation with different aluminium alkyl derivatives such as tri(isobutyl) aluminium, triethylaluminium and diethylaluminium chloride. The influence of the polymerization conditions, i.e . hydrogen pressure, 1-butene content, temperature, cocatalyst and ethyl benzoate, on the specific activity, polymerization rate and molecular weight has been investigated. The behaviour of the Hf catalyst has been compared to that of similar systems based on titanium or zirconium. The results obtained suggest that the selection of appropriate transition metal derivatives provides the further possibility of tailoring the characteristics of polyethylene.

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.

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.