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Abstract

This dialkylmagnesium is represented by the formula R--(CH.sub.2 --CH.sub.2).sub.n --Mg--(CH.sub.2 --CH.sub.2).sub.n' --R', (E--O--E').sub.x, where R and R' each independently represent a C.sub.1 -C.sub.20 hydrocarbon residue, branched or not, substituted or not, of the alkyl, cycloalkyl or aralkyl type; n and n', identical or substantially identical, represent an average number of --CH.sub.2 --CH.sub.2 --linkages such that the average number of carbon atoms in each of the two chains borne by Mg is greater than 20; E and E', identical or different, each represent an alkyl radical, linear or cyclical, branched or not; and 0.ltoreq.x.ltoreq.2, x being the average value of the number of moles of E--O--E' ether complexed to the dialkylmagnesium. In order to prepare a dialkylmagnesium of this type, ethylene is polymerized in an anhydrous, de-oxygenated hydrocarbonaceous solution by means of a catalyst Cp*.sub.2 MCl.sub.2 Li(OEt.sub.2).sub.2 (Cp*=a pentamethylcyclopentadienyl ligand; M=scandium, yttrium or a metal of the lanthanide series; Et=ethyl), in the presence of a dialkylmagnesium, soluble in the solvent, of the formula R--Mg--R', (E--O--E').sub.x, where R, R', E, E' and x are as defined above, as a chain-transfer agent, and the reaction is stopped before the polydispersity of the polymer product obtained exceeds 2, and preferably before it exceeds 1.5.

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