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.

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.

Abstract: This catalyst has the general formula VX.sub.3, mAIX.sub.3, nZ, wherein X is a halogen atom, Z is at least one at least partially halogenated, branched or unbranched, saturated hydrocarbon, m is between 0.1 and 10, and n is between 1 and 300. To prepare it, a vanadium halide VX.sub.2 and/or VX.sub.3 is coground with an aluminum halide AlX.sub.3, and then at least one halogenated hydrocarbon Z is added to the mixture obtained in proportions corresponding to the chosen values of m and n. The invention also relates to the (co)polymerization of alpha-olefins at 20.degree.-350.degree. C. in the presence of a catalyst system comprising at least one catalyst as defined above and at least one organometallic activator.