Abstract: (I) The (co)polymerisation of ethylene is carried out in at least one reaction zone (R) under high pressure, at 150.degree.-320.degree. C.; (II) the reaction product is routed into a zone (S), under an intermediate pressure P, for separation of the (co)polymer, in the molten state, from the unreacted (co)monomer(s), or the reaction product is routed into a first separation zone (S1), under an intermediate pressure (P1); the (co)polymer issuing from (S1) is then routed into a second separation zone (S2), under a pressure P2<P1; (III) the (co)polymer is routed into a zone (SF), under low pressure, for final separation of the (co)polymer from the residual gases. Let-down zones (D), (D1) and (D2) are provided along the path of the (co)polymer between the zones (S) and (SF), (S1) and (S2), and (S2) and (SF) respectively, the grafting monomer or monomers being introduced on-line, in the path of the (co)polymer, at at least one point located at the level of the zone (D) or at at least one zone (D1, D2).

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.

Abstract: (I) The (co)polymerisation of ethylene is carried out in at least one reaction zone (R) under high pressure, at 150.degree.-320.degree. C.; (II) the reaction product is routed into a zone (S), under an intermediate pressure P, for separation of the (co)polymer, in the molten state, from the unreacted (co)monomer(s), or the reaction product is routed into a first separation zone (S1), under an intermediate pressure (P1); the (co)polymer issuing from (S1) is then routed into a second separation zone (S2), under a pressure P2<P1; (III) the (co)polymer is routed into a zone (SF), under low pressure, for final separation of the (co)polymer from the residual gases. Let-down zones (D), (D1) and (D2) are provided along the path of the (co)polymer between the zones (S) and (SF), (S1) and (S2), and (S2) and (SF) respectively, the grafting monomer or monomers being introduced on-line, in the path of the (co)polymer, at at least one point located at the level of the zone (D) or at at least one zone (D1, D2).

Abstract: The compositions contain a) less than 90% and at least 2% by weight of at least one ethylene polymer (A), and optionally, at least one .alpha.-olefin having from 3 to 12 carbon atoms, said polymer containing at least 70% in moles of patterns derived from ethylene, and b) more than 10% and up to 98% by weight of at least one polymer (B) containing at least 85% in moles of patterns derived from at least one .alpha.-olefin having from 3 to 12 carbon atoms and at the most 15% in moles of patterns derived from ethylene, and are characterized in that the polymer (A) is a modified polymer of which the measured limit viscosity is comprised between 1.3 and 100 times its limit viscosity calculated from the distribution of molecular masses. The compositions are obtained by a process consisting in providing a polymer of ethylene and, optionally, of at least one .alpha.-olefin having from 3 to 12 carbon atoms in the presence, on the one hand, of at least one initiator of free radicals in a quantity comprised between 0.