Catalyst system, use of this catalyst system for the (co)polymerization of olefins, process for preparing this catalyst system and olefin (co) polymerization process

A process for preparing a catalyst system comprising at least one aluminium derivative of formula AlX.sub.n T.sub.3-n, in which X denotes a halogen, T denotes a hydrocarbon radical which may optionally include oxygen and n is a number from 0 to 3, at least one ionizing agent selected from the group consisting of tris(pentafluorophenyl)boron, triphenylboron, trimethylboron, tris(trimethylsilyl) borate, and organoboroxines or an ionizing agent consisting of a first part which exhibits the properties of a Lewis acid selected from the group consisting of carbenium, sulfonium, and oxonium and capable of ionizing a neutral halogenated metallocene, and a second part which is capable of stabilizing said ionic metallocene and which is inert toward the ionized metallocene, and at least one halogenated neutral metallocene derived from a transition metal, comprising:

• in a first stage, preparing a mixture of the halogenated neutral metallocene with a catalytic solid containing 15 to 20% by weight of titanium, 0.5 to 20% by weight of magnesium and 30 to 40% by weight of chlorine and,
• in a second stage, first adding to said mixture an aluminium derivative in at least one hydrocarbon diluent and then adding said ionizing agent to said mixture to form said catalyst system.

A precursor of a catalyst system comprising (a) at least one neutral halogenated metallocene derived from a transition metal, (b) at least one catalytic solid containing 15 to 20% by weight of titanium, 0.5 to 20% by weight of magnesium and 30 to 40% by weight of chlorine and (c) at least one aluminium derivative of formula AlX.sub.n T.sub.3-n in which X denotes a halogen, T denotes a hydrocarbon radical which may optionally include oxygen, and n is a number from 0 to 3.