Abstract: Support for catalysts, containing at least two constituents chosen from silica, alumina and aluminium phosphate, having a specific surface of 100 to 800 m.sup.2 /g, a crystallization temperature greater than or equal to 700.degree. C. and a pore volume of 1.5 to 4 cm.sup.3 /g, the specific surface (SS) and the pore volume (PV) corresponding to the relationship:SS<(PV.times.564-358).Process for the manufacture of such a support, according to which an alcohol, water, a silicon alkoxide and an acid are mixed under conditions such that gelling or precipitation of silica is prevented, an acidic solution of an aluminium compound and/or a solution of a source of phosphate ions are added thereto, a gelling agent is added thereto, a gel is recovered which is washed with water and then by means of an organic liquid, the gel is then dried until a powder is obtained, and the powder is calcined. Polymerization of olefins in the presence of a catalyst containing chromium on a support as described above.

Abstract: Cocatalytic composition resulting from bringing an organoaluminium halide (A) into contact with an electron-donating organic compound (ED) selected from esters, amides and ketones, the organoaluminium halide (A) possessing an atomic ratio halogen (X)/aluminium (Al) of greater than 1 and less than 1.3, the halide (A) and the compound (ED) being employed in a mole ratio halide (A)/compound (ED) of greater than 20. Catalytic systems which are usable for the polymerization of alpha-olefins, comprising such a cocatalytic composition and a solid based on titanium trichloride complexed with an electron-donating compound.

Abstract: (Co)polymerisation process according to which a mixture of a halogenated neutral metallocene as defined above and of an organoaluminium compound is prepared, the olefin is brought into contact with this mixture and an ionising agent is added thereto.

Abstract: Process for the polymerization of alpha-olefins, which is carried out in the presence of a catalyst system comprising:(1) a solid based on completed titanium trichloride;(2) an organometallic compound of metals of groups IA, IIA, IIB and IIIB of the Periodic Table, and(3) an electron-donor organic compound, the solid (1) based on complexed titanium trichloride having been preactivated by being brought into contact with a preactivator comprising the product of reaction of a trialkylaluminum or alkylaluminum chloride compound (a) and of a compound (b) chosen from hydroxyaromatic compounds whose hydroxyl group is sterically hindered, then isolating the predetermined solid (1) from the medium in which it was found.This process enables propylene to be polymerized with an improved stereospecificity without any significant decrease in catalyst activity.

Abstract: Solid precursor of a catalytic system for the polymerization of olefins, containing at least one neutral halogenated metallocene derived from a transition metal, the latter being linked to at least one halogen atom, and at least one ionizing agent.Process for the preparation of this solid precursor, according to which a compound based on the neutral halogenated metallocene is mixed with a compound based on the ionizing agent in a heterogeneous medium. Catalytic system comprising an organometallic compound derived from a metal chosen from groups IA, IIA, IIB, IIIA and IVA of the Periodic Table, and a solid precursor described above. Process for the polymerization of olefins, in which this catalytic system is used.

Abstract: Catalyst support containing an .alpha.-olefin polymer which is in the form of particles of mean size from 5 to 350 .mu.m in which the pore volume generated by the pores of radius from 1,000 to 75,000 .ANG. is at least 0.2 cm.sup.3 /g.Catalyst usable for the polymerization of .alpha.-olefins, including a compound containing at least one transition metal belonging to groups IIIb, IVb, IVb and VIb of the Periodic Table, bound in or on this support.

Abstract: Propylene block copolymer containing per:100 parts by weight of a polymer (a) of propylene chosen from homopolymers and copolymers of propylene not containing more than 6% by weight of ethylene and/or of an alpha-olefin having from 4 to 6 carbon atoms,from 1 to 100 parts by weight of a polymer (b) of ethylene chosen from homopolymers and copolymers of ethylene not containing more than 90% by weight of propylene and/or of another alpha-olefin having from 4 to 6 carbon atoms,the said block copolymer additionally containing from approximately 0.001 to approximately 20% by weight of .alpha.,.omega.-diene-derived monomer units with respect to the total weight of the block copolymer.