Abstract: A process for polymerizing ethylene feedstock to produce branched polymer of ethylene suitable for blow molding applications and having improved environmental stress crack resistance (ESCR). For example, a catalyst comprising chromium on silica is (1) calcined in air and (2) reduced in carbon monoxide and then used with hydrogen to polymerize ethylene feedstock.

Abstract: A process for producing polyethylene having improved crack resistance, the process comprising copolymerizing ethylene and an alpha-olefinic comonomer comprising from 3 to 8 carbon atoms, in the presence of a chromium-based catalyst in a first reactor, feeding the polyethylene copolymer thereby produced and the chromium-based catalyst to a second reactor, and in the second reactor copolymerizing ethylene and an alpha-olefinic comonomer comprising from 3 to 8 carbon atoms, in the presence of the chromium-based catalyst thereby to produce a second polyethylene product, the first and second polyethylene products forming a combined polyethylene having a monomodal molecular weight distribution.

Abstract: A process for producing medium density polyethylene by copolymerizing ethylene and an alpha-olefinic comonomer comprising from 3 to 10 carbon atoms in the presence of a chromium-based catalyst having a support comprising silica and titania, the catalyst being fluorinated and having been chemically reduced by carbon monoxide, and in the presence of a co-catalyst selected from at least one of an aluminum alkyl and a zinc alkyl to produce a polyethylene copolymer having a density of from 0.930 to 0.945 g/cc and a dispersion index (D) of from 9 to 13.

Abstract: A process for producing polyethylene in a two step polymerisation process that comprises the steps of: (a) homopolymerising ethylene in the presence of a first catalyst comprising an activated chromium catalyst in a first reactor to produce a first polyethylene fraction; (b) transferring the first polyethylene fraction produced in the first reactor and at least a portion of the first catalyst to a second reactor; (c) in the second reactor homopolymerising ethylene or copolymerising ethylene and an alpha-olefinic comonomer having from 3 to 10 carbon atoms in the presence of a second catalyst under conditions which suppress residual activity in the first catalyst to produce a second polyethylene fraction; and (d) retrieving from the second reactor polyethylene comprising a blend of the first and second polyethylene fractions.

Abstract: A process of producing polyethylene, the process comprising copolymerizing ethylene and an alpha-olefinic comonomer comprising from 3 to 8 carbon atoms in the presence of a chromium-based catalyst in a main polymerization reactor and, in a gas-phase preliminary reactor upstream of the main polymerization reactor, chemically treating the chromium-based catalyst with at least one treatment agent prior to introduction of the catalyst into the main polymerization reactor and releasing from the preliminary reactor waste gases produced during the chemical treatment.

Abstract: A process to produce ethylene polymers is provided.

Abstract: An ethylene homopolymer having a density of from about 0.956 to about 0.970 g/cm3 and a high load melt index (190° C. and 21.6 kg) of from about 7 to about 21 g/10 min. A continuous polymerization process for the production of the ethylene homopolymer, in the presence of a diluent chosen from the group consisting essentially of n-butane, n-hexane, n-heptane, isobutane, isopentane, isooctane, 2,2-dimethylpropane, cyclopentane, cyclohexane and their mixtures, wherein ethylene is brought into contact with a chromium oxide catalyst deposited on an inorganic support containing from about 0.1 to about 5 wt % chromium and activated at a temperature from about 600 to about 900° C., at a polymerization temperature from about 55 to about 150° C., the ethylene concentration in the diluent being from about 1 to about 10 mol %. A composition consisting essentially of the ethylene homopolymer, the composition having a density of from about 0.956 to about 0.

Abstract: A process for preparing a supported chromium-based catalyst for the production of high density polyethylene, by polymerising ethylene, or copolymerising ethylene and an alpha-olefinic comonomer comprising 3 to 10 carbon atoms, which comprises the steps of: a) providing an alumina-containing support; b) depositing a chromium compound on the support to form a chromium-based catalyst; c) dehydrating the chromium-based catalyst to remove physically adsorbed water by heating the catalyst at a temperature of at least 300° C. in an atmosphere of dry, inert gas; d) titanating the chromium-based catalyst at a temperature of at least 300° C.

Abstract: Polymerization processes for ethylene and at least one mono-1-olefin comonomer having from about three to eight carbon atoms per molecule in the presence of a twice-aged catalyst system comprising chromium supported on a silica-titania support and a trialkylboron compound is provided. Novel ethylene copolymers also are produced.

Abstract: The present invention relates to a supported chromium-based catalyst titanated under specific conditions and used for the homopolymerization or the copolymerization of ethylene. The polyethylene obtained with this catalyst has high shear resirance and environmental stress crack resistance, and can be used for manufacturing films with improved tear properties.

Abstract: A process comprising polymerizing ethylene, or polymerizing ethylene and at least one other olefin, to produce a polymer is provided.

Abstract: A process for producing polyethylene having impact resistance, the process comprising polymerizing ethylene, or copolymerizing ethylene and an alpha-olefinic comonomer comprising from 3 to 10 carbon atoms, in the presence of a chemically reduced chromium-based catalyst containing in a support thereof from 2 to 3 wt % of titanium, based on the weight of the catalyst. The invention also provides a chromium-based catalyst for the production of polyethylene by polymerizing ethylene or copolymerizing ethylene and an alpha-olefinic comonomer comprising from 3 to 10 carbon atoms, the catalyst being chemically reduced and containing in a support from 2 to 3 wt % of titanium, based on the weight of the catalyst.

Abstract: Polymerization processes for ethylene and at least one mono-1-olefin comonomer having from about three to eight carbon atoms per molecule in the presence of a catalyst system comprising chromium supported on a silica-titania support and a trialkylboron compound is provided. Novel ethylene copolymers also are produced.

Abstract: An olefin polymerization process in which at least two introductions of hydrogen are made during the olefin polymerization reaction. Suitable catalysts include metallocenes of the general formula (Cp).sub.m TiX.sub.n, wherein Cp is a substituted or unsubstituted cyclopentadienyl ring, X is a halogen, m=1-2, n=2-3, and wherein m+n=4, and conventional Ziegler-Natta catalysts blended with or modified by such metallocenes.

Abstract: A process for producing high density polyethylene suitable for blow molding and having high environmental stress cracking resistance and rigidity, comprising copolymerizing ethylene and a C.sub.3 -C.sub.10 alpha-olefinic comonomer in the presence of a chromium-based catalyst having an alumina-containing support, and an organoboron compound, wherein the atomic ratio of boron in the organoboron compound to the chromium in the chromium-based catalyst is from around 0.2 to around 0.4.

Abstract: Catalyst systems which are suitable for polymerizing ethylene and 1-alkenes comprise as active constituentsA) a chromium-containing component obtainable by reducing an oxidized Phillips catalyst with organic compounds selected from the group of linear C.sub.4 -C.sub.10 - or cyclic C.sub.3 -C.sub.10 -alkenes with at least one internal C.dbd.C double bond, of 1,3-butadiene, of C.sub.2 -C.sub.10 -alkynes and of C.sub.1 -C.sub.10 -aldehydes,with or withoutB) an organometallic compound of the first, second, third or fourth main group or of the second subgroup of the Periodic Table of the Elements.

Abstract: A blow-moldable or extrudable ethylene polymer having an excellent moldability which gives a molded product free of roughened surface, having: a number-average molecular weight Mn (V1) as determined from the amount of a terminal vinyl group by .sup.13 C-NMR measurement with respect to a fraction having a number-average molecular weight Mn of 100,000 according to GPC; and a number-average molecular weight Mn (V2) as determined from the amount of a terminal vinyl group by .sup.13 C-NMR measurement with respect to a fraction having a number-average molecular weight Mn of 10,000 according to GPC, Mn(V1) and Mn(V2) satisfying the following relationship:1.01.ltoreq.log.sub.10 {Mn(V1)/Mn(V2)}.ltoreq.3.0and having a melt flow rate (HLMFR) of from 0.1 to 70 g/10 min.

Abstract: An amorphous aluminophosphate composition is provided which exhibits a microstructure of sheets and spheres of silica-modified aluminophosphate. Also provided is a method for preparing the composition comprising mixing an aqueous solution containing aluminum ions, phosphate ions and silica ions, with a neutralizing solution, wherein the mixing is conducted with sufficient shear to produce on a microlevel, sheets of silica-modified aluminophosphate and spheres of silica-modified aluminophosphate. A polymerization catalyst and a polymerization process are also provided.

Abstract: Process for the preparation of polyethylene by the homopolymerization of ethylene or by the copolymerization of ethylene and of at least one alpha-olefinic comonomer containing from 3 to 10 carbon atoms, in two reactors for loop polymerization in liquid phase which are connected in series in the presence of a chromium catalyst, in which ethylene and the chromium catalyst are first introduced into the first reactor at a temperature of 95 to 110.degree. C. to produce an ethylene homopolymer of low molecular weight of a melt index MI.sub.2 of at least 1 g/10 minutes and in which the ethylene homopolymer thus obtained is next transferred into the second reactor to which ethylene and optionally the alpha-olefinic comonomer is added, at a temperature of 80 to 90.degree. C. to obtain an ethylene copolymer of high molecular weight.

Abstract: An ethylene base polymer and a preparation method of an ethylene base polymer characterized in use of a catalyst comprising a chromium amide compound, a solid inroganic oxide calcined at 100 to 500.degree. C. and alumoxane. According to the present invention, an ethylene base polymer with a wide molecular weight distribution, excellent shock and environmental stress cracking resistances, causing no melt fracture, and suitable for film forming and blow molding can be effectively prepared.

Abstract: A process for producing high density polyethylene, the process comprising polymerising ethylene, or copolymerising ethylene and an alpha-olefinic comonomer comprising from 3 to 10 carbon atoms, in the presence of a catalyst system comprising first and second chromium-based catalysts, the first chromium-based catalyst having been reduced and reoxidised and the second chromium-based catalysts having been activated, fluordised before or during the activation step, and reduced, the first and second chromium-based catalysts having a pore volume difference of at least O.8 cc/g.

Abstract: A polymerization process is provided comprising contacting under polymerization conditions a catalyst, a taming agent, ethylene and optionally a comonomer, wherein the catalyst comprises chromium oxide, titanium oxide and an inorganic refractory oxide, wherein the taming agent is selected from the group consisting of water, alcohols, aldehydes, ketones, esters, and organic acids and mixtures thereof. The process is especially suited for polymerizing ethylene resins suitable for blow molding use.

Abstract: Novel catalysts are provided for the polymerization of olefins such as ethylene. These catalysts comprise a novel amorphous aluminophosphate comprising, on a microlevel, sheets of aluminophosphate as well as spheres of aluminophosphate plus a chromium compound. These new catalysts are surprisingly active; result in more co-monomer incorporation than prior art catalysts and reduce the amount of low molecular weight material in the product.

Abstract: Catalyst compositions are provided that comprise (a) discrete silica particles; (b) chromium; and (c) a support comprising silica. These composition are useful in polymerizing ethylene or copolymerizing ethylene with a comonomer and provide better control over the molecular weight and the molecular weight distribution of a resulting polymer.

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: A continuous process for the polymerization of one or more alpha-olefins in the gas phase in the presence of a chromium based catalyst, under polymerization conditions including the reaction temperature, the pressure, and the target oxygen to alpha-olefin molar ratio operating point required for desired resin properties.

Abstract: Process for the production of a microspheroidal powder of low dispersity, according to which a hydrogel is prepared, an aqueous suspension of the ground hydrogel is subjected to atomisation in a stream of gas, gelled particles containing moisture are collected from the atomisation, the particles are brought into contact with an organic liquid in order to remove at least some of the moisture and the particles are then subjected to drying. The process applies to the production of silica powders intended to serve as support for chromium oxide catalysts which may contain titanium oxide and are suitable for the polymerisation of ethylene.

Abstract: Chromium catalyst compositions are provided. These chromium catalyst compositions comprise at least two chromium catalyst systems that have a certain amount of chromium and a support that comprises silica and titania. the supports have certain pore volumes and surface areas. These catalyst compositions are useful for homopolymerizing ethylene or copolymerizing ethylene with a comonomer. The resins produced are useful in film applications.

Abstract: A process is provided to produce polyolefins having a multimodal molecular weight distribution at the molecular level by contacting at least one mono-1-olefin in a polymerization zone, under polymerization conditions, with two different types of supported chromium catalyst systems. The first catalyst system comprises a chromium oxide catalyst system which comprises chromium oxide supported on an inorganic oxide. The second catalyst system comprises a chromocene catalyst system which comprises chromocene supported on an inorganic oxide. The resultant, recovered polymer has a multimodal, or at least a bimodal, molecular weight distribution.

Abstract: A process is provided comprising copolymerizing, under copolymerizing conditions, ethylene with one or more alpha-olefins, using a catalyst system, where said catalyst system has been sized, to produce a lower-density-copolymer, where said lower-density-copolymer has a density lower than a similar copolymer produced by copolymerizing, under said copolymerizing conditions, ethylene with said one or more alpha-olefins, using said catalyst system, where said catalyst system has not been sized, where said catalyst system that has been sized has had substantially all particles removed that produce, under said copolymerizing conditions, a higher-density-copolymer, where said higher-density-copolymer has a density higher than said similar copolymer.

Abstract: A process for the preparation of a supported catalyst for the polymerization of .alpha.-olefins, in which(1) a silicon dioxide-containing support gel is prepared by(1.1) introducing a sodium water glass or potassium water glass solution into a swirling stream of an aqueous mineral acid longitudinally and tangentially to the stream, spraying the resultant silica hydrosol in drop form into a gaseous medium and allowing it to solidify to form a hydrogel, and freeing the resultant hydrogel from salts by washing without prior ageing,(1.2) drying the hydrogel resulting from (1.1) to form the support gel,(2) the support gel (1) is charged with chromium trioxide or a chromium compound which can be converted into chromium trioxide under the conditions of process step (3), giving a chromium-containing support gel, and(3) the chromium-containing support gel (2) is heated at from 400.degree. to 1100.degree. C.

Abstract: A process for the polymerization of at least one olefin in the presence of an activated catalyst, includes (a) providing an activated catalyst by a method including (1) mixing, in the absence of a solvent, at least one chromium salt with a support composition comprised of at least one compound (A) which is an inorganic, oxygen containing compound of at least one element selected from the group consisting of Group IVb, IIIa and IVa, and at least one compound (B) which is an inorganic compound containing at least one element selected from the group consisting of Group IVb and IIIa, the at least one compound (B) being different from the at least one compound (A), to provide a mixture; (2) preactivating the mixture by heating for a period ranging from 0.5 to 18 hours without calcining in an oxidizing atmosphere to a temperature ranging from at least 30.degree. C. above room temperature to a temperature which is lower than the decomposition temperature of the at least one chromium salt and which is 5.degree. C.

Abstract: This invention provides a chromium catalyst system that comprises[a] a support that comprises silica, wherein said support has a surface area to pore volume relationship as follows0.5.ltoreq.Pore volume.ltoreq.f(SA)wherein said f(SA) is(1.14.times.10.sup.-7)(SA).sup.3 -(1.19.times.10.sup.-4)(SA).sup.2 +(3.19.times.10.sup.-2)(SA)+0.20;and[b] a hexavalent chromium compound;wherein the surface concentration of said hexavalent chromium on said support is from 0.25 to 1 hexavalent chromium atoms per square nanometer.Another embodiment of this invention provides a process to homopolymerize ethylene, or copolymerize ethylene with a comonomer, said process comprises polymerizing ethylene with the above chromium catalyst composition.

Abstract: Novel homopolymer and copolymer compositions produced from the polymerization of non-conjugated dienes have been discovered. The novel compositions are produced by an equally novel process comprising cyclopolymerization of non-conjugated dienes employing reduced valence state Group VIB metal oxide catalyst on porous support. The preferred catalyst comprises carbon monoxide reduced chromium on silica. Copolymers are formed by copolymerization of non-conjugated dienes with 1-alkenes in contact with the activated chromium on silica catalyst. The compositions are useful as lubricants and additives.

Abstract: This invention provides a chromium catalyst system that comprises[a] a support that comprises silica, wherein said support has a surface area to pore volume relationship as follows0.5.ltoreq.Pore volume.ltoreq.f(SA)wherein said f(SA) is(1.14.times.10.sup.-7)(SA).sup.3 -(1.19.times.10.sup.-4)(SA).sup.2 +(3.19.times.10.sup.-2)(SA)+0.20;and[b] a hexavalent chromium compound;wherein the surface concentration of said hexavalent chromium on said support is from 0.25 to 1 hexavalent chromium atoms per square nanometer.Another embodiment of this invention provides a process to homopolymerize ethylene, or copolymerize ethylene with a comonomer, said process comprises polymerizing ethylene with the above chromium catalyst composition.

Abstract: A process is provided to produce polyolefins having a multimodal molecular weight distribution at the molecular level by contacting at least one mono-1-olefin in a polymerization zone, under polymerization conditions, with dual site supported chromium polymerization catalyst system. Such a polymerization catalyst system comprises an inorganic oxide supported chromium oxide catalyst system and a chromocene compound, wherein the supported catalyst system and chromocene compound are contacted in a polymerization reactor. The resultant, recovered polymer has at least a bimodal, or broad, molecular weight distribution.

Abstract: Disclosed herein are a novel process for the preparation of alkylsilanes and alkenyl silanes by hydrosilylation of their corresponding silanes and unsaturated hydrocarbons in the presence or absence of a catalyst, a process for the preparation of polyalkenylsilanes (some of which are novel high molecular compounds) by the anionic coordination polymerization, radical polymerization or ionic polymerization of the alkenylsilanes, and a process for the production of silicon carbide by using these polyalkenylsilanes as prepolymers.

Abstract: Chromium catalyst compositions are provided. Theses chromium catalyst compositions can be used to polymerized olefins. The resulting polymerization product can have improved properties.

Abstract: A process for the polymerization of at least one olefin is a suspension process employing a hydrocarbon diluent such that at least 70% of the polymer formed is insoluble therein, and employs at least two polymerization reactors. The process includes introducing a first portion of the at least one olefin and a catalyst into a first reactor. The catalyst is composed of bis(cyclopentadienyl)chromium, which may be substituted, supported on an inorganic oxide support and is supplied solely into the first reactor. The at least one olefin is polymerized in the first reactor to provide a first composition composed of a polymer and the catalyst. The first composition is then drawn off from the first reactor and the first composition and another portion of the at least one olefin are introduced into a second reactor and the another portion of the at least one olefin is polymerized in the second reactor in the presence of the first composition.

Abstract: Disclosed is a process for making polyethylene resin by reacting ethylene or ethylene and a C.sub.3 -C.sub.8 alpha-olefin in the presence of a CrO.sub.3 /TiO.sub.2 catalyst on a silica support in a gas-phase polymerization reactor and in the presence of a catalyst productivity reduction agent such as oxygen. The resin in one embodiment has a melt flow ratio between 65 and 140 and a melt index between 0.1 and 0.6 g/10 min. The resin is particularly suitable for use in blow-molding applications.

Abstract: A process for the preparation of a supported catalyst for the polymerization of .alpha.-olefins, in which(1) a silicon dioxide-containing support gel is prepared by(1.1) introducing a sodium water glass or potassium water glass solution into a swirling stream of an aqueous mineral acid longitudinally and tangentially to the stream, spraying the resultant silica hydrosol in drop form into a gaseous medium and allowing it to solidify to form a hydrogel, and freeing the resultant hydrogel from salts by washing without prior ageing,(1.2) drying the hydrogel resulting from (1.1) to form the support gel,(2) the support gel (1) is charged with chromium trioxide or a chromium compound which can be converted into chromium trioxide under the conditions of process step (3), giving a chromium-containing support gel, and(3) the chromium-containing support gel (2) is heated at from 400.degree. to 1100.degree. C.

Abstract: An improved supported chromium oxide catalyst, a process for preparing the catalyst and a process for polymerizing olefin monomers utilizing the catalyst are provided. In accordance with the preparation process, chromium oxide and a support containing one or more of silica, titania, thoria, alumina, zirconia or aluminophosphates are combined, impregnated with a metal salt and activated. The metal salt is selected from the group consisting of alkali metal salts, alkaline earth metal salts and mixtures thereof.

Abstract: Microspherical cogel particles comprising silica and at least one other metal oxide are prepared by forming a slurry of porous particulate material comprising silica and a liquid medium comprising at least 90%, by weight of the liquid medium, of an organic liquid and subjecting the slurry to spray drying. The method enables agglomeration of particulate material of especially small particle size to obtain microspherical particles of a size especially suitable for us as a catalyst support. The particles have a pore size of at least 2 cc/g and, especially when the particles contain titanium atoms, are especially suitable for use in the preparation of a catalyst for the polymerization of olefins.

Abstract: Olefins can be polymerized in the presence of a catalyst system composition comprising chromium oxides supported on a inorganic oxide support, a pyrrole-containing compound and a non-hydrolyzed metal alkyl. The use of this type of polymerization catalyst in a polymerization process can produce an olefin comonomer in-situ, resulting in polymers having decreased density and increased branching.

Abstract: A process to produce a polyethylene composition that has less than 2 weight percent fines, based on the weight of said polyethylene composition, and a fluff bulk density greater than 25 pounds per cubic foot, said process comprising polymerizing ethylene in the presence of a trialkylaluminum compound, and a diluent that is essentially saturated with ethylene under slurry polymerization conditions that comprise a polymerization temperature from 104.degree. C. to 116.degree. C., with an activated catalyst system comprising (a) a catalyst support that comprises a silica compound, wherein said catalyst support has a pore volume less than 2 cubic centimeters per gram, (b) a catalyst that comprises a chromium compound, wherein the amount of chromium in said catalyst system is from about 0.1 to about 20 weight percent, wherein said weight percent is based on the total weight of said catalyst and catalyst support, and (c) a fluorine compound, wherein the amount of fluorine used is from about 0.

Abstract: Olefins can be polymerized in the presence of a catalyst system composition comprising chromium supported on an inorganic oxide support, a pyrrole-containing compounds, and a metal alkyl. The use of this type of polymerization process can produce an olefin comonomer in-situ, resulting in polymers having decreased density and increased branching.

Abstract: In a novel process for the preparation of a supported catalyst for the polymerization of .alpha.-olefins, a carrier gel (1) is first prepared by a method in which a sodium or potassium waterglass solution is introduced into a stream of an aqueous mineral acid which is subjected to angular momentum and may contain aluminum ions, longitudinally and tangentially to the stream, the resulting silica hydrosol sprayed in the form of drops into a gaseous medium and allowed to solidify into a hydrogel, and the resulting hydrogel is freed from salts by washing without prior aging and, if necessary, freed from some of the water by extraction with alcohols and/or ketones and is then dried. This gives the carrier gel (1), which is milled and is fractionated according to particle size. In a second process step, the carrier gel (1) is laden with chromium trioxide or with a chromium compound which can be converted into chromium trioxide, after which the resulting chromium-containing carrier gel (2) is heated at from 400.

Abstract: The present invention relates to a continuous process for the polymerization of an alpha-olefin having from 2 to 12 carbon atoms, which is carried out in a gas phase polymerization reactor by bringing a gaseous reaction mixture containing the alpha-olefin to be polymerized into contact with a catalyst based on chromium oxide associated with a catalyst based on chromium oxide associated with a granular support and activated by a heat treatment in which the polymerization reactor is fed with (a) alpha-olefin and (b) catalyst at constant rates.

Abstract: The present invention relates to a process for the polymerization of ethylene or copolymerization of ethylene and at least one other alpha-olefin, in the gas phase in a fluidized bed reactor, in the presence of a catalyst comprising chromium compounds associated with a granular support comprising a refractory oxide and activated by thermal treatment, which process is characterized in that the catalyst is used in the form of active particles of prepolymer, obtained by bringing the said catalyst in contact with ethylene, alone, or in a mixture with at least one higher alpha-olefin, in such a way that the prepolymer contains from 4.times.10.sup.-5 to 3 milligram atoms of chromium in the catalyst per gram.

Abstract: The present invention relates to a process for the polymerisation of ethylene or copolymerisation of ethylene and at least one other alpha-olefin, in the gas phase in a fluidised bed reactor, in the presence of a catalyst comprising chromium compounds associated with a granular support comprising a refractory oxide and activated by thermal treatment, which process is characterised in that the catalyst is used in the form of active particles of prepolymer, obtained by bringing the said catalyst in contact with ethylene, alone, or in a mixture with at least one higher alpha-olefin, in such a way that the prepolymer contains from 4.times.10.sup.-5 to 3 milligram atoms of chromium in the catalyst per gram.