Abstract: A semi-continuous process and system thereof, for the synthesis of a narrow particle size distribution Zeigler Natta procatalyst for use in the manufacture of polyolefins. The process comprises: (a) mixing a reaction mixture containing a titanium compound; (b) charging a first reactor with said reaction mixture; (c) removing excess reactants from said first reactor as a filtrate; (d) feeding said filtrate to at least one further reactor; and continuously removing excess reactants from said at least further reactor.

Abstract: The present invention relates to a process of producing an ethylene polymer composition in multiple stages, of which the first stage is preferably a slurry polymerization stage, in the presence of a catalyst system, comprising: a) a solid catalyst precursor, comprising a transition metal selected from titanium and vanadium; magnesium; a halide, optionally an electron donor; and a solid particulate material comprising an inorganic oxide, and wherein the median particle diameter, D50, of the solid catalyst precursor, based upon the total volume of solid catalyst precursor, is from 1 to 13 micrometers; and b) an organoaluminium compound.

Abstract: The present invention relates to a process for preparing a bimodal polyethylene product in a single loop reactor, comprising polymerizing ethylene monomer and optionally one or more olefin co-monomers in the presence of a single heterogeneous polymerization catalyst consisting of a metallocene-alumoxane catalyst immobilized on a porous support wherein said metallocene comprises only one transition metal. Said polymerization catalyst consists of two physically different fractions of support particles onto which said metallocene-alumoxane catalyst is immobilized.

Abstract: The invention relates to a process for the preparation of a particulate bimodal polyethylene product having a median particle diameter of less than 300 ?m in a serially connected double loop reactor, wherein said polymerization catalyst applied in the polymerization process comprises a particulate metallocene-alumoxane catalyst immobilized on a porous silica support, wherein said polymerization catalyst has a median particle diameter of less than 50 ?m; and whereby the ratio of the median particle diameter of the obtained bimodal polyethylene product to the median particle diameter of the applied polymerization catalyst is less than 30.

Abstract: A method for producing a polyethylene or polypropylene polymer, or co-polymer thereof, comprises contacting a monomer-containing medium with polymerization catalyst particles in at least one high shear mixing device to form a nanodispersion, wherein the particles have a mean diameter less than 1 micron. The monomer is selected from the group consisting of ethylene, propylene, and combinations thereof. The method further includes subjecting the nanodispersion to polymerization conditions comprising pressure in the range of about 203 kPa to about 6080 kPa (about 2 atm to about 60 atm) and temperature in the range of about 20° C. to about 230° C., whereby at least a portion of the monomer is polymerized. A system for carrying out the method is also disclosed.

Abstract: A fuel reforming apparatus includes a reforming catalyst, a filtering member, a raw material supply flow passage and a processed gas flow passage. The filtering member has a plurality of cells. A reforming catalyst is carried on a surface of a partition on the side of the processed gas flow passage. If raw gas including hydrocarbon fuel is supplied to the fuel reforming apparatus and filtered by the filtering member, soot included in the raw gas is trapped by gaps in the partition, and the hydrocarbon fuel is reformed into reformed gas including hydrogen and carbon monoxide on the reforming catalyst. By increasing the amount of air supplied from a blower at intervals of time, the soot trapped by the partition is removed by combustion.

Abstract: The present invention relates to a method for preparing neodymium-carbon-nanotube and a process for preparing 1,4-cis-polybutadiene utilizing the same and more particularly, neodymium-carbon-nanotube. Neodymium is introduced by coordination with carboxylic acid, which is formed on the surface of carbon nanotube. In 1,3-butadiene polymerization with the neodymium-catalyst comprising the neodymium-carbon nanotube, a particular halogen compound, and a particular organometallic compound in an appropriate ratio, high 1,4-cis polybutadiene having molecular weight of 10,000 to 2,000,000 is produced, which exhibits excellent mechanical properties such as elasticity and durability.

Abstract: A propylene block copolymer with a high proportion of ethylene-propylene copolymer particles (rubber component) well dispersed in a propylene polymer exhibiting well-balanced rigidity and impact resistance can be obtained by using a solid catalyst for polymerization of olefins comprising (a) a solid catalyst component with controlled morphology, comprising magnesium, titanium, and a halogen atom, having an average particle diameter, specific surface area, and pore volume in a specific range, and having a pore size distribution in which an cumulative pore volume with a pore size of 100 ? or less is more than 50%, (b) an organoaluminum compound, and (c) an organosilicon compound. The block copolymer is very useful particularly for the application of vehicle parts such as a bumper and parts for household electric appliances.

Abstract: Olefin polymers produced using components of catalysts obtained by contacting a Ti compound of formula Ti(OR)n-yXy, where n is the valence of titanium and y is a number between 1 and n, with a pre-polymer having a porosity higher than 0.3 cc/g and containing from 0.5 to 100 g of polymer per g of solid catalyst component, the pre-polymer being obtained by (co)polymerizing an olefin or a diolefin in the presence of a catalyst comprising a Ti, V, Zr or Hf compound supported on a Mg dihalide having a mean crystallite dimension lower than 30 nm.

Abstract: There is provided a process for producing an olefin polymer, which comprises a step of polymerizing an olefin in a gas phase in the presence of a homogeneous solid catalyst having a content of particles having a particle size of not more than 180 &mgr;m of not more than 15% by weight based on 100% by weight of the total weight of the homogeneous solid catalyst.

Abstract: A novel loop/slurry olefin polymerization process is provided which produces ultra-high molecular weight ethylene homopolymer.

Abstract: A prepolymerization catalyst for use in a gas phase polymerization of an olefin or combinations of olefins which comprises (A) a solid catalyst component comprising magnesium, halogen, titanium and an electron donor and having a weight-average particle diameter of 15 to 45 &mgr;m, (B) at least one organoaluminum compound and (C) a prepolymer of an ethylene and/or at least one &agr;-olefin, wherein the molar ratio of aluminum to titanium in the prepolymerization catalyst (Al/Ti ratio) is 3 to 11 (mol/mol), the weight ratio of the prepolymerization catalyst to the solid catalyst component (prepolymerization catalyst/solid catalyst component) is 2 to 35 (g/g), the content of volatile materials (VM) in the prepolymerization catalyst is 2.0% by weight or less, and the intrinsic viscosity [&eegr;] measured in tetralin at 135° C. is 2.0 dl/g or less, and a process for a production thereof.

Abstract: Described is the use of nanoscale metal oxide particles as catalysts for the thermal and/or photochemical polymerization of species having at least one polymerizable carbon-carbon multiple bond and/or at least one carbon containing ring capable of undergoing a ring opening polymerization.

Abstract: A novel loop/slurry olefin polymerization process is provided which produces ultra-high molecular weight ethylene homopolymer.

Abstract: Components of catalysts for the polymerization of olefins are obtained by contacting a Ti compound of formula Ti(OR)n−yXy, where n is the valence of titanium and y is a number between 1 and n, with a pre-polymer having a porosity higher than 0.3 cc/g and containing from 0.5 to 100 g of polymer per g of solid catalyst component, the pre-polymer being obtained by (co)polymerizing an olefin or a diolefin in the presence of a catalyst comprising a Ti, V, Zr of Hf compound supported on an Mg dihalide having a mean crystallite dimension lower than 30 nm.

Abstract: Catalyst support containing an &agr;-olefin polymer which is in the form of particles of mean size from 5 to 350 &mgr;m in which the pore volume generated by the pores of radius from 1,000 to 75,000 Å is at least 0.2 cm3/g. Catalyst usable for the polymerization of &agr;-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: Components of catalysts for the polymerization of olefins are obtained by contacting a Ti compound of formula Ti(OR)n−yXy, where n is the valence of titanium and y is a number between 1 and n, with a pre-polymer having a porosity higher than 0.3 cc/g and containing from 0.5 to 100 g of polymer per g of solid catalyst component, the pre-polymer being obtained by (co)polymerizing an olefin or a diolefin in the presence of a catalyst comprising a Ti, V, Zr of Hf compound supported on an Mg dihalide having a mean crystallite dimension lower than 30 nm.

Abstract: The present invention relates to a solid catalyst component for &agr;-olefin polymerization having a narrow particle size distribution of not less than 6.0 in terms of the value of N in a Rosin-Rammler function of particle size distribution and giving a catalytic activity of not less than 10,000 ((g-polymer produced/g-solid catalyst component)/hour) in polymerization. According to the present invention, there can be provided a solid catalyst component for &agr;-olefin polymerization capable of giving a poly-&agr;-olefin having the high bulk density and favorable particle properties with high catalytic activity.

Abstract: The present invention relates to a process to produce polyethylene through homopolymerization or copolymerization of ethylene with alpha-olefins in the presence of a titanium amide catalyst supported by an organic polymer material, for the production of moldings, such as through extrusion, injection molding, film blowing, sintering under pressure or ram extrusion. The catalyst according to the present invention contains a partially chloromethylated styrene divinyl benzene copolymer as the organic polymer material, a complex compound supported by it, which contains Mg, Al and Ti.

Abstract: A novel loop/slurry olefin polymerization process is provided which produces ultra-high molecular weight ethylene homopolymer.

Abstract: A novel loop/slurry olefin polymerization process is provided which produces ultra-high molecular weight ethylene homopolymer.

Abstract: Catalysts for producing methylamines are provided, which comprise mordenite, as the essential component, of spherical fine particles having a crystal diameter of not more than 0.5 &mgr;m. When the catalysts are used for the production of dimethylamine from methanol and ammonia or through a disproportionation of methylamines, they exhibit a high dimethylamine productivity with the formation of few amount of trimethylamine whose demand is very small. The process can be operated for a long period of time without decrease in the catalyst activity.

Abstract: The present invention relates to a solid catalyst component for &agr;-olefin polymerization having a narrow particle size distribution of not less than 6.0 in terms of the value of N in a Rosin-Rammler function of particle size distribution and giving a catalytic activity of not less than 10,000 ((g-polymer produced/g-solid catalyst component)/hour) in polymerization. The catalyst component is obtained by a process which comprises reducing a titanium compound with an organomagnesium compound in the presence of an organosilicon compound and an ester to obtain a solid product, then treating the solid product with: (a) a mixture of titanium tetrachloride and an ether, (b) an organic acid halide, and (c) a mixture of titanium tetrachloride and an ether optionally containing an ester.

Abstract: Process for the preparation of a catalyst component for the polymerization and copolymerization of ethylene to give ultrahigh-molecular-weight ethylene polymers. Reaction of a Grignard compound with a halogenating agent, a titanium compound, a perhalogen compound and an electron-donor compound and subsequent comminution of the resultant solid to a mean particle size of from 0.5 to 5 .mu.m gives a catalyst component which, together with an organoaluminum compound, results, in the polymerization and copolymerization of ethylene, in ultrahigh-molecular-weight ethylene polymers having a mean particle diameter of from 50 to 200 .mu.m and a viscosity index of greater than 2,000 cm.sup.3 /g. The mean particle diameter can be adjusted by means of the catalyst particle diameter and by means of the catalyst productivity.

Abstract: A catalyst system for olefin polymerization, including a catalytic solid based on chromium deposited on a support containing silica, alumina and aluminium phosphate and a cocatalyst chosen from organoaluminium compounds. One of the essential characteristics of the invention is the combination of an organoaluminium compound as cocatalyst with a catalytic solid based on chromium deposited on a support containing silica, alumina and aluminium phosphate.

Abstract: Maleic anhydride can be hydrogenated with hydrogen to give succinic anhydride by a catalyzed liquid-phase hydrogenation, by carrying out the hydrogenation continuously at a pressure of 50 to 400 bar and a reaction temperature of 60.degree. to 180.degree. C. on oxygen-free and support-free shaped bodies arranged in the fixed bed, which shaped bodies are made of pressed powders of the elements of the iron subgroup of subgroup VIII of the Periodic Table of the Elements or of their alloys with each other or of their alloys with elements of subgroup VI; in addition, hydrogenation-inert elements can be present. The shaped bodies have a compressive strength of 20 to 250N and an internal surface area of 10 to 90 m.sup.2 /g.

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: A process is provided for preparing a shape-shifted catalyst component comprising (1) contacting a dihydrocarbyloxide magnesium compound with carbon dioxide in the presence of a slurrying agent to form a slurry of a carboxylated dihydrocarbyloxide magnesium compound; (2) adding a filler to the slurry either before or after the carboxylation of step (1); (3) spray drying the slurry of step (2) to evaporate the slurrying agent and to produce solid particles of the carboxylated dihydrocarbyloxide magnesium compound incorporating the filler; and, optionally, (4) heating the solid particles to remove carbon dioxide to produce a shape-shifted dihydrocarbyloxide magnesium compound component. A catalyst system using the component and a polymerization process employing the catalyst system are also provided.

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 is provided for preparing a shape-shifted catalyst component comprising (1) contacting a dihydrocarbyloxide magnesium compound with carbon dioxide in the presence of a slurrying agent to form a slurry of a carboxylated dihydrocarbyloxide magnesium compound; (2) adding a filler to the slurry either before or after the carboxylation of step (1); (3) spray drying the slurry of step (2) to evaporate the slurrying agent and to produce solid particles of the carboxylated dihydrocarbyloxide magnesium compound incorporating the filler; and, optionally, (4) heating the solid particles to remove carbon dioxide to produce a shape-shifted dihydrocarbyloxide magnesium compound component. A catalyst system using the component and a polymerization process employing the catalyst system are also provided.

Abstract: A process is provided for preparing a shape-shifted catalyst component comprising (1) contacting a dihydrocarbyloxide magnesium compound with carbon dioxide in the presence of a slurrying agent to form a slurry of a carboxylated dihydrocarbyloxide magnesium compound; (2) adding a filler to the slurry either before or after the carboxylation of step (1); (3) spray drying the slurry of step (2) to evaporate the slurrying agent and to produce solid particles of the carboxylated dihydrocarbyloxide magnesium compound incorporating the filler; and, optionally, (4) heating the solid particles to remove carbon dioxide to produce a shape-shifted dihydrocarbyloxide magnesium compound component. A catalyst system using the component and a polymerization process employing the catalyst system are also provided.

Abstract: According to the present invention, in a vapor phase polymerization of olefins, there is provided an efficient polyolefin manufacturing process capable of preventing the formation of a sheet-like polymer and allowing an olefin polymerization reaction to be started stably.

Abstract: It is possible to obtain globular polymers in the polymerization of alpha-olefins by means of a highly active, spherical Ziegler catalyst. The polymers have a high bulk density and can be processed very easily.

Abstract: A process for producing sticky polymers at polymerization reaction temperatures in excess of the softening temperatures of the sticky polymers in a fluidized bed reactor in the presence of an inert particulate material which comprises utilizing as a support material for the catalyst, a support which includes silica having an average particle size of from about 60 to about 200 microns.

Abstract: Disclosed is a particle-bearing composite comprising a solid carrier and polymer-protected particles adsorbed thereon, wherein the polymer has a number average molecular weight of 3,000 to 300,000 and the particles are of a metal and/or a metal compound. The particle-bearing composite of the present invention is produced by a method in which a polymer-protected colloidal dispersion, which is comprised of a dispersion medium, colloidal metallic particles and/or metal compound particles dispersed in the dispersion medium and a protective polymer having a number average molecular weight of 3,000 to 300,000 and serving as a protective colloid for protecting the colloidal particles, is contacted with a solid carrier.

Abstract: The present invention relates to a process for polymerizing or copolymerizing alpha-olefins comprising, on the one hand, a prepolymerization stage by contacting one or more alpha-olefins with a Ziegler catalyst system consisting of a solid catalyst containing essentially atoms of halogen, magnesium and transition metal, and a co-catalyst consisting of an organometallic compound, and, on the other hand, a polymerization or copolymerization stage in a gas phase process by means of a fluidized bed, by contacting one or more alpha-olefins with the prepolymer produced in the first stage, process characterized in that the alpha-olefin prepolymer and polymer or copolymer are in the form of a powder consisting of particles having specific values for the transition metal content and the mean diameter by mass, and characterized in that the particle size distribution of the prepolymer is narrow and easily controlled.

Abstract: The present invention relates to a process for polymerizing or copolymerizing alpha-olefins comprising, on the one hand, a prepolymerization stage by contacting one or more alpha-olefins with a Ziegler catalyst system consisting of a solid catalyst containing essentially atoms of halogen, magnesium and transition metal, and a co-catalyst consisting of an organomatellic compound, and, on the other hand, a polymerization or copolymerization stage in a gas phase process by means of a fluidized bed, by contacting one or more alpha-olefins with the prepolymer produced in the first stage, process characterized in that the alpha-olefin prepolymer and polymer or copolymer are in the form of a powder consisting of particules having specific values for the transition metal content and the mean diameter by mass, and characterized in that the particle size distribution of the prepolymer is narrow and easily controlled.

Abstract: A process for (co)polymerizing alpha-olefins in the gas phase in a fluidized bed or mechanically stirred bed reactor using a Ziegler-Natta type catalyst or a heat activated chromium oxide/refractory oxide catalyst wherein the (co)polymerization is carried out in the presence of 0.005 to 0.2 weight %, based on the weight of the (co)polymer bed, of an inert pulverulent inorganic substance (e.g. silica) having a mean particle diameter between 0.5 and 20 microns. The process is capable of being operated at temperatures closer to the softening point of the produced polyolefin than in the absence of the pulverulent solid.

Abstract: A solid catalyst component for .alpha.-olefin polymerization containing as catalyst components at least titanium, magnesium and chlorine which are impregnated into an organic porous polymer carrier having a mean particle diameter of 5 to 1,000 .mu.m and a pore volume of 0.1 ml/g or above at a pore radius of 100 to 5,000 .ANG., a catalyst system comprising at least said solid catalyst component (A) and an organoaluminum compound (B), as well as a process for producing .alpha.-olefin polymers using said catalyst system.

Abstract: Ultra-high-molecular-weight polyolefin fine powder characterized in that said fine powder has an intrinsic viscosity measured in decalin at 135.degree. C. of at least 10 dl/g and an average particle diameter in the range of 1-80 .mu.m, at least 20 weight % of the powder passing through 350 Tyler mesh screen.

Abstract: A modified supported catalyst for polymerizing alpha-olefins, for example, ethylene, is prepared by combining a chromium compound-containing catalyst with a modifier which is an oxide of an element of Group II A of the Periodic Chart of the Elements. The resulting modified catalyst composition, when used to polymerize alpha-olefins, produces polymer resins having higher values of High Load Melt Index (I.sub.21) than catalyst compositions without the modifier.There is also disclosed a process for polymerizing alpha-olefins, at relatively lower temperatures than was heretofore possible, to obtain resins of relatively high HLMI, using the above-identified modified catalyst composition. The resulting polymers can be used for the fabrication of blow-molded products, e.g., household and industrial containers.

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.

Abstract: The present invention relates to a process for the polymerisation or copolymerisation of alpha-olefins in the presence of a Ziegler-Natta catalyst system comprising on the one hand a catalyst solid comprising compounds of halogens, transition metals of Groups IV, V, VI of the Periodic Table of Elements, magnesium and optionally aluminum, free from a metal/carbon bond and on the other hand a catalyst consisting of organometallic compounds of metals of Group II or III of the said Table. This process is characterised in that before its use in polymerisation or copolymerisation, the catalyst solid is treated by water in a given quantity, this treatment making it possible to improve the conditions for obtaining polymers, particularly in a process of polymerisation or copolymerisation in the gas phase.

Abstract: Ethene homopolymers and ethene copolymers with minor amounts of higher .alpha.-monoolefins are prepared by polymerization of the monomer or monomers using a chromium trioxide catalyst which is obtained by a method in which (1) a finely divided, porous, silicate carrier is laden with (2) chromium trioxide or a chromium compound which is converted to chromium trioxide under the conditions of stage (3), and then (3) the intermediate resulting from stage (2) is heated in an oxygen-containing gas stream. In this process, a cocatalyst which consists of a lithiumalkyl used in addition to the chromium trioxide catalyst.

Abstract: The present invention relates to a process for polymerizing or copolymerizing alpha-olefins comprising, on the one hand, a prepolymerization stage by contacting one or more alpha-olefins with a Ziegler catalyst system consisting of a solid catalyst containing essentially atoms of halogen, magnesium and transition metal, and a co-catalyst consisting of an organomatellic compound, and, on the other hand, a polymerization or copolymerization stage in a gas phase process by means of a fluidized bed, by contacting one or more alpha-olefins with the prepolymer produced in the first stage, process characterized in that the alpha-olefin prepolymer and polymer or copolymer are in the form of a powder consisting of particles having specific values for the transition metal content and the mean diameter by mass, and characterized in that the particle size distribution of the prepolymer is narrow and easily controlled.

Abstract: Novel high pore volume, high purity alumina gels and catalysts prepared therefrom are provided by a unique alumina gel preparation comprising hydrolyzing an aluminum alcoholate in a medium of a mixture of water and acetone, aging the resultant product and thereafter washing with acetone. Processes for the polymerization of olefins utilizing the afore-described alumina gels are also included herein.

Abstract: Catalyst systems for the polymerization of .alpha.-olefins, as well as process for making and using the same, which comprises an organometallic compound, like an organoaluminum compound, and a solid catalyst complex formed by reacting together at least one compound selected from organic oxygen-containing and halogen-containing compounds of each of the metals of magnesium and titanium and at least one aluminum halide selected from organoaluminum chlorides of formula AlR.sub.n Cl.sub.3-n wherein R is an alkyl radical featuring at least 4 carbon atoms and 1.ltoreq.n.ltoreq.2. Particles of polyolefins of higher average dimension are recoverable, when polymerized in the presence of such catalyst systems.

Abstract: Highly active catalysts for the polymerization of olefins are disclosed. The catalysts are prepared by mixing a hydride or organometallic compound of a metal belonging to Groups I, II or III of the Mendelyeev Periodic Table with the product obtained by contacting an oxygenated Ti compound in which one or more Ti atoms are bound through oxygen atoms to organic radicals with an activated anhydrous Mg dihalide. The polymerization of ethylene by means of said catalysts is also disclosed.

Abstract: Film grade ethylene copolymers are prepared by copolymerizing ethylene with at least one C.sub.3 to C.sub.8 alpha olefin employing a catalyst formed from an organo aluminum activator compound and a precursor composition impregnated in very fine particle sized porous silica, said precursor composition having the formulaMG.sub.m Ti.sub.1 (OR).sub.n X.sub.p [ED.sub.q ]wherein R is a C.sub.1 to C.sub.14 aliphatic or aromatic hydrocarbon radical, or COR' wherein R' is a C.sub.1 to C.sub.14 aliphatic or aromatic hydrocarbon radical; X is selected from the group consisting of Cl, Br, I or mixtures thereof; ED is an electron donor compound; m is .gtoreq.0.5 to .ltoreq.56; n is 0, 1 or 2; p is .gtoreq.2 to .ltoreq.116; and q is .gtoreq.2 to .ltoreq.85; and said organo aluminum activator compound having the formulaAl(R").sub.c X'.sub.d H.sub.ewherein X' is Cl or OR'"; R" and R'" are the same or different and are C.sub.1 to C.sub.14 saturated hydrocarbon radicals; d is 0 to 1.5; e is 1 or 0; and c+d+e=3.

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.