WO2005021610A1

WO2005021610A1 - Process for the polymerization of olefins

Info

Publication number: WO2005021610A1
Application number: PCT/IB2004/002964
Authority: WO
Inventors: Stephan Detournay; Dominique Jan
Original assignee: Innovene Manufacturing Belgium Nv
Priority date: 2003-09-01
Filing date: 2004-08-26
Publication date: 2005-03-10
Also published as: CN100591701C; EP1660546A1; CN1875037A; GB0320473D0

Abstract

The invention relates to a process for the polymerization of olefins in which at least one olefin is placed in contact with a catalytic system comprising: a) a catalytic solid comprising magnesium, at least on transition metal selected from the group consisting of titanium and zirconium and halogen, prepared by successively: - reacting, in a first step (1), at least one magnesium compound (M) chosen from oxygen-containing organic magnesium compounds with at least on compound (T) selected from the group consisting of oxygen-containing or halogen-containing tetravalent titanium and zirconium compounds, until a liquid complex is obtained; - treating, in a second step (2), the said liquid complex with an electron donor (ED); - treating, in a third step (3), the complex obtained in step (2) with a halogen-containing aluminic compound of formula A1RnX3-n, in which R is a hydrocarbon radical comprising up to 20 carbon atoms, X is halogen and n is less than 3, and B) an organometallic compound of a metal chosen from lithium, magnesium, zinc, aluminum or tin, wherein the complex obtained in step (2) of the preparation of the catalytic solid (a) is a substantially solid complex and the electron donor (ED) is a benzoyl halide.

Description

PROCESS FOR THE POLYMERIZATION OF OLEFINS. The present invention relates to a process for the polymerization of olefins, more precisely to a polymerization process carried out in the presence of a catalytic system comprising a catalytic solid based on magnesium, a transition metal and halogen together with an organometallic compound (cocatalyst). In catalytic processes for polymerizing olefϊnic monomers, the catalyst morphology is of prime importance in order to ensure the production of olefins-based polymers,- especially ethylene-based polymers, with suitable powder properties. Indeed, polymer powders with high fines content lead to severe drawbacks: accumulation of fines in the recycling units, during the transfer and packing of the powder, sampling, risk of accumulation of static electricity in the conveyors and the storage tanks. These issues usually have major impacts on the polymerisation and .the compounding processes. The most obvious way to overcome these problems is to use a catalyst with an appropriate granulometry: high average diameter preferably combined with narrow particle size distribution. In this connection, some very efficient catalytic systems comprising a solid catalytic complex based on magnesium, on a transition metal and on halogen, like those described e.g. in documents EP-A-0703 247 and EP-A-0703 248, the entire contents of which being enclosed herewith by reference, are not totally satisfactory as far as the morphology of the recovered polymer is concerned. Furthermore, it is well known that the rheological properties of the recovered polymer also have a great influence on its processability and/or on its mechanical properties. In this context, another drawback of the catalytic systems disclosed in EP- A-0 703 247 is that the balance between the melt-index (MI₂) values and the corresponding dynamic viscosity (μ) values of the polymers synthesized in then- presence is not optimal. An optimal balance between these properties is, however, a key factor for accurate die swell, melt strength, and other aspects related to the rheology and mechanical properties of the recovered olefins-based polymer. It has now been found that the use of a very specific class of electron donors, incorporated in a specific way during the preparation of the catalytic system in accordance with EP-A-0 703 247, allows to overcome the above mentioned drawbacks. The main object of the present invention is thus a process for the polymerization of olefins in which at least one olefin is placed in contact with a catalytic system comprising :

(a) a catalytic solid comprising magnesium, at least one transition metal selected from the group consisting of titanium and zirconium and halogen, prepared by successively ; reacting, in a first step (1), at least one magnesium compound (M) chosen from oxygen-containing organic magnesium compounds with at least one compound (T) selected from the group consisting of oxygen-containing organic tetravalent titanium and zirconium compounds, until a liquid complex is obtained; treating, in a second step (2), the said liquid complex with an electron donor (ED); treating, in a third step (3), the complex obtained in step (2) with a halogen- containing aluminic compound of formula AlR_nX_3-n, in which R is a hydrocarbon radical comprising up to 20 carbon atoms, X is a halogen and n is less than 3, and (b) an organometallic compound of a metal chosen from lithium, magnesium, zinc, aluminium or tin, wherein the complex obtained in step (2) of the preparation of the catalytic solid (a) is a substantially solid complex and the electron donor (ED) is a carboxylic acid halide. One of the essential features of the process in accordance with the invention is that the addition of the electron donor (ED), being a carboxylic acid halide, to the liquid complex .obtained in step (1) of the preparation of the catalytic solid, induces a precipitation of said liquid complex, before the further treatment thereof (step (3)) with the halogen-containing aluminic compound. The first known step (1) of the preparation of the catalytic solid lies in the preparation of a liquid complex by reacting at least one magnesium compound (M) chosen from oxygen-containing organic magnesium compounds with at least one compound (T) selected from the group consisting of oxygen-containing or halogen- containing tetravalent titanium and zirconium compounds. Of course, several different compounds (M) may be used simultaneously. Similarly, it is also possible to use simultaneously several different compounds (T), even several compounds (T) wherein the metals are simultaneously tetravalent titanium and zirconium. The reaction of the first step may be performed by any suitable known method, provided that it makes it possible to obtain a complex in the liquid state. When the compound (M) and/or the compound (T) are liquid under the operating conditions of the reaction, it is desirable to perform the reaction simply by mixing these reactants together in the absence of solvent or diluent. However, the reaction may be performed in the presence of a diluent when the amount of liquid is not sufficient for the reaction to be complete or when the reactants are solid under the operating conditions of the reaction. The diluent is generally chosen among liquid aliphatic, cycloaliphatic and aromatic hydrocarbons, preferably containing up to 20 carbon atoms, and in particular among linear alkanes (such as n-butane, n-hexane and n-heptane), branched alkanes (such as isobutane, isopentane and isooctane) or cycloalkanes (such as cyclopentane and cyclohexane). Good results are obtained with linear alkanes, especially n-hexane. The amount of compound (T) used is defined relative to the amount of compound (M) used. This amount may vary within a wide range. In general, it is at least 0.01 mol of titanium or zirconium present in compound (T) per mole of magnesium present in compound (M), in particular at least 0.02 mol, values of at least 0.05, more specifically of at least 0.2 mol, being preferred. The amount is usually not more than 20 mol of titanium or zirconium present in compound (T) per mole of magnesium present in compound (M), more particularly not more than 10 mol, values of not more than 2 mol being especially preferred. The temperature at which compound (M) is contacted with compound (T) in step (1) of the preparation of the catalytic solid depends on the nature of the reactants and is preferably below the decomposition temperature of the reactants and of the liquid complex obtained after the reaction. It is generally at least - 20°C, preferably at least 0°C, temperatures of at least 20°C being the most common. The temperature is usually not more than 200°C more especially not more than 180°C, temperatures of not more than 150°C being advantageous, for example of about 140°C. The duration of step (1) in the preparation of the catalytic solid depends on the nature of the reactants and on the operating conditions; and is advantageously long enough to obtain a complete reaction between the reactants. The duration ranges generally from 10 minutes to 20 hours, more particularly from 2 to 15 hours, for example from 4 to 10 hours. The pressure at which the reaction of step (1) is carried out and the rate of addition of the reactants are not critical factors. For reasons of convenience, the process is generally performed at atmospheric pressure; the rate of addition is generally chosen so as not to cause sudden heating of the reaction medium due to a possible self- acceleration of the reaction. The reaction medium is generally stirred so as to promote its homogenization throughout the reaction. The reaction may be performed in a continuous or batchwise manner. At the end of step ( 1 ) of the preparation of the catalytic solid, a liquid complex of compound (M) and of compound (T) is collected, which liquid complex may be used as it is in the subsequent step or may optionally be stored in a diluent, preferably an inert diluent, so as to subsequently use it in the presence of the diluent. The diluent is generally chosen from those disclosed above as possible medium for the reaction of step (1). Compound (M) is chosen from oxygen-containing organic magnesium compounds. The term "oxygen-containing organic magnesium compound" is understood to define all the compounds in which an organic radical is bonded to magnesium via oxygen, that is to say all the compounds comprising at least one magnesium-oxygen-organic radical bonding sequence per magnesium atom. The organic radicals bonded to the magnesium via oxygen are generally chosen from radicals comprising up to 20 carbon atoms and, more particularly, from those comprising up to 10 carbon atoms. Good results are obtained when these radicals comprise from 2 to 6 carbon atoms. These radicals may be saturated or unsaturated, containing a branched chain or containing a straight or cyclic chain. They are preferably chosen from hydrocarbon radicals and in particular from alkyl (linear or branched), alkenyl, aryl, cycloalkyl, arylalkyl, alkylaryl and acyl radicals and the substituted derivatives thereof. In addition to organic radicals bonded to magnesium via oxygen, the compound (M) may include other radicals. These other radicals are preferably the radicals -OH, - (SO₄) ₂, -NO3, -(PO₄)!/₃, ~{C0₃) and -ClO . They may also be organic radicals which are bonded directly to the magnesium via carbon. Among the compounds (M) which may be used, there may be mentioned alkoxides (such as ethoxide and cyclohexanolate, alkylalkoxides (such as ethylethoxide), hydroxyalkoxides (such as hydroxymethoxide), phenoxides (such as naphtoxide) and optionally hydrated carboxylates (such as acetate and benzoate). They may also be oxygen- and nitrogen-containing organic compounds, that is to say compounds comprising magnesium-oxygen-nitrogeri-organic radical sequences (such as oximates, in particular butyloxymate, and hydroxylamine acid salts, in particular the derivative of N-nitroso-N-phenyl-hydroxylamine), chelates, that is to say oxygen- containing organic compounds in which the magnesium possesses at least one normal bonding sequence of the magnesium-oxygen-organic radical type and at least one coordination bond so as to form an heterocycle in which the magnesium is included

(such as enolates, in particular acetylacetonate) and silanolates, that is to say compounds comprising magnesium-oxygen-silicon-hydrocarbon radical bonding sequences (such as triphenylsilanolate). Examples of compounds (M) which may also be mentioned are those comprising several different organic radicals (such as magnesium methoxyethoxide, alkoxide and phenoxide complexes of magnesium and another metal (such as Mg[Al(OR) ]₂) and mixtures of two or more of the compounds (M) defined above. Among all the compounds (M) which are suitable, it is preferred to use those which contain, on each magnesium atom, only magnesium-oxygen-organic radical bonds, to the exclusion of any other bonding. Among these, magnesium alkoxides are particularly preferred. The best results are obtained with magnesium dialkoxi'des, in particular magnesium diethoxide. Compound (T) is chosen from the group consisting of oxygen-containing . organic tetravalent titanium and zirconium compounds. The term "oxygen-containing organic tetravalent titanium arid zirconium compound" is understood to define all the compounds in which an organic radical is bonded to tetravalent titanium or zirconium (said metals being hereinafter collectively designated hereafter under the term "transition metal") via oxygen, that is to say all the compounds comprising at least one transition metal-oxygen-organic radical bonding sequence per transition metal atom. The organic radicals are in accordance with those defined above for the compound (compound (M)). It goes without saying that the compound (T) may comprise several different organic radicals. The compound (T) which may be used may also comprise transition metal - oxygen-transition metal bonds or transition metal -halogen-transition metal bonds. Compound (T) may be represented by the general formula TX_x(OR')_4-2x where compound (T) represents the transition metal, X represents oxygen or an halogen, preferably chlorine, R' represents an organic radical as defined above and x is a number such that 0 < x < 3/2. It is preferred to use compounds (T) of the said formula wherein x is such that 0 < x < 1. Among the compounds (T) which may be mentioned are alkoxides (such as Ti(O-nC₄H )₄), phenoxides (such as Zr(OC₆H₅) ), oxyalkoxides (such as TiO(OC H₅)₂), haloalkoxides (such as Ti(OC₂H₅)₂Cl₂ or Zr(OiC₃H₇)₃Cl), condensed alkoxides (such as Ti₂O(O-iC₃H₇)₆ ) and enolates (such as titanium acetylacetonate). It goes without saying that several compounds (compound (T)) maybe used simultaneously. When it is desired to obtain a polyolefin having a wide molecular weight distribution, it may be preferable to use a titanium compound and a zirconium compound. Among all the compounds (compound (T)) which are suitable, it is preferred to use those which contain, on each transition metal atom, only transition metal-oxygen-organic radical bonds, to the exclusion of any other bonding. Alkoxides are suitable for use. The best results are obtained with the tetraalkoxides of titanium or of zirconium, in particular titanium or zirconium tetrabutoxide. During the second step (2) of the preparation of the catalytic solid, the liquid complex obtained in step (1) is treated with an electron donor (ED), said electron donor being a carboxylic acid halide, preferably an aromatic carboxylic acid halide. As electron donor (ED), any carboxylic acid halide able to transform the liquid complex obtained in step (1) into a substantally solid complex is usable for practicing the process of the invention. While not wishing to be bound by theory, Applicant believes that carboxylic acid halides act towards the liquid complex obtained in step (1) as mild non reducing halogenating agents, inducing the selective precipitation of a magnesium halide, especially MgCl₂. In the course of the reaction, the carboxylic acid halide would be consumed, and esters formed in situ. It has been experimentally verified that a significant amount of these esters (being preferably alkyl benzoates when the preferred compounds (M), (T) and electron donor (ED) are used), are incorporated in the solid, and esters account for up to 50% wt of the solid obtained after washings and drying operations. Due to several equilibria, a magnesium halide in a solid form should be progressively obtained. The carboxylic acid halide, which is preferably a chloride, can be derived from mono- and polycarboxylic acids, preferably from mono- and polycarboxylic aromatic acids. Examples of halides derived from the monocarboxylic acids are benzoyl chloride; o-, m-, or p-toluylchlorides; halobenzoyl chlorides; nitrobenzoyl chlorides; aminobenzoyl chlorides and salycylil chloride. Examples of halides derived from the polycarboxylic acids are phtaloyl mono- and dichloride and the chlorides derived from isophtalic, terephtalic, trimellitic, trimesic, hemimellitic and prehnitic acids. Chlorides, derived from the monocarboxylic aromatic acids are preferred, and among them, benzoyl chloride leads to especially good results. The treatment using the electron donor (ED) may be carried out by any suitable known means. The electron donor may be added in the pure state to the liquid complex or in the form of a solution in a solvent generally chosen from the diluents disclosed above as possible medium for the reaction of step (1). Good results are obtained with linear alkanes. Hexane is preferred. The temperature at which the treatment using the electron donor is carried out in the first variant is generally below the decomposition temperatures of the electron donor and of the liquid complex. It is in particular at least -20°C, more precisely at least 0°C, values of at least 20°C being more common. The temperature is usually not more than 150°C, more particularly not more than 120°C, temperatures of not more than 100°C being recommended, for example not more than 70°C. The duration of the treatment using the electron donor in the first variant is commonly from 1 minute to 50 hours, preferably from 45 minutes to 30 hours, for example from 120 minutes to 24 hours. The pressure at which the treatment is performed is not critical; the process is preferably performed at atmospheric pressure. The amount of electron donor (ED) used is usually at least 0.01 mol per mole of compound (M) used, more precisely at least 0.1 mol, values of at least 0.5 mol being the most advantageous. The amount of electron donor used usually does not exceed 20 mol per mole of transition metal used, and preferably does not exceed 10 mol, values of not more than 5 mol being the most recommended. Amounts from 1 to 4 mol are particularly suitable. At the end of step (2), the substantially solid complex obtained may be used as such in its preparation medium, possibly after an ageing step, generally carried out under stirring during 1 to 30 hours, preferably during 5 to 25 hours at a temperature generally comprised between 0 and 100°C, preferably between 25 and 90°C. It may also been separated and washed by means of a diluent like those disclosed above, wherein it may be resuspended for the third step of preparation of the catalytic solid. The solid formed during step (2), when isolated for characterisation, exhibits markedly improved particle size distribution, narrower particle size distribution (PSD), higher average diameter, lower fines content when compared to the known solid catalytic complex obtained in accordance with EP-A-0703247. The preparation of the solid catalytic complex comprises a subsequent third step (3), which has the main function of reducing the valency of the transition metal and simultaneously of more halogenating, if necessary, the magnesium compound and/or the transition metal compound, that is to say of substituting the alkoxy groups still possibly present in the magnesium compound and/or in the transition metal compound by halogens, such that the substantially solid complex obtained after step (2) is transformed in a catalytically active precipitated as a catalytic solid. The reduction and the possible further halogenation are performed simultaneously using the halogen-containing aluminic compound which thus acts as a reductive halogenating agent. The treatment using the halogen-containing aluminic compound in step (3) of the preparation of the catalytic solid may be carried out by any suitable known means, and preferably by gradually adding the halogen-containing organόaluminium compound to a suspension of the complex obtained in step (2). The amount of halogen-containing aluminic compound to be used depends on the amounts of magnesium compound and of transition metal compound used and is advantageously sufficient to obtain the desired rate of reduction and, where appropriate, the desired rate of halogenation. In practice, there is no advantage in using an amount greater than the minimum amount required to obtain complete reduction and, where appropriate, complete halogenation, since any excess used leads to an increase in the aluminium content in the solid catalytic complex, which is not desirable. In general, the amount is at least 0.5 mol of aluminium per mole of magnesium used, preferably at least 1 mol, values of at least 1.5 mol being the most common; it is commonly not more than 50 mol of aluminium per mole of transition metal used, in particular not more than 30 mol, values of not more than 20 mol, especially not more than 10 mol, being advantageous. Particularly good results have been obtained for amounts of aluminium being comprised between 2 and 9 mol per mol of magnesium. The treatment using the halogen-containing aluminic compound in step (3) in may be carried out either in one step or in two successive steps as disclosed in EP-A-0703248. The temperature at which step (3) is performed is advantageously below the boiling point, at ordinary pressure, of the halogen-containing aluminic compound. It is usually at least -20°C, more particularly at least 0°C, temperatures of at least 20°C being recommended. The temperature usually does not exceed 150°C, and more especially does not exceed 100°C, temperatures of not more than 80°C being the most common. Step (3) is preferably long enough to obtain complete reduction and possible further halogenation of the substantially solid complex of step (2). It may range from 1 minute to 10 hours, more precisely from 10 minutes to 8 hours, for example from 0.5 to

5 hours. The pressure at which step (3) is carried out is not a critical factor. For reasons of convenience, the process is generally performed at atmospheric pressure. The rate of addition of the reactants is generally chosen constant enough as not to cause sudden heating of the reaction medium due to a possible self-acceleration of the reaction. The reaction medium is generally stirred so as to promote its homogenization throughout the reaction. The reaction may be performed in a continuous or batchwise manner. The halogen-containing aluminic compound corresponds to the formula AlR_nX_3-n in which R is a hydrocarbon radical comprising up to 20 carbon atoms and preferably up to

6 carbon atoms. Good results are obtained when R is an alkyl (linear or branched), cycloalkyl, arylalkyl, aryl or alkylaryl radical. The best results are obtained when R represents a linear or branched alkyl radical. X is generally chosen from fluorine, chlorine, bromine and iodine. Chlorine is particularly suitable. Preferably, n does not exceed 1.5 and more especially does not exceed 1. As examples of halogen-containing aluminic compounds which may be used in the invention, there may be mentioned aluminium trichloride [A1C1₃], ethylaluminium dichloride [A1(C₂H₅)C1₂], ethylaluminium sesquichloride [A1₂(C₂H₅)₃C1₃], diethylaluminium chloride [A1(C₂H₅)₂C1] and isobutylaluminium dichloride [Al(iC H )Cl₂]. Isobutylaluminium dichloride is preferred. After step (3) of the preparation of the catalytic solid, the said solid is collected, consisting of a homogeneous precipitate (the constituents being coprecipitated from a liquid complex) of a mixture of a magnesium halide, a transition metal halide and, where appropriate, partially reduced and/or partially halogenated compounds, as well as residues of esters generated by the consumption of the electron donor (ED). These are chemically bonded complexes, produced by chemical reactions and not as a result of mixing or of adsorption phenomena. Indeed, it is impossible to dissociate either of the constituents of these complexes by using purely physical methods of separation. Step (3) of the preparation of the catalytic solid may advantageously be followed by a maturation treatment whose function is to make it possible to obtain a catalytic solid having an improved resistance to uncontrolled breakdown in polymerization. The maturation is carried out at a temperature generally equivalent to or above that at which step (3) takes place. It is carried out for a non-critical period ranging from 5 minutes to 12 hours in general, preferably for at least 0.5 hour. Step (3) may also be followed, preferably after the optional maturation step, by a washing step so as to remove the excess reactants and the possible by-products formed during the preparation, with which the catalytic solid may still be impregnated. Any inert diluent maybe used for this washing and, for instance, those disclosed above as possible medium for the reaction of step (1). After washing, the catalytic solid may be dried, for example by flushing with a stream of an inert gas such as nitrogen, which is preferably dry. The catalytic system, with which the olefin to polymerize is contacted, in accordance with the process of the invention, also contains, besides the catalytic solid (a) described above, an organometallic compound (b), which serves as activator for the catalytic solid and is commonly referred to as the "cocatalyst". It is chosen among organometallic compounds of lithium, magnesium, zinc, aluminium or tin. The best results are obtained with organoaluminium compounds. As organometallic compound, it is possible to use totally alkylated compounds in which the alkyl chains comprise up to 20 carbon atoms and are straight or branched, such as, for example, n-butyllithium, diethylmagnesium, diethylzinc, tetraethyltin, tetrabutyltin and trialkylaluminiums. It is also possible to use alkyl metal hydrides in which the alkyl radicals also comprise up to 20 carbon atoms, such as diisobutylaluminium hydride and trimethyltin hydride. Alkylmetal halides in which the alkyl radicals also comprise up to 20 carbon atoms are equally suitable, such as ethylaluminium sesquichloride, diethylaluminium chloride and diisobutylaluminium chloride. It is also possible to use organoaluminium compounds obtained by reacting trialkylaluminiums or dialkylaluminium hydrides, in which the radicals comprise up to 20 carbon atoms, with diolefins comprising from 4 to 20 carbon atoms, and more particularly the compounds known as isoprenylaluminiums. In general, preference is given to trialkylaluminiums and in particular to those in which the alkyl chains are straight and comprise up to 18 carbon atoms, more particularly from 2 to 8 carbon atoms. Triethylaluminium and triisobutylaluminium are preferred. The total amount of organometallic compound used in the polymerization process of the invention may vary within a wide range. It is generally from 0.02 to 50 mmol per litre of solvent, of diluent or of reactor volume and preferably from 0.2 to 2.5 mmol per 1. The amount of solid catalytic complex used in the polymerization process of the invention is determined as a function of the transition metal content of the said complex. It is generally chosen such that the concentration is from 0.001 to 2.5 and preferably from 0.01 to 0.25 mmol of transition metal per litre of solvent, of diluent or of reactor volume. The molar ratio of the. total amount of the metal present in the organometallic compound to the total amount of the transition metal present in the transition metal compound is usually at least 1, values of at least 5 being advantageous. The ratio is generally not more than 100, values of not more than 50 being recommended. The polymerization process of the invention may be carried out according to any known process, in solution in a solvent which may be the olefin itself in the liquid state, or in suspension in a hydrocarbon diluent, or alternatively in the gas phase. Good results are obtained in suspension polymerizations. The polymerization is performed by placing the olefin in contact with the catalytic system comprising the solid catalytic complex, the organometallic compound and the electron donor. ' The olefin which is polymerized may be chosen from olefins containing from 2 to 20 carbon atoms, and preferably from 2 to 6 carbon atoms, such as ethylene, propylene, 1- butene, 4-methyl-l-pentene and 1-hexene. Ethylene, 1-butene and 1-hexene are suitable for use: Ethylene is particularly preferred. Obviously, several different olefins maybe used simultaneously in order to obtain copolymers, for example mixtures of two of the olefins mentioned above or mixtures of one or more of these olefins with one or more diolefins preferably comprising from 4 to 20 carbon atoms. These diolefins may be non- conjugated aliphatic diolefins such as 1,4-hexadiene, monocyclic diolefins such as 4- vinylcyclohexene, 1,3-divinylcyclohexane, cyclopentadiene or 1,5-cyclooctadiene, alicyclic diolefins having an endocyclic bridge, such as dicyclopentadiene or norbornadiene, and conjugated aliphatic diolefins such as butadiene and isoprene. The process according to the invention applies particularly well to the manufacture of ethylene homopolymers and copolymers containing at least 90 mol % of ethylene and preferably 95 mol % of ethylene. The suspension polymerization is generally carried out in a hydrocarbon diluent such as liquid aliphatic, cycloaliphatic and aromatic hydrocarbons, at a temperature such that at least 80 % (preferably at least 90 %) of the polymer formed is insoluble therein. The preferred diluents are linear alkanes such as n-butane, n-hexane and n-heptane or branched alkanes such as isobutane, isoperitane, isooctane and 2,2-dimethylprόpane or cycloalkanes such as cyclopentane and cyclohexane or mixtures thereof. The best results are obtained with hexane and isobutane. The polymerization temperature is generally chosen to be between 20 and 200°C, preferably between 50 and 150°C, in particular between 65 and 115°C. The partial pressure of the olefin is usually chosen to be between atmospheric pressure and 5 MPa, preferably between 0.2 and 2 MPa, more particularly between 0.4 and 1.5 MPa. The polymerization process of the invention may optionally be carried out in the presence of a molecular weight regulator such as hydrogen. The polymerization process of the invention may be carried out in a continuous or batchwise manner, in a single reactor or in several reactors arranged in series; the polymerization conditions (temperature, possible comonomer content, possible hydrogen content, type of polymerization medium) in one reactor being different from those used in the other reactors. The polymerization process of the invention makes it possible, thanks to the improved morphology of the catalytic solid, to improve the particle size distribution and the powder bulk density of the resulting polyolefin powder. Furthermore, the polymerization process of the invention makes it is also possible to obtain polyolefms of improved rheological properties. The examples which follow are intended to illustrate the invention. The meaning of the symbols used in these examples, the units expressing the mentioned properties and the methods of measuring these properties are explained herebelow. Span : figures the particle size distribution by the relationship (D90 - D 10)/ D50 wherein the meanings of the respective D, measured by laser granulometry and expressed in μm are :

D 10 : value under which 10 % by volume of the particles are collected; D50 : value under which 50 % by volume of the particles are collected; D90 : value under which 90 % by volume of the particles are collected. MI₂.₁₆ : nielt index of the polymer, measured at 190 °C under a 2.16 kg load, and expressed in g/10 min,according to ASTM standard D 1238 (condition E) (1986). HLMI : melt index of the polymer, measured at 190°C under a 21.6 kg load , and expressed in g/10 min, according to ASTM standard D 1238. Density (D) : standard density of the polymer, expressed in kg/m³ and measured according to ISO standard 1183 (1987). SCB : short chain branch content of the polymer, measured by ¹³C MR. Bulk density (BD) : measured according to the principle set forth in ASTM standard D 1895 (1979) and ISO 60 (1977) by using the following procedure : the polymer powder is poured into a cylindrical container with a capacity of 50 ml, taking care not to pack it down, from a hopper whose lower edge is arranged 20 mm above the upper edge of the container. The container filled with powder is then weighed, the tare is deducted from the read weight and the result obtained (expresssed in g) is divided by 50). μ : dynamic viscosity of the polymer, expressed in dPa.s and measured at a shear rate of 100 s^"1 and at 190 °C. Fines (%) : volume % of polymer particles featuring an average diameter lower than 125 μm (determined by laser granulometry). Example 1

1. Preparation of the catalytic solid

Step (1). Preparation of the liquid complex Magnesium diethoxide, which was prepared in situ by reacting magnesium metal with ethanol, was reacted under stirring for 4 hours at 140°C with titanium tefrabutoxide in amounts such that the molar ratio of titanium to magnesium was equal to 1. Step (2). Treatment using the electron donor (ED) A solution of 16.3 ml of benzoyl chloride in 50 ml of hexane was added to a solution of 70 mmol of the liquid complex in 135 ml of hexane under stirring at 60°C. The addition was completed within 45 min. The reaction medium was kept under stirring at 60°C for 6 hours, and a white solid was formed. The resulting hexane suspension was directly used for the next step of preparation of the catalytic solid. Step (3). Treatment with the halogen-containing aluminic compound Iso-butylaluminium dichloride (IBADIC) was added at 45°C to the suspension produced in step (2) with a molar ratio aluminium:titanium of 8 : 1. The addition was completed within 135 min, and the temperature was increased to 60°C for 60 min. The resulting brown solid was then decanted and washed with several fractions of hexane.

2. Polymerization test The general operating conditions under which the polymerization tests of this and the following examples have been conducted were selected in order to produce the polymer of ethylene with a catalytic productivity comprised between about 15 and 20 kg of polymer per g of catalytic solid. A 5 liter stainlees steel autoclave equipped with a mechanical stirrer was heated treated overnight at 90°C under flowing of nitrogen and cooled to room temperature. Then, 1.5 1 of hexane and 2 mmol of triethylaluminium were introduced, and the autoclave was heated at 85°C. After pressure stabilisation, hydrogen under a partial pressure of 0.25 MPa and ethylene under a partial pressure of 0.6 MPa were introduced. The polymerization was initiated by flushing 30 mg of catalytic solid suspended in 100 ml of hexane. Ethylene was continuously and automatically metered via mass flowmeter so as to keep constant the monomer partial pressure. After 90 minutes, the polymerization was stopped by rapid venting and the autoclave cooled down to room temperature (hereinafter referred to as polymerization conditions I). Example 2 A polymerisation test was carried out with the catalytic solid disclosed in example 1 and in the same conditions as described in this example, but the hydrogen partial pressure was set to 0.4 MPa (hereinafter referred to as polymerization conditions II).

Example 3 A 5 liter stainlees steel autoclave equipped with a mechanical stirrer was heated treated overnight at 90°C under flowing of nitrogen and cooled to room temperature.

Then, 1.5 1 of hexane and 2 mmol of triethylaluminium were introduced, and the autoclave was heated at 75°C. After pressure stabilisation, ethylene was introduced under a partial pressure of 0.4 MPa and hydrogen was introduced in order to obtain a hydrogen / ethylene ratio of 0.003 / 1.

25 g of butene were also adde. The polymerization was initiated by flushing 30 mg of the catalytic solid disclosed in example 1, suspended in 100 ml of hexane. Ethylene was continuously and automatically metered via mass flowmeter so as to keep constant the monomer partial pressure. After 90 minutes, the polymerization was stopped by rapid venting and the autoclave cooled down to room temperature (hereinafter referred to as polymerization conditions III).

Example 4 The preparation of the catalytic solid of example 1 was reproduced except that, in step (3), IBADIC was added to the slurry produced in step (2) with a molar ratio aluminiumrtitanium of 6:1. The addition was completed within 100 min, and the temperature was increased to 60°C for 60 min. The resulting brown solid was then decanted and washed with several fractions of hexane. A polymerisation test was carried out with this catalytic solid under polymerization conditions I.

Example 5 A polymerisation test was carried out with the catalytic solid of example 4 under polymerization conditions II. Example 6 The preparation of the catalytic solid of example 1 was repeated under the following specific conditions for steps (2) and (3) : a solution of 16.3 ml of benzoyl chloride in 50 ml of hexane was added to a solution of 70 mmol of the liquid complex of step (1) in 135 ml of hexane under stirring at 30°C. The addition was completed within 45 min. The reaction was kept under stirring at 30°C for 24 hours, and a white solid was formed. The resulting hexane suspension was directly used for step (3) of the preparation of the catalytic solid, wherein IBADIC was added at 45°C to this suspension with a molar ratio aluminium:titanium of 3.5 : 1. The addition was completed within 60 min, and the temperature was kept constant for 60 min. The resulting brown solid was then decanted and washed with hexane. IBADIC was then again added at 45 °C within 40 min with a molar ratio aluminium:titanium of 2.5 : 1. An ageing was performed at 60°C for 45 min, and the resulting dark brown solid was washed with several fractions of hexane. A polymerisation test was carried out with this catalytic solid under polymerization conditions I. Example 7R (given for comparison) The preparation of the catalytic solid of example 1 was reproduced except that, in step (2), a solution containing 16 ml of tetrachlorosilane and 50 ml of hexane was added to a solution of 70 mmol of the liquid complex of step (1) in 80 ml of hexane under stirring at 45°C. The addition was completed within 45 min, and the reaction was kept under stirring at 45°C for 1 hour. The resulting hexane suspension was directly used for the next step (3), carried out under the following specific conditions: IBADIC was added to the suspension prepared in step (2) at 45 °C with a molar ratio aluminium:titanium of 3.5 : 1. The addition was completed within 60 min and the temperature kept constant for 60 min. The resulting brown solid was then decanted and washed with hexane. IBADIC was then added at 45°C within 40 min with a molar ratio aluminium.-titanium of 2.5 : 1. An ageing was performed at 60°C for 45 min, and the resulting dark brown solid was washed with several fractions of hexane.

A polymerisation test was carried out with this catalytic solid under polymerization conditions I. The measurements made on the product of step (2) and on the catalytic solid of the above examples are collected in Table 1 hereunder. The physical, morphological and rheological properties of the polymers obtained in accordance with these examples are collected in Table 2 hereunder. Example 8R (given for comparison) The preparation of the catalytic solid of example 1 was reproduced except that, in step (2), 19 ml of ethyl benzoate were mixed with 70 mmol of the liquid complex obtained in step (1) and diluted with 120 ml of hexane. Then, IBADIC in a ratio aluminium:titanium of 3 : 1 was added dropwise under stirring at 45°C. The addition was completed within 50 min, and a white solid was formed. The resulting hexane suspension was directly used for the next step (3), carried out under the following specific conditions : IBADIC was added dropwise to the suspension prepared in step (2) at 45°C with a molar ratio aluminium:titanium of 4:1. The addition was completed within 67 min and the temperature kept constant for 60 min. An ageing was performed at 60°C for 45 min, and the resulting dark brown solid was washed with several fractions of hexane. A polymerisation test was carried out with this catalytic solid under polymerization conditions I. The measurements made on the product of step (2) and on the catalytic solid of the above examples are collected in Table 1 hereunder. The physical, morphological and rheological properties of the polymers obtained in accordance with these examples are collected in Table 2 hereunder. Table 1

This Table shows that the products of step (2) in accordance with the invention have either an improved particle size distribution (expressed by a lower span value) (comparison of examples 1,4 and 6 with 8R), or a higher average diameter (expressed by a higher D50 value) (comparison of examples 1,4 and 6 with 7R). The catalytic solids in accordance with the invention, in turn, have a higher average diameter (expressed by a higher D50 value). Table 2

* HLMI

This Table shows the improvement in the balance of physical and morphological properties of the polymers obtainable in accordance with the invention: under comparable polymerization conditions, fines contents are reduced and bulk density is increased; furthermore, for comparable MI_2jl6 values, lower dynamic viscosities are obtained, which balance improves the processability of the polymer. Finally, this Table also shows that, under comparable polymerization conditions, higher melt-indexes are obtained with the catalytic system in accordance with the invention; that means that its sensitivity to the molecular weight regulator ("hydrogen response" of the catalyst) is higher.

Claims

Claims:

1. A process for the polymerization of olefins in which at least one olefin is placed in contact with a catalytic system comprising : a) a catalytic solid comprising magnesium, at least one transition metal selected from the group consisting of titanium and zirconium and halogen, prepared by successively : - reacting, in a first step (1), at least one magnesium compound (M) chosen from oxygen-containing organic magnesium compounds with at least one compound (T) selected from the group consisting of oxygen-containing organic tetravalent titanium and zirconium compounds, until a liquid complex is obtained; - treating, in a second step (2), the said liquid complex with an electron donor (ED); treating, in a third step (3), the complex obtained in step (2) with a halogen- containing aluminic compound of formula AlR_nX_3-n, in which R is a hydrocarbon radical comprising up to 20 carbon atoms, X is a halogen and n is less than 3, and b) an organometallic compound of a metal chosen from lithium, magnesium, zinc, aluminium or tin, wherein the complex obtained in step (2) of the preparation of the catalytic solid (a) is a substantially solid complex and the electron donor (ED) is a carboxylic acid halide.

2. The process of claim 1, wherein the electron donor (ED) is an aromatic carboxylic acid halide.

3. The process of claim 2, wherein the electron donor (ED) is derived from a monocarboxylic acid halide.

4. The process of claim 3, wherein the electron donor (ED) is benzoyl chloride.

5. The process of claims 1 to 4, wherein the amount of electron donor (ED) used is 1 to 4 mol per mole of compound (M).

6. The process of claims 1 to 5, wherein the at least one olefin is ethylene.