Process for the polymerization of olefins

The present invention relates to a process for the polymerization of olefins which makes it possible in particular to manufacture ethylene polymers having an advantageous combination of characteristics which makes them particularly suitable for extrusion and extrusion blow molding for the manufacture of articles (eg pipes) having excellent mechanical properties and in particular high resistance to stress cracking.

It is generally known that resins having a high elongational viscosity (which results in a high swelling rate) are particularly suitable for use by extrusion and extrusion blow molding. For example, Belgian Patent BE 840378 (SOLVAY & CIE) describes polyethylenes obtained by polymerization in a single reactor in the presence of a catalytic solid which is prepared by reacting an organic oxygenated magnesium compound with an organic oxygenated titanium compound and an oxygenated organic zirconium compound, and then treating the reaction product thus obtained with an aluminum halide. Known polyethylenes have a high swelling rate. However, their mechanical properties are such that the resistance to stress cracking of pipes extruded from these polyethylenes is low.

Furthermore, polyethylenes of improved mechanical properties and in particular of high resistance to stress cracking, are known. For example, the patent application EP 603935 (SOLVAY) discloses polymers of ethylene obtained by polymerization in at least two reactors in series in the presence of a titanium catalyst. The ethylene polymers thus obtained have good mechanical properties (high resistance to stress cracking). However, the ethylene polymers have a low swelling rate.

The present invention aims to overcome the aforementioned drawbacks by providing a new polyethylene ethylene having both a high swelling rate and a high resistance to stress cracking, which is particularly suitable for extrusion and extrusion processing. blowing.

Accordingly, the invention relates to an ethylene polymer having a swelling ratio (T_G) of at least 1.4, a stress crack resistance (ESCR) of at least 55 h and a melt index (MI₅) at least 0.2 g / 10 min.

One of the essential characteristics of the ethylene polymer according to the invention therefore lies in the combination of a high swelling rate with a high resistance to stress cracking.

The swelling rate of the ethylene polymer according to the invention is measured by extruding at 190 ° C. and at a speed gradient of 100 s^-1, the ethylene polymer through a 30 mm long and 2 mm diameter die and at a constant extrusion rate, and by measuring the displacement of the piston required to extrude a length of rod 70 mm. The swelling rate is defined by the relationship<maths id="math0001" num=""><math display="inline"><mrow><msub><mrow><mtext>T</mtext></mrow><mrow><mtext>G</mtext></mrow></msub><mtext> = 0.5707 √e</mtext></mrow></math><img file="EP0908474A2_D0001.tif" /></maths>where e represents the piston displacement expressed in mm. The cylinder and the piston of the rheometer used for this measurement meet the criteria of that used for the measurement of the melt index according to ASTM D1238 (1986).

The resistance to stress cracking of the ethylene polymer is measured according to the following procedure. Ten plates of dimensions 125 mm × 12.7 mm × 3.2 mm are pressed from a sheet of ethylene polymer. Two cuts are made, the first 60 mm from one end of the plate and the second 15 mm from the other end of the plate. The notched plates are subjected to a constant bending force of 7.36 N, corresponding to a stress less than the plastic flow threshold stress, and immersed simultaneously in a surfactant solution comprising 3 ml of nonylphenoxypoly (ethyleneoxy ) ethanol per liter of water at a temperature of 60 ° C. The time at which the test pieces break is recorded and the average time corresponding to the failure of 50% of the test pieces is calculated.

For the purpose of the present invention, the term "ethylene polymers" refers to homopolymers of ethylene as well as copolymers of ethylene with at least one comonomer. Ethylene copolymers are the most advantageous. As the comonomer, there may be mentioned alpha-olefins containing from 3 to 8 carbon atoms. Butene, hexene and mixtures thereof are preferred. The comonomer content in the ethylene polymer is generally at least 0.1% by weight, in particular at least 0.5% by weight, the values ​​of at least 1% by weight being favorable. The comonomer content is usually at most 10% by weight, more precisely at most 8% by weight, the values ​​of at most 5% by weight being the most common.

The ethylene polymers according to the invention usually have a melt index measured at 190 ° C. under a load of 5 kg according to ASTM D 1238 - Condition P (1986) (hereinafter referred to as MI).₅) at least 0.3 g / 10 min, in particular at least 0.6 g / 10 min. MI₅ does not generally exceed 10 g / 10 min, most often not 5 g / 10 min, and especially not 2 g / 10 min.

Preferred ethylene polymers according to the invention are furthermore characterized by a dynamic viscosity η expressed in dPa.s and measured at a speed gradient of 100 s^-1 at 190 ° C as the ratio<maths id="math0002" num=""><img file="EP0908474A2_D0002.tif" /></maths> at least 0.55. Preferably, this ratio is at least 0.59, the values ​​of at least 0.61 being particularly advantageous. In most cases, this ratio is at most 0.73, and most often at most 0.70.

The ethylene polymers according to the invention commonly have a standard density (or density) measured according to ISO 1183 (1987) of at least 945 kg / m.³, in particular of at least 950 kg / m³, values ​​of at least 952 kg / m³ being preferred. The standard density does not exceed, in general, 965 kg / m³, more precisely not 960 kg / m³, values ​​not exceeding 958 kg / m³ being the most preferred.

The invention also relates to various processes for the preparation of the ethylene polymer described above.

In a first process for the preparation of ethylene polymer according to the invention, a single catalytic solid containing titanium and zirconium is used as active elements in a polymerization in two reactors arranged in series.

The first preparation method consists more particularly of polymerizing ethylene optionally with one or more comonomers in two reactors in series in the presence of a catalytic solid comprising titanium and zirconium in a molar ratio Zr / Ti of at least 2 and a cocatalyst, the first reactor being fed with ethylene, optionally in comonomer and / or in hydrogen, as a catalytic solid and cocatalyst, the reaction medium of the first reactor being transferred to the second reactor, and the second reactor is further fed with ethylene and optionally with comonomer. Preferably, hydrogen is introduced into at least one of the two reactors.

The catalytic solid used in the first process according to the invention advantageously comprises from 0.5 to 10% by weight of titanium (preferably from 1 to 6% by weight), from 5 to 40% by weight of zirconium (preferably from 10 to 25% by weight), the Zr / Ti molar ratio being at least 2, from 20 to 80% by weight of halogen (preferably from 40 to 60% by weight), from 1 to 30% by weight of magnesium (preferably from 5 to 15% by weight), and from 0.5 to 10% by weight of aluminum (preferably from 1 to 3% by weight). The balance consists of residual organic groups from the reagents used, in particular alkoxy and alkyl groups. The halogen is preferably chlorine.

The ratio Zr / Ti in the catalytic solid is preferably at least 2.5, values ​​of at least 3 being particularly preferred. The ratio Zr / Ti does not exceed more often 10, more precisely not 8, the values ​​of at most 6 being preferred.

In a second process for the preparation of ethylene polymer according to the invention, a mixture of two catalytic solids is used, the first containing a single active element, namely titanium, and the second containing two active elements, namely titanium and zirconium, in a polymerization in two reactors in series, and a cocatalyst, the first reactor being fed with ethylene, optionally in comonomer and / or in hydrogen, first and second catalytic solids and cocatalyst, the reaction medium of the first reactor being transferred to the second reactor, and the second reactor is further supplied with ethylene and optionally with comonomer and / or hydrogen.

The second method for preparing the ethylene polymers according to the invention more particularly consists in polymerizing ethylene optionally with one or more comonomers in two reactors in series, in the presence of a first catalytic solid consisting essentially of 10 to 30% by weight of titanium, from 20 to 60% by weight of halogen, from 0.5 to 20% by weight of magnesium and from 0.1 to 10% by weight of aluminum, a second catalytic solid consisting essentially of 0.5 to 10% by weight of titanium, from 5 to 40% by weight of zirconium, from 20 to 80% by weight of halogen, from 1 to 30% by weight of magnesium and from 0.5 to 10% by weight of aluminum. Preferably, hydrogen is introduced into at least one of the two reactors.

The two catalytic solids may optionally be mixed beforehand in the polymerization process. The premix is ​​then advantageously carried out at room temperature.

Preferably, the first catalytic solid consists essentially of 15 to 20% by weight of titanium, 30 to 50% by weight of halogen, 1 to 10% of magnesium and 0.5 to 5% by weight of aluminum. The balance consists of residual organic groups from the reagents used, in particular alkoxy and alkyl groups. Halogen is usually chlorine.

Most often, the second catalytic solid consists essentially of 1 to 6% by weight of titanium, 10 to 25% by weight of zirconium, 40 to 60% by weight of halogen, 5 to 15% by weight of magnesium and from 1 to 3% by weight of aluminum. The balance consists of residual organic groups from the reagents used, in particular alkoxy and alkyl groups. In most cases, halogen is chlorine.

In the second process according to the invention, the two catalytic solids are generally used in amounts such that the molar ratio of titanium from the first titanium catalytic solid from the second catalytic solid is at least 1, in particular from at least 1.25, values ​​of at least 1.50 being preferred. The ratio is usually at most 10, more specifically at most 5, values ​​of at most 4 being preferred.

The cocatalyst used in the first or in the second process can be any cocatalyst known in the art, in particular organoaluminium compounds. By way of example, mention may be made of trialkylaluminums, in particular those in which the alkyl group contains up to 20 carbon atoms (preferably from 2 to 8 carbon atoms), such as triethylaluminium and triisobutylaluminium. Triethylaluminum is preferred.

According to a particular embodiment, the catalytic solid (s) used in the first and second processes for preparing ethylene polymers according to the invention are prepared by reacting, in a first step, an oxygenated organic magnesium compound with an organic oxygenated titanium compound, and with, where appropriate, an oxygenated organic zirconium compound until a liquid complex is obtained, and treating said liquid complex, in a second step, by means of a halogenated organoaluminium compound of general formula AlR_notX_3-n wherein R is a hydrocarbon radical, X is halogen and n is less than 3, to precipitate the liquid complex into a catalytic solid.

For the purposes of the present invention, the term "organic oxygenated magnesium compound" is intended to mean compounds comprising at least one magnesium-oxygen-organic radical bonding sequence per magnesium atom. The organic radical generally comprises up to 20 carbon atoms and more particularly up to 10 carbon atoms, preferably from 2 to 6 carbon atoms. The organic radical may be chosen from alkyl radicals (linear or branched), alkenyls, aryls, cycloalkyls, arylalkyls, alkylaryls, acyls and their substituted derivatives. The best results are obtained with magnesium alkoxides. Magnesium dialkoxides are preferred, especially magnesium diethylate.

The term "organic oxygen compound of titanium or zirconium" is intended to denote compounds comprising at least one titanium (or zirconium) -oxygen-organic radical bonding sequence per titanium or zirconium atom. The organic radical is in accordance with those defined above for organic oxygenated magnesium compounds. The tetravalent titanium or zirconium compounds are preferably used. Among the organic oxygen compounds of titanium or zirconium, mention may be made of alkoxides, phenoxides, oxyalkoxides, condensed alkoxides, carboxylates and enolates. The best results are obtained with alkoxides. Preferred are the tetraalkoxides of titanium or zirconium, especially titanium tetrabutylate or zirconium.

The first step of preparation of the catalytic solid (s) consists in preparing a liquid complex by reacting the organic oxygenated magnesium compound with the organic oxygenated titanium compound and, when the catalytic solid also comprises zirconium, with the organic oxygenated zirconium compound. The reaction can be carried out in the presence of a diluent. The diluent is generally selected from linear or branched alkanes or cycloalkanes containing up to 20 carbon atoms. Hexane is good.

The amount used of the organic oxygenated titanium compound is generally at least 0.01 mole of titanium per mole of magnesium employed, in particular of at least 0.02 mole, the values ​​of at least 0 Moles being preferred. The amount is usually at most 20 moles of titanium per mole of magnesium employed, more precisely at most 10 moles, values ​​of at most 5 moles being preferred. The amount used of the organic oxygenated zirconium compound then depends on the desired Zr / Ti molar ratio.

The second stage of preparation of the catalytic solid (s), which is called the precipitation stage, has the function of reducing the valence of the transition metal and simultaneously halogenating the organic oxygenated magnesium compound, the organic oxygenated titanium compound and, where appropriate, the oxygenated organic zirconium compound, that is to say to substitute the alkoxy groups present in these compounds by halogens, so that the liquid complex obtained at the end of the first step is precipitated into a catalytic solid. The reduction and halogenation are carried out simultaneously by means of the halogenated organoaluminum compound acting as a reducing-halogenating agent causing the precipitation of a catalytic solid.

The treatment with the halogenated organoaluminum compound in the precipitation step is carried out by bringing the liquid complex from the first step into contact with the halogenated organoaluminum compound, and preferably by gradually adding the halogenated organoaluminium compound to the liquid complex.

The halogenated organoaluminium compound advantageously corresponds to the AlR formula_notX_3-n in which R is a hydrocarbon radical comprising up to 20 carbon atoms and preferably up to 6 carbon atoms. The best results are obtained when R represents a linear or branched alkyl radical. X is usually chlorine. Preferably n does not exceed 1.5, especially not 1. Ethylaluminum dichloride or isobutylaluminum dichloride is preferred.

The amount of halogenated organoaluminum compound to be used is generally at least 0.5 mole of aluminum per mole of titanium and zirconium, preferably at least 1 mole, the values ​​of at least 2 moles being the most common; it is commonly at most 50 moles of aluminum per mole of titanium and zirconium, in particular at most 30 moles, values ​​of at most 20 moles being advantageous.

At the end of the step of precipitation of the liquid complex by means of the halogenated organoaluminium compound, a catalytic solid consisting of a homogeneous precipitate (the constituents being coprecipitated from a liquid complex) of an essentially amorphous mixture is collected. a magnesium halide, a titanium halide and, where appropriate, a zirconium halide and optionally partially reduced and / or partially halogenated compounds. These are chemically bound complexes produced by chemical reactions, and not the result of mixtures or adsorption phenomena. Indeed, it is impossible to dissociate one or the other of the constituents of these complexes by using purely physical separation methods.

The catalytic solid comprising titanium and zirconium obtained according to the particular method of preparation described above also makes it possible, when used in a process for the polymerization of olefins in two reactors in series, to obtain polyolefins other than the ethylene polymers according to the invention. The invention therefore also relates to a process for the polymerization of olefins, according to which the olefin is optionally polymerized with one or more comonomers in two reactors in series, in the presence of a catalytic solid comprising titanium and zirconium in a molar ratio Zr / Ti of at least 2 and a cocatalyst, the first reactor being supplied with olefin and optionally with comonomer and / or hydrogen, as a catalytic solid and cocatalyst, the reaction medium of the first reactor being transferred to the second reactor, and the second reactor is further supplied with olefin and optionally with comonomer and / or hydrogen. The catalytic solid is prepared by reacting, in a first step, an organic oxygenated magnesium compound with an organic oxygenated titanium compound and with an organic oxygenated zirconium compound to obtain a liquid complex, and treating said liquid complex, in a second step, using a halogenated organoaluminium compound of general formula AlR_notX_3-n wherein R is a hydrocarbon radical, X is a halogen and n is less than 3 to precipitate the liquid complex into a solid catalytic complex. The process makes it possible to obtain, with high productivity, particularly homogeneous polymers.

The olefin may be selected from olefins containing from 2 to 20 carbon atoms, and preferably from 2 to 6 carbon atoms, such as ethylene, propylene, butene-1, 4-methylpentene-1 and hexene-1. Ethylene, butene-1 and hexene-1 are suitable. Ethylene is particularly preferred. The comonomer may be chosen from the olefins mentioned above and from diolefins comprising from 4 to 20 carbon atoms. It goes without saying that the comonomer introduced to the second reactor may be different from that introduced to the first reactor.

The mixture of the two catalytic solids used in the second process for preparing an ethylene polymer in accordance with the invention can also be used in other olefin polymerization processes in a single reactor or in two reactors arranged in a single reactor. series. The invention thus also relates to a catalyst system for the polymerization of olefins comprising:<ul id="ul0001" list-style="none" compact="compact"><li>(a) a first catalytic solid consisting essentially of 10 to 30% by weight of titanium, 20 to 60% by weight of halogen, 0.5 to 20% by weight of magnesium and 0.1 to 10% by weight of weight of aluminum,</li><li>(b) a second catalytic solid consisting essentially of 0.5 to 10% by weight of titanium, 5 to 40% by weight of zirconium, 20 to 80% by weight of halogen, 1 to 30% by weight of magnesium and 0.5 to 10% by weight of aluminum, and</li><li>(c) a cocatalyst.</li></ul>

The use of a mixture of two catalytic solids makes it possible to modify very quickly the properties of the polymer obtained by adjusting the composition of said mixture.

The polymerization processes 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 in the gas phase. Good results are obtained in suspension polymerizations.

The principle of a polymerization in two reactors arranged in series is that described in the patent application EP 603935 (SOLVAY). The installation can obviously include more than two reactors connected in series. The polymerization processes in two reactors in series are advantageously carried out so as to use in the second reactor polymerization conditions (temperature, concentration of transfer agent such as hydrogen, concentration of possible comonomer, concentration of cocatalyst possible, .. .) different from those used in the first reactor. Thus, the polymer produced in the second reactor has a melt flow rate different from that produced in the first reactor. It can therefore be ensured that the melt index obtained in the first reactor is lower than that obtained in the second reactor. Alternatively, a higher melt index can be obtained in the first reactor than that obtained in the second reactor.

The following examples are intended to illustrate the invention. The meaning of the symbols used in these examples, the units expressing the quantities mentioned and the methods of measurement of these quantities are explained below.<dl id="dl0001" compact="compact"><dt>MID₂</dt><dd>melt index of polyethylene measured at 190 ° C. under a load of 2.16 kg according to ASTM D 1238 (condition E) (1986).</dd><dt>MID₅</dt><dd>melt index of polyethylene measured at 190 ° C. under a load of 5 kg according to ASTM D 1238 (condition P) (1986).</dd><dt>MVS</dt><dd>= standard density of polyethylene expressed in kg / m³ and measured according to ISO 1183 (1987).</dd><dt>η</dt><dd>= dynamic viscosity of polyethylene expressed in dPa.s and measured at a speed gradient of 100 s^-1 at 190 ° C.</dd><dt>ESCR</dt><dd>= resistance to stress cracking expressed in hours and measured by the following method: Ten plates of dimensions 125 mm × 12.7 mm × 3.2 mm are pressed from a sheet of ethylene polymer. Two cuts are made, the first 60 mm from one end of the plate and the second 15 mm from the other end of the plate. The notched plates are subjected to a constant bending force of 7.36 N, corresponding to a stress less than the plastic flow threshold stress, and immersed simultaneously in a surfactant solution comprising 3 ml of nonylphenoxypoly (ethyleneoxy ) ethanol per liter of water at a temperature of 60 ° C. The time at which the test pieces break is recorded and the average time corresponding to the failure of 50% of the test pieces is calculated.</dd><dt>T_G</dt><dd>= swelling rate of the ethylene polymer (without unit). The measurement method consists of extruding at 190 ° C and a speed gradient of 100 s^-1, the ethylene polymer through a die 30 mm in length and 2 mm in diameter and at a constant extrusion rate, and to measure the displacement of the piston necessary to extrude a length of rod 70 mm. The swelling rate is defined by the relationship<maths id="math0003" num=""><math display="inline"><mrow><msub><mrow><mtext>T</mtext></mrow><mrow><mtext>G</mtext></mrow></msub><mtext>= 0.5707 √e</mtext></mrow></math><img file="EP0908474A2_D0003.tif" /></maths>where e represents the piston displacement expressed in mm. The cylinder and the piston of the rheometer used for this measurement meet the criteria of that used for the measurement of the melt index according to ASTM D 1238 (1986).</dd><dt>P</dt><dd>= productivity of the catalytic solid expressed in kg of polyethylene produced per gram of titanium used.</dd></dl>

Example 1 (reference)

In this example, an ethylene polymer was prepared in two reactors in series using a titanium catalyst according to the procedure described in the patent application EP 603935 and its swelling rate and its resistance to stress cracking.

AT.

Preparation of the catalytic solid

Magnesium diethylate was reacted at 150 ° C. for 4 hours with titanium tetrabutylate in such amounts that the molar ratio of titanium to magnesium was 2. Then, the reaction product thus obtained was chlorinated and precipitated by contacting it with a solution of ethylaluminum dichloride in such a quantity that the molar ratio of aluminum to magnesium was equal to 6.5, during 90 minutes at 45 ° C. The solid thus obtained comprised 15.8% by weight of Ti, 36.0% by weight of Cl, 2.2% by weight of Al and 4.4% by weight of Mg.

B.

Polymerization of ethylene into two reactors

Ethylene was polymerized in an installation comprising two reactors arranged in series. Hexane, triethylaluminum as cocatalyst, ethylene and hydrogen were fed continuously into the first reactor in a hydrogen / ethylene mole ratio of 0.27 and the catalytic solid obtained in A. The temperature was kept constant at 85 ° C. The polymerization medium of the first reactor was continuously removed from the first reactor and transferred to the second reactor, which is further fed with ethylene, hydrogen in a hydrogen / ethylene molar ratio of 0.0085 and butene in a molar ratio. butene / ethylene of 0.31. The temperature in the second reactor was 70 ° C. The productivity P was 200. The polymer obtained had the following characteristics:<dl id="dl0002" compact="compact"><dt>MID₅</dt><dd>= 1.3</dd><dt>η</dt><dd>= 15400</dd><dt>T_G</dt><dd>= 1.34</dd><dt>ESCR</dt><dd>= 128</dd><dt>MVS</dt><dd>= 956.</dd></dl>

The polymer obtained has a swelling rate of less than 1.4 while the ethylene polymers according to the invention have a swelling ratio of at least 1.4.

Example 2 (reference)

In this example, an ethylene polymer was prepared in a single reactor using a titanium and zirconium catalyst according to the procedure described in Belgian Patent BE 840378 and its swelling rate and its resistance were measured. to stress cracking.

AT.

Preparation of the catalytic solid

Magnesium diethylate was reacted for 4 hours at 150 ° C. with titanium tetrabutylate and with zirconium tetrabutyalte in amounts such that the Ti / Mg molar ratio was 0.6 and the Zr / mol ratio was Ti is equal to 1.2. Then, the reaction product thus obtained was chlorinated and precipitated by contacting it with a solution of isobutylaluminum dichloride in an amount such that the Al / Mg molar ratio was 11 to 45 ° C. The catalytic solid was mixed with titanium tetraisopropoxide at a rate of 150 g per kg of catalytic solid. The solid thus obtained comprised 6.4% by weight of Ti, 12.6% by weight of Zr, 55.2% by weight of Cl, 2.5% by weight of Al and 5.8% by weight of Mg. .

B.

Polymerization of ethylene in a single reactor

Ethylene was polymerized in a single reactor. Hexane, triisobutylaluminum as cocatalyst, ethylene and hydrogen in a molar ratio of hydrogen to ethylene of 0.09 and the catalytic solid obtained in A. were introduced into it. a butene / ethylene molar ratio of 0.07. The temperature was kept constant at 87 ° C. The productivity P was 100. The polymer obtained had the following characteristics:<dl id="dl0003" compact="compact"><dt>MID₅</dt><dd>= 1.1</dd><dt>η</dt><dd>= 18300</dd><dt>T_G</dt><dd>= 1.59</dd><dt>ESCR</dt><dd>= 38</dd><dt>MVS</dt><dd>= 954.</dd></dl>

The obtained polymer has a stress crack resistance of less than 55 h while the ethylene polymers according to the invention have a stress crack resistance of at least 55 h.

Example 3 (in accordance with the invention)

In this example, an ethylene polymer according to the invention has been manufactured by means of the first preparation method according to the invention.

AT.

Preparation of the catalytic solid

Magnesium diethylate was reacted for 4 hours at 150 ° C. with titanium tetrabutylate and with zirconium tetrabutylate in amounts such that the Ti / Mg molar ratio was 0.4 and the Zr / mol ratio was Ti is equal to 3. Then, the reaction product thus obtained was chlorinated and precipitated by contacting it at 45 ° C with a solution of isobutylaluminum dichloride in an amount such that the Al / Mg molar ratio was 8.4. The solid thus obtained comprised 4.4% by weight of Ti, 14.9% by weight of Zr, 50.2% by weight of Cl, 2.4% by weight of Al and 8.0% by weight of Mg. .

B.

Polymerization of ethylene into two reactors

Ethylene was polymerized in an installation comprising two reactors arranged in series. Hexane, triethylaluminum as co-catalyst, ethylene and hydrogen were fed continuously into the first reactor in a molar hydrogen / ethylene ratio of 0.37 and the catalytic solid obtained in A. The temperature was kept constant at 85 ° C. The polymerization medium of the first reactor was continuously removed from the first reactor and transferred to the second reactor, which is further fed with ethylene, hydrogen in a hydrogen / ethylene molar ratio of 0.0125 and butene in a molar ratio. butene / ethylene 0.2. The temperature in the second reactor was 80 ° C. The productivity P was 213. The weight ratio of the polymer obtained in the first reactor to the polymer obtained in the second reactor was 45.6 / 54.4. The polymer obtained had the following characteristics:<dl id="dl0004" compact="compact"><dt>MID₅</dt><dd>= 1.5</dd><dt>η</dt><dd>= 12800</dd><dt>T_G</dt><dd>= 1.49</dd><dt>ESCR</dt><dd>= 143</dd><dt>MVS</dt><dd>= 955.</dd></dl>

Example 4 (in accordance with the invention)

In this example, an ethylene polymer according to the invention has been manufactured using the second preparation method according to the invention.

AT.

Preparation of the mixture of catalytic solids

A.1.

Preparation of the first titanium catalytic solid

Magnesium diethylate was reacted at 150 ° C. for 4 hours with titanium tetrabutylate in such amounts that the molar ratio of titanium to magnesium was 2. The reaction product thus obtained was then chlorinated and precipitated by contacting it with a solution of ethylaluminum dichloride in such a quantity that the Al / Mg molar ratio was 6.5, for 90 minutes at 45.degree. ° C. The solid thus obtained comprised 15.8% by weight of Ti, 36.0% by weight of Cl, 2.2% by weight of Al and 4.4% by weight of Mg.

A.2.

Preparation of the second catalytic solid titanium and zirconium

Magnesium diethylate was reacted for 4 hours at 150 ° C. with titanium tetrabutylate and with zirconium tetrabutyalte in amounts such that the Ti / Mg molar ratio was 0.6 and the Zr / mol ratio was Ti is equal to 2. Then, the reaction product thus obtained was chlorinated and precipitated by contacting it with a solution of isobutylaluminum dichloride in such a quantity that the Al / Mg molar ratio was 14, first at 45 ° C. then at 60 ° C. The solid thus obtained comprised 5.4% by weight of Ti, 16.3% by weight of Zr, 52.6% by weight of Cl, 2.4% by weight of Al and 4.1% by weight of Mg. .

A3.

Preparation of the mixture

The solid obtained in A was mixed with the solid obtained in B at such amounts that the molar ratio of titanium from the first titanium catalytic solid from the second catalytic solid was 1.5.

B.

Polymerization of ethylene into two reactors

Ethylene was polymerized in an installation comprising two reactors arranged in series. Hexane, triethylaluminum as cocatalyst, ethylene and hydrogen in a hydrogen / ethylene mole ratio of 0.32 and the catalytic solid mixture obtained in A were continuously fed to the first reactor. .3. Total reactor pressure and temperature were held constant at 3.2 MPa and 85 ° C, respectively. The polymerization medium of the first reactor was continuously removed from the first reactor and transferred to the second reactor, which is further fed with ethylene, hydrogen in a hydrogen / ethylene molar ratio of 0.0185 and butene in a molar ratio. butene / ethylene of 0.35. The total pressure in the reactor was 3.0 MPa. The temperature in the second reactor was 75 ° C. The productivity P was 111. The polymer obtained had the following characteristics:<dl id="dl0005" compact="compact"><dt>MID₂</dt><dd>= 0.32</dd><dt>η</dt><dd>= 15300</dd><dt>T_G</dt><dd>= 1.43</dd><dt>ESCR</dt><dd>= 109</dd><dt>MVS</dt><dd>= 956.4.</dd></dl>