GB2034337A - Preparation of ethylene-propylene - Google Patents

Abstract

A soft or semi-hard ethylene- propylene copolymer having a melt index of 0.01 to 10 and a density of 0.850 to 0.910 is obtained by copolymerising ethylene, 15 to 250 mol% thereof of propylene, and optionally a diene in a substantially solvent-free vapor phase condition and in the presence of a catalyst consisting of a solid substance and an organoaluminium compound, said solid substance containing a magnesium-containing inorganic solid compound and a titanium compound and/or a vanadium compound.

Description

SPECIFICATION Process for preparing a copolymer This invention relates to a process for preparing a low density ethylene copolymer by the vapor phase polymerization process using a Ziegler catalyst of high activity.

More particularly, this invention is concerned with a process for preparing a soft or semi-hard ethylene-propylene copolymer having a melt index of 0.01 to 10 and a density of 0.850 to 0.910, characterised in that ethylene and 1 5 to 250 mol% thereof of propylene are copolymerized in a substantially solvent-free vapor phase condition and in the presence of a catalyst consisting of a solid substance and an organoaluminium compound, said solid substance containing a magnesium-containing inorganic solid compound and a titanium compound and/or a vanadium compound.

Polyethylenes obtained by polymerization using a catalyst consisting of a transition metal compound and an organometallic compound are generally prepared by the slurry polymerization process, and usually produced are limited to those having a density not lower than 0.945 which value is considered to be the limit of preventing polymer deposition or fouling on the inner wall or stirrer in the reactor inside at the time of polymerization.

Medium or low density polyethylene having a density below 0.945 g/cm3 are usually prepared by the so-called high pressure process using a radical catalyst. Quite recently, however, there has also been tried a high temperature solution process using a Ziegler catalyst. Also tried is to copolymerize ethylene with other a-olefin using a vanadium compound to prepare an elastomer.

However, polymers prepared by the aforesaid processes are either crystalline resins or amorphous elastomers, and their characters are definite. These polyolefin series plastics and elastomers each exhibit excellent performances and are used in various applications.

In some applications, however, we often experience that an improvement in the resistance to environmental stress cracking is desired by somewhat imparting an elastomeric character to plastics for example, or conversely a strength based on crystallinity is required of elastomers. It is well known, however, that if both components are mixed together in an attempt to achieve such objects, it will in many cases result in deterioration in physical properties such as tensile strength and rigidity.

However, if it is possible to prepare a soft or semi-hard resin which resin itself is neither a crystalline plastic nor an elastomer, having an intermediate structure, and exhibits a highgrade of elongation, the said resin itself will become suited for the above-mentioned objects, or by mixing it with other plastics it is made possible to impart an elastomeric character to the plastics whereby the latter can be improved in properties. But such a soft or semi-hard resin is not well known.

Recently there have been made some reports on the method of producing a resin which exhibits such an intermediate physical property. But those proposed methods have various drawbacks and their industrial application involves many problems to be solved.

For example, Japanese Patent Publication No.11028/71 discloses a solution polymerization using an aromatic hydrocarbon solvent in the production of an ethylene-a-olefin copolymer. This method, however, is disadvantageous in that the catalytic efficiency is low and, because of a solution polymerization, it is troublesome to separate and recover solvent.

Japanese Patent Publication No.

26185/72 proposes a method of copolymerizing ethylene and an a-olefin using an aliphatic hydrocarbon halide as solvent. But this method has the drawback that molded articles of the said copolymer have sticky surfaces due to the production of a low molecular weight copolymer in a large amount, probably because the hydrocarbon halide solvent acts as a molecular weight adjuster. The said patent publication also discloses the use of lower hydrocarbons of C3 to C5 as solvent but the polymerization using these solvents requires an increase in reaction pressure in the presence of vapor pressures from these solvents; besides, in the solvent recovering step it is ncessary to compress and cool the recovered solvent for the liquefaction thereof.

Furthermore, Japanese Patent Laying Open Print No. 41784/76 discloses a slurry copolymerization of ethylene and butene-1. But this proposed method is also disadvantageous in that the polymerization temperature and the composition of starting materials are specified minutely and at values outside such specified range the slurry becomes milky or mushy resulting in the reactor operation and the transport of slurry becoming difficult.

In summary, the foregoing drawbacks are based on the low catalyst activity, the troublesomeness of separating and recovering solvent because of a solution polymerization, a large volume production of a low molecular weight copolymer because of a chain transfer with solvent and, in the case of a slurry polymerization, the necessity of specifying the polymerization temperature and the starting materials composition for maintaining the slurried condition of polymer. As a further drawback associated with the above-proposed methods, the comonomer to be copolymerized has to be provided in an extremely large amount.

Through many studies made recently about the improvement of catalyst activity, it is known that if a transition metal is attached to a magnesium-containing solid carrier, e.g.

MgO, Mg(OH)2, MgCI2, MgCO3 and Mg(OH)Cl, and then combined with an organometallic compound, the resulting catalyst system can act as a catalyst of an extremely high activity in the olefin polymerization. It is also known that the reaction product of an organomagnesium compound, e.g. RMgX, R2Mg and RMg(OR), and a transition metal compound, can act as a superior, high polymerization catalyst for olefins (see, for example, Japanese Patent Publication No. 12105/64, Belgian Patent No. 742,112, Japanese Patent Publications No. 12105/64, Belgian Patent No. 742,112, Japanese Patent Publications Nos. 13050/68 and 9548/70).

However, even if such high activity catalysts with carrier are used in order to attain the lowering in density according to the slurry polymerization or solution polymerization, the foregoing drawbacks heretofore have not been remedied at all.

This invention provides a new process which has solved all the above-described problems associated with the solutions or slurry polymerization such as low activity, low bulk density, polymer adhesion to reactor and the production of coarse particles, whereby an ethylene-propylene copolymer having excellent physical properties could be obtained.

According to the process of this invention, a vapor phase polymerization reaction can be conducted in an extremely stable manner and the catalyst removing step can be omitted, so there could be completed a vapor phase polymerization process for ethylene and propylene which process as a whole is very simple.

In more particular terms, this invention relates to a process for preparing an ethylenepropylene copolymer having a melt index of 0.01 to 10 and a density of 0.850 to 0.910 by contacting in vapor phase condition a mixture of ethylene and 1 5 to 250 mol% thereof of propylene with a catalyst consisting of a solid substance and an organoaluminium compound, said solid substance containing a magnesium-containing inorganic solid compound and a titanium compound and/or a vanadium compound.It has become clear that if a vapor phase polymerization reaction is carried out according to the process of this invention, that is, using ethylene and propylene in a quantitative ratio within the range specified herein and in the presence of a catalyst consisting of a solid substance and an organoaluminium compound, said solid substance containing a magnesium-containing inorganic solid compound and a titanium compound and/or a vanadium compound, such vapor phase polymerization reaction can be made very stably in an extremely high activity and, despite high stickiness and low density of the resulting polymer, with reduced production ratio of coarse or ultra-fine particles, improved particle properties, increased bulk density and minimized adhesion to reactor and conglomeration of polymer particles.It cannot help being considered quite unexpected and surprising that according to the process of this invention, not only a vapor phase polymerization reaction can be carried out extremely smoothly, but also a low density ethylene copolymer can be obtained easily.

In this invention, the copolymerization can be conducted even at relatively low temperatures to easily afford a low density ethylene copolymer, which is advantageous for preventing the adhesion to reactor and conglomeration of product. This point is another advantage of the invention. According to the process of this invention, moreover, a low density ethylene copolymer of a high melt index can be obtained easily, and this point is a further advantage of the invention. Thanks to these advantages, as previously explained, there can be obtained such a soft or semi-hard copolymer as referred to herein efficiently by a vapor phase polymerization.

The propylene to be copolymerized with ethylene in the process of this invention adjusts the density and molecular weight of copolymer, and the resulting copolymer has a high transparency, good appearance and gloss, and its flexibility and rubber-like elasticity are superior at low temperatures, not to mention at room temperatures.

The copolymer obtained according to the process of this invention, despite such a flexibility, has a strength equal or even superior to that of conventional polyolefin resins. Furthermore, it scarcely contains unsaturated bond, residual catalyst or other impurities, so is very superior in weathering- and chemicals-resistance, as well as in electrical characteristics such as dielectric loss, bread-down voltage and resistivity. Also in impact resistance and resistance to environmental stress cracking, the copolymer prepared according to the process of this invention exhibits very superior characteristics. Thanks to these characteristics, the said copolymer can be formed into films, sheets, hollow containers, electric wires and various other products by known methods such as extrusion, blow, injection, press and vacuum moldings. And thus it can be used in various applications.

The copolymer prepared according to the process of this invention contains olefins as the component, so is very similar in composition to that of polyolefin resins; besides, because of a low crystallinity, it is compatible with other polyolefin resins, especially with high and low density polyethylenes, poiypropylenes and ethylene-vinyl acetate copolymer.

Blending it into these resins can improve resistance to impact, to coid and to environmental stress cracking.

The catalyst system used in this invention comprises the combination of a solid sub stance and an organoaluminium compound, said solid substance containing a magnesiumcontaining inorganic solid compound and a titanium compound and/or a vanadium compound. Such solid substance is obtained by attaching a titanium compound and/or a vanadium compound by a known method to an inorganic solid carrier typical of which are metallic magnesium, magnesium hydroxide, magnesium carbonate, magnesium oxide and magnesium chloride, or a double salt, double oxide, carbonate, chloride or hydroxide containing a metal selected from silicon, aluminium and calcium, and magnesium atom, or these inorganic solid carriers treated or reacted with an oxygen-containing compound, a sulfur-containing compound, a hydrocarbon or a halogen-containing substance.

To illustrate the titanium compound and/or vanadium compound referred to herein, mention may be made of halides, alkoxyhalides, oxides and halogenated oxides of titanium and/or vanadium, e.g. tetravalent titanium compounds such as titanium tetrachloride, titanium tetrabromide, titanium tetraiodide, monoethoxytrichlorotitanium, diethoxydichlorotitanium, thiethoxymonochlorotitanium,tetraethoxytitanium, monoisopropoxytrichlorotitanium, diisopropoxydichlorotitanium, and tetraisopropoxytitanium; various titanium trihalides obtained by reducing titanium tetrahalides with hydrogen, aluminium, titanium or an organometallic compound; trivalent titanium compounds such as those obtained by reducing various tetravalent alkoxytitanium halides with an organometallic compound; tetravalent vanadium compounds, e.g. vanadium tetrachloride; pentavalent vanadium compounds such as vanadium trichloride and vanadium triethoxide.

Among the above-exemplified titanium compounds and vanadium compounds, tetravalent titanium compounds are specially preferred.

The catalyst used in this invention comprises the combination of a solid substance, which is obtained by attaching a titanium compound and/or a vanadium compound to the solid carriers previously exemplified, and an organoaluminium compound.

By way of illustrating preferred catalyst systems, mention may be made of an organoaluminium compound combined with the following solid substances (the R in the following formulae represents an organic radical and X represents halogen): MgO-RX-TiCI4 system (see Japanese Patent Publication No.

3514/76), Mg-SiCI4-ROH-TiCI4 system (see Japanese Patent Publication No. 23864/75), MgCI2-Al(OR)3-TiCI4 system (see Japanese Patent Publications Nos. 152/76 and 15111 /77), MgCI2-SiCI4-ROH-TiCI4 system (see Japanese Patent Laying Open Print No.

106581 /74), Mg(OOCR)2-Al(OR)3-TiCl4 system (see Japanese Patent Publication No.

11710/77), Mg-POCI3-TiCI4 system (see Japanese Patent Publication No.153/76), and MgCI2-AIOCI-TiCI4 system (see Japanese Patent Laying Open Print No. 133386/76).

In these catalyst systems, a titanium compound and/or a vanadium compound may be used as the addition product with an organocarboxylic acid ester, or the foregoing magnesium-containing inorganic solid carriers may be contacted with an organocarboxylic acid ester before use. Also, an organoaluminium compound may be used as the addition product with an organocarboxylic acid ester, and this causes no trouble. Furthermore, in every case in this invention, the catalyst system used in the invention may be prepared in the presence of an organocarboxylic acid ester without causing any trouble.

As the organocarboxylic acid ester there may be used various aliphatic, alicyclic and aromatic carboxylic acid esters, and preferably aromatic carboxylic acids of C7 to C,2, for example, alkylesters such as methyl and ethyl of benzoic acid, anisic acid and toluic acid.

Examples of an organoaluminium compound used in the invention are those represented by the general formulae R3AI, R2AIX, RAIX2, R2AIOR, RAI(OR)X and R3AI2X3 wherein R is C, to C20 alkyl or aryl, X is halogen and R may be same or different, such as triethylaluminium, triisobutylaluminium, trihexylaluminium, trioctylaluminium, diethylaluminium chloride, ethylaluminium sesquichloride, and mixtures thereof.

The amount of an organoaluminium compound to be used in the invention is not specially limited, but usually it is in the range of from 0.1 to 1000 mols per mol of a transition metal compound.

In the polymerization reaction, a mixture of ethylene and propylene is allowed to polymerize in vapor phase. A known type of reactor, such as a fluidized bed or an agitation vessel, may be used.

Polymerization conditions involve temperatures usually ranging from 20 to 110"C, preferably from 50 to 100"C, and pressures from atmospheric pressure to 70 kg/cm2.G, preferably from 2 to 60 kg/cm2G. The molecular weight may be adjusted by changing the polymerization temperature, the molar ratio of catalyst or the amount of comonomer, but the addition of hydrogen into the polymerization system is more effective for this purpose. Of course, using the process of this invention there may be carried out, without any trouble, two or more stage polymerization reactions involving different polymerization conditions such as different hydrogen and comonomer concentrations and different polymerization temperatures.

In this invention, moreover, the foregoing catalyst systems may be contacted with an CL- olefin and thereafter used in the vapor phase polymerization reaction, whereby the polymerization activity can be largely improved and the operation performed more stably than in untreated condition. In this case, various aolefins may be used, preferably those having 3 to 12 carbon atoms and more preferably those having 3 to 8 carbon atoms, e.g. propylene, butene-1, pentene-1, 4-methylpentene1, heptene-1, hexene-1, octene-1, and mixtures thereof. The temperature and time of contact between the catalyst used in the invention and an a-olefin can be selected in a wide range, for example, from 0" to 200"C, preferably from 0" to 110"C, and from 1 minute to 24 hours.

The amount of an a-olefin to be brought into contact with the catalyst can also be selected in a wide range, but usually it is desired that the catalyst be treated with the aolefin in an amount of ranging from 1 g. to 50,000 g., preferably from 5 g. to 30,000 g., per gram of the aforesaid solid substance and that 1 g. to 500 g. of the a-olefin per gram of the solid substance be reacted. In this case, the pressure may be selected optionally, but desirably it is in the range of from - 1 to 100 kg/cm2G.

In the treatment with an a-olefin, the total amount of an organoaluminium compound to be used may be combined with the foregoing solid substance and then contacted with the a-olefin, or part of the organoaluminium compound may be combined with the solid substance, then contacted with the a-olefin in gaseous state and thereafter the remaining portion of the organoaluminium compound may be added separately in the vapor phase polymerization of ethylene. In the contact treatment for the catalyst with an a-olefin there may be present, without any trouble, a hydrogen gas or other inert gas such as nitrogen, argon or helium.

The amount of propylene used in the process of this invention may range from 1 5 to 250 mol%, preferably from 60 to 250 mol% based on the amount of ethylene. Outside this range, it is impossible to obtain the object product of this invention, namely an ethylenepropylene copolymer having a melt index of 0.01 to 10 and a density of 0.850 to 0.910.

The amount of propylene to be used can be easily adjusted according to the composition ratio of the vapor phase in the polymerization vessel.

In the copolymerization according to the process of this invention, moreover, there may be added various dienes as termonomers, such as butadiene, 1 ,4-hexadiene, 1 ,5-hexadi- ene, vinylnorbornene, ethylidenenorbornene and dicyclopentadiene.

Working examples of this invention are given below, but it is to be understood that these examples are for illustration only for working the invention and are not intended to place limitation thereon.

EXAMPLE 1 100 g. of anhydrous magnesium chloride, 50 g. of 1 ,2-dichloroethane and 1 70 g. of titanium tetrachloride were subjected to ball milling for 1 6 hours at room temperature in a nitrogen atmosphere to allow the titanium compound to be attached to the carrier. The resulting solid substance contained 35 mg. of titanium per gram thereof.

A stainless steel autoclave was used as the apparatus for the vapor phase polymerization, and there were used a blower, a flow rate regulating valve and a dry cyclone for separation of the resulting polymer, to form a loop.

And the temperature of the autoclave was controlled by passing a warm water through jacket.

The solid substance prepared above and triethylaluminium were fed into the autoclave at the rates of 250 mg/hr and 50 mmol/hr, respectively, and a polymerization was made while making adjustment so that the proportions (in molar ratio) of ethylene, propylene and hydrogen in the gases fed into the autoclave with the blower were 55%, 35% and 10%, respectively.

The resulting polymer had a melt index (Ml) of 1.1, a bulk density of 0.375 and a density of 0.895. Although the density was very low, the polymer was unsticky and the greater part thereof was composed of powders with particle sizes ranging from 300 to 600,u. The polymerization activity was very high, 269,000 gpolyethylene/gTi.

After 10 hours of continuous operation, the polymerization was discontinued and the interior of the autoclave was checked to find that there was no polymer adhesion to the inner wall, stirrer and polymer withdrawing pipe.

That is, although in the slurry polymerization shown in Comparative Example 1 below it was impossible to effect a continuous operation stably for a long time, it is now apparent that according to the process of this invention, a continuous operation can be made extremely stably for a long time.

The copolymer prepared above was pressed into shape and the resulting shaped article was transparent and unsticky on its surface, having a breaking point strength of 220 kg/cm2 and an elongation of 600%. Also, the copolymer could be granulated easily through an extruder. Furthermore, it could be formed into a film of uniform thickness by means of an inflation film forming machine for high pressure process polyethylene.

Comparative Example 1 A continuous slurry polymerization was carried out at 80"C using the same catalyst as that used in Example 1 and hexane as solvent.

Hexane containing 5 mg/l of the solid substance and 1 mmol/l of triethylaluminium, ethylene, propylene (80 mol% of ethylene) and hydrogen were fed at the rates of 40 I/hr, 8 kg/hr, 9.6 kg/hr and 3Nm3/hr, respectively, and a continuous polymerization was conducted.

The resulting polymer was continuously withdrawn as slurry, but the polymer particles of the withdrawn slurry were remarkably swollen from the beginning of polymerization, and the hexane layer was milky. After 2 hours, the slurry withdrawing pipe was obturated, so the polymerization was compelled to be discontinued. The interior of the reactor was checked to find that a large amount of polymer adhered to the inner wall and also to the stirrer.

The polymer prepared above had a Ml of 1.4, a bulk density of 0.253 and a density of 0.930. Thus, despite of a very large amount of the comonomer propylene added, the density of the resulting polymer was not sufficiently lowered, it being apparent that this is an example of a very disadvantageous polymerization.

The copolymer after subjected to press forming had a sticky surface, whose breaking point strength was 1 60 kg/cm2 and elongation 800%.

EXAMPLE 2 830 g. of anhydrous magnesium chloride, 50 g. of aluminium oxychloride and 1 70 g. of titanium tetrachloride were subjected to ball milling in the same manner as in Example 1.

The resulting solid substance contained 41 mg. of titanium per gram thereof.

The solid substance just prepared above and triethylaluminium were fed at the rates of 200 mg/hr and 50 mmol/hr, respectively, and a polymerization was made at 80"C in the same way as in Example 1 with the proviso that the proportions of ethylene, propylene and hydrogen in the vapor phase were 45:40:15 (in molar ratio), respectively.

After 10 hours of continuous operation, the polymerization was discontinued and the interior of the reactor was checked to find no polymer adhesion therein.

The resulting polymer had a Ml of 2.9, a bulk density of 0.411 and a density of 0.865, and the polymerization activity was 318,000 gpolymer/gTi. The copolymer after subjected to press forming was transparent and its surface was unsticky, having a breaking point strength of 1 90 kg/cm2 and an elongation of 700%.

The copolymer was granulated through an extruder, then blended with a high density polyethylene in the form of pellets and formed into a film. With 10% addition thereof, the haze value of the film 30ju thick dropped from 63% to as low as 10%.

Comparative Example 2 Using the same catalyst as that used in Example 2 and n-paraffin as solvent, there was carried out a continuous solution polym erization.

That is, n-paraffin containing 25 mg/l of the solid substance prepared in Example 2 and 5 mmol/l of triethylaluminium was fed at the rate of 40 I/hr, and also added were ethylene, propylene (117 mol% of ethylene) and hydrogen at the rates of 8 kg/hr, 14 kg/hr and 0.1 Nm3/hr, respectively, and a continuous polymerization was made at 160"C.

The resulting copolymer had a Ml of 1.5 and a density of 0.931, and the polymerization activity was 97,000 gpolymer/gTi.

Thus, despite of a large excess of propylene used with respect to ethylene, the density of the resulting copolymer is not lowered so much, and the polymerization activity is low, it being apparent that this is an example of an inefficient polymerization. The copolymer after subjected to press forming had a breaking point strength of 1 70 kg/cm2 and an elongation of 480%.

EXAMPLE 3 830 g. of anhydrous magnesium chloride, 1 20 g. of anthracene and 1 80 g. of titanium tetrachloride were subjected to ball milling in the same manner as in Example 1. The resulting solid substance contained 40 mg. of titanium per gram thereof.

Using the same apparatus as that used in Example 1 and at a temperature of 80"C, the solid substance prepared above and triisobutylaluminium were fed at the rates of 500 mg/hr and 1 50 mmol/hr, respectively, and a continuous polymerization was made while making adjustment so that the proportions of ethylene, propylene and hydrogen in the vapor phase were 43:42:1 5 (in molar ratio), respectively. After completion of the polymerization the reactor was opened to find no polymer adhesion therein.

The resulting polymer had a Ml of 2.3, a bulk density of 0.398 and a density of 0.880, and the polymerization activity was 141,000 gpolymer/gTi. The copolymer after subjected to press forming was transparent and its surface was unsticky, having a breaking point strength of 1 90 kg/cm2 and an elongation of 850%.

EXAMPLE 4 400 g. of magnesium oxide and 1.3 kg. of aluminum chloride were reacted together for 4 hours at 300"C, then 950 g. of the reaction product and 1 80 g. of titanium tetrachloride were treated in the same manner as in Example 1 to give a solid substance which contained 39 mg. of titanium per gram thereof.

Using the same apparatus as that used in Example 1, there were fed as catalyst the solid substance just prepared above and triisobutylaluminum at the rates of 500 mg/hr and 250 mmol/hr, respectively, and further introduced were ethylene, propylene and hydrogen so that they were 60 :35:5 respectively in the vapor phase, and a continuous polymerization was made at 80"C.

After 1 6 hours of continuous operation, the polymerization was discontinued and the inter ior of the reactor was checked to find no polymer adhesion therein.

The resulting polymer was composed of nearly true spherical particles of a narrow particle size distribution with an average particle diameter of 600ju, and had a bulk density of 0.384, Ml of 0.65 and a density of 0.908.

The polymerization activity was 246,000 g polymer /g Ti. The copolymer after subjected to press forming was transparent and its surface was unsticky, having a breaking point strength of 240 kg/cm2 and an elongation of 650%.

Claims (16)

1. A process for preparing a soft or semihard ethylenepropylene copolymer having a melt index of 0.01 to 10 and a density of 0.850 to 0.910, characterized in that ethylene and 1 5 to 250 mol% thereof of propylene are copolymerized in a substantially solvent-free vapor phase condition and in the presence of a catalyst consisting of a solid substance and an organoaluminium compound, said solid substance containing a magnesium-containing inorganic solid compound and a titanium compound and/or a vanadium compound.

2. A process according to claim 1, in which said titanium compound and/or vanadium compound is a halide, alkoxyhalide, oxide, or halogenated oxide of titanium and/or vanadium.

3. A process according to claim 1 or claim 2 in which said magnesium-containing inorganic solid compound is selected from metallic magnesium, magnesium hydroxide, magnesium carbonate, magnesium oxide and magnesium chloride.

4. A process according to claim 1 or claim 2 in which said magnesium-containing inorganic solid compound is selected from double salt, double oxide, carbonate, chloride and hydroxide containing magnesium atom and a metal selected from silicon, aluminium and calcium.

5. A process according to any one of claims 1 to 4 in which said magnesiumcontaining inorganic solid compound is further treated or reacted with an oxygen-containing compound, a sulfur-containing compound, a hydrocarbon and halogen-containing substance.

6. A process according to any one of claims 1 to 5 in which said titanium compound and/or vanadium compound is used as the addition product with an organocarboxylic acid ester.

7. A process according to any one of claims 1 to 6 in which said magnesiumcontaining inorganic solid compound is contacted with an organocarboxylic acid ester before use.

8. A process according to any one of claims 1 to 7 in which said organoaluminium compound is used as the addition product with an organocarboxylic acid ester.

9. A process according to any one of claims 1 to 8 in which said catalyst system is prepared in the presence of an organocarboxylic acid ester.

10. A process according to claim 6, 7, 8 or 9 in which said organocarboxylic acid ester is selected from alkyl-esters of benzoic acid, anisic acid and toluic acid.

11. A process according to any one of claims 1 to 10 in which said copolymerization is carried out at a temperature in the range of from 20 to 110"C and at a pressure in the range of from atmospheric to 70 kg/cm2G.

1 2. A process according to any one of claims 1 to 11 in which said copolymerization is carried out in the presence of hydrogen.

1 3. A process according to any one of claims 1 to 1 2 in which, before initiation of the copolymerization, said catalyst system is contacted with an a-olefin having 3 to 1 2 carbon atoms for 1 minute to 24 hours at a temperature in the range of from 0 to 200"C and at a pressure in the range of from - 1 to 100 kg/cm2-G.

14. A process as claimed in claim 1, substantially as hereinbefore described with particular reference to the Examples.

1 5. A process as claimed in claim 1, substantially as illustrated in any one of the Examples.

16. A copolymer of ethylene and propylene when prepared by the process claimed in any one of the preceding claims.

1 7. An article fabricated from the copolymer claimed in claim 16.