GB2051094A - Process for Preparing Ethylene Copolymers - Google Patents

Abstract

Medium and low density ethylene copolymers are obtained by the copolymerization of ethylene and an alpha -olefin using a catalyst composed of (1) a solid component obtained by supporting a titanium compound or a vanadium compound on a magnesium-containing inorganic solid carrier and (2) an organoaluminium compound; a first stage of the copolymerization being carried out at a temperature of 0 DEG to 120 DEG C and in the presence of a C3 or C4 inert hydrocarbon to produce a 0.1 to 60% by weight based of the amount of ethylene copolymer to be finally produced at a catalyst activity of 100 to 10,000 g. copolymer per gram of the solid catalyst component and a second stage of the copolymerization being carried out in vapor phase while evaporating the hydrocarbon.

Description

SPECIFICATION Process for Preparing Polyolefins This invention relates to a new process for preparing polyolefins. More particularly, it is concerned with a process for preparing medium and low density ethylene copolymers in high efficiency in which there is first made a copolymerization of ethylene and an a-olefin according to the slurry polymerization process and then the copolymerization is further conducted according to the vapor phase polymerization process.

Conventional methods of preparing polyethylenes include a slurry polymerization of ethylene (hereinafter referred to as the "slurry polymerization process") which is carried out in a hydrocarbon solvent such as propane, butane, hexane or heptane and at a temperature below 1 200C at which polyethylenes do not dissolve, usually below 1 000C, and a solution polymerization of ethylene (hereinafter referred to as the "solution polymerization process") which is carried out at a temperature above 1 200C at which polyethylenes dissolve, usually above 1 30 C. These processes are widely adopted industrially.

The so-called high density polyethylenes with a density above 0.945 usually are prepared by the homopolymerization of ethylene or by the copolymerization of ethylene and a small amount of an a-olefin. In many cases in these processes, by-production of polymers soluble in solvent is low, the bulk density of the resulting polymers is not much reduced, and the fouling of the reactor wall is minimized; furthermore the increase in viscosity and stickiness of the polymerization system are reduced. Consequently, an industrial production of such high density polyethylenes does not involve any particularly technical problem.

On the other hand, when preparing the socalled medium and low density ethylene copolymers with a density below 0.945 by the use of a large amount of an a-olefin, there are byproduced solvent-soluble polymers in large amounts, so that tha bulk density of the resulting polymers is reduced, the fouling of the reactor wall becomes conspicuous, and the polymerization system is liable to become more viscous and sticky. According to the slurry polymerization process, therefore, it is very difficult to industrially prepare medium and low density ethylene copolymers.

In the solution polymerization process, the resulting polymers are in a dissolved state and there is used a polymer solution handling process, so the problems such as fouling to the reactor wall and stickness of the polymerization do not arise. However, since a high viscosity solution is stirred, the higher the molecular weight grade with a small melt index the lower must be the polymer concentration, resulting in the productivity being lowered. Therefore, it is practically impossible to industrially produce ethylene copolymers of a high molecular weight grade with a melt index below 0.1.

Also known as a preparation method for polyethylenes is a vapor phase polymerization in which, however, usually it is difficult to remove the polymerization heat, there easily occurs flocculation or coagulation of polymer within the reaction vessel, and further there often occur serious troubles in continuous operation such as the plugging of pipe and discontinuance of agitation.

This invention reduces or eliminates the aforesaid drawbacks associated with the prior art processes in the preparation of medium and low density ethylene copolymers and provides a new process for preparing medium and low density ethylene copolymers whereby the polymerization reaction can be carried out extremely smoothly and the physical properties of the resulting polymer can be adjusted easily and in a wide range.

According to this invention there is provided a preparation process for polyolefins such that, in the copolymerization of ethylene and an a-olefin of Cs to C10 using a catalyst which comprises (1) a solid component obtained by supporting on a magnesium-containing inorganic solid carrier at least one compound selected from titanium compounds and vanadium compounds and (2) an organoaluminium compound, there is made in a first stage a copolymerization of ethylene and an a-olefin having 3 to 10 carbon atoms at a temperature of 00 to 1200C and in the presence of an inert hydrocarbon having 3 to 4 carbon atoms to produce a copolymer in an amount of 0.1 to 60% by weight based on the amount of an ethylene copolymer to be finally produced at a catalyst activity of 100 to 10,000 g. copolymer per gram of the solid catalyst component and, there is further made in a second stage the copolymerization of the remaining and optionally newly added ethylene and an a-olefin of C3 to C,O in vapor phase and in a substantial absence of liquid solvent, whereby medium and low density ethylene copolymers with a density of 0.900 to 0.945 are produced.

According to the process of this invention, the polymerization reaction in each stage can be conducted smoothly and in extremely high activity without the problem of fouling within the reaction vessel in the first-stage slurry polymerization and without the problem of polymer coagulation in the second-stage vapor phase polymerization. Furthermore, the inert hydrocarbon used as solvent in the first stage can further be used as quench liquid for the removal of heat in the next stage vapor phase polymerization, so that there can be produced medium and low density ethylene copolymers in an extremely economical manner.In addition, the physical properties of the final products obtained by the process of this invention cover a wide range; that is, the molecular weight, molecular weight distribution, density, and branch distribution can be changed in a wide range by suitably selecting the conditions for the slurry polymerization and for the vapor phase polymerization, whereby there can be prepared medium and low density ethylene copolymers having excellent physical properties for example in impact resistance, environmental stress cracking resistance and processability. As compared with a single slurry polymerization process or a single vapor phase polymerization process, the process of this invention involving slurry polymerization and subsequent vapor phase polymerization is, as a whole, an extremely superior preparation process for medium and low density ethylene copolymers.

In the process of this invention there is carried out in the first stage a siurry polymerization in the presence of an inert hydrocarbon having 3 to 4 carbon atoms. Examples of such an inert hydrocarbon are those having 3 to 4 carbon atoms such as propane, n-butane, i-butane, transand cis-butene-2. Of course, there may be contained small amounts of other inert hydrocarbon solvents such as pentane and hexane. The polymerization temperature is in the range of from 0 to 1 2O0C, preferably from 0 to 900C and more preferably from 100 to 7O0C. As to the polymerization pressure, the range of from atmospheric pressure to 70 kg/cm2.G is preferred.

The molecular weight of the resulting polymer can 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.

a-Olefins having 3 to 10 carbon atoms are desirable as (z-olefins to be used as comonomer, for example propylene, butene-1, hexene-1, 4 methylpentene-1, heptene-1, octene-1, and decene-1, and mixtures of these are also employabie. The amount of such at-olefins to be used is in the range of from 1 to 250 mole%, preferably from 2 to 200 mole%, based on the amount of ethylene used.

In the process of this invention it is essential for attaining the objects of the invention that in the stage of slurry polymerization reaction there be produced a copolymer in an amount of 0.1 to 60% by weight based on the amount of an ethylene copolymer to be finally produced at a catalyst activity of 100 to 10,000 g. copolymer per gram of the solid catalyst component. And the density of the polymer resulting from the slurry polymerization is desirably in the range of from 0.900 to 0.945.

After completion of the slurry polymerization, the slurry thus obtained is introduced into the subsequent vapor phase polymerization reaction zone and the vapor phase polymerization reaction is carried out under vapor phase polymerization conditions in a substantial absence of liquid solvent.

The slurry polymerization and the vapor phase polymerization may be conducted using the same reaction vessel, or more preferably both polymerizations may be carried out in such a manner that firstly the slurry polymerization is made in one or more than one reaction vessels and then the vapor phase polymerization is made in a separate one or more than one reaction vessels.

Where the slurry polymerization and the vapor phase polymerization are to be carried out in the same vessel, the slurry polymerization is first made and then the polymerization system is brought under vapor phase polymerization conditions to allow vapor phase polymerization to take place in a substantial absence of a liquid solvent while allowing the hydrocarbon solvent to be evaporated outside the system. The hydrocarbon solvent which has left the polymerization system is liquefied, and a portion of the liquefied solvent is recycled to the vapor phase polymerization vessel. Unreacted olefins are also recycled to the vapor phase polymerization vessel.

On the other hand, where the slurry polymerization and the vapor phase polymerization are to be carried out in separate vessels, the slurry polymerization is made in a first stage and then the slurry solution containing polymer is introduced to the second stage where a vapor phase polymerization is allowed to take place together with evaporation of unreacted olefins and the hydrocarbon solvent. The hydrocarbon solvent which has left the polymerization system is liquefied, and at least a portion of the liquefied solvent is recycled to the vapor phase polymerization vessel. Unliquefied materials such as unliquefied ethylene and hydrogen are usually recycled to the vapor phase polymerization vessel together with separately added ethylene, a-olefin and hydrogen.

Specially preferred is a direct continuous operation such that the polymer-containing slurry resulting from the slurry polymerization is introduced directly into a vapor phase polymerization vessel which is placed under vapor phase polymerization conditions, within which vessel the hydrocarbon solvent contained in the slurry is evaporated, and the resulting vapor leaving the vapor phase polymerization vessel is subjected to a liquefying treatment and at least a portion of the liquefied hydrocarbon is recycled as a quench liquid into the vapor phase polymerization vessel, while unliquefied olefins are recycled into the vapor phase polymerization vessel together with newly added ethylene and a- olefin. Any type of vapor phase polymerization apparatus may be used, including conventional ones such as an agitation vessel and a fluidized bed.

In the vapor phase polymerization reaction no further catalyst is usually added, but it is recommended to add further ethylene and ar- olefin.

The conditions for the vapor phase polymerization reaction involve temperatures in the range usually from 20 to 1 00C and preferably from 500 to 1 000C, and pressures from atmospheric pressure to 70 kg/cm2.G, preferably from 2 to 60 kg/cm2.G. The adjustment of molecular weight can be made by changing the polymerization temperature, the molar ratio of catalyst, or the amount of comonomer used, but a more effective method is to add hydrogen into the polymerization system.

Preferred c4-olefins used as comonomer in the vapor phase polymerization reaction are those having 3 to 10 carbon atoms, and they may be the same as or different from the cr-olefin used in the initial stage of slurry polymerization. And the amount thereof is 1 to 250 mole%, preferably from 2 to 200 mole%, based on the amount of ethylene used.

The catalyst system used in the process of this invention comprises the combination of a solid component, which is obtained by supporting on a magnesium-containing inorganic solid carrier at least one compound selected from titanium compounds and vanadium compounds, and an organoaluminium compound. Examples of the said magnesium containing inorganic solid carrier are metallic magnesium, magnesium hydroxide, magnesium carbonate, magnesium oxide, and magnesium chloride; further, double salts, double oxides, carbonates, chlorides and hydroxides containing a metal selected from silicon, aluminium and calcium and a magnesium atom; and also these inorganic solid carriers after treatment or reaction with an oxygen-containing compound, a sulfur-containing compound, a hydrocarbon, or a halogen-containing substance.

By way of illustrating the above oxygencontaining compound, mention may be made of organic oxygen-containing compounds such as water, alcohols, phenols, ketones, aldehydes, carboxylic acids, esters, and acid amides, as well as inorganic oxygen-containing compounds such as metallic alkoxides and oxychlorides of metals.

Examples of the above sulfur-containing compound are organic sulfur containing compounds such as thiols and thio ethers, as well as inorganic sulfur-containing compounds such as sulfur dioxide, sulfur trioxide, and sulfuric acid. To exemplify the aromatic hydrocarbon, mention may be made of various monocyclic and polycyclic aromatic hydrocarbons such as benzene, toluene, xylene, anthracene, and phenanthrene. Examples of the halogen-containing substance are chlorine, hydrogen chloride, metallic chlorides, and organic halides.

In the process of this invention, a titanium compound and/or a vandium compound is attached by known means to a magnesium-containing inorganic solid carrier which has just been exemplified above, and the resulting product is used as one catalyst component.

As the titanium compound and vanadium compound as referred to herein, mention may be made of halides, alkoxyhalides, and halogenated oxides of titanium and vanadium, for example, tetravalent titanium compounds such as titanium tetrachloride, titanium tetrabromide, titanium tetraiodide, monoethoxytrichlorotitanium, diethoxydichlorotitanium, triethoxymonochlorotitanium, tetraethoxytitanium, monoisopropoxytrichlorotitanium, diisopropoxydichlorotitanium, and tetraisopropoxytitanium; titanium trihalides obtained by reducing titanium tetrahalides with hydrogen, aluminium, titanium or an organometallic compound; trivalent titanium compounds such as those obtained by reducing tetravalent alkoxytitanium halides with an organometallic compound; tetravalent vanadium compounds such as vanadium tetrachloride; pentavalent vanadium compounds such as vanadium oxytrichloride and orthoalkylvanadate; and trivalent vanadium compounds such as vanadium trichloride and vanadium triethoxide.

Preferably, the amount of a titanium compound and/or a vanadium compound to be supported is adjusted so that the titanium and/or vanadium content of the resulting solid catalyst component is in the range of from 0.5 to 10% by weight, the range of from 1 to 8% by weight being specially desirable in order to attain a well-balanced activity per titanium and/or vanadium and that per solid.

The catalyst used in the process of this invention comprises the combination of a solid catalyst component obtained by supporting a titanium compound and/or a vanadium compound on a solid carrier both exemplified above, and an organoaluminium compound.

For example, preferred catalyst systems are obtained by combining an organoaluminium compound with the following solid substances (in the formulae here shown R represents an organic radical): MgO-RX-TiCI4 system (see Japanese Patent Publication No. 3514/1976), Mg-SiCI4 ROH-TiCI4 system (see Japanese Patent Publication No. 23864/1975), MgCI2-Al(OR)3- TiCI4 system (see Japanese Patent Publications Nos. 152/1976 and 15111/1977), MgCI2-SiCI4 ROH-TiCI4 system (see Japanese Patent Laying Open Print No. 106581/1974), Mg(OOCR)2 Al(OR)3-TiCI4 system (see Japenese Patent Publication No. 11 710/1 977), Mg-POCI3-TiCI4 system (see Japanese Patent Publication No.

153/1976), MgCI2-AIOCI-TiCI4 system (see Japanese Patent Laying Open Print No.

133386/1976), MgCI2-polycyclic aromatic-TiCI4 system (see Japanese Patent Laying Open Prints Nos. 80382/1976 and 22080/1977), and MgCI2 RX-TiCI4 system (see Japanese Patent Laying Open Prints Nos. 42584/1977 and 65592/1977).

In the catalyst systems exemplified above the titanium compounds and/or vanadium compounds may be used as addition products with an organocarboxylic acid ester, and the foregoing magnesium-containing solid carriers may be used after their contact with an organocarboxylic acid ester, and further the use of organoaluminium compounds as addition products with an organocarboxylic acid ester would cause no trouble. Moreover, in every case where the process of this invention is applied, the catalyst system used therein may be one which has been prepared in the presence of an organocarboxylic acid ester.

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

By way of illustrating organoaluminium compounds used in the process of this invention, mention may be made of those represented by the general formulae R3AI, R2AIX, RAiX2,R2AIOR, RAI(OR)X and R3AI2X3 wherein R, which may be same or different, is an alkyl group of C, to C20 and X is a haiogen atom; for example, triethylaluminium, triisobutylaluminium, trihexylaluminium, trioctylaluminium, diethylaluminium chloride, ethylaluminium sesquichloride, and mixtures thereof.

The amount of an organoaluminium compound to be used in the process of this invention is not specially limited, but usually it is 0.1 to 1000 moles per mole of the transition metal compound.

Working examples of this invention are given below to describe the invention more concretely, but it is to be understood that the invention is not limited thereto.

Example 1 Into a first stage agitation type reaction vessel having a content volume of 5 liters were continuously charged 3.6 kg/hr of isobutane, 6.6 mmol/hr of triethylaluminium, and 0.26 g/hr of a solid component with titanium tetrachloride supported on a solid carrier containing anhydrous magnesium chloride as one component, in an added state into 300 ml/hr of hexane.

Also added continuously were 220 g/hr of ethylene, 40 g/hr or propylene, and 10 Nl/hr of hydrogen, and the reaction vessel was kept full of liquid under temperature and pressure conditions of 550C and 9.5 kg/cm2.G, respectively.

The slurried polymerization reaction mixture from the first stage reaction vessel was conducted by virtue of a differential pressure and through a pipe line into a second stage vapor phase reaction vessel having a reaction zone volume of 30 liters, where the isobutane and the hexane evaporated and a polymerization reaction was continued in vapor phase at 800C and at a total pressure of 9.0 kg/cm2.G, while hydrogen, ethylene and propyiene were introduced so as to maintain the hydrogen/ethylene mole ratio, propylene/ethylene mole ratio and the total pressure at 0.50, 0.25, and 9.0 kg/cm2.G, respectively.Removal of the polymerization heat was effected through the evaporation heat of the isobutane used as solvent in the first stage reaction vessel and also by cooling the gases within the vapor phase reaction vessel cyclically at a rate of 30 m3/hr. Furthermore, the recycle gases were taken out of the polymerization system as necessary so as not to allow isobutane and hexane to accumulate within the system.

The above polymerization reaction was continued for 72 hours, which could be carried out in an extremely stable manner to yield 170 kg. of polymer having a melt index of 2.5 and a density of 0.9320. The polymer powder was dry and easy to handle, the bulk density of which was 0.38 g/cm3.

Example 2 Into a first stage agitation type reaction vessel having a content volume of 5 liters were continuously charged 3.6 kg/hr of isobutane, 5.3 mmol/hr of triethylaluminium, and 0.1 8 g/hr of a solid component with titanium tetrachloride supported on a solid carrier containing anhydrous magnesium chloride as one component, in an added state into 300 ml/hr or hexane.

Also added continuously were 110 g/hr of ethylene, 28 g/hr of butene-1, and 6 Nl/hr of hydrogen, and the reaction vessel was kept full of liquid under temperature and pressure conditions of 500C and 9.5 kg/cm2.G, respectively.

The slurried polymerization reaction mixture from the first stage reaction vessel was conducted by virtue of a differential pressure and through a pipe line into a second stage vapor phase reaction vessel having a reaction zone volume of 30 liters, where the isobutane and hexane evaporated and a polymerization reaction was continued in vapor phase at 800C and at a total pressure of 9.0 kg/cm2.G, while hydrogen, ethylene and propylene were introduced so as to maintain the hydrogen/ethylene mole ratio, butene-l/ethylene mole ratio and the total pressure at 0.28, 0.30, and 9.0 kg/cm2.G, respectively.Removal of the polymerization heat was effected through the evaporation heat of the isobutane used as solvent in the first stage reaction vessel and also by cooling the gases within the vapor phase reaction vessel cyclically at a rate of 28 m3/hr. Furthermore, the recycle gases were taken out of the polymerization system as necessary so as not to allow isobutane and hexane to accumulate within the system.

The above polymerization reaction was continued for 65 hours, which could be carried out in an extremely stable manner, to yield 137 kg. of polymer having a melt index of 1.8 and a density of 0.9251. The polymer powder was dry and easy to handle, the bulk density of which was 0.37 g/cm3.

Example 3 Into a 3 liter stainless steel autoclave was placed 600 g. of butane, then was introduced 30 mg. of a solid catalyst component obtained by ball-milling 10 g. of anhydrous magnesium chloride, 0.5 g. of n-butyl chloride and 1.7 g. of titanium tetrachloride for 1 6 hours at room temperature in a nitrogen atmosphere, and was also introduced 1 mmol of triethylaluminium, thereafter the temperature was held at 200C resulting in that the pressure was found to be 1.5 kg/cm2.G. Then, after introducing hydrogen until the pressure was 1.9 kg/cm2.G, a mixed ethylenepropylene gas containing 15 mol% of propylene was introduced until the pressure was 2.1 kg/cm2.G, thereafter a polymerization was allowed to take place for 40 minutes while continuously introducing the said mixed ethylenepropylene gas so as to maintain the total pressure at 2.1 kg/cm2.G, resulting in that there was produced a polymer in an amount of 200 times by weight the amount of the solid catalyst component.

Then, purging off the butane, hydrogen was introduced at 700C until the pressure was 2.5 kg/cm2.G and further introduced a mixed ethylenepropylene gas containing 7 moi% of propylene until the pressure was 8.5 kg/cm2.G, thereafter a polymerization was made for 2 hours in vapor phase while introducing the said mixed ethylene-propylene gas containing 7 mol% propylene so as to maintain the total pressure at 8.5 kg/cm2.G. The vapor phase polymerization was carried out in an extremely stable manner to yield 144 g. of polymer with no agglomerated mass of polymer produced.

The polymer thus prepared had a melt index of 1.2, a density of 0.913, a flow parameter of 1.54 and a very high bulk density of 0.40.

Example 4 Into a 3 liter stainless steel autoclave was placed 470 g. of propane, then was introduced 20 mg. of a solid catalyst component obtained by bali-milling 10 g. of anhydrous magnesium chloride, 1.2 g. of anthracene and 2.4 g. of titanium trichloride for 1 6 hours at room temperature in a nitrogen atmosphere, and was also introduced 1 mmol of triethylaluminium, thereafter the temperature was held at 450C resulting in that the pressure was found to be 1 6 kg/cm2.G. Then, after introducing 500 ml. of hydrogen, a mixed gas of ethylene and butene-1 containing 1 3 mol% of butene-1 was introduced until the pressure was 18.5 kg/cm2.G, thereafter a polymerization was allowed to take place for 30 minutes while introducing the said mixed gas of ethylene and butene-1 so as to maintain the total pressure at 1 8.5 kg/cm2.G, resulting in that there was produced a polymer in an amount of 1000 times by weight the amount of the solid catalyst component.

Then, purging off the propane, hydrogen was introduced at 700C until the pressure was 8 kg/cm2.G and further introduced a mixed gas of ethylene and butene-1 containing 1 5 mol% of butene-1 until the pressure was 10 kg/cm2.G, thereafter a polymerization was made for 2 hours in vapor phase while introducing the said mixed gas of ethylene and butene-1 so as to maintain the total pressure at 10 kg/cm2.G. The vapor phase polymerization was carried out in an extremely stable manner to yield 52 g. of polymer with no agglomerated mass of polymer produced.

The polymer thus prepared, having a melt index of 0.05, a density of 0.940 and a flow parameter of 2.30, was superior in processability and impact strength for its low melt index.

Example 5 Into a 3 liter stainless steel autoclave was placed 600 g. of butane, then was introduced 20 mg. of a solid catalyst component-which had obtained by ball-milling 10 g. of the reaction product from reaction of 400 g. magnesium oxide and 1.3 kg. of aluminium chloride at 3000C for 4 hours, 1 g. of chloroform and 2.1 g. titanium tetrachloride together for 1 6 hours at room temperature, in a nitrogen atmosphere--, and was also introduced 1 mmol of triethylaluminium, thereafter the temperature was held at 650C resulting in that the pressure was found to be 7 kg/cm2.G. Then, after introducing 300 ml. of hydrogen and 10 ml. of "Dialen" (a mixture of hexene-1, octene-1, and decene-1, manufactured by Mitsubishi Chemical Industries Ltd.), ethylene was introduced so that the total pressure was 11 kg/cm2.G, under which condition a polymerization was allowed to take place for 10 minutes, resulting in that there was produced a polymer in an amount of 500 times by weight the amount of the solid catalyst component.

Then, purging off the butane, hydrogen was introduced at 700C until the pressure was 1.8 kg/cm2.G and further introduced continuously a mixed gas of ethylene and butene-1 containing 8 mol% of butene-1 so that the total pressure was held at 10 kg/cm2.G, under which condition a polymerization was made for 2 hours in vapor phase. The vapor phase polymerization was carried out in an extremely stable manner to yield 173 g. of polymer with no agglomerated mass of polymer produced.

The polymer thus prepared, having a melt index of 0.35, a density of 0.920 and a flow parameter of 1.81, was superior in processability and anti-environmental stress cracking.

Claims (13)

Claims

1. A process for preparing an ethylene copolymer having a density of 0.900 to 0.945, which process comprises copolymerizing ethylene and an a-olefin having 3 to 10 carbon atoms at a temperature in the range of from 0 to 1200C in the presence of an inert hydrocarbon solvent having 3 to 4 carbon atoms and using a catalyst composition, said catalyst comprising (1) a solid component obtained by supporting on a magnesium-containing inorganic solid carrier at least one compound selected from the group consisting of titanium compounds and vanadium compounds and (2) an organoaluminium compound, to produce a copolymer in an amount of 0.1 to 60% by weight based on the amount of the ethylene copolymer as the final product at a catalyst activity of 100 to 10,000 g. copolymer per gram of said solid catalyst component and then conducting the copolymerization reaction in vapor phase while evaporating the hydrocarbon.

2. The process as defined in claim 1, in which said inert hydrocarbon solvent is propane, nbutane, i-butane, or butene-2.

3. The process as defined in claim 1 or claim 2, in which said a-olefin is propylene, butene-1, hexene-1, 4-methylpentene-1, heptene-1, octene-1, or decene-1.

4. The process as defined in claim 1, 2 and 3 in which further ethylene and a-olefin are added in the vapor phase polymerization step.

5. The process as defined in any one of claims 1 to 4 in which said vapor phase polymerization is carried out at a temperature in the range of from 200 to 1 00C and at a pressure in the range of from atmospheric to 70 kg/cm2.G.

6. The process as defined in any one of claims 1 to 5 in which the amount of -olefin added is 1 to 250 mol% based on the amount of ethylene added.

7. The process as defined in claim 6, in which the amount of a-olefin added is 2 to 200 mol% based on the amount of ethylene added.

8. The process as defined in any one of claims 1 to 7, in which hydrogen is introduced into the polymerization system.

9. The process as defined in any one of claims 1 to 8, in which the polymer-containing slurry resulting from the first stage slurry polymerization is continuously fed to the second stage vapor phase polymerization system which is held under vapor phase polymerization conditions, the resulting evaporated hydrocarbon solvent is liquefied and at least a portion of the liquefied hydrocarbon is recycled to the vapor phase polymerization system.

10. The process as defined in claim 9, in which to the second stage vapor phase polymerization system is recycled a gas of evaporated, unreacted olefins and also are newly fed ethylene and the a olefinofC3toC10.

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

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

13. A copolymer of ethylene and an a-olefin, when prepared by the process claimed in any one of the preceeding claims.