JP3269376B2 - Gas phase polymerization of olefins - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a process for gas phase polymerization of olefins in a fluidized bed reactor. Specifically, using a gas-phase fluidized-bed reactor, a bulk polymer generated when olefin is continuously polymerized and copolymerized, particularly an olefin that can be stably produced without generating a sheet-like mass or the like. It relates to a gas phase polymerization method.

[0002]

2. Description of the Related Art Hitherto, olefin polymers have been produced by solution, bulk, slurry or gas phase polymerization methods. In recent years, processes that do not require the step of separating catalyst residues from the produced polymer due to the remarkable improvement in the activity of the catalyst have become widespread.In particular, the method of gas phase polymerization of olefins in a fluidized bed reactor has industrial value. High and attracting attention.

When producing polymers in a fluidized bed reactor,
Because it is difficult to remove the heat of polymerization compared to solution polymerization methods, etc.
Due to the local accumulation of polymerization heat, the temperature in the fluidized bed becomes unstable, forming a bulk polymer, making it difficult to maintain a fluid state.Furthermore, since the formed bulk polymer blocks the product extraction line, continuous In some cases, stable production was difficult.

Japanese Patent Application Laid-Open No. Sho 56-4608 discloses that an inert hydrocarbon compound having 3 or more carbon atoms coexists in a liquid state in a polymerization reactor so that a polymer can be deposited on the polymerization reactor wall and other parts of the reactor. It is disclosed to prevent electrostatic adhesion of particles. Also, JP-A-58-201802 discloses a method of cooling an unreacted olefin circulating gas to form a two-phase mixture of a gas and an entrained liquid, re-introducing the fluid into a fluidized bed reactor, and removing heat of polymerization. Is disclosed. In these methods, since a liquid substance is present in the polymerization reactor, measurement of a differential pressure gauge or the like necessary for control of the reaction may be disturbed and control of the polymerization reaction may be difficult.

On the other hand, Japanese Patent Application Laid-Open No. 57-67612 discloses that a gas-phase polymerization method of ethylene and an α-olefin has 3 to 3 carbon atoms.
There is disclosed a method in which a saturated hydrocarbon of No. 6 is present in an amount of at least equimolar to ethylene, and the gaseous mixture is cooled so as not to be liquefied and circulated to a gas phase polymerization system.

However, in this method, since the saturated hydrocarbon is used in a large amount of equimolar or more with respect to ethylene,
It is more economically disadvantageous than using inexpensive nitrogen as an inert substance. Also, if a large amount of saturated hydrocarbon having a large number of carbon atoms is used as a saturated hydrocarbon, the dew point of the circulating gas rises, and if the liquefied gas is to be cooled within a range that does not liquefy, the circulating gas temperature must be increased. Becomes lower.

Further, Japanese Patent Application Laid-Open No. 7-62009 describes a polymerization method for cooling the inner wall temperature of a reactor to below the dew point of a fluidized gas in a fluidized-bed gas-phase polymerization. A method for preventing generation and stably performing gas phase polymerization is disclosed.

An object of the present invention is to provide a method for preventing a bulk polymer from being generated in a conventional fluidized-bed gas-phase polymerization process and enabling a long-term stable polymerization reaction.

[0008]

SUMMARY OF THE INVENTION The present invention relates to a method for continuously vapor-phase polymerizing olefins in a fluidized-bed reactor, wherein a circulating gas containing unreacted monomers and replenished monomers discharged from the reactor is represented by the following formula (1). The present invention relates to a gas phase polymerization method for olefins, characterized in that it is cooled so as to satisfy the relationship of (1) and then introduced into the reactor, and the temperature of the inner wall of the fluidized bed reactor is cooled below the dew point of the circulating gas. Td <T <Td + 5 ° C. (1) (where Td is the dew point of the circulating gas, and T is the cooling temperature of the circulating gas.)

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0009] The polymerization of the olefins of the present invention is carried out continuously in the gas phase in a fluidized bed reactor. That is, a catalyst component, an olefin monomer, an inert gas, an additive, and the like are continuously introduced into a fluidized bed reactor, and polymerization is performed while the catalyst particles and the polymer particles are fluidized by a gas phase component. In continuous polymerization, the resulting polymer is continuously or intermittently withdrawn from the reactor, and unreacted monomer gas is continuously discharged from the fluidized bed reactor. Unreacted monomer gas is preferably replenished with an olefin monomer and the like as appropriate via a heat exchanger and a compressor through a circulation line, after the accompanying fine powder polymer is removed by a cyclone or the like, and reintroduced into the fluidized bed reactor. Is done.

As the reactor used in the present invention, for example, JP-A-58-201802, JP-A-59-126406, JP-A-2-233708, JP-A-4-234409, The fluidized bed reactor described in, for example, Kohei 7-62009 can be used.

In the present invention, a Ziegler catalyst or a metallocene catalyst is used as a polymerization catalyst. A combination of a metallocene compound of a transition metal of Group IV or V of the periodic table with an organoaluminum compound and / or an ionic compound is used.

The Ziegler-type catalyst comprises a transition metal catalyst solid component and an organoaluminum compound. As the transition metal catalyst solid component, transition metal compounds of Groups IV to VI of the periodic table such as titanium, vanadium, chromium and zirconium can be used, and these compounds, for example, magnesium compounds such as magnesium chloride, Alternatively, a highly active catalyst supported on a carrier such as an inorganic oxide such as silicon dioxide, alumina and zirconia is preferably used.

Methods for producing these solid catalyst components are described in JP-A-59-8706, JP-A-59-22907 and JP-A-59-2907.
No. 2908, No. 59-64611, No. 59-71309, No.
No. 60-42404, No. 60-133011, No. 60-215006, No. 62-232405, No. 62-297304, Japanese Patent Laid-Open No. 1-256502, Japanese Patent No. 1-289809, JP-A-3-81
No. 303, JP-A-3-88808, JP-A-3-93803, JP-B-56-18132, JP-B-56-15807,
JP-B-61-50964, JP-B-61-363, JP-B-62
A method proposed in JP-A-56885 can be adopted.
A typical production method includes a method of treating a magnesium chloride carrier obtained by reacting a specific organomagnesium compound, a chlorinated organic compound, and an electron addition donor under specific conditions with titanium tetrachloride.

Examples of the transition metal compound include titanium, vanadium, chromium, zirconium, hafnium and the like, halides, hydrocarbyl oxyhydrides, hydrocarbyloxyhalides, hydrocarbyloxyhydrocarbylates, hydrocarbyl oxides of transition metals of Groups IV to VI of the periodic table. Halide and hydrocarbyloxyhydrocarbyl halide.

Among them, a titanium compound can be preferably used. Specific examples thereof include methoxytrichlorotitanium, dimethoxydichlorotitanium, trimethoxychlorotitanium, ethoxytrichlorotitanium, diethoxydichlorotitanium, propoxytrichlorotitanium, dipropoxydichlorotitanium, butoxytrichlorotitanium, dibutoxydichlorotitanium, and phenoxy. Trichlorotitanium, diphenoxydichlorotitanium, methoxytribromotitanium,
Phenoxytribromotitanium, methoxytriioditanium, phenoxytriiodotitanium, tetrachlorotitanium, tetrabromotitanium, tetraiodotitanium, tetramethoxytitanium, tetraethoxytitanium, tetrapropoxytitanium and the like.

Examples of the organoaluminum compound used together with the solid catalyst component include trialkylaluminum, dialkylaluminum halide, dialkylaluminum hydride and alkylalumoxane. Specific examples thereof include trimethylaluminum, triethylaluminum, tributylaluminum, triisobutylaluminum, trihexylaluminum,
Trioctyl aluminum, diethyl aluminum chloride, diethyl aluminum hydride, isoprenyl aluminum, methylalumoxane are exemplified.

Various electron donors may be further added to the above polymerization catalyst system. As electron donors, organic acid esters, inorganic acid esters, acid halides, ethers, acid amides, N, N-dialkyl amides, amines, nitriles, acid anhydrides, silane compounds, ketones, alcohols, aldehydes, carboxylic acids, Isocyanate and the like can be used. Particularly, a silane compound is preferable.

Examples of the metallocene catalyst include those in the Periodic Table IV.
Alternatively, a combination of a metallocene compound of a group V transition metal with an organoaluminum compound and / or an ionic compound is used.

The transition metals of Groups IV or V of the Periodic Table include:
Titanium (Ti), zirconium (Zr), hafnium (H
f) and vanadium (V) are preferred.

The metallocene compound includes at least one cyclopentadienyl group, a substituted cyclopentadienyl group (for example, an alkyl-substituted cyclopentadienyl group such as methyl, dimethyl and pentamethyl, an indenyl group,
Fluorenyl group) as a ligand, or a cyclopentadienyl group thereof is a hydrocarbyl group (eg, an alkylene group, a substituted alkylene group), a hydrocarbyl silicon (eg, a silanylene group, a substituted silanylene group, a silaalkylene group, Cross-linked by silaalkylene) or the like, and further, cyclopentadienyl group is oxygen,
Those cross-linked to nitrogen and phosphorus atoms (for example, oxasilanylene group, substituted oxasilanylene group, oxasilaalkylene group, substituted oxasilaalkylene group, aminosilyl group,
Any known metallocene compounds having a monosubstituted aminosilyl group, phosphinosilyl group or monosubstituted phosphinosilyl group as a ligand can be used.

Specific examples thereof are disclosed in Japanese Patent Application Laid-Open No. 58-19309.
No. 60-35006, No. 61-130314, No. 61
JP-A-264010, JP-A-61-296008, JP-A-63-222177, JP-A-63-251405, JP-A-1-66214,
Nos. 1-74202, 1-275609, 1-301704, 1-319489, 2-41303 and 2-131488
Publication Nos., 3-12406, 3-139504, 3-17
No. 9006, No. 3-185005, No. 3-188092,
No. 3-197514, No. 3-207703, No. 5-2009013, Japanese Translation of PCT International Publication No. 1-501950, No. 1-502036, and
No. 5,505,593.

In the present invention, catalysts other than those described above are disclosed in JP-A-61-130314, JP-A-61-264010,
JP 63-142004, JP-A 1-129004, 1-301704
Gazette, Gazette 2-75605, Gazette 3-12406, 3-12
Nos. 407, 4-227708, 4-268308,
Examples thereof include metallocene compounds described in JP-A Nos. 4-300887 and 6-25343.

These metallocene compounds are themselves
C ₂ symmetry elements capable of forming a complex with crosslinked, and / or have a multi-substituted ligands. As a specific example, dimethylsilyl (2,4-dimethylcyclopentadienyl) (3 ', 5'-
Silicon-bridged metallocene compounds such as zirconium dichloride, zirconium dichloride, dimethylsilyl (2,4-dimethylcyclopentadienyl) (3 ', 5'-dimethylcyclopentadienyl) hafnium dichloride, ethylenebisindenyl Zirconium dichloride, ethylenebisindenyl hafnium dichloride,
Examples include indenyl-based cross-linked metallocene compounds such as ethylenebis (methylindenyl) zirconium dichloride and ethylenebis (methylindenyl) hafnium dichloride.

The organoaluminum compound used in combination with the metallocene compound in the present invention includes a compound represented by the general formula:
Al (R) O-) _{n represents} a linear or cyclic polymer (R
Is a hydrocarbon group having 1 to 10 carbon atoms, including those partially substituted with a halogen atom and / or an RO group. n is the degree of polymerization and is 5 or more, preferably 10 or more), and specific examples include methylalumoxane, ethylalumoxane, isobutylethylalumoxane, and the like, in which R is a methyl, ethyl, or isobutyl group, respectively. Can be

As the ionic compound, a compound represented by the general formula: C ⁺ A
^- , C ⁺ is an oxidizing cation of an organic compound, an organometallic compound, or an inorganic compound, or a Bronsted acid comprising a Lewis base and a proton, and reacts with an anion of a metallocene ligand to form a metallocene cation. Can be generated.

A ^- is a bulky, non-coordinating anion capable of stabilizing the metallocene cation without coordinating to the metallocene. As specific examples thereof, those described in JP-A-4-253711, JP-A-4-305585, JP-A-5-507756 and JP-A-5-502906 can be used.

In particular, an ionic compound of a tetrakis (pentafluorophenyl) borate anion and a triphenylcarbonium cation or a dialkylanilinium cation is preferred. These ionic compounds can be used in combination with the aforementioned organic aluminum compounds.

The above-mentioned polymerization catalyst can be used by being supported on an inorganic compound or an organic polymer compound. As the inorganic compound as the carrier, inorganic oxides, inorganic chlorides and inorganic hydroxides are preferable, and those containing a small amount of carbonate and sulfate can also be used. Particularly preferred are inorganic oxides, such as silica, alumina, magnesia, titania, zirconia, and calcia. These inorganic oxides have an average particle size of 5 to 150 μ and a specific surface area of 2
Preferably porous microparticles of ~800m ² / g, for example from 100 to 80
It can be used after heat treatment at 0 ° C. The organic polymer compound preferably has an aromatic ring, a substituted aromatic ring, or a functional group such as a hydroxy group, a carboxyl group, an ester group, or a halogen atom in a side chain. As specific examples, ethylene, propylene, α-olefin homopolymer having the functional group by chemical modification such as polybutene, α-olefin copolymer, acrylic acid, methacrylic acid, vinyl chloride, vinyl alcohol, styrene, homopolymer such as divinylbenzene, Copolymers and their chemically modified products can be mentioned. As these organic polymer compounds, spherical fine particles having an average particle diameter of 5 to 250 μm are used.

The olefin used in the present invention includes:
Linear α such as ethylene, propylene, butene-1, pentene-1, 4-methylpentene-1, hexene-1, octene-1
-Cyclic α-olefins such as olefins, cyclopentene and cyclohexene.

The method of the present invention can be suitably used for homopolymerization of ethylene or propylene or copolymerization of ethylene or propylene with another α-olefin. In particular, by copolymerizing ethylene and another α-olefin,
It can be suitably used in a method for producing linear low density polyethylene (LLDPE). As the α-olefin used for copolymerization with ethylene, propylene, butene-1, pentene-1, hexene-1, 4-methylpentene-1,
Octene-1 and the like. Among them, olefins having 4 or more carbon atoms can be suitably used.

The circulating gas may contain an inert gas such as hydrogen or nitrogen as a molecular weight regulator.

In the gas-phase polymerization method of the present invention, the circulating gas preferably contains a saturated hydrocarbon, preferably a saturated hydrocarbon having 4 to 6 carbon atoms. Saturated hydrocarbons have little effect unless they are introduced in large amounts with small molecular weights such as propane, and high molecular weight saturated hydrocarbons are difficult to remove from the product powder. Carbon number 4
To 6 as saturated hydrocarbons, n-butane, i-butane,
Examples include n-pentane, i-pentane, hexanes and mixtures thereof.

The total amount of the saturated hydrocarbon having 4 to 6 carbon atoms and the olefin monomer having 4 or more carbon atoms in the circulating gas is preferably 1.0 mol% or more, more preferably 1.0 to 15 mol%. Less than the above range has less effect,
If the amount is too large, the dew point increases, so that it is difficult to remove heat of polymerization.

In the fluidized-bed gas-phase polymerization, the cooling temperature (T) of the circulating gas fluctuates depending on conditions such as polymerization temperature, gas flow rate (superficial velocity), and gas specific heat. Circulating gas with respect to the dew point (Td) of the circulating gas in a relationship of Td <T <Td + 5 ° C., preferably Td <Td.
The mixture is cooled so as to satisfy the relationship of <Td + 3 ° C. and introduced into the reactor.

If T is lower than the above range, the amount of liquids present in the polymerization reactor increases, and the measurement of the differential pressure gauge required for controlling the reaction is disturbed, so that it is difficult to control the polymerization reaction. is there. In addition, in the copolymerization of ethylene and an olefin having a large number of carbon atoms, the amount of the olefin or hydrocarbon introduced as a liquid increases, so that the copolymerization becomes uneven, which is not preferable in terms of the properties of the product.

The dew point of the circulating gas is the temperature at which the precipitation of the liquid material starts, and varies depending on conditions such as the composition of the circulating gas and the polymerization pressure, but is usually 20 to 80 ° C., preferably 35 to 70 ° C.
In this case, the temperature is adjusted to be higher than the minimum cooling temperature and lower than the reaction temperature.

As the polymerization conditions, the polymerization temperature is usually 5 to 1
00 ° C, preferably 60 to 95 ° C, the atomic ratio of transition metal in the solid catalyst to the organoaluminum compound (Al / M) is 1 to 1000
0, The polymerization pressure can be usually carried out under the conditions of normal pressure to 100 kg / cm ² · G.

[0038] It is also intended to improve the polymerization activity, maintain the shape of the produced polymer, facilitate the introduction of the catalyst into the polymerization reactor, prevent the catalyst from adhering to the polymerization reactor, and improve the fluidity in the fluidized bed reactor. After preliminarily polymerizing an olefin or a certain amount of an olefin and another olefin, the prepolymer can be used as a catalyst in the main polymerization. The preliminary polymerization is performed, for example, in a slurry method in an inert hydrocarbon solvent, usually at a polymerization temperature of 5 to 80 ° C, preferably 10 to 80 ° C, and a polymerization time.
The reaction is carried out for 5 minutes to 20 hours under the condition that 1 to 100 g of polymer is obtained per 1 mg atom of transition metal in the catalyst solid.

An embodiment of the present invention will be schematically described with reference to FIG. The gas phase reaction mixture is polymerized in the gas phase fluidized bed part 1 of the reactor, withdrawn from the top of the reactor, and reintroduced from the lower part of the reactor via the heat exchanger 6 and the compressor 7 through the circulation line 5. Is done. The supplementary feedstock is
It is supplied from a raw material supply line 8. A prepolymer preliminarily polymerized with a small amount of olefin in a catalyst or another container is supplied from a supply line 9. The produced polymer is continuously or intermittently withdrawn from the reactor by a withdrawal line 10 so that the height of the gas-phase fluidized bed portion becomes constant.

In the present invention, the effect can be exerted by cooling the inner wall temperature of the reactor to a temperature not higher than the dew point of the circulating gas during polymerization.

As a cooling method, a cooling device such as an air-cooling fin, a cooling jacket, a cooling tube, etc.
As shown in FIG. 1, a method of attaching to the outer wall of the reactor can be used. In the gas-phase fluidized-bed polymerization, since the polymer attachment site differs depending on the shape of the reactor, the injection method of the catalyst used, the particle shape of the prepolymer, etc., there is no particular limitation on the position where the cooling device is attached, It can be attached to a portion of the reactor, such as the cylinder portion of the reactor or the calming compartment at the top of the reactor. Under normal polymerization conditions, it is preferable to carry out polymerization by providing a cooling device from the bottom of the reactor to a height of about 1 to 2 times the diameter of the reactor.

[0042]

【Example】

Example 1 (Synthesis of solid support) In a 100 L reactor previously filled with nitrogen, equipped with a mechanical stirring system and a double jacket,
Charge 26.2 L of toluene and 1.2 mol of triethylaluminum, and maintain tert-butyl alcohol while maintaining at 26 ° C.
1.2 moles were added. 4.02 L of a diisoamyl ether solution containing 6 mol of n-butylmagnesium chloride was added to the mixture. 6.6 mol of tert-butyl chloride was added dropwise while heating to 40 ° C. over 30 minutes. In addition, 40
The reaction was carried out for 5 hours while maintaining the temperature. The obtained carrier solid was spherical particles having a particle diameter (radius) of 30 to 40 μm and a relatively narrow distribution.

(Synthesis of solid catalyst) 500 g of the obtained carrier was slurried with 7.5 L of toluene, and 7.5 M of methyltriethoxysilane was added as an electron donor at 40 ° C.
, And then 10 mol of tetrachlorotitanium was added, followed by stirring at 40 ° C. for 60 minutes. Thereafter, the solid was separated by filtration, washed three times with 5 L of toluene and once with 5 L of heptane, and the solid was dried. The Ti content of the obtained spherical solid is 0.
The Mg content was 61% and the Mg content was 15.7%.

(Preliminary polymerization) 200 L filled with nitrogen gas
The autoclave was charged with 80 L of n-heptane, and 0.75 mol of tri-n-octylaluminum, 0.25 mol of triethylaluminum and the above-mentioned catalyst solid (titanium content: 0.25 mol) were added. After injecting 1.5 kg / cm ² · G of hydrogen gas, the temperature was raised to 60 ° C. to initiate polymerization. 67- during polymerization
Keep at 80 ° C, introduce ethylene at 100 L / min.
Polymerization was carried out until 40 g of a prepolymer was produced per M.

(Gaseous phase polymerization of ethylene and butene-1) FIG.
Was used to copolymerize ethylene and butene-1. As the polymerization catalyst, a solid catalyst prepared by the method described above and a catalyst system comprising triethylaluminum were used. The composition of the flowing gas and the polymerization conditions are summarized in Table 1. Polymerization could be carried out stably without forming lumps.

Example 2 An inner wall of a fluidized-bed reactor was cooled from the bottom of the reactor to a height of 1 to 2 times the diameter of the reactor by a cooling device laid in the fluidized-bed reactor. Polymerization was carried out in the same manner as in Example 1 using the conditions, but the polymerization could be carried out stably without forming lumps.

Example 3 The following metallocene catalyst was used as a polymerization catalyst. (Synthesis of solid catalyst) Toluene 100L, silica 10kg, 3
1 L of a toluene solution of mol / L methylalumoxane was mixed and heated at 60 ° C. for 2 hours. Filtration, separation,
After washing three times with 30 L of toluene, again, 100 L of toluene
Was added, and 10 L of a toluene solution containing 1 mol of dimethylcyclopentadienyl zirconium dichloride was further added, followed by heat treatment. Filtration, separation, and washing with 30 L of toluene three times gave a supported catalyst. Zirconium loading is 0.5
% By weight.

(Preliminary polymerization) 200 L filled with nitrogen gas
The autoclave was charged with 80 L of n-heptane, 1.0 mol of triethylaluminum, and the above-mentioned catalyst solid (zirconium content: 0.25 mol) were added. Polymerized at 35-50 ° C,
Ethylene was introduced at a rate of 100 L / min., And polymerization was carried out until 40 g of a prepolymer was produced per 1 mmol of zirconium.

Using the above catalyst system and under the polymerization conditions shown in Table 1, polymerization was carried out in the same manner as in Example 2, but the polymerization could be carried out stably without forming lumps.

Comparative Examples 1-2 Using the same catalyst system as in Example 1, the temperature of the circulating gas in Example 1 or 2 was increased, and polymerization was carried out in the same manner as in Example 1 using the polymerization conditions shown in Table 2. However, the temperature in the fluidized-bed reactor during polymerization was unstable and fluctuated, such as abruptly rising.Lumps were mixed in the extracted product, and the extraction line was eventually blocked, making it difficult to continue the polymerization. The reaction was stopped.

Comparative Example 3 Using the same catalyst system as in Example 3, the dew point of the circulating gas was lowered by adjusting the amount of pentane, and polymerization was carried out in the same manner as in Example 1 using the polymerization conditions shown in Table 2. However, the temperature in the fluidized bed reactor during polymerization was unstable and fluctuated, such as abruptly rising.Lumps were mixed into the extracted product, and the extraction line was eventually blocked, making it difficult to continue the polymerization. Was stopped.

[0052]

[Table 1]

[0053]

[Table 2]

[0054]

According to the present invention, when a polymer is produced in a fluidized-bed reactor, a bulk polymer produced by destabilization of the temperature in the fluidized bed due to local accumulation of polymerization heat, especially a sheet-like polymer, is produced. Gas phase polymerization can be stably performed without generating lump or the like.

[Brief description of the drawings]

FIG. 1 is a drawing showing an embodiment of the present invention. DESCRIPTION OF SYMBOLS 1 ・ ・ gas phase fluidized bed part of reaction 2 ・ ・ fluidized bed cylinder 3 ・ ・ settlement compartment 4 ・ ・ fluidization grid 5 ・ ・ circulation line 6 ・ ・ heat exchanger 7 ・ ・ compressor 8 ・ ・Raw material supply line 9. Prepolymer supply line 10. Polymer extraction line 11. Cooling unit

──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Tadashi Shinoda 8-1, Goi-minamikachi, Ichihara-shi, Chiba Examiner at Ube Industries, Ltd. Chiba Works Eiichi Yoshi ▼ (56) References JP-A-7- 62009 (JP, A) JP-A-10-259204 (JP, A) JP-A-58-201802 (JP, A) JP-T10-503799 (JP, A) (58) Fields investigated (Int. ⁷ , DB name) C08F 2/00-2/60

Claims (1)

(57) [Claims] 1. A method for continuously vapor-phase polymerizing olefins in a fluidized-bed reactor, wherein a circulating gas containing unreacted monomers and replenished monomers discharged from the reactor is cooled so as to satisfy the following formula (1). Gas-phase polymerization of olefins, wherein the temperature of the inner wall of the fluidized-bed reactor is cooled below the dew point of the circulating gas. Td <T <Td + 5 ° C. (1) (where Td is the dew point of the circulating gas, and T is the cooling temperature of the circulating gas.)