JPH0377865A - Production of aromatic cyclic polyimide - Google Patents

Abstract

PURPOSE:To obtain the title high purity compound useful as an adhesive in high yield and to suppress by-products by continuously or intermittently adding a specific aromatic polyamide to a reaction system comprising an alpha,beta-unsaturated dicarboxylic acid anhydride and a mixed solvent in the presence of an acid catalyst under reflux and dehydration under heating. CONSTITUTION:100 pts.wt. aromatic polyamide containing two or more primary amino group in the molecule is continuously or intermittently added to a reaction system comprising an alpha,beta-unsaturated dicarboxylic acid anhydride (the ratio to primary amino group of the aromatic polyamide is preferably 1-20 times mol) and 100-2,000 pts.wt. mixed solvent (comprising 60-99wt.% nonpolar solvent having azeotrope with water and 40-1wt.% aprotic polar solvent) in the presence of an acid catalyst under reflux and dehydration under heating to give an aromatic polyimide. The polyimide is crystallized and separated by cooling the reaction mixture prepared by the method to increase profits such as elimination of large amount of waste water treatment.

Description

DETAILED DESCRIPTION OF THE INVENTION (Industrial Application Field) The present invention relates to a method for producing aromatic cyclic polyimides. Aromatic cyclic polyimides are useful as raw materials for adhesives, WJ layer materials, sealing materials, sliding materials, and structural materials for electrical parts, aircraft, vehicles, and the like.

(Prior art) A method for producing corresponding cyclic polyimides from aromatic polyamines having 2 or more primary amino groups and α,β-unsaturated dicarboxylic acid anhydrides is to produce the corresponding cyclic polyimides via polyamic acid. A common method is to produce by dehydration imidization.

In this method, the intermediate polyamic acid is easily produced by mixing a polyamine and an α,β-unsaturated dicarboxylic acid anhydride under mild conditions, but the subsequent dehydration and imidization is easy. Therefore, there are problems with rFX1, such as low yield and low purity, making it insufficient as a raw material for resin.

Although various methods have been proposed to solve these problems, there is still no method that is fully satisfactory.

For example, a method in which a dehydrating agent such as acetic anhydride and a dehydrating catalyst such as sodium acetate or a tertiary amine are added to bisamidic acid obtained from an aromatic diamine and anhydrous 71/ynic acid to imidize it at a relatively low temperature (for example, 46-29140. Special Publication No. 49-40231, Special Publication No. 52-2913. Japanese Patent Application Publication No. 5
5-13202. JP-A-59-2i2470, JP-A-Sho 6
1-24564) is disclosed. However, this method is industrially disadvantageous because it consumes a large amount of expensive dehydrating agent.

For this reason, various methods without using a dehydrating agent have been proposed.

For example, hydrocarbons and halogenated hydrocarbon solvents that are azeotropic with water, N,N-dimethylformamide (hereinafter abbreviated as DMF) and N-methylpyrrolidone (hereinafter abbreviated as NMP)
For example, if, special rM, 1987-68770. tel! 1982-159764, JP 60-260623. JP-A-62-123169, JP-A-63-20i 166)
is disclosed.

However, both methods use a solvent that has high solubility for both the raw material amic acid and the target polyimide, so in order to remove the target polyimide from the reaction solution, after the reaction is complete, the target polyimide is poured into a large amount of water or methanol. has been adopted.

For this reason, problems such as the inability to recover and reuse the catalyst and solvent and the generation of a large amount of waste water arise, making this production method unsatisfactory industrially.1. :.

On the other hand, if a nonpolar solvent such as a hydrocarbon is used as the reaction solvent, the solubility of the polyimide produced is low, and the target product is produced as a solid, which can be separated from the reaction solution by an easy operation such as I-filtration. However, the solubility of the starting amic acid in these solvents is also low, and only extremely slow reaction rates can be obtained. Therefore, if the reaction is unavoidably carried out at high temperatures or in the presence of a large amount of catalyst, polymeric by-products will be produced, making it impossible to continue the reaction. It was completely unworkable.

Furthermore, in addition to the above-mentioned problems, all of these production methods either synthesize polyamic acid as a reaction intermediate in a separate reactor, or perform the first step of producing amic acid using the same reactor. This is followed by a second imidization reaction. This two-step method may complicate the reaction operation,
There is a problem that a complicated device for handling the slurry is required, and a one-step method for producing the polyimide using an easier and more convenient device is desired.

On the other hand, as a one-step method in which no amic acid is prepared, an aromatic polyamine is gradually added to a molten solution of maleic anhydride (JP-A-61-225215).

62-77363> is disclosed. However, these methods involve heating maleic anhydride under reduced pressure to a high temperature to react, and then increasing the degree of reduced pressure to remove excess maleic anhydride or extracting it with hot water, resulting in sublimation. There are problems such as the need for special equipment for the anhydrous maleic acid/inic acid, and the need to treat washing wastewater in which a large amount of maleic acid is dissolved, so there is no practical manufacturing method. there were.

(Means for Solving the Problem) The present inventors have intensively studied a one-step reaction process that is easy to separate the produced polyimide, has high reactivity and good selectivity, and is simple in operation and equipment. Using a mixed solvent of an aprotic polar solvent and a nonpolar solvent, the
- Continuously or intermittently adding an aromatic polyamine to a reaction system undergoing heating and reflux dehydration in the presence of a predetermined amount of an unsaturated dicarboxylic anhydride significantly increases the reaction rate;
The present invention was achieved by discovering that it is extremely effective in suppressing polymeric by-products and can improve the yield and purity of the target polyimide without impairing its crystallization properties.

That is, when producing a cyclic polyimide by heating and dehydrating an α,β-unsaturated dicarboxylic acid anhydride and an aromatic polyamine having two primary amino groups in the molecule,
100 parts by weight of aromatic polyamine, 1 part by weight of α,β-unsaturated dicarboxylic acid anhydride (the ratio of the aromatic polyamine to primary amino groups is in the range of ~20 times the mole) and 100 to 2000 parts by weight of a mixed solvent (60-99% by weight of a non-polar solvent that has azeotropy with water and 40-1% of an aprotic polar solvent)
A dehydration imidization method characterized by continuously or intermittently adding an acid catalyst and an acid catalyst to the reaction system undergoing heating reflux dehydration, and This is a method for producing an aromatic polyimide, which is characterized by crystallizing and separating polyimide from a reaction mixture.

Conventionally, when an aromatic polyamine, which can be considered as a curing agent, is directly introduced into the reaction system during the synthesis of a compound that produces a polymer by migel addition, such as the cyclic polyimide produced in the present invention, the addition reaction to the existing imide can be prevented. Priority was given to the fact that by-products of polymeric products were unavoidable. On the other hand, the presence of an excess of α,β-unsaturated dicarboxylic acids and their anhydrides may cause the formation of fumaric acid through thermal isomerization or the formation of polymers through intermolecular dehydration with polyamines such as the isomerized acid. It has been f::.

However, as a result of research to solve these rays, the present inventors found that acid catalysts and α,β-
By sequentially adding an aromatic polyamine while heating and dehydrating in the presence of an unsaturated dicarboxylic acid anhydride, it is possible to perform an imidization reaction in one step and at a relatively low temperature, resulting in the production of polymeric by-products and the isomerization of dicarboxylic acids. This led to the invention of a synthetic method that suppresses this.

The first feature of the present invention is the reaction method with high reaction rate and high selectivity in the one-step reaction proposed here. The second feature is that while taking advantage of the above properties, after the reaction is complete, the D-type polyimide is crystallized by cooling and is required for a solvent system that can be easily separated. Not only is the benefit eliminated, but the crystallization mother liquor can be subjected to the reaction again, making efficient use of the excess amount of α,β-unsaturated dicarboxylic acid anhydride dissolved in the solution. This makes it possible to increase the benefits of the present invention.

The aromatic polyamines used in the present invention are
It is an aromatic amine that has two primary amino groups. Examples include polyamines represented by general formulas (A), (B), (C) and (D).

(In the formula, X is a hydrogen atom, a halogen atom, an alkyl group having 1 to 8 carbon atoms, an alkoxy group, a hydroxyl group;
An integer of 3, n゛ goes to O ~ an integer of 10 5Y goes to -0-, S
-, -S O2-1-S O- or -CO- may be the same or different, respectively).

More specifically, as the polyamine represented by formula (A), m-phenylenediamine, p-phenylenediamine, 2.4-diaminotoluene, 2.6-diaminotoluene, 2-chloro-p-phenylenediamine, Polyamines represented by the general formula (B) include 2,2-bis(4-aminophenyl)propane, 4゛4゛-diaminodiphenylmethane, 4.4゛-diaminodiphenyl ether, 4.4'
-diaminodiphenylsulfide, 4.4'-diaminodiphenylsulfone, 4.4'-diaminodiphenylmethylethylmethane, 4.4'-diaminotoluenyl di(
trifluoromethyl)methane 3,3'-diaminodiphenyl ether, 3,3'-diaminodiphenyl sulfide, 3,3''-diaminodiphenylsulfone condensate of aniline and formaldehyde,
The polyamine represented by the general formula (C) is exemplified by a condensate of aniline and acetaldehyde, a condensate of toluidine and formaldehyde, etc.
-) Unishin diisopropylidene) dianiline, 3.4'-(i3-phenylene diisopropylidene) dianiline, 3.3'-(p-phenylene diisopropylidene) dianiline, 1.4'-bis(p -aminophenoxy)benzene1.
3'-bis(ρ-aminophenoxy)benzene 1.4'
-bis(p-aminophenoxythioether)benzene, 1.3゛-bis(p-aminophenoxythioether)
Benzene, 1.4'-bis(p-aminophenyl)benzene, 1.
Examples include 3'-bis(p-aminophenyl)benzene and meta-units of each of the above diamines.

As the polyamine represented by the general formula (D), 2.2-
Bis[4-(p-aminophenylthioether)phenyl]propane, 2.2-bis[3-(p-aminophenoxythioether)phenyl]propane, 4.4'-bis(p-aminophenoxy)diphenylsulfone, 3 .3'-bis(p-aminophenoxy)diphenylsulfone, 4.4'-bis(p-aminophenylthioether)diphenylsulfone, 3.3'-t's(p-aminophenylthioether)
Diphenylsulfone, 4.4'-bis(p-aminophenoxy) diphenyl ether, 3.3'-bis(p-aminophenoxy) diphenyl ether, 4.4'-bis(p-aminophenoxy) diphenyl sulfide, 3.3' -bis(p-aminophenylthioether) diphenyl sulfide, 4.4°-bis(p-aminophenylthioether) diphenyl sulfide, 3.3'-bis(p-aminophenylthioether) diphenyl sulfide, 4.4'-bis( p-aminophenylthioether) diphenyl ether, 3.3′-bis(p-aminophenylthioether) diphenyl ether, 4.4′-bis(p-aminophenoxy)benzophenone, 3.3′-bis(p-aminophenoxy)benzophenone , 4.4'-bis(p-aminophenylthioether)benzophenone, 3.3'-bis(p-aminophenylthioether)benzophenone, 4.4'-bis(p-aminophenylthioether)diphenyl, 3.3゛-bis(p-7'minophenylthioether)
Diphenyl and monomers of each of the above diamines are exemplified.

These polyamines represented by general formulas (A) to (D) can be used alone or in combination of two or more, and as long as the desired effects of the present invention can be obtained,
In addition to those exemplified above, other aromatic polyamines may be used in combination.

The aromatic polyamine may be added in the form of a solid, a melt, or a solution in the above-mentioned solvent. The rate of sequential addition varies depending on the type and amount of the dicarboxylic acid anhydride, acid catalyst, and solvent composition present in the reaction system, but the concentration of the intermediate maleamic acid present in the reaction solution is 0.1 mol/'1 or less. It is best to adjust the addition rate so that the If it exceeds this range, by-products due to isomerized acid and polymerization reactions due to Migel addition reactions tend to occur. As a guide, it is best to add at a rate that causes the reaction solution to become homogeneous or slightly cloudy, and the addition is usually completed in 1 to 10 hours.

Next, specific examples of α,β-unsaturated dicarboxylic acid anhydrides include maleic acid, 3-methylmaleic acid, 3-ethylmaleic acid, 3,4-dimethylmaleic acid, 3.4-diethylmaleic acid, - phenylmaleic acid, 3-chloromaleic acid, 3.4=dichloromaleic acid, tetrahydrophthalic acid, methyltetrahydrophthalic acid, 5-norbornene-2,3-dicarboxylic acid and acid anhydrides thereof,
Further examples include maleated alloocimene and maleated mil7ane. Naturally, it is also possible to use anhydrides obtained by preparing in advance a hydroacid corresponding to these acids and dehydrating them by heating.

In the present invention, the Brønsted acids used as catalysts include inorganic acids such as phosphoric acid, phosphorous acid, hypophosphorous acid, metaphosphoric acid, birophosphoric acid, tripolyphosphoric acid, and sulfuric acid, methanesulfonic acid, p-toluenesulfonic acid, and benzenesulfonic acid. , naphthalenesulfonic acid, and other organic acids can be used. Further, among the above-mentioned Bronsted acid catalysts, in which a small amount of a dehydrating agent such as phosphorus pentoxide can be used in combination with these acid catalysts, organic sulfonic acids are particularly preferred because they cause fewer side reactions and have a good hue.

,: These catalysts have a ratio of 1 to
The amount of solvent used in the reaction is 50% by weight.
About 1 part by weight of 100 to 2000 is used per part by weight of O. Preferably, the amount is 200 to 600, or more than this, the amount of dissolved polyimide produced increases, resulting in a large loss. Conversely, if the amount is less than this, a large amount of produced polyimide will precipitate, causing problems in completing the reaction.

The aprotic polar solvent in the present invention is, for example, DM
F,N,N-dimethylacetamide, N.

N-diethylformamide, NMP, dimethyl sulfoxide (hereinafter abbreviated as DMSO), hexamethylphosphoramide, γ-butyrolactam, tetramethylurea, 1.
Examples include 3-dimethyl-2-imidazolidinone, diglyme, and dioxane.

On the other hand, a non-polar solvent is preferably 60" 200℃
It is a solvent that has a boiling point range of about 100 mL and can azeotropically distill off produced water, specifically toluene, xylene, ethylbenzene, hexane, octane, decane, cyclohexane, methylcyclohexane, ethylcyclohexane, gas oil, and hydrides of gas oil. and halogenated hydrocarbons such as chlorobenzene, dichloroethane, trichloroethane, perchlorethylene, etc., and can be used alone or in a mixture of two or more of them.

The mixing ratio of the aprotic polar solvent and the water azeotropic nonpolar solvent is in the range of 60 to 99% by weight of the nonpolar solvent and 40 to 1% by weight of the aprotic polar solvent. If the proportion of the aprotic polar solvent is too small, the reaction rate will drop to a large extent, and a large amount of highly sticky polymer will be produced as a by-product, making it difficult to continue the reaction. On the other hand, if there are too many
The solubility of the target cyclic polyimide increases, making it difficult to separate and recover the imide, and at the same time, the selectivity of the reaction (3) decreases.

The amount of α5β-unsaturated dicarboxylic acid anhydride to be dissolved in the above solvent is 1 to 20 times the mole of the primary amino group of the wholly aromatic polyamine to be added to the reaction system. If the charging molar ratio is less than 1, excess aromatic polyamine will cause a migel addition reaction to the polyimide produced, significantly reducing the purity of the polyimide. The tendency becomes stronger as the molar ratio approaches 1, so the preferred molar ratio is 162 to 2.
An excessive 0 molar ratio, which is 0 times the molar ratio, may not cause any problem in terms of the reaction, but it is undesirable from an economic standpoint. α,
It is convenient to charge the β-unsaturated dicarbonate@anhydride by dissolving the entire amount in the solvent at the start of the reaction, but it may be added at the same time while taking care not to reverse the ratio with the aromatic polyamine.

Furthermore, the reaction can also be carried out in the presence of a stabilizer in order to prevent coloring of the reactants and obtain a high quality cyclic polyimide. As the stabilizer, hydroquinone, methoxybenzoquinone, phenothiazine, t-butylcatechol, dimethylcarbamic acid, etc. are suitable, and the amount added is generally preferably 0.001 to 1% by weight in the reaction system. .

The reaction is carried out under heating and reflux conditions, specifically 60 to
The temperature is 200°C, more preferably 100°C to 130°C.

The method of the invention is carried out as follows: - Boat fishing. That is, an acid catalyst and an α,β-unsaturated dicarboxylic acid anhydride are dissolved and charged in a solvent of a predetermined composition in a reactor equipped with a decanter, and the aromatic polyamine is sequentially added while dehydrating under heating under reflux for 1 to 10 hours. . The polyamine is supplied as a powder, in the form of mineral solution, or dissolved in the above-mentioned solvent, and supplied continuously or intermittently. 7. After the reaction is completed, stirring is stopped and the mixture is turned on. When the reaction solution separates into layers, the polyimide-containing layer and the catalyst-containing layer are separated at an arbitrary temperature at which polyimide does not precipitate. Polyimide crystals can be precipitated by cooling a polyimide-containing solution, or by distilling off a portion of the solvent and then cooling. The separated catalyst layer can be used repeatedly as is.If the reaction solution does not separate into layers,
Polyimide crystals can be precipitated by cooling the reaction solution as it is, or by distilling off a portion of the solvent and then cooling it.

The precipitated polyimide can be obtained separately from the r solution by passing through or centrifuging. In some cases, the polyimide can be removed by washing with a suitable solvent such as a hydrocarbon solvent, water, aqueous sodium carbonate, and/or methanol. Acid catalyst and α.

The β-unsaturated dicarboxylic anhydride is removed. I can do two things.

The r solution can be used repeatedly as it is or by adding the dicarboxylic anhydride.

Hereinafter, the present invention will be explained in detail by giving Examples and Comparative Examples.

(Example) Practical 1 In a four-day flask equipped with a condenser with a water separator, a dropping funnel, a thermometer, and a stirrer, rt1 luenesulfone lllll4
g, toluene 100g, DMFiOg, and maleic anhydride 33.8g (0°34 mol) were charged, heated to continuous flow temperature with stirring, and while removing the produced water, 4.4'-diaminodiphenylmethane 19.8g (0.1 A solution of mol) dissolved in 80 g of toluene was added slowly for 4 hours. The maleamic acid concentration in this funnel was 0.05 moi/l or less. The reaction was further continued by refluxing and dehydrating for 1 hour. After the reaction is completed, 30
Cooled to ℃. The precipitated crystals were separated and washed with a 2% aqueous sodium carbonate solution, dried, and a pale yellow powder of N,
N'-4,4°-diphenylmethane bismaleimide 29
．． 8g was obtained. High performance liquid chromatography (hereinafter referred to as HP
Purity was measured by LC (abbreviated as LC). The results are shown in Table 1.

Example 2 The same procedure as Example 1 was carried out except that the amount of maleic anhydride charged was changed to 60 g (0.61 mol), and yellow powder N,
N''-4,4°-diphenylmethane bismaleimide was obtained. The results are shown in Table 1. Same as Example 3 except that the same amount of NMP was used instead of DMF as the solvent. Reaction 12, yellow powder N
, N'-4,4°-diphenylmethane bismaleimide was obtained. The results are shown in Table 1.

Example 4 The amount of maleic anhydride charged was 33□8g (0.34 mol)
The experiment was carried out in the same machine as Example 1 except that 300 g of toluene and 30 g of DMF were used, and yellow powder N, N'-4,
4'-diphenylmethane bismaleimide was obtained. The results are shown in Table 1.

Example 5 A reaction was carried out in the same manner as in Example 1 except that 6 g of 85% phosphoric acid was used as the acid catalyst. After the reaction, the reaction mixture was cooled to 570° C. while being kept still and separated into two layers.

The lower catalyst layer was separated, and the upper layer was further cooled to 30° C. to precipitate crystals. The crystals were separated from P, washed with an aqueous sodium carbonate solution, and then dried to obtain a yellow powder of N,N-4,4'-diphenylmethane bismaleimide. The results are shown in Table 1. Example 6 210 g of Ffi containing bismaleimide recovered in Example 1 was put into a reactor similar to Example 1, and the liquid composition was 1
-Luene 85.2t%, DMF 4゜8wt,%,p
-)-Luenesulfonic acid Q, gwt%, maleic anhydride 6.8wt%, the mal/imide 2°4 wt%) and maleic anhydride 21.4g (0°22 mol) were charged,
While stirring and heating to reflux temperature to remove the produced water, 19.8 g of 4.4"-diaminodiphenylmethane (
0.1 mol) of the melt was continuously fed 17 over a period of 4 hours. During this time, MaI/amic acid concentration 0.05 mol/
It was less than l. The reaction was further continued by refluxing and dehydrating for 1 hour.

After the reaction was completed, the mixture was cooled to 30°C (17), and the precipitated crystals were separated in a furnace, washed with a 2% aqueous sodium carbonate solution, and dried to obtain a pale yellow powder of N,N''4.4'-diphenylmethane bismaleimide. The results are shown in Table 1. Example 7 The reaction was carried out in the same manner as in Example 1 except that the same amount of dimethyl sulfoxide was used instead of DMF as a solvent.゛-Diphenylmethane bismaleimide was obtained 2 The results are shown in Table 1.

Examples 8 to 14 Using various α, β-unsaturated dicarboxylic acid anhydrides and aromatic polyamines, changing the acid catalyst and the α, β-unsaturated dicarboxylic acid anhydride to be added as shown in Table 2, α, β- The molar ratio of the unsaturated dicarboxylic acid anhydride to the aromatic polyamine and the operation were the same as in Example 1 to obtain an aromatic cyclic polyimide. In addition, when sulfuric acid or phosphoric acid was used as a catalyst, the catalyst layer was separated and crystallized after the reaction. The results are shown in Table 2.

Comparative Example 1 The same reaction as in Example 1 was carried out except that no acid catalyst was used. The solution became a slurry, and even after 8 hours of reaction, only a small amount of the produced water distilled out, and HPLC analysis showed that According to N, N"4.4'-diphenylmethane bisma l/
The imide production rate was 10% or less.

Reaction 1. was carried out in the same manner as above. , Q Tokoro, HPi-4
According to C analysis, the production rate of N,N"4.4"-diphenylmethane bismaleimide was 46%. In addition, many peaks caused by addition reactions and isomerized acids were observed.

Comparative Example 2 In Example 1, a toluene solution of 4,4'-diaminodiphenylmethane was charged at 30°C, and then the temperature was raised and dehydration was carried out under reflux for 5 hours. occured. According to HPLC analysis of the dissolved content, the production rate of N,N'-4,4'diphenylmethane bismaleimide was 38%. In addition, many peaks due to isomerized acids were observed.

Comparative Example 3 In Example 1, the toluene solution of 4,4'-diaminodiphenylmethane was charged at a concentration of amic acid in the reaction solution of 0.
．． Comparative Example 4 carried out at a rate of 2 mol/1 The same operation as in Example 8 was carried out except that 400 g of toluene and 40 g of DMF were used as reaction solvents.After the reaction, the reaction solution was cooled, but the target N, No crystals of N'4.4'-diphenylmethane bismaleimide were obtained. As a result of 14 PLC analysis of the solution, N,N'-4,4'
-The production rate of diphenylmethane bisma l/imide is 90%
Met.

Comparative Example 5 The same operation as in Example 1 was performed except that 15 g of toluene and 5 g of DMF were used as the reaction solvent, and a pale yellow powder of N, N'-
4,4'-diphenylmethane bismuth/imide was obtained.

According to HP t,C analysis, the production rate of N, N'4.4'-diphenylmethane bisma l/imide was 26%, and many other by-product peaks were observed.

Comparison Fuchi 6 The same operation as in Example 1 was performed except that 40 g of toluene and 40 g of DMF were used as reaction solvents. After the reaction, the reaction solution was cooled, but no crystals of N,N'-4,4'-diphenylmethane bismaleimide were obtained. The solution was subjected to HPLC
As a result of analysis, the production rate of N,N-4,4°-diphenylmethane bismaleimide was 76%.

(Effects of the Invention) According to the method of the present invention, a highly pure cyclic polyimide can be obtained in high yield by a simple one-step method, and the solvent and catalyst can be used repeatedly, which is advantageous over Hongji.

Claims (2)

[Claims] (1) a, β-unsaturated dicarboxylic acid anhydride and 2 in the molecule
When producing a cyclic polyimide by heating and dehydrating aromatic polyamines having 1 or more primary amino groups, 100 parts by weight of the aromatic polyamine is mixed with α,β-unsaturated dicarboxylic acid anhydride (of the aromatic polyamine). The ratio to the primary amino group is 1 to 20 times by mole) and the mixed solvent is 100 to 20 times
00 parts by weight (60 to 99 parts by weight of a nonpolar solvent that has azeotropy with water)
An aromatic compound characterized by being added continuously or intermittently to a reaction system in which an aprotic polar solvent (by weight % and 40 to 1 weight %) and an acid catalyst are present and being dehydrated under heating under reflux. Method for manufacturing polyimide. (2) A method for producing aromatic polyimide, which comprises crystallizing and separating polyimide from the reaction mixture obtained by the reaction method according to claim 1.