JPS63273621A - Production of heat-resistant polymer - Google Patents

Abstract

PURPOSE:To obtain the titled polymer, having excellent heat resistance, mechanical and electrical characteristics, etc., and suitable as composite and electrical insulating materials, adhesives, etc., by reacting an organic polyfunctional isocyanate with an organic polyfunctional carboxylic acid (anhydride) using a specific catalyst. CONSTITUTION:(A) An organic polyfunctional isocyanate (e.g. diisocyanatoe thane) and (B) an organic polyfunctional carboxylic acid (anhydride) (e.g. malonic acid) are dissolved in, e.g. an inert organic solvent (e.g. N,N- dimethylacetamide) and then (C) an alkali metal fluoride (e.g. lithium fluoride) and (D) a crown compound (e.g. dibenzo-14-crown-4) as a catalyst are added to carry out reaction at 20-250 deg.C in an atmosphere of an inert gas, e.g. nitro gen, to afford the aimed polymer.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention relates to a method for producing a heat-resistant polymer obtained from an organic polyvalent incyanate and an organic polyvalent carboxylic acid or an organic polyvalent carboxylic acid anhydride. . In addition to heat resistance, these polymers also have excellent heat insulation properties, radiation resistance, dimensional stability under heat, mechanical properties, electrical properties, chemical resistance, and flame retardancy, so they are used in various industrial materials and protective materials. , high-performance industrial materials such as composite materials, reinforcing materials, and electrical insulation materials, as well as molded products, films, paper, fibers, festivals, and adhesives in the electrical, electronic, automobile, vehicle, and aircraft industries, as well as clothing and interior industries. It can be widely used for drugs, etc.

[Conventional technology]

It is well known that heat-resistant polymers can be produced by reacting organic polyvalent isocyanates with organic polyvalent carboxylic acids or organic polyvalent carboxylic acid anhydrides. It is difficult to obtain a polymer with a high molecular weight that can be processed into a form and exhibit sufficient physical properties, and therefore adhesives, wax, etc. have been used in most cases. In addition, when the polyvalent isocyanate used in the reaction reacts,
Particularly at high temperatures, various side reactions occur, which often causes problems such as gelation during the reaction, and side-reactants mixed into the polymer reduce the heat resistance and physical properties of the polymer. Ta. For this reason, various catalysts have been developed for the above-mentioned reaction system.
．． 186.4.061.622 and 4.061°623
, +21 How to use Lactamate: USP.

4.021.4) 2.4.094.864 and 4.09
4.866, +31 Method using cyclic phosphorus oxide: USP, 4゜156.065 Furthermore, (4) Method using alkali metal salt of polyhydric carboxylic acid: JP-A-57
-151615, (51 Method using alkali metal carbonate or hydrogen carbonate: JP-A-58-18629, (
6) Method using alkali metal hydroxide: JP-A-58
-67723 etc. However, even when the above catalysts are used, they often gel due to side reactions of organic polyvalent isocyanates, or tend to form polyisocyanates. There were problems such as not being able to obtain a polymer with good physical properties.

[Problem that the invention seeks to solve]

An object of the present invention is to provide a method for producing a linear high molecular weight polymer that does not undergo gelation during the reaction and suppresses side reactions caused by organic polyvalent isocyanates.

[Means for solving problems]

The present inventors investigated a method for producing a heat-resistant polymer by reacting such an organic polyvalent isocyanate with an organic polyvalent carboxylic acid or an organic polyvalent carboxylic acid anhydride.
This has led to the completion of the present invention.

That is, the present invention provides a method for producing a heat-resistant polymer by reacting an organic polyvalent isocyanate with one or more compounds selected from the group consisting of an organic polyvalent carboxylic acid or an organic polyvalent carboxylic acid anhydride. This is a method for producing a heat-resistant polymer, characterized by using an alkali metal fluoride represented by (1) and a crown compound as a catalyst.

MF... (1) Organic polyvalent isocyanates that can be used in the present invention include:
-M All known organic polyvalent isocyanates can be used, but the following can be particularly exemplified.

Examples of diisocyanates include those described in JP-A-57-151615, such as 1,2-diisocyanateethane, cyclohexane-1,4-diisocyanate, 4,4°-methylenebis(cyclohexylisocyanate), m-xylene diisocyanate, and phenylene. −
1,4-diisocyanate, phenylene-1,3-diisocyanate, tolylene-2,4-diisocyanate,
Examples include tolylene-2,6-diisocyanate, diphenylmethane-4,4'-diisocyanate, diphenyl ether-4,4'-diisocyanate, and 1,5-naphthalene diisocyanate.

In addition, the following are examples of organic polycarboxylic acids or organic polycarboxylic acid anhydrides that can be used in the present invention.
For example, as an organic polycarboxylic acid, JP-A-57-179
No. 223, for example, dicarboxylic acids include oxalic acid, malonic acid, succinic acid, glutaric acid, adipic acid, pimelic acid, superric acid, azelaic acid, sebacic acid, terephthalic acid, isophthalic acid, and hexahydroterephthalic acid. , diphenylsulfone-4,4'-dicarboxylic acid, biphenyl-4,4'-dicarboxylic acid, thiophene-2,5-dicarboxylic acid, pyridine-2,6-dicarboxylic acid, naphthalene-2,6-dicarboxylic acid, 4 ,4°−
diphenylmethane-bis-trimellitic imide acid,
4.4'-diphenyl ether bistrimeric imide acid, etc. Tricarboxylic acids include butane-1,2,4-)licarboxylic acid, cyclohexane-1
,2,3-)licarboxylic acid, cyclopenkungenyl-
3,4,4'-1-licarboxylic acid, cyclopentadienyl-1,2,4-tricarboxylic acid, benzene-1,2
, 4-tricarboxylic acid, naphthalene-1,4,5-1-
Ricarboxylic acid, biphenyl-3,4,4°-tricarboxylic acid, diphenylsulfone-3,4,3'-tricarboxylic acid, diphenyl ether-3,4,3°-tricarboxylic acid, benzophenone-3,4,4'-) There are recarboxylic acids, etc. Examples of tetracarboxylic acids include butane-1,2,3,4-tetracarboxylic acid, pentane-1
, 2,4,5-tetracarboxylic acid, cyclohexane 1.
2,3.4-tetracarboxylic acid, benzene-1,2,4
, 5-tetracarboxylic acid, naphthalene 2.3.6.7-
Tetracarboxylic acid, biphenyl-3,3', 4.4"
-Tetracarboxylic acid, benzophenone-3,3°, 4.
4′-tetracarboxylic acid, diphenyl ether-3,3
', 4.4°-tetracarboxylic acid, diphenylsulfone-3,3', 4.4'-tetracarboxylic acid, 2.2-bis(3,4-dicarboxyphenyl)propane, furan-2,3, 4,5-tetracarboxylic acid, pyridine-2,
Examples include 3,5,6-tetracarboxylic acid.

In addition, the organic polycarboxylic acid anhydride is, for example, an acid anhydride derived from tricarboxylic acid, in which case 1
An acid anhydride containing one carboxyl group and one acid anhydride group and further derived from a tetracarboxylic acid, in this case,
There are those containing two acid anhydride groups in the molecule, and those containing one acid anhydride group and two carboxyl groups, and the following are examples. Examples of organic polycarboxylic anhydrides include trimellidic anhydride, benzene-1,2,
3-tricarboxylic anhydride, butane-1,2,3,4-
Tetracarboxylic dianhydride, pyromellitic dianhydride, diphenyl-3,3',4,4'-tetracarboxylic dianhydride, naphthalene-2,3,6,7-tetracarboxylic dianhydride, naphthalene -1,4,5,8-tetracarboxylic dianhydride, diphenyl ether-3,3',4
, 4°-tetracarboxylic dianhydride, diphenylsulfone-3,3°, 4,4′-tetracarboxylic dianhydride.

Diphenylketone 3,3°,4,4''-tetracarboxylic dianhydride, 2,2-bis(3,4-dicarboxyphenyl)propane dianhydride, furan-2,3,4,
5-tetracarboxylic dianhydride, pyridine-2,3,5
, 6-tetracarboxylic dianhydride, and the like.

Further, the alkali metal fluoride used in the present invention includes lithium fluoride, sodium fluoride, potassium fluoride, cesium fluoride, rubidium fluoride, etc., and potassium fluoride and cesium fluoride are particularly preferred.

The crown compound used in the present invention is a cyclic polyether in which an oxygen atom constitutes a part of the ring, and it three-dimensionally incorporates cations in water into the ring to increase its affinity with hydrophobic organic solvents. Specifically, crown ethers, aza crown ethers, thiocrown ethers, cyclic azathia ether azathia crown ether, multicyclic crown ethers, isolated multicyclic composite donor crown compounds, cryptants, such as dibenzo-14-crown-4,15-crown-5,
18-crown-6, dicyclohexyl-24-talalan-8, dibenzo-24-crown-8, dicyclohexyl-24-crown-6, dibenzo-24-crown-8, dicyclohexyl-18-crown- 6, dibenzo-18-crown-6, etc. Other 5 scientists
e 174,459 (1971) and those shown in "Crown Compounds" written by Michio Hiraoka and published by Kodansha in 1978.

Heat-resistant polymers obtained from organic polyvalent isocyanates and organic polyvalent carboxylic acids or organic polyvalent carboxylic acid anhydrides include (
1) Organic polyvalent isocyanate and organic polyvalent carboxylic acid, (
2) Organic polyvalent isocyanates, organic polyvalent carboxylic acids and organic polyvalent carboxylic acid anhydrides, and (3) organic polyvalent isocyanates and organic polyvalent carboxylic acid anhydrides, all of which are useful as heat-resistant polymers. In the case of (1), the skeleton of the polymer produced is formed from an amide group, in the case of (2), it is formed of an amide group and an imide group, and in the case of (3), it is formed of an imide group.

The reaction according to the method of the present invention involves reacting a mixture of an organic polyvalent isocyanate, an organic polyvalent carboxylic acid or an organic polyvalent carboxylic acid anhydride, an alkali metal fluoride, and a crown compound in an inert organic solvent in a substantially anhydrous state. inert gas,
For example, heating is performed at a temperature of 20° C. to 250° C., preferably 100° C. to 200° C., for 1 to 20 hours in a nitrogen atmosphere. The molar ratio of the organic polyisocyanate to the organic polycarboxylic acid or polycarboxylic acid anhydride used in the reaction is used in the range of 0.70 to 1.30, particularly 0.95.
It is preferable to use it within the range of 1.10 to 1.10. Outside this range, a high molecular weight heat-resistant polymer cannot be obtained. The total amount of the alkali metal fluoride and crown compound used as a catalyst is preferably 0.01 to 10 mol%, particularly 0.1 to 5 mol%, based on the total amount of the raw material carboxylic acid or acid anhydride. is preferable.If the amount is less than this range, it will be difficult to obtain a high molecular weight polymer, and if the amount is more than this range, the heat resistance of the polymer will decrease due to catalyst residue remaining in the produced polymer, resulting in a decrease in quality. Also, the amount of the crown compound used relative to the alkali metal fluoride is in the range of 1 to 500 mol%, preferably 10 to 300 mol%, more preferably 30 to 200 mol%.
The range is mole %. The raw material isocyanate and carboxylic acid or acid anhydride, as well as the alkali metal fluoride and crown compound may be charged into the reaction system at the same time.
Although they may be added to the reaction system in any order, it is usually best to add the raw materials to the reaction system at the same time at room temperature, or by using a solvent and feeding the raw materials into the solvent. Alternatively, one of the acid anhydrides, preferably an isocyanate, may be continuously added and reacted at a predetermined reaction temperature. Further, the amount of the solvent to be used can be appropriately selected depending on the performance of the final polymer and the reaction temperature. Generally, it is preferable to select conditions that do not interfere with stirring due to thickening during polymerization.

Examples of organic solvents used in the present invention include N, N-
Chain or cyclic amides or phosphorylamides such as dimethylacetamide, N,N-dimethylformamide, N-methylpyrrolidone, γ-butyrolactone, hexamethylphosphoric triamide, or sulfoxides or sulfones such as dimethylsulfoxide and diphenylsulfone. , ureas such as tetramethylurea, N,
Cyclic ureas such as N'-dimethylethylene urea, benzene, toluene, xylene, decalin, cyclohexane, heptane, hexane, pentane, methylene chloride, chlorobenzene, dichlorobenzene, tetrahydrofuran, etc. are used.

After completion of polymerization, in order to separate the polymer as a solid, the reaction solution is poured into a non-solvent for the polymer to precipitate the polymer. The precipitated polymer is also thoroughly washed with the same non-solvent to remove remaining catalyst and other impurities. After washing, the polymer is dried at room temperature or high temperature, and in some cases under reduced pressure.

(Examples) The method of the present invention will be explained below with reference to Examples, but the present invention is not limited to these.

[Example 1] Terephthalic acid 12.45 g (0,0750 mol) and isophthalic acid 12.47 g (0,0751 mol) were placed in a 500 ml separable flask equipped with a stirrer, thermometer, condenser, dropping funnel, and nitrogen introduction tube. , potassium fluoride 0.
0407 g (7, OxlO-4 mol), dibenzo-18
- Crown-60,252 (7,0Xl0-4 mol) and anhydrous sulfolane 3501 were charged, and the mixture was heated to 200°C.
Heat to. While maintaining this temperature, trilene-2,4
-diisocyanate-) 26.20g (0,1505 mol)
was added dropwise over 2 hours.

The mixture was cooled to room temperature where stirring was continued for an additional 2 hours. During cooling, the polymer began to precipitate, and at room temperature it almost became a slurry. The polymer was filtered off, thoroughly washed with a large amount of methanol, and then dried under reduced pressure at 150 tons for 3 hours to obtain a milky white polymer powder. Logarithmic viscosity of the obtained polymer (using concentrated sulfuric acid as a solvent, Poly?-0.1g/100cc
, 30°C, the same conditions hereinafter) was 3.0. Of this polymer! R spectrum is 1.660 cm-', 1.5
Amide absorption was observed at 30 c+s-'. A dope made by dissolving this aromatic polyamide in dimethylacetamide (
10 wt. Got the film. The tensile strength of this film was 1280 Kg/and the elongation was 12%. Further, the glass transition temperature T of the film was 260° C. (TMA method).

5% by weight determined from a thermobalance (10°C/l1in in air)
The weight loss temperature was 4) 5°C.

[Comparative Example 1] Fluoride compound and dibenzo-18-crown- as a catalyst
Polymerization was carried out in the same manner as in Example 1, except that 6 was not used.

A mixture of 12.46 g (0.0750 mol) of terephthalic acid, 12.46 g (0.0750 mol) of isophthalic acid and 350 Ill of sulfolane anhydride is heated to 200°C, and at this temperature 26.2 g of tolylene-2,4-diisocyanate is dissolved.
5 g (0.1507 mol) was added dropwise over 2 hours. After continuing stirring for another 2 hours, the mixture was cooled to room temperature. During cooling, the polymerization solution became milky and became a suspension at room temperature. This product was poured into a large amount of methanol, and the product was filtered, thoroughly washed with methanol, and dried under reduced pressure at 150°C for 3 hours. The obtained polymer was a white fine powder with a low molecular weight and a logarithmic viscosity of 0.26.

[Comparative example 2] The effect of only the crown compound as a catalyst was examined.

Polyamide was polymerized using the same apparatus as in Example 1.

12.40 g (0,0746 mol) of terephthalic acid, 12.53 g (0,0754 mol) of isophthalic acid and 60,252 g (7,0×10-
4 mol) and 350 ml of anhydrous sulfolane.
The mixture was heated to 0° C., and at this temperature 26.28 g (0.1509 mol) of tolylene-2,4-diisocyanate was added dropwise over 2 hours. After continuing stirring for another 2 hours, the mixture was cooled to room temperature. The cooled mixture was milky and was poured into a large amount of methanol to precipitate the product.The filtrate was thoroughly washed with methanol and dried under reduced pressure at 150°C for 3 hours. The obtained polymer was a white fine powder with a low molecular weight and a logarithmic viscosity of 0.32.

Example 2 Polyamide was polymerized using the same apparatus as in Example 1.

22.43 g (0,1350 mol) of isophthalic acid, 2.821 g (0,0150 mol) of azelaic acid, 0.2450 g (1,6xlO-3 mol) of cesium fluoride, 18-
Crown - 60,3965g (1,5Xl0-3 mol)
and N,N'-dimethylethylene urea 3501, the mixture was heated to 200°C, and at this temperature tolylene-2
, 26.29 g (0.1510 mol) of 4-diisocyanate was also added dropwise over 2 hours, and the reaction was continued for an additional 2 hours. A portion of the polymerization solution cooled to room temperature was poured into a large amount of methanol to coagulate the polymer, which was then sufficiently washed with methanol and dried under reduced pressure at 150° C. for 3 hours to obtain a milky white polymer powder. The logarithmic viscosity of this polyamide was 1.4, and the 5% weight loss temperature on heating was 393°C as measured by thermobalance. Furthermore, the Tg of the film made in the same manner as in Example 1 was 2.
The temperature was 25°C.

Comparative Example 3 Polyamide was polymerized using sodium methoxide in place of cesium fluoride and 18-crown-6.

22.42 g (0,1350 mol) of isophthalic acid, 2.821 g (0,0151 mol) of azelaic acid, 0.0860 g (1,6X 10-3 mol) of sodium methoxide and 350 ml of N,N'-dimethylethylene urea. charge, maintain the mixture at 200°C, and add trilene-2,4-
26.29 g (0.1501 mol) of diisocyanate was added dropwise over 2 hours, and the reaction was continued for an additional 2 hours. This polymerization liquid was treated in the same manner as in Example 2 to obtain polyamide powder. This polyamide had a logarithmic viscosity of 0.27 and an extremely low molecular weight. This powder was heat pressed at 280°C and 100K to make a 0.5sn thick product.

The pressed sheet was too brittle to be used for physical property measurements.

Example 3 Using the same apparatus as in Example 1, polyamideimide was polymerized. 19.30 g of trimellidic anhydride (0,10
05 mol), cesium fluoride 0.1575 g (0,001
0 mol), dicyclohexyl-18-crown-60,
0633g (1,5Xl0-4 mol) and 350+el of anhydrous sulfolane were charged and the mixture was heated to 200°C with stirring under nitrogen atmosphere. Maintaining this temperature, diphenylmethane-4,4'-diisocyanate 7.575
A solution of g (0,0303-El) dissolved in anhydrous sulfolane 301 was added dropwise over 2 hours. After further reaction for 1 hour, tolylene-2,4-diisocyanate 12.31
g (0,07070 mol) of anhydrous sulfolane 20+m
A solution dissolved in 1 ml was added dropwise over 2 hours. After further reaction for 1 hour, it was cooled to room temperature. Approximately 130' during cooling
The polymer began to precipitate from around c, resulting in turbidity, and when it was cooled to room temperature, it took on the form of a slurry. The material was filtered, washed thoroughly with more methanol, and then washed with methanol, and the product was dried under reduced pressure at 150'C for 3 hours. The logarithmic viscosity of the obtained polymer was 1.25. The IR spectrum of this polymer is 1780 cm
-', 1720c+m-', imide bond absorption at 1370cm-', 1670cm-', 1530cm-'
Absorption of amide bonds was observed. A cast film made using a solution (10% by weight) of this polymer dissolved in N-methylpyrrolidone in the same manner as in Example 1 was a strong film with a pale yellow edge and a tensile strength of 1200 kg/cs.
, the Tg of the film (TMA method) is 267 at an elongation of 14%.
It was ℃. Further, the 5% weight loss temperature of this polymer was 435°C as measured by thermobalance.

Comparative Example 4 Cesium fluoride and dicyclohexyl-18-crown-6
Polycondensation of polyamideimide was carried out in the same manner as in Example 3 except that the polyamideimide was not used. The raw materials used are as follows.

Trimelidic anhydride 19.31g (0,1005 mol)
, diphenylmethane-4,4-diisocyanate7.
572g (0,0303 mol), tolylene-2,4-diisocyanate) 12.30g (0,0707 mol)
, anhydrous sulfolane 4001, respectively, became emulsified during cooling, and a portion of this was poured into a large amount of methanol to solidify the polymer. After thoroughly washing with methanol, it was heated at 150°C under reduced pressure for 3 hours. After drying, a pale yellow polymer was obtained. The logarithmic viscosity of this polymer was 0.21.

Example 4 Polyimide was polymerized using the same apparatus as in Example 1.

Benzophenone tetracarboxylic dianhydride 25.08g
(0,0779 mol), potassium fluoride 0.10110 l
6.0017 mol), dibenzo-18-crown-60
, 6300 g (0,0017 mol) and 300 ml of N,N'-dimethylethylene urea were charged, and the mixture was heated to 200° C. with stirring under a nitrogen atmosphere. While maintaining this temperature, 15.66 g (0,0626 mol) of diphenylmethane-4,4)-diisocyanate was added to N,N'
- A solution of 20 parts of dimethylethylene urea was added dropwise over 1 hour. After further reaction for 1 hour, tolylene-
A solution of 2.73 g (0,0157 mol) of 2,4-diisocyanate dissolved in 20@l of anhydrous N,N"-dimethylethylene urea was added dropwise over 1 hour. After further reaction for 1 hour, the mixture was cooled to room temperature. This material was poured into a large amount of water with strong stirring, the precipitate was filtered, and further washed thoroughly with water, then washed with methanol, and dried under reduced pressure at 150°C for 3 hours to obtain a pale yellow color. A powder was obtained. The logarithmic viscosity of this polymer was 0.99. This polymer was dissolved in N-methylpyrrolidone and prepared in the same manner as in Example 1. The tensile strength of the film was 1120 kg/cm". is 26%
Met. The 5% weight loss temperature of this polymer was 471°C as measured by thermobalance.

Comparative Example 5 Polyamideimide was polymerized in the same manner as in Example 4 except that potassium fluoride was not used. The raw materials used are as follows.

Benzophenone tetracarboxylic dianhydride 24.97g
(0,0775 mol), dibenzo-18-crown-6
0,6295 g (0,0017 mol), diphenylmethane-4,4''-diisocyanate) 15.58 g (0,
623 mol), tolylene-2,4-diisocyanate 2
．． 71 g (0,0156 mol), anhydrous N, N'-dimethylethylene urea 340 ml. The logarithmic viscosity of the polymer obtained by post-treatment in the same manner as in Example 4 was 0.28.

Example 5 Polyimide was polymerized using the same apparatus as in Example 1.

Butanetetracarboxylic dianhydride 20.13g (0,1
016 mol), fluoridation power'J ram 0.1772 (0,0
0306 mol), 18-crown-60,3432 g (
0,0013 mol) and 350 ml of N,N'-dimethylethyleneurea were charged and the mixture was heated to 200°C under nitrogen atmosphere. While maintaining this temperature, diphenylmethane-4,4''-diisocyanate 20.45
A solution prepared by dissolving N,N'-dimethylethyleneurea (0,0818 mol) in 50 ml of N,N'-dimethylethylene urea was added dropwise over 1 hour.

After further reaction for 1 hour, 3.55 g (0,0204 mol) of tolylene-2,4-diisocyanate was added with anhydrous N,
A solution dissolved in N"-dimethylethylene urea 21)++1 was added dropwise over 1 hour. After reacting for another 1 hour, the 0 reactant cooled to room temperature was poured into a large amount of water with strong stirring. The precipitated polymer was filtered, further washed with a large amount of water, and then dried under reduced pressure at 150°C for 3 hours to obtain a pale yellow powder.The logarithmic viscosity of this polyimide was 0.96, and the method was the same as in Example 1. The cast film made was strong, with a tensile strength of 920 kg/ca+' and an elongation of 33%.

〔effect〕

The polymer according to the present invention can be subjected to melt molding, or in some cases can be redissolved in a solvent to be used as a face or adhesive, or can be used in the production of cast films or fibers. Further, the polymerization solution can be used as it is as a dope for spinning.

Claims (5)

[Claims] (1) In a method for producing a heat-resistant polymer by reacting an organic polyvalent isocyanate with one or more compounds selected from the group consisting of organic polyvalent carboxylic acids or organic polyvalent carboxylic acid anhydrides, a method of producing a heat-resistant polymer with the general formula ( A method for producing a heat-resistant polymer, which comprises using an alkali metal fluoride represented by 1) and a crown compound as a catalyst. MF・・・・・・(1) (2) The method according to claim 1, wherein a polyamide is produced by reacting an organic diisocyanate and an organic dicarboxylic acid. (3) The method according to claim 1, wherein polyamide or polyamide-imide is produced by reacting an organic diisocyanate with a compound selected from the group consisting of organic tricarboxylic acid or tricarboxylic acid anhydride. (4) The method according to claim 1, wherein polyimide or polyamide-imide is produced by reacting an organic diisocyanate with a compound selected from the group consisting of organic tetracarboxylic acids or tetracarboxylic dianhydrides. (5) The method according to claim 1, wherein the alkali metal fluoride is potassium fluoride or cesium fluoride.