JPH02117921A - New aromatic polyether sulfone copolymer and production thereof - Google Patents

Abstract

PURPOSE:To obtain the subject copolymer, excellent in heat resistance well balanced with moldability and useful for electrical insulating applications, etc., by reacting a specific sulfone compound with a dihydric phenolic mixture in the presence of an alkali metal carbonate. CONSTITUTION:A dihalodiphenyl sulfone compound expressed by formula I (X is halogen; R<1> and R<2> are H, 1-8C hydrocarbon or halogen; a and b are 1-4) is reacted with a dihydric phenolic mixture consisting of 1-99mol% dihydroxydiphenyl sulfone compound expressed by formula II (R<3> to R<6> are H, 1-8C hydrocarbon, halogen and at least one of the R<3> to R<6> is 1-8C hydrocarbon) and the remainder 99-1mol% dihydroxydiphenyl compound expressed by formula III in the presence of an alkali metal carbonate in an aprotic polar solvent to afford the objective copolymer, containing 1-99mol% mol fraction of recurring units expressed by formula IV and 99-1mol% mol fraction of recurring units expressed by formula V, linked through ether bonds and having >=0.1dl/g reduced viscosity.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a novel aromatic polyethersulfone copolymer having excellent heat resistance and moldability, and a method for producing the same.

[Conventional technology]

Polyether sulfone, which has the following repeating structural unit, has been known as an engineering plastic with excellent heat resistance.

The polyether sulfone has a glass transition temperature (Tg) of 220 to 226°C, has excellent mechanical properties at relatively high temperatures, and has good electrical properties, chemical resistance, and flame retardancy, so it is already widely used. However, in recent years, there has been an increase in the number of application fields that require even greater hardness and heat resistance, and polymers with even greater heat resistance are desired from all sides.

[Problem to be solved by the invention]

Although polyether sulfone has excellent mechanical and electrical properties as mentioned above, its use has been limited due to insufficient heat resistance to meet recent requirements. The method for producing polyether sulfone is as follows: in the presence of a dehalogenating agent;
4.4”-dihydroxydiphenylsulfone and 4.4”
There is a method of polycondensation with -dichlorodiphenylsulfone, but in order to improve the heat resistance of the polyethersulfone, for example, 3.3' 5.5°-tetra instead of the above 4,4'-dihydroxydiphenylsulfone is used. A method using alkyl-4,4'-dihydroxydiphenyl (JP-A-52-27500) was proposed. Although this method improves the Tg to 270-275°C, the resulting polymer may generate decomposition gas during molding,
It had the disadvantage of causing burns. That is, there was a problem in that thermal decomposition occurred during molding, and the original characteristics of polyether sulfone were lost.

[Means to solve the problem]

In view of the above problems, the present inventors aimed to improve the heat resistance of polyether sulfone without impairing its excellent properties such as mechanical properties, electrical properties, and thermal stability during molding.
We conducted extensive research to obtain a polymer with a good balance between heat resistance and moldability, and finally found a polymer structure that meets the objectives of the present invention.As a result of pursuing a good industrial method for producing it,
The present invention has now been completed.

That is, the present invention relates to the general formula (1) (wherein R' and R'' may be the same or different from each other, a hydrogen atom, a straight or branched hydrocarbon group having 1 to 8 carbon atoms, or a halogen represents an atom.

a and b are each integers of 1 to 4, and may be the same or different from each other. ) The molar fraction of the repeating unit represented by formula (II) is 1 to 99 mol%, and R'+ R2+ a+ b represents the same meaning as above, and R3-R6 also represents R1 or It has the same meaning as R2, and at least one of R3-R6 is a straight or branched hydrocarbon group having 1 to 8 carbon atoms. It is a copolymer in which repeating units (I) and repeating units (II) are arranged in a disordered or orderly manner and connected to each other by ether bonds, and the concentration is 0.
25°C with 5g/dl N-methyl-2-pyrrolidone solution
The reduced viscosity of the copolymer measured in
A novel aromatic polyether sulfone copolymer characterized by the above characteristics and a method for producing the same are firstly expressed by the general formula (III) (wherein lit, R2, a, b represent the same meanings as above, X represents a halogen atom.) A dihalodiphenylsulfone compound represented by
9 mol% is a dihydroxydiphenyl sulfone compound represented by general formula (TV), and the remaining 99 to 1 mol% is general formula (V) (in which R3 to R
6 represents the same meaning as above. ), and a dihydric phenol mixture consisting of a dihydroxydiphenyl compound shown in
Second, the method for producing the above-mentioned novel aromatic polyether sulfone copolymer, which is characterized by reacting in an aprotic polar solvent in the presence of an alkali metal carbonate, is carried out using the general formula (
III) A dihalodiphenylsulfone compound represented by (wherein R', R2, a, b represent the same meanings as above, and X represents a halogen atom), and 1 to 9
9 mol% is a dihydroxydiphenyl sulfone compound represented by the general formula (1'V) U, and the remaining 99 to 1 mol% is a dihydroxydiphenyl sulfone compound represented by the general formula (V) (in which R3 to R
6 represents the same meaning as above. ) with a dihydric phenol alkali metal di-salt obtained by reacting an alkali metal salt-forming agent with a dihydric phenol mixture consisting of a dihydroxydiphenyl compound shown in ) in an aprotic polar solvent. The present invention provides a method for producing a novel aromatic polyether sulfone copolymer.

The novel aromatic polyether sulfone copolymer (hereinafter abbreviated as copolymer) according to the present invention is produced by
Compared to the polymer described in Japanese Patent No. 27500, this polymer has significantly improved thermal stability and moldability during molding. Furthermore, it has improved heat resistance than the conventional polyether sulfone, and by adjusting the ratio of the dihydric phenol component, the glass transition temperature can be raised within an arbitrary range up to about 60°C. It is possible. Therefore, the copolymer according to the present invention maintains its mechanical and electrical properties up to higher temperatures than polyether sulfone, has good thermal stability during molding, and has a good combination of heat resistance and moldability. It has excellent balance and can now be used in many fields of application that were previously difficult to use.

The copolymer according to the present invention has a connecting chain in which one or more repeating units represented by the general formula (N) are linearly connected through an ether bond, and a repeating unit represented by the general formula (II). One or more linking chains are connected in a linear manner through an ether bond, and the connecting chains are arranged randomly or in an orderly manner and connected in a linear manner through an ether bond.

In addition, the molar fraction of the repeating unit (r) is 1 to 99 mol%
and preferably 5 to 85 mol%. If the molar fraction is less than 1 mol %, the effect of improving moldability and thermal stability during molding is insufficient.

The molecular weight of the copolymer according to the invention can be expressed as a reduced viscosity measured in N-methyl-2-pyrrolidone at 25° C. at a concentration of 0.5 g, 1, 0.1 d1. /g or more. If the reduced viscosity is less than O.Ia/g, the mechanical properties will deteriorate, making it impractical.

Examples of the halogen atom represented by X in the dihalodiphenylsulfone compound represented by the general formula (II[) used in the present invention include fluorine, chlorine, and iodine, with fluorine and chlorine being particularly preferred. Also, R',
R2 is a hydrogen atom, a linear or branched hydrocarbon group having 1 to 8 carbon atoms, or a halogen atom. Preferred halogen atoms are chlorine and bromine. Specific examples of hydrocarbon groups include methyl, ethyl, n-propyl, isopropyl, n-butyl, isobutyl, t-butyl, n-amyl, isoamyl, neopentyl, hexyl, heptyl, n
-octyl, inoctyl, etc. can be mentioned.

Preferred compounds include those represented by the following formulas.

Preferred compounds as the dihydroxydiphenyl sulfone compound represented by the general formula (TV) used in the present invention include those represented by the following formula.

R3-R6 in the dihydroxydiphenol compound represented by the general formula (V) used in the present invention are a hydrogen atom, a linear or branched hydrocarbon group having 1 to 8 carbon atoms, and a halogen atom; At least one of -R6 is a linear or branched hydrocarbon group having 1 to 8 carbon atoms, and the halogen atom is preferably chlorine or bromine, and specific examples of the hydrocarbon group include: All examples of R1 and R2 are included. Preferred compounds include those represented by the following formula.

CH.

CI, CH.

The alkali metal carbonate used in the present invention is preferably carbonic acid, since it reacts with phenol to form an alkali metal salt of phenol.
These are IJium, potassium carbonate, rubidium 12, and cesium carbonate. Particularly preferred are potassium carbonate or sodium carbonate. Anhydrous alkali metal carbonates are preferred, although hydrated carbonates can be used if desired.

Potassium bicarbonate or sodium bicarbonate can also be used to produce carbonate through the thermal decomposition reaction shown in the formula below.

Δ 2+, IH ('03 M2C0+ +H
20+CO2 (M represents the above-mentioned alkali metal) At least 2 moles of the alkali metal carbonate must be used per mole of dihydric phenol. In particular, in order to obtain a high molecular weight polymer and to increase the polymerization reaction rate, it is preferable to use an excess of 0.5 to 200 mol % of carbonate. If the amount of alkali metal carbonate used is small, the reaction for producing the alkali metal salt of phenol will not be completed, and only a product with a low molecular weight will be obtained due to free phenol groups, which is not preferable.

The dihydric phenol alkali metal di-salt forming agent used in the present invention is not particularly limited as long as it reacts with dihydric phenol to form dihydric phenol alkali metal di-salt, but examples of such agents include: , alkali metals, alkali metal hydrides, alkali metal hydroxides, alkali metal carbonates, alkali metal sulfides, alkali metal hydrogen sulfides, alkali metal alkoxides, and the like. Among these, it is preferable to use alkaline earth hydroxides, which are inexpensive and highly reactive, particularly sodium hydroxide and potassium hydroxide.

The amount of the alkali metal di-salt forming agent for dihydric phenol used in the present invention is preferably such that there is substantially one alkali metal atom to react with one hydroxyl group of the dihydric phenol. Specifically, the alkali metal di-salt forming agent for dihydric phenol is used in an amount of 0.95 mol to 1.05 mol per mol of hydroxide of dihydric phenol. If the amount is less than this range, it will be difficult to obtain a high molecular weight polymer, while if it is more than this range, side reactions will occur during polymerization, resulting in decreased physical properties and increased degree of coloration of the resulting polymer, which is undesirable.

The aprotic polar solvent used in the present invention is not particularly limited as long as it is stable in the presence of an alkali at the polymerization temperature and has high solubility for the dihydric phenol, dihalodiphenylsulfone compound, and the resulting polymer. For example, N,N-dimethylformamide, N,N-dimethylacetamide, N-methyl-2-pyrrolidone, 1,3-dimethyl-2-imidazolidinone,
Dimethyl sulfoxide, sulfolane, diphenyl sulfone, etc. can be mentioned.

The copolymer of the present invention can be produced by any of the methods described below, but is not limited thereto.

b) A method in which dihydric phenol, dihalodiphenylsulfone compound, and hyalkali metal carbonate are placed in an aprotic polar solvent, heated to a predetermined temperature, and polymerized while distilling off the produced water.

(1) A method in which a divalent phenonopedihalodiphenylsulfone compound and an alkali metal carbonate are placed in an aprotic polar solvent, an azeotropic dehydration solvent is added thereto, and the mixture is polymerized while being azeotropically dehydrated at a predetermined temperature.

c) In an aprotic polar solvent, dihydric phenol is reacted with an alkali metal salt forming agent as it is or in the form of an aqueous solution, and the water in the system is distilled off by heating or by azeotropic distillation by adding an azeotropic dehydration solvent. After removal, a dihalodiphenylsulfone compound is added and polymerized at a predetermined temperature.

2) In an aprotic polar solvent, dihydric phenol is reacted with an alkali metal salt forming agent as it is or in the form of an aqueous solution, an azeotropic dehydration solvent and a dihalodiphenylsulfone compound are added thereto, and the mixture is heated to a predetermined temperature. A method of polymerizing with azeotropic dehydration.

e) A method in which a separately synthesized alkali metal di-salt of dihydric phenol and a dihalodiphenylsulfone compound are placed in an aprotic polar solvent and polymerized at a predetermined temperature.

f) A dihalodiphenylsulfone compound, an alkali metal carbonate, and one dihydric phenol (dihydroxydiphenylsulfone compound or dihydroxydiphenyl compound) are placed in an aprotic polar solvent, heated to a predetermined temperature, and the produced water is After synthesizing one polymer block by polymerizing while distilling it off or adding an azeotropic dehydration solvent and performing azeotropic dehydration at a predetermined temperature, the other dihydric phenol (dihydroxydiphenyl compound or dihydroxydiphenyl sulfone compound) is synthesized. A method of preparing and further polymerizing to obtain a block copolymer.

The azeotropic dehydration solvent used in the present invention is one that forms an azeotropic mixture by azeotroping with water, such as benzene, toluene, xylene, chlorobenzene, monochloroethane, dichloroethane, trichloroethyl, tetrachloroethane, monochloroethylene, Examples include dichloroethylene and trichloroethylene.

The amount of the dihalodiphenyl sulfone compound used in the present invention is 90 to 110% based on the dihydric fer.
It is preferable to use it within the range of mol%. In order to obtain a polymer with a higher molecular weight, it is preferably used within the range of 95 to 105 mol%.

In the present invention, the polymerization reaction temperature is determined by the type of reaction raw material components,
Although it varies depending on the type of polymerization reaction, etc., it is usually 80 to 400.
℃ range, preferably in the range of 100 to 350℃. If the reaction temperature is lower than the above-mentioned temperature range, the desired polymerization reaction will hardly proceed at a rate that can be used practically, and it will be difficult to obtain a polymer with the required molecular weight. On the other hand, when the reaction temperature is higher than the above range, side reactions other than the desired polymerization reaction cannot be ignored, and the resulting polymer becomes significantly colored. Further, the reaction may be carried out at a constant temperature, or the temperature may be gradually changed or the temperature may be changed stepwise.

In the method of the present invention, the time required for the polymerization reaction varies greatly depending on the types of reaction raw materials, the type of polymerization reaction, the reaction temperature, etc., but is usually in the range of 10 minutes to 100 hours, preferably 30 minutes to 100 hours. It will be carried out over a 24 hour period.

The reaction atmosphere in carrying out the reaction in the method of the present invention is preferably free of acid poisons, and good results are obtained when the reaction is carried out in nitrogen or other inert gas.

Alkali metal salts of dihydric phenols are easily oxidized when heated in the presence of oxygen, hindering the desired polymerization reaction, making it difficult to increase the molecular weight, and also causing coloration of the resulting polymer.

In the method of the present invention, in order to stop the polymerization reaction, it is usually sufficient to cool the reactants. However, in order to stabilize phenoxide groups that may be present at the terminals of the polymer, an addition reaction of an aliphatic halide, an aromatic halide, etc. may be carried out as necessary. Specific representative examples of the halides include methyl chloride, ethyl chloride, methyl bromide, 4-chlorodiphenylsulfone, 4-talolobenzophenone, 4.
Examples include 4''-dichlorodiphenylsulfone and 4-chloronitrobenzene.

Known methods can be applied to the separation and purification of the polymer after the completion of the polymerization reaction. For example, after filtering the salt (alkali halide) and excess alkali metal carbonate precipitated in the reaction solvent, the polymer solution (liquid A) is usually added dropwise to the non-solvent of the polymer, or conversely, By adding a nonsolvent to the polymer solution, the desired polymer can be precipitated. Typical examples of non-solvents commonly used for polymers include methanol, ethanol, impropatol, acetone, methyl ethyl ketone, water, etc., but these can be used alone or as a mixture of two or more. It's okay.

〔Effect of the invention〕

The copolymer according to the present invention maintains the advantages of conventional BoIJ ether sulfone and has improved heat resistance, and has an extremely good balance between heat resistance and moldability, and has excellent mechanical properties. Due to its physical properties, electrical properties, thermal stability, moldability, etc., it can be used in a wide range of applications such as electrical insulation, heat-resistant parts, cooking utensils, coating materials, precision parts, etc., and has great industrial value.

〔Example〕

The present invention will be explained in detail in the following Examples and Comparative Examples, but the present invention is not limited thereto.

Example 1 In a lβ5tlS 316 flask equipped with a stirrer, a gas inlet tube, a thermometer and a condenser with a receiver at the tip, 4.
4”-dihydroxydiphenylsulfone 45.05g
(0,180 mol), 3.3',5,5°-tetramethyl-4,4°-dihydroxydiphenyl4,80
(0,0198 mol), 59.15 g (0,206 mol) of 4.4''-dichlorodiphenylsulfone, 24.38 g (0,230 mol) of anhydrous sodium carbonate, and 235 g of sulfolane were charged and the atmosphere was replaced with nitrogen. Stirring and temperature raising were started under a nitrogen-exposed atmosphere, and the system temperature was raised to 235°C over about 1 hour, and the reaction was carried out at this temperature for about 6 hours. During this time, about 3.6 g of water was added to the receiver. After the reaction was completed, the reaction solution was cooled to 150°C, and at this temperature methyl chloride gas was blown in at 300 mj'/min for 30 minutes.In addition, sodium chloride, excess sodium carbonate, etc. precipitated in the reaction solution. was removed by filtration, and the filtrate was cooled to room temperature.Next, this filtrate was poured into a large amount of methanol to precipitate the polymer.The polymer was filtered off, washed several times with water, and then cooled under reduced pressure. It was dried by heating at 150°C.The reduced viscosity of the obtained polymer was 52a/g (25°C, 0.5g/J in N-methyl-2-pyrrolidone).Formula (1) and (I[ ) was found to be 50,150 by measurement of proton nuclear magnetic resonance spectrum (1H-NMR').

The physical property measurement results are shown in Table 1.

Examples 2 to 6 and Comparative Example 1 4.4°-dihydroxydiphenyl sulfone and 3,3
Example 1 was carried out in the same manner as in Example 1, except that the amount of 5,5'-tetramethyl-4,4°-dihydroxydiphenyl was changed to the amount shown in Table 1.

Table 1 shows the results of measuring the physical properties of the obtained polymer.

Comparative Example 2 3,3', 5,5'-tetramethyl-4,4'-dihydroxydiphenyl 48.46 g (0,200 mol), 4,4'-dichlorodiphenyl sulfone 59.15
g (0,206 mol), anhydrous sodium carbonate 24.38
Example 1 was carried out in the same manner as in Example 1, except that 300 g of 1,3-dimethyl-2-imidazolidinone were used. The reduced viscosity of the obtained polymer was 0.56J
/g (25°C, 0.0% in N-methyl-2-pyrrolidone).
5g/dl).

The physical property measurement results are shown in Table 1.

As shown in Table 1, it can be seen that the polymer obtained according to the present invention has a high Tg and excellent moldability.

Example 7 4,
4°-dihydroxydiphenylsulfone 35.04g (
0,140 mol), 59.15 g (0,206 mol) of 4,4'-dichlorodiphenylsulfone, 24.38 g (0,230 mol) of anhydrous sodium carbonate, and 150 g of sulfolane were charged and purged with nitrogen. Next, the temperature was raised to 235℃, and after reacting at this temperature for 4 hours, 3.3℃,
A solution prepared by dissolving 14.54 g (0.0600 mol) of 5,5°-tetramethyl-4,4°-dihydroxydiphenyl in 85 g of sulfolane was added, and the mixture was allowed to react for an additional 4 hours. Approximately 3.5 g of water has been captured in the receiver so far. Thereafter, the reaction solution was cooled to 150° C., and methyl chloride gas was blown into the reactor at 300 d/min for 30 minutes. Furthermore, sodium chloride and excess sulfur carbonate precipitated in the reaction solution.
-IJum etc. were removed by filtration, and the filtrate was cooled to room temperature.

Next, this filtrate was poured into a large amount of methanol to precipitate the polymer.

After separating the polymer by filtration and washing with water several times, 1
It was dried by heating at 50°C. The reduced viscosity of the resulting polymer was 0.53 rinsings/g (25 DEG C., 0.5 g/dl in N-methyl-2-pyrrolidone).

The composition ratio (mole fraction) of the formula date) and (I[) is 'I
I-N! , was found to be 70/30 by IR measurement.

Example 8 In a 1β5ljS 316 flask equipped with a stirrer, a gas inlet tube, a thermometer and a condenser with a receiver at the tip, 4.
35.04g of 4'-dihydroxydiphenylsulfone (
0,140 mol), 3.3″, 5,5″-tetramethyl-4,4°-dihydroxydiphenyl 14,54 g (
0,0600 mol), 48.55% by weight potassium hydroxide aqueous solution 47.06 g (0,407 mol), l, Ilk
Dimethyl-2-imidazolidinone (DMJ) 372, O
g and 23 g of water were added, and the mixture was stirred at room temperature until the mixture became homogeneous. Next, the temperature was raised to 200°C and stirred for 20 minutes. Approximately 90% of water was distilled out during this time. Thereafter, the temperature was raised to the boiling point of DMI, and .DMr90d was distilled out. As a result, more than 98% of water was distilled out, making it virtually anhydrous. Next, 4.4'-dichlorodiphenylsulfone 57
．． 42g (0,200mol) of DML 100m+?
The solution was added and polymerized at 226°C for 6 hours. Thereafter, the reaction solution was cooled to 150° C., and methyl chloride gas was blown into the solution for 30 minutes to stop the polymerization. The reaction solution was cooled to room temperature, potassium chloride precipitated in the reaction solution was removed by filtration, and the filtrate was poured into a large amount of methanol to precipitate the polymer. The precipitated polymer was separated by filtration, washed with water several times, and then heated and dried under reduced pressure.

The yield of the produced polymer was 95%, and the reduced viscosity of the obtained polymer was 0.43 d/g (25°C, N-methyl-2-
0.5 g/a) in pyrrolidone. The above formula (1)
The composition ratio (mole fraction) of and (II) is 'H-\1. )
It was found to be 70/30 by R measurement.

Claims (1)

[Claims] 1 General formula (I) ▲There are mathematical formulas, chemical formulas, tables, etc.▼The molar fraction of the repeating unit represented by (I) is 1 to 99 mol%, and the general formula (II) (II) A copolymer in which the molar fraction of the repeating unit represented by and the concentration is 0.
25°C with 5g/dl N-methyl-2-pyrrolidone solution
The reduced viscosity of the copolymer measured in
A novel aromatic polyether sulfone copolymer characterized in that it has a molecular weight of at least g. (However, in the above formula, R^1 to R^6 represent a hydrogen atom, a linear or branched hydrocarbon group having 1 to 8 carbon atoms, or a halogen atom, which may be the same or different from each other, and R ^3
At least one of ~R^6 is a linear or branched hydrocarbon group having 1 to 8 carbon atoms. a and b are each from 1 to
They are an integer of 4 and may be the same or different from each other. ) 2 General formula (III) ▲There are mathematical formulas, chemical formulas, tables, etc.▼ (III) (In the formula, X is a halogen atom, R^1 and R^2 are hydrogen atoms that may be the same or different from each other, and have a carbon number of 1 ~8 linear or branched hydrocarbon group or a halogen atom, a and b are each an integer of 1 to 4) and a dihalodiphenylsulfone compound represented by 1 to 9.
9 mol% is a dihydroxydiphenyl sulfone compound represented by the general formula (IV) ▲ There are mathematical formulas, chemical formulas, tables, etc. ▼ (IV) The remaining 99 to 1 mol% is the general formula (V) ▲ Numerical formulas, chemical formulas, tables, etc. etc.▼(V) (In the formula, R^3 to R^6 are hydrogen atoms, which may be the same or different from each other, a linear or branched hydrocarbon group having 1 to 8 carbon atoms, or a halogen atom. and at least one of R^3 to R^6 is a linear or branched hydrocarbon group having 1 to 8 carbon atoms. , in the presence of an alkali metal carbonate,
2. The method for producing a novel aromatic polyethersulfone copolymer according to claim 1, wherein the reaction is carried out in an aprotic polar solvent. 3 General formula (III) ▲There are mathematical formulas, chemical formulas, tables, etc.▼ (III) (In the formula, 8 represents a linear or branched hydrocarbon group or a halogen atom, and a and b are each an integer of 1 to 4.) and a dihalodiphenylsulfone compound represented by 1 to 9.
9 mol% is a dihydroxydiphenyl sulfone compound represented by the general formula (IV) ▲ There are mathematical formulas, chemical formulas, tables, etc. ▼ (IV) The remaining 99 to 1 mol% is the general formula (V) ▲ Numerical formulas, chemical formulas, tables, etc. etc.▼(V) (In the formula, R^3 to R^6 are hydrogen atoms, which may be the same or different from each other, a linear or branched hydrocarbon group having 1 to 8 carbon atoms, or a halogen atom. and at least one of R^3 to R^6 is a linear or branched hydrocarbon group having 1 to 8 carbon atoms. Production of the novel aromatic polyethersulfone copolymer according to claim 1, characterized in that the dihydric phenol alkali metal di-salt obtained by reacting a metal salt-forming agent is reacted in an aprotic polar solvent. Method.