JPS61241358A - Polyimide precursor solution - Google Patents

Abstract

PURPOSE:A polyimide precursor solution, obtained by using a specific diamino compound and aromatic tetracarboxylic acid dianhydride as starting materials, and dissolving specific components in a specific proportion in a general-purpose solvent and having improved handleability without problems in safety and hygiene. CONSTITUTION:A polyimide precursor solution obtained by reacting (A) a diamino compound expressed by formula [X is C(CH3)2, etc.] with a specific aromatic tetracarboxylic acid dianhyride (derivative) in a polyhydric alcohol expressed by formula II (m is >=4), and containing 100-80mol% total of (A) of a component consisting of repeating units expressed by formula III and/or formula IV (Ar1 is, 3,3',4,4'-biphenyltetracarboxylic acid residue; Ar2 is pyromellitic acid residue) and (B) a component consisting of repeating units expressed by formula V (Ar3 is tetracarboxylic acid residue other than Ar1 and Ar2; Y is diamino compound residue) and (C) 0-20mol% component consisting of repeating units expressed by formula VI, etc. The ratio of the components (A) to (B) is 100-80mol% component (A) and 0-20mol% component (B).

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Application Field] The present invention relates to a polyimide precursor solution using a polyhydric alcohol derivative, which is a general-purpose solvent, as a solvent for the polyimide precursor.

[Conventional technology]

Generally, aromatic polyimides with high heat resistance do not require organic solvents, and therefore cannot be processed into molded products such as films as they are. Therefore, N is usually used in the form of polyamic acid, which is a precursor of polyimide.
-Methyl-2-pyrrolidone, N,N'-dimethylacetamide, N.

N-dimethylformamide, dimethyl sulfoxide,
Hexamethylphosphoramide dissolved in an organic polar solvent,
It is cast onto a glass plate or the like, and heat-treated at high temperatures to cause dehydration and ring closure, thereby producing a polyimide film or the like. However, since all of the organic polar solvents described above are skin-absorbable and highly toxic, handling them is complicated, such as having to wear solvent-resistant gloves, and there are difficulties in handling them. In addition, these organic polar solvents have strong dissolving power, so when a solution of the above-mentioned polyamic acid in an organic polar solvent is applied to a material to be coated and cured by heating to form a polyimide film on the surface of the material to be coated, If an organic material is used as a coating material, the material will be attacked by the above-mentioned organic polar solvent, so inorganic materials such as metals and metal oxides must be used as the coating material, which limits the scope of application of polyimide. Narrowed by organic polar solvent.

Since organic polar solvents have the above-mentioned drawbacks, polyimide precursors that dissolve in other solvents have been researched.
Polyimide precursors that are soluble in phenolic solvents such as phenol and cresol have been developed. A polyimide precursor solution using such a phenolic solvent has health and safety problems because the phenolic solvent has a strong odor (for example, cresol odor) and can cause burns if it comes in contact with the skin.

[Problem that the invention seeks to solve]

As mentioned above, in the past, no polyimide precursor has been developed that is soluble in general-purpose solvents that are free of toxicity and odor, resulting in the various problems described above. Therefore, the purpose of the present invention is to provide a polyimide precursor solution that is easy to handle, does not cause health and safety problems, and has a wide range of applications by developing a polyimide precursor that can be dissolved in a general-purpose solvent. It is something to do.

[Means for solving problems]

In order to achieve the above object, the polyimide precursor of the present invention contains 100 to 80 mol% of the following components A and B in total, and 0 to 20 mol% of the following C component, and When the mutual ratio of A component and B component is 100 mol% of the total amount of A component and daily component, B component is 0 to 2 to 100 to 80 mol% of A component.
a polyimide precursor selected in a proportion of 0 mol%;
This is a polyimide precursor solution consisting of the solvent, and the solvent has the following component D as a main component.

(A) Repeating unit represented by general formula (1) and general formula (
At least one of the repeating units represented by 2).

(B) One or more repeating units represented by general formula (3).

(C) Repeating unit represented by general formula (4), general formula (5)
At least one of a repeating unit represented by and a repeating unit represented by general formula (6).

] (D) A polyhydric alcohol derivative represented by general formula (7).

)1sC+0-CHxCHi'')1-OCH3----
−−−−−−−・ (7) [In formula (7), m is 4
is an integer greater than or equal to ] As a result of a series of studies to develop a polyimide precursor that dissolves in general-purpose solvents, the inventors found that the raw materials used to synthesize the polyimide precursor have a large effect on the solubility of the polyimide precursor. , especially in diamino compounds, and certain aromatic 4
Using a nuclear diamine and combining it with a specific aromatic tetracarboxylic dianhydride or its derivative,
The present invention was achieved by discovering that a polyimide precursor having excellent solubility in the polyhydric alcohol m-conductor represented by the above general formula (7), which is a general-purpose solvent, can be obtained.

Since the polyimide precursor solution of this invention uses a polyhydric alcohol derivative, which is a general-purpose solvent, as the solvent for the polyimide precursor, there is no need to wear solvent-resistant gloves when handling it, and it does not adhere to the skin. It does not cause burns, and the polyhydric alcohol derivative does not have a strong dissolving power in the organic polar solvent, so it can be applied to organic materials as well. Moreover, the polyimide precursor solution of the present invention can form a polyimide having excellent properties equivalent to those of conventional polyimides. In other words, in order to improve the solubility of wholly aromatic polyimide precursors, bulky substituents are generally introduced into the polymer to reduce its crystallinity, or ortho- or meta- It is conceivable to use substituted diamines to make the molecular chains of the polymer zigzag-shaped to reduce its crystallinity. However, in this way, properties such as toughness of polyimide deteriorate.

However, this invention does not rely on the conventional method of reducing the crystallinity of the polymer as described above, but instead selects a specific diamino compound as a starting material for a polyimide precursor, and combines this with a specific aromatic tetracarboxylic acid. The solubility of the polyimide precursor is improved by combining it with a dianhydride or its derivative, and the polyimide precursor solution is manufactured by selecting and using a general-purpose solvent that is optimal for dissolving the generated precursor. The produced polyimide is of good quality and has properties such as toughness that are equivalent to conventional polyimides.

The polyimide precursor used in this invention usually uses a diamino compound represented by the following general formula (IO), and in some cases, a part of it is added to the following general formula (11)% formula% (10
) [In the formula, χ is as in the above general formula (1). ] Substituted with cyanosiloxane represented by the following formula (12), aromatic tetracarboxylic dianhydride or derivative thereof represented by the following general formula (12), and aromatic tetracarboxylic dianhydride represented by the general formula (13) In some cases, aromatic tetracarboxylic dianhydrides or derivatives thereof represented by the following general formulas (12) and (13) may be reacted with one or both of the tetracarboxylic dianhydrides or derivatives thereof. It is obtained by substituting a part with an aromatic tetracarboxylic dianhydride represented by the following general formula (14) or a derivative thereof and reacting it.

[^r is as in the general formula (1) above,] (Ar
m is as shown in the above general formula (2), [Arc is as shown in the above general formula (3)],] the above general formula (lO)
Typical examples of diamino compounds represented by 2.
2-bis(4-(4-aminophenoxy)phenyl)propane, 2.2-bis(4-(4-aminophenoxy)
phenyl]hexafluoropropane, 4,4'-di(4
-aminophenoxy)diphenyl sulfone. Preferred is 2,2-bis(4-(4-aminophenoxy)phenyl)propane.The aromatic tetranuclear diamines listed above may be used alone or in combination.

Further, typical examples of the diaminosiloxane represented by the general formula (11) are as follows.

CH2CI.

CHs CH3 CaHs CJs cns　C■.

Cs, Hs 0CHs CJs The above-mentioned diaminosiloxane is used when it is necessary to specifically impart adhesion to silicon-containing materials such as silicon wafers and glass to polyimide. Therefore, the amount of diaminosiloxane represented by the above general formula (11) needs to be suppressed within the range of 1 to 20 mol % based on the total diamino compound. The preferred range is 1-4 mol%, and the most preferred range is 2-3.5 mol%. When polyimide is used in fields where adhesion to silicon-containing materials is not required, such as in polyimide films, it is not necessary to use the diaminosiloxane represented by the above general formula (11).

The aromatic tetracarboxylic dianhydride represented by the above general formula (12) is 3.3°, 4.4'-biphenyltetracarboxylic dianhydride, and the aromatic tetracarboxylic dianhydride represented by the general formula (13) The group tetracarboxylic dianhydride is pyromellitic dianhydride. These may be used alone or in combination, and derivatives such as esters and acid chlorides thereof may also be used. However, particularly preferred are dianhydrides.

The use of dianhydrides makes it possible to obtain polyimide precursors with particularly excellent properties.

Further, the aromatic tetracarboxylic dianhydride represented by the above general formula (14) is represented by the above general formula (12), (13)
It is an aromatic tetracarboxylic dianhydride other than the aromatic dianhydride represented by, and derivatives such as esters and acid chlorides of these are also used. The aromatic tetracarboxylic dianhydride represented by the general formula (14) or its derivative is used as appropriate as necessary, and becomes a constituent component of the repeating unit represented by the general formula (3). It is something. If the proportion of repeating units represented by the general formula (3) is too large, the solubility of the polyimide precursor in the solvent will decrease, so the proportion should be When the total of the repeating unit (A component) and the repeating unit (B unit) represented by the general formula (3) is 100 mol%, the repeating unit (B component) represented by the general formula (3) is 100 mol%. ) is 0
~20 mol% (this is taken as 2 mol%), and the repeating unit (A component) represented by general formula (1) and general formula (2) is 100 to 80 mol%, which is 100-1 mol%. mol %.

Suitable examples of the aromatic tetracarboxylic dianhydride represented by the general formula (14) include 2,3.3°,4'-biphenyltetracarboxylic dianhydride and 3.3'',4,4
``-benzophenonetetracarboxylic dianhydride.These may be used alone or in combination, and derivatives such as esters and acid chlorides thereof may also be used.

In addition, the aromatic tetracarboxylic dianhydride represented by the above general formula (14) or its derivative is represented by the general formula (12),
When used in place of a part of the aromatic tetracarboxylic dianhydride represented by (13) or its derivative, a diamino compound that reacts with the aromatic dianhydride represented by general formula (14) or its derivative; is usually the general formula (10)
The diamino compound represented by the formula (11) or the diamino compound represented by the general formula (11) is obtained.

Therefore, in all of the repeating units represented by general formulas (1), (2) and (3), the diamino compound residue is derived from general formulas (10) and (11) and becomes common. However, the diamino compounds are not limited to those represented by the above general formulas (10) and (11), and instead of some of them,
Other diamino compounds other than the diamino compounds represented by the above general formulas (10) and (11) may be used.

However, in this case, it is necessary to react the whole in the same reaction vessel so that the structural parts derived from the other diamino compounds mentioned above are introduced into the polyimide molecular chain here and there. This ensures the solubility of the polyimide.

The polymerization reaction between the aromatic tetracarboxylic dianhydride or its derivative and the diamino compound can be carried out according to a conventionally known method (usually, the polyhydric alcohol derivative represented by the general formula (7) is , the aromatic tetracarboxylic dianhydride or its derivative and the diamino compound are charged in equal moles or approximately equal moles, preferably under a nitrogen gas stream, and at a temperature of 80° C. or lower, preferably 60° C. or lower, taking into account the exotherm of polymerization. The reaction is carried out under limited conditions until a high degree of polymerization is obtained. This degree of polymerization can be easily detected by examining the intrinsic viscosity η of the reactant, and the intrinsic viscosity is 0.3 to 3. It is preferable to keep it within the range of 0.

The above intrinsic viscosity [η] is determined by reprecipitating the polyimide precursor with water from the obtained polyimide precursor solution, drying it under reduced pressure at a temperature of 40°C or less, and drying the obtained dry product with N-methyl- Dissolved in 2-pyrrolidone, measurement temperature 30±0.0
At 1°C (thermal bath), the following formula: % formula %) t: Falling time of the polymer solution measured with an Ubbelohde viscometer tO; Falling time of the solvent measured in the same manner as above C: Concentration of the polyimide precursor (0.5% by weight).

The polyhydric alcohol derivative represented by the above general formula (7) is
In the above formula (7), m is an integer of 4 or more, usually up to 6, and particularly preferably 4.

Typical examples include glycol dialkyl ethers such as tetraethylene glycol dimethyl ether, pentaethylene glycol dimethyl ether, and hexaethylene glycol dimethyl ether, which can be used alone or in combination.

These polyhydric alcohol derivatives are chemically inert compared to organic polar solvents such as N,N'-dimethylformamide, and have low volatility and low flammability, making them easy to handle. be.

In this way, the polyimide precursor solution of the present invention is obtained. This polyimide precursor solution is usually 15 to 3
Used at a concentration of 0% by weight. In this case, as a diluting solvent for the polyimide precursor solution, it goes without saying that the polyhydric alcohol derivative represented by the above general formula (7) can be used as is, and within a range that does not impair handling properties. It is also possible to use conventionally used organic polar solvents or phenolic solvents (usually up to 50% by weight of the total solvent composition).

In this case, since the amount of organic polar solvent or phenolic solvent used is less than half of the conventional amount, the harmful effects thereof are also significantly reduced. In addition, hydrocarbon solvents such as benzene and xylene, ketone solvents, alcohol solvents, ester solvents, ether solvents, and glycol monoether solvents that do not have the ability to dissolve the polyimide precursor, and m= Various general-purpose solvents such as glycol ether solvents shown in Nos. 1 to 3 can also be used as diluting solvents within a range that does not precipitate the polyimide precursor.

In addition, in this invention, the polyimide precursor includes one partially imidized by dehydration and ring closure,
It refers to something that has not yet gone through the imidization process. In addition, the polyimide precursor solution of the present invention uses an organic polar solvent as a polymerization solvent, and after synthesizing the polyimide precursor, solvent substitution is performed to convert the polyhydric alcohol derivative represented by the general formula (7) into a synthetic polyimide precursor. It may also be produced by dissolving.

The concentration of the polyimide precursor to be charged during synthesis is generally in the range of 5 to 50% by weight, but this can be arbitrarily selected depending on the economical point of view or the purpose of use. In addition, since the polymer solution after the reaction has a high degree of polymerization, the viscosity of the solution may be considerably higher than the intended viscosity in some cases, but even in this case, it is usually 40 to 8
The viscosity can be lowered to a desired level by heating it to 0°C and passing through an aging process.

The polyimide precursor solution of the present invention uses the above-mentioned general-purpose solvent, so it has excellent handling properties, and after applying it to an adherend, it can be heated at high temperature for dehydration and ring closure. It can be converted into polyimide with excellent properties such as heat resistance, toughness, and so on. It goes without saying that it can also be used to produce polyimide films. In these cases, the imidization method is not limited to heating imidization as described above; for example, the polyimide precursor solution is cast on a glass plate and dried to form a film, and the resulting film is mixed with pyridine. Chemical imidization can also be carried out, such as by immersing the material in a benzene solution of acetic anhydride to perform a solvent removal and imidization reaction to form a polyimide film. In particular, when the polyimide precursor solution of the present invention is used as a protective film, a passivation film, an α-ray shielding film, etc. on the surface of a semiconductor element, adhesion to a silicon-containing material as an adherend is required. In such cases,
As already mentioned, the diaminosiloxane represented by the general formula (11) is an aromatic 4 represented by the general formula (10).
It is used together with a nuclear diamine, but rather than reacting these with an aromatic tetracarboxylic dianhydride or its derivative in the same reaction vessel to synthesize a polyimide precursor consisting of a copolymer (see above). Formula (11)
The diaminosiloxane represented by the above general formula (10) is reacted with the aromatic tetranuclear diamine represented by the above general formula (10) in separate reaction vessels to synthesize two types of polyimide precursors, and these are mixed after synthesis. By doing so, the desired polyimide precursor solution may be manufactured. However, rather than doing this, it is preferable to make the generated polyimide precursor a copolymer as described above.
This makes it possible to produce a polyimide precursor solution that can form a polyimide with stable quality without causing variations in adhesiveness and moisture resistance properties.

In addition, the polyimide precursor solution of the present invention is very useful as a padding film on the surface of a semiconductor element, etc.
It is often used when assembling semiconductor devices, but if ionic impurities are contained in the passivation film etc., they will have an adverse effect on the electrical characteristics, so it is necessary to remove the ionic impurities. As a method for this removal, each monomer component and the solvent used may be purified by known purification methods such as recrystallization and distillation, which allows almost complete removal of the ionic impurities.

As a guide to the degree of purification, Na ion, Cj! Both ions are 5 ppm or less, preferably 1 ppm or less.

〔Effect of the invention〕

As described above, since the polyimide precursor solution of the present invention uses a polyhydric alcohol derivative, which is a general-purpose solvent, it does not have the toxicity and skin absorption properties of conventionally used organic polar solvents. Therefore, it is not necessary to wear solvent-resistant gloves or the like when handling it, and it is easy to handle. Additionally, there is no odor or risk of burns like with phenolic solvents, so there are no health and safety issues.
It can be suitably used for various conventionally known applications. Furthermore, since the above-mentioned polyhydric alcohol derivatives do not have strong dissolving power like organic polar solvents, even if the polyimide precursor solution of the present invention is applied to an adherend made of an organic material such as polyester, There is no fear of melting the material to be adhered, and it can be applied to organic material adherends, which was previously impossible, and the range of application of polyimide can be greatly expanded. Moreover, this invention prevents the introduction of bulky substituents into the molecular structure of the polyimide precursor, or the use of ortho- or meta-substituted diamines as starting materials, which would reduce the crystallinity of the polymer. First, a polyimide precursor soluble in a polyhydric alcohol derivative, which is a general-purpose solvent, is produced by combining a specific aromatic tetranuclear diamine and an aromatic tetracarboxylic dianhydride or its derivative, and a solution thereof is prepared. Therefore, the polyimide produced by imidization is of good quality, and therefore, a polyimide having properties equivalent to conventional polyimides in terms of heat resistance, toughness, etc. can be obtained. Since the polyimide precursor solution of the present invention has such excellent characteristics, it can be used in a wide range of applications and can exhibit excellent effects. For example, passivation films on the surface of semiconductor devices, protective films, junction protective films for P-N junctions of diodes, cyclists, transistors, etc., α-ray shielding films for VLSI, glabella insulating films,
It can be suitably used for liquid crystal alignment films of liquid crystal cells made of organic film substrates such as glass substrates or polyester substrates containing transparent electrodes, laminates, adhesives for sandwich structures of various substrates, various insulating films, etc. In addition, the polyimide precursor solution of this invention can be used not only for the above purposes, but also for inorganic,
A paste binder in which various organic fillers are dispersed (
For example, it can be used as a binder for heat-resistant conductive pastes containing conductive fillers such as silver powder, gold powder, palladium powder, etc. for chip bonding, electrodes of electronic parts, etc. Furthermore, it can be used as a screen printing paste for preventing soft errors by dispersing an organic or inorganic filler with a uranium or thorium content of 59 pb or less, and exhibits excellent effects in these applications.

Next, examples will be described together with comparative examples.

In addition, in the following examples and comparative examples, the viscosity of the solution is a value measured at 25 ° C. using an EMD type rotational viscosity needle manufactured by Tokyo Keiki Co., Ltd., and the tensile strength 2 volume resistivity of the film is JIS K6760 and JIS K648, respectively. This is the value measured according to. In addition, adhesion is determined by forming a film on a silicon wafer (mirror silicon) and meeting JIS 0
The test was carried out according to 0202, and the measurement was made by examining (number of peelings)/(total number).

[Examples 1 to 5, Comparative Examples 1 to 3] Using the diamines shown in Table 1 as diamino compounds, solutions of polyimide precursors having no siloxane bonds in their molecular structures were produced in the following manner.

That is, into a 500 m It flask equipped with a stirrer, a cooling tube, a thermometer, and a nitrogen purging device, the organic solvent shown in Table 1 purified by distillation was added in the amount shown in the table, and nitrogen gas was introduced. did. Next, the diamino compounds shown in Table 1 above were added in the amounts shown in the table and stirred, and then the acid dianhydrides shown in Table 1 were gradually added little by little over 5 minutes. As a result, the viscosity of the reaction system gradually increased and the temperature rose to 38°C. Then, stirring was continued for 6 hours while keeping the temperature at 30° C., and as a result, the reaction system became a solution with the viscosity and state shown in Table 1, and the desired polyimide precursor solution was obtained.

Next, the obtained polyimide precursor solution was applied onto a glass plate, heated at 150°C for 1 hour, 200°C for 1 hour, and then heated in a dryer for 25 hours (1°C for 6 hours) to form a transparent polyimide film. Next, the obtained polyimide film was peeled off from the glass plate, and the tensile strength and volume resistivity were measured.The results are also shown in Table 1.

As can be seen from the comparison between Examples 1 to 5 and Comparative Example 1, the polyimide films obtained using the polyimide precursor solutions of Examples 1 to 5 and the comparative example 1 obtained using the organic polar solvent It can be seen that the properties of the polyimide film are similar, and that the polyimide obtained from the polyimide precursor solution of the present invention has the same excellent properties as conventional ones. Furthermore, in Comparative Examples 2 and 3, the resulting polyimide precursor solution became opaque due to polymer precipitation, and therefore could not be formed into a polyimide film.

(Left below) [Examples 6 to 8, Comparative Example 4] Using diaminosiloxane together with aromatic tetranuclear diamine as the diamino compound, a solution of a polyimide precursor having a siloxane bond in its molecular structure was prepared as follows. It was manufactured as follows.

That is, the organic solvents, acid dianhydrides, and diamino compounds shown in Table 1 were replaced with the raw materials shown in Table 2, and the above Examples 1 to 5. A solution of a polyimide precursor having a siloxane bond in its molecular structure was produced in the same manner as in Comparative Examples 1 to 5. Then, the obtained solution was applied onto a glass plate in the same manner as above and cured by heating to form a polyimide film. In this case, the polyimide film obtained in Example 6.7 had good adhesion, but the polyimide film obtained in Example 8
The polyimide film obtained in Example 6 did not have as much adhesion as Examples 6 and 7 because the amount of diaminosiloxane used was small. On the other hand, although the polyimide film obtained in Comparative Example 6 had good adhesion to the glass plate, the thermal decomposition temperature (measured in air) was 320 ° C.
41 of the polyimide films obtained in Examples 6 and 7
The heat resistance is considerably inferior to 5'C, 415℃,
It turns out that there are practical problems.

(Margin below) Procedural amendment (voluntary) May 13, 1986

Claims (1)

[Claims] (1) The following A component and B component are 100 to 80 in total
Contains 0 to 20 mol% of the following C components.
When the total amount of A component and B component is 100 mol%, the B component is 0 to 100 to 80 mol% of the A component. ~
A polyimide precursor solution consisting of a polyimide precursor selected at a ratio of 20 mol% and a solvent thereof,
A polyimide precursor solution in which the solvent has the following component D as a main component. (A) Repeating unit represented by general formula (1) and general formula (
At least one of the repeating units represented by 2). ▲There are mathematical formulas, chemical formulas, tables, etc.▼......(1) ▲There are mathematical formulas, chemical formulas, tables, etc.▼......(2) [In the above formulas (1) and (2), Ar_1 is 3, 3'
, 4,4'-biphenyltetracarboxylic acid residue, Ar_
2 is a pyromellitic acid residue, X is C(CH_3)_2, C
(CF_3)_2 or SO_2. ] (B) One or more repeating units represented by general formula (3). ▲There are mathematical formulas, chemical formulas, tables, etc.▼……(3) [In the above formula (3), Ar_3 is the formula (1), (2)
is a tetracarboxylic acid residue other than Ar_1 and Ar_2 shown in , and Y is a diamino compound residue. ](C
) At least one of a repeating unit represented by general formula (4), a repeating unit represented by general formula (5), and a repeating unit represented by general formula (6). ▲There are mathematical formulas, chemical formulas, tables, etc.▼......(4) ▲There are mathematical formulas, chemical formulas, tables, etc.▼......(5) ▲There are mathematical formulas, chemical formulas, tables, etc.▼......(6) [Above In formulas (4), (5), and (6), Ar_1,
Ar_2 and Ar_3 are as shown in formulas (1), (2), and (3), R is a divalent organic group, R' is a monovalent organic group, and n is an integer of 1 or more. ] (D) A polyhydric alcohol derivative represented by general formula (7). ▲There are mathematical formulas, chemical formulas, tables, etc.▼……(7) [In formula (7), m is an integer of 4 or more. ](2
2. The polyimide precursor solution according to claim 1, wherein component B is at least one of a repeating unit represented by the following general formula (8) and a repeating unit represented by the general formula (9). ▲There are mathematical formulas, chemical formulas, tables, etc.▼...(9) [In the above formulas (7) and (8), Ar_4 is 2, 3,
3',4'-biphenyltetracarboxylic acid residue, Ar_
5 is a 3,3',4,4'-benzophenonetetracarboxylic acid residue, X is C(CH_3)_2, C(CF_3)_
2 or SO_2. ]