JP7549100B2 - Polyimide and polyimide film - Google Patents

Description

(1) Proceedings of the 68th Annual Meeting of the Society of Polymer Science, Japan, published on May 14, 2019 (2) Poster presented at the 68th Annual Meeting of the Society of Polymer Science, Japan, on May 29, 2019

The present invention relates to a polyimide having high transparency, a high glass transition temperature, a low coefficient of linear thermal expansion and sufficient film toughness, which is suitable as an alternative material to the current glass substrates in image display devices such as liquid crystal displays (LCDs), organic light-emitting diode displays (OLEDs) and electronic paper (EPs), i.e. as a transparent heat-resistant plastic substrate material, a heat-resistant film made of said polyimide, and a varnish for forming said polyimide film.

Currently, inorganic glass substrates (e.g., alkali-free glass substrates, hereafter simply referred to as "glass substrates") are used in image display devices such as LCDs, but active research is being conducted into the use of plastic substrates instead of glass substrates to make image display devices lighter, thinner, and more flexible.

Polyethersulfone (hereinafter referred to as "PES") is excellent in toughness, flame retardancy and processability in addition to transparency, and has the highest physical heat resistance (i.e., glass transition temperature: _Tg = 225°C) among currently available super engineering plastics. However, even PES is not necessarily sufficient in terms of physical heat resistance (also referred to as short-term heat resistance) against various high-temperature processes such as the formation of transparent electrodes and thin film transistors (TFTs) in the manufacturing process of image display devices.

In the manufacturing process of image display devices, there are multiple temperature cycles in which the above-mentioned high-temperature process and cooling to room temperature are repeated, but recently, there has been a strong demand for plastic substrate materials to have excellent dimensional stability against temperature cycles. One of the most effective ways to increase dimensional stability is to reduce the thermal expansion characteristics of the plastic substrate material in the film surface direction (XY direction), specifically, the XY direction linear thermal expansion coefficient (hereinafter referred to as "CTE") below the glass transition temperature (glass region) as much as possible. The lower the CTE, the more the reversible thermal expansion-contraction of the film that follows the temperature cycle can be reduced. This makes it possible to avoid serious problems such as cracks in the element layer, poor interlayer adhesion, or element misalignment.

In addition, if the reversible dimensional changes associated with thermal expansion and contraction are large, there is a high risk that irreversible dimensional changes will accumulate as thermal expansion and contraction are repeated. From these perspectives, it is preferable to reduce the CTE of the transparent plastic substrate as much as possible.

However, most organic polymer films, including PES, have a high CTE value of 60 to 100 ppm/K, and the reality is that there are no transparent resin materials that meet the above dimensional stability requirements.

On the other hand, wholly aromatic polyimides are currently widely used mainly in the electronics field as the most reliable heat-resistant insulating resin material in terms of physical and chemical heat resistance, electrical insulation, mechanical properties, flame retardancy, and ease of production process. However, current wholly aromatic polyimide films, such as "KAPTON-H" (product name) manufactured by Toray DuPont Co., Ltd. and "UPILEX-S" (product name) manufactured by Ube Industries, Ltd. and represented by the following formula (14) and (15), respectively, are strongly colored due to charge transfer interactions derived from chains in which electron donors (aromatic groups derived from diamines) and electron acceptors (bisimide structural units) are alternately linked (see, for example, Non-Patent Document 1), and are not suitable for the present purpose.

In view of this situation, colorless and transparent polyimides based on molecular design that interferes with charge transfer interactions have been studied. Among all aromatic polyimides, those represented by the following formula (16):

Among all aromatic polyimides, the polyimide represented by the formula (I) is a limited case that gives a colorless and transparent film (see, for example, Non-Patent Document 2). The transparency of this polyimide film is due to the effect of the trifluoromethyl group (CF ₃ ), which acts as an electron-withdrawing group and has the function of weakening the cohesive force between polymer chains, and due to this effect, this polyimide also has excellent solvent solubility, i.e., solution processability. However, this polyimide film does not show low thermal expansion properties (see, for example, Non-Patent Document 2).

In general, for a polyimide film to exhibit a low CTE, the polyimide main chain must be highly molecular oriented in the XY directions (referred to as "in-plane orientation"), and it has been reported that linearity and rigidity of the polyimide main chain are essential for this (see, for example, Non-Patent Document 3). In the polyimide represented by formula (16), the bent structure at the trifluoroisopropylidene group site causes the polyimide main chain to have a non-linear structure, hindering the in-plane orientation of the main chain, which is the reason why this polyimide film does not exhibit low thermal expansion properties.

An effective method for making polyimides colorless and transparent is to use non-aromatic, i.e. aliphatic, monomers as either the tetracarboxylic dianhydride monomer or the diamine monomer, or both. From the viewpoint of heat resistance, cyclic, rather than linear, aliphatic monomers (called "alicyclic diamines") are usually selected.

For example, the following formula (17):

and a general-purpose alicyclic diamine represented by the following formula (18):

The following formula (19):

Polyimides represented by the formula (I) give a colorless and transparent film. However, due to the fact that the basicity of aliphatic diamines is much higher than that of aromatic diamines, when an equimolar polyaddition reaction (hereinafter simply referred to as "polymerization reaction") is carried out using aliphatic diamines, a salt is formed at the early stage of the polymerization reaction (see, for example, Non-Patent Document 4). This salt has a crosslinked structure and is difficult to dissolve in anhydrous polymerization solvents (usually amide-based solvents), so the salt may precipitate and the polymerization reaction may not proceed at all. If the salt formed is soluble in the polymerization solvent even slightly, the polymerization may proceed gradually by stirring at room temperature after the precipitation, and a very long polymerization time is required until the salt is completely dissolved and a uniform varnish is formed. In addition, the reproducibility of the polymerization reaction time required for homogenization and the reproducibility of the molecular weight of the polyimide precursor formed are low.

In addition, the polyimide film represented by the above formula (19) does not exhibit low thermal expansion characteristics. This is because the bending of the main chain at the methylene bond site and the mixture of trans-cis isomers at the cyclohexyl group site reduce the linearity of the polyimide main chain and hinder in-plane orientation.

The only alicyclic diamine that is advantageous in reducing the thermal expansion of a polyimide film is represented by the following formula (20):

However, when attempting to react it with PMDA by a normal polymerization method, the salt formed in the initial stage is extremely strong, and the precipitated salt does not dissolve at all under any reaction conditions, making it impossible to obtain a polyimide precursor (see, for example, Non-Patent Document 4).

The above t-CHDA is represented by the following formula (21):

Since the polyimide precursor reacts with an aromatic tetracarboxylic dianhydride represented by the formula (22), i.e., 3,3',4,4'-biphenyltetracarboxylic dianhydride (hereinafter referred to as "s-BPDA"), it is possible to obtain a uniform and highly viscous polyimide precursor varnish in the end. However, this process is inconvenient for large-scale production because a short heating operation is required to dissolve the salt that precipitates at the beginning of the polymerization. When the obtained polyimide precursor is applied to a substrate and dried, and then heated to 300°C or higher to cause a dehydration ring-closing reaction (imidization reaction), the following formula (22):

The resulting polyimide film is relatively transparent and exhibits low thermal expansion (see, for example, Non-Patent Document 5). However, this polyimide film does not have sufficient membrane toughness for practical use. In addition, this polyimide is completely insoluble in solvents and has poor solution processability.

On the other hand, in the combination of an alicyclic tetracarboxylic dianhydride and an aromatic diamine, the above-mentioned salt formation does not occur, and a polyimide precursor varnish can be obtained by a normal method. By selecting monomers used in the polymerization reaction, i.e., an alicyclic tetracarboxylic dianhydride and an aromatic diamine, that have a linear, planar, and rigid structure, it is possible to obtain a transparent polyimide with low thermal expansion. The available alicyclic tetracarboxylic dianhydrides that have a linear, planar, and rigid structure are very limited, and examples thereof include those represented by the following formula (23):

The only known alicyclic tetracarboxylic dianhydride having a rigid structure represented by the formula (24): i.e., 1,2,3,4-cyclobutanetetracarboxylic dianhydride (hereinafter referred to as "CBDA").

A high molecular weight polyimide precursor can be easily obtained by polymerization with an aromatic diamine having a rigid and linear structure represented by the following formula (25):

A polyimide film represented by the formula (1) is colorless, transparent, and exhibits a low CTE (see, for example, Non-Patent Document 5). However, although this polyimide film can be a free-standing film, it does not have sufficient flexibility, and the polyimide itself does not have solution processability (see, for example, Non-Patent Document 6).

CBDA is produced by the photodimerization of maleic anhydride using an ultraviolet irradiation device (see, for example, Non-Patent Document 7). For this reason, large-scale production methods using large heating reaction vessels cannot be applied, and it is not easy to reduce costs.

In contrast, the following formula (26):

(In formula (26), the central cyclohexane moiety is a boat structure.)
Cis,cis,cis-1,2,4,5-cyclohexanetetracarboxylic dianhydride represented by the following formula (hereinafter referred to as "H-PMDA" or "(1S,2R,4S,5R)-cyclohexanetetracarboxylic dianhydride") can be obtained by catalytic reduction of inexpensive PMDA and can be produced on a large scale, and is therefore the most cost-effective and practical of currently available alicyclic tetracarboxylic dianhydrides.

When H-PMDA is polymerized at room temperature with an aromatic diamine containing an ether bond, which is a flexible linking group, to obtain a polyimide precursor, it is known that a polyimide film that is transparent and has excellent toughness is obtained, although the degree of polymerization estimated from the intrinsic viscosity is not necessarily high (see, for example, Non-Patent Document 8).

The polyimides represented by the following formula are representative examples, but these polyimide films do not exhibit low thermal expansion properties because the in-plane orientation of the main chain is hindered due to the nonlinear structure of the main chain.

On the other hand, when a diamine with a rigid and highly linear structure, such as TFMB represented by formula (24), is used in the hope of achieving low thermal expansion, the polyimide film becomes very fragile and difficult to form due to the low degree of polymerization and the rigid main chain structure, which is unfavorable for entanglement of polymer chains (see, for example, non-patent document 8).

If a diamine component with an extremely rigid and highly linear structure that is advantageous for achieving a low CTE and is also highly reactive were available, it could be combined with H-PMDA to potentially produce a low-cost, practical, transparent polyimide with low thermal expansion; however, no such diamine is currently known.

To increase the polymerization reactivity of H-PMDA with diamines, it is often effective to carry out the polymerization reaction at high temperatures (sometimes called "one-pot polymerization"). In this polymerization method, the imidization reaction proceeds simultaneously without stopping at the polyimide precursor, producing polyimide in the solution. If a stable polyimide varnish can be obtained by one-pot polymerization, polyimide films can be produced simply by applying the varnish to a substrate and drying it (i.e., without the need for a thermal imidization reaction at higher temperatures), which offers a great advantage in terms of shortening the process.

However, if one-pot polymerization is performed using a rigid, linear diamine that is advantageous for achieving a low CTE, the resulting polyimide loses solubility, and the reaction solution often becomes inhomogeneous due to gelation, precipitation, etc., making it impossible to carry out the subsequent steps, i.e., isolating the polyimide, redissolving it, and forming it into a film to produce a high-quality film. Therefore, diamines suitable for this purpose must have a rigid, linear structure to reduce the CTE of the polyimide film as much as possible, while at the same time must not deteriorate the solubility of the resulting polyimide in the polymerization solvent, so that it is not easy to solve these problems with current technology.

Prog. Polym. Sci., 26, 259-335 (2001). Eur. Polym. J., 49, 3657-3672 (2013). Macromolecules, 29, 7897-7909 (1996). High Performance. Polym., 19, 175-193 (2007). High Performance. Polym., 15, 47-64 (2003). Polymer, 74, 1-15 (2015). J. Polym. Sci., Part A, 38, 108-116 (2000). J. Polym. Sci., Part A, 51, 575-592 (2013).

An object of the present invention is to provide a transparent, heat-resistant resin material that can contribute to reducing the weight and reducing the fragility of image display devices.
That is, the problem to be solved by the present invention is to provide a polyimide capable of forming a film having excellent colorless transparency, heat resistance (physical heat resistance), dimensional stability, thermal stability (chemical heat resistance), mechanical properties and toughness, as well as a polyimide varnish and a polyimide film containing the polyimide.

As a result of intensive research conducted by the inventors in order to achieve the above object, they discovered that a polyimide obtained from H-PMDA represented by the above formula (26) and a novel diamine containing an alicyclic structure and an amide group or an imide group simultaneously has excellent properties suitable for use as a plastic substrate material for image display devices, namely, excellent solution processability, high transparency, a very high glass transition temperature, a relatively low coefficient of linear thermal expansion, and sufficient film toughness, and thus completed the present invention.

That is, the present invention is as follows.
<1> The following formula (1):

Or the following formula (2):

Diamine represented by the formula:
<2> The following general formula (3):

In the formula (3), the divalent organic group Ar is a structural unit represented by any one of the following formulas (4) to (7):




<3> A varnish obtained by dissolving the polyimide according to <2> above in an organic solvent.
<4> A heat-resistant film comprising the polyimide according to <2> above.
<5> The heat-resistant film according to the above <4>, having a glass transition temperature of 320° C. or higher, a linear thermal expansion coefficient of 40 ppm/K or lower, and a light transmittance at a wavelength of 400 nm of 70% or higher.
<6> A plastic substrate for an image display device, comprising the heat-resistant film according to <4> or <5> above.

According to the present invention, it is possible to form a film that is excellent in colorless transparency, heat resistance (physical heat resistance), dimensional stability, thermal stability (chemical heat resistance), mechanical properties and toughness.
The diamine of the present invention has an unprecedentedly high polymerization reactivity with tetracarboxylic dianhydride and has a bulky substituent, so that even when reacted with H-PMDA, a polyimide having a sufficiently high molecular weight can be obtained while suppressing gelation and precipitation of the reaction solution. In addition, since the polyimide of the present invention is compatible with existing film-forming processes, high-quality polyimide can be easily produced, and further, since the obtained polyimide film has excellent properties, i.e., high transparency, high glass transition temperature, low thermal expansion coefficient, and sufficient flexibility, the polyimide of the present invention can be used as a plastic substrate material instead of the glass substrate used in current image display devices, and a useful material for realizing lighter and more flexible display devices can be provided.

<Diamine>
The diamine of the present invention is represented by the following formula (1):

Or the following formula (2):

Hereinafter, the diamine represented by formula (1) may be referred to as "AMB-mTOL," and the diamine represented by formula (2) may be referred to as "AB-MP-HPMDI."
An amidation reaction will be described below as an example of the method for producing a diamine of the present invention, but the present invention is not limited thereto.

(Production of diamine "AMB-mTOL" represented by formula (1))
m-Tolidine is dissolved in a solvent, and the container is sealed with a septum cap to obtain liquid A. Next, 3-methyl-4-nitrobenzoyl chloride (hereinafter referred to as "3M4NBC") is dissolved in the same solvent, and an appropriate amount of base (deoxidizer) is added to the solution, and the solution is sealed in the same manner to obtain liquid B. Liquid A is cooled in an ice bath, and while stirring with a stirrer, liquid B is slowly dropped into liquid A using a syringe, and the solution is reacted for several hours, and then stirred at room temperature for 12 hours. After the reaction, the precipitate is filtered off, washed with a small amount of reaction solvent, and then washed with water to remove the water-soluble hydrochloride by-product, and vacuum dried at a temperature range of 50 to 120°C for 5 to 12 hours to obtain a dinitro product. This can be purified by recrystallization from an appropriate solvent and then used in the next hydrogen reduction step, but since a dinitro product of sufficiently high purity can be obtained only by the above washing and drying steps, the purification step may be omitted.
The reduction reaction of the terminal nitro group of the obtained dinitro compound can be carried out as follows, for example. First, the dinitro compound is dissolved in a solvent, and an appropriate amount of palladium/carbon (Pd/C) catalyst is added thereto. The reaction solution is reacted for 2 to 24 hours while being heated at a constant temperature in the range of room temperature to 150°C at a temperature below the boiling point of the solvent used, i.e., in a hydrogen atmosphere, at a temperature below the boiling point of the solvent used. The end point of the reaction can be confirmed by thin layer chromatography when the dinitro compound completely disappears and only one new spot appears. After the reaction is completed, the catalyst residue is removed by filtration. The filtrate may be appropriately concentrated with an evaporator. If a precipitate is deposited by concentration, it is filtered off, washed thoroughly with a small amount of the reaction solvent, followed by water and methanol, and finally vacuum dried for 5 to 12 hours at a temperature below the melting point of the product, i.e., in the temperature range of 50 to 120°C, to obtain the diamine represented by formula (1). This may be used as it is in the next polymerization step, or may be further purified by recrystallization from an appropriate solvent.

(Production of diamine "AB-MP-HPMDI" represented by formula (2))
In the production of the above AMB-mTOL, 4-nitrobenzoyl chloride (hereinafter referred to as "4-NBC") not containing a methyl substituent is used instead of 3M4NBC, and a diamine represented by the following formula (8) is subjected to an amidation reaction in the same manner as above to obtain a dinitro form, which is then subjected to catalytic reduction in the same manner as above to reduce the nitro group, thereby obtaining a diamine represented by formula (2).

The diamine represented by the above formula (8) can be synthesized, for example, as follows. 2-Methoxy-4-nitroaniline (hereinafter referred to as "2MeO-4NA", X mmol) is dissolved in N,N-dimethylacetamide (DMAc). H-PMDA powder (0.5X mmol) is added to this solution, and the solution is refluxed in a nitrogen atmosphere at 150 to 180°C for 1 to 12 hours. After the reaction, the reaction solution is concentrated with an evaporator, or a precipitant (poor solvent) that is miscible with DMAc is added to precipitate, and the precipitate is filtered and washed, and vacuum dried at a temperature range of 50 to 120°C for 5 to 12 hours to obtain a dinitro product. The dinitro product can be purified by recrystallization before being used in the next hydrogen reduction step, but since a dinitro product of sufficiently high purity can be obtained only by the above washing and drying steps, the purification step may be omitted. The reduction of the nitro group of the dinitro product can be performed by the same method as above.

In the above amidation reaction, the amount (mol) of the acid chloride (3M4NBC or 4-NBC) may be twice (equivalent) the amount (mol) of the diamine (m-tolidine or the compound represented by formula (8)), but in some cases it may be 2 to 5 times as much to increase the yield.

In addition, the solvent that can be used in the amidation reaction is not particularly limited as long as it dissolves the reaction raw materials, and examples thereof include ether solvents such as tetrahydrofuran (THF), 1,4-dioxane, diglyme, and triglyme, ester solvents such as γ-butyrolactone and ethyl acetate, amide solvents such as N,N-dimethylformamide (DMF), N,N-dimethylacetamide (DMAc), N-methyl-2-pyrrolidone, 1,3-dimethyl-2-imidazolidinone, and hexamethylphosphoramide, ketone solvents such as acetone, cyclopentanone, and cyclohexanone, sulfone solvents such as dimethylsulfoxide and sulfolane, chlorine solvents such as dichloromethane, chloroform, and 1,2-dichloroethane, toluene, and xylene. These solvents may be used alone or in combination of two or more. From the viewpoint of the solubility of the reaction raw materials and the ease of removal, at least one selected from THF, DMF, and DMAc is preferably used.

The reaction temperature for the amidation reaction is preferably -10 to 50°C, more preferably 0 to 30°C. If the reaction temperature is 50°C or less, side reactions are unlikely to occur and there is no risk of a decrease in yield.

The amidation reaction is carried out at a solute concentration of preferably 5 to 50% by mass, more preferably 10 to 40% by mass, taking into consideration the control of side reactions and the filtration process of the precipitate.

The deacidification agent used in the amidation reaction is not particularly limited, and organic tertiary amines such as pyridine, triethylamine, and N,N-dimethylaniline, and inorganic bases such as potassium carbonate and sodium hydroxide can be used. From the viewpoint of ease of removing the hydrochloride, pyridine is preferably used.

The precipitate produced by the amidation reaction contains not only the dinitro form, but also water-soluble pyridine hydrochloride when pyridine is used as a deoxidizer. Since the dinitro form is insoluble in water, the hydrochloride can be removed by simply washing the precipitate thoroughly with water. Completion of hydrochloride removal can be easily confirmed by the presence or absence of a white precipitate of silver chloride using a 1% aqueous solution of silver nitrate as a washing solution.

Amidation reaction can be carried out by a known method, other than the reaction of a dicarboxylic acid dichloride with an amine in the presence of a base (acid acceptor) as described above. For example, the amidation reaction can be carried out from trans-1,4-cyclohexanedicarboxylic acid (hereinafter referred to as "t-CHDCA") and 2MeO-4NA using a condensation agent such as N,N'-dicyclohexylcarbodiimide (hereinafter referred to as "DCC"), triphenyl phosphite/pyridine, diphenyl(2,3-dihydro-2-thioxo-3-benzoxazolyl)phosphonate/triethylamine, 1-hydroxybenzotriazole/DCC, 1-hydroxy-7-azabenzotriazole, or N-hydroxysuccinimide/DCC.

The method of the reduction reaction of the nitro group to the amino group is not particularly limited, and known methods can be applied. In addition to the above-mentioned method using hydrogen and Pd/C, a contact reduction method using metal powder such as tin, zinc, iron, etc. in hydrochloric acid acidic water or a method using an ethanol solution of tin chloride dihydrate can also be applied. The solvent that can be used in the above contact reduction reaction using Pd/C as a catalyst in a hydrogen atmosphere is not particularly limited as long as it dissolves the dinitro body, and examples of the solvent include amide solvents such as N,N-dimethylformamide (DMF), N,N-dimethylacetamide (DMAc), N-methyl-2-pyrrolidone, 1,3-dimethyl-2-imidazolidinone, and hexamethylphosphoramide, alcohol solvents such as methanol, ethanol, and propanol, ether solvents such as tetrahydrofuran, 1,4-dioxane, diglyme, and triglyme, ester solvents such as γ-butyrolactone and ethyl acetate, toluene, and xylene. These solvents may be used alone or in a mixture of two or more types. DMF and DMAc are preferably used from the viewpoint of the solubility of the reaction raw materials and ease of removal. It is also preferable that the solvent has high solubility for the diamine product. If the solubility is extremely poor, some of the diamine may precipitate at the monoamine stage, preventing the reduction reaction from proceeding completely.

<Polyimide>
The polyimide of the present invention is represented by the following general formula (3):

In the formula (3), the divalent organic group Ar is a structural unit represented by any one of the following formulas (4) to (7). That is, the polyimide of the present invention has a structural unit derived from H-PMDA, and also has a structural unit represented by any one of the following formulas (4) to (7).

The polyimide of the present invention is obtained by a polymerization reaction between a tetracarboxylic dianhydride and a diamine.

(Tetracarboxylic acid dianhydride)
As the tetracarboxylic dianhydride, H-PMDA is used.
During the above polymerization reaction, an alicyclic tetracarboxylic dianhydride other than H-PMDA can be used as a copolymerization component together with H-PMDA, within the scope that does not impair the polymerization reactivity with diamine and the required properties of the polyimide.
The alicyclic tetracarboxylic dianhydride that can be used in this case is not particularly limited, and examples thereof include (1S,2S,4R,5R)-cyclohexanetetracarboxylic dianhydride (hereinafter referred to as "H'-PMDA"), (1R,2S,4S,5R)-cyclohexanetetracarboxylic dianhydride (hereinafter referred to as "H"-PMDA"), bicyclo[2.2.2]oct-7-ene-2,3,5,6-tetracarboxylic dianhydride (hereinafter referred to as "BTA"), bicyclo[2.2.2]octane-2,3,5,6-tetracarboxylic dianhydride, bicyclo[2.2.2]heptanetetracarboxylic dianhydride, 5-(dioxotetrahydro Examples of such an anhydride include furyl-3-methyl-3-cyclohexene-1,2-dicarboxylic anhydride, 4-(2,5-dioxotetrahydrofuran-3-yl)-tetralin-1,2-dicarboxylic anhydride, tetrahydrofuran-2,3,4,5-tetracarboxylic dianhydride, bicyclo-3,3',4,4'-tetracarboxylic dianhydride, 3c-carboxymethylcyclopentane-1r,2c,4c-tricarboxylic acid 1,4:2,3-dianhydride, 1,2,3,4-cyclobutane tetracarboxylic dianhydride, and 1,2,3,4-cyclopentane tetracarboxylic dianhydride. These may be used alone or in combination of two or more.
When these alicyclic tetracarboxylic dianhydrides are used as copolymerization components, the amount used is preferably in the range of 1 to 70 mol %, more preferably 10 to 50 mol %, of the total amount of alicyclic tetracarboxylic dianhydrides including H-PMDA.

Additionally, aromatic tetracarboxylic dianhydrides can be used as copolymerization components together with H-PMDA within the limits that do not impair the polymerization reactivity with diamines and the required properties of polyimide.
The aromatic tetracarboxylic dianhydride that can be used in this case is not particularly limited, but examples thereof include pyromellitic dianhydride, 3,3',4,4'-biphenyltetracarboxylic dianhydride, 2,3,3',4'-biphenyltetracarboxylic dianhydride, 2,2',3,3'-biphenyltetracarboxylic dianhydride, 4,4'-oxydiphthalic anhydride, 3,4'-oxydiphthalic anhydride, 3,3'-oxydiphthalic anhydride, hydroquinone-diphthalic anhydride, 4,4'-biphenol-diphthalic anhydride, 3,3',4,4'-benzophenonetetracarboxylic dianhydride, 4,4'-(hexafluoroisopropylidene)diphthalic anhydride, etc. These may be used alone or in combination of two or more.
When these aromatic tetracarboxylic dianhydrides are used as copolymerization components, the amount used is preferably in the range of 1 to 30 mol %, more preferably 1 to 20 mol %, of the total amount of tetracarboxylic dianhydrides including H-PMDA.

(Diamine)
As the diamine, a compound that gives a structural unit represented by any one of the above formulas (4) to (7) is used.
The compound that provides the structural unit represented by the above formula (4) is AMB-mTOL (the diamine represented by the above formula (1)).
The compound that provides the structural unit represented by the above formula (5) is AB-MP-HPMDI (the diamine represented by the above formula (2)).
The compound which provides the structural unit represented by the above formula (6) is represented by the following formula (9):

The method for producing AB-mTOL is not particularly limited, but it can be obtained, for example, by the method of Synthesis Example 1 described later.
The compound which provides the structural unit represented by the above formula (7) is represented by the following formula (10):

The method for producing AB-44ODA is not particularly limited, but it can be obtained, for example, by the method of Synthesis Example 2 described below.

An aliphatic diamine can be used as a copolymerization component together with a compound that provides a structural unit represented by any one of the above formulas (4) to (7), within a range that does not impair the polymerization reactivity with a tetracarboxylic dianhydride and the required properties of a polyimide.
The aliphatic diamine that can be used in this case is not particularly limited, and examples thereof include 4,4'-methylenebis(cyclohexylamine), 4,4'-methylenebis(3-methylcyclohexylamine), 4,4'-methylenebis(3-ethylcyclohexylamine), 4,4'-methylenebis(3,5-dimethylcyclohexylamine), 4,4'-methylenebis(3,5-diethylcyclohexylamine), isophoronediamine, trans-1,4-cyclohexanediamine, cis-1,4-cyclohexanediamine, 1,4-cyclohexanebis(methylamine), and 2,5-bis(aminomethyl). Examples of such compounds include bicyclo[2.2.1]heptane, 2,6-bis(aminomethyl)bicyclo[2.2.1]heptane, 3,8-bis(aminomethyl)tricyclo[5.2.1.0]decane, 1,3-diaminoadamantane, 2,2-bis(4-aminocyclohexyl)propane, 2,2-bis(4-aminocyclohexyl)hexafluoropropane, 1,3-propanediamine, 1,4-tetramethylenediamine, 1,5-pentamethylenediamine, 1,6-hexamethylenediamine, 1,7-heptamethylenediamine, 1,8-octamethylenediamine, and 1,9-nonamethylenediamine. These compounds may be used alone or in combination of two or more.
When these aliphatic diamines are used as copolymerization components, the amount used is preferably in the range of 1 to 50 mol %, more preferably 5 to 30 mol %, of the total amount of diamines including the compound that provides the structural unit represented by any one of the above formulas (4) to (7).

In addition, aromatic diamines other than the compounds giving the structural units represented by any one of the above formulas (4) to (7) can be used as copolymerization components together with the compounds giving the structural units represented by any one of the above formulas (4) to (7), within the scope of not impairing the polymerization reactivity with tetracarboxylic dianhydrides and the required properties of the polyimide.
The aromatic diamine that can be used in this case is not particularly limited, but examples thereof include p-phenylenediamine, m-phenylenediamine, 2,4-diaminotoluene, 2,5-diaminotoluene, 2,4-diaminoxylene, 2,4-diaminodurene, 4,4'-methylenedianiline, 4,4'-methylenebis(3-methylaniline), 4,4'-methylenebis(3-ethylaniline), 4,4'-methylenebis(2-methylaniline), 4,4'-methylenebis(2-ethylaniline), 4,4'-methylenebis(3,5-dimethylaniline), 4,4'-methylenebis(3,5-diethylaniline), 4,4'-methylenebis(2,6-dimethylaniline), 4,4'-methylenebis(2,6-diethylaniline), 4,4'-diaminodiphenyl ether, 3,4'-diaminodiphenyl ether, 3,3'-diaminodiphenyl ether, and the like. nyl ether, 2,4'-diaminodiphenyl ether, 2,2'-diaminodiphenyl ether, 4,4'-diaminodiphenyl sulfone, 3,3'-diaminodiphenyl sulfone, 4,4'-diaminobenzophenone, 3,3'-diaminobenzophenone, 4,4'-diaminobenzanilide, benzidine, 3,3'-dihydroxybenzidine, 3,3'-dimethoxybenzidine, o-tolidine, m-tolidine, 1,4-bis(4-aminophenoxy)benzene, 1,4-bis(3-aminophenoxy)benzene, 1,3-bis(4-aminophenoxy)benzene, 1,3-bis(3-aminophenoxy)benzene, 4,4'-bis(4-aminophenoxy)biphenyl, bis(4-(3-aminophenoxy)phenyl)sulfone, bis(4-(4-aminophenoxy)phenyl)sulfone, 2,2-bis(4-(4-aminophenoxy)phenyl)sulfone, 2,2'-bis(4-aminophenoxy)phenyl)propane, p-terphenylenediamine, 2,2'-bis(trifluoromethyl)benzidine, 2,2-bis(4-(4-aminophenoxy)phenyl)hexafluoropropane, 2,2-bis(4-aminophenyl)hexafluoropropane, 4,4'-diaminodiphenyl ether, 3,4'-diaminodiphenyl ether, 3,3'-diaminodiphenyl ether, 2,4'-diaminodiphenyl ether, 2,2'-diaminodiphenyl ether, 1,4-bis(4-aminophenoxy)benzene, 1,4-bis(3-aminophenoxy)benzene, 1,3-bis(4-aminophenoxy)benzene, 1,3-bis(3-aminophenoxy)benzene, 4,4'-bis(4-aminophenoxy)biphenyl, bis(4-(3-aminophenoxy)phenyl)sulfone, bis(4-(4-amino 2,2-bis(4-(4-aminophenoxy)phenyl)sulfone, 2,2-bis(4-(4-aminophenoxy)phenyl)propane, 2,2-bis(4-(4-aminophenoxy)phenyl)hexafluoropropane, 4,4'-diaminodiphenylsulfone, 3,3'-diaminodiphenylsulfone, 2,2-bis(4-aminophenyl)hexafluoropropane, 4,4'-methylenedianiline, 4,4'-methylenebis(3-methyl Examples of the aniline include 4,4'-methylenebis(3-ethylaniline), 4,4'-methylenebis(2-methylaniline), 4,4'-methylenebis(2-ethylaniline), 4,4'-methylenebis(3,5-dimethylaniline), 4,4'-methylenebis(3,5-diethylaniline), 4,4'-methylenebis(2,6-dimethylaniline), 4,4'-methylenebis(2,6-diethylaniline), etc. These may be used alone or in combination of two or more.
When these aromatic diamines are used as copolymerization components, the amount used is preferably in the range of 1 to 50 mol %, more preferably 5 to 30 mol %, of the total amount of diamines.

The method for reacting the tetracarboxylic dianhydride with the diamine is not particularly limited, and any known method can be used.
For example, a polyimide varnish can be obtained by dissolving a diamine, an azeotropic agent and an imidization catalyst in a polymerization solvent at room temperature in a reaction vessel equipped with a nitrogen inlet tube, a stirrer, a Dean-Stark trap and a condenser, adding a tetracarboxylic dianhydride powder with stirring, and refluxing for 0.5 to 12 hours at 150 to 250° C. From the viewpoint of suppressing coloration of the varnish, it is preferable to carry out the polymerization reaction in an atmosphere of an inert gas such as nitrogen, but the introduction of the inert gas can be omitted.

In the above polymerization reaction, the molar ratio of diamine to tetracarboxylic dianhydride can be 0.8 to 1.1 per 1 of the total amount of diamine, but is preferably 0.9 to 1.1, and more preferably 0.95 to 1.05. From the viewpoint of obtaining a product with as high a molecular weight as possible, the monomers are charged in substantially equimolar amounts.

The initial monomer (solute) concentration during the polymerization reaction is preferably 5 to 60% by mass, and more preferably 10 to 50% by mass. By initiating polymerization with a monomer concentration in this range, the molecular weight of the polyimide can be sufficiently increased, and the solubility of the monomer and the resulting polyimide can be sufficiently ensured, making it possible to suppress non-uniformity of the reaction solution, such as gelation or precipitation. If the molecular weight of the polyimide increases too much and the reaction solution becomes difficult to stir, the reaction solution can be diluted with an appropriate amount of the same solvent.

The solvent used in polymerizing polyimide is not particularly limited as long as it sufficiently dissolves the raw material monomer and the resulting polyimide and has a boiling point of 150° C. or higher from the viewpoint of completing the imidization reaction. For example, amide-based solvents such as N,N-dimethylformamide, N,N-dimethylacetamide, N-methyl-2-pyrrolidone, 1,3-dimethyl-2-imidazolidinone, and hexamethylphosphoramide, cyclic ester solvents such as γ-butyrolactone, γ-valerolactone, δ-valerolactone, and γ-caprolactone, sulfone-based solvents such as dimethylsulfoxide and sulfolane, ether-based solvents such as diglyme and triglyme, phenol-based solvents such as m-cresol, p-cresol, 3-chlorophenol, and 4-chlorophenol, and ketone-based solvents such as cyclopentanone and cyclohexanone can be used. These solvents may be used alone or in combination of two or more. From the viewpoint of the solubility of the reaction raw materials and the boiling point, N,N-dimethylacetamide, N-methyl-2-pyrrolidone and γ-butyrolactone are preferably used.
In some cases, it is preferable that the solvent used has low hygroscopicity. By using a low hygroscopic solvent, the risk of the coating film whitening due to partial precipitation of polyimide caused by moisture absorption during coating is reduced, and it is also advantageous in terms of cost reduction, since humidity control during coating is no longer necessary. From this viewpoint, γ-butyrolactone, cyclopentanone, cyclohexanone, diglyme, triglyme, etc. are suitable as the solvent to be used.

Entrainers used to remove water produced during the imidization reaction include toluene, xylene, benzene, cumene, cyclohexane, ethyl acetate, pyridine, etc. From the standpoint of boiling point and ease of removal, toluene and xylene are preferably used.

In the above polymerization reaction, an imidization catalyst can be used as appropriate. Examples of usable catalysts include organic bases such as 1-ethylpiperidine, pyridine, bipyridine, picoline, pyrimidine, pyrazine, pyridazine, triazine, quinoline, quinoxaline, acridine, phenazine, benzimidazole, benzoxazole, and their isomers and derivatives, as well as organic acids such as benzoic acid and its analogs. These basic catalysts and acidic catalysts may be used alone or in combination. There are no particular restrictions on the amount of these catalysts added, but the amount is preferably in the range of 0.1 to 10 times the molar amount per theoretical dehydration amount of 1. However, the above imidization catalysts may color the polyimide varnish, which may result in a deterioration in the transparency of the polyimide film, so it is preferable to use them with care to avoid coloration.

Completion of the imidization reaction of the polyimide obtained by the polymerization reaction can be confirmed by measuring the ^1H -NMR spectrum of the polyimide obtained by dissolving the polyimide powder isolated in a deuterated solvent and observing the complete disappearance of NHCO protons and COOH protons derived from the polyimide precursor. Completion of the imidization can also be confirmed by preparing a 4-5 μm-thick polyimide thin film or measuring the FT-IR spectrum of the polyimide powder by the KBr method, observing the complete disappearance of the amide C═O stretching vibration band derived from the polyimide precursor and the appearance of an imide characteristic absorption band.

(Physical properties of polyimide)
The intrinsic viscosity of the polyimide of the present invention is preferably in the range of 0.2 to 5 dL/g, more preferably in the range of 0.5 to 2 dL/g. If the intrinsic viscosity is 0.2 dL/g or more, the molecular weight of the polyimide is sufficiently high, so that the polymer chains are sufficiently entangled with each other, and the occurrence of cracks and the like during film formation can be suppressed. On the other hand, if the intrinsic viscosity is 5 dL/g or less, the viscosity of the varnish is appropriate, so that degassing does not require a long time, and handling during coating is also good.

By using the polyimide of the present invention, a film that is excellent in colorless transparency, heat resistance (physical heat resistance), dimensional stability, thermal stability (chemical heat resistance), mechanical properties and toughness can be formed, and the preferable physical properties of the film are as follows:

The light transmittance (T ₄₀₀ ) at a wavelength of 400 nm is preferably 70% or more, more preferably 75% or more, and even more preferably 80% or more, when made into a film having a thickness of about 20 μm. When in this range, the transparency is excellent.
The total light transmittance (T _tot ) of the film having a thickness of about 20 μm is preferably 85% or more, more preferably 86% or more, and even more preferably 87% or more. When in this range, the transparency is excellent.
The yellowness index (YI) of the film having a thickness of about 20 μm is preferably 5.0 or less, more preferably 4.0 or less, and even more preferably 3.0 or less. When the YI is in this range, the film has excellent colorless transparency.
The haze, when made into a film having a thickness of about 20 μm, is preferably 2.0 or less, more preferably 1.8 or less, and even more preferably 1.5 or less. When in this range, the transparency is excellent.

The glass transition temperature (Tg) is preferably 320°C or higher, more preferably 340°C or higher, and even more preferably 360°C or higher. Within this range, the polyimide substrate has suitable heat resistance (physical heat resistance) for use in manufacturing image display devices such as liquid crystal displays and OLED displays.

The coefficient of linear thermal expansion (CTE) is preferably 40 ppm/K or less, more preferably 35 ppm/K or less, and even more preferably 30 ppm/K or less, when made into a film having a thickness of about 20 μm. When in this range, the dimensional stability is excellent.

The 5% mass reduction temperature (T _d ⁵ ) is the temperature at which the mass of a polyimide film (20 μm thick) is reduced by 5% of its initial mass during a heating process in nitrogen at a heating rate of 10° C./min, and is preferably 400° C. or higher, more preferably 420° C. or higher, and even more preferably 440° C. or higher. Also, the temperature at which the mass of a polyimide film (20 μm thick) is reduced by 5% of its initial mass during a heating process in air at a heating rate of 10° C./min is preferably 400° C. or higher, more preferably 410° C. or higher, and even more preferably 420° C. or higher. Within this range, the thermal stability (chemical heat resistance) is excellent.

The tensile modulus (E) is preferably 2.7 GPa or more, more preferably 3.0 GPa or more, and even more preferably 3.2 GPa or more.
The breaking strength (σ _b ) is preferably 0.08 GPa or more, more preferably 0.10 GPa or more, and even more preferably 0.12 GPa or more.
The breaking elongation (ε _b ) has an average value (av) of preferably 5% or more, more preferably 7% or more, and even more preferably 10% or more, and a maximum value (max) of preferably 10% or more, more preferably 12% or more, and even more preferably 15% or more.
Within these ranges, the film has excellent mechanical properties, particularly excellent toughness.

The pencil hardness is preferably 4H or more, and more preferably 5H or more. This range provides excellent hardness.

The above-mentioned physical properties in the present invention can be measured specifically by the method described in the Examples.

<Polyimide varnish>
The polyimide of the present invention has sufficiently high solvent solubility, so that it can be made into a varnish with a high solid content concentration that is stable at room temperature. The polyimide varnish of the present invention is obtained by dissolving the polyimide of the present invention in an organic solvent. That is, the polyimide varnish of the present invention contains the polyimide of the present invention and an organic solvent, and the polyimide is dissolved in the organic solvent. The organic solvent may be any one that dissolves the polyimide, and is not particularly limited, but it is preferable to use the above-mentioned compounds alone or in a mixture of two or more kinds as the reaction solvent used in the production of the polyimide.
The polyimide varnish of the present invention may be a polyimide solution itself in which a polyimide obtained by a polymerization method is dissolved in a reaction solvent, or may be a polyimide solution to which a dilution solvent is further added.

The method for producing the polyimide varnish is not particularly limited, and any known method can be applied.
The polyimide varnish obtained in one step as described above can be used as it is, or it can be appropriately diluted with the same solvent and slowly dropped into a large amount of poor solvent to precipitate, and then filtered, washed, and dried to isolate it as a polyimide powder. The poor solvent that can be used in this case is not particularly limited as long as it is a solvent that is well miscible with the polymerization solvent and does not dissolve polyimide, and examples of such poor solvents include water, methanol, ethanol, and propanol. Two or more of these may be mixed and used. The obtained polyimide powder may be redissolved in a solvent at a solid content concentration of 5 to 40 mass % to obtain a polyimide varnish. The solvent that can be used in this case is the same as the above-mentioned solvent that can be used in the polymerization reaction of polyimide. When redissolving the polyimide powder in a solvent, it may be heated at 40 to 200 ° C for 1 minute to 24 hours as long as the varnish is not significantly colored.

Various additives such as inorganic fillers, adhesion promoters, release agents, flame retardants, UV stabilizers, surfactants, leveling agents, defoamers, fluorescent brighteners, crosslinking agents, polymerization initiators, and photosensitizers may be added to the polyimide varnish obtained as described above, so long as they do not impair the required properties of the polyimide film.

<Heat-resistant film>
The heat-resistant film of the present invention contains the polyimide of the present invention. Therefore, the polyimide film of the present invention is excellent in colorless transparency, heat resistance (physical heat resistance), dimensional stability, thermal stability (chemical heat resistance), mechanical properties and toughness. The preferable physical properties of the film of the present invention are as described above.
The method for producing the polyimide film of the present invention is not particularly limited, and a known method can be used. For example, the heat-resistant film of the present invention can be obtained by applying the polyimide varnish obtained by the above method onto a substrate and drying it. If necessary, the film can be further heat-treated at a high temperature.
Specifically, a polyimide varnish is applied onto a substrate such as glass, copper, aluminum, stainless steel, or silicon, and dried at preferably 40 to 220° C., more preferably 60 to 200° C., for preferably 10 minutes to 4 hours, more preferably 0.5 to 2 hours. The temperature is then further increased, and the polyimide film is obtained by heat-treating the substrate at preferably 200 to 350° C., more preferably 250 to 330° C., for preferably 10 minutes to 2 hours, more preferably 0.5 to 1 hour. From the viewpoint of suppressing coloration of the polyimide film, the heat treatment temperature is preferably 350° C. or less, and further, the heat treatment is preferably performed in a vacuum or in an inert gas such as nitrogen. In addition, from the viewpoint of smoothness and low thermal expansion properties of the polyimide film surface, the drying and heat treatment steps are preferably performed in as many stages as possible so that the temperature is raised gradually, and further, the drying and heat treatment steps exceeding 200° C. are preferably performed in a vacuum or in an inert gas such as nitrogen.

The polyimide film of the present invention is suitable for use as a film for various components such as color filters, flexible displays, semiconductor parts, and optical components. The polyimide film of the present invention is particularly suitable for use as a substrate for image display devices such as liquid crystal displays and OLED displays.

The thickness of the polyimide film of the present invention is not particularly limited and can be adjusted appropriately depending on the purpose of use. When used as a plastic substrate material to replace glass substrates in image display devices such as LCD, OLED, and EP, the film thickness is preferably in the range of 20 to 100 μm, and when used as a flexible circuit board, the film thickness is preferably in the range of 30 to 200 μm.

[Evaluation of physical properties]
The present invention will be described in detail below with reference to examples, but is not limited to these examples. The physical properties in the following examples were measured by the following methods.

<Infrared absorption (FT-IR) spectrum>
The infrared absorption spectrum of the diamine was measured by the KBr plate method using a Fourier transform infrared spectrophotometer "FT/IR-4100" (manufactured by JASCO Corporation).

< ^1H -NMR spectrum>
^{The 1} H-NMR spectrum of the diamine was measured using deuterated dimethyl sulfoxide (DMSO-d ₆ ) as a solvent and an NMR spectrophotometer "ECP400" (manufactured by JEOL Ltd.).

<Differential scanning calorimetry (melting point and melting curve)>
The melting point and melting curve of the diamine were measured using a differential scanning calorimeter "DSC3100" (manufactured by Netzsch Japan Co., Ltd.) in a nitrogen atmosphere at a temperature rise rate of 5°C/min.

<Intrinsic viscosity (η _inh )>
The reduced viscosity of the polyimide was measured using an Ostwald viscometer at a solids concentration of 0.5% by mass and at 30° C. This value can be regarded as the intrinsic viscosity, and a higher value indicates a higher molecular weight.

<Transparency: Light transmittance, cutoff wavelength, yellowness, haze>
The transparency of the polyimide film was evaluated from the following optical properties. The light transmittance curve of the polyimide film (about 20 μm thick) was measured in the wavelength range of 200 to 800 nm using an ultraviolet-visible spectrophotometer "V-530" (manufactured by JASCO Corporation), and the light transmittance at a wavelength of 400 nm (T ₄₀₀ ) and the wavelength at which the light transmittance is practically zero (cutoff wavelength, λ _cut ) were obtained. Based on this spectrum, the yellowness index (YI) was obtained based on the ASTM E313 standard using a color calculation program (manufactured by JASCO Corporation). Furthermore, the total light transmittance (T _tot ) and turbidity (haze) were obtained based on the JIS K7361-1 and JIS K7136 standards using a haze meter "NDH4000" (manufactured by Nippon Denshoku Industries Co., Ltd.).

<Glass transition temperature ( _Tg )>
The glass transition temperature ( _Tg ) of the polyimide film (approximately 20 μm thick) was determined from the peak temperature of the loss energy curve at a frequency of 0.1 Hz and a heating rate of 5° C./min using a thermomechanical analyzer "TMA4000" (manufactured by Netzsch Japan Co., Ltd.) The higher _{the Tg} , the higher the physical heat resistance.

<Coefficient of Linear Thermal Expansion (CTE)>
The CTE of the polyimide film (approximately 20 μm thick) was determined as the average value in the range of 100 to 200° C. from the elongation of the test piece at a temperature rise rate of 5° C./min per load of 0.5 g/film thickness of 1 μm using a thermomechanical analyzer "TMA4000" (manufactured by Netzsch Japan Co., Ltd.). The closer the CTE value is to 0, the better the dimensional stability.

<Tensile modulus, elongation at break, and strength at break>
The mechanical properties of the polyimide film (approximately 20 μm thick) were evaluated using a tensile tester "Tensilon UTM-2" (manufactured by A&D Co., Ltd.). Test pieces (30 mm long x 3 mm wide x approximately 20 μm thick) were prepared and subjected to a tensile test (stretching speed: 8 mm/min) to determine the tensile modulus (E) from the initial gradient of the stress-strain curve, the breaking strength (σ _b ) from the stress at break, and the breaking elongation (ε _b ) from the elongation at the time of judgment. A higher breaking elongation indicates a higher toughness of the film.

<5% mass reduction temperature (T _d ⁵ )>
The 5% mass loss temperature (T _d ⁵ ) of a polyimide film (approximately 20 μm thick) was measured in nitrogen and air at a heating rate of 100° C. using a thermogravimetric analyzer "TG-DTA2000" (manufactured by Netsch Japan Co., Ltd.). The mass of a polyimide film (20 μm thick) was determined from the temperature at which it decreased by 5% of its initial mass during a temperature increase at 10° C./min. The higher the T _d ⁵ value, the better the chemical heat resistance (thermal stability). ) is high.

<Pencil hardness>
The surface hardness of the polyimide film was evaluated based on the presence or absence of scratches on the film surface by performing a pencil scratch test (Mitsubishi Pencil Uni, pencil hardness range: 6B to 6H) on the polyimide film formed on a glass substrate using a pencil hardness tester "BEVS 1301/750" (manufactured by BEVS, pencil tip load: 750 g) in accordance with the ASTM D3363 standard.

[Synthesis of diamine]
The process for producing the diamine of the present invention will be specifically described below, but the present invention is not limited to these examples.

Example 1
<Synthesis of AMB-mTOL>
The diamine (AMB-mTOL) of the present invention represented by the above formula (1) was synthesized as follows.
First, in a reaction vessel, m-tolidine (25 mmol) was dissolved in well-dehydrated DMAc (20 mL), to which 6.0 mL of pyridine was added as an acid scavenger, and the vessel was sealed with a septum cap to obtain liquid A. Next, in a separate vessel, 3-methyl-4-nitrobenzoyl chloride (3M4NBC, 60 mmol) was dissolved in dehydrated DMAc (10 mL), and the vessel was sealed in the same manner to obtain liquid B. Liquid B was cooled in an ice bath and stirred, while liquid A was gradually added dropwise thereto using a syringe, and the mixture was stirred for several hours, and then further stirred at room temperature for 24 hours. The precipitated yellow precipitate was filtered off, washed with toluene to remove excess 3M4NBC, and then thoroughly washed with water to remove the by-product pyridine hydrochloride, and vacuum dried at 120° C. for 12 hours to obtain a yellow powder with a yield of 85%. The analytical results of this product are shown below.
Melting point (DSC): 251°C.
FT-IR spectrum (KBr plate method, cm ^-1 ): 3274 (amide group, N--H stretching vibration), 3103/3033 (aromatic C--H stretching vibration), 2984/2929/2857 (aliphatic C--H stretching vibration), 1651/1588 (amide group, C=O stretching vibration), 1523/1343 (nitro group, N--O stretching vibration).
¹ H-NMR spectrum (400 MHz, DMSO-d ₆ , δ, ppm): 10.48 [s, 2H (measured integral intensity: 2.00) NHCO], 8.13 [d, 2H (1.98H), J = 8.4 Hz, 6,6'-protons of terminal nitrobenzene (NB), 8.06 [sd, 2H (2.00H), J = 1.4 Hz, 3,3'-protons of NB], 7.99 [dd, 2H (1.95H), J = 8.4, 1.6 Hz, 5,5'- protons], 7.73 [sd, 2H (2.05H), J = 1.8 Hz, 3,3'-protons of the central biphenyl (BP)], 7.68 [dd, 2H (2.02H), J = 8.3, 2.0 Hz, 5,5'-protons of BP], 7.09 [d, 2H (2.06H), J = 8.2 Hz, 6,6'-protons of BP], 2.61 [s, 6H (5.97H), 2,2'-CH ₃ of NB]. 2.05 [s, 6H (6.01H), 2,2'-CH ₃ of BP].

From the above analytical results, this product was found to be the target compound of the following formula (11):

It was identified as a dinitro form represented by the formula:

The dinitro compound was reduced as follows. First, this dinitro compound (11.283 g) was dissolved in DMAc (30 mL), Pd/C (1.204 g) was added as a catalyst, and the mixture was refluxed at 80° C. for 6 hours in a hydrogen atmosphere. Completion of the reduction reaction was confirmed by thin layer chromatography. After the reaction solution was cooled to room temperature, the Pd/C residue was separated and removed by filtration. The filtrate was dropped into a large amount of saturated saline to cause a precipitate to separate out. The precipitate was filtered off, washed with water and a small amount of methanol, and vacuum dried at 120° C. for 12 hours, obtaining a white powder with a yield of 87%. The analytical results of this product are shown below.
Melting point (DSC): 210°C.
FT-IR spectrum (KBr plate method, cm ^-1 ): 3442 (amino group, N--H stretching vibration), 3350/3291 (amino group + amide group, N--H stretching vibration), 3026 (aromatic C--H stretching vibration), 2917/2859 (aliphatic C--H stretching vibration), 1625/1578 (amide group, C=O stretching vibration), 1500 (1,4-phenylene group).
^1H -NMR spectrum (400 MHz, DMSO- _d6 , δ, ppm): 9.76 [s, 2H (2.00H), NHCO], 7.70-7.62 [m, 8H (8.07H), 3,3',5,5'-protons of terminal aniline (AN) + 3,3',5,5'-protons of central BP], 7.01 [d, 2H (1.99H), J = 8.3 Hz, 6,6'-protons of BP], 6.66 [d, 2H (1.98H), J = 8.3 Hz, 6,6'-protons of AN], 5.53 [s, 4H (3.99H), _NH2 ], 2.13 [s, 6H (6.07H), 2,2'- _CH3 of BP]. 2.02 [s, 6H (5.89H), 2,2'-CH ₃ of AN].
Elemental analysis (molecular weight 478.59): Estimated (%) C: 75.29, H: 6.32, N: 11.71, Found C: 74.67, H: 6.46, N: 11.55.

From the above analytical results, the product was identified as the desired diamine (AMB-mTOL) of the present invention represented by the above formula (1).

Example 2
<Synthesis of AB-MP-HPMDI>
The diamine (AB-MP-HPMDI) of the present invention represented by the above formula (2) was synthesized as follows.
First, as a starting material,

The dinitro compound represented by the formula was synthesized as follows. In a three-neck flask, 11.271 g (67 mmol) of 2-methoxy-4-nitroaniline (2MeO-4NA) was dissolved in DMAc (30 mL), and 6.763 g (30 mmol) of H-PMDA powder was added to this solution, followed by refluxing at 180° C. for 5 hours in a nitrogen atmosphere. After the reaction, the mixture was cooled to room temperature, and the reaction solution was concentrated using an evaporator, and then ethanol (200 mL) was added to cause a precipitate to form. The precipitate was filtered off, washed with ethanol, and then vacuum-dried at 120° C. for 12 hours, yielding a pale yellow powder in a 53% yield.
FT-IR spectrum (KBr plate method, cm ^-1 ): 3088/3006 (aromatic C-H stretching vibration), 2953/2879 (aliphatic C-H stretching vibration), 1777/1718 (imido group, C=O stretching vibration), 1527/1350 (nitro group, N-O stretching vibration), 1390 (imido group, N-Ar stretching vibration), 1255/1194 (ether group, C-O stretching vibration).

The dinitro compound was reduced in the same manner as described in Example 1, by refluxing in DMAc in the presence of Pd/C in a hydrogen atmosphere at 80° C. for 3.5 hours. Completion of the reduction reaction was confirmed by thin layer chromatography. A white powder was obtained in a yield of 92%.
FT-IR spectrum (KBr plate method, cm ^-1 ): 3463/3370/3234 (amino group, N--H stretching vibration), 2943/2874 (aliphatic C--H stretching vibration), 1774/1709 (imido group, C=O stretching vibration), 1398 (imido group, N--Ar stretching vibration), 1254/1193 (ether group, C--O stretching vibration).

The following formula (8) obtained as above:

and 4-NBC were subjected to an amidation reaction in DMF in the same manner as described in Example 1 to obtain a compound represented by the following formula (13):

A pale yellow powder product represented by the formula:
FT-IR spectrum (KBr plate method, cm ^-1 ): 3344 (amide group, N-H stretching vibration), 3111/3078 (aromatic C-H stretching vibration), 2942/2867 (aliphatic C-H stretching vibration), 1775/1718 (imide group, C=O stretching vibration), 1685 (amide group, C=O stretching vibration), 1516/1347 (nitro group, N-O stretching vibration), 1389 (imide group, N-Ar stretching vibration), 1200 (ether group, C-O stretching vibration).

The reduction of the dinitro compound was carried out in the same manner as described in Example 1, by refluxing in DMAc in the presence of Pd/C in a hydrogen atmosphere at 80° C. for 3.5 hours. Completion of the reduction reaction was confirmed by thin layer chromatography. A white powder was obtained in a yield of 76%. The analytical results of this product are shown below.
FT-IR spectrum (KBr plate method, cm ^-1 ): 3451 (amino group, N-H stretching vibration), 3358/3227 (amino group + amide group, N-H stretching vibration), 3137/3033 (aromatic C-H stretching vibration), 2938/2875 (aliphatic C-H stretching vibration), 1775/1711 (imide group, C=O stretching vibration), 1656 (amide group, C=O stretching vibration), 1390 (imide group, N-Ar stretching vibration), 1254/1202 (ether group, C-O stretching vibration).
¹ H-NMR spectrum (400 MHz, DMSO-d ₆ , δ, ppm): 9.91 [s, 2H (2.00H), NHCO], 7.75-7.64 [m, 6H (6.03H), 3,3',5,5'-protons of terminal AN + N-(6,6'-protons of 2-methoxyphenyl group)], 7.48-7.40 [m, 2H (1.98H), N-(3,3'-protons of 2-methoxyphenyl group)], 7.24-7.01 [m, 2H (1.99H), N-(5,5'-protons of 2-methoxyphenyl group)], 6.62 [d, 4H (4.01H), J=10.8 Hz, 2,2',6,6'-protons of terminal AN], 5.80 [s, 4H (3.95H), NH ₂ )], 3.74 [s, 6H (5.84H), CH ₃ ], 3.40-1.80 [m, 8H, central cyclohexyl group].

From the above analytical results, the product was identified as the desired diamine (AB-MP-HPMDI) of the present invention represented by formula (2).

Synthesis Example 1
<Synthesis of AB-mTOL>
The diamine (AB-mTOL) represented by the above formula (9) was synthesized by carrying out an amidation reaction between m-tolidine and 4-NBC, followed by a reduction reaction of the nitro group, in the same manner as described in Example 1. The analytical results of this product are shown below.
Melting point (DSC): 295°C.
FT-IR spectrum (KBr plate method, cm ^-1 ): 3458 (amino group, N--H stretching vibration), 3376/3305 (amino group + amide group, N--H stretching vibration), 3037 (aromatic C--H stretching vibration), 2917 (aliphatic C--H stretching vibration), 1650/1530 (amide group, C=O stretching vibration), 1508 (1,4-phenylene group).
¹ H-NMR spectrum (400 MHz, DMSO- _d , δ, ppm): 9.75 [s, 2H (2.00H), NHCO], 7.74 [d, 4H (4.03H), J = 8.6 Hz, 3,3',5,5'-protons of terminal AN], 7.69 [sd, 2H (1.93H), J = 1.9 Hz, 3,3'-protons of central BP], 7.61 [dd, 2H (1.99H), J = 8.3, 2.0 Hz, 5,5'-protons of central BP], 7.00 [d, 2H (1.96H), J = 8.2 Hz, 6,6'-protons of BP], 6.61 [d, 4H (4.02H), J = 8.6 Hz, 2,2',6,6'-protons of terminal AN], 5.75 [s, 4H (4.10H), NH ₂ ], 2.01 [s, 6H (6.03H), CH ₃ ].
Elemental analysis (molecular weight 450.54): Estimated (%) C: 74.65, H: 5.82, N: 12.44, Found C: 74.44, H: 5.99, N: 12.38.

From the above analytical results, the product was identified as the desired diamine (AB-mTOL) represented by formula (9).

Synthesis Example 2
<Synthesis of AB-44ODA>
The diamine (AB-44ODA) represented by the above formula (10) was synthesized by carrying out an amidation reaction between 4,4'-oxydianiline and 4-NBC, followed by a reduction reaction of the nitro group, in the same manner as described in Example 1. The analytical results of this product are shown below.
Melting point (DSC): 282°C.
FT-IR spectrum (KBr plate method, cm ^-1 ): 3409 (amino group, N--H stretching vibration), 3375/3322/3221 (amino group + amide group, N--H stretching vibration), 3036 (aromatic C--H stretching vibration), 1658/1572 (amide group, C=O stretching vibration), 1509 (1,4-phenylene group), 1257 (ether group, C--O stretching vibration).
^1H -NMR spectrum (400 MHz, DMSO- _d6 , δ, ppm): 9.78 [s, 2H (2.00H), NHCO], 7.76-7.71 [m, 8H (8.04H), 3,3',5,5'-protons of terminal AN + 3,3',5,5'-protons of central biphenyl ether (BE) group], 6.97 [d, 4H (3.92H), J = 9.1 Hz, 2,2',6,6'-protons of central BE], 6.61 [d, 4H (4.08H), J = 8.6 Hz, 2,2',6,6'-protons of terminal AN], 5.75 [s, 4H (4.02H), _NH2 ].
Elemental analysis (molecular weight 438.49): Estimated (%) C: 71.22, H: 5.06, N: 12.78, Found C: 71.21, H: 5.16, N: 12.64.

From the above analytical results, the product was identified as the desired diamine (AB-44ODA) represented by formula (10).

<Polymerization, film formation, and evaluation of polyimide film properties>
Example 3
The diamine of the present invention represented by the formula (1) obtained in Example 1 (5 mmol) was placed in a separable flask, and γ-butyrolactone (GBL) sufficiently dehydrated with molecular sieves 4A was added and dissolved. At this time, the solvent was added in a calculated manner so that the total solute concentration would be 40 mass%. 1-Ethylpiperidine (1-EPD, 10 mmol) and benzoic acid (10 mmol) were added as catalysts to this diamine solution, and then H-PMDA (manufactured by Mitsubishi Gas Chemical Co., Ltd.) powder (5 mmol) was added, and the mixture was heated to 200°C while stirring, and stirred in a nitrogen atmosphere for 4 hours to obtain a uniform and viscous polyimide varnish (solution). This polyimide varnish was appropriately diluted with the same solvent, and then slowly dropped into a large amount of poor solvent (methanol) to precipitate a white, fibrous polyimide, which was filtered and washed, and then vacuum dried at 120°C for 12 hours to obtain a polyimide powder. This was redissolved in pure GBL to obtain a homogeneous polyimide varnish having a solid content of 10% by mass. The reduced viscosity of the polyimide measured in GBL at a concentration of 0.5% by mass at 30° C. using an Ostwald viscometer was 3.82 dL/g.
This polyimide varnish was applied to a glass substrate and dried in a hot air dryer at 80° C. for 2 hours, and then heated stepwise in a vacuum at 150° C. for 30 minutes, 200° C. for 30 minutes, and 250° C. for 1 hour. To remove residual stress, the film was peeled off from the substrate and further heat-treated in a vacuum at 3250° C. for 1 hour, yielding a transparent and flexible polyimide film with a thickness of approximately 20 μm.

The film properties are shown in Table 1. From the dynamic viscoelasticity measurement, this polyimide film had a very high _Tg of 367°C. In addition, the linear thermal expansion coefficient was 24.3 ppm/K, and it also showed low thermal expansion properties. This is because the diamine of the present invention described in Example 1 was used. In addition, the ⁵ % mass loss temperature ( _Td5 ) was 448°C in nitrogen and 428°C in air, and the thermal stability was also good. In addition, the light transmittance at a wavelength of 400 nm was 80.3%, the yellowness index was 2.8, and the haze was 1.79%, and it had high transparency. Furthermore, when the mechanical properties were evaluated, it showed a high tensile modulus of elasticity of 5.46 GPa, and the breaking elongation was a maximum of 13%, and it had flexibility. In addition, it had high pencil hardness. Other properties are also shown in Table 1.

Example 4
Instead of the diamine represented by the above formula (1), the diamine of the present invention represented by the above formula (2) obtained in Example 2 was used, and polymerization was carried out in the same manner as in Example 3, except that 1-EPD was used alone as the catalyst, and a film was formed and its physical properties were evaluated. The film physical properties are shown in Table 1. This polyimide film exhibited a relatively low CTE and also maintained high transparency and heat resistance. This is because the diamine of the present invention described in Example 2 was used.

Example 5
Instead of the diamine represented by the above formula (1), the diamine represented by the above formula (9) (AB-mTOL) obtained in Synthesis Example 1 was used, and polymerization was carried out in the same manner as in Example 3, except that 1-EPD was used alone as the catalyst, and a film was formed and its physical properties were evaluated. The film properties are shown in Table 1. This polyimide film exhibited a relatively low CTE and also maintained high transparency and heat resistance. This effect was first achieved by combining H-PMDA and AB-mTOL.

Example 6
Instead of the diamine represented by the above formula (1), the diamine represented by the above formula (10) (AB-44ODA) obtained in Synthesis Example 2 was used, and polymerization was carried out in the same manner as in Example 3, except that 1-EPD was used alone as the catalyst, and a film was formed and its physical properties were evaluated. The film properties are shown in Table 1. This polyimide film exhibited a relatively low CTE, and also maintained high transparency and heat resistance. This effect was only achieved by combining H-PMDA and AB-44ODA.

Comparative Example 1
Polymerization was carried out in the same manner as in Example 3, except that 4,4'-oxydianiline (4,4'-ODA) was used instead of the diamine represented by the above formula (1), and 1-EPD was used alone as the catalyst, and a film was formed and its physical properties were evaluated. The film properties are shown in Table 1. The CTE of this polyimide film was 46.6 ppm/K, and it did not exhibit low thermal expansion properties. This is because the molecular structure of the diamine does not contain a precisely molecular-designed structural unit.

Comparative Example 2
Polymerization was carried out in the same manner as in Example 3, except that m-tolidine was used instead of the diamine represented by the above formula (1), and 1-EPD was used alone as the catalyst, and a film was formed and its physical properties were evaluated. The film properties are shown in Table 1. This polyimide film had a very high Tg, but the CTE was 57.5 ppm/K, and it did not exhibit low thermal expansion properties. This is because the molecular structure of m-tolidine does not contain a precisely molecular-designed structural unit.

Comparative Example 3
Polymerization was carried out in the same manner as in Example 3, except that 4,4'-diaminobenzanilide (DABA) was used instead of the diamine represented by the above formula (1), and 1-EPD was used alone as the catalyst. However, precipitates were formed during the polymerization reaction, and a uniform varnish was not obtained. This is because the molecular structure of DABA does not contain a precisely molecular-designed structural unit, and the solubility of the produced polyimide is insufficient.

Claims (5)

The following general formula (3):

In the formula (3), the divalent organic group Ar is a structural unit represented by the following formula (7):
A varnish obtained by dissolving the polyimide according to claim 1 in an organic solvent. A heat-resistant film comprising the polyimide according to claim 1. The heat-resistant film according to claim 3, which has a glass transition temperature of 320°C or higher, a linear thermal expansion coefficient of 40 ppm/K or lower, and a light transmittance of 70% or higher at a wavelength of 400 nm. A plastic substrate for an image display device, comprising the heat-resistant film according to claim 3 or 4.