Amide-modified diamine monomer, method for preparing the same, and polyimide

Abstract

The application belongs to the technical field of polymer materials, and specifically discloses an amide modified diamine monomer, a preparation method thereof and a polyimide. The amide modified diamine monomer is prepared by stirring and reacting 4-nitrobenzoyl chloride and 4,4'- (cyclohexyl methylene) bis (2- (tert-butyl) aniline) in a mixed solvent containing an alkaline compound to obtain a dinitro compound; then stirring and reacting the dinitro compound and a catalyst in an organic solvent to obtain the amide modified diamine monomer. The amide modified diamine monomer is subjected to polycondensation reaction with an aromatic dianhydride monomer to obtain the polyimide by a high-temperature one-step method. The resin film prepared from the polyimide has a low thermal expansion coefficient, improves the dimensional stability of the film, and the film has high transparency, and has a wide application prospect in the fields of flexible transparent substrates for optoelectronic devices, flat panel displays and the like.

Description

Technical Field

[0001] This invention belongs to the field of polymer materials technology, specifically relating to an amide-modified diamine monomer and its preparation method, and polyimide. Background Technology

[0002] Polyimide is a class of high-performance engineering plastics. Its molecular chain contains a characteristic imide ring structure, endowing the material with excellent heat resistance, mechanical strength, and chemical stability. It is now widely used in high-end fields such as aerospace, electrical engineering, machinery, chemical engineering, and microelectronics. However, traditional aromatic polyimides, due to their rigid main chain, easily form charge-transfer complexes (CTCs) within and between molecules. This results in a generally darker color, making it difficult to meet the high transparency requirements of the optoelectronic field.

[0003] Meanwhile, the coefficient of thermal expansion is a key indicator affecting the processing and molding performance of polyimide and its dimensional stability during use. Reducing the coefficient of thermal expansion is also an important technical direction for the modification of polyimide materials. How to achieve a synergistic balance between high transparency and low coefficient of thermal expansion, while maintaining the original excellent thermal stability of the material, has become a technical problem that urgently needs to be solved in the field of polyimide.

[0004] To address the technological need for improved transparency in polyimides, existing technologies mainly employ three improvement pathways, but all have significant limitations: First, introducing strong electron-withdrawing groups such as fluorinated substituents can suppress CTC formation by interrupting the conjugation effect of the electron cloud, but this method involves complex synthesis processes and high costs, limiting its industrial application; Second, introducing alicyclic structures to reduce the proportion of aromatic structures in the molecule can reduce CTC formation, but the flexibility of alicyclic structures weakens the rigidity of the main chain, leading to a decrease in the thermal stability of the material; Third, introducing non-coplanar structures or large-volume side groups can inhibit CTC formation by hindering the flow of charge in the molecular chain through steric hindrance, but large-volume side groups increase the inter-chain spacing, thereby increasing the coefficient of thermal expansion and affecting the dimensional stability of the material.

[0005] To address the technical need for reducing the coefficient of thermal expansion (CTE) of polyimide, existing technologies mainly employ two modification methods, but neither can solve the problem of synergistic effects between CTE and transparency: One method involves physical doping with rigid fillers such as nanoparticles to restrict the thermal movement of molecular chains and reduce the CTE. However, these fillers are prone to aggregation and have poor compatibility with the polyimide matrix, leading to a significant decrease in material transparency and failing to meet the high transparency requirements of the optoelectronic field. The second method involves chemical modification by introducing amide bonds and other groups into the molecule. This utilizes polar interactions such as hydrogen bonds to enhance intermolecular forces and improve the compactness of molecular chain stacking, thereby effectively reducing the CTE. However, this modification strategy, which relies solely on introducing amide bonds to enhance intermolecular interactions, still suffers from CTC formation, resulting in insufficient polyimide transparency and making it difficult to achieve a balance between high transparency and a low CTE.

[0006] In summary, existing polyimide modification technologies cannot simultaneously achieve synergistic optimization of multiple properties, including improved transparency, reduced coefficient of thermal expansion, and maintained thermal stability. Developing novel monomers through molecular structure design to prepare polyimide materials that combine high transparency, low coefficient of thermal expansion, and excellent thermal stability has become an important research direction in the field of high-performance polyimides. However, such monomers are currently scarce and still in the early stages of research and development. Summary of the Invention

[0007] To address the aforementioned technical problems, this invention provides an amide-modified diamine monomer, its preparation method, and a polyimide. This invention develops a novel monomer through molecular structure design, thereby preparing a polyimide material that combines high transparency, low coefficient of thermal expansion, and excellent thermal stability.

[0008] The technical solution provided by this invention has the following objectives: (1) An amide-modified diamine monomer and its preparation method are provided. This diamine monomer integrates amide bonds, tert-butyl groups, and aliphatic ring groups simultaneously in its main chain structure through molecular design. The polar nature of the amide bond enhances the intermolecular hydrogen bonding, effectively reducing the coefficient of thermal expansion and improving dimensional stability. The steric hindrance effect of the tert-butyl group and the aliphatic ring weakens the conjugation of the molecular chain, inhibiting CTC formation, thereby significantly improving transparency while maintaining the thermal stability of the material. This invention further provides a specific preparation process for this amide-modified diamine monomer, achieving precise synthesis of the target structure.

[0009] (2) Provides polyimides prepared from the above-mentioned amide-modified diamine monomers and their preparation methods. A novel type of polyimide resin with a main chain containing amide bonds, tert-butyl groups, and aliphatic ring structures can be obtained by polycondensation reaction of the amide-modified diamine monomers with various commercially available aromatic dianhydride monomers. This series of resins achieves performance breakthroughs through the synergistic effect of multiple structures: amide bonds enhance intermolecular forces, reduce the coefficient of thermal expansion, and improve dimensional stability; tert-butyl groups and aliphatic rings inhibit CTC formation, ensuring high transparency; and the aromatic main chain and imide ring structure maintain the material's high heat resistance.

[0010] (3) Provide the application and processing methods of the above-mentioned polyimide in products. This material can be widely used in the fields of packaging materials for optoelectronic components such as flexible OLED substrates, high-temperature insulating coatings, and precision mechanical parts. Its processing methods are compatible with conventional polymer material molding processes, including compression molding, solution casting, and injection molding. By adjusting the processing temperature of 200-300℃, solvent ratio, and pressure parameters, high-precision molding of products can be achieved, and the material is not easily decomposed during processing, meeting the needs of industrial production.

[0011] In summary, this invention, through innovative molecular structure of diamine monomers, breaks through the performance bottlenecks of traditional polyimides in terms of coefficient of thermal expansion, transparency, and thermal stability, providing a new technical path for the application of high-performance polyimide materials in high-end fields such as optoelectronics.

[0012] To achieve the above objectives, the technical solution adopted by the present invention is as follows: This invention provides an amide-modified diamine monomer, the structural formula of which is as follows: .

[0013] This invention provides a method for preparing the above-mentioned amide-modified diamine monomer, comprising the following steps: Under an inert atmosphere and in a tetrahydrofuran solvent, using 4-nitrobenzoyl chloride and 4,4'-(cyclohexylmethylene)bis(2-(tert-butyl)aniline) as raw materials, a nucleophilic addition-elimination reaction between an aromatic primary amine and an acyl chloride occurs under the catalysis of pyridine to yield a dinitro compound.

[0014] In a reducing atmosphere, a dinitro compound and a palladium catalyst on carbon are added to a tetrahydrofuran solvent. The nitro group in the dinitro compound is reduced to a primary amino group through a heterogeneous catalytic hydrogenation reduction reaction to obtain an amide-modified diamine monomer.

[0015] Furthermore, in the preparation steps of the dinitro compound, the nucleophilic addition-elimination reaction is carried out at room temperature for 12-24 hours.

[0016] Furthermore, the molar ratio of 4-nitrobenzoyl chloride to 4,4'-(cyclohexylmethylene)bis(2-(tert-butyl)aniline) is 7.1~7.3:3, and the molar ratio of 4,4'-(cyclohexylmethylene)bis(2-(tert-butyl)aniline) to pyridine is 0.03:0.1, i.e. 0.3:1.

[0017] Furthermore, in the step of reducing nitro to primary amino, the reaction temperature is 50℃~100℃ and the time is 24h~48h; the reducing atmosphere is hydrogen atmosphere, and the mass ratio of dinitro compound to palladium catalyst on carbon is 8~12:1.

[0018] The specific steps for preparing the above-mentioned amide-modified diamine monomer are as follows: (1) Under the protection of an inert gas, 4-nitrobenzoyl chloride and tetrahydrofuran were added to a three-necked flask and stirred to obtain a clear liquid. The system was then cooled to below room temperature to obtain the first mixture.

[0019] The core purpose of the inert gas protection here is to isolate oxygen and moisture from the air, preventing 4-nitrobenzoyl chloride from hydrolyzing and becoming ineffective upon contact with water. It also prevents the aromatic primary amine raw material from being oxidized and discolored, ensuring the activity of the reactive functional groups. This operation directly affects the purity of the subsequent monomers, thus determining the molecular weight of the polymer and the transparency of the film. Existing technologies often neglect strict inert atmosphere protection, leading to acyl chloride hydrolysis rates exceeding 10%, an increase in monosubstituted byproducts, and monomer purity below 90%, ultimately reducing the visible light transmittance of the polymer film. 4-Nitrobenzoyl chloride, as an acylation reagent, is a core raw material for constructing the target diamine monomer. It provides acyl chloride functional groups that react with amino groups, while introducing a rigid aromatic structure to ensure the thermal stability of the subsequent polyimide. The introduced nitro group can be subsequently reduced to an amino group, providing reaction sites for the synthesis of the diamine monomer. Tetrahydrofuran, as a homogeneous reaction solvent, can simultaneously dissolve 4-nitrobenzoyl chloride and the subsequently added diamine raw material, ensuring the reaction proceeds uniformly in a homogeneous system and avoiding side reactions caused by excessively high local concentrations. Its low boiling point also facilitates solvent removal in the subsequent purification process. Cooling the system to below room temperature is crucial for controlling the rate of the subsequent amidation reaction. This prevents the reaction rate between the acyl chloride and amino group from being too rapid at room temperature, which could lead to excessively high local diamine concentrations and the formation of monosubstituted byproducts. Simultaneously, it further suppresses the hydrolysis side reaction of the acyl chloride. This temperature control is essential for ensuring high selectivity in the diamidation reaction and directly determines the purity of the intermediate dinitro compound. The purity of the intermediate is critical for achieving high molecular weight polycondensation of the subsequent diamine monomer. Current technologies often use room temperature feeding, resulting in high monosubstituted byproduct content, which leads to end-capping effects during subsequent polycondensation, preventing polymer molecular weight growth and significantly reducing film-forming properties and mechanical properties.

[0020] (2) Dissolve 4,4'-(cyclohexylmethylene)bis(2-(tert-butyl)aniline) in a mixed solution of pyridine and tetrahydrofuran to obtain a second mixture. Add the second mixture slowly dropwise to the first mixture after cooling in step (1) over 1-2 hours using a constant pressure dropping funnel. Then restore the reaction mixture to room temperature and continue stirring for 12-24 hours.

[0021] 4,4'-(cyclohexylmethylene)bis(2-(tert-butyl)aniline) serves as the core skeleton material for the target diamine monomer. The primary amino groups at both ends of its molecular structure provide active sites for the amidation reaction. The cyclohexyl aliphatic ring and the sterically hindered side group of the ortho-tert-butyl group are the core functional groups for achieving high transparency of polyimide. The aliphatic ring can reduce the conjugation degree of the aromatic structure in the molecular chain, and the steric hindrance effect of the tert-butyl group can hinder charge transfer between molecular chains and inhibit the formation of CTC. This solves the problem of dark color and poor transparency of traditional aromatic polyimide from the molecular root. This structural design is different from the modification scheme of simply introducing amide bonds or aliphatic ring structures in the existing technology. It can improve transparency while reserving structural space for subsequent introduction of amide bonds to reduce the coefficient of thermal expansion. Pyridine, as an acid-binding agent and a weakly basic catalyst, can rapidly neutralize the hydrogen chloride byproduct generated in the amidation reaction to form pyridine hydrochloride, thus promoting the reversible amidation reaction to proceed completely in the forward direction. On the other hand, its weak basicity can activate the nucleophilic activity of amino groups, increasing the reaction rate, while avoiding the excessive hydrolysis of acyl chlorides caused by strongly basic systems. The amount of pyridine used is directly related to the acid neutralization efficiency of the reaction system. If the amount of pyridine is insufficient, hydrogen chloride cannot be completely neutralized, resulting in insufficient reaction conversion. Excessive amounts will increase the difficulty of subsequent purification and introduce colored impurities. The core purpose of the slow, dropwise addition of the diamine feedstock over 1-2 hours is to strictly control the feed rate of the diamine, ensuring that the acyl chloride in the reaction system is always in excess. This allows the added diamine to preferentially undergo disubstituted reactions, minimizing the formation of monosubstituted byproducts. If the addition time is too short, such as less than 30 minutes, the local diamine concentration will be too high instantaneously, and the content of monosubstituted byproducts will exceed 25%. If the addition time is too long, such as more than 3 hours, the system will be in prolonged contact with trace amounts of air, increasing the risk of acyl chloride hydrolysis and amine oxidation. This rate-controlled addition is one of the innovative aspects of this invention in achieving high purity of the intermediate. Existing technologies often use a one-step feeding method, which cannot control the reaction selectivity. The operation of restoring stirring to room temperature for 12-24 hours is to provide sufficient activation energy for the reaction after the low-temperature dropwise addition, ensuring the complete completion of the diamidation reaction. This step involves a nucleophilic addition-elimination reaction between an aromatic primary amine and an acyl chloride. Specifically, the primary amino nitrogen atoms at both ends of the 4,4'-(cyclohexylmethylene)bis(2-(tert-butyl)aniline) group act as nucleophilic centers, attacking the electron-deficient carbonyl carbon atom of the 4-nitrobenzoyl chloride acyl chloride group to form a tetrahedral transition state. Subsequently, the transition state undergoes an elimination reaction, losing chloride ions to generate an amide bond, while releasing hydrogen chloride as a byproduct. The two amino groups of the starting material react with two molecules of 4-nitrobenzoyl chloride. -Nitrobenzoyl chloride undergoes a disubstituted reaction, ultimately generating a dinitro intermediate with a diamide bond. When the reaction time is less than 12 hours, the conversion rate is less than 80%, and a large amount of monosubstituted intermediate remains. When the reaction time exceeds 30 hours, the generated amide bond will undergo a hydrolysis side reaction, reducing the product yield and purity. This reaction time setting can ensure a conversion rate of over 99%, laying the foundation for the subsequent preparation of high-purity diamine monomers.

[0022] (3) After the reaction is complete, pour the mixture into deionized water, precipitate and filter to collect the yellow solid, and then vacuum dry the yellow solid.

[0023] This step utilizes the low solubility of the target product in water to precipitate it from the tetrahydrofuran solvent system, achieving preliminary separation from water-soluble pyridine hydrochloride and unreacted small molecule raw materials. It is the core pre-step for crude product purification and can remove more than 90% of water-soluble impurities from the system.

[0024] (4) The dry yellow solid was purified by vacuum column chromatography with a volume ratio of tetrahydrofuran to ethyl acetate as the developing solvent, and a pale yellow solid was obtained. The solid was further recrystallized with toluene to obtain pale yellow crystals, namely the intermediate dinitro compound.

[0025] This dual purification process, combining reduced-pressure column chromatography and recrystallization, is key to ensuring high purity of the intermediate. Column chromatography efficiently separates monosubstituted byproducts with small polarity differences from the target disubstituted product, while recrystallization further removes trace amounts of residual impurities and colored substances. The combination of these two steps can increase the purity of the intermediate to over 99%. This process differs from existing purification methods that only use recrystallization. Existing purification methods cannot effectively remove monosubstituted byproducts, resulting in intermediate purity generally below 95%. Monosubstituted byproducts contain only a single nitro group, and after reduction, only a single amino group. In subsequent polycondensation reactions, they act as end-capping agents, limiting the growth of polymer molecular weight, leading to a decrease in the polymer's glass transition temperature and an increase in its coefficient of thermal expansion. The high-purity intermediate of this invention completely avoids this problem, ensuring an equimolar ratio of functional groups in subsequent polycondensation reactions, and achieving the preparation of high molecular weight polymers.

[0026] (5) Add dinitro compound, tetrahydrofuran and palladium on carbon catalyst to the high-pressure reactor, and simultaneously add magnetic stirring to make the system uniform. Introduce reducing atmosphere hydrogen into the high-pressure reactor to make the pressure reach 5.0MPa~10.0MPa, and then heat to 50℃~100℃ and stir the reaction for 24h~48h.

[0027] This step involves the heterogeneous catalytic hydrogenation reduction of aromatic nitro compounds. The core objective is to completely reduce the nitro groups at both ends of the intermediate to primary amino groups, yielding a diamine monomer with dual-terminated amino groups. Palladium on carbon is the key catalytic material in this heterogeneous hydrogenation reduction reaction. Its surface palladium active sites adsorb hydrogen molecules and dissociate them into highly reactive hydrogen atoms, while simultaneously adsorbing nitro groups, lowering the activation energy for the reduction of nitro groups to amino groups, thus ensuring high selectivity and completeness of the reduction reaction. The mass ratio of the intermediate dinitro compound to the catalyst is set to 8-12:1, with an optimal ratio of 10:1. Insufficient catalyst leads to insufficient active sites, incomplete reduction of nitro groups, and the residue of large amounts of hydroxylamine and nitroso intermediates, resulting in low purity of the reduction product. Excessive catalyst increases the difficulty of subsequent filtration and separation, causing palladium metal residue in the diamine monomer, affecting monomer purity and consequently leading to a lower polymer molecular weight. High-pressure hydrogen gas serves as the reducing agent in the reduction reaction, providing a hydrogen source to completely reduce aromatic nitro groups to primary amines. The high-pressure environment enhances the solubility of hydrogen in tetrahydrofuran solvent, ensuring sufficient active hydrogen in the reaction system for complete reduction of dinitro groups. Below 5 MPa, hydrogen solubility is insufficient, resulting in low conversion rates over 24 hours and a large number of nitro groups remaining unreduced. Above 10 MPa, a side reaction involving the hydrogenation of the benzene ring occurs, disrupting the rigid aromatic structure of the monomer and leading to a decrease in the rigidity of the polymer backbone, a lower glass transition temperature, and an increased coefficient of thermal expansion. The reaction temperature (50℃~100℃) and time (2...) are related. Setting the reaction time from 4h to 48h ensures both the reduction rate and high selectivity, preventing side reactions. Below 50℃, the reaction rate is too slow, with a conversion rate below 70% after 24h, resulting in incomplete reduction. Above 100℃, side reactions such as amide bond hydrolysis can occur, damaging the core functional groups in the monomer and preventing the subsequent polymer from achieving the synergistic optimization of high transparency and low thermal expansion coefficient. The reaction time setting ensures complete reduction of dinitrogen groups; too short a time results in incomplete reduction, while too long a time increases the risk of product oxidation and discoloration, leading to a decrease in the transmittance of the subsequent polymer film. The reaction mechanism of this step is that hydrogen molecules are adsorbed and dissociated into active hydrogen atoms on the surface of the palladium active site. At the same time, the nitro group of the dinitro intermediate is adsorbed on the catalyst surface. The active hydrogen atoms attack the nitro group stepwise, passing through the nitroso and hydroxylamine intermediates in sequence, and are finally completely reduced to the primary amino group. Water molecules are generated at the same time. After the complete reduction of the dinitro group, the target diamine monomer with double-terminated amino groups is obtained. The completeness of this reduction reaction directly determines the purity of the diamine monomer, and the integrity of the double-terminated amino functional groups of the diamine monomer is the core prerequisite for subsequent stepwise polycondensation and preparation of high molecular weight polymers.

[0028] (6) After the reaction is complete, the gas is discharged and the pressure inside the vessel is restored to atmospheric pressure. N,N-dimethylformamide is added to the system for solubilization, the mixture is filtered, and the filtrate is collected. Tetrahydrofuran in the filtrate is removed by rotary evaporation. The concentrated liquid is then poured into deionized water, and a pale orange solid is precipitated. The solid is collected by filtration and dried under vacuum at 100℃~120℃ to obtain the amide-modified diamine monomer.

[0029] The addition of N,N-dimethylformamide for solubilization is necessary because the target diamine monomer has low solubility in tetrahydrofuran after the reduction reaction. Adding N,N-dimethylformamide completely dissolves the product, achieving efficient separation from the solid palladium-on-carbon catalyst and preventing catalyst residue in the product. This method differs from direct filtration in existing technologies, significantly improving product recovery while completely removing residual metal catalyst, ensuring the thermal stability of the subsequent polymer. Subsequent rotary evaporation and water precipitation separate the product from the solvent, yielding high-purity diamine monomer. Vacuum drying completely removes residual solvent and water from the product, preventing them from affecting the functional group molar ratio in the subsequent polycondensation reaction.

[0030] The preferred purification method for the crude intermediate of this invention is as follows: the crude intermediate is poured into deionized water for precipitation, followed by chromatography separation and recrystallization purification to obtain the intermediate dinitro compound. This purification process can remove monosubstituted byproducts and small molecule impurities to the maximum extent, ensuring that the purity of the intermediate reaches more than 99%, thus providing high-quality raw materials for the subsequent synthesis of diamine monomers.

[0031] The preferred purification method for the crude product of this invention is as follows: the concentrated crude product is poured into deionized water, a pale orange solid precipitates, and then vacuum dried. This method can efficiently separate the product from the solvent, while removing water-soluble impurities, to obtain a high-purity diamine monomer that meets the raw material requirements for subsequent polycondensation reactions.

[0032] This invention provides a polyimide, which is prepared using the above-mentioned amide-modified diamine monomer as a raw material, comprising the following steps: Under a protective atmosphere, the above-mentioned amide-modified diamine monomer, aromatic dianhydride monomer, and catalyst are subjected to a stepwise polycondensation reaction in a phenolic solvent by stepwise heating to obtain polyimide.

[0033] Furthermore, the catalyst is quinoline or isoquinoline; the molar ratio of the amide-modified diamine monomer to the aromatic dianhydride monomer is 1:1.

[0034] Furthermore, the aromatic dianhydride monomer is selected from one of 4,4'-oxobisphthalic anhydride, 4,4'-(hexafluoroisopropylidene)phthalic anhydride, 3,3',4,4'-biphenyltetracarboxylic anhydride, 3,3',4,4'-benzophenonetetracarboxylic anhydride, or pyromellitic dianhydride.

[0035] Furthermore, the stepwise heating reaction is as follows: first, the reaction is carried out at 20℃~100℃ for 1h~4h to produce polyamic acid through ring-opening polycondensation of diamine monomer and dianhydride monomer; then, the temperature is raised to 120℃~150℃ for 5h~12h to produce pre-iminoization of polyamic acid; finally, the temperature is raised to 180℃~220℃ for 12h~24h to produce complete iminoization of polyamic acid.

[0036] The specific steps for preparing the above-mentioned polyimide are as follows: (1) Phenolic solvent, catalyst quinoline or isoquinoline, the amide-modified diamine monomer of the present invention and aromatic dianhydride are added to the reactor and stirred at 20℃~100℃ to obtain a viscous polyamic acid solution. The temperature is further increased to 120℃~150℃ for 5h~12h, and then at 180℃~220℃ for 12h~24h to obtain a transparent polyimide solution.

[0037] Phenolic solvents, particularly m-cresol (with a boiling point of 202℃), are used as high-boiling-point solvents in the one-step polycondensation reaction. M-cresol, for example, maintains a homogeneous state during the high-temperature imidization stage (180℃~220℃), dissolving the diamine monomer, dianhydride monomer, polyamic acid intermediate, and the final polyimide product throughout the process. This prevents product precipitation and chain termination during imidization, a crucial step in preparing high-molecular-weight polyimides. Existing technologies often use low-boiling-point solvents such as N,N-dimethylformamide and N-methylpyrrolidone, which are prone to evaporation during the high-temperature imidization stage, leading to a heterogeneous system, lower polymer molecular weight, and significant performance fluctuations. Quinoline or isoquinoline are used in the imidization reaction. The catalyst can lower the activation energy of intramolecular dehydration cyclization of ammonium acids, allowing the imidization reaction to proceed completely below 200℃, avoiding the problems of molecular chain thermal degradation and terminal amino oxidation caused by high-temperature treatment above 300℃. Simultaneously, it can suppress the hydrolysis side reaction of polyamic acids, ensuring stable molecular chain growth. Existing catalyst-free systems require temperatures above 300℃ to achieve complete imidization, resulting in high energy consumption and easy polymer thermal degradation, leading to a decrease in optical properties. The molar ratio of amide-modified diamine monomer to aromatic dianhydride monomer is 1:0.95~1.05, with an optimal ratio of 1:1. This parameter setting stems from the basic principle of stepwise polycondensation reactions, where the polymer molecular weight is determined by the functional groups... The molar ratio determines the highest degree of polymerization, resulting in high-molecular-weight polymers with equimolar ratios of diamine and dianhydride. This ensures the material's mechanical strength, thermal stability, and dimensional stability. A molar ratio deviation exceeding 5% leads to an overabundance of functional groups, causing end-capping effects that prevent the polymer's molecular weight from exceeding 20,000 and result in complete loss of film-forming properties. Precise control of this molar ratio is a core prerequisite for achieving high-performance polymer preparation. This stepwise heating process is an innovative process innovation in this invention, achieving synergistic optimization of high transparency, low thermal expansion coefficient, and high heat resistance in polyimides. It precisely controls the polymerization and imidization reaction processes, achieving a balance between high molecular weight and complete imidization. Specifically, it consists of three core stages, the first stage being... The reaction occurs at 0℃~100℃. Within this temperature range, the diamine and dianhydride preferentially undergo a rapid ring-opening polycondensation reaction. The primary amino group of the diamine attacks the anhydride group of the dianhydride, resulting in a nucleophilic addition-ring-opening reaction to generate a linear polyamic acid intermediate. The imidization and cyclization reactions occur at a very low rate, almost not at all. This ensures the formation of high molecular weight polyamic acid first, avoiding premature imidization which leads to product precipitation and termination of molecular chain growth. This lays the foundation for the final preparation of high-performance polymers. If the temperature exceeds 100℃ in this stage, premature imidization will occur, causing the product to precipitate from the system, terminating molecular chain growth, and significantly reducing the polymer molecular weight. If the temperature is too low, the ring-opening reaction will be incomplete, resulting in more monomer residue and a lower molecular weight.The second stage involves reacting at 120℃~150℃ for 5h~12h. Within this temperature range, the imidization reaction accelerates, and polyamic acid gradually undergoes intramolecular dehydration and cyclization to form an imide ring. Simultaneously, the water molecules generated during the cyclization reaction can be slowly removed by boiling at this temperature, while the high-boiling-point phenolic solvent does not volatilize, maintaining a homogeneous state of the system. The core function of this stage is to remove most of the water generated during cyclization, avoiding system inhomogeneity and product precipitation caused by violent boiling of water when directly raised to high temperatures. It also prevents water retention in the system from leading to polyamic acid hydrolysis, ensuring stable molecular chain growth. If the temperature in this stage is below 120℃, the imidization rate is slow, water removal is incomplete, and polyamic acid is prone to hydrolysis in subsequent high-temperature stages, resulting in a decrease in molecular weight. If the temperature is above 150℃, the imidization rate is too fast, water boils violently, the system becomes inhomogeneous, and the product easily precipitates. The third stage involves reacting at 180℃~220℃. The reaction is carried out at ℃ for 12-24 hours. This temperature range represents the complete imidization stage. Under the action of the catalyst, 100% of the residual ammonium acid groups are converted into imide rings. Simultaneously, the gradual polymerization reaction reaches equilibrium, and the molecular chain achieves sufficient growth, resulting in high molecular weight polyimide. Complete imidization is the core to ensure the material's high heat resistance, low coefficient of thermal expansion, and solvent resistance. Only when the imide ring is fully formed can its rigid structure provide the necessary heat resistance and dimensional stability. If the temperature is below 180℃ in this stage, imidization is incomplete, and the residual ammonium acid groups will cause the polymer's glass transition temperature to decrease, the coefficient of thermal expansion to increase, and the solvent resistance to deteriorate. If the temperature is above 220℃, the solvent will evaporate rapidly, and side reactions such as molecular chain thermal degradation and amide bond hydrolysis will increase, leading to a decrease in molecular weight and a decline in material performance. If the reaction time is too short, imidization will be incomplete, and optimal material performance cannot be achieved.

[0038] In this process, phenolic solvents are used as solvents, and a catalytic amount of isoquinoline or quinoline is used as catalyst. In laboratory synthesis reactions, the amount is usually 1-5 drops. The specific amount is determined by those skilled in the art based on general experience and common knowledge in the field. The time required to obtain the polyamic acid solution can be determined by those skilled in the art based on experience and common knowledge in the field, usually 1h~12h. The process of heating the polyamic acid solution to 120℃~150℃ for 5h~12h can be a one-step heating reaction process or a step-by-step heating reaction process. It can be flexibly adjusted according to the actual production system scale to ensure that the imidization process proceeds smoothly. The solid content of the polyamic acid solution in step (1) is preferably 5wt%-30wt%. The solid content directly affects the polymerization rate and the molecular weight distribution of the polymer. When the solid content is too low, the monomer concentration is low, the probability of collision between molecular chains is small, the polymerization rate is slow, and the molecular weight of the polymer is low. When the solid content is too high, the viscosity of the system is too high, the reaction is uneven, the molecular weight distribution becomes wider, and even gelation occurs. This solid content range can ensure that the system remains in a homogeneous state throughout the reaction process, realize controllable step-by-step polymerization, and prepare a high molecular weight polymer with a narrow distribution.

[0039] (2) After cooling to room temperature, the polyimide solution is slowly poured into the precipitant to obtain a white fibrous precipitate, which is then separated, washed and dried to obtain polyimide.

[0040] This step separates the polymer from small molecule impurities such as unreacted monomers, residual catalysts, and solvents, resulting in purified polyimide resin and ensuring the stability of material properties.

[0041] The specific precipitation method is as follows: the obtained polyimide solution is diluted and slowly poured into a precipitant, resulting in a fibrous precipitate. This precipitate is then filtered and vacuum dried to constant weight. The precipitant can be an alcohol such as methanol or ethanol, or a ketone such as acetone. This type of precipitant is miscible with good solvents of polyimide but does not dissolve the polyimide, thus achieving efficient precipitation and purification of the polymer.

[0042] In the preparation method of polyimide of this invention, the aromatic dianhydride used is a commercially available reagent. Specifically, it can be selected based on the Ar group on the target polyimide. The aromatic dianhydride can be selected from one of the following commercially available aromatic dianhydrides: 4,4'-oxybisphthalic anhydride, 4,4'-(hexafluoroisopropylidene)phthalic anhydride, 3,3',4,4'-biphenyltetracarboxylic anhydride, 3,3',4,4'-benzophenonetetracarboxylic anhydride, and pyromellitic dianhydride. By selecting aromatic dianhydrides with different structures, the rigidity, heat resistance, and solubility of the polymer can be flexibly controlled to adapt to different application requirements.

[0043] This invention also provides a method for using the aforementioned polyimide in the production of resin products. The polyimide is prepared into a solution, then cast into a film or applied directly. The solid content of the polyimide solution is 0.5 wt%-20 wt%. The polyimide can be dissolved in polar organic solvents such as dimethylacetamide, N,N-dimethylformamide, N-methylpyrrolidone, chloroform, tetrahydrofuran, or m-cresol at room temperature. This room-temperature dissolution film-forming process differs from the traditional two-step process requiring high-temperature imidization at over 300°C. Room-temperature film formation completely avoids internal stress and warping deformation caused by high-temperature treatment, further reducing the coefficient of thermal expansion and improving dimensional stability. It also significantly reduces processing energy consumption and is suitable for industrial roll-to-roll production processes. This improved processing performance stems from the monomer structure design of this invention. The synergistic effect of amide bonds, tert-butyl groups, and aliphatic rings ensures material rigidity and heat resistance while improving the polymer's solubility in organic solvents, solving the problems of insolubility, infusibility, and processing difficulties associated with traditional high-rigidity polyimides.

[0044] Compared with the prior art, the present invention has the following beneficial effects: I. This invention provides a diamine monomer containing tert-butyl and aliphatic rings with amide bonds. By introducing amide bonds, tert-butyl and aliphatic rings in the form of diamine monomers into the polyimide backbone, the aromatic content is reduced. While maintaining the thermal stability of the material, the formation of charge transfer complexes is inhibited, significantly improving the transparency of the polyimide. Furthermore, by utilizing intermolecular forces, the coefficient of thermal expansion is reduced, and the dimensional stability is improved.

[0045] II. Through rational molecular design and a one-step method, this invention produces a class of soluble, highly transparent polyimides containing tert-butyl groups and aliphatic rings with amide bonds. Due to the special structure of the designed amide-modified diamine monomer, the resulting polyimide resin not only has good heat resistance but also good solubility. It is soluble in common solvents such as N,N-dimethylformamide at room temperature, thus it can be processed and molded at room temperature, making processing more convenient. Moreover, the preparation process is simple and the cost is low.

[0046] Third, the soluble, highly transparent polyimide prepared by this invention not only has good heat resistance, but also high transparency and low coefficient of thermal expansion, and has broad application prospects in the fields of flexible transparent substrates for optoelectronic devices and flat panel displays. Attached Figure Description

[0047] Figure 1 This is the mass spectrum of the amide-modified diamine monomer prepared in Example 1.

[0048] Figure 2 The amide-modified diamine monomer prepared in Example 1 1 H-NMR spectrum.

[0049] Figure 3 The infrared spectrum of the polyimide film prepared in Application Example 2.

[0050] Figure 4 The image shows the infrared spectrum of PAI-2 resin.

[0051] Figure 5 The image shows the infrared spectrum of PAI-3 resin.

[0052] Figure 6 The image shows the infrared spectrum of PAI-4 resin.

[0053] Figure 7 The image shows the infrared spectrum of PAI-5 resin. Detailed Implementation

[0054] The specific embodiments of the present invention are described in detail below, but it should be understood that the scope of protection of the present invention is not limited to the specific embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention. Unless otherwise specified, the experimental methods described in the embodiments of the present invention are conventional methods, and the materials and reagents used in the following embodiments are commercially available unless otherwise specified.

[0055] Example 1 A method for preparing an amide-modified diamine monomer, the synthetic route is shown below: .

[0056] The specific steps are as follows: (a) Under a nitrogen atmosphere, 0.072 mol of 4-nitrobenzoyl chloride and 40 mL of tetrahydrofuran were added to a 500 mL three-necked flask equipped with a magnetic stirrer and a constant-pressure dropping funnel. The mixture was stirred to obtain a clear liquid, and then the system was cooled to 0 °C. 0.03 mol of 4,4'-(cyclohexylmethylene)bis(2-(tert-butyl)aniline) was dissolved in a mixed solution of 8.7 mL of pyridine and 35 mL of tetrahydrofuran. This solution was slowly added dropwise to the cooled clear liquid through a constant-pressure dropping funnel over 1 h. The reaction mixture was then brought back to room temperature, and the reaction was stirred for another 20 h. After the reaction was completed, the resulting mixture was poured into deionized water, the precipitate was filtered and the yellow solid was collected. It was dried in a vacuum oven at 120°C for 12 hours. The dried yellow solid was then purified by vacuum column chromatography with tetrahydrofuran and ethyl acetate as the developing solvent in a volume ratio of 1:1 to obtain a pale yellow solid. Further recrystallization with toluene yielded pale yellow crystals, which were the intermediate dinitro compound with a yield of 74.0%.

[0057] (b) 10.0 g of the intermediate dinitro compound, 60 mL of tetrahydrofuran, and 1.0 g of palladium on carbon catalyst were added to a 250 mL high-pressure reactor, and a magnetic stirrer was used to homogenize the system. Hydrogen gas was introduced into the high-pressure reactor to achieve a pressure of 6.0 MPa, and the reaction was carried out at 60 °C with stirring for 24 h. After the reaction was completed, the gas was vented and the pressure inside the reactor was restored to atmospheric pressure. 40 mL of N,N-dimethylformamide was added to the system for solubilization, and the filtrate was collected by filtration. The tetrahydrofuran in the filtrate was removed by rotary evaporation, and the concentrated liquid was poured into deionized water, precipitating a pale orange solid. The solid was collected by filtration and dried under vacuum at 120 °C for 12 h to obtain the amide-modified diamine monomer with a yield of 85.0%.

[0058] Application Example 1 Polyimide was prepared using the amide-modified diamine monomer prepared in Example 1, and the reaction route is as follows: .

[0059] The specific steps are as follows: Under nitrogen protection, 1.586 mmol of the amide-modified diamine monomer prepared in Example 1, 1.586 mmol of pyromellitic dianhydride, 9.6 mL of m-cresol, and 10 drops of isoquinoline were added to a 100 mL three-necked flask equipped with a reflux condenser and a mechanical stirrer. The mixture was heated to 50 °C and stirred for 1 h, then successively heated to 80 °C and stirred for 4 h, 150 °C and stirred for 12 h, and 180 °C and stirred for 12 h. It should be noted that the ring-opening condensation reaction between the diamine monomer and the dianhydride monomer occurred during the heating process of heating to 50 °C and stirring for 1 h, followed by successively heating to 80 °C and stirring for 4 h, to generate polyamic acid. After the reaction solution cooled to room temperature, 8 mL of N,N-dimethylformamide was added for dilution. Then, it was slowly poured into 400 mL of vigorously stirred methanol, resulting in the precipitation of a white fibrous precipitate. After filtration, the precipitate was collected and dried under vacuum at 100 °C for 24 h. After being dissolved in N,N-dimethylformamide, it was reprecipitated and purified twice to obtain polyimide.

[0060] Application Example 2 A method for preparing a resin product includes the following steps: At room temperature, the polyimide prepared in Example 1 was dissolved in chloroform to prepare a mixed solution with a solid content of 6 wt%. Then, at room temperature, the solution was cast into a film on a clean glass slide with a size of 3 cm. 2 ×6cm 2 Finally, vacuum drying at 150℃ for 12 hours yields a colorless and transparent polyimide film.

[0061] Figure 1 The mass spectrum shows the amide-modified diamine monomer obtained in Example 1. The mass spectrum shows a molecular ion peak that perfectly matches the theoretical molecular weight of the target compound, with no obvious impurity molecular ion peaks, proving that Example 1 successfully synthesized the target diamine monomer with the correct molecular weight. Specifically, the signal peak at 316.2012 corresponds to the molecular weight of the target product bound with two protons; the signal peak at 631.3998 corresponds to the molecular ion molecular weight bound with one proton, which is very close to the theoretical value of 631.4012.

[0062] Figure 2 The amide-modified diamine monomer prepared in Example 1 1 1H-NMR spectrum. All proton signal peaks in the 1H spectrum were assigned, and the chemical shifts, peak shapes, and integral area ratios perfectly matched the theoretical hydrogen atom distribution of the target compound, confirming the correct chemical structure of the target monomer at the hydrogen atom level. The amide bond, amino group, tert-butyl group, cyclohexyl group, and aromatic ring functional groups were all present intact.

[0063] Figure 3The infrared spectrum of the polyimide film prepared in Example 2 is shown, with 1779 cm⁻¹. -1 and 1725cm -1 The absorption peak is a characteristic absorption peak of the carbonyl group on the imide ring in polyimide, at 3333 cm⁻¹. -1 The broad peak at 1667 cm⁻¹ corresponds to the NH stretching vibration of the amide bond. -1 The absorption peak at the point corresponds to the C=O stretching vibration of the amide bond, proving that the amide bond was successfully introduced into the polymer backbone, indicating that the present invention successfully prepared a polyimide resin with solubility and high transparency.

[0064] Example 2 A method for preparing an amide-modified diamine monomer, the reaction route is the same as that in Example 1, and the specific steps are as follows: (a) Under a nitrogen atmosphere, 0.072 mol of 4-nitrobenzoyl chloride and 40 mL of tetrahydrofuran were added to a 500 mL three-necked flask equipped with a magnetic stirrer and a constant-pressure dropping funnel. The mixture was stirred to obtain a clear liquid, and then the system was cooled to -10 °C. 0.03 mol of 4,4'-(cyclohexylmethylene)bis(2-(tert-butyl)aniline) was dissolved in a mixed solution of 8.7 mL of pyridine and 40 mL of tetrahydrofuran. This solution was slowly added dropwise to the cooled clear liquid over 1.5 h using a constant-pressure dropping funnel. The reaction mixture was then brought back to room temperature, and the reaction was stirred for another 24 h. After the reaction was completed, the resulting mixture was poured into a large amount of deionized water, the precipitate was filtered and the yellow solid was collected. The solid was dried in a vacuum oven at 120°C for 24 hours. The dried yellow solid was then purified by vacuum column chromatography with tetrahydrofuran and ethyl acetate as the developing solvent in a volume ratio of 1:1 to obtain a pale yellow solid. The solid was further recrystallized from xylene to obtain pale yellow crystals, which yielded the intermediate dinitro compound with a yield of 77.0%.

[0065] (b) Add 10.0 g of the intermediate dinitro compound obtained in step (a), 60 mL of tetrahydrofuran, and 1.0 g of palladium on carbon catalyst to a 250 mL high-pressure reactor, and simultaneously add magnetic stirring to homogenize the system. Introduce hydrogen gas into the high-pressure reactor to achieve a pressure of 6.5 MPa, and then stir the reaction at 60 °C for 24 h. After the reaction is complete, vent the gas and restore the pressure inside the reactor to atmospheric pressure. Add 30 mL of N,N-dimethylformamide to the system for solubilization, and filter to collect the filtrate. Remove the tetrahydrofuran from the filtrate by rotary evaporation, and then pour the concentrated liquid into a large amount of deionized water, precipitating a pale orange solid. Filter and collect the solid, and dry it under vacuum at 120 °C for 24 h to obtain the amide-modified diamine monomer, with a yield of 88.0%.

[0066] Application Example 3 Polyimide was prepared using the amide-modified diamine monomer prepared in Example 2. The reaction route was the same as that in Application Example 1, and the specific steps were as follows: Under nitrogen protection, 1.586 mmol of the amide-modified diamine monomer prepared in Example 2, 1.586 mmol of pyromellitic dianhydride, 9.6 mL of m-cresol, and 10 drops of isoquinoline were added to a 100 mL three-necked flask equipped with a reflux condenser and a mechanical stirrer. The mixture was heated to 50 °C and stirred for 1 h, then successively heated to 80 °C and stirred for 4 h, 150 °C and stirred for 12 h, and 180 °C and stirred for 12 h. After the reaction solution cooled to room temperature, 8 mL of N,N-dimethylformamide was added for dilution, and then slowly poured into 400 mL of vigorously stirred methanol. A white fibrous precipitate was formed. After filtration, the precipitate was collected and dried under vacuum at 100 °C for 24 h. After being dissolved in N,N-dimethylformamide and reprecipitated twice, the polyimide was obtained and named PAI-1.

[0067] Application Example 4 A method for preparing a resin product includes the following steps: At room temperature, the PAI-1 obtained in Example 3 was dissolved in chloroform to prepare a mixed solution with a solid content of 6 wt%. Then, at room temperature, the solution was cast into a film on a clean glass slide with a size of 3 cm. 2 ×6cm 2 Finally, vacuum drying at 150℃ for 12 hours yields a colorless and transparent PAI-1 film, which is named PAI-1 resin.

[0068] Example 3 A method for preparing an amide-modified diamine monomer differs from Example 2 in that the molar ratio of 4-nitrobenzoyl chloride to 4,4'-(cyclohexylmethylene)bis(2-(tert-butyl)aniline) in step (a) is adjusted to 7.1:3, while other conditions remain the same as in Example 2. The yield of the intermediate dinitro compound was 75.5%, and the yield of the amide-modified diamine monomer was 88.0%.

[0069] Example 4 A method for preparing an amide-modified diamine monomer differs from Example 2 in that the molar ratio of 4-nitrobenzoyl chloride to 4,4'-(cyclohexylmethylene)bis(2-(tert-butyl)aniline) in step (a) is adjusted to 7.3:3, while other conditions remain the same as in Example 2. The yield of the intermediate dinitro compound was 76.0%, and the yield of the amide-modified diamine monomer was 88.0%.

[0070] Example 5 A method for preparing an amide-modified diamine monomer differs from Example 2 in that the stirring reaction time in step (a) is adjusted to 12 hours, while other conditions remain the same as in Example 2. The yield of the intermediate dinitro compound was 75.0%, and the yield of the amide-modified diamine monomer was 88.0%.

[0071] Example 6 A method for preparing an amide-modified diamine monomer, differing from Example 2 in that the stirring reaction time in step (a) is adjusted to 20 h, while other conditions remain the same as in Example 2. The yield of the intermediate dinitro compound was 76.5%, and the yield of the amide-modified diamine monomer was 88.0%.

[0072] Example 7 A method for preparing an amide-modified diamine monomer differs from Example 2 in that, in step (b), the mass ratio of the intermediate dinitro compound to the palladium-on-carbon catalyst is adjusted to 9.95:1, while other conditions remain the same as in Example 2. The yield of the intermediate dinitro compound was 77.0%, and the yield of the amide-modified diamine monomer was 86.0%.

[0073] Example 8 A method for preparing an amide-modified diamine monomer differs from Example 2 in that, in step (b), the mass ratio of the intermediate dinitro compound to the palladium-on-carbon catalyst is adjusted to 10.05:1, while other conditions remain the same as in Example 2. The yield of the intermediate dinitro compound was 77.0%, and the yield of the amide-modified diamine monomer was 87.8%.

[0074] Example 9 A method for preparing an amide-modified diamine monomer differs from Example 2 in that, in step (b), the stirring reaction temperature is adjusted to 50°C, while other conditions remain the same as in Example 2. The yield of the intermediate dinitro compound was 77.0%, and the yield of the amide-modified diamine monomer was 86.5%.

[0075] Example 10 A method for preparing an amide-modified diamine monomer differs from Example 2 in that, in step (b), the stirring reaction temperature is adjusted to 100°C, while other conditions remain the same as in Example 2. The yield of the intermediate dinitro compound was 77.0%, and the yield of the amide-modified diamine monomer was 84.5%.

[0076] Example 11 A method for preparing an amide-modified diamine monomer differs from Example 2 in that, in step (b), the stirring reaction time is adjusted to 36 hours, while other conditions remain the same as in Example 2. The yield of the intermediate dinitro compound was 77.0%, and the yield of the amide-modified diamine monomer was 88.5%.

[0077] Example 12 A method for preparing an amide-modified diamine monomer differs from Example 2 in that, in step (b), the stirring reaction time is adjusted to 48 hours, while other conditions remain the same as in Example 2. The yield of the intermediate dinitro compound was 77.0%, and the yield of the amide-modified diamine monomer was 88.7%.

[0078] Example 13 A method for preparing an amide-modified diamine monomer differs from Example 2 in that, in step (b), the mass ratio of the intermediate dinitro compound to the palladium-on-carbon catalyst is adjusted to 8:1, while other conditions remain the same as in Example 2. The yield of the intermediate dinitro compound was 73.0%, and the yield of the amide-modified diamine monomer was 80.2%. The properties of the subsequently prepared PAI-1 resin showed no significant difference from those in Application Example 4.

[0079] Example 14 A method for preparing an amide-modified diamine monomer differs from Example 2 in that, in step (b), the mass ratio of the intermediate dinitro compound to the palladium-on-carbon catalyst is adjusted to 12:1, while other conditions remain the same as in Example 2. The yield of the intermediate dinitro compound was 76.8%, and the yield of the amide-modified diamine monomer was 83.6%. The performance of the subsequently prepared PAI-1 resin showed no significant difference from that of Application Example 4.

[0080] Application Example 5 The difference from Application Example 3 is that the imidization catalyst isoquinoline was replaced with an equal amount of quinoline, while other conditions were the same as in Application Example 3, to obtain PAI-1. PAI-1 resin was prepared according to the method of Application Example 4, and the properties of the prepared PAI-1 resin were not significantly different from those of Application Example 4.

[0081] Application Example 6 The difference from Application Example 3 is that pyromellitic dianhydride was replaced with 1.586 mmol of 3,3',4,4'-benzophenone tetracarboxylic acid dianhydride, while other conditions remained the same as in Application Example 3, to prepare PAI-2. The reaction route is as follows: .

[0082] Following the method described in Application Example 4, PAI-2 was prepared into PAI-2 resin. Figure 4 The infrared spectrum of PAI-2 resin is shown, with 1781 cm⁻¹. -1 and 1720cm -1 The absorption peak is a characteristic absorption peak of the carbonyl group on the imide ring in polyimide, at 3333 cm⁻¹. -1 The broad peak at 1668 cm⁻¹ corresponds to the NH stretching vibration of the amide bond. -1The absorption peak at the point corresponds to the C=O stretching vibration of the amide bond, proving that the amide bond was successfully introduced into the polymer backbone, indicating that a polyimide resin with solubility and high transparency was successfully prepared.

[0083] Application Example 7 The difference from Application Example 3 is that pyromellitic dianhydride was replaced with 1.586 mmol of 3,3',4,4'-biphenyltetracarboxylic acid dianhydride, while other conditions remained the same as in Application Example 3, to prepare PAI-3. The reaction route is as follows: .

[0084] Following the method described in Application Example 4, PAI-3 was prepared into PAI-3 resin. Figure 5 The infrared spectrum of this polymer film of PAI-3 resin is shown, with the value at 1777 cm⁻¹. -1 and 1718cm -1 The absorption peak at 3343 cm⁻¹ is a characteristic absorption peak of the carbonyl group on the imide ring in polyimide. -1 The broad peak at 1667 cm⁻¹ corresponds to the NH stretching vibration of the amide bond. -1 The absorption peak at the point corresponds to the C=O stretching vibration of the amide bond, proving that the amide bond was successfully introduced into the polymer backbone, indicating that a polyimide resin with solubility and high transparency was successfully prepared.

[0085] Application Example 8 The difference from Application Example 3 is that pyromellitic dianhydride was replaced with 1.586 mmol of 4,4'-oxobisphthalic anhydride, while other conditions remained the same as in Application Example 3, to prepare PAI-4. The reaction route is as follows: .

[0086] Following the method described in Application Example 4, PAI-4 was prepared into PAI-4 resin. Figure 6 The image shows the infrared spectrum of PAI-4 resin, with the 1780 cm⁻¹ value being [missing information]. -1 and 1720cm -1 The absorption peak is a characteristic absorption peak of the carbonyl group on the imide ring in polyimide, at 3333 cm⁻¹. -1 The broad peak at 1668 cm⁻¹ corresponds to the NH stretching vibration of the amide bond. -1 The absorption peak at the point corresponds to the C=O stretching vibration of the amide bond, proving that the amide bond was successfully introduced into the polymer backbone, indicating that a polyimide resin with solubility and high transparency was successfully prepared.

[0087] Application Example 9 The difference from Application Example 3 is that pyromellitic dianhydride was replaced with 1.586 mmol of 4,4'-(hexafluoroisopropylidene) diaphthalic anhydride to prepare PAI-5. Other conditions were the same as in Application Example 3, and the synthetic route is as follows: .

[0088] Following the method described in Application Example 4, PAI-5 was prepared into PAI-5 resin. Figure 7 The infrared spectrum of PAI-5 resin is shown, with 1786 cm⁻¹ as an example. -1 and 1723cm -1 The absorption peak is a characteristic absorption peak of the carbonyl group on the imide ring in polyimide, at 3335 cm⁻¹. -1 The broad peak at 1672 cm⁻¹ corresponds to the NH stretching vibration of the amide bond. -1 The absorption peak at the point corresponds to the C=O stretching vibration of the amide bond, proving that the amide bond was successfully introduced into the polymer backbone, indicating that a polyimide resin with solubility and high transparency was successfully prepared.

[0089] Application Example 10 The difference from Application Example 3 is that the heating program is as follows: heat to 50°C and stir for 1 hour, then heat to 100°C and stir for 4 hours, 120°C and stir for 12 hours, and 220°C and stir for 12 hours in sequence. Polyimide was successfully synthesized. After the polyimide was made into resin according to the method of Application Example 4, its properties were similar to those of PAI-1 resin. The resin has solubility and high transparency.

[0090] Application Comparative Example 1 The difference between this and Application Example 3 is that the amidation reaction in step (a) of Example 2 is not performed. Instead, 4,4'-(cyclohexylmethylene)bis(2-(tert-butyl)aniline) is directly used as the diamine monomer and polycondensed with pyromellitic dianhydride. The remaining operation steps and process parameters are the same as in Application Example 3, and the comparative sample PI-1 is obtained.

[0091] Application Comparative Example 2 The difference between this and Application Example 3 is that the polyimide synthesis does not use a stepwise heating process. Instead, the reaction system is directly heated to 180°C and kept at a constant temperature for 24 hours. The polyamic acid synthesis stage at 50°C and 80°C and the pre-imidization stage at 150°C are not performed. All other operation steps and process parameters are the same as in Application Example 3, and the comparative sample PI-2 is obtained.

[0092] Application Comparative Example 3 The difference between Example 2 and Application Example 3 is that the hydrogenation reduction reaction temperature in step (b) of Example 2 was set to 30°C, while the remaining operation steps and process parameters were the same as in Application Example 3, and the comparative sample PI-3 was obtained.

[0093] Application Comparative Example 4 The difference between Example 2 and Application Example 3 is that the hydrogenation reduction reaction temperature in step (b) of Example 2 was set to 120°C, while the remaining operation steps and process parameters were the same as in Application Example 3, and the comparative sample PI-4 was obtained.

[0094] Application Comparative Example 5 The difference between this and Application Example 3 is that, in the preparation of the amide-modified diamine monomer, the molar ratio of 4-nitrobenzoyl chloride to 4,4'-(cyclohexylmethylene)bis(2-(tert-butyl)aniline) in step (a) was adjusted to 6:3. The preparation of the remaining monomers and the polyimide synthesis process were the same as in Application Example 3, and the comparative sample PI-5 was obtained.

[0095] Application Comparative Example 6 The difference between this and Application Example 3 is that the molar ratio of the amide-modified diamine monomer to pyromellitic dianhydride is adjusted to 1:0.9. All other operating steps and process parameters are the same as in Application Example 3, and the comparative sample PI-6 is obtained.

[0096] The PAI-1 to PAI-5 resins prepared in Application Examples 4, 6 to 9, and the comparative samples PI-1 to PI-6 prepared in Comparative Examples 1 to 6 were subjected to performance tests. The performance test methods are as follows: Molecular weight and molecular weight distribution were determined using a PE Series 200GPC instrument equipped with a differential refractive index detector. The test conditions were as follows: N,N-dimethylformamide was used as the eluent, the column temperature was 25℃, the flow rate was 1.0 mL / min, and polystyrene was used as the standard.

[0097] The glass transition temperature was determined using a TA Q2000 differential scanning calorimeter. The test conditions were: under a nitrogen atmosphere, a heating rate of 20℃ / min, and a temperature range of 40℃~430℃. The thermal history of the sample was eliminated before the test.

[0098] Thermal stability was tested using a TA Q5000 IR thermogravimetric analyzer. Test conditions: under nitrogen or air atmosphere, heating rate of 20℃ / min, temperature range of 50℃~800℃, and the temperature T at which a weight loss of 5wt% was selected. 5wt% As its decomposition temperature.

[0099] Transmittance was measured using a Lambda 35 UV-Vis spectrophotometer under the following conditions: scanning wavelength range of 200 nm to 900 nm.

[0100] The coefficient of thermal expansion was measured using a TA DMA Q800 dynamic thermomechanical analyzer in static tensile mode, with a prestress of 0.02 N, a heating range of 50℃ to 200℃, and a heating rate of 5℃ / min.

[0101] The test results are shown in Table 1 below.

[0102] Table 1 Properties of polyimide film materials

[0103] The solubility of the resin samples in each group is shown in Table 2. The specific test method for solubility is as follows: weigh 10 mg of sample, place it in 1 mL of the corresponding solvent, add a stir bar and stir at room temperature, and observe the degree of solubility.

[0104] Table 2 Solubility of resin samples in each group

[0105] Note: ++ indicates complete dissolution at room temperature, - indicates no dissolution, and +- indicates partial solubility or swelling at 60°C. As can be seen from the data in Table 1, the polyimide resin obtained by this invention has significant advantages in the synergistic optimization of thermal expansion coefficient, transparency, and heat resistance. The comparative examples, lacking the key structure of this invention or deviating from the core process parameters, all exhibited obvious performance defects. Compared with Comparative Example 1, Application Example 4 of this invention, by introducing amide bonds into the diamine monomer and utilizing intermolecular hydrogen bonds to enhance the molecular chain packing density, significantly reduced the resin's thermal expansion coefficient from 33.7 ppm / K to 12.0 ppm / K, improving dimensional stability. In terms of optical performance, due to the influence of amide bond hydrogen bonding, the degree of intermolecular charge transfer complex formation slightly increased, resulting in a slight decrease in the average visible light transmittance compared to PI-1 without amide bonds. However, the average visible light transmittance of all sample examples remained in the range of 68% to 81%, still falling within the category of high-transparency polyimides, far superior to the transmittance level of traditional aromatic polyimides, fully meeting the transparency requirements of flexible optoelectronic devices for substrate materials. This performance improvement has clear technical rationale: the invention achieves a significant optimization of the coefficient of thermal expansion (CTE) by slightly reducing transmittance within a controllable range. This solves the core failure problems of existing high-transparency polyimides, such as device warping and interlayer delamination caused by excessively high CTE. Compared to transparency, CTE matching is a more critical performance bottleneck in current flexible optoelectronic processes. Furthermore, by selecting dianhydride monomers with different structures, flexible control over transparency and dimensional stability can be achieved. For example, PAI-5 prepared using fluorinated dianhydrides can achieve a visible light transmittance of 81%, making it suitable for applications with higher transparency requirements. Regarding thermal performance, although the introduction of amide bonds lowers the thermal decomposition temperature compared to PI-1 without amide bonds, the glass transition temperature of all sample examples remains above 270°C, and the thermal decomposition temperature remains above 447°C, still belonging to the high-heat-resistant engineering plastic category, fully meeting the heat resistance requirements for processing flexible OLED substrates, thin-film transistor arrays, and other optoelectronic devices. In addition, the visible light transmittance remains above 68%, reaching up to 81%, meeting the requirements of optoelectronic applications. Combined with controllable high transmittance and significantly optimized low thermal expansion characteristics, it solves the technical problem that traditional amide-free polyimide structures cannot simultaneously achieve transparency and dimensional stability.

[0106] Compared to Comparative Example 2, the three-stage stepwise heating process employed in this invention first synthesizes high molecular weight polyamic acid at a low temperature, then removes moisture through medium-temperature pre-iminolation, and finally achieves complete iminolation at a high temperature. This avoids the problems of incomplete iminolation and premature molecular chain termination caused by a single high-temperature step, resulting in a 31°C increase in the polymer's glass transition temperature and a 16.5 ppm / K decrease in the coefficient of thermal expansion, significantly improving overall performance. Compared to Comparative Example 3, this invention controls the hydrogenation reduction temperature within a reasonable range of 50°C-100°C, ensuring complete reduction of nitro groups to amino groups and achieving a monomer purity of over 99%. This avoids the problems of low polymer molecular weight and decreased transparency caused by residual reduction intermediates. Compared to Comparative Example 4, this invention strictly controls the upper limit of the hydrogenation reduction temperature, effectively avoiding side reactions such as amide bond hydrolysis and tert-butyl removal, fully preserving the three major functional structures in the monomer, and effectively ensuring both the polymer's transparency and heat resistance.

[0107] Compared with Comparative Example 5, this invention precisely controls the molar ratio of 4-nitrobenzoyl chloride to diamine raw material within the range of 7.1~7.3:3, ensuring a moderate excess of acyl chloride. This maximizes the suppression of the formation of monosubstituted byproducts, improves the purity of intermediates and diamine monomers, and avoids monosubstituted impurities acting as end-capping agents to limit the growth of polymer molecular weight. Ultimately, this increases the polymer's glass transition temperature by 38°C, reduces the coefficient of thermal expansion by 16.2 ppm / K, and significantly improves transparency.

[0108] Compared with Comparative Example 6, this invention controls the molar ratio of amide-modified diamine monomer to aromatic dianhydride to be near an equimolar ratio, which conforms to the molecular weight control law of stepwise polycondensation reaction. This can achieve the highest degree of polymerization and the preparation of high molecular weight polymers, avoid the end-capping effect caused by excessive functional groups, increase the glass transition temperature of the polymer by 49°C, reduce the coefficient of thermal expansion by 16.8 ppm / K, and significantly improve the heat resistance and dimensional stability of the material.

[0109] As shown in Table 2, the polyimide resins obtained in the embodiments of the present invention are all soluble in common polar organic solvents such as dimethyl sulfoxide, N-methylpyrrolidone, and dimethylacetamide at room temperature, exhibiting good processing performance. They can be cast into films at room temperature, avoiding internal stress and warping deformation caused by high-temperature processing, and further improving the dimensional stability of the products. Comparative Example 1, lacking amide bonds, has weak intermolecular forces and is soluble in low-boiling-point solvents such as acetone, chloroform, and tetrahydrofuran, but its coefficient of thermal expansion is too high. Comparative Examples 2 to 6, due to process defects, have lower polymer molecular weights or incomplete structures. While their solubility is basically the same as in the embodiments of the present invention, their thermal and optical properties are significantly reduced.

[0110] The highly transparent polyimide containing tert-butyl and aliphatic rings with amide bonds provided by this invention has both excellent high-temperature resistance and a low coefficient of thermal expansion, making it easy to process and mold at room temperature. In addition, the polyimide film also has high transparency, which has broad application prospects in the fields of flexible transparent substrates for optoelectronic devices and flat panel displays. Furthermore, the preparation method of the soluble highly transparent polyimide containing tert-butyl and aliphatic rings with amide bonds provided by this invention is simple to operate, has few steps, and is convenient to implement.

[0111] Obviously, those skilled in the art can make various modifications and variations to this invention without departing from its spirit and scope. If such modifications and variations fall within the scope of equivalents of this invention, then this invention also intends to include these modifications and variations.

Claims

1. An amide-modified diamine monomer, characterized in that, The structural formula of the amide-modified diamine monomer is: 。 2. A method for preparing the amide-modified diamine monomer according to claim 1, characterized in that, Includes the following steps: Under an inert atmosphere, using tetrahydrofuran as a solvent, and 4-nitrobenzoyl chloride and 4,4'-(cyclohexylmethylene)bis(2-(tert-butyl)aniline) as raw materials, a nucleophilic addition-elimination reaction between an aromatic primary amine and an acyl chloride occurs in the presence of pyridine as a catalyst to yield a dinitro compound. In a reducing atmosphere, a dinitro compound and a palladium catalyst on carbon are added to a tetrahydrofuran solvent. The nitro group in the dinitro compound is reduced to a primary amino group through a heterogeneous catalytic hydrogenation reduction reaction to obtain an amide-modified diamine monomer.

3. The method for preparing the amide-modified diamine monomer according to claim 2, characterized in that, In the preparation steps of the dinitro compound, the nucleophilic addition-elimination reaction is carried out at room temperature for 12 h to 24 h. The molar ratio of 4-nitrobenzoyl chloride to 4,4'-(cyclohexylmethylene)bis(2-(tert-butyl)aniline) is 7.1~7.3:

3.

4. The method for preparing the amide-modified diamine monomer according to claim 2, characterized in that, The specific steps in the preparation of dinitro compounds are as follows: Under the protection of an inert gas, 4-nitrobenzoyl chloride and tetrahydrofuran were mixed and the system was cooled to below room temperature to obtain the first mixture; 4,4'-(cyclohexylmethylene)bis(2-(tert-butyl)aniline) was dissolved in a mixed solution of pyridine and tetrahydrofuran to obtain a second mixture. The second mixture was added dropwise to the first mixture over 1-2 hours. The reaction mixture was then brought to room temperature and the reaction was stirred.

5. The method for preparing the amide-modified diamine monomer according to claim 2, characterized in that, In the step of reducing nitro to primary amino, the reaction temperature is 50℃~100℃ and the time is 24h~48h; the reducing atmosphere is hydrogen atmosphere, and the mass ratio of dinitro compound to palladium on carbon catalyst is 8~12:1; After the reaction is complete, the reaction system is restored to atmospheric pressure, and N,N-dimethylformamide is added to the reaction system for solubilization. The mixture is filtered, the filtrate is collected, and the tetrahydrofuran in the filtrate is concentrated. The concentrated liquid is poured into water, the solid is collected, and the solid is dried.

6. A polyimide, characterized in that, The preparation method involves reacting the amide-modified diamine monomer of claim 1 with an aromatic dianhydride monomer, and includes the following steps: Under a protective atmosphere, the amide-modified diamine monomer, aromatic dianhydride monomer, and catalyst described in claim 1 are subjected to a stepwise polycondensation reaction in a phenolic solvent by stepwise heating to obtain polyimide.

7. The polyimide according to claim 6, characterized in that, The catalyst is quinoline or isoquinoline, and the molar ratio of amide-modified diamine monomer to aromatic dianhydride monomer is 1:

1.

8. The polyimide according to claim 6, characterized in that, The aromatic dianhydride monomer is selected from one of 4,4'-oxobisphthalic anhydride, 4,4'-(hexafluoroisopropylidene)phthalic anhydride, 3,3',4,4'-biphenyltetracarboxylic anhydride, 3,3',4,4'-benzophenonetetracarboxylic anhydride, or pyromellitic dianhydride.

9. The polyimide according to claim 6, characterized in that, The stepwise heating reaction is as follows: first, the ring-opening polycondensation reaction of the diamine monomer and the dianhydride monomer occurs at 20℃~100℃ to generate polyamic acid; then, the temperature is successively increased to 120℃~150℃ to carry out the pre-iminoization of polyamic acid; finally, the temperature is increased to 180℃~220℃ to carry out the complete iminoization of polyamic acid.

10. The polyimide according to claim 6, characterized in that, Polyimide was dissolved in a polar organic solvent to prepare a polyimide solution, which was then cast into a film at room temperature.

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