Method for synthesizing reprocessable and reusable bismaleimide monomer, monomer synthesized by same method, and curing composition and cured product comprising same monomer

Abstract

The present invention relates to a method for synthesizing a reprocessable and reusable bismaleimide (BMI) monomer having a covalent adaptable network and excellent thermal properties after an initial curing reaction, using a BMI monomer including an exchangeable ester functional group and a biphenyl mesogenic core and a diamine curing agent, to a monomer synthesized by the method, and to a curing composition and a cured product comprising same. By introducing a functional group into a BMI resin, which is a thermosetting resin known to be difficult to reprocess and recycle, to enable reuse and conversion into a reprocessable thermosetting vitrimer resin via an ester functional group exchange reaction, thereby imparting reprocessability in addition to excellent heat resistance of a conventional BMI resin, the present invention has the effect of being recognized as an eco-friendly material that can serve as an alternative to existing resins in advanced industries, including the aerospace sector, as an eco-friendly high-performance thermosetting resin.

Description

Method for synthesizing a bismaleimide monomer capable of being reprocessed and reused, a monomer synthesized by the method, and a cured composition and cured product containing the same The present invention relates to a method for synthesizing a reprocessable and reusable bismaleimide monomer, and a curing composition and a cured product comprising the same, and more particularly, to a method for synthesizing a reprocessable and reusable bismaleimide monomer characterized by having a covalent bond adaptive network and excellent thermal properties after an initial curing reaction using a BMI (bismaleimide) monomer including an exchangeable ester functional group and a biphenyl mesogenic core and a diamine curing agent, and a monomer synthesized by the method, and a curing composition and a cured product comprising the same. Plastics are widely used in all industrial fields, and are classified into thermoplastics and thermosetting resins according to the characteristics of the microstructure and appearance of the polymer resin. Thermoplastic resins are composed of numerous linear polymer chains that are interconnected by weak intermolecular forces such as van der Waals forces, dipole-dipole interactions, and hydrogen bonds. Therefore, when the temperature rises, they can lose almost all physical properties, and can even change into a liquid-like form, and can recover all functions after cooling. Due to these characteristics, thermoplastic resins include polyethylene terephthalate (PET), nylon (polyamide), polyvinyl chloride (PVC), polystyrene (PS), and polypropylene (PP), and can be repeatedly reprocessed and reused. In addition, the advantages of this product are that it has excellent impact resistance, is recyclable, and is an environmentally friendly material. Due to these properties, it is widely used in our daily lives as a core component of synthetic resin bottles, packaging materials, office supplies, and various tools and devices around us, but it has the disadvantage of being unsuitable for use in harsh environments where thermosetting is preferred due to its low glass transition temperature (Tg) and poor chemical, physical, and thermal stability. In contrast, thermosetting resins form 'cross-linkers' between polymer chains in the form of irreversible covalent bonds. These are formed through a process called 'curing' to become three-dimensionally stable materials, so unlike thermoplastic resins that can be reshaped and reused through heating and cooling, thermosetting resins are materials that cannot be reprocessed after the initial curing. Thermosetting resins include epoxy resins, BMI (bismaleimide) thermosetting resins, polyimides, and polyurethane. Due to their excellent durability, they are not easy to recycle, making them materials that cause serious environmental pollution problems. In recent years, various studies have been conducted to design sustainable and reprocessable thermosets to extend the life of these materials. Instead of relying on irreversible three-dimensional covalent networks, researchers have introduced covalent adaptable networks (CANs) into thermosets to solve the problem of disposable materials. CANs were reported in Non-Patent Document 1 to allow the reversion or exchange of connections when triggered by an external stimulus while maintaining the number of covalent bonds in the network. As a result, these materials exhibit high ductility and fluidity under certain conditions, allowing re-molding. Some of the initial properties can be preserved to some extent even after reprocessing cycles. This development suggests the possibility of making more environmentally friendly and sustainable thermoset materials. Based on the concept of associated CANs, Leibler and co-workers first discovered and defined 'vitrimers' in 2011 in networks composed of epoxy and acid polyester in the presence of a transesterification catalyst. Due to development and evolution, vitrimers are now considered a separate material class, along with thermoplastics and thermosets. The study of vitrimer chemistry has expanded to include various exchange reactions, such as transesterification, amide exchange, ester amidation, imine exchange, and disulfide metathesis. As a representative vitrimer resin, bismaleimide (BMI) thermosetting resin belongs to a class of high-performance materials known to have excellent chemical, physical, and thermal stability, low dielectric properties, and minimal moisture absorption, as described in Non-Patent Document 2. Due to these advantageous properties, BMI resin is widely used in aerospace and space flight fields where the operating temperature can exceed 250℃. The BMI monomer is an irreplaceable component described as an organic compound terminating with two identical maleimide rings as shown in the following [chemical formula 1]. In the case of a general route for synthesizing the BMI monomer, a diamine is combined with two equivalents of maleic anhydride to form bismaleamic acid as an intermediate as illustrated in the following [reaction scheme 1]. Subsequently, a dehydration cyclization reaction proceeds in the presence of a dehydrating agent such as acetic anhydride and a catalyst such as sodium acetate. [Chemical Formula 1] [Reaction Formula 1] Due to the diversity of maleimide functional groups, BMI monomers can undergo chain extension through various reactions. These reactions include free radical homopolymerization, coupling with other types of monomers, aza-Michael reaction, ene-Alder reaction, and Diels-Alder reaction, as shown in Scheme 2 below, and further reactions, leading to the formation of the final BMI resin. [Reaction Formula 2] There has been a notable trend toward the development of sustainable materials in recent years, and this trend has also extended to BMI resins. However, there is a significant challenge in preserving the excellent properties generally associated with bismaleimide resins while incorporating exchangeable covalent bonding structures, and various studies have attempted to develop reprocessable BMI systems, including systems based on disulfide metathesis or esterification in some research processes. However, BMI resin is a high-performance material with excellent mechanical properties, chemical resistance, and thermal stability, and is widely used in high-performance fields such as aerospace based on various excellent properties, but existing BMI resin is known to be difficult to process and causes environmental burden. Accordingly, the present applicant has completed the present invention by developing a technology for synthesizing a new BMI monomer including a variable ester bond that can be reprocessed and reused to solve the problems of the BMI resin as described above. The present invention is a solution to the above problems, and aims to provide a method for synthesizing a reprocessable and reusable bismaleimide monomer having a covalent bond adaptive network and excellent thermal properties after an initial curing reaction using a bismaleimide (BMI) monomer containing an exchangeable ester functional group and a biphenyl mesogenic core and a diamine curing agent, and a monomer synthesized by the method, and a cured composition and a cured product containing the same. A preferred embodiment of the present invention for achieving the above-described task is a method for synthesizing a bismaleimide monomer that is reprocessable and reusable, characterized by including the steps of: (S100) dissolving 4-nitrobenzoyl chloride, pyridine and 4,4'-dihydroxybiphenyl in a first organic solvent to synthesize BP.NBC; (S200) adding the BP.NBC and SnCl2 to a second organic solvent, stirring under reflux, and neutralizing the mixture in a NaHCO3 solution to synthesize BP.ABA; and (S300) dissolving maleic anhydride and the BP.ABA in a third organic solvent to synthesize a maleic acid intermediate, and then dissolving the maleic acid intermediate and sodium acetate in a fourth organic solvent to synthesize SBMI. In the above BP.NBC synthesis step (S100), 4-nitrobenzoyl chloride, pyridine and 4,4'-dihydroxybiphenyl are characterized by mixing 35 to 36 mmol: 42 to 44 mmol; 14 to 16 mmol in 100 ml of the first organic solvent. The above first organic solvent is characterized by selecting one or more from the group consisting of tetrahydrofuran (THF), dimethylformamide (DMF), and dimethyl sulfoxide (DMSO). In the above BP.ABA synthesis step (S200), BP.NBC and SnCl2 are characterized by being mixed in 100 ml of the second organic solvent at 8 to 12 mmol: 90 to 110 mmol. The second organic solvent is characterized by selecting one or more from the group consisting of methanol (MtOH), ethanol (EtOH), and tetrahydrofuran (THF). In the above SBMI synthesis step (S300), maleic anhydride and BP.ABA are characterized by being mixed in 100 ml of a third organic solvent at 48 to 52 mmol: 4 to 6 mmol. The third organic solvent is characterized by being selected from one or more selected from the group consisting of dimethyl sulfoxide (DMSO), dimethylformamide (DMF), and toluene. In the above SBMI synthesis step (S300), the maleic acid intermediate and sodium acetate are characterized by being mixed in 100 ml of the fourth organic solvent at 9 to 11 mmol: 18 to 22 mmol. The fourth organic solvent is characterized in that it is a mixture of acetic anhydride and acetone (1:1 v / v). *And another preferred embodiment of the present invention for achieving the above-mentioned task is a curable composition including a reprocessable and reusable bismaleimide monomer characterized in that SBMI, 4,4'-bismaleimidodiphenylmethane (CBMI), and 4,4'-diaminodiphenylmethane (DDM) are mixed in a molar ratio of 1.0 to 1.2: 1.0 to 1.2: 1.0 to 1.2, which is another means for solving the task. And another preferred embodiment of the present invention for achieving the above-mentioned task is another means for solving the task by forming a cured product characterized in that the cured composition is placed in a steel mold, heated continuously at 160±10°C for 1±0.1 hours and at 250±10°C for 2±0.2 hours, and then the molded product is taken out of the mold and post-cured at 275±5°C. The above-mentioned hardened product is crushed into powder and placed in a steel mold and heated at 290±10℃ for 4±0.5 hours under a pressure of 5±0.5 MPa, so that it can be reused. The present invention introduces a functional group into a bismaleimide resin, a thermosetting resin known to be difficult to reprocess and recycle, through an ester functional group exchange reaction, so as to enable reuse, thereby imparting reprocessability in addition to the excellent heat resistance of existing bismaleimide resins, thereby resulting in an eco-friendly, high-performance thermosetting resin that can be noted as an alternative to existing resins as an eco-friendly material in cutting-edge industries led by the aerospace field. The effects of the present invention are not limited to the effects mentioned above, and other effects not mentioned will be clearly understood by those skilled in the art from the description of the claims. Figure 1 is a process block diagram for explaining a method for synthesizing a bismaleimide monomer according to a preferred embodiment of the present invention. FIG. 2 is a drawing for explaining a process of curing using a curing composition containing a bismaleimide monomer according to a preferred embodiment of the present invention. FIG. 3 is a drawing for explaining a process for reprocessing a cured product containing a bismaleimide monomer according to a preferred embodiment of the present invention. Figure 4 is (a) of BP.NBC 1 A diagram showing (a) H NMR and (b) ATR-FTIR spectra. Figure 5 is (a) of BP.ABA 1 A diagram showing (a) H NMR and (b) ATR-FTIR spectra. Figure 6 is (a) of SBMI. 1 H NMR spectrum and (b) 13 This is a diagram showing the C NMR spectrum. Figure 7 is a diagram showing (a) ATR-FTIR and (b) enlarged ATR-FTIR spectra of SBMI. Figure 8 is a diagram showing (a) the TGA curve and (b) the DSC curve of SBMI. Figure 9 is a diagram showing the DSC curve of the cured mixture. Figure 10 is a diagram showing the TGA curve of a thermosetting system. Figure 11 is a diagram showing (a) DSC and (b) DMA curves of the original thermosetting specimen. Figure 12 is a drawing showing (a) the ATR-FTIR spectrum and (b) the chemical structure of the original specimen according to the present invention. Figure 13 is a comparison diagram of (a) DSC and (b) DMA curves of the original and processed specimens. Figure 14 is a comparison diagram of the chemical structure change and ATR-FTIR spectrum of BMI resin during reprocessing compared to a commercial base system that cannot be processed. FIG. 15 is a drawing for explaining a reprocessing mechanism of a cured product according to a preferred embodiment of the present invention. Figure 16 is a diagram showing the stress-relaxation curve and Arrhenius plot of ln(t*) versus reverse temperature. Hereinafter, the present invention will be described in detail with reference to the attached drawings according to preferred embodiments of the present invention, but a detailed description of the configuration and operation that can be easily understood by those skilled in the art of the present invention will be omitted. In addition, it should be noted that the present invention is not necessarily limited to the following embodiments, and that those skilled in the art can make various modifications to the present invention within a scope that does not depart from the technical spirit of the present invention. The terms used in this specification are selected from the most widely used general terms possible while considering the functions of the present invention, but they may vary depending on the intention of engineers working in the field, precedents, the emergence of new technologies, etc. In addition, in certain cases, there are terms arbitrarily selected by the applicant, and in this case, the meanings thereof will be described in detail in the description of the relevant invention. Therefore, the terms used in the present invention should be defined based on the meanings of the terms and the overall contents of the present invention, rather than simply the names of the terms. The numerical ranges are inclusive of the numbers defined in the above ranges. Every maximum numerical limitation given throughout this specification includes every lower numerical limitation, as if that lower numerical limitation were expressly written out. Every minimum numerical limitation given throughout this specification includes every higher numerical limitation, as if that higher numerical limitation were expressly written out. Every numerical limitation given throughout this specification will include every better numerical range within that broader numerical range, as if that narrower numerical limitation were expressly written out. The term 'BP.NBC' described in the specification of the present invention is an abbreviation for 1,1'-biphenyl]-4,4'-diyl bis(4-nitrobenzoate, 'BP.ABA' is an abbreviation for 1,1'-biphenyl]-4,4'-diyl bis(4-aminobenzoate, and 'SBMI' is an abbreviation for synthesized bismaleimide monomer. 1. Synthesis of bismaleimide monomer Hereinafter, a method for synthesizing a bismaleimide monomer that can be reprocessed and reused according to a preferred embodiment of the present invention will be described with reference to FIG. 1 attached to the specification. The method for synthesizing a bismaleimide monomer according to the present invention includes a BP.NBC synthesis step (S100); a BP.ABA synthesis step (S200); and a SBMI synthesis step (S300). The BP.NBC synthesis step (S100) is a step of synthesizing BP.NBC by dissolving 4-nitrobenzoyl chloride, pyridine, and 4,4'-dihydroxybiphenyl in a first organic solvent. It is preferable to mix 35 to 36 mmol: 42 to 44 mmol; 14 to 16 mmol of 4-nitrobenzoyl chloride, pyridine, and 4,4'-dihydroxybiphenyl in 100 ml of the first organic solvent, and specifically, it is more preferable that it is 35.7 mmol: 42.9 mmol; 14.4 mmol. If the molar ratio of the above compounds is outside the range limited above, there is a concern that unreacted compounds may remain in the solution, resulting in insufficient synthesis of BP.NBC. In this step, the above compounds are slowly added dropwise to a mixture dissolved in the first organic solvent in a flask stabilized in an ice bath under a nitrogen atmosphere at room temperature with constant stirring. After 24±2 hours, the first organic solvent is vaporized using a rotary evaporator. Afterwards, the generated product is washed 2-3 times with hot ethanol (EtOH) to separate monofunctional byproducts and unreacted precursors. After drying in a vacuum oven at 50±2℃ for 2±0.5 hours, the product is recrystallized from dioxane. After this step, the final white crystalline product is filtered, washed again with ethanol (EtOH), and dried in a vacuum oven for 10±2 hours to produce BP.NBC. In this step, if the process conditions fall below the conditions defined above, there is a concern that BP.NBC may not be sufficiently generated, and if the conditions defined above are exceeded, there is a concern that an inefficient process may be performed since BP.NBC is no longer generated in proportion to the process conditions exceeding the conditions. However, the process conditions are not necessarily limited to the range of the process conditions defined above, and may be appropriately adjusted outside the range of the process conditions as needed. It is preferable that the above first organic solvent is selected from one or more of the group consisting of tetrahydrofuran (THF), dimethylformamide (DMF), and dimethyl sulfoxide (DMSO). Below [Reaction Scheme 3] is a reaction scheme showing the synthesis process of BP.NBC. [Reaction Formula 3] The BP.ABA synthesis step (S200) is a step of synthesizing BP.ABA by adding BP.NBC and SnCl2 to a second organic solvent, stirring under reflux, and neutralizing in a NaHCO3 solution. It is preferable to mix 8 to 12 mmol: 90 to 110 mmol of BP.NBC and SnCl2 in 100 ml of the second organic solvent, and more preferably 10 mmol: 100 mmol. If the molar ratio of the above compounds is outside the range limited above, there is a concern that unreacted compounds may remain in the solution, resulting in insufficient synthesis of BP.ABA. In this step, the above compounds are placed in a flask and stirred under reflux for 24±2 hours using a second organic solvent. Afterwards, the precipitate is collected by filtration, washed 2~3 times more with hot EtOH, and neutralized in 5 wt% NaHCO3 solution. The resulting light yellow precipitate is filtered and washed 2~3 times with DI water and EtOH to remove impurities. Finally, the pure diamine product is dried in a vacuum oven at 50±2℃ for 10±2 hours to produce BP.ABA. In this step, if the process conditions fall below the conditions defined above, there is a concern that BP.ABA may not be sufficiently produced, and if the conditions defined above are exceeded, there is a concern that an inefficient process may be performed since BP.ABA is no longer produced in proportion to the exceeding process conditions. However, the process conditions are not necessarily limited to the range of the process conditions defined above, and may be appropriately adjusted outside the range of the process conditions as needed. It is preferable that the second organic solvent is selected from one or more selected from the group consisting of methanol (MtOH), ethanol (EtOH), and tetrahydrofuran (THF). Below [Reaction Scheme 4] is a reaction scheme showing the synthesis process of BP.ABA. [Reaction Formula 4] The SBMI synthesis step (S300) is a step of synthesizing a maleic acid intermediate by dissolving maleic anhydride and the BP.ABA in a third organic solvent, and then synthesizing SBMI by dissolving the maleic acid intermediate and sodium acetate in a fourth organic solvent. It is preferable to mix the above maleic anhydride and BP.ABA in 100 ml of a third organic solvent at 48 to 52 mmol: 4 to 6 mmol, and more preferably 50 mmol: 5 mmol. In this step, the above compounds are slowly added dropwise to a mixture dissolved in a third organic solvent in a flask while stirring constantly at room temperature under a nitrogen atmosphere. After 24±2 hours, the first organic solvent is vaporized using a rotary evaporator. It is preferable to prepare a maleic anhydride solution dissolved in a third organic solvent in a flask, then add a diamine compound, BP.ABA, to the solution and stir at 100±5℃ for 1±0.1 hour. Then, the reaction solution is poured into 2 L of ethanol (EtOH) to precipitate a maleic acid intermediate. After refrigerating for 4±0.5 hours, the precipitate is collected by filtration, washed with acetone, and dried in a vacuum oven at 50±2℃ for 10±2 hours to prepare a yellow powder. It is preferable that the third organic solvent is selected from one or more selected from the group consisting of dimethyl sulfoxide (DMSO), dimethylformamide (DMF), and toluene. In the next step, it is preferable to mix the maleic acid intermediate and sodium acetate in 100 ml of the fourth organic solvent at 9 to 11 mmol: 18 to 22 mmol, more preferably 10 mmol: 20 mmol. It is preferable to introduce the above intermediate and sodium acetate into the fourth organic solvent and heat it at 75±5℃ for 12±1 hours. Then, the precipitate is filtered, washed sequentially with acetone and water, and then dried in a vacuum oven at 80±5℃ for 10±2 hours to produce the final fine yellow powder of SBMI. It is preferable that the fourth organic solvent is a mixture of acetic anhydride and acetone (1:1 v / v). If the molar ratio of the above compounds is outside the range limited above, there is a concern that unreacted compounds may remain in the solution, resulting in insufficient synthesis of SBMI. In addition, in this step, if the process conditions fall below the conditions limited above, there is a concern that SBMI may not be sufficiently generated, and if the conditions limited above are exceeded, there is a concern that an inefficient process may be performed since SBMI is no longer generated in proportion to the exceeding process conditions. However, the process conditions are not necessarily limited to the range of the process conditions limited above, and may be appropriately adjusted outside the range of the process conditions as needed. Below [Reaction Scheme 5] is a reaction scheme showing the synthesis process of SBMI. [Reaction Formula 5] 2. Manufacturing process of hardened product As shown in Fig. 2, SBMI and 4,4'-bismaleimidodiphenylmethane (CBMI) and 4,4'-diaminodiphenylmethane (DDM) manufactured in the above 1 are mixed in an agate mortar at a molar ratio of 1.0 to 1.2 : 1.0 to 1.2 : 1.0 to 1.2. Next, the fine powder mixture is placed into a steel mold and heated continuously at 160±10℃ for 1±0.1 hours and at 250±10℃ for 2±0.2 hours, then the molded product is taken out of the mold and post-cured at 275±5℃ to produce a hardened product. If the above curing conditions fall below the above-mentioned limited range, there is a risk that the cured product may not develop a crosslinking network and may not exhibit sufficient properties. If the conditions exceed the above-mentioned limited range, there is a risk that over-curing may occur and deterioration may occur. 3. Reprocessing process The powder from which the hardened product has been crushed is crushed into small pieces using a coffee grinder, and then crushed into an even finer powder using an agate mortar, and then, as shown in Fig. 3, is placed into a steel mold and heated at 290±10℃ for 4±0.5 hours under a pressure of 5±0.5 MPa so that it can be molded for reuse. Hereinafter, the method for synthesizing a bismaleimide monomer capable of being reprocessed and reused according to the present invention, the monomer synthesized by the method, and the cured composition and cured product containing the same will be specifically described through the following examples, and the present invention is not necessarily limited to the following examples. 1. Raw materials The compounds used in this example were 4,4'-dihydroxybiphenyl (TCI, >99 wt%), 4-nitrobenzoyl chloride (TCI, >98 wt%), maleic anhydride (Alfa Aesar, 98 wt%), tin (II) chloride (SnCl2, JUNSEI, 97 wt%), pyridine (Alfa Aesar, 99.5 wt%), tetrahydrofuran (THF, DAEJUNG, 99 wt%), ethanol (EtOH, SAMCHUN, 99.9 wt%), acetone (SAMCHUN, 99.5 wt%), dioxane (SAMCHUN, 99.5 wt%), dimethyl sulfoxide (DMSO, SAMCHUN, 99.5 wt%), acetic anhydride (SAMCHUN, 97 wt%), sodium acetate (NaOAc, SAMCHUN, 98.5 wt%), sodium bicarbonate (NaHCO3, SAMCHUN, 99 wt%). 2. Synthesis of bismaleimide monomer (Example 1) Synthesis of BP.NBC In a 500-mL flask stabilized in an ice bath, 35.7 mmol of 4-nitrobenzoyl chloride was dissolved in 100 mL of THF. To this solution, a mixture of 42.9 mmol of pyridine and 14.4 mmol of 4,4'-dihydroxybiphenyl in 80 mL of THF was slowly added dropwise with constant stirring under a nitrogen atmosphere at room temperature. After 24 hours, the THF solvent was evaporated using a rotary evaporator. After that, the generated product was washed twice with hot EtOH to separate the single functional byproduct and unreacted precursor. After drying in a vacuum oven at 50°C for 2 hours, the product was recrystallized from dioxane. After this step, the final white crystalline product was filtered, washed again with EtOH, and dried in a vacuum oven for 10 hours to produce BP.NBC (yield: 76%). 1 H-NMR (500 MHz, DMSO d6) δ(ppm) = 7.48(d, 4H), 7.84(d, 4H), 8.42(dd, 8H). (Example 2) Synthesis of BP.ABA Dinitro compound BP.NBC 10 mmol, SnCl2 100 mmol, and EtOH 100 ml were placed in a 300 ml flask and refluxed for 24 hours. The precipitate was collected by filtration, washed twice more with hot EtOH, and neutralized in 5 wt% NaHCO3 solution. The resulting light yellow precipitate was filtered and washed twice with DI water and EtOH. Finally, the pure diamine product was dried in a vacuum oven at 50 °C for 10 hours to produce BP.ABA (yield: 97%). 1 H-NMR (500 MHz, DMSO d6) δ(ppm) = 6.18(s, 4H), 6.64(d, 4H), 7.30(d, 4H), 7.74(d, 4H), 7.82(d, 4H). (Example 3) Synthesis of SBMI A solution of 50 mmol of maleic anhydride dissolved in 100 ml of DMSO was prepared in a 500 ml flask. 5 mmol of diamine compound BP.ABA was added to the solution and stirred at 100°C for 1 hour. Then, the reaction solution was poured into 2 L of EtOH to precipitate the maleic acid intermediate. After refrigerating for 4 hours, the precipitate was collected by filtration, washed with acetone, and dried in a vacuum oven at 50°C for 10 hours to obtain a yellow powder (yield: 94%). 10 mmol of the intermediate and 20 mmol of sodium acetate were introduced into 100 ml of a mixture of acetic anhydride and acetone (1:1 v / v) and heated at 75°C for 12 h. The precipitate was then filtered and washed sequentially with acetone and water. After drying in a vacuum oven at 80°C for 10 h, SBMI was obtained as a final fine yellow powder. 1 H-NMR (500 MHz, DMSO d6, δ(ppm) = 7.26 (s, 4H), 7.43 (m, 4H), 7.65 (m, 4H), 7.82 (m, 4H), 8.28 (m, 4H). 3. Curing process and reprocessing process (Example 4) Curing process A mixture of SBMI (1.6 g, 2.7 mmol) as a curing agent, 4,4'-bismaleimidodiphenylmethane (CBMI) (0.9809 g, 2.7 mmol), and 4,4'-diaminodiphenylmethane (DDM) (0.5427 g, 2.7 mmol) was thoroughly mixed in an agate mortar at a molar ratio of 1:1:1. Then, the fine powder mixture was put into a steel mold and successively heated at 160 °C for 1 h, 200 °C for 1 h, and 250 °C for 2 h. Thereafter, the specimen was taken out of the mold and post-cured at 275 °C. The cured thermosetting resin was characterized using DSC, TGA, thermal conductivity analyzer, FTIR, and DMA. (Example 5) Reprocessing process The bismaleimide resin specimen was ground into small pieces using a coffee grinder and then ground into a finer powder using an agate mortar. The resulting powder was placed into a steel mold and heated at 290°C for 4 hours under a pressure of 5 MPa. 4. Measurement method All compounds dissolved in DMSO d6 solvent 1 H NMR and 13C NMR spectra were recorded at 500 MHz using a Bruker (AVANCE III 500) nuclear magnetic resonance spectrometer. The curing reaction and thermal properties of the thermosetting samples were investigated using differential scanning calorimetry (DSC) on a TA DSC 25 instrument at a heating rate of 10 °C / min in a nitrogen atmosphere. Thermogravimetric analysis (TGA) results were collected using a TGA Discovery TGA 55 instrument at a heating rate of 20 °C / min in a nitrogen atmosphere. Thermal conductivity studies were performed using a Hot Disk TPS 3500, and dynamic mechanical analysis (DMA) and stress relaxation results were performed using a TA Discovery DMA 850 instrument. Fourier transform infrared (FTIR) spectra were recorded in the range of 4000–600 cm -1 The data were collected on a PerkinElmer Spectra Two instrument using a KBr IR source. 5. Evaluation 5.1. Synthesis and characterization of BMI monomer Dinitro compounds were synthesized through the esterification reaction of 4,4'-dihydroxybiphenyl, a diol compound, and 4-nitrobenzoyl chloride, an acid chloride. The reaction is highly exothermic and can produce HCl. Therefore, the reactor was stabilized in an ice bath and pyridine was used as a catalyst to form a reactive acyl pyridinium ion and absorb HCl. For reference, Fig. 4 shows the proton NMR and ATR-FTIR spectra of BP.NBC. In Fig. 4(a), the signals could be accurately assigned to all proton atoms and functional groups of BP.NBC. Because of the symmetric structure and the presence of only aromatic hydrogens in BP.NBC, 1The H NMR spectrum shows a doublet at 8.42 ppm (representing eight hydrogens) and two doublets at 7.84 and 7.48 ppm (representing four hydrogens, respectively). In Figure 4(b), a doublet at 1728 cm -1 The ester stretch of is confirmed, and also the CN stretch as well as the NO asymmetric and symmetric stretch are 1520, 1347 and 1263 cm, respectively. -1 It was well confirmed in . And the dinitro compound was reduced using SnCl2 as a reducing agent to synthesize a diamine compound. In Fig. 5(a), all proton signals were correctly assigned. The aromatic proton peaks of BP.ABA, which have a symmetric structure similar to that of BP.NBC, can be found as doublets especially at 7.82, 7.74, 7.30, and 7.64 ppm, which indicate four hydrogens for each peak. The primary amine protons were accurately assigned as singlets at 6.18 ppm, which indicate four hydrogens. In the ATR-FTIR spectrum, the asymmetric and symmetric NH stretchings were located at 3410 and 3338 cm, respectively. -1 was found in about 100 cm. -1 Below, a shoulder band peak is clearly seen, which is due to the interaction between symmetric NH stretching and NH bending vibrations at 1595 cm. -1 In the low-wavelength region, the CN stretching and NH rocking vibrations are at 1276 and 765 cm, respectively. -1 It is designated as 1701cm and shows a strong signal. -1 The peak is attributed to the C=O ester group, similar to the peak of BP.NBC. In Fig. 6(a), SBMI, a BMI monomer, 1In the H NMR spectrum, vinyl hydrogens are clearly assigned at 7.26 ppm, indicating four protons. Similar to the previous precursor, the symmetric structure is due to the reduced peaks at 7.43, 7.65, 7.82, and 8.28 ppm, each peak representing four hydrogens. In Fig. 6(b), there are 11 distinct peaks assigned to 11 carbon positions as indicated in the schematic compound structure. Aromatic carbon peaks were found in the range of 122.45–150.1 ppm, and the vinyl carbon peak was located at 134.95 ppm. Finally, the carbonyl carbons of the main chain and maleimide group were found at 164.03 and 169.48 ppm, respectively. Even after evolution to bismaleimide as shown in Fig. 7(a), the C=O stretch and CN stretch of the ester linkage still remain at 1728 and 1263 cm -1 , but no amine signal was present in the ATR-FTIR spectrum of SBMI as shown in Fig. 7(b). In addition, the C=O and C=C stretching of maleimide was found at about 1710 cm -1 By fitting several peaks to each of 1709 cm -1 and 1690cm -1 The similar peak positions for the extension of the maleimide functional group can be compared with the ATR-FTIR spectrum of 4,4'-bismaleimidodiphenylmethane, the most widely used commercial bismaleimide monomer, as reported in Non-Patent Document 4. 5.2. Thermal properties of monomers The thermal properties of the synthesized bismaleimide monomer, SBMI, are shown in Fig. 8. As can be seen from the TGA curve in Fig. 8(a), the monomer has high thermal stability, and the temperatures at which the residual weight is 95% (Td5%) and 90% (Td10%) are determined to be 402.8 and 434.8°C, respectively. The Td10% of SBMI is 4,4'-bismaleimidodiphenylmethane (506.2°C).

[0021] Although lower than Td10%, the thermal durability of SBMI is still similar to that of conventional BMI monomers. It will be demonstrated later that the final thermoset exhibits notable advanced properties. Differential scanning calorimetry is a technique that can investigate the thermal properties of a specimen by detecting changes in the heat flow released or absorbed by the sample. This information can be used to investigate phase changes such as melting points and glass transition temperatures, as well as thermally induced reactions. The DSC curve in Fig. 8(b) shows one exothermic peak due to thermally stimulated radical polymerization, with the onset and peak temperatures observed just above 200°C and 290°C, respectively. The enthalpy change of this homopolymerization was recorded as 52.8 J / g. To promote the crosslinking reaction at lower temperatures and lower energy requirements, SBMI is combined with a curing agent, which is discussed in the following section. 5.3. BMI thermosetting system design Among various processes for curing the BMI system, the use of aza-Michael addition polymerization and a diamine curing agent was selected due to its ease of operation, easy processability, and various curing agent structures that can be used in the design. Diaminodiphenyl sulfone (DDS), p-phenylenediamine (PDA), sulfanilamide (SAA), diaminodiphenylmethane (DDM), 4,4'-oxydianiline (ODA), Jeffamine D-230 (JA) and its structure is as shown in the following [Chemical Formula 2]. [Chemical formula 2] Among the various systems considered, the goal is to find a balance between processability and thermal properties. Some of the candidates have a rigid structure, such as DDS and PDA, which can contribute to good toughness and high glass transition temperature (Tg). However, the resulting thermoset products are very brittle and have poor processability. On the other hand, those with more flexible structures, such as ODA and JA, which have ether bonds, can facilitate specimen processing. However, these characteristics are also the reason why the Tg of the thermosetting product is lowered later. Therefore, the best performing curing agent is DDM. 5.4. Characteristics of BMI pristine thermosetting resin Figure 9 shows the DSC curve of the prepared curing mixture, where the molar ratio of SBMI:CBMI:DDM is 1:1:1. The ratio was controlled by the total mole number of BMI monomers compared to DDM (2:1), with the expectation that it would generate a tertiary amine structure and produce a high crosslinking density system. The presence of CBMI in the mixture can make the thermal properties of the designed thermosetting resin closer to the commercial thermal properties. In addition, SBMI is not only a high molecular weight compound (about 584 g / mol) compared to DDM (about 198 g / mol) but also insoluble. For these reasons, it is difficult to prepare a homogeneous mixture of SBMI and DDM, which causes difficulties during processing. In fact, the specimen obtained by curing this mixture was brittle and easily broken after removing from the mold. Therefore, CBMI acts as a blender and reduces the weight ratio of SBMI. In the DSC curve, two endothermic peaks are observed at about 90°C and 150°C, which represent the melting points of DDM and CBMI, respectively. The broad exothermic peak then corresponds to the aza-Michael addition reaction with an enthalpy change of 35.8 J / g, which is lower than that of the homopolymerization of SBMI. Based on this evidence, this strategy can be considered easier to implement than radical polymerization. The specimens cured at the mentioned ratio are denoted as 'Pristine' when compared with the samples of other reprocessing cycles. At the same time, other models consisting of SBMI or CBMI and DDM in a molar ratio of 2:1 were prepared for comparison with the Pristine specimens. These are denoted as SBMI.DDM.21 and CBMI.DDM.21 as shown in Fig. 10. Based on the TGA curves, the following [Table 1] is plotted, which shows that CBMI provides much higher thermal stability to the system compared to SBMI. The Td5% of CBMI.DDM.21 (415.5°C) is much higher than that of SBMI.DDM.21 (356.3°C). This observation is consistent with the difference in decomposition temperatures between SBMI and CBMI mentioned earlier. Consequently, the presence of CBMI in the system increases the thermal decomposition temperature of the designed system to Td5% (385.6°C) and Td10% (403.1°C). The following [Table 1] shows the decomposition temperatures of the thermosetting systems at 5% and 10% residual weight and at 800°C of residue. Psalm T g (DSC), ℃T g (DMA), ℃Pristine258324.61 st Cycle255321.62 nd Cycle250313.4 The pristine specimens were examined using DSC and DMA analysis techniques after curing. In Fig. 11(a), the DSC curve showed a shift indicating the change in the heat capacity of the sample, and the Tg was observed at 258°C. DMA was used to verify these thermal properties because of its sensitivity and accuracy in measuring mechanical changes, which are more dramatic than heat capacity changes. Like DSC, DMA can provide insight into the transition of a material by applying oscillatory stress while increasing the temperature, as shown in Fig. 11(b). The glass transition temperature can be determined by monitoring the change in storage modulus, loss modulus, or tan δ. Of these methods, the Tg of the original specimen was determined to be 324.6°C, observed from the peak of tan δ versus temperature. To investigate the chemical structure and explain the reprocessing mechanism, the ATR-FTIR spectrum of the original (Pristine) specimen was collected as shown in Fig. 12(a). 3000–3600 cm -1A broad and weak peak is clearly visible within the wavenumber range, indicating the presence of secondary amines. It is worth noting that the molar ratio of the BMI monomer mixture to DDM is 2:1 for the purpose of generating tertiary amine functionality (see Figure 12a). However, the fast reaction kinetics and the low fluidity of the curing mixture may explain why the curing reaction did not proceed as expected initially. Based on these observations, the proposed chemical structure of pure BMI resin is shown in Figure 12(b). At the same time, free radical polymerization may be initiated at high curing temperature and at the post-cure stage to compensate for the initial composition. 3.5. Reprocessability of designed BMI thermosetting materials To investigate the reprocessability and retention of thermal properties of the BMI resin, DSC and DMA curves were collected as shown in Fig. 13. These analytical techniques allowed the determination of the glass transition temperatures (Tg) listed in Table 2 below. The different analytical principles between DSC, which is based on changes in thermal conductivity, and DMA, which is related to changes in physical properties, may explain the differences in the investigated Tg values. However, the observed changes in Tg over two reprocessing cycles within a range of 10°C demonstrate the reprocessability of the system. Table 2 below shows the Tg comparison between the commercial BMI derived resin, pristine specimens, and reprocessed specimens. Psalm T g (DSC), ℃T g (DMA), ℃Pristine258324.61 st Cycle255321.62 nd Cycle250313.4 To further elucidate the chemical structure evolution and reprocessing mechanism, ATR-FTIR spectra were collected as shown in Fig. 14. Over time, the C=C stretch present in the commercial system and the original sample were shifted to 1640 cm due to the pressure effect at the reprocessing temperature. -1was consumed as the peak disappeared. Additionally, 1660cm -1 A new peak shoulder appeared nearby, which may be due to the newly formed amide bond. For this reason, the ester-amide exchange phenomenon was proposed, as shown in Fig. 14. For reference, Figure 14 shows (a) the chemical structural changes of BMI resin during reprocessing compared to a non-processable commercial base system and (b) the change in the temperature range of 1200–2000 cm -1 The diagram shows the enlarged ATR-FTIR spectrum. The spectra of the second reprocessing cycle did not show significant changes compared to the first reprocessing cycle. Moreover, the broad peaks at high wavenumbers remained relatively consistent over time. Based on these observations, it was proposed that the first reprocessing cycle involves a bonded ester-amide exchange domain between the ester and the secondary amide. Subsequently, free OH groups are formed, which can induce the exchange of ester bonds by a domain mechanism in the subsequent reprocessing cycles. In addition to the dissociative ester-amide exchange between esters and amides, esterification between esters can also occur simultaneously. However, it is worth noting that such dissociative bond exchange generally requires more activation energy, so it may not be the primary mechanism. Stress-relaxation measurements were performed using the same DMA machine setup, with initial stress applied at both ends of the specimen. As a result, a decrease in internal stress was observed over time. In typical ductile and reprocessable thermosetting systems, relaxation times are expected to vary significantly with temperature, and these results should follow Arrhenius' law, as shown in Equation 1 below, as shown in Fig. 15. [Mathematical Formula 1] In the above, to is the characteristic relaxation time at infinite temperature, T is the experimental temperature, Ea is the activation energy (kJ / mol) of the bond exchange process, and R is the universal gas constant (see Non-Patent Document 5). Based on the Arrhenius plot, the reprocessing activation energy was determined to be 209 kJ / mol, which is relatively high compared to other reprocessable thermosetting systems. This value may explain why the reprocessing process for the designed BMI system must be performed at a high temperature of 290°C under a pressure of 5 MPa. Information on the operating conditions of various types of aircraft varies from reference material to reference material. Therefore, some properties of common BMIs used in aerospace applications are described here for comparison with our material. [Table 3] below shows the typical cured BMI properties [bis(4-maleimidophenyl)methane (CBMI), a first-generation non-reinforced BMI] compared to the BMI system designed in the present invention (see Non-Patent Document 4). Characteristic Hardened BMI Value Designed BMI Color (Colour) BrownBrownT g (℃)342324.6 Fracture toughness (J / m 2 )25-34- In the present invention, a novel bismaleimide (BMI) monomer and a reprocessable BMI thermosetting resin were successfully synthesized and designed. The BMI monomer contains an exchangeable ester functional group and a biphenyl mesogenic core. After an initial curing reaction using a diamine curing agent, a system having a covalently bonded adaptive network and excellent thermal properties was generated. The original (Pristine) BMI resin was ground into powder form and successfully re-molded through two cycles. The processability was confirmed by maintaining the glass transition temperature and storage modulus measured using DSC and DMA analysis techniques. In addition, the stress relaxation results, temperature-dependent relaxation time dependence, and linear Arrhenius plot further support this conclusion. The reprocessing activation energy was determined to be 209 kJ / mol. The proposed reprocessing mechanism involves ester-amide exchange and esterification, which is supported by ATR-FTIR analysis. The curing system containing the synthesized BMI monomer was investigated and the thermal properties and chemical structures were compared between the as-prepared and reprocessed samples to demonstrate the possibility of reprocessing. The reprocessing mechanism was suggested to involve ester-amide exchange and transesterification based on the ATR-FTIR analysis results. In addition, the activation energy of the reprocessing reaction was determined from the stress relaxation test results. As described above, the method for synthesizing a bismaleimide monomer capable of being reprocessed and reused according to a preferred embodiment of the present invention, the monomer synthesized by the method, and the cured composition and cured product containing the same have been described, but this has been described only as an example, and those skilled in the art will readily understand that various changes and modifications are possible within a scope that does not depart from the technical spirit of the present invention.

Claims

1. A step of synthesizing BP.NBC by dissolving 4-nitrobenzoyl chloride, pyridine, and 4,4'-dihydroxybiphenyl in a first organic solvent (S100); Step (S200) of adding the BP.NBC and SnCl2 to the second organic solvent, stirring under reflux, and then neutralizing in a NaHCO3 solution to synthesize BP.ABA; and A step (S300) of synthesizing a maleic acid intermediate by dissolving maleic anhydride and the BP.ABA in a third organic solvent, and then synthesizing SBMI by dissolving the maleic acid intermediate and sodium acetate in a fourth organic solvent; A method for synthesizing a reprocessable and reusable bismaleimide monomer, characterized by including:

2. In paragraph 1, In the above BP.NBC synthesis step (S100), A method for synthesizing a bismaleimide monomer that can be reprocessed and reused, characterized in that 4-nitrobenzoyl chloride, pyridine and 4,4'-dihydroxybiphenyl are mixed in 100 ml of a first organic solvent at an amount of 35 to 36 mmol: 42 to 44 mmol; 14 to 16 mmol.

3. In paragraph 2, A method for synthesizing a bismaleimide monomer that can be reprocessed and reused, characterized in that the first organic solvent is one or more selected from the group consisting of tetrahydrofuran (THF), dimethylformamide (DMF), and dimethyl sulfoxide (DMSO).

4. In paragraph 1, In the above BP.ABA synthesis step (S200), A method for synthesizing a bismaleimide monomer that can be reprocessed and reused, characterized in that BP.NBC and SnCl2 are mixed in 100 ml of a second organic solvent at 8 to 12 mmol: 90 to 110 mmol.

5. In paragraph 4, A method for synthesizing a bis monomer that can be reprocessed and reused, characterized in that the second organic solvent is one or more selected from the group consisting of methanol (MtOH), ethanol (EtOH), and tetrahydrofuran (THF).

6. In paragraph 1, In the above SBMI synthesis step (S300), A method for synthesizing a bismaleimide monomer that can be reprocessed and reused, characterized in that maleic anhydride and BP.ABA are mixed in 100 ml of a third organic solvent at a concentration of 48 to 52 mmol: 4 to 6 mmol.

7. In paragraph 6, A method for synthesizing a bismaleimide monomer that can be reprocessed and reused, characterized in that the third organic solvent is one or more selected from the group consisting of dimethyl sulfoxide (DMSO), dimethylformamide (DMF), and toluene.

8. In paragraph 1, In the above SBMI synthesis step (S300), A method for synthesizing a bismaleimide monomer that can be reprocessed and reused, characterized in that a maleic acid intermediate and sodium acetate are mixed in 100 ml of a fourth organic solvent at 9 to 11 mmol: 18 to 22 mmol.

9. In paragraph 8, A method for synthesizing a bismaleimide monomer that can be reprocessed and reused, characterized in that the fourth organic solvent is a mixture of acetic anhydride and acetone (1:1 v / v).

10. A curing composition comprising a reprocessable and reusable bismaleimide monomer, characterized in that SBMI, 4,4'-bismaleimidodiphenylmethane (CBMI), and 4,4'-diaminodiphenylmethane (DDM) are mixed in a molar ratio of 1.0 to 1.2: 1.0 to 1.2: 1.0 to 1.

2.

11. A cured product characterized by being formed by putting the curing composition of claim 10 into a steel mold, heating it continuously at 160±10°C for 1±0.1 hours and at 250±10°C for 2±0.2 hours, then taking the molded product out of the mold and post-curing it at 275±5°C.

12. In paragraph 11, A cured product characterized in that it can be reused by placing the powder obtained by crushing the above-mentioned cured product into a steel mold and heating it at 290±10℃ for 4±0.5 hours under a pressure of 5±0.5 MPa.

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