Process for the preparation of a bismaleimide series product

Abstract

This invention discloses a method for preparing a series of bismaleimide products, relating to the field of high-performance thermosetting resin monomer synthesis technology. The method includes a pre-reaction of maleic anhydride and aromatic diamine at a molar ratio of 2.0–2.2:1 under nitrogen protection at 60–90°C. The resulting amic acid intermediate is pumped into a twin-screw extruder. Irradiation is performed in the vacuum dehydration ring-closing zone (or imidization zone) using a 365 nm UV LED array, coupled with a 0.3–1.0 wt% benzophenone / triethanolamine photoinitiator system. The ring-closing reaction is completed at 110±5°C and a vacuum of -0.090 to -0.098 MPa, with a total residence time of 30–90 seconds, yielding bismaleimide products with an acid value ≤0.1 mg KOH / g and a ring closure degree ≥98.5%. Rapid product switching for a series of products can be achieved by changing different aromatic diamines. This invention eliminates the use of organic solvents, reduces energy consumption by more than 55%, and has a batch-to-batch acid value variation coefficient of less than 2%, meeting the requirements of electronic-grade packaging materials. Furthermore, the process conditions are fundamentally different from existing technologies, demonstrating significant technological advancements.

Description

Technical Field

[0001] This invention relates to the field of high-performance thermosetting resin monomer synthesis technology, specifically to a method for preparing bismaleimide series products. This method is a green synthesis method for efficiently preparing high-purity bismaleimide (BMI) series products in a continuous twin-screw extrusion reactor by combining a solvent-free melting system with an ultraviolet light-induced dehydration and ring-closing reaction. Background Technology

[0002] Bismaleimide resins are widely used as advanced composite matrix, printed circuit board substrates, and high-temperature adhesives due to their excellent heat resistance (Tg>250℃), dimensional stability, and good processability. Traditional BMI preparation mainly employs a two-step solution method: first, aromatic diamines are amidated with maleic anhydride (maleic anhydride) at low temperature in a polar aprotic solvent (N,N-dimethylformamide, N-methylpyrrolidone, etc.) to generate bismaleimide acid; then, a dehydrating agent (acetic anhydride, trifluoroacetic anhydride) or an azeotropic dehydrating agent (toluene, xylene) is added to complete dehydration and ring closure at high temperature (120–160℃). This method suffers from the following insurmountable drawbacks:

[0003] According to existing continuous flow synthesis technologies, although continuous process operation is achieved through tubular reactors, toluene / DMF mixed solvents and strong protic acid catalysts are still required, with solvent recovery energy consumption accounting for more than 35% of the total energy consumption. In Chinese invention patent CN111423584A, although an adamantane structure is introduced to improve solubility, the reaction temperature is still as high as 140℃×5h, leading to significant issues with maleic anhydride sublimation clogging the pipelines. Chinese invention patent CN108395534A employs a polyamic acid chemical imidization route, which involves complex process steps and incomplete ring closure, resulting in an acid value as high as 0.5 mg KOH / g, making it difficult to meet electronic-grade packaging requirements (i.e., an acid value ≤0.2 mg KOH / g).

[0004] The non-patent literature *Green Chemistry* (2024, 26, 3185-3194) reports a solvent-free mechanochemical synthesis of BMI, but the ball milling batch size is small (≤50g), the scale-up effect is significant, and the product has a wide particle size distribution, requiring additional grinding and classification. While the literature *Reactive & Functional Polymers* (2025, 192, 105732) introduces dynamic disulfide bonds to achieve a biodegradable BMI network, the monomer synthesis still uses the traditional thermal shrinkage method (or thermal dehydration method), failing to address fundamental process innovation.

[0005] In summary, existing technologies have not achieved a true unity of solvent-free, low-energy, high-purity, and continuous preparation of organic compounds. There is an urgent need to develop a new BMI synthesis method that can both ensure product quality and meet the principles of green chemistry. Summary of the Invention

[0006] The purpose of this invention is to overcome the shortcomings of existing technologies and provide a method for preparing bismaleimide series products. This method does not use any organic solvents, and uses an ultraviolet light free radical induction mechanism to rapidly complete dehydration and ring closure at low temperatures. A twin-screw extrusion reactor is used to achieve continuous material transport, reaction, and online monitoring. The resulting product has an acid value ≤0.1 mgKOH / g, a ring closure degree ≥98.5%, batch-to-batch quality fluctuation <2%, and energy consumption reduced by more than 55%.

[0007] To achieve the above objectives, the present invention adopts the following technical solution:

[0008] A method for preparing a series of bismaleimide products includes the following steps:

[0009] S1: Pre-melt amidation: Maleic anhydride and aromatic diamine at a molar ratio of 2.0–2.2:1 are added to a stirred pre-melting vessel and heated to 60–90°C under nitrogen protection, with stirring for 10–30 minutes to allow the maleic anhydride and amino group to fully open the ring and form a bismaleamic acid prepolymer. This step is carried out in a homogeneous molten state without any solvent. A slight excess of maleic anhydride ensures complete amino group conversion and avoids subsequent crosslinking.

[0010] S2: Continuous Conveying and Segmented Temperature Control: The above-mentioned amic acid prepolymer is continuously pumped into a twin-screw extruder reactor at a precise flow rate of 5–15 kg / h using a gear metering pump. The reactor screw has a length-to-diameter ratio of 40–60:1 and is divided into three zones: a melt amidation zone (70–80℃) to ensure stable material viscosity; a vacuum dehydration closed-loop zone (105–115℃) as the core reaction section; and a cooling and pulverizing zone (30–50℃) to embrittle the product for easy pulverization. The screw speed is 100–300 rpm, enhancing mass and heat transfer through shearing action.

[0011] S3: UV-induced loop closure: A 365 nm UV LED array with an intensity of 80–120 mW / cm² is arranged circumferentially on the reactor wall of the vacuum dehydration loop closure zone, and a photoinitiator system of 0.3–1.0 wt% (preferably benzophenone / triethanolamine = 1:1.2) is injected. Under vacuum conditions of -0.090 to -0.098 MPa, the hydroxyl groups in the amide acid are photoexcited to generate free radicals, which abstract adjacent carboxyl protons and induce dehydration, forming a maleimide ring. This process takes only 30–90 seconds, which is three orders of magnitude shorter than the thermal method (>240 minutes).

[0012] S4: Online Monitoring and Quality Control: A near-infrared spectroscopy probe is installed at the end of the vacuum dehydration closed-loop zone to monitor the peak area ratio of 1660 cm⁻¹ (C=C double bond) to 1710 cm⁻¹ (anhydride side peak) in real time. When the ratio is ≥0.97, the closed-loop degree is determined to be ≥98.5%, and the system automatically records the batch data; if the ratio is <0.97, the system will adjust the ultraviolet light intensity or extend the residence time to ensure that the acid value of each batch is ≤0.1 mg KOH / g.

[0013] S5: Post-processing and product switching: Closed-loop products are cooled to 30–50℃ in the cooling and pulverizing zone and then continuously discharged through a pelletizer. Switching between different aromatic diamines (such as 4,4′-ODA, fluorinated diamine TFDB, phosphorus-containing diamine DOPO-ODA, etc.) can be done quickly on the same equipment without cleaning, with a switching time of <30 minutes. If necessary, a vacuum flash devolatilization device (120–130℃, -0.099 MPa, 1–2 minutes) can be added after the cooling zone to remove residual photoinitiators, ensuring their content is ≤100 ppm.

[0014] The aromatic diamine is selected from at least one of 4,4′-diaminodiphenyl ether, 4,4′-diaminodiphenylmethane, 3,3′-diaminodiphenyl sulfone, 2,2-bis(4-aminophenyl)hexafluoropropane, 1,5-naphthyldiamine or phosphorus-containing diamine DOPO-ODA.

[0015] The photoinitiator system is a mixture of benzophenone and triethanolamine in a mass ratio of 1:1–1.5, or a mixture of 2-hydroxy-2-methylphenylacetone and triethylamine in a mass ratio of 1:2–3. / 4. A method for preparing a series of bismaleimide products according to claim 1, characterized in that a near-infrared online monitor is installed at the end of the vacuum dehydration closed-loop zone of the twin-screw extrusion reactor to monitor the peak area ratio of the characteristic peak of the C=C double bond at 1660 cm⁻¹ to the peak of the acid anhydride byproduct at 1710 cm⁻¹. When the ratio is ≥0.97, the closed-loop reaction is determined to be complete; otherwise, the ultraviolet light intensity is automatically adjusted or the residence time is extended.

[0016] The temperature of the melt amidation zone is controlled at 70–80℃, the temperature of the vacuum dehydration closed-loop zone is controlled at 105–115℃, the cooling and pulverizing zone is cooled to 30–50℃ by circulating water, and the screw speed is 100–300 rpm.

[0017] A vacuum flash evaporation devolatilization device is added after the cooling and pulverizing zone, and the photoinitiator is treated at 120–130℃ and -0.099 MPa for 1–2 minutes to ensure that the residual amount of photoinitiator is ≤100 ppm.

[0018] The obtained bismaleimide is copolymerized with diallylbisphenol A or allylphenyl compound at a mass ratio of 1:0.8–1.2. The cured product has a glass transition temperature ≥280℃ and a dielectric constant ≤3.5 (10 GHz), making it suitable for 5G high-frequency substrate materials.

[0019] The yield of the bismaleimide product is ≥96%, the purity is ≥99.2% (HPLC area normalization method), and the melting point fluctuation range is within ±2℃.

[0020] Beneficial effects

[0021] Compared with the prior art, the present invention has the following significant advantages:

[0022] 1. Completely solvent-free: The entire process is carried out in the molten state, completely eliminating the use of organic solvents such as DMF and toluene and the energy consumption of recycling, which complies with the twelve principles of green chemistry and reduces VOC emissions by 100%.

[0023] 2. Extremely fast reaction: The UV free radical induction mechanism shortens the dehydration loop closure time from 4–6 hours in the traditional thermal method to 30–90 seconds, increases the space-time yield to 2.8–4.5 kg L⁻¹ h⁻¹, and reduces the equipment volume by 80%.

[0024] 3. Product quality leap: closed-loop degree ≥98.5%, acid value stable ≤0.1 mg KOH / g, which is 80% lower than the 0.5 mg KOH / g of patent CN108395534A, meeting the stringent requirements of electronic-grade packaging and high-frequency substrates.

[0025] Energy consumption and cost are reduced: the reaction temperature is reduced by 30-40℃, and the total energy consumption is reduced by more than 55%; continuous operation reduces the batch-to-batch acid value variation coefficient (CV) to less than 2%, and labor costs are reduced by 60%.

[0026] 4. Intrinsically safe process: Low-temperature operation inhibits maleic anhydride sublimation, reducing pipeline blockage rate from 8% in the traditional thermal method to <0.5%; online monitoring enables closed-loop quality control upfront, with a non-conforming rate of <0.1%.

[0027] 5. Serialized flexible production: By quickly switching diamine monomers, five major categories of BMI products, namely aliphatic, aromatic, fluorine-containing, silicon-containing, and phosphorus-containing, can be produced on the same equipment, covering multiple application scenarios such as dielectric, flame retardant, and high Tg, and possessing the characteristics of Industry 4.0 flexible manufacturing. Attached Figure Description

[0028] Figure 1 This is a flowchart of the equipment of the present invention;

[0029] Figure 2 This is a flowchart of the preparation process of the present invention;

[0030] Figure 3 This is a reaction mechanism diagram of the present invention;

[0031] Figure 4 This is a bar chart comparing the key performance characteristics of embodiments and comparative examples of the present invention.

[0032] Figure 5 This is a schematic diagram of the near-infrared online monitoring spectrum of the present invention. Detailed Implementation

[0033] The present invention will be described in detail below with reference to specific embodiments, but the present invention is not limited to these embodiments. All raw materials are commercially available industrial grade with a purity ≥99.5%. In the embodiments, acid value determination was performed according to GB / T 12008.5-2010, ring closure was calculated by ¹H NMR integration, and dielectric properties were tested using an Agilent N5247A vector network analyzer.

[0034] Example 1 (Basic Example)

[0035] Weigh 196.2 g (2.0 mol) maleic anhydride and 200.2 g (1.0 mol) 4,4′-diaminodiphenyl ether (4,4′-ODA), and add them together to a 1 L sealed pre-melting reactor. After purging the reactor with high-purity nitrogen three times, turn on the heating and stirring device, slowly raise the temperature to 80°C, and stabilize the stirring speed at 150 rpm. Stir the reaction at this constant temperature for 20 minutes until the system is completely melted and the reaction is uniform, yielding a uniform dark amber molten ammonium acid intermediate.

[0036] The viscosity of the intermediate was measured to be 2800 mPa·s using a rotational viscometer. A high-precision gear pump was then started, pumping the molten ammonium acid into a co-rotating twin-screw extruder with a length-to-diameter ratio of 50:1 at a constant material flow rate of 10 kg / h. The process temperatures of each section of the screw were strictly set: a constant temperature of 75℃ in the melt amidation zone, a constant temperature of 110℃ in the vacuum dehydration closed-loop zone, and a constant temperature of 40℃ in the cooling and pulverizing zone. A 365 nm UV LED strip with a power density of 100 mW / cm² was installed around the outer wall of the equipment in the vacuum dehydration closed-loop zone, with a wrapping angle of 270° to ensure uniform UV irradiation of the reactants. Simultaneously, a benzophenone / triethanolamine composite photoinitiator was injected via a micro-feed pump at a mass ratio of 1:1.2, with a total addition of 1.2 g, representing 0.5 wt% of the total reactant mass. The vacuum level of the extruder vacuum system was maintained stably at -0.095. The pressure was controlled at 60 MPa, and the total residence time of the material in the extruder was set at 60 seconds. The reaction process was monitored online via near-infrared spectroscopy (NIR). When the monitoring showed that the area ratio of the 1660 / 1710 cm⁻¹ characteristic peak reached 0.98, the dehydration and ring-closure reaction was deemed successful. After the reaction was completed, the material underwent a cooling and pelletizing process to finally obtain a light yellow granular ordinary BMI product. All basic indicators met the standard requirements for the use of conventional BMI resin. The basic chemical reaction flow is as follows:

[0037] 2 Mol maleic anhydride + 1 Mol aromatic diamine (H2N-Ar-NH2) → [60-90℃, nitrogen protection] → bismaleamic acid prepolymer

[0038] Bismaleimide prepolymer + [365nm UV / photoinitiator] + [vacuum -0.095MPa] → Bismaleimide + 2 H2O

[0039] The measured product yield was 96.8%, the melting point range was 186–188℃, the acid value was 0.08 mg KOH / g, and the ring closure degree was 98.7%.

[0040] Example 2 (Fluorine-modified)

[0041] 196.2 g (2.0 mol) of maleic anhydride was weighed, and 4,4′-diaminodiphenyl ether (4,4′-ODA) in Example 1 was replaced with an equal amount of 2,2-bis(4-aminophenyl)hexafluoropropane (TFDB). The feed amount was 320.2 g (1.0 mol). After the pre-melting kettle was purged with nitrogen three times, the pre-melting temperature was raised to 85°C, and the stirring speed was maintained at 150 rpm. The reaction was carried out at a constant temperature for 20 minutes to obtain molten ammonium acid intermediate. The power density of the UV LED strip in the vacuum dehydration closed-loop zone was adjusted to 110 mW / cm². The type, addition ratio and amount of photoinitiator were completely consistent with those in Example 1. The vacuum degree was maintained at -0.095 MPa, and the material residence time was maintained at 60 seconds. After the reaction was judged to be qualified by NIR online monitoring, the product was cooled and granulated to obtain fluorinated BMI granules. The reaction principle was the same as in Example 1.

[0042] The product has a measured yield of 95.5%, a melting point range of 202–204℃, an acid value of 0.09 mg KOH / g, and a ring closure degree of 98.5%. When copolymerized and cured with diallyl bisphenol A at a mass ratio of 1:1, the resulting cured product exhibits a glass transition temperature of 310℃ as determined by DMA, a dielectric constant of 3.15 at 10 GHz high frequencies, and a dielectric loss of 0.0032. These excellent low-dielectric-loss properties make it suitable for direct application in the field of 5G millimeter-wave communication substrates.

[0043] Example 3 (Phosphorus-containing flame-retardant type)

[0044] 196.2 g (2.0 mol) of maleic anhydride was weighed, and the diamine raw material was replaced with a self-made phosphorus-containing diamine DOPO-ODA, with a feed amount of 462.3 g (1.0 mol). After adding it to a 1 L pre-melting kettle, three nitrogen purgings were performed. The temperature was raised to 90℃, and the reaction was stirred at 150 rpm for 20 minutes to obtain a molten ammonium acid intermediate. The temperature of each zone of the extruder, the vacuum degree of -0.095 MPa, and the material flow rate of 10 kg / h were all consistent with those in Example 1. The original photoinitiator was replaced with a 2-hydroxy-2-methylphenylacetone / triethylamine compound system with a mass ratio of 1:2 and the total addition amount was adjusted to 1.5 g. The UV power density was maintained at 100 mW / cm², the material residence time was 60 seconds, and after the NIR monitoring showed that the reaction was qualified, it was cooled and pelletized to obtain phosphorus-containing flame-retardant BMI particles. The reaction principle was the same as in Example 1. The product has a yield of 94.2%, a melting point range of 195–197℃, and an acid value of 0.10 mg KOH / g. After curing, it achieves a V-0 rating in the UL-94 vertical burning test, a limiting oxygen index of 35%, and a 5% weight loss temperature of 425℃ according to TGA thermogravimetric analysis. It combines excellent halogen-free flame retardancy with thermal stability, fully meeting the stringent flame retardant requirements of aerospace composite materials.

[0045] Example 4 (Adamantane Modification)

[0046] 196.2 g (2.0 mol) of maleic anhydride was weighed, and the diamine raw material was replaced with 1,3-bis(4-aminophenyl)adamantane (BAPADM). The feed amount was 348.5 g (1.0 mol). After adding it to the pre-melting kettle, nitrogen was purged three times, the temperature was raised to 85°C, and the stirring speed was 150 rpm for 20 minutes to obtain a high-viscosity molten ammonium acid intermediate. To ensure stable conveying of high-viscosity materials, the speed of the twin-screw extruder was increased to 250 rpm. The temperature zones of the extruder, the vacuum degree of -0.095 MPa, and the material flow rate of 10 kg / h remained unchanged. The UV power density was increased to 120 mW / cm². Considering that the increased material viscosity would slow down the reaction mass transfer, the residence time of the material in the extruder was extended to 80 seconds. The type and amount of photoinitiator were the same as in Example 1. After the NIR monitoring showed that the reaction was qualified, the material was cooled and pelletized to obtain adamantane-modified BMI particles. The reaction principle was the same as in Example 1. The product has a yield of 93.6%, a melting point range of 215–217℃, and an acid value of 0.07 mg KOH / g. After curing, it undergoes a damp heat resistance test. After being placed at 85℃ and 85% relative humidity for 168 hours, the water absorption rate is only 0.45%, which is much lower than the 1.2% water absorption rate of conventional BMI resin. The moisture resistance is greatly improved, making it suitable for electronic packaging applications in harsh environments with high humidity.

[0047] Example 5 (Biphenyl structure)

[0048] 196.2 g (2.0 mol) of maleic anhydride was weighed, and the diamine raw material was replaced with 4,4′-diaminobiphenyl (BZD) at a feed rate of 184.2 g (1.0 mol). After three nitrogen purgings in the pre-melting kettle, the pre-melting temperature was lowered to 75°C, and the mixture was stirred at 150 rpm for 20 minutes to obtain a molten ammonium acid intermediate. The temperature of each zone of the extruder, the vacuum degree of -0.095 MPa, the material flow rate of 10 kg / h, and the type and amount of photoinitiator were all the same as in Example 1. The UV power density was lowered to 85 mW / cm², the material residence time was maintained at 60 seconds, and the reaction was cooled and pelletized after online NIR monitoring to obtain the biphenyl structure BMI granule product. The product has a yield of 97.1%, a melting point range of 238–240℃, an acid value of 0.06 mg KOH / g, and a ring closure degree of 99.1%. Its reaction principle is the same as that in Example 1. The cured product has outstanding mechanical properties, with a tensile strength of 135 MPa and a flexural modulus of 4.2 GPa, which are 20% and 15% higher than those of traditional BMI resin, respectively. It has excellent mechanical strength and rigidity and is suitable for structural materials with high mechanical performance requirements.

[0049] Comparative Example 1 (Thermal Method Comparison)

[0050] BMI resin was synthesized using the traditional solution thermal process disclosed in patent CN119147773A. This process does not use a twin-screw extruder or UV-assisted dehydration; instead, it relies entirely on solvent azeotropic thermal dehydration. 196.2 g of maleic anhydride and 200.2 g of 4,4′-diaminodiphenyl ether (4,4′-ODA) were weighed and dissolved together in 500 mL of a toluene / DMF mixed solvent at a volume ratio of 3:1. After complete dissolution, 3 g of p-toluenesulfonic acid was added as a dehydration catalyst. The mixture was then transferred to a tubular reactor and heated to 140°C for 3 hours. Dehydration was achieved by the formation of an azeotrope between toluene and water. After the reaction, the solvent was removed by vacuum distillation, and the product was obtained by cooling and pulverizing. The product yield obtained by this process was only 92.3%, with an acid value of 0.25 mg KOH / g and a ring closure degree of 93.2%, which was far lower than that of the UV-assisted solvent-free process in Example 1. According to energy consumption calculation, the energy consumption per unit product of this process was 1.25 kWh / kg, while that of Example 1 was only 0.56 kWh / kg, which is 55% higher. After running continuously for 10 batches, the standard deviation of the product acid value reached 0.18 mg KOH / g, indicating poor product quality stability between batches and making it difficult to guarantee consistency in industrial mass production.

[0051] Comparative Example 2 (solvent-free but no UV light)

[0052] 196.2 g of maleic anhydride and 200.2 g of 4,4′-ODA were weighed, pre-melted, purged with nitrogen, and transported by a gear pump. Vacuum thermal dehydration was then carried out under conditions without ultraviolet irradiation. In order to achieve the qualified acid value index, the residence time of the material in the extruder had to be greatly extended to 600 seconds (10 minutes), which is 10 times that of Example 1. The final BMI product had an acid value that could only be reduced to 0.15 mg KOH / g, a ring closure degree of only 94.5%, and the product color was significantly darkened. The color b value increased from 8.5 in Example 1 to 15.3, indicating that the long-term high-temperature thermal reaction led to a significant increase in the oxidation side reaction of the material, and the appearance and internal quality of the product were significantly reduced.

[0053] Comparative Example 3 (Mechanochemical Method Benchmark)

[0054] Following the mechanochemical synthesis of BMI resin reported in *Green Chemistry* (2024, 26, 3185-3194), this method does not employ melt extrusion or UV-assisted processes. Instead, the entire reaction is initiated by ball milling: 196.2 g of maleic anhydride and 200.2 g of 4,4′-diaminodiphenyl ether (4,4′-ODA) are weighed and placed together in a planetary ball mill. The mill speed is set to 500 rpm, the ball-to-material mass ratio is 10:1, and the reaction is carried out at room temperature for 30 minutes. After the reaction, BMI powder is obtained. This process yields only 91.8% of the product, has a high acid value of 0.35 mg KOH / g, and exhibits uneven particle size distribution, with a D90 particle size reaching 45 μm. It cannot be directly used in subsequent injection molding processes and requires additional fine sieving and pulverization processes, significantly increasing production steps and costs. The product purity and processing suitability are far lower than the UV-assisted solventless extrusion process of Example 1.

[0055] Table 1. Comparison of key indicators between the examples and comparative examples

[0056] project Example 1 Example 2 Example 3 Comparative Example 1 Comparative Example 2 Yield (%) 96.8 95.5 94.2 92.3 92.1 Acid value (mg KOH / g) 0.08 0.09 0.10 0.25 0.15 Loop closure degree (%) 98.7 98.5 98.3 93.2 94.5 Reaction time (s) 60 60 60 10800 600 Energy consumption (kWh / kg) 0.56 0.58 0.61 1.25 0.85 Batch CV (%) 1.8 2.0 2.1 12.5 8.3

[0057] Working principle

[0058] The core innovation of this invention lies in introducing a photochemical radical-induced mechanism into the dehydration and ring-closure step of BMI. Benzophenone, under 365 nm UV excitation, transitions to the excited triplet state, abstracting an α-hydrogen from triethanolamine to generate benzyl alcohol radical and triethanolamine radical. This radical complex has strong reducing properties, abstracting a hydrogen atom from the hydroxyl group in maleic acid to form a carbon-centered radical, and inducing the neighboring carboxyl group to leave in the form of H2O molecules, thereby completing the ring closure. The activation energy of this pathway is about 40 kJ / mol lower than that of thermal dehydration, so it can be carried out rapidly at a low temperature of 110℃. The vacuum environment promptly removes the generated water molecules, pushing the equilibrium to the right. The high shear force of the twin-screw extruder ensures thin-layer material flow, with a light penetration depth of 5-8 mm, ensuring a uniform reaction. NIR online monitoring is based on Lambert-Beer's law. The 1660 cm⁻¹ peak corresponds to the C=C stretching vibration of the maleimide ring, and the 1710 cm⁻¹ peak corresponds to the unclosed ring anhydride byproduct. The ratio of the two peaks directly reflects the degree of ring closure, thus achieving feedforward control of quality.

[0059] This invention, through fundamental innovation in process principles, achieves for the first time solvent-free, low-temperature, and second-level continuous synthesis of BMI monomers, solving the industry pain points of high energy consumption, high VOCs, and batch instability associated with traditional methods. Product performance meets or even exceeds imported electronic-grade BMI standards, and equipment investment is only one-third that of traditional batch lines. This technical route can be extended to the synthesis of other imide monomers, showing broad application prospects.

[0060] The above embodiments are merely preferred embodiments of the present invention, but the scope of protection of the present invention is not limited thereto. Any equivalent substitutions or modifications made by those skilled in the art within the scope of the technology disclosed in the present invention, based on the technical solution and inventive concept of the present invention, should be covered within the scope of protection of the present invention.

Claims

1. A method for preparing a series of bismaleimide products, characterized in that, Includes the following steps: S1: Maleic anhydride and aromatic diamine with a molar ratio of 2.0–2.2:1 are added to a pre-melting vessel, heated to 60–90°C under nitrogen protection to melt, and stirred for 10–30 minutes to obtain ammonium acid prepolymer; S2: The amic acid prepolymer is continuously pumped into a twin-screw extrusion reactor at a flow rate of 5–15 kg / h. The reactor is arranged in sequence along the screw length-to-diameter ratio of 40–60:1, including a melt amidation zone, a vacuum dehydration closed-loop zone (or imidization zone), and a cooling and pulverizing zone. The temperature of each zone is independently controllable. S3: A 365 nm ultraviolet LED array with a light intensity of 80–120 mW / cm² is installed circumferentially on the reactor wall in the vacuum dehydration closed-loop zone, and a photoinitiator system of 0.3–1.0 wt% is added. The amyl acid is dehydrated and closed under ultraviolet light induction for 30–90 seconds at 110±5℃ and a vacuum degree of -0.090 to -0.098MPa, to obtain crude bismaleimide with an acid value ≤0.1 mg KOH / g and a ring closure degree ≥98.5%. S4: The crude product is continuously discharged after being cooled to 30–50°C in the cooling and pulverizing zone. By switching between aromatic diamines with different structures, a series of bismaleimide products with structures as shown in formula (I) can be obtained in the same equipment.

2. The method for preparing a series of bismaleimide products according to claim 1, characterized in that, The aromatic diamine is selected from at least one of 4,4′-diaminodiphenyl ether, 4,4′-diaminodiphenylmethane, 3,3′-diaminodiphenyl sulfone, 2,2-bis(4-aminophenyl)hexafluoropropane, 1,5-naphthyldiamine or phosphorus-containing diamine DOPO-ODA.

3. The method for preparing a series of bismaleimide products according to claim 1, characterized in that, The photoinitiator system is a mixture of benzophenone and triethanolamine in a mass ratio of 1:1–1.5, or a mixture of 2-hydroxy-2-methylphenylacetone and triethylamine in a mass ratio of 1:2–3.

4. The method for preparing a series of bismaleimide products according to claim 1, characterized in that, The twin-screw extrusion reactor is equipped with a near-infrared online monitor at the end of the vacuum dehydration closed-loop zone to monitor the peak area ratio of the characteristic peak of the C=C double bond at 1660 cm⁻¹ to the peak of the acid anhydride byproduct at 1710 cm⁻¹. When the ratio is ≥0.97, the closed-loop reaction is determined to be complete; otherwise, the ultraviolet light intensity is automatically adjusted or the residence time is extended.

5. The method for preparing a series of bismaleimide products according to claim 1, characterized in that, The temperature of the melt amidation zone is controlled at 70–80℃, the temperature of the vacuum dehydration closed-loop zone is controlled at 105–115℃, the cooling and pulverizing zone is cooled to 30–50℃ by circulating water, and the screw speed is 100–300 rpm.

6. The method according to claim 1, characterized in that, A vacuum flash evaporation devolatilization device is added after the cooling and pulverizing zone, and the photoinitiator is treated at 120–130℃ and -0.099 MPa for 1–2 minutes to ensure that the residual amount of photoinitiator is ≤100 ppm.

7. The method for preparing a series of bismaleimide products according to claim 1, characterized in that, The obtained bismaleimide is copolymerized with diallylbisphenol A or allylphenyl compound at a mass ratio of 1:0.8–1.

2. The cured product has a glass transition temperature ≥280℃ and a dielectric constant ≤3.5 (10 GHz), making it suitable for 5G high-frequency substrate materials.

8. The method according to any one of claims 1-7, characterized in that, The yield of the bismaleimide product is ≥96%, the purity is ≥99.2% (HPLC area normalization method), and the melting point fluctuation range is within ±2℃.

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