Biimidazole catalyst for synthesizing liquid crystal polyarylester and application of biimidazole catalyst

Abstract

The invention belongs to the field of liquid crystal polyarylester synthesis, and particularly relates to a biimidazole catalyst for liquid crystal polyarylester synthesis and application of the biimidazole catalyst. The catalyst is a biimidazole compound, and the thermal stability of the catalyst is quantitatively evaluated. The Gibbs free energy at the second set temperature is lower than the Gibbs free energy at the first set temperature; wherein the second set temperature is higher than the first set temperature. The specific biimidazole catalyst obtained through screening shows excellent thermal stability in synthesis of II-type liquid crystal polyarylester and can keep activity in the high-temperature melt polycondensation process of 200-350 DEG C, so that the reaction time is shortened, side reactions caused by catalyst decomposition are effectively inhibited, and the yield of II-type liquid crystal polyarylester is increased. Finally, the prepared liquid crystal polyarylester product has higher intrinsic viscosity and higher L value, and the quality of the product is improved.

Description

Technical Field

[0001] This invention belongs to the field of synthesis of liquid crystal polyarylates, and specifically relates to a biimidazole catalyst for the synthesis of liquid crystal polyarylates and its application. Background Technology

[0002] Liquid crystalline polyarylates (LCPs) are intermediate polymers between solid crystals and liquids. When LCP exists as a liquid crystal phase, it has low viscosity and high orientation; upon cooling and solidification, its morphology remains stable. This property endows LCP materials with a range of excellent properties, such as high strength, high modulus, excellent molding and processing performance, outstanding heat resistance, low water absorption, excellent flame retardancy, extremely low coefficient of linear expansion, excellent flame resistance, electrical insulation, chemical corrosion resistance, weathering resistance, microwave transmittance, and low dielectric constant and dielectric loss factor. These characteristics have led to their widespread application in high-tech industries such as electronics, automotive parts, aerospace, and defense. Type II liquid crystalline polyarylates, by introducing some flexible segments, significantly improve processing performance while maintaining high performance, exhibiting a wider processing window, higher toughness, and controllable dielectric properties, giving them unique advantages in high-end fields such as high-frequency circuit boards and 5G communication equipment.

[0003] Melt polycondensation is the main method for the industrial synthesis of type II LCPs, widely used due to its advantages such as high degree of polymerization, speed, and simple equipment. The reactant monomer is typically p-acetoxybenzoic acid, which reacts with its acetoxy and carboxyl groups in the molten state to form a polymer, releasing acetic acid as a byproduct. Currently, major domestic and international manufacturers all adopt this technology. The significant advantages of this process are that it eliminates the need for solvents, reducing environmental pollution and recycling costs, and the process flow is simple with relatively low equipment investment. Simultaneously, the product has high purity, and the molten product can be directly obtained for subsequent processing. However, this process also has several problems. First, the reaction conditions are harsh, requiring prolonged high temperatures, leading to high energy consumption. Second, side reactions are difficult to control; at high temperatures, end-group decomposition, molecular chain breakage, and cross-linking are prone to occur. Furthermore, the viscosity of the reaction system increases sharply with the degree of polymerization during the polycondensation stage, easily causing difficulties in stirring and uneven mass transfer, thus affecting the molecular weight distribution of the polymer and consequently the material properties. These defects result in large batch-to-batch performance fluctuations, becoming a major factor restricting the industry's development. Oligomers can be obtained through melt polycondensation, and LCPs of the desired molecular weight can be obtained through solid-state polymerization under nitrogen or vacuum conditions (CN109535403A). However, solid-state polymerization is time-consuming, energy-intensive, and the molecular weight distribution is uncontrollable, resulting in limited improvement. Currently, the industry is working to develop new catalyst systems to address these issues.

[0004] Besides metal compounds, most commonly used catalyst systems are organic base catalysts. According to their reaction mechanisms, the basicity and nucleophilicity of the catalyst can effectively promote the reaction, especially imidazole catalysts such as N-methylimidazole (CN116199865A). However, N-methylimidazole has a boiling point of only 198 °C. Under the high-temperature conditions of type II LCP synthesis, it will volatilize, break bonds, undergo alkyl substitution or dimerization, resulting in catalytic failure. This seriously affects the low polymerization efficiency, leading to long reaction time, increased side reactions, and ultimately affecting the performance of the final polymer. Summary of the Invention

[0005] This application aims to overcome the shortcomings of existing technologies where catalysts used in the synthesis of liquid crystal polyarylates have insufficient thermal stability at high temperatures, leading to decreased catalytic efficiency, easy initiation of side reactions, and impact on product quality. It provides a biimidazole catalyst for the synthesis of liquid crystal polyarylates and its application.

[0006] To achieve the above-mentioned objectives, the present invention is implemented through the following technical solution: In a first aspect, the present invention provides a biimidazole catalyst for the synthesis of liquid crystal polyarylates. The catalyst is a biimidazole compound, and its thermal stability has been quantitatively evaluated, resulting in a Gibbs free energy at the second set temperature that is lower than that at the first set temperature; wherein, The second set temperature is higher than the first set temperature.

[0007] Preferably, the biimidazole compound has a structure as shown in formula (I), (II), or (III): Formula (I); Formula (II); Formula (Ⅲ); Among them, the R substituent can be hydrogen, alkyl, alkoxy, or halogen; R ， Examples include alkyl, phenyl, biphenyl, benzyl, pyridyl, pyridinyl, and pyrazinyl.

[0008] Preferably, the R substituent can be any one of hydrogen, methyl, ethyl, propyl, isopropyl, hexyl, methoxy, ethoxy, fluorine, chlorine, and bromine, including but not limited to the listed substituents, and the R substituent can independently replace one or more hydrogen atoms on the conjugated ring. R ， It can be any one of methyl, ethyl, propyl, phenyl, biphenyl, benzyl, pyridyl, pyrazinyl, or pyrazinyl, including but not limited to the listed substituents.

[0009] Preferably, the quantitative evaluation includes the following steps: S1. Construct molecular models of candidate catalysts, optimize their geometry and analyze their vibrational frequencies to obtain the optimized molecular configuration; S2. Based on the optimized molecular configuration, calculate the Gibbs free energy of the candidate catalyst at different temperatures; S3. Using the Gibbs free energy at the first set temperature as a benchmark, calculate the relative Gibbs free energy change of the candidate catalyst at the second set temperature, and evaluate the thermal stability of the candidate catalyst based on the relative Gibbs free energy change; wherein, a more negative value of the relative Gibbs free energy change indicates higher thermal stability.

[0010] As shown in the background section, existing screening methods primarily focus on assessing the initial activity of catalysts, such as predicting reaction rates by calculating their interaction energy with monomers or simulating simple reaction barriers. However, these methods often use 0 K or a specific temperature for simulation, neglecting the structural integrity of the catalyst itself during long-term operation at high temperatures of 200 to 350 °C in actual polymerization. A catalyst exhibiting high activity at 0 K may decompose or deactivate in the later stages of the reaction if its thermal stability is insufficient. This not only leads to a decrease in the polymerization rate but also degrades the color and properties of the final polymer due to impurities introduced by the decomposition products. Therefore, developing a screening method that can directly and effectively assess the thermal stability of catalysts is crucial for improving the product quality and process reliability of liquid crystal polyarylates.

[0011] The technical solution of this invention is conceived based on the aforementioned clearly defined technical problems, and it ingeniously selects the "Gibbs free energy," a thermodynamic state function, as the core evaluation index. Since the Gibbs free energy is directly related to the thermodynamic stability of matter, its change can sensitively reflect the stability trend of molecular structure with increasing temperature.

[0012] Specifically, this method first obtains the stable configuration of the candidate catalyst molecule through standard computational chemistry steps (S1), which ensures the fundamental reliability of subsequent calculations. Then (S2), instead of simulating complex catalytic reaction pathways, it calculates the Gibbs free energy of the molecule itself at different temperatures (especially the high-temperature range covering the actual polymerization temperature). The most crucial step is (S3), where this method introduces the concept of "relative Gibbs free energy change," that is, using the free energy at a reference temperature (e.g., 298 K) as a reference to examine the magnitude of its change at the target high temperature.

[0013] Compared to existing technologies, this invention represents a significant technological advancement. For the first time, it shifts the focus of screening from the "activity" of the catalyst to its intrinsic "thermal stability," which better aligns with the stringent requirements for catalyst durability in the high-temperature synthesis of liquid crystal polyarylates. By calculating the relative change in Gibbs free energy—a simple and physically clear parameter—this method avoids the enormous computational costs and uncertainties associated with constructing complex multi-scale reaction models, providing a more direct and efficient screening path. Catalysts screened using this method are expected to maintain more stable catalytic performance during actual polymerization, thereby helping to shorten reaction time, significantly improve the hue of the final polymer, and enhance product consistency and high-end application potential.

[0014] Preferably, in step S2, the different temperatures include temperature points in the range of 0-1000 K; In step S3, the first temperature is 298 K and the second temperature is 300-1000 K.

[0015] Secondly, the present invention also provides an application of the aforementioned biimidazole catalyst in the synthesis of liquid crystal polyarylates. The application includes using the biimidazole catalyst in the melt polycondensation reaction of liquid crystal polyarylates to directly obtain products with the corresponding intrinsic viscosity.

[0016] Thirdly, the present invention also provides a method for preparing liquid crystal polyarylate, comprising acetylation of an aromatic hydroxycarboxylic acid in the presence of the aforementioned biimidazole catalyst, followed by melt polycondensation after acetylation, and obtaining the liquid crystal polyarylate after the reaction is completed.

[0017] As a preferred option, the specific preparation method is as follows: A certain proportion of monomers, acylation reagents, and specific catalysts are added to a Hastelloy reactor. The reactor is then purged with nitrogen to fully replace the atmosphere with nitrogen protection, and an acetylation reaction is carried out under nitrogen protection. After acetylation, the reaction system is slowly and uniformly heated to the temperature at which byproduct acetic acid and excess unreacted acetic anhydride are distilled off. Once the amount of acetic acid distilled off reaches more than 90% of the theoretical acetic acid yield, the temperature is raised to the polycondensation temperature. The reaction continues until the distilled byproduct acetic acid reaches 90% of the theoretical yield, at which point the system pressure is reduced. The reaction is terminated when a specific torque is reached, yielding a liquid crystal copolyester of the desired viscosity.

[0018] Preferably, the liquid crystal polyarylate is a type II liquid crystal polyarylate obtained by copolymerizing p-hydroxybenzoic acid and 6-hydroxy-2-naphthoic acid.

[0019] Preferably, the temperature of the acetylation reaction in the polymerization step is 100-150 °C, and the reaction time is 0.5-2 hours; The temperature of the melt polycondensation reaction is 200-350 ℃.

[0020] Preferably, the amount of catalyst added is 10-1000 ppm of the total mass of the monomer reactants.

[0021] Preferably, the pressure of the melt polymerization reaction system is between 0.01 and 100 kPa.

[0022] Therefore, the present invention has the following beneficial effects: (1) The catalyst screening method provided by the present invention evaluates the thermal stability by calculating the relative Gibbs free energy of candidate catalysts at different temperatures, providing a more direct and reliable theoretical prediction tool for screening catalysts suitable for high-temperature polymerization processes, which helps to significantly reduce the blindness and cost of experimental screening. (2) The specific biimidazole catalysts obtained by this method exhibit excellent thermal stability in the synthesis of liquid crystal polyarylates (especially type II). They can maintain activity during high-temperature melt polycondensation at 200-350 °C, which helps to shorten the reaction time and effectively suppress side reactions caused by catalyst decomposition. Ultimately, the prepared liquid crystal polyarylate products have higher intrinsic viscosity and higher L value, thus improving the quality of the products. Attached Figure Description

[0023] Figure 1 These are models of different catalysts used in this invention.

[0024] Figure 2 This is a graph showing the temperature-Gibbs free energy curves for different catalysts in this invention.

[0025] Figure 3 This is a graph showing the reaction time and distillate volume under different catalysts in this invention. Detailed Implementation

[0026] The present invention will be further described below with reference to specific embodiments. Those skilled in the art will be able to implement the present invention based on these descriptions. Furthermore, the embodiments of the present invention described below are generally only some, not all, of the embodiments of the present invention. Therefore, all other embodiments obtained by those skilled in the art based on the embodiments of the present invention without inventive effort should fall within the scope of protection of the present invention.

[0027] In this embodiment, the viscosity of the liquid crystal polyarylate is measured using an automatic viscometer. The instruments and testing conditions used for obtaining viscosity data are listed below: Test solvent: pentafluorophenol / 1,1,2,2-tetrachloroethane; Test concentration: 1 mg / mL.

[0028] In this embodiment, the color value of the liquid crystal polyarylate is detected using a colorimeter. The instruments and testing conditions used for color value data are listed below: Test instrument: Hunterlab LabScan XE.

[0029] Unless otherwise specified, all goods or reagents used in this invention were purchased through market channels. Specifically: p-Hydroxybenzoic acid: HBA, purchased from Shengxiao Company; 6-Hydroxy-2-naphthoic acid: HNA, purchased from Shengxiao Company; Acetic anhydride: Ac2O, purchased from Sinopharm Group; 1,1'-Dimethyl-1H,1'H-[2,2']biimidazole (catalyst 1), 2,6-bis-(1H-imidazol-2-yl)pyridine (catalyst 2), 3,6-bis(1H-imidazol-1-yl)pyridazine (catalyst 3), N-methylimidazolium (comparative catalyst): purchased from Adamas.

[0030] Example Catalyst stability calculation: S1. Using Materials Studio software, reasonable molecular models were established for catalyst 1, catalyst 2, catalyst 3, and the control catalyst. Based on the included H, C, and N atoms, GGA / PBE functionals were selected, and DNP was used as the computational basis set for structure optimization calculations. The energy convergence criterion was 1.0 e. -5 Ha, after the calculation converges, the optimized model is obtained. Calculate the vibration frequency. If there is an imaginary frequency, repeat the structural optimization and vibration frequency calculation steps until the vibration frequency calculation shows no imaginary frequencies, then determine it as the optimal model.

[0031] S2. For the optimal model of the catalyst described in S1, the calculated functional, basis set, and calculation accuracy are the same as in S1. The calculation task is geometry optimization + frequency. By submitting a task, the Gibbs free energy at different temperatures (0-1000 K) can be obtained, achieving efficient calculation.

[0032] S3. Based on the Gibbs free energy at 298 K, calculate the relative Gibbs free energy at other temperatures for the four catalysts. Compare the relative Gibbs free energies of the four catalysts at the same temperature, compare the data, and determine the stability of the catalysts.

[0033] Table 1. Gibbs free energy of the catalyst at 298 K

[0034] Synthesis of Type II liquid crystal polyarylate: The monomer, acetic anhydride, and a specific catalyst were added to a reactor equipped with a stirrer, nitrogen inlet device, vacuum system, thermometer, and reflux cooling device. The reactor was purged with nitrogen to fully replace the nitrogen atmosphere. Under a nitrogen flow, the temperature was initially increased to 140 °C at 2 °C / min for 2 hours for acetylation. After acetylation, the temperature was increased again at 2 °C / min to initiate a polycondensation reaction. A certain conversion rate was first achieved at 240 °C, then the temperature was increased to 330 °C, and the reactor was evacuated to 0.1 kPa for final polycondensation. The reaction was terminated when the torque reached the desired value. The resulting liquid crystal copolyester was then removed and analyzed using an automatic viscometer and colorimeter.

[0035] Table 2. Feed amounts and reaction times for the examples and comparative examples.

[0036] Note: The polycondensation reaction time refers to the time from when the temperature starts to rise from 240 ℃ until the reaction ends.

[0037] Table 3 Test results of the obtained slices .

[0038] The Gibbs free energy at 298 K for each catalyst was calculated using Materials Studio software. A more negative Gibbs free energy value indicates greater stability under the same temperature conditions. Table 1 and... Figure 2 The data clearly show that the stability of biimidazole catalysts is higher than that of N-methylimidazole catalysts, especially 2,6-bis-(1H-imidazol-2-yl)-pyridine (catalyst 2) and 3,6-bis(1H-imidazol-1-yl)pyridazine (catalyst 3), which have much higher stability than N-methylimidazole, especially under high temperature conditions, where their stability is significantly better.

[0039] Based on the calculation results, it can be concluded that at the higher synthesis temperature of LCP, biimidazole catalysts are more stable than traditional N-methylimidazole catalysts, enabling sustained and efficient catalytic polymerization reactions. This is also evidenced by the polymerization data. Firstly, regarding the polycondensation reaction time, under the same feed ratio and polymerization process, the polycondensation reaction times of Examples 1-3 are significantly shorter than those of Comparative Example 1, indicating that the catalysts in the examples have better catalytic performance at high temperatures than the catalysts in the comparative example, resulting in a faster reaction rate under the same conditions and making it easier to achieve the desired power or viscosity of the material. Furthermore, the comparison of the examples also shows that catalysts 2 and 3, which have better stability, have shorter polycondensation reaction times than catalyst 1, further demonstrating the important role of catalyst stability in the polycondensation reaction.

[0040] Secondly, the color value data also shows that the color results of Examples 1-3 are significantly better than those of the comparative example. Side reactions during polymerization, such as the decomposition of acyl groups to produce ketone derivatives with a certain color, and the decarboxylation reaction to produce phenol, also exhibits a certain color under high temperature or oxidative conditions, are also possible. Furthermore, oxidation and thermal degradation may occur under high temperature conditions, affecting the color value of the polymer. Catalysts play a role in accelerating the reaction rate during polycondensation, effectively reducing the overall polycondensation reaction time and the residence time of the polymer melt at high temperatures, thereby reducing side reactions and improving the final polymer color. The catalysts in the examples, especially catalysts 2 and 3, exhibit better stability than other catalysts. Even under the high temperature conditions of the polycondensation reaction, they still have a good catalytic effect on the reaction, resulting in superior polymer color value data. The L value is higher than that of the comparative example, while the a and b values ​​are lower than those of the examples, indicating a superior color.

[0041] Furthermore, intrinsic viscosity data also demonstrates the excellent catalytic effect of the catalyst selected in this patent. There is a quantitative relationship between intrinsic viscosity and the molecular weight of the polymer; generally, the higher the intrinsic viscosity, the larger the molecular weight. Although the examples and comparative examples were produced under the same torque conditions, there were still some differences in the intrinsic viscosity tests of the polymers. The viscosity of the examples was slightly higher than that of the comparative examples, indicating that the molecular weight of the examples was slightly higher than that of the comparative examples. This means that the catalyst in the examples had a superior catalytic effect, effectively promoting molecular chain growth and increasing the molecular weight of the polymer. Examples 2 and 3 were slightly higher than Example 1, further illustrating this point.

[0042] In the examples, the polycondensation reaction times and test results of Examples 2 and 3 are quite similar because their structures are similar, and their stability calculation results also highly overlap, indicating that the catalytic effects of catalysts 2 and 3 are quite consistent, corresponding to the obtained slice results. Furthermore, the catalyst addition amount in the examples was 200 ppm, while the addition amount in the comparative example was 400 ppm, twice that of the examples. The overall catalytic effect of the comparative example was far lower than that of the examples, further demonstrating the excellent catalytic effect and high-temperature stability of the catalysts in the examples.

[0043] Also attached Figure 3 A graph showing the relationship between reaction time and distillate volume under different catalysts is provided. The graph clearly shows that the overall distillation time and distillate volume of Examples 2 and 3 are quite similar, indicating that the catalytic effects of catalysts 2 and 3 are consistent, which is consistent with the results calculated using Materials Studio software. Furthermore, the distillate volumes of catalysts 1, 2, and 3 are significantly faster than those of the control catalyst, suggesting that the control catalyst may have decomposed or volatilized due to its poor stability in the higher-temperature polycondensation reaction, failing to provide the expected catalytic effect, thus slowing down the subsequent polymerization rate and affecting the final polymer properties.

[0044] In summary, this invention provides a novel organic base catalyst for the synthesis of liquid crystal polyarylates. This biimidazolium catalyst, while retaining the existing imidazole ring, possesses a more stable structure, higher decomposition temperature, higher boiling point, and more active sites. It can continuously and efficiently catalyze the melt polymerization of liquid crystal polyarylates, directly obtaining the melt of the desired viscosity. Furthermore, it features sustained polymerization effect, fewer polymerization side reactions, and a one-step process to obtain liquid crystal polyarylates with excellent color values. Simultaneously, this invention proposes a theoretical calculation method for determining catalyst stability. This calculation method is applicable to the stability calculation of catalysts in polymerization reactions and has a certain degree of general applicability.

Claims

1. A biimidazolium-based catalyst for the synthesis of liquid crystal polyarylates, characterized in that, The catalyst is a biimidazole compound, and its thermal stability has been quantitatively evaluated, resulting in a Gibbs free energy at the second set temperature that is lower than that at the first set temperature; wherein, The second set temperature is higher than the first set temperature.

2. The catalyst according to claim 1, characterized in that, The biimidazole compounds have structures as shown in formula (I), (II), or (III): Equation (I); Formula (II); Formula (Ⅲ); Among them, the substituent R can be hydrogen, alkyl, alkoxy, or halogen; R ， Examples include alkyl, phenyl, biphenyl, benzyl, pyridyl, pyridinyl, and pyrazinyl.

3. The screening method as described in claim 1 or 2, characterized in that, The quantitative evaluation includes the following steps: S1. Construct molecular models of candidate catalysts, optimize their geometry and analyze their vibrational frequencies to obtain the optimized molecular configuration; S2. Based on the optimized molecular configuration, calculate the Gibbs free energy of the candidate catalyst at different temperatures; S3. Using the Gibbs free energy at the first set temperature as a benchmark, calculate the relative Gibbs free energy change of the candidate catalyst at the second set temperature, and evaluate the thermal stability of the candidate catalyst based on the relative Gibbs free energy change; wherein, a more negative value of the relative Gibbs free energy change indicates higher thermal stability.

4. The screening method as described in claim 3, characterized in that, In step S2, the different temperatures include temperature points within the range of 0-1000 K; In step S3, the first temperature is 298 K and the second temperature is 300-1000 K.

5. The application of a biimidazole catalyst according to any one of claims 1-4 in the synthesis of liquid crystal polyarylates, characterized in that, The application includes using the biimidazole catalyst in the melt polycondensation reaction of type II liquid crystal polyarylate to directly obtain a product with the corresponding intrinsic viscosity.

6. A method for preparing liquid crystal polyarylate, characterized in that, The reaction includes acetylation of an aromatic hydroxycarboxylic acid in the presence of a biimidazole catalyst as described in any one of claims 1 to 4, followed by melt polycondensation after acetylation, and the liquid crystal polyarylate with the intrinsic viscosity is directly obtained after the reaction.

7. The method as described in claim 6, characterized in that, The liquid crystal polyarylate is a type II liquid crystal polyarylate obtained by copolymerizing p-hydroxybenzoic acid and 6-hydroxy-2-naphthoic acid.

8. The method as described in claim 6 or 7, characterized in that, The acetylation reaction in the polymerization step is carried out at a temperature of 100-150 °C for 0.5-2 hours. The temperature of the melt polycondensation reaction is 200-350 ℃.

9. The method as described in claim 6 or 7, characterized in that, The amount of catalyst added is 10-1000 ppm of the total mass of the monomer reactants.

10. The method as described in claim 6 or 7, characterized in that, The pressure of the melt polymerization reaction system is 0.01-100 kPa.

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