Imidazopyridine catalyst for synthesizing liquid crystal polyarylester and application of imidazopyridine catalyst

Abstract

The invention relates to the field of polymer preparation, in particular to an imidazopyridine catalyst for liquid crystal polyarylester synthesis and application of the imidazopyridine catalyst, the catalyst is an imidazopyridine 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. According to the imidazopyridine catalyst provided by the invention, the thermal stability at a high temperature is evaluated by calculating the relative Gibbs free energy of the imidazopyridine catalyst at different temperatures, so that the imidazopyridine catalyst shows excellent thermal stability in melt polymerization of liquid crystal polyarylester (particularly II type), and can keep activity in a high-temperature melt polycondensation process of 200-350 DEG C; the liquid crystal polyarylester has the advantages that the reaction time is shortened, the side reaction is effectively inhibited, the finally prepared liquid crystal polyarylester product has higher color value (Lgt; 75), and the quality of the product is improved.

Description

Technical Field

[0001] This invention relates to the field of polymer preparation, and more particularly to an imidazopyridine catalyst for the synthesis of liquid crystal polyarylates and its application. Background Technology

[0002] In the melt polycondensation process for preparing liquid crystal polyarylates, the choice of catalyst is one of the key factors affecting polymer performance. Highly active catalysts can effectively lower the activation energy of the reaction, accelerate the polymerization rate, and thus reduce the residence time of the polymer in the high-temperature molten state. This helps to reduce the occurrence of side reactions such as thermal degradation and has a decisive impact on the molecular weight, color, and processing properties of the final product.

[0003] Currently, commonly used organic catalysts in this field include nitrogen-containing heterocyclic compounds such as N-methylimidazole. However, in actual industrial production, these catalysts still face some challenges. For example, most catalysts contain only a single imidazole ring in their molecular structure, resulting in a single active site. More importantly, some catalysts lack sufficient thermal stability under the high-temperature polycondensation environment required for liquid crystal polyarylates, and may decompose or volatilize. This not only leads to a decrease in catalytic efficiency at high temperatures, but may even require subsequent solid-state polymerization processes to compensate for the insufficient molecular weight, increasing process complexity and cost. Furthermore, catalyst decomposition products or excessive residence time at high temperatures may trigger more side reactions, affecting the polymer color.

[0004] To screen for more efficient catalysts, researchers have explored various evaluation methods. Kinetic experiments are a traditional approach, indirectly assessing catalyst performance by monitoring the composition and concentration of gaseous products during the reaction. However, in the polycondensation reaction of liquid crystal polyarylates, the complex gas phase composition and the fact that major byproducts are not always direct indicators of catalytic activity limit the accuracy of this method, while also being time-consuming and resource-intensive. Computer-aided screening shows promise as an alternative, for example, predicting activity by calculating the adsorption energy or reaction barrier of small molecules on the catalyst surface. These methods are effective for systems with relatively simple reactants and mechanisms, but their applicability is challenged in complex multi-step polymerization reactions such as the synthesis of liquid crystal polyarylates. This reaction involves a wide range of temperature variations and involves small molecule cyclization, oligomer conversion, and small molecule chain growth, making it unsuitable for using the reaction barrier or adsorption energy at a single temperature to reflect catalyst activity.

[0005] Therefore, developing a screening method that can balance computational efficiency and effectively reflect the long-term thermal stability of catalysts in complex polymerization systems, especially under high-temperature conditions, is of great significance for advancing the research and development of catalysts for high-performance liquid crystal polyarylates. Summary of the Invention

[0006] 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 an imidazopyridine catalyst for the synthesis of liquid crystal polyarylates and its application.

[0007] To achieve the above-mentioned objectives, the present invention is implemented through the following technical solution: In a first aspect, the present invention provides an imidazopyridine catalyst for the synthesis of liquid crystal polyarylates. The catalyst is an imidazopyridine 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.

[0008] Preferably, the imidazopyridine compound has a structure as shown in formula (I) or formula (II): Formula (I); Formula (II); R is selected from hydrogen, alkyl, alkoxy, or halogen.

[0009] Preferably, R can be hydrogen, methyl, ethyl, propyl, isopropyl, methoxy, ethoxy, fluorine, chlorine, bromine, etc., including but not limited to the substituents listed, and each of the R substituents can independently replace one or more hydrogen atoms on the conjugated ring.

[0010] 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.

[0011] 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 energies with monomers or simulating simple reaction barriers. However, these methods often neglect the structural integrity of the catalyst itself during prolonged operation at high temperatures of 200 to 350 °C in actual polymerization. A catalyst exhibiting high activity at room temperature 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.

[0012] 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.

[0013] 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., 298.15 K, or 25 °C) as a reference, examining the magnitude of its change at the target high temperature.

[0014] 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.

[0015] Preferably, in step S2, the different temperatures include temperature points in the range of 0-1000 K; In step S3, the first temperature is 298.15 K and the second temperature is 300-1000 K.

[0016] Secondly, the present invention also provides an application of the catalyst in the synthesis of liquid crystal polyarylates. The application includes using the catalyst in the melt polycondensation reaction of liquid crystal polyarylates.

[0017] Thirdly, the present invention also provides a method for preparing liquid crystal polyarylate, comprising an acetylation reaction of an aromatic hydroxycarboxylic acid in the presence of the catalyst, followed by a melt polycondensation reaction after acetylation, and obtaining the liquid crystal polyarylate after the reaction is completed.

[0018] 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 corrosion-resistant reactor. The reactor is then purged with nitrogen to fully replace the atmosphere with nitrogen protection. Under nitrogen protection, the temperature is raised to carry out an acetylation reaction for 10 minutes to 2 hours. 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.

[0019] Preferably, the liquid crystal polyarylate is a type II liquid crystal polyarylate obtained by polymerizing p-hydroxybenzoic acid and 6-hydroxy-2-naphthoic acid.

[0020] Preferably, the temperature of the acylation reaction in the polymerization step is 100-150 °C.

[0021] Preferably, the temperature of the melt polycondensation reaction is 200-350 °C.

[0022] Preferably, the pressure of the melt polymerization reaction system is between 0.01 and 100 kPa.

[0023] Preferably, the amount of catalyst added is 10-1000 ppm of the total mass of the reactants.

[0024] 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 imidazopyridine 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 color values ​​(L > 75), thus improving the quality of the products. Attached Figure Description

[0025] Figure 1 These are models of different catalysts used in this invention.

[0026] Figure 2 This is a graph showing the temperature-Gibbs free energy curves for different catalysts in this invention.

[0027] Figure 3 This is a graph showing the reaction time and distillate volume under different catalysts in this invention. Detailed Implementation

[0028] 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.

[0029] In this embodiment, the viscosity of the liquid crystal polyarylate is measured using an automatic viscometer.

[0030] The instruments and testing conditions used for viscosity data can be listed below: Test solvent: pentafluorophenol / 1,1,2,2-tetrachloroethane; Test concentration: 1 mg / mL.

[0031] In this embodiment, the color value of the liquid crystal polyarylate is detected by a colorimeter. The instruments and test conditions used to obtain the color value data can be listed below: Test instrument: Hunterlab LabScan XE.

[0032] Unless otherwise specified, all goods or reagents used in this invention were purchased through market channels, wherein: p-Hydroxybenzoic acid: HBA, purchased from Shengxiao Company; 6-Hydroxy-2-naphthoic acid: HNA, purchased from Shengxiao Company; Acetic anhydride: purchased from Sinopharm Group; 3H-imidazo[4,5-b]pyridine (catalyst a), 3H-imidazo[4,5-c]pyridine (catalyst b), 3-methyl-3H-imidazo[4,5-b]pyridine (catalyst c), N-methylimidazolium (comparative catalyst): purchased from Sigma-Aldrich.

[0033] Example Calculation of the thermal stability of the catalyst: S1. Using Materials Studio software, reasonable molecular models were established for catalysts a, b, and c. Based on the H, C, and N atoms they contain, GGA / PBE functionals were selected, and DNP was chosen as the computational basis set for structure optimization calculations. The energy convergence criterion was 1.0 e. -5 Ha, after convergence, the optimized model is obtained. The vibration frequency is calculated. If imaginary frequencies are present, the structural optimization and vibration frequency calculation steps are repeated until the vibration frequency calculation shows no imaginary frequencies. This is then determined to be the optimal model, and its structure is as follows: Figure 1 As shown.

[0034] 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.

[0035] The calculation results for the three catalysts were used as a benchmark, with the Gibbs free energy at 298.15 K as the baseline. The relative Gibbs free energies at other temperatures were then calculated. The relative Gibbs free energies of the three catalysts at the same temperature were compared, and the numerical values ​​of the Gibbs free energies at the same temperature were compared to determine the thermal stability of the catalysts.

[0036] Table 1. Gibbs free energy of the catalyst at 298.15 K .

[0037] Synthesis of Type II liquid crystal polyarylate: 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 polycondensation. A certain conversion rate was first achieved at 240 °C, then the temperature was increased to 330 °C, and the reactor was evacuated to 100 Pa 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.

[0038] Table 2. Feed amounts and reaction times for the examples and comparative examples.

[0039] Note: The polycondensation reaction time refers to the time from when the temperature starts to rise from 240 ℃ until the reaction ends.

[0040] Table 3 Test results of the obtained slices .

[0041] 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 thermal stability of imidazopyridine catalysts is higher than that of N-methylimidazolium catalysts, especially 3-methyl-3H-imidazo[4,5-b]pyridine catalyst c, which has a much higher thermal stability than N-methylimidazolium, particularly under high temperature conditions.

[0042] Based on the calculation results, it can be concluded that at the higher synthesis temperature of LCP, imidazopyridine catalysts are more stable than traditional N-methylimidazolium catalysts, enabling sustainable 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 time of Examples 1-3 is significantly shorter than that of Comparative Example 1, indicating that the catalysts in the examples have better catalytic performance at high temperatures than the comparative catalyst, 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 catalyst c, with better thermal stability, has a shorter polycondensation reaction time compared to catalysts a and b, further demonstrating the important role of catalyst thermal stability in the polycondensation reaction.

[0043] 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 certain colors, can also occur under high-temperature conditions, such as oxidation and thermal degradation, affecting the polymer's color value. 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's color. The catalysts in the examples, especially catalyst c, exhibit better thermal 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.

[0044] Furthermore, intrinsic viscosity data also demonstrates the excellent catalytic effect of the catalyst selected in this patent. Intrinsic viscosity has a quantitative relationship with 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. This is particularly evident in Example 3, which was slightly higher than that of Examples 1 and 2.

[0045] In the examples, the polycondensation reaction times and test results of Examples 1 and 2 are quite similar because their structures are similar, their catalytic effects are consistent, and their calculated thermal stability results also highly overlap, indicating that the catalytic effects of catalysts a and b are basically the same, corresponding to the catalytic results. Furthermore, the catalyst addition amount in the examples is 200 ppm, while the addition amount in the comparative example is 400 ppm, twice that of the examples. The overall catalytic effect of the comparative example is far lower than that of the examples, further demonstrating the excellent catalytic effect and high-temperature thermal stability of the catalysts in the examples.

[0046] Also attached Figure 3 The graphs show the relationship between reaction time and distillate volume under different catalysts. The graphs clearly show that catalysts a and b have relatively consistent catalytic effects, consistent with the results calculated using Materials Studio software. Meanwhile, the polymerization reaction catalyzed by catalyst c shows a significantly faster distillate flow than the others, further demonstrating the excellent catalytic effect of catalyst c. Furthermore, the distillate flow of catalysts a, b, and c is significantly faster than that of the control catalyst, indicating that the control catalyst may have decomposed or volatilized due to its poor thermal stability in the higher-temperature polycondensation reaction, thus failing to achieve the desired catalytic effect.

[0047] In summary, this invention provides a novel organic-base catalyst for the synthesis of liquid crystal polyarylates. This catalyst exhibits a more stable structure, high decomposition temperature, high boiling point, and high activity, enabling continuous and efficient catalysis of the melt polymerization of liquid crystal polyarylates to directly obtain melts of the desired viscosity. It also features sustained polymerization effect, fewer side reactions, and a one-step process to obtain liquid crystal polyarylates with excellent color values. Furthermore, this invention proposes a theoretical calculation method for determining the thermal stability of the catalyst. This calculation method is applicable to the calculation of the thermal stability of catalysts in polymerization reactions and has a certain degree of general applicability.

[0048] The specific embodiments described herein are merely illustrative of the spirit of the invention. Those skilled in the art to which this invention pertains may make various modifications or additions to the described specific embodiments or use similar methods to substitute them, without departing from the spirit of the invention or exceeding the scope defined by the appended claims.

Claims

1. An imidazopyridine catalyst for the synthesis of liquid crystal polyarylates, characterized in that, The catalyst is an imidazopyridine 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 imidazopyridine compounds have structures as shown in formula (I) or formula (II): Equation (I); Formula (II); R is selected from hydrogen, alkyl, alkoxy, or halogen.

3. The catalyst according to 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 catalyst according to 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 298.15 K and the second temperature is 300-1000 K.

5. The application of the catalyst according to any one of claims 1-4 in the synthesis of liquid crystal polyarylates, characterized in that, The application includes using the catalyst in the melt polycondensation reaction of liquid crystal polyarylates to directly obtain products 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 catalyst as described in any one of claims 1 to 4, followed by a melt polycondensation reaction at a higher temperature after acetylation, until the liquid crystal polyarylate with the intrinsic viscosity is directly obtained after the reaction is completed.

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 temperature of the acetylation reaction in the polymerization step is 100-150 °C; 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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