Preparation method of polycarbonate

Abstract

The invention relates to the technical field of catalytic melt ester exchange, in particular to a preparation method of polycarbonate. The invention provides a preparation method of polycarbonate, which comprises the following steps: under the action of a catalyst, diphenyl carbonate and bisphenol A are sequentially subjected to melt transesterification and condensation polymerization to obtain polycarbonate, the catalyst is an azole alkali metal salt; the number-average molecular weight of polycarbonate is gt; 20,000, the weight-average molecular weight is gt; 30 thousand, PDIlt; 2, 3; the temperature of the melt ester exchange reaction is 140-160 DEG C, and the time is more than or equal to 40 minutes; the temperature of the polycondensation reaction is 200-250 DEG C, and the time is 1.5-3 hours. According to the preparation method, the polycarbonate prepared by taking the azoles alkali metal salt as the catalyst is narrow in molecular weight distribution and high in thermal stability.

Description

Technical Field

[0001] This invention relates to the field of catalytic melt transesterification technology, and more particularly to a method for preparing polycarbonate. Background Technology

[0002] Polycarbonate (PC), as one of the most important engineering plastics, has always had its green synthesis process as a research focus in the global chemical industry. Melt transesterification (non-phosgene method), using diphenyl carbonate (DPC) and bisphenol A (BPA) as raw materials, is widely recognized as the future direction to replace the traditional phosgene method due to its high atom economy, solvent-free process, and complete avoidance of the use of highly toxic phosgene. However, the industrial application success of the melt transesterification method is highly dependent on the performance of the catalyst system. The process involves two steps: first, transesterification to generate oligomers, followed by a polycondensation reaction under high temperature and high vacuum. The second step has harsh reaction conditions, placing extremely stringent requirements on the catalyst. Existing catalyst systems have a significant drawback: while traditional alkali metal catalysts such as sodium hydroxide and potassium hydroxide have high initial activity, their excessive alkalinity can violently catalyze side reactions such as Fries rearrangement during the high-temperature polycondensation stage. This leads to branched structures in the product molecular chains, resulting in a wider molecular weight distribution and a significant decrease in thermal stability. Summary of the Invention

[0003] In view of this, the purpose of the present invention is to provide a method for preparing polycarbonate, wherein the polycarbonate prepared by using azole alkali metal salts as catalysts has a narrow molecular weight distribution and high thermal stability.

[0004] To achieve the above-mentioned objectives, the present invention provides the following technical solution: This invention provides a method for preparing polycarbonate, comprising the following steps: With the aid of a catalyst, diphenyl carbonate and bisphenol A undergo sequential melt transesterification and polycondensation reactions to obtain polycarbonate. The catalyst is a nitrogen-azole alkali metal salt; The polycarbonate has a number-average molecular weight > 20,000, a weight-average molecular weight > 30,000, and a PDI < 2.3. The temperature of the molten transesterification reaction is 140~160℃, and the time is ≥40min; The polycondensation reaction is carried out at a temperature of 200-250°C for 1.5-3 hours.

[0005] Preferably, the alkali metal salt of the nitrazole class comprises an alkali metal ion and a nitrazole organic ligand.

[0006] Preferably, the azole organic ligands include one or more of triazole, tetrazolium, aminotriazole, aminotetrazole and benzotriazole.

[0007] Preferably, the alkali metal ions include one or more of potassium ions, sodium ions, rubidium ions, and cesium ions.

[0008] Preferably, the preparation method of the azole alkali metal salt includes the following steps: An alkali metal hydroxide, a nitrogen azole organic ligand, and a solvent are mixed and neutralized to obtain the nitrogen azole alkali metal salt.

[0009] Preferably, the molar ratio of the alkali metal hydroxide to the nitrogen azole organic ligand is 1:1.

[0010] Preferably, the solvent includes one or more of ethanol, methanol, and water.

[0011] Preferably, the neutralization reaction is carried out under stirring conditions, the temperature of the neutralization reaction is room temperature, and the time is 4 hours.

[0012] Preferably, the molar ratio of diphenyl carbonate to bisphenol A is (1~1.1):1.

[0013] Preferably, the mass of the catalyst accounts for 0.05% to 1% of the mass of the bisphenol A.

[0014] This invention provides a method for preparing polycarbonate, comprising the following steps: under the action of a catalyst, diphenyl carbonate and bisphenol A are subjected to a melt transesterification reaction and a polycondensation reaction sequentially to obtain polycarbonate; the catalyst is a nitrogen-azole alkali metal salt; the polycarbonate has a number-average molecular weight > 20,000, a weight-average molecular weight > 30,000, and a PDI < 2.3; the temperature of the melt transesterification reaction is 140~160℃, and the time is ≥ 40 min; the temperature of the polycondensation reaction is 200~250℃, and the time is 1.5~3 h.

[0015] Compared with the prior art, the technical solution of the present invention has the following beneficial effects: 1) The preparation method described in this invention selects azole-based alkali metal salts as catalysts, representing a paradigm shift from "inorganic strong bases" to "organic weak base coordination salts." This method chooses azole-based alkali metal salts as a specific type of compound, whose structure combines a weakly basic organic anion and a Lewis acidic metal center, exhibiting synergistic catalysis and effective suppression of side reactions. The synergistic catalysis mechanism is that the catalyst does not rely on the traditional strong base proton abstraction mechanism, but rather uses its alkali metal cation as a Lewis acid to activate the phenolic hydroxyl group of BPA or the carbonyl group of DPC. Simultaneously, the azole-based organic ligands in the catalyst act as both organic bases and potential ligands, synergistically acting on the reaction center to form a unique "acid-base synergy" or "coordination activation" mechanism. This mechanism is milder and more precise than simple strong base catalysis. The mechanism for suppressing side reactions is that, due to its overall basicity being much lower than that of traditional inorganic bases (such as sodium hydroxide / potassium hydroxide), the probability of initiating base-driven side reactions such as Fries rearrangement is significantly reduced thermodynamically. Meanwhile, its unique structure may stabilize reaction intermediates through coordination, guiding the reaction to proceed in a directional chain growth rather than disordered side reactions; 2) The present invention uses azole alkali metal salts as catalysts to avoid the defects of the existing technology that must use a complex "catalyst + co-catalyst / stabilizer" compound system, which can combine "activation, selectivity control and stability" in one. 3) The preparation method is simple, greatly simplifying the catalyst feeding, storage, and mixing process, and reducing process complexity and human error. At the same time, it reduces the introduction of foreign ions, which is beneficial for obtaining polymers with higher purity. 4) Excellent stability: The azole heterocyclic structure has excellent chemical stability at high temperatures, ensuring the continuity and stability of catalytic activity throughout the reaction process and avoiding deactivation or new side reactions caused by catalyst decomposition. 5) Green and economical: The entire process complies with the principles of green chemistry, and the catalyst is inexpensive, easy to industrialize, and has extremely high commercial value. Attached Figure Description

[0016] Figure 1 The infrared spectrum of the KATriz; Figure 2 Thermogravimetric spectrum of the KATriz; Figure 3 DSC curves of the polycarbonates prepared in Examples 1 and 4 (the polycarbonate in Example 1 is denoted as PC200, and the polycarbonate in Example 4 is denoted as PC240). Figure 4 The infrared spectra of the polycarbonates prepared in Examples 1 and 4 are shown below (the polycarbonate in Example 1 is denoted as PC200, and the polycarbonate in Example 4 is denoted as PC240). Detailed Implementation

[0017] This invention provides a method for preparing polycarbonate, comprising the following steps: With the aid of a catalyst, diphenyl carbonate and bisphenol A undergo sequential melt transesterification and polycondensation reactions to obtain polycarbonate. The catalyst is a nitrogen-azole alkali metal salt; The polycarbonate has a number-average molecular weight > 20,000, a weight-average molecular weight > 30,000, and a PDI < 2.3. The temperature of the molten transesterification reaction is 140~160℃, and the time is ≥40min; The polycondensation reaction is carried out at a temperature of 200-250°C for 1.5-3 hours.

[0018] In this invention, unless otherwise specified, all raw materials used in the preparation are commercially available products well known to those skilled in the art.

[0019] In this invention, the azole-based alkali metal salt preferably comprises an alkali metal ion and an azole-based organic ligand; the molar ratio of the alkali metal ion to the azole-based organic ligand is preferably 1:1; the azole-based organic ligand preferably comprises one or more of triazole, tetrazolium, aminotriazole (3-amino-1,2,4-triazole), aminotetrazole, and benzotriazole. When the azole-based organic ligand is two or more of the above-mentioned specific selections, this invention does not impose any special limitation on the ratio of the specific substances, and they can be mixed in any ratio. In this invention, the alkali metal ion preferably comprises one or more of potassium ions, sodium ions, rubidium ions, and cesium ions. When the alkali metal ion is two or more of the above-mentioned specific selections, this invention does not impose any special limitation on the ratio of the specific substances, and they can be mixed in any ratio. In an embodiment of this invention, the azole-based organic ligand can be aminotriazole; the alkali metal ion can be potassium ion.

[0020] In this invention, the preparation method of the azole alkali metal salt preferably includes the following steps: An alkali metal hydroxide, a nitrogen azole organic ligand, and a solvent are mixed and reacted to obtain the nitrogen azole alkali metal salt.

[0021] In this invention, the alkali metal hydroxide preferably includes one or more of lithium hydroxide, sodium hydroxide, potassium hydroxide, rubidium hydroxide, and cesium hydroxide. When the alkali metal hydroxide is two or more of the above-mentioned specific selections, this invention does not impose any special limitation on the ratio of the above-mentioned specific substances, and they can be mixed in any ratio. In an embodiment of this invention, the alkali metal hydroxide can be potassium hydroxide.

[0022] In this invention, the azole organic ligand preferably includes one or more of triazole, tetrazolium, aminotriazole (3-amino-1,2,4-triazole), aminotetrazole, and benzotriazole. When the azole organic ligand is two or more of the above-mentioned specific selections, this invention does not impose any special limitation on the ratio of the specific substances, and they can be mixed in any ratio. In an embodiment of this invention, the azole organic ligand may be aminotriazole.

[0023] In this invention, the solvent preferably includes one or more of ethanol, methanol, and water. When the solvent is two or more of the specific choices mentioned above, this invention does not have any special limitations on the ligands of the specific substances, and they can be mixed in any proportion. In an embodiment of this invention, the solvent can be ethanol.

[0024] In this invention, the molar ratio of the alkali metal hydroxide to the nitrogen azole organic ligand is preferably 1:1.

[0025] The present invention does not impose any special limitation on the amount of solvent used; any amount known to those skilled in the art can be used.

[0026] In this invention, the neutralization reaction is preferably carried out under stirring conditions, the temperature of the neutralization reaction is preferably room temperature, and the time is preferably 4 hours. In this invention, room temperature can be understood as no additional heating or cooling is performed.

[0027] After the neutralization reaction is completed, the present invention preferably includes drying, and the drying method is preferably vacuum drying. The vacuum drying temperature is preferably 80°C. The present invention does not have any special limitation on the vacuum drying time, and a time known to those skilled in the art can be used to remove the solvent and water.

[0028] In this invention, the azole-based heterocyclic structure of the azole-based organic ligand in the catalyst enables it to have a certain coordination ability. During the reaction, it may stabilize the reactant species by coordinating with the reaction intermediate or the active chain end, guiding the reaction in the direction of ordered chain growth, rather than causing random chain breaking or branching reactions.

[0029] In this invention, the molar ratio of diphenyl carbonate to bisphenol A is preferably (1~1.1):1, more preferably 1:1, 1.02:1, 1.05:1, 1.08:1 or 1.1:1. In an embodiment of this invention, the molar ratio of diphenyl carbonate to bisphenol A can be 1.02:1.

[0030] In this invention, the catalyst preferably accounts for 0.05% to 1% of the mass of bisphenol A, more preferably 0.05%, 0.1%, 0.15%, 0.2%, 0.25%, 0.3%, 0.35%, 0.4%, 0.45%, 0.5%, 0.55%, 0.6%, 0.65%, 0.7%, 0.75%, 0.8%, 0.85%, 0.9%, 0.95%, or 1%. In an embodiment of this invention, the catalyst accounts for 0.44% of the mass of bisphenol A.

[0031] In this invention, the temperature of the molten transesterification reaction is 140~160℃, preferably 140℃, 145℃, 150℃, 155℃ or 160℃; the time is ≥40min, more preferably 40min, 35min, 40min, 45min, 50min, 55min or 60min; the absolute pressure is preferably ≤5kPa.

[0032] In this invention, the temperature of the polycondensation reaction is 200~250℃, preferably 200℃, 210℃, 220℃, 230℃, 240℃ or 250℃; the time is 1.5~3h, more preferably 1.5h, 2h, 2.5h or 3h; the absolute pressure is preferably ≤1.5kPa.

[0033] In this invention, the polycarbonate preferably has a number-average molecular weight > 20,000, a weight-average molecular weight > 30,000, and a molecular weight distribution index (PDI) preferably ≤ 2.3.

[0034] The technical solutions of this invention will be clearly and completely described below with reference to the embodiments thereof. Obviously, the described embodiments are only a part of the embodiments of this invention, and not all of them. All other embodiments obtained by those skilled in the art based on the embodiments of this invention without creative effort are within the scope of protection of this invention.

[0035] Examples 1-4 and Comparative Examples 1-2 0.84 g of aminotriazole and 0.56 g of potassium hydroxide were added to a 100 mL round-bottom flask, and 50 mL of anhydrous ethanol was added. The mixture was stirred at room temperature for 4 h, and then dried under vacuum at 80 °C to remove the ethanol, thus obtaining the catalyst (potassium aminotriazole salt, denoted as KATriz, and the reaction process is shown in Formula 1). Formula 1; 10.9 g of diphenyl carbonate, 11.4 g of bisphenol A and 0.05 g of catalyst were added sequentially to a 100 mL three-necked flask, and the mixture was subjected to a melt transesterification reaction (140 °C, 5 kPa, 40 min) and a polycondensation reaction (reaction conditions are shown in Table 1) to obtain polycarbonate (yield, number average molecular weight, weight average molecular weight and molecular weight distribution index are shown in Table 1). Table 1. Physicochemical parameters of the polycondensation reaction and polycarbonate described in Examples 1-4 and Comparative Examples 1-2.

[0036] Figure 1 The infrared spectrum of the KATriz is shown below. Figure 2 The thermogravimetric spectrum of the KATriz is given by... Figures 1-2 It can be seen that the decomposition temperature of KATriz is above 350℃, indicating high thermal stability; As shown in Table 1, and based on Examples 1, 3, 4, and Comparative Example 1, polycarbonates obtained when the polycondensation reaction temperature is in the range of 200–220°C have a lower weight-average molecular weight (M). w <50000), but with a narrow distribution (PDI 2.0~2.23); at this temperature, the chain growth reaction kinetics are slow, failing to reach higher molecular weight levels. Side reactions have not yet occurred significantly, hence the narrow molecular weight distribution, indicating a relatively uniform chain growth process. The optimal reaction temperature is 240℃, at which the weight-average molecular weight reaches its peak (Mw=67876) and maintains a narrow distribution (PDI=2.16). This shows that 240℃ provides sufficient energy for the chain growth reaction to dominate, significantly increasing the degree of polymerization. Simultaneously, degradation reactions have not posed a serious threat, and the reaction is well controlled. However, in the high-temperature region (260℃), the number-average molecular weight decreased sharply, while the weight-average molecular weight increased abnormally, resulting in a sharp widening of the PDI to 4.70, indicating severe degradation and irregular chain growth. As shown in Examples 1 and 2 (same temperature, different time), extending the time from 2h to 3h did not increase the molecular weight but decreased it. This shows that at low temperatures, extending the reaction time is not only not beneficial, but also provides time for trace degradation reactions, resulting in a slight decrease in molecular weight. As shown in Examples 4 and Comparative Example 2 (same temperature, different time), shortening the time from 2h to 0.5h resulted in extremely low molecular weight (Mw=13711). This proves that 2h is a necessary condition for reaching reaction equilibrium at this temperature. At 0.5h, the reaction was far from over, and the chain growth process was interrupted midway. The yields of all reactions were extremely high (98%~99%), indicating that the KATriz used in this invention has extremely high catalytic activity and can almost quantitatively drive the reaction to completion. Figure 3The DSC curves of the polycarbonates prepared in Examples 1 and 4 (the polycarbonate in Example 1 is denoted as PC200, and the polycarbonate in Example 4 is denoted as PC240) are obtained from... Figure 3 It can be seen that the glass transition temperature (T) of the obtained product g The temperature is basically consistent with the literature report of 145℃, and the T value of PC240 is also consistent with the literature report. g The temperature of 146℃ indicates that the synthesized PC has standard or even better heat resistance; Figure 4 The infrared spectra of the polycarbonates prepared in Examples 1 and 4 are shown below (the polycarbonate in Example 1 is denoted as PC200, and the polycarbonate in Example 4 is denoted as PC240). Figure 4 It can be known that at approximately 1764 cm -1 A strong stretching vibration absorption peak belonging to the carbonyl group of carbonate appears at approximately 1500 cm⁻¹; -1 ~1600 cm -1 A vibrational absorption peak belonging to the benzene ring skeleton appears at approximately 1212 cm⁻¹; -1 A strong stretching vibration absorption peak at approximately 2967 cm⁻¹ appears, attributed to the COC bond in the carbonate; -1 and 2873 cm -1 An absorption peak attributable to the CH stretching vibration of methyl groups appears, and these characteristic peaks are consistent with the infrared spectrum of standard bisphenol A polycarbonate.

[0037] Comparative Examples 3-6 0.84 g of aminotriazole and 0.24 g of potassium hydroxide were added to a 100 mL round-bottom flask, followed by 50 mL of anhydrous ethanol. The mixture was stirred at room temperature for 4 h, and then dried under vacuum at 80 °C to remove the ethanol, yielding the catalyst (potassium aminotriazole salt, denoted as LiATriz). 10.9 g of diphenyl carbonate, 11.4 g of bisphenol A and 0.05 g of catalyst were added sequentially to a 100 mL three-necked flask, and the molten transesterification reaction (reaction conditions are shown in Table 2) and polycondensation reaction (reaction conditions are shown in Table 2) were carried out sequentially to obtain polycarbonate (yield, number average molecular weight, weight average molecular weight and molecular weight distribution index are shown in Table 2). Table 2 Physicochemical parameters of the polycondensation reaction and polycarbonate described in Comparative Examples 3-6

[0038] As shown in Table 2, in Comparative Examples 3 and 4 (with the same polycondensation conditions but different melt transesterification times), the molecular weight increased from 1630 to 5239, representing a leap of orders of magnitude. This means that transesterification at 20 min may be insufficient, generating a large amount of oligomers or even unreacted raw materials. These "raw materials" cannot effectively grow their chains after polycondensation, resulting in extremely low final molecular weights. After 40 min, more prepolymers with more uniform chain lengths were generated, laying the foundation for chain growth. In Comparative Examples 5 and 6 (with different transesterification times), extending the transesterification time from 40 min to 60 min resulted in a significant increase in the number-average molecular weight from 6398 to 17011.

[0039] The above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention in any way. It should be noted that those skilled in the art can make various improvements and modifications without departing from the principles of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. A method for preparing polycarbonate, characterized in that, Includes the following steps: With the aid of a catalyst, diphenyl carbonate and bisphenol A undergo sequential melt transesterification and polycondensation reactions to obtain polycarbonate. The catalyst is a nitrogen azole alkali metal salt; The polycarbonate has a number-average molecular weight > 20,000, a weight-average molecular weight > 30,000, and a PDI < 2.

3. The temperature of the molten transesterification reaction is 140~160℃, and the time is ≥40min; The polycondensation reaction is carried out at a temperature of 200-250°C for 1.5-3 hours.

2. The preparation method according to claim 1, characterized in that, The alkali metal salts of the nitrazole class include alkali metal ions and nitrazole organic ligands.

3. The preparation method according to claim 2, characterized in that, The azole organic ligands include one or more of triazole, tetrazolium, aminotriazole, aminotetrazole, and benzotriazole.

4. The preparation method according to claim 2, characterized in that, The alkali metal ions include one or more of potassium ions, sodium ions, rubidium ions, and cesium ions.

5. The preparation method according to any one of claims 2 to 4, characterized in that, The preparation method of the azole alkali metal salt includes the following steps: An alkali metal hydroxide, a nitrogen azole organic ligand, and a solvent are mixed and neutralized to obtain the nitrogen azole alkali metal salt.

6. The preparation method according to claim 5, characterized in that, The molar ratio of the alkali metal hydroxide to the nitrogen azole organic ligand is 1:

1.

7. The preparation method according to claim 5, characterized in that, The solvent includes one or more of ethanol, methanol, and water.

8. The preparation method according to claim 5, characterized in that, The neutralization reaction was carried out under stirring conditions at room temperature for 4 hours.

9. The preparation method according to claim 1, characterized in that, The molar ratio of diphenyl carbonate to bisphenol A is (1~1.1):

1.

10. The preparation method according to claim 5, characterized in that, The catalyst accounts for 0.05% to 1% of the mass of bisphenol A.

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