A kind of high purity lactide and its synthesis method

Abstract

The present invention relates to a high-purity lactide and a synthesis method thereof, wherein a lactic acid oligomer and a depolymerization catalyst are mixed and added to a reaction kettle, heated and heated, and a diamine solvent is added at the same time to carry out a depolymerization reaction, and after the reaction is completed, a crude lactide product is collected and refined to obtain a high-purity lactide product. The present invention reduces the degree of racemization in the depolymerization process by adding a diamine solvent in the crude lactide synthesis process, thereby avoiding m The production of lactide improves the purity of the product; at the same time, it can inhibit the polymerization frequency between lactic acid oligomer molecular chains and increase the reaction rate of the depolymerization process.

Description

Technical Field

[0001] The invention belongs to the technical field of degradable materials, and particularly relates to high-purity lactide and a synthesis method thereof. Background Art

[0002] Polylactic acid is a kind of bio-based degradable material. It is made from lactic acid through chemical synthesis. After use, polylactic acid products can be quickly degraded into carbon dioxide and water under composting conditions without polluting the environment. Industrial polylactic acid synthesis is mainly obtained through the ring-opening polymerization of lactide: the first step is to prepare lactide from lactic acid; the second step is to prepare polylactic acid through the ring-opening polymerization of lactide. The molecular weight of PLA obtained in this process can reach 100,000 to 1 million.

[0003] Among them, lactide is the key to the entire synthesis process, and the process barriers are relatively high. It is usually prepared by condensation polymerization and depolymerization under a catalyst, high temperature, and high vacuum system, and this process is prone to cause the racemization of lactide. This is mainly because the depolymerization process of lactide generation is mainly a "back-biting" ester catalytic reaction that occurs on the lactic acid oligomer molecular chain. The specific process is: under the action of catalyst, high temperature, and high vacuum, the carbonyl group on the lactic acid oligomer chain is activated, and the hydroxyl group at the beginning of the chain attacks the positively charged carbonyl group, causing the ester bond to break ("positive bite" process) to form L / D-lactide; however, in the presence of excessive catalyst or excessively high temperature, the carboxylic acid anion at the end of the lactic acid oligomer attacks the chiral carbon atom on the unit adjacent to the lactic acid unit, thereby breaking the bond between the methyl carbon and the ester oxygen bond ("back bite" process), and the configuration is reversed to obtain meso-lactide ( m -lactide). m -The presence of lactide, on the one hand, will affect the optical purity of lactide, and then affect the ring-opening polymerization process of lactide, making the molecular weight of the obtained PLA low; on the other hand, it will destroy the regularity of the PLA structure, reduce its crystallinity and mechanical properties.

[0004] Therefore, the crude lactide obtained by depolymerization generally needs to be purified and refined through processes such as solvent recrystallization, water extraction, distillation, and melt crystallization to reduce the presence of m-lactide in the product; and because L-lactide and m-lactide have similar physical and chemical properties, and lactide itself has high freezing point, boiling point, and thermal sensitivity, it is difficult to separate them, and the overall yield is low, only about 40-60%, and the overall economic efficiency is also low. Therefore, racemization in the lactide synthesis process is a key factor affecting the quality and yield of lactide, and it is also the focus and difficulty of current foreign lactide technology research.

[0005] CN104710401A discloses a high-purity lactide and a preparation method thereof, comprising the following steps: raw material addition, free water removal, polycondensation, depolymerization, distillation, and secondary distillation. In the raw material addition step, a catalyst is added to L-lactic acid or D-lactic acid, and the catalyst is any one or more of zinc oxide, stannous oxide, and stannous octoate. The preparation process does not use organic solvents, has no environmental pollution, does not use N2 or inert gas, has a moderate vacuum degree, reduces purification losses, reduces production costs, and is suitable for industrial production. The L-lactide or D-lactide product prepared by the method has a high yield and good quality, and the optical purity of the lactide after purification is greater than 99.45%. However, the method does not take relevant measures to reduce m-lactide during the preparation process, and mainly relies on the subsequent distillation method to remove m-lactide, which is difficult to separate and increases the refining cost.

[0006] CN110128397A discloses a method for preparing high-purity lactide, comprising the following steps: weighing 500g of D,L-lactic acid, pouring it into a three-necked flask with a thermometer, a condensing device and a stirrer, slowly heating to boiling, dehydrating under normal pressure, adding 5g of stannous octoate when the temperature rises to 130°C, wherein the mass ratio of lactic acid to stannous octoate is 100, maintaining reduced pressure dehydration for 10h, the pressure is 0.08-0.09MPa, so that the lactic acid is polycondensed into lactic acid oligomers, after the dehydration is completed, changing the condensation receiving device, so that the reaction liquid is rapidly heated to 250°C, and the pressure is maintained at 2000Pa-0.1MPa, collecting crude lactide through a three-stage cooling and collecting system, the product is a yellow solid, and the crude product yield is 93%, dissolving the crude product in water, cooling, suction filtering, and then recrystallizing three times with anhydrous ethanol, and vacuum drying to constant weight to obtain a finished product, the yield is 77%, and the purity of lactide is 99.96% after inspection. However, this method uses a tin catalyst and does not take any measures to reduce the racemization of lactide during the synthesis process. Therefore, the subsequent product purification is mainly carried out through multiple solvent recrystallizations, with a low single-pass product yield and a high refining cost.

[0007] In the existing industrial preparation process of lactide, crude lactide is usually first obtained through polycondensation-depolymerization, and then refined lactide is obtained through a series of purification processes. In the process of preparing crude lactide, in order to ensure the conversion rate of lactic acid oligomers, the purpose is usually to increase the crude product yield. Effective measures to inhibit the racemization of lactide are not taken during the preparation process, resulting in a high content of m-lactide, which increases the difficulty of purification. Summary of the invention

[0008] In view of the shortcomings of the prior art, the present invention provides a high-purity lactide and a method for synthesizing the same. The present invention reduces the degree of racemization during the depolymerization process by adding a diamine solvent during the synthesis of crude lactide, thereby avoiding m-The formation of lactide improves the purity of the product; at the same time, it can inhibit the polymerization frequency between lactic acid oligomer molecular chains and increase the reaction rate of the depolymerization process.

[0009] The present invention provides a method for synthesizing high-purity lactide, comprising the following contents:

[0010] The lactic acid oligomers and the depolymerization catalyst are mixed and added into a reaction kettle, heated and heated, and a diamine solvent is added at the same time to carry out a depolymerization reaction. After the reaction is completed, the crude lactide product is collected and refined to obtain a high-purity lactide product.

[0011] In the present invention, the molecular weight of the lactic acid oligomer is 800-4000, preferably 1200-2800, and is generally prepared by dehydration and polycondensation process using L-lactic acid or / and D-lactic acid as raw materials. The dehydration process is mainly to remove free water in the lactic acid, which can be in the form of normal pressure or reduced pressure. The reaction temperature of the polycondensation process is 140-170°C, the vacuum degree is 1000-2000Pa, and the reaction time is 0.5-4.0h.

[0012] In the present invention, the depolymerization catalyst is a tin compound, such as at least one selected from stannous octoate, stannous chloride, stannous oxide, stannous sulfate, stannous oxalate, etc., preferably stannous octoate.

[0013] In the present invention, the amount of the depolymerization catalyst is 0.2%-5.0%, preferably 0.8%-3.0%, based on the amount of the lactic acid oligomers.

[0014] In the present invention, the lactic acid oligomer and the depolymerization catalyst are mixed and added into a reaction kettle, and heated to 100-160°C.

[0015] In the present invention, the carbon number of the diamine solvent is not less than 12, and it may be at least one of C12-C18 diamine solvents, preferably at least one of dodecane diamine, tetradecane diamine, hexadecane diamine and the like.

[0016] In the present invention, the diamine solvent is used in an amount of 0.1%-5.0% of the amount of the lactic acid oligomer, preferably 0.5%-3.0%.

[0017] In the present invention, the protonated solvent is heated and melted before use, and the melting temperature is 80-160°C, preferably 100-160°C.

[0018] In the present invention, the depolymerization reaction temperature is 190-240° C., the vacuum degree is 200-1500 Pa, and the reaction time is 0.5-1.5 h.

[0019] In the present invention, in the crude lactide product, the L-lactide content is 80%-88%, the m-lactide content is less than 5%, preferably 3.0%-4.5%, the L-lactic acid content is less than 5%, preferably 1.5%-4.5%, and the dimer and trimer content is less than 2.5%-6.0%.

[0020] In the present invention, the single-pass conversion rate of lactic acid oligomers in the depolymerization process can reach more than 92%.

[0021] In the present invention, the undepolymerized lactic acid oligomers and diamine solvents at the bottom of the reactor can be directly mixed with fresh lactic acid oligomer raw materials for crude lactide preparation, or can be separated and reused from the solvent and lactic acid after hydrolysis.

[0022] In the present invention, the refining can adopt the refining method conventionally used in the art, such as distillation or solvent water extraction. If the distillation process is used for refining, the number of plates of the distillation tower is 10, the temperature of the bottom reboiler is controlled at 160-180°C, the tower top temperature is 110-130°C, the tower top vacuum is 200-300Pa, and the lactide product is continuously produced.

[0023] In the present invention, the chemical purity and optical purity of the refined lactide product can reach more than 99.0%, the m-lactide content is not more than 1.2%, and the product quality meets the requirements of polymerization-grade monomers. The total yield of the product in a single pass of the entire synthesis process can reach more than 74%.

[0024] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0025] (1) Based on the traditional lactide synthesis process, the present invention effectively inhibits the occurrence of the "backbite" process during the depolymerization process by adding a diamine solvent, reduces the degree of racemization during the synthesis process, and avoids m -The production of lactide reduces the subsequent refining cost. Compared with the crude lactide without solvent addition, m -The lactide content is not more than 5%, and the degree of racemization can be reduced by more than 60%.

[0026] (2) Adding diamine solvents during the depolymerization process can effectively inhibit the probability of polymerization reaction, reduce the frequency of repolymerization between lactic acid oligomer molecular chains, reduce the impact of the excessive molecular weight of oligomers on the reaction rate of the depolymerization process, increase the reaction rate of the depolymerization process in the middle and late stages of the reaction, and ensure the reaction efficiency of the entire depolymerization process. Compared with no solvent added, the depolymerization reaction efficiency can be increased by more than 30%.

[0027] (3) The crude lactide product obtained by the present invention does not require a specific (melt crystallization) separation process for refining. A high-purity lactide product can be obtained by distillation or solvent water extraction, which meets the requirements of polymerization-grade products and reduces the refining cost. In addition, the solvent can be recycled, further reducing the cost of the entire synthesis process. DETAILED DESCRIPTION

[0028] The method and its effects of the present invention are further illustrated by the following examples. The examples are implemented based on the technical solution of the present invention, and detailed implementation methods and specific operation processes are given, but the protection scope of the present invention is not limited to the following examples.

[0029] The experimental methods in the following examples, unless otherwise specified, are all conventional methods in the art. The experimental materials used in the following examples, unless otherwise specified, can all be purchased from biochemical reagent stores.

[0030] The lactic acid used in the embodiment of the present invention is heat-resistant grade L-lactic acid with a lactic acid content of 88% or more, and an optical purity of not less than 99.0%.

[0031] The present invention adopts Malvern Viscotek OMNISEC GPC / SEC gel chromatograph to analyze the molecular weight of lactic acid oligomers. The conventional calibration method is adopted, polystyrene (PS) is used as the internal standard, the chromatographic column model is T3000, the size is 300mmL×8.0mm, the column temperature is 40℃, the flow rate is 1.0mL / min, the sample concentration is 2-5mg / mL, and the single injection volume is 500μL.

[0032] The present invention adopts Agilent high performance liquid chromatograph to analyze the chemical purity of lactide, L-lactic acid and dimer and trimer content, ultraviolet detector, phosphoric acid and acetonitrile as mobile phase, chromatographic column model is ZORBAX SB-Aq, column length is 250mm, column inner diameter is 4.6mm, internal filler particle size is 5μm, detection wavelength is 200nm, column temperature is 40℃, flow rate is 1mL / min, injection volume is 5μL.

[0033] The present invention adopts Agilent gas chromatograph to analyze the content of different optical isomers of lactide, selects CYCLOSIL-B model chromatographic column, vaporization chamber temperature of 250°C, detector temperature of 280°C, hydrogen flame ionization detector, column temperature program heating initial temperature of 100°C, maintained for 5 minutes, heated to 140°C at a rate of 4°C / min, maintained for 7 minutes, heated to 200°C at a rate of 8°C / min, maintained for 20 minutes, carrier gas N2 flow rate of 1.4mL / min, hydrogen flow rate of 30mL / min, air flow rate of 400mL / min, injection volume of 0.5μL.

[0034] The yield Y of the lactide purification process and the yield Y of the product in the entire preparation and purification process总 The calculation formula is as follows:

[0035]

[0036] in, m 0 is the mass of crude lactide, y 0 is the purity of L-lactide in crude lactide, m is the mass of lactide product, M is the mass of lactide that a certain amount of lactic acid oligomers can theoretically be converted into, that is, the mass of lactic acid oligomers.

[0037] The specific rotation of the sample was analyzed by WZZ-2S automatic polarimeter to characterize the optical purity of the sample. The specific rotation of pure L-lactide was -278, and the specific rotation of pure D-lactide was +278. m -The specific rotation of lactide is 0, and the calculation formula of the sample optical purity X is as follows;

[0038]

[0039] Among them, α 纯物质 Represents the specific rotation of pure lactide, α 被测样品 Indicates the specific optical rotation of the substance being measured.

[0040] Example 1

[0041] Preparation of lactic acid oligomers: (1) Dehydration of lactic acid: Take 600g of L-lactic acid (lactic acid content of about 88.0%) and add it to a four-necked flask with a stirring system. Use a vacuum circulating water pump to maintain the system pressure at about 50kPa. Start heating under vacuum and gradually heat to 110-120℃. Dehydrate for 2h. At this time, the free water in the reaction system is slowly evaporated out of the reaction system. (2) Preparation of lactic acid oligomers: After the free water in the system is almost completely removed, increase the vacuum degree of the system and slowly reduce the pressure of the system to about 1.0kPa. The temperature of the feed liquid is gradually raised to 160℃ and reacted for 3h. At this time, a polycondensation reaction occurs between lactic acid molecules, and the water generated by the reaction in the system is evaporated out of the system to obtain a lactic acid oligomer with a molecular weight of 2214.

[0042] Take 500g of the lactic acid oligomer obtained above, transfer it to a kettle reactor, add 4.0g of stannous octoate, heat it to 160°C, add 5.0g of dodecane diamine melted at 140°C, control the vacuum degree of the depolymerization reaction at 500Pa, the reaction temperature at 200°C, react for 1.0h, and obtain a crude L-lactide product. The product was analyzed to have an L-lactide content of 86.6%, an m-lactide content of 3.1%, an L-lactic acid content of 3.0%, and a dimer and trimer content of 4.4%. The single-pass conversion rate of lactic acid oligomers during the depolymerization process was 94.1%.

[0043] The crude lactide prepared above was purified by distillation process, the number of distillation tower plates was 10, the temperature of the bottom reboiler was controlled at 175°C, the top temperature was 118°C, the top vacuum was 200Pa, and the light components and lactide products were continuously produced. After analysis, the product yield of lactide at the top of the tower was 84.3%, the chemical purity of the product was 99.3%, the optical purity was 99.3%, the m-lactide content was 0.7%, and the total single-pass lactide yield of the entire synthesis process was 76.3%.

[0044] Example 2

[0045] The preparation process of lactic acid oligomers is the same as that in Example 1.

[0046] Take 500g of the lactic acid oligomer obtained above, transfer it to a kettle reactor, add 25g of stannous octoate, heat it to 160°C, add 25g of dodecane diamine melted at 140°C, control the vacuum degree of the depolymerization reaction at 1000Pa, the reaction temperature at 190°C, react for 1.0h, and obtain a crude L-lactide product. The L-lactide content of the product is 82.8%, the m-lactide content is 4.7%, the L-lactic acid content is 4.1%, and the dimer and trimer content is 3.9%. The single-pass conversion rate of lactic acid oligomers in the depolymerization process is 92.9%.

[0047] The crude lactide prepared above was purified by distillation process, the number of distillation tower plates was 10, the temperature of the bottom reboiler was controlled at 180°C, the top temperature was 118°C, the top vacuum was 200Pa, and the light components and lactide products were continuously produced. The product yield of the top lactide was 81.4%, the chemical purity of the product was 99.2%, the optical purity was 99.1%, the m-lactide content was 0.9%, and the total single-pass lactide yield of the entire synthesis process was 74.2%.

[0048] Example 3

[0049] The preparation process of lactic acid oligomers is the same as that in Example 1.

[0050] Take 500g of the lactic acid oligomer obtained above, transfer it to a kettle reactor, add 2.0g of stannous octoate, heat it to 140°C, add 2.0g of dodecane diamine melted at 140°C, control the vacuum degree of the depolymerization reaction at 300Pa, the reaction temperature at 240°C, react for 1.0h, and obtain a crude L-lactide product. The L-lactide content of the product is 83.7%, the m-lactide content is 5.0%, the L-lactic acid content is 4.0%, and the dimer and trimer content is 3.6%. The single-pass conversion rate of lactic acid oligomers in the depolymerization process is 92.1%.

[0051] The crude lactide prepared above was purified by distillation process, the number of distillation tower plates was 10, the temperature of the bottom reboiler was controlled at 180°C, the top temperature was 118°C, the top vacuum was 200Pa, and the light components and lactide products were continuously produced. After analysis, the product yield of lactide at the top of the tower was 82%, the chemical purity of the product was 99.3%, the optical purity was 99.0%, the m-lactide content was 1.0%, and the total single-pass lactide yield of the entire synthesis process was 74.1%.

[0052] Example 4

[0053] The preparation process of lactic acid oligomers is the same as that in Example 1.

[0054] The lactide preparation process was the same as in Example 1, except that the same mass of hexadecane diamine was used. The crude L-lactide obtained was analyzed, and the L-lactide content was 85.1%, the m-lactide content was 3.5%, the L-lactic acid content was 3.7%, and the dimer and trimer content was 4.6%. The single-pass conversion rate of lactic acid oligomers in the depolymerization process was 93.1%.

[0055] The crude lactide purification process was the same as in Example 1, and the lactide product yield was 83.5%. The product chemical purity was 99.3%, the optical purity was 99.1%, the m-lactide content was 1.0%, and the total lactide yield per pass in the entire synthesis process was 74.8%.

[0056] Example 5

[0057] The preparation of lactic acid oligomer was the same as that in Example 1, except that the final temperature of the polycondensation process was 150° C. and the reaction time was 2 h, thereby obtaining a lactic acid oligomer with a molecular weight of 917.

[0058] The lactide preparation process was the same as in Example 1, except that the depolymerization reaction was carried out using a lactic acid oligomer with a molecular weight of 917 as a raw material. The crude L-lactide obtained was analyzed, and the L-lactide content was 80.3%, the m-lactide content was 3.0%, the L-lactic acid content was 5.0%, and the dimer and trimer content was 3.7%. The single-pass conversion rate of the lactic acid oligomer in the depolymerization process was 92.8%.

[0059] The crude lactide purification process was the same as in Example 1, and the lactide product yield was 81.1%. The chemical purity of the product was 99.2%, the optical purity was 99.3%, the m-lactide content was 0.7%, and the total lactide yield per pass in the entire synthesis process was 74.1%.

[0060] Example 6

[0061] The preparation of lactic acid oligomers was the same as in Example 1, except that the final temperature of the polycondensation process for preparing lactic acid oligomers was 170° C., and the reaction time was 4 h, to obtain lactic acid oligomers with a molecular weight of 3814.

[0062] The lactide preparation process is the same as that in Example 1, except that a lactic acid oligomer with a molecular weight of 3814 is used as a raw material for depolymerization reaction. The crude lactide obtained is analyzed to have an L-lactide content of 87.8%, an m-lactide content of 4.8%, an L-lactic acid content of 2.7%, and a dimer and trimer content of 3.8%. The single-pass conversion rate of lactic acid oligomers in the depolymerization process is 92.4%.

[0063] The crude lactide purification process was the same as in Example 1, and the yield of the obtained lactide product was 82.2%. The chemical purity of the product was 99.5%, the optical purity was 99.2%, the m-lactide content was 1.0%, and the total lactide yield per pass of the entire synthesis process was 74.9%.

[0064] Example 7

[0065] The preparation process of lactic acid oligomers is the same as that in Example 1.

[0066] The lactide preparation process was the same as in Example 1, except that the catalyst used the same mass of stannous chloride to prepare crude L-lactide. The product was analyzed to have an L-lactide content of 85.8%, an m-lactide content of 3.5%, an L-lactic acid content of 3.2%, and a dimer and trimer content of 5.0%. The single-pass conversion rate of lactic acid oligomers in the depolymerization process was 93.2%.

[0067] The crude lactide purification process was the same as in Example 1, and the lactide product yield was 83.5%. The chemical purity of the product was 99.1%, the optical purity was 99.2%, the m-lactide content was 0.8%, and the total lactide yield per pass in the entire synthesis process was 75.1%.

[0068] Comparative Example 1

[0069] The preparation process of lactic acid oligomers and crude lactide was the same as that of Example 1, except that dodecadiamine was not added. The crude lactide obtained had an L-lactide content of 78.3%, an m-lactide content of 5.9%, an L-lactic acid content of 4.7%, and a dimer and trimer content of 7.1%. The single-pass conversion rate of lactic acid oligomers in the depolymerization process was 66.8%.

[0070] The crude lactide purification process was the same as in Example 1, and the lactide product yield was 70.1%. The chemical purity of the product was 97.6%, the optical purity was 97.5%, and the m-lactide content was 2.5%. The product could not meet the requirements of polymerization-grade monomers and still needed to be further refined by melt crystallization.

[0071] Comparative Example 2

[0072] The preparation process of lactic acid oligomers and crude lactide is the same as that of Example 1, except that the same mass of decanediamine is used instead of dodecanediamine. The crude lactide obtained has an L-lactide content of 82.2%, an m-lactide content of 5.3%, an L-lactic acid content of 4.5%, and a dimer and trimer content of 6.1%. The single-pass conversion rate of lactic acid oligomers in the depolymerization process is 80.3%.

[0073] The crude lactide purification process was the same as in Example 1. The lactide product yield of the entire purification process was 72.3%, the chemical purity of the product was 98.6%, the optical purity was 97.2%, and the m-lactide content was 2.4%. The product could not meet the requirements of polymerization-grade monomers and still needed to be refined in conjunction with the melt crystallization process.

[0074] Comparative Example 3

[0075] The preparation process of lactic acid oligomers and crude lactide was the same as that of Example 1, except that zinc oxide was used as a catalyst. The crude L-lactide obtained was analyzed to have an L-lactide content of 80.5%, an m-lactide content of 5.7%, an L-lactic acid content of 3.6%, and a dimer and trimer content of 5.7%. The single-pass conversion rate of lactic acid oligomers in the depolymerization process was 82.9%.

[0076] The crude lactide purification process was the same as in Example 1, and the lactide product yield was 73.1%. The chemical purity of the product was 98.6%, the optical purity was 97.6%, and the m-lactide content was 2.2%. The product could not meet the requirements of polymerization-grade monomers, and melt crystallization was still required for product refining.

Claims

1. A method for synthesizing high-purity lactide, characterized in that The method comprises the following contents: adding lactic acid oligomers and a depolymerization catalyst to a reaction kettle, heating and raising the temperature, adding a diamine solvent at the same time, carrying out a depolymerization reaction, collecting a crude lactide product after the reaction is completed, and obtaining a high-purity lactide product after refining; the diamine solvent is at least one of dodecane diamine, tetradecane diamine and hexadecane diamine; the molecular weight of the lactic acid oligomer is 800-4000; the depolymerization catalyst is at least one of stannous octoate, stannous chloride, stannous oxide, stannous sulfate and stannous oxalate; the depolymerization reaction temperature is 190-240°C, the vacuum degree is 200-1500Pa, and the reaction time is 0.5-1.5h.

2. The method according to claim 1, characterized in that: The molecular weight of the lactic acid oligomer is 1200-2800.

3. The method according to claim 1 or 2, characterized in that: The lactic acid oligomer is prepared by dehydration and polycondensation process with L-lactic acid or / and D-lactic acid as raw materials; the dehydration process mainly removes free water in the lactic acid under normal pressure or reduced pressure; the polycondensation process has a reaction temperature of 140-170°C, a vacuum degree of 1000-2000Pa, and a reaction time of 0.5-4.0h.

4. The method according to claim 1, characterized in that: The depolymerization catalyst is used in an amount of 0.2%-5.0% of the amount of the lactic acid oligomers.

5. The method according to claim 4, characterized in that: The depolymerization catalyst is used in an amount of 0.8%-3.0% of the amount of the lactic acid oligomers.

6. The method according to claim 1, characterized in that: The lactic acid oligomer and the depolymerization catalyst are mixed and added into a reaction kettle, and heated to 100-160°C.

7. The method according to claim 1, characterized in that: The diamine solvent is used in an amount of 0.1%-5.0% of the lactic acid oligomer mass.

8. The method according to claim 7, characterized in that: The diamine solvent is used in an amount of 0.5%-3.0% of the lactic acid oligomer mass.

9. The method according to claim 1, characterized in that: The diamine solvent is heated and melted before use, and the melting temperature is 80-160°C.

10. The method according to claim 9, characterized in that: The melting temperature is 100-160℃.

11. The method according to claim 1, characterized in that: In the crude lactide product, the L-lactide content is 80%-88%, the m-lactide content is less than 5%, the L-lactic acid content is less than 5%, and the dimer and trimer content is 2.5%-6.0%.

12. The method according to claim 11, characterized in that: In the crude lactide product, the content of m-lactide is 3.0%-4.5%, and the content of L-lactic acid is 1.5%-4.5%.

13. The method according to claim 1, characterized in that: The single-pass conversion rate of lactic acid oligomers in the depolymerization process can reach more than 92%.

14. The method according to claim 1, characterized in that: The undepolymerized lactic acid oligomers and diamine solvents at the bottom of the reactor are directly mixed with fresh lactic acid oligomer raw materials for crude lactide preparation, or are separated and reused as solvents and lactic acid after hydrolysis.

15. The method according to claim 1, characterized in that: The refining adopts a distillation process, the number of distillation tower plates is 10, the temperature of the bottom reboiler is controlled at 160-180°C, the tower top temperature is 110-130°C, the tower top vacuum is 200-300Pa, and the lactide product is continuously produced.

16. The method according to claim 1, characterized in that: The chemical purity and optical purity of the refined lactide product can reach above 99.0%, and the m-lactide content is no more than 1.2%. The product quality meets the requirements of polymerization-grade monomers.

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