A method for reducing racemization in the synthesis process of lactide

Abstract

The present invention relates to a method for reducing racemization in the synthesis process of lactide, mainly including: (1) mixing a lactic acid oligomer with a depolymerization catalyst and adding them into a reaction kettle, heating and raising the temperature; (2) adding a molten long-chain diol solvent into the reaction kettle to carry out a depolymerization reaction, and collecting crude lactide after the reaction ends. By adding a long-chain diol solvent in the process of synthesizing crude lactide, the present invention reduces the degree of racemization in the depolymerization process, reduces the formation of m-lactide, and reduces the subsequent refining cost.

Description

Technical Field

[0001] The invention belongs to the technical field of degradable materials, and particularly relates to a method for reducing racemization in a lactide synthesis process. Background Art

[0002] Polylactic acid material is a kind of bio-based degradable material. It is made of lactic acid as raw material through chemical synthesis. Polylactic acid products can be quickly degraded into carbon dioxide and water under composting conditions after use, without polluting the environment. In 1932, Carothers et al. of DuPont proved that high molecular weight polylactic acid can be obtained by the ring-opening polymerization of lactic acid cyclic dimer-lactide, commonly known as the two-step method. It is the origin of the two-step method for producing polylactic acid and is also the polylactic acid preparation method currently used at home and abroad.

[0003] 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 from the ring-opening polymerization of lactide. The molecular weight of PLA obtained in this process can reach 100,000 to 1 million. Among them, lactide is the key to the entire synthesis process, and the process barriers are relatively high. It is usually prepared by polycondensation 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 carbonyl group on the lactic acid oligomer chain is activated under the action of catalyst, high temperature, and high vacuum, 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 a strong alkaline catalyst or at an 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 ("reverse 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, crude lactide usually 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.

[0005] CN110128397A discloses a method for preparing high-purity lactide, which includes the following steps: Weigh 500 g of D,L-lactic acid and pour it into a three-necked flask equipped with a thermometer, a condensing device and a stirrer. Slowly heat it to boiling and dehydrate it under normal pressure. When the temperature rises to 130 °C, add 5 g of stannous octoate, where the mass ratio of lactic acid to stannous octoate is 100. Keep dehydrating under reduced pressure for 10 h, with the pressure being 0.08 - 0.09 MPa, to make lactic acid polycondense into lactic acid oligomers. After the dehydration ends, change the condensing and receiving device, quickly raise the temperature of the reaction solution to 250 °C, and keep the pressure at 2000 Pa - 0.1 MPa. Collect the crude lactide through a three-stage cooling and collection system. The product is a yellow solid, and the crude product yield is 93%. The crude product is dissolved in water, cooled, filtered by suction, and then recrystallized three times with absolute ethanol, and dried in vacuum to constant weight to obtain the finished product, with a yield of 77%. After inspection, the purity of lactide is 99.96%. However, this method uses a tin-based catalyst and does not take any measures to reduce the racemization of lactide during the synthesis process. Therefore, subsequent product purification is mainly carried out through multiple solvent recrystallizations, resulting in a low single-pass product yield and a high purification cost.

[0006] In the existing industrial two-step method for preparing lactide, usually, crude lactide is first prepared through polycondensation - depolymerization, and then a series of purification processes are carried out to obtain refined lactide; and during the preparation of crude lactide, to ensure the conversion rate of lactic acid oligomers, usually with the aim of increasing the crude product yield, no effective measures are taken to inhibit the racemization of lactide during the preparation process, resulting in a relatively high content of m-lactide, increasing the difficulty of purification, and affecting the product quality, production cost and the product yield of the entire synthesis process. Summary of the Invention

[0007] Aiming at the deficiencies of the existing technology, the present invention provides a method for reducing the racemization during the synthesis of lactide. By adding a long-chain diol solvent during the synthesis of crude lactide, the present invention reduces the degree of racemization during the depolymerization process, reduces the generation of m-lactide, and reduces the subsequent purification cost.

[0008] A method for reducing the racemization during the synthesis of lactide provided by the present invention mainly includes the following content:

[0009] (1) Mix the lactic acid oligomer with a depolymerization catalyst and add them to a reaction kettle, and heat and raise the temperature.

[0010] (2) Add a molten long-chain diol solvent to the reaction kettle, carry out a depolymerization reaction, and collect the crude lactide after the reaction ends.

[0011] Further, it also includes step (3) of refining the crude lactide obtained in step (2) to obtain a high-purity lactide product.

[0012] In the present invention, the molecular weight of the lactic acid oligomer is 800 - 4000, preferably 1200 - 2800, and it is usually prepared from L-lactic acid or / and D-lactic acid as raw materials through dehydration and polycondensation processes. The dehydration process mainly removes the free water in lactic acid, and it can be carried out under normal pressure or reduced pressure; the reaction temperature in the polycondensation process is 140 - 170°C, the vacuum degree is 1000 - 2000 Pa, and the reaction time is 0.5 - 4.0 h.

[0013] In the present invention, the depolymerization catalyst is a tin compound catalyst, such as at least one of stannous octoate, stannous chloride, stannous oxide, stannous sulfate, stannous oxalate, etc., and stannous octoate is preferred.

[0014] In the present invention, the dosage of the depolymerization catalyst is 0.2% - 5.0% of the mass of the lactic acid oligomer, preferably 0.8% - 3.0%.

[0015] In the present invention, after the lactic acid oligomer and the depolymerization catalyst are mixed, they are added to a reaction kettle and heated to 100 - 160°C.

[0016] In the present invention, the number of carbon atoms of the long-chain diol solvent is not less than 12, preferably at least one of long-chain diol solvents with C13 - C18, and more preferably at least one of tetradecane diol, hexadecane diol, etc.

[0017] In the present invention, the dosage of the long-chain diol solvent is 0.3% - 6.0% of the mass of the lactic acid oligomer, preferably 0.8% - 4.0%.

[0018] In the present invention, the long-chain diol solvent is heated and melted before use, and the melting temperature is 80 - 160°C, preferably 100 - 160°C.

[0019] In the present invention, the depolymerization reaction temperature is 190 - 240°C, the vacuum degree is 200 - 1500 Pa, and the reaction time is 1.0 - 3.0 h.

[0020] In the present invention, in the crude lactide product, the content of L-lactide is 78% - 86%, the content of m-lactide is not more than 5.5%, preferably 3.5% - 5.0%, the content of L-lactic acid is not more than 6%, preferably 1.5% - 4.0%, and the content of dimer and trimer is 2.0% - 6.0%.

[0021] In the present invention, the single-pass conversion rate of the lactic acid oligomer in the depolymerization process can reach more than 90%.

[0022] In the present invention, the un-depolymerized lactic acid oligomer and the long-chain diol solvent at the bottom of the reaction kettle can be directly mixed with fresh lactic acid oligomer raw materials for the preparation of crude lactide, or they can be separated by hydrolysis to recycle the solvent and lactic acid.

[0023] In the present invention, the purification can adopt the purification methods commonly used in the art, such as rectification or solvent water extraction. If rectification process is used for purification, the number of trays in the rectification column is 8 - 10, the temperature of the reboiler at the bottom of the column is controlled at 160 - 180 °C, the temperature at the top of the column is 110 - 130 °C, and the vacuum degree at the top of the column is 200 - 300 Pa. The lactide product is continuously withdrawn.

[0024] In the purified lactide product of the present invention, both the chemical purity and the optical purity reach over 99%, the content of m-lactide is not more than 1.5%, and the product quality meets the requirements of the polymerization-grade monomer.

[0025] In the present invention, the overall single-pass total yield of the product in the whole synthesis process can reach over 70%.

[0026] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0027] (1) Based on the traditional lactide synthesis process, the present invention reduces the degree of racemization in the synthesis process by adding a long-chain diol solvent, avoids the formation of m-lactide, and reduces the subsequent purification cost. Compared with the process without adding the solvent, the content of m -lactide in the crude lactide is not more than 5.5%, and the degree of racemization can be reduced by more than 50%.

[0028] (2) By using the long-chain diol solvent, the influence of the depolymerization of tin-based catalysts or too high temperature on the racemization in the lactide preparation process can be significantly reduced. Therefore, the reaction can be carried out at a higher catalyst concentration and a higher temperature, which not only ensures the product quality but also improves the depolymerization reaction rate.

[0029] (3) The crude lactide product prepared by the present invention does not need to be refined by a specific (melt crystallization) separation process. High-purity lactide products can be obtained through rectification or solvent water extraction processes, meeting the requirements of polymerization-grade products and reducing the subsequent purification cost. Moreover, the solvent can be recycled, further reducing the cost of the whole synthesis process. Specific Embodiments

[0030] The method and its effects of the present invention will be further illustrated by the following examples. The examples are implemented on the premise of the technical solution of the present invention, and the detailed implementation manners and specific operation processes are given. However, the protection scope of the present invention is not limited to the following examples.

[0031] In the following examples, the experimental methods, unless otherwise specified, are all conventional methods in the art. The experimental materials used in the following examples, unless otherwise specified, can all be obtained from biochemical reagent stores.

[0032] The lactic acid used in the embodiments of the present invention is heat-resistant L-lactic acid with a lactic acid content of 88% or more, and its optical purity is not less than 99%.

[0033] The present invention uses a Malvern Viscotek OMNISEC GPC / SEC gel permeation chromatograph to analyze the molecular weight of lactic acid oligomers. Using the traditional calibration method, with polystyrene (PS) as the internal standard, the chromatographic column model is T3000, the size is 300 mm L × 8.0 mm, the column temperature is 40 °C, the flow rate is 1.0 mL / min, the sample concentration is 2 - 5 mg / mL, and the single injection volume is 500 μL.

[0034] The present invention uses an Agilent high performance liquid chromatograph to analyze the chemical purity of lactide. An ultraviolet detector is used, phosphoric acid and acetonitrile are used as the mobile phase, the chromatographic column model is ZORBAX SB-Aq, the column length is 250 mm, the column inner diameter is 4.6 mm, and the packing particle size inside is 5 μm. Detection wavelength: 200 nm, column temperature: 40 °C, flow rate: 1 mL / min, injection volume: 5 μL.

[0035] The present invention uses an Agilent gas chromatograph to analyze the content of different optical isomers of lactide. The CYCLOSIL-B model chromatographic column is selected, the vaporization chamber temperature is 250 °C, the detector temperature is 280 °C, a hydrogen flame ionization detector is used, the initial temperature of the column temperature programmed heating is 100 °C, held for 5 min, heated at a rate of 4 °C / min to 140 °C, held for 7 min, heated at a rate of 8 °C / min to 200 °C, held for 20 min, carrier gas N 2 Flow rate 1.4 mL / min, hydrogen flow rate 30 mL / min, air flow rate 400 mL / min, injection volume 0.5 μL.

[0036] The yield Y of the lactide purification process and the product yield Y of the entire preparation and purification process 总 The calculation formulas are as follows:

[0037]

[0038] Among them, m 0 is the mass of crude lactide, y 0 is the purity of L-lactide in crude lactide, m is the mass of the lactide product, and 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.

[0039] The specific rotation of the sample is analyzed using a WZZ-2S automatic polarimeter to characterize the optical purity of the sample. The specific rotation of pure L-lactide is -278, and the specific rotation of pure D-lactide is +278. mThe specific rotation of -lactide is 0, and the calculation formula for the optical purity X of the sample is as follows;

[0040]

[0041] Among them, α 纯物质 represents the specific rotation of pure lactide, and α 被测样品 represents the specific rotation of the substance to be measured.

[0042] Example 1

[0043] Preparation of lactic acid oligomer: (1) Removal of free water from lactic acid: Take 600 g of L-lactic acid (the lactic acid content is about 88.0%), add it into a four-necked flask with a stirring system, use a vacuum circulating water pump to maintain the pressure of the system at about 50 kPa, start heating under vacuum, gradually heat to 110 - 120 °C, and dehydrate for 2 h. At this time, the free water in the reaction system is slowly distilled out of the reaction system. (2) Preparation of lactic acid oligomer: After almost all the free water in the system is removed, increase the vacuum degree of the system, slowly reduce the pressure of the system to about 1.0 kPa, and gradually raise the temperature of the liquid material to 160 °C. React for 3 h. At this time, polycondensation reaction occurs between lactic acid molecules, and the water generated in the system is distilled out of the system, obtaining a lactic acid oligomer with a molecular weight of 2214.

[0044] Take 500 g of the above-prepared lactic acid oligomer and 4.0 g of stannous octoate, add them into a kettle reactor, heat up to 160 °C, and add 5.0 g of tetradecane diol heated and melted at 140 °C. Control the vacuum degree of the depolymerization reaction at 500 Pa, the reaction temperature at 200 °C, and react for 1.5 h to obtain a crude L-lactide product. After analysis of the product, the content of L-lactide is 85.1%, the content of m-lactide is 3.8%, the content of L-lactic acid is 3.1%, and the content of dimer and trimer is 4.6%. The single-pass conversion rate of lactic acid oligomer in the depolymerization process is 92.9%.

[0045] Take the crude lactide obtained in the above preparation process, and carry out purification by distillation process. The number of plates of the distillation column is 10. Control the temperature of the reboiler at the bottom of the column at 175 °C, the temperature at the top of the column at 118 °C, and the vacuum degree at the top of the column at 200 Pa, and continuously extract light components and lactide products. After analysis, the product yield of lactide at the top of the column is 83.6%, the chemical purity of the product is 99.3%, the optical purity is 99.2%, the content of m-lactide is 0.8%, and the single-pass total yield of lactide in the whole synthesis process is 74.1%.

[0046] Example 2

[0047] The preparation process of lactic acid oligomer is the same as that in Example 1.

[0048] Take 500 g of the above-prepared lactic acid oligomer and 25 g of stannous octoate, add them to a kettle reactor, heat up to 160 °C, and add 25 g of tetradecane diol heated and melted at 140 °C. Control the vacuum degree of the depolymerization reaction at 1000 Pa, the reaction temperature at 190 °C, and react for 1.5 h to obtain a crude L-lactide product. After analysis, the content of L-lactide in the product is 80.9%, the content of m-lactide is 5.5%, the content of L-lactic acid is 4.3%, and the content of dimers and trimers is 5.9%. The single-pass conversion rate of the lactic acid oligomer during the depolymerization process is 91.3%.

[0049] Take the crude lactide obtained in the above preparation process and purify it by distillation. The number of plates in the distillation column is 10. Control the temperature of the reboiler at the bottom of the kettle at 180 °C, the temperature at the top of the column at 118 °C, and the vacuum degree at the top of the column at 200 Pa. Continuously extract light components and lactide products. After analysis, the yield of the lactide product at the top of the column is 79.3%. Among them, the chemical purity of the product is 99.2%, the optical purity is 99.0%, and the content of m-lactide is 1.3%. The single-pass total yield of lactide in the whole synthesis process is 71.2%.

[0050] Example 3

[0051] The preparation process of the lactic acid oligomer is the same as that in Example 1.

[0052] Take 500 g of the above-prepared lactic acid oligomer and 2.0 g of stannous octoate, add them to a kettle reactor, heat up to 160 °C, and add 2.5 g of tetradecane diol heated and melted at 140 °C. Control the vacuum degree of the depolymerization reaction at 300 Pa, the reaction temperature at 240 °C, and react for 1.5 h to obtain a crude L-lactide product. After analysis, the content of L-lactide in the product is 81.5%, the content of m-lactide is 5.3%, the content of L-lactic acid is 4.1%, and the content of dimers and trimers is 5.8%. The single-pass conversion rate of the lactic acid oligomer during the depolymerization process is 90.4%.

[0053] Take the above-prepared crude lactide and purify it by distillation. The number of plates in the distillation column is 10. Control the temperature of the reboiler at the bottom of the kettle at 180 °C, the temperature at the top of the column at 118 °C, and the vacuum degree at the top of the column at 200 Pa. Continuously extract light components and lactide products. After analysis, the yield of the lactide product at the top of the column is 80.2%. Among them, the chemical purity of the product is 99.3%, the optical purity is 99.0%, and the content of m-lactide is 1.4%. The single-pass total yield of lactide in the whole synthesis process is 70.1%.

[0054] Example 4

[0055] The preparation process of the lactic acid oligomer is the same as that in Example 1.

[0056] The preparation process of lactide was the same as that in Example 1, except that the same mass of hexadecane diol was used. The obtained crude L-lactide was analyzed, and the content of L-lactide was 84.2%, the content of m-lactide was 4.1%, the content of L-lactic acid was 3.2%, and the content of dimers and trimers was 4.8%. The single-pass conversion rate of lactic acid oligomers in the depolymerization process was 91.9%.

[0057] The purification process of crude lactide was the same as that in Example 1, and the product yield of lactide obtained was 83.0%. The chemical purity of the product was 99.2%, the optical purity was 99.1%, the content of m-lactide was 1.0%, and the single-pass total yield of lactide in the whole synthesis process was 72.5%

[0058] Example 5

[0059] The preparation of lactic acid oligomers was the same as that in Example 1, except that: the final temperature in the polycondensation process was 150 °C, and the reaction was carried out for 2 h to obtain lactic acid oligomers with a molecular weight of 917.

[0060] The preparation process of lactide was the same as that in Example 1, except that the depolymerization reaction was carried out with lactic acid oligomers with a molecular weight of 917 as the raw material. The obtained crude L-lactide was analyzed, and the content of L-lactide was 78.81%, the content of m-lactide was 3.4%, the content of L-lactic acid was 5.8%, and the content of dimers and trimers was 5.9%. The single-pass conversion rate of lactic acid oligomers in the depolymerization process was 91.8%.

[0061] The purification process of crude lactide was the same as that in Example 1, and the product yield of lactide obtained was 79.3%. The chemical purity of the product was 99.1%, the optical purity was 99.0%, the content of m-lactide was 0.9%, and the single-pass total yield of lactide in the whole synthesis process was 70.5%.

[0062] Example 6

[0063] The preparation of lactic acid oligomers was the same as that in Example 1, except that: the final temperature in the polycondensation process for preparing lactic acid oligomers was 170 °C, and the reaction was carried out for 4 h to obtain lactic acid oligomers with a molecular weight of 3814.

[0064] The preparation process of lactide was the same as that in Example 1, except that the depolymerization reaction was carried out with lactic acid oligomers with a molecular weight of 3814 as the raw material. The obtained crude lactide was analyzed, the content of L-lactide was 86.0%, the content of m-lactide was 5.3%, the content of L-lactic acid was 2.8%, and the content of dimers and trimers was 3.9%. The single-pass conversion rate of lactic acid oligomers in the depolymerization process was 90.6%.

[0065] The purification process of crude lactide was the same as that in Example 1, and the yield of the obtained lactide product was 81.8%. The chemical purity of the product was 99.4%, the optical purity was 99.0%, the content of meso-lactide was 1.4%, and the overall single-pass total yield of lactide in the whole synthesis process was 71.3%.

[0066] Example 7

[0067] The preparation process of lactic acid oligomer was the same as that in Example 1.

[0068] The preparation process of lactide was the same as that in Example 1, except that stannous chloride with the same mass was used as the catalyst to prepare crude L-lactide. After analyzing the product, the content of L-lactide was 84.7%, the content of meso-lactide was 3.9%, the content of L-lactic acid was 3.4%, and the content of dimer and trimer was 5.1%. The single-pass conversion rate of lactic acid oligomer in the depolymerization process was 91.2%.

[0069] The purification process of crude lactide was the same as that in Example 1, and the product yield of the obtained lactide was 83.3%. The chemical purity of the product was 99.2%, the optical purity was 99.0%, the content of meso-lactide was 1.0%, and the overall single-pass total yield of lactide in the whole synthesis process was 71.8%.

[0070] Comparative Example 1

[0071] The preparation processes of lactic acid oligomer and crude lactide were the same as those in Example 1, except that tetradecane diol was not added. In the obtained crude lactide, the content of L-lactide was 81.3%, the content of meso-lactide was 7.6%, the content of L-lactic acid was 3.8%, and the content of dimer and trimer was 6.2%. The single-pass conversion rate of lactic acid oligomer in the depolymerization process was 83.5%.

[0072] The purification process of crude lactide was the same as that in Example 1, and the product yield of the obtained lactide was 72.3%. The chemical purity of the product was 98.6%, the optical purity was 97.3%, the content of meso-lactide was 3.1%, and the product could not meet the requirements of polymerization-grade monomer and still needed to be further refined by combining with melt crystallization.

[0073] Comparative Example 2

[0074] The preparation processes of lactic acid oligomer and crude lactide were the same as those in Example 1, except that decane diol with the same mass was used instead of tetradecane diol. In the obtained crude lactide, the content of L-lactide was 81.9%, the content of meso-lactide was 6.6%, the content of L-lactic acid was 3.8%, and the content of dimer and trimer was 5.2%. The single-pass conversion rate of lactic acid oligomer in the depolymerization process was 89.3%.

[0075] The purification process of crude lactide was the same as that in Example 1. The product yield of the obtained lactide was 67.2%, the chemical purity of the product was 98.2%, the optical purity was 97.1%, and the content of m-lactide was 3.3%. The product could not meet the requirements of polymerization-grade monomers, and the product still needed to be refined by combining with the melt crystallization process.

[0076] Comparative Example 3

[0077] The preparation processes of lactic acid oligomers and crude lactide were the same as those in Example 1, except that zinc oxide was used as the catalyst. The obtained crude L-lactide was analyzed, and the content of L-lactide was 79.6%, the content of m-lactide was 7.3%, the content of L-lactic acid was 3.5%, and the content of dimer and trimer was 5.8%. The single-pass conversion rate of lactic acid oligomers in the depolymerization process was 80.9%.

[0078] The purification process of crude lactide was the same as that in Example 1. The product yield of the obtained lactide was 70.1%. The chemical purity of the product was 98.0%, the optical purity was 94.1%, and the content of m-lactide was 3.8%. The product could not meet the requirements of polymerization-grade monomers, and the product still needed to be refined by combining with the melt crystallization process.

Claims

1. A method for reducing the racemization in the synthesis of lactide, characterized in that it comprises the following steps: (1) Mix the lactic acid oligomer with a depolymerization catalyst and add them to a reaction kettle, then heat up; the molecular weight of the lactic acid oligomer is 800 - 4000; (2) Add a molten long-chain diol solvent to the reaction kettle for depolymerization reaction, and collect the crude lactide after the reaction ends; the long-chain diol solvent is at least one of C13 - C18 long-chain diol solvents; the depolymerization catalyst is a tin compound catalyst; the depolymerization reaction temperature is 190 - 240 °C, the vacuum degree is 200 - 1500 Pa, and the reaction time is 1.0 - 3.0 h.

2. The method according to claim 1, characterized in that: It further comprises step (3) of refining the crude lactide in step (2) to obtain a high-purity lactide product.

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

4. The method according to claim 1 or 3, characterized in that: The lactic acid oligomer is prepared from L-lactic acid or / and D-lactic acid as raw materials through dehydration and polycondensation processes; the dehydration process mainly removes the free water in lactic acid, adopting atmospheric pressure or reduced pressure forms; the reaction temperature in the polycondensation process is 140 - 170 °C, the vacuum degree is 1000 - 2000 Pa, and the reaction time is 0.5 - 4.0 h.

5. The method according to claim 1, characterized in that: The depolymerization catalyst is at least one of stannous octoate, stannous chloride, stannous oxide, stannous sulfate, and stannous oxalate.

6. The method according to claim 1 or 5, characterized in that: The dosage of the depolymerization catalyst is 0.2% - 5.0% of the mass of the lactic acid oligomer.

7. The method according to claim 6, characterized in that: The dosage of the depolymerization catalyst is 0.8% - 3.0% of the mass of the lactic acid oligomer.

8. The method according to claim 1 or 5, characterized in that: After the lactic acid oligomer and the depolymerization catalyst are mixed, they are added to the reaction kettle and heated to 100 - 160 °C.

9. The method according to claim 1, characterized in that: The long-chain diol solvent is at least one of tetradecane diol and hexadecane diol.

10. The method according to claim 1 or 9, characterized in that: The dosage of the long-chain diol solvent is 0.3% - 6.0% of the mass of the lactic acid oligomer.

11. The method according to claim 1 or 9, characterized in that: The dosage of the long-chain diol solvent is 0.8% - 4.0% of the mass of the lactic acid oligomer.

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

13. The method according to claim 12, characterized in that: The melting temperature is 100 - 160 °C.

14. The method according to claim 1, characterized in that: In the crude lactide product, the content of L-lactide is 78% - 86%, the content of meso-lactide is not more than 5.5%, the content of L-lactic acid is not more than 6%, and the content of dimers and trimers is 2.0% - 6.0%.

15. According to the method described in claim 14, it is characterized in that: in the crude lactide product, the content of meso-lactide is 3.5% - 5.0%, and the content of L-lactic acid is 1.5% - 4.0%.

16. According to the method described in claim 1, it is characterized in that: the single-pass conversion rate of the lactic acid oligomer in the depolymerization process reaches more than 90%.

17. According to the method described in claim 1, it is characterized in that: the un-depolymerized lactic acid oligomer at the bottom of the reaction kettle is mixed with a long-chain diol solvent, or directly with a fresh lactic acid oligomer raw material for the preparation of crude lactide, or the solvent and lactic acid are separated and recycled after hydrolysis.

18. According to the method described in claim 2, it is characterized in that: the refining is carried out by a distillation process. The number of trays in the distillation column is 8 - 10. The temperature of the reboiler at the bottom of the column is controlled at 160 - 180 °C, the temperature at the top of the column is 110 - 130 °C, and the vacuum degree at the top of the column is 200 - 300 Pa. The lactide product is continuously withdrawn.

19. According to the method described in claim 2, it is characterized in that: in the purified and refined lactide product, both the chemical purity and the optical purity reach more than 99%, the content of meso-lactide is not more than 1.5%, and the product quality meets the requirements of polymer-grade monomers.

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