An amine-based metal catalyst composition and a method of catalyzing the polycondensation of lactic acid monomers and / or the synthesis of lactide

Abstract

The application discloses an amine metal catalyst composition and a method for catalyzing polycyclization of lactic acid monomers and / or synthesis of lactide, wherein the amine metal catalyst composition comprises components and their molar ratios are as follows: stannous octoate: 2-[[bis(2-methoxyethyl)aminyl]methyl]-4,6-bis(1,1-dimethylethyl)phenol:dihydroxytoluene:(triphenylphosphine)acetyl ethyl ester=1:(1.1-3):(1.1-4):(1.1-6). The catalyst composition is used in the method for catalyzing polycyclization of lactic acid monomers and / or synthesis of lactide, solves the problem that L-lactide is generated while more D-, MESO-lactide is generated in the existing lactide synthesis process, can significantly reduce the hetero-acid value in the system, reduce the color value of the lactide product, increase the number of catalyst reuse, significantly enhance the process economy, and has a strong industrial application prospect.

Description

Technical Field

[0001] This invention belongs to the technical field of polylactic acid biodegradable materials, specifically relating to an amine-based metal catalyst composition and a method for catalyzing the polycondensation of lactic acid monomers and / or the synthesis of lactide. Background Technology

[0002] Polylactic acid, also known as polylactide, is a high-performance polymer with excellent biocompatibility and biodegradability. It is mainly used in biodegradable packaging materials, drug microsphere carriers, anti-adhesive films, biological catheters, orthopedic fixation devices, orthopedic surgical instruments, artificial bones, and other applications.

[0003] Polylactic acid (PLA) can be synthesized through two pathways: direct polycondensation of lactic acid monomers, which generally struggles to produce polymers with high relative molecular weights; and a two-step method, where lactic acid is first dehydrated and polycondensed to obtain lactic acid oligomers, which then undergo self-cyclization to synthesize the intermediate product lactide. Lactide is then ring-opened and polymerized to generate PLA. This method, through ionic or coordination polymerization, can yield high molecular weight products with millions of molecules, making it the preferred method for PLA preparation. In current lactide synthesis processes, the reuse of reactor residue is a key step in cost reduction. Improving efficiency and reusing the residue in this step is a core direction for process optimization. Simultaneously, developing environmentally friendly synthesis processes and achieving green and environmentally friendly polyester production is of great significance to green chemistry. Most of the reported synthesis processes focus on catalyst optimization. For example, the Mehrkhodavandi group improved the catalytic activity of the reaction by introducing secondary amines or imines to coordinate with zinc metal (Inorg. Chem., 2016, 55, 9445-9453). However, most of these processes do not pay attention to the economic efficiency of product acid value, color, and residue reuse, as well as their impact on product indicators. Furthermore, traditional industrial catalysts do not have high optical purity, resulting in higher costs.

[0004] To address the aforementioned problems, this invention proposes adding reaction promoters to existing industrial catalysts to obtain a catalyst composition that not only enhances catalyst stability (increasing the number of re-applications) and ensures good performance under high-temperature reaction conditions, but also significantly improves the selectivity of lactide. Furthermore, it maintains high optical purity and low impurity acid content even after multiple re-applications. This is a simple and efficient new method for preparing lactide materials, which is of great significance for achieving highly active and controllable synthesis of lactones and promoting the development of sustainable polymers. Summary of the Invention

[0005] The purpose of this invention is to provide an amine-based metal catalyst composition that can be used to catalyze the polycondensation of lactic acid monomers and / or the synthesis of lactide. In the process of lactide synthesis, it can increase the number of catalyst reuses, reduce the heteroacid value of the reaction products, significantly promote monomer polycondensation and oligomer cyclization reactions, especially inhibit the occurrence of side reactions during cyclization, and improve the ring-opening conversion rate of lactide.

[0006] Another object of the present invention is to provide a method for the use of the aforementioned amine-based metal catalyst composition for the polycondensation of lactic acid monomers and / or the synthesis of lactide, which has the advantages of high reactivity, high product purity and high raw material conversion rate.

[0007] To achieve the above-mentioned objectives, the present invention adopts the following technical solution:

[0008] An amine-based metal catalyst composition comprising the following components: stannous octoate, 2-[[bis(2-methoxyethyl)amino]methyl]-4,6-bis(1,1-dimethylethyl)phenol, dihydroxytoluene, and ethyl (triphenylphosphine)pyruvate.

[0009] In one specific implementation, the molar ratio of each component is stannous octoate: 2-[[bis(2-methoxyethyl)amino]methyl]-4,6-bis(1,1-dimethylethyl)phenol: dihydroxytoluene: ethyl (triphenylphosphine)pyruvate = 1:(1.1-3):(1.1-4):(1.1-6).

[0010] In one specific embodiment, the amine-based metal catalyst composition is prepared by the following steps: adding stannous octoate, 2-[[bis(2-methoxyethyl)amino]methyl]-4,6-bis(1,1-dimethylethyl)phenol, dihydroxytoluene, and ethyl (triphenylphosphine)pyruvate to a reactor, introducing a solvent under inert gas protection, stirring the mixture and refluxing, and removing excess solvent to obtain the amine-based metal catalyst composition; preferably, the solvent is selected from acetone, lactic acid, toluene, tetrahydrofuran, or ethylene glycol dimethyl ether, preferably lactic acid or acetone.

[0011] In one specific embodiment, the molar ratio of stannous octoate, 2-[[bis(2-methoxyethyl)amino]methyl]-4,6-bis(1,1-dimethylethyl)phenol, dihydroxytoluene, and ethyl (triphenylphosphine)pyruvate is 1:(1.1-1.5):(1.1-2):(1.1-4).

[0012] In one specific implementation, the reaction temperature of the mixture stirring reaction is 40–60°C, and the reaction time is 1–6 h.

[0013] In one specific implementation, the reaction temperature of the mixture stirring reaction is 50-55°C, and the reaction time is 1-2 hours.

[0014] On the other hand, a method for catalytic polycondensation of lactic acid monomers and / or synthesis of lactide, wherein the catalyst comprises the aforementioned amine metal catalyst composition; preferably, the catalyst is the aforementioned amine metal catalyst composition.

[0015] In one specific implementation, during the lactic acid monomer polycondensation stage, the reaction temperature is 140–170°C, preferably 150–160°C, the reaction pressure is 30–50 hPa, and the reaction time is 0.5–3 h.

[0016] In one specific implementation, during the lactide synthesis stage, the reaction temperature is 140–200°C, preferably 150–160°C, the reaction pressure is 10–25 hPa, and the reaction time is 0.5–1 h.

[0017] In one specific embodiment, the amount of the amine metal catalyst composition added during the catalytic polycondensation of lactic acid monomers and / or lactide synthesis stage is 0.1% to 1%, preferably 0.4% to 1%, based on the mass of stannous octoate in the amine metal catalyst composition relative to the 87 wt% aqueous lactic acid solution added in the reaction.

[0018] Compared with the prior art, the present invention has the following beneficial effects:

[0019] The amine-based metal catalyst composition of the present invention is used for the polycondensation of lactic acid monomers and / or the synthesis of lactide. It can significantly suppress the formation of D-,MESO-lactide during the reaction process and convert it into L-lactide, thus solving the problem that the formation of L-lactide is accompanied by the formation of a large amount of D-,MESO-lactide in the existing lactide synthesis process. In addition, it can significantly reduce the value of impurities and color in the system, increase the number of catalyst reuses, significantly enhance the economic efficiency of the process, and has the potential for industrial scale-up. Detailed Implementation

[0020] To better understand the technical solution of the present invention, the following embodiments will further illustrate the method provided by the present invention. However, the present invention is not limited to the listed embodiments, but should also include any other well-known modifications within the scope of the claims of the present invention.

[0021] The amine-based metal catalyst composition of the present invention is used for the polycondensation of lactic acid monomers and / or the synthesis of lactide, replacing the metal catalysts in the prior art. In the amine-based metal catalyst composition, the components and molar ratios are stannous octoate: 2-[[bis(2-methoxyethyl)amino]methyl]-4,6-bis(1,1-dimethylethyl)phenol: dihydroxytoluene: ethyl (triphenylphosphine)pyruvate = 1:(1.1-3):(1.1-4):(1.1-6), for example 1:1.1:1.1:1.1, 1:1.2:1.5:1.5, 1:1.3:1.8:2, 1:1.5:1.1:2, 1:1.5:2:4, 1:2:3:5, 1:3:4:6, etc.

[0022] 2-[[bis(2-methoxyethyl)amino]methyl]-4,6-bis(1,1-dimethylethyl)phenol, also known as {phenol, 2-[[bis(2-methoxyethyl)amino]methyl]-4,6-bis(1,1-dimethylethyl}, has CAS number 870680-83-4. Suppliers include, for example, Chemieliva Pharmaceutical. Its structural formula is as follows:

[0023]

[0024] This invention obtains an amino-based metal catalyst composition by pretreating a stannous octoate metal catalyst with 2-[[bis(2-methoxyethyl)amino]methyl]-4,6-bis(1,1-dimethylethyl)phenol, dihydroxytoluene, and ethyl (triphenylphosphine)pyruvate. Compared with metal catalysts used in the prior art, by using N and O electron-donating groups, not only is the catalytic stability of the catalyst improved, but the charge distribution at the reaction point can also be controlled, allowing Sn to... 2+ It is easier to induce carbonyl group to form a ring; at the same time, it is speculated that the steric hindrance formed by this composition can effectively control the methyl form of lactide during the ring formation process, which is beneficial to inhibiting the formation reaction of R-lactide and M-lactide.

[0025] Furthermore, the preparation method of the amine-based metal catalyst composition includes the following steps: adding stannous octoate, 2-[[bis(2-methoxyethyl)amino]methyl]-4,6-bis(1,1-dimethylethyl)phenol, dihydroxytoluene, and ethyl (triphenylphosphine)pyruvate to a reactor in a molar ratio of 1:(1.1-3):(1.1-4):(1.1-6), introducing lactic acid as a solvent under inert gas protection, stirring the mixture and refluxing, and then evaporating excess solvent to obtain the amine-based metal catalyst composition.

[0026] The amount of solvent lactic acid added is based on the complete dissolution of all components, with no particular limitation. Excessive addition will not have any impact, as the excess solvent will be removed by rotary evaporation in subsequent post-treatment. The mixed solution formed by the aforementioned mixture and solvent is stirred and refluxed. The reaction temperature is 40–60°C, for example, 41°C, 42°C, 43°C, 44°C, 45°C, 46°C, 47°C, 48°C, 49°C, 50°C, 51°C, 52°C, 53°C, 54°C, 55°C, 56°C, 57°C, 58°C, 59°C, 60°C, etc., preferably 50–55°C. The reaction time is 1–6 hours, for example, 1 hour, 1.5 hours, 2 hours, 3 hours, 4 hours, 5 hours, 6 hours, etc., preferably 1–2 hours.

[0027] This invention also provides the use of the aforementioned amine-based metal catalyst composition for catalyzing the polycondensation of lactic acid monomers and the synthesis of lactide. Specifically, in the process of catalyzing the polycondensation of lactic acid monomers and the synthesis of lactide, the amine-based metal catalyst composition of this invention is used instead of the metal catalyst in the prior art. Unless otherwise specified, reference can be made to the prior art, such as CN202210876468.1. Relevant contents of the prior art in the polycondensation of lactic acid monomers and the synthesis of lactide can be incorporated into this invention, which is understandable to those skilled in the art.

[0028] Specifically, in the lactic acid polycondensation stage, the amount of the amine-based metal catalyst composition added is 0.1% to 1%, based on the mass of stannous octoate in the amine-based metal catalyst composition relative to the mass of the 87 wt% lactic acid aqueous solution added in the reaction. For example, the amount added is 0.1 wt%, 0.2 wt%, 0.3 wt%, 0.4 wt%, 0.5 wt%, 0.6 wt%, 0.7 wt%, 0.8 wt%, 0.9 wt%, 1 wt%, etc., preferably 0.4 wt% to 1 wt%. In the lactide preparation reaction, 87 wt% lactic acid aqueous solution is usually added as a raw material. The amount of the amine-based metal catalyst composition added in this invention is based on the mass of stannous octoate contained therein relative to the mass of the 87 wt% lactic acid aqueous solution added in the reaction. The reaction temperature is 140–170℃, such as 140℃, 145℃, 150℃, 155℃, 160℃, 165℃, 170℃, etc., preferably 150℃–160℃; the reaction pressure is 30–50 hpa, such as 30 hpa, 35 hpa, 40 hpa, 45 hpa, 50 hpa, etc.; and the reaction time is 0.5–3 h, such as 0.5 h, 1 h, 1.5 h, 2 h, 2.5 h, 3 h, etc.

[0029] In actual production, the lactic acid polycondensation product from the first step is usually heated to carry out the second step of lactide synthesis. Under this process, the amine metal catalyst composition of the present invention does not need to be added again in the second step.

[0030] In the independent synthesis process of lactide, the amount of the amine metal catalyst composition added is 0.1% to 1%, based on the mass of stannous octoate in the catalyst composition relative to the lactic acid aqueous solution (87 wt%) added in the first step reaction. The reaction temperature is 140 to 200°C, for example, 140°C, 150°C, 160°C, 165°C, 170°C, 175°C, 180°C, 185°C, 190°C, 195°C, 200°C, etc., preferably 150°C to 160°C. The reaction pressure is 10 to 25 hPa, for example, 10 hPa, 15 hPa, 20 hPa, 25 hPa, etc., and the reaction time is 0.5 to 1 h, for example, 0.5 h, 0.6 h, 0.7 h, 0.8 h, 0.9 h, 1 h, etc.

[0031] Using the amine-based metal catalyst composition of the present invention as a catalyst, the optical purity of crude lactide obtained in the lactide reaction process under optimal conditions can reach 97.8-99.9%, which is higher than the 90% of the prior art.

[0032] The present invention will be further explained and illustrated below through more specific embodiments, but these do not constitute any limitation.

[0033] The analytical and testing methods involved in the following implementation are as follows:

[0034] 1) GPC testing of molecular weight and distribution

[0035] GPC measurements were performed using a combination of an LC-20AD solvent delivery pump, a Wyatt OPTILAB rEX refractive index detector, and Styragel P8512-10E3A10, P8512-10E4A10, and P8512-10E5A10 detectors with effective molar mass ranges of 100–40,000, 400–500,000, and 10,000–2,000,000, respectively. THF was used as the eluent (flow rate 1 mL / min, T = 40°C).

[0036] 2) GC test for miscellaneous acid content

[0037] Agilent 6820 gas chromatograph; column: OV-1 capillary column (50m*0.25mm); column temperature: (programmed temperature) initial temperature 129℃, hold for 4 min, temperature ramp rate 0.5℃ / min, final temperature 132℃, hold for 35 min; vaporization temperature: 280℃; detector temperature: 250℃; split ratio: 80:1; carrier gas: high-purity hydrogen, pressure 0.1MPa; make-up gas: 29ml / min; injection mode: split injection; injection volume: 0.2uL.

[0038] 3.) Colorimetric testing: Liquid colorimeter - KREDI D040 colorimeter (high-precision platinum cobalt Gardner colorimeter), platinum cobalt colorimetric values ​​are expressed in Hazen units, with a result range of 1 to 500.

[0039] Preparation Example 1: Amine-based metal catalyst composition 1

[0040] Stannous octoate, 2-[[bis(2-methoxyethyl)amino]methyl]-4,6-bis(1,1-dimethylethyl)phenol, dihydroxytoluene, and ethyl (triphenylphosphine)pyruvate were added sequentially in a molar ratio of 0.003:0.0033:0.006:0.0033 to a 150 mL acetone solution. The mixture was heated to 50 °C and stirred under reflux for 1 h. The excess solvent was then evaporated to obtain 3.9 g of a pale yellow liquid. This liquid was then added to a lactic acid solution to prepare a 25 wt% solution of 15.7 g, which was used as composition solution 1.

[0041] Preparation Example 2: Amine-based metal catalyst composition 2

[0042] Stannous octoate, 2-[[bis(2-methoxyethyl)amino]methyl]-4,6-bis(1,1-dimethylethyl)phenol, dihydroxytoluene, and ethyl (triphenylphosphine)pyruvate were added sequentially in a molar ratio of 0.003:0.0045:0.0033:0.012 to a 150 mL lactic acid solution. The mixture was heated to 55 °C and stirred under reflux for 5 hours. Excess solvent was then evaporated to obtain 6.9 g of a pale yellow liquid. This liquid was then added to the lactic acid solution to prepare a 25 wt% solution of 27.8 g, which was used as composition solution 2.

[0043] Preparation Example 3: Amine-based metal catalyst composition 3

[0044] Stannous octoate, 2-[[bis(2-methoxyethyl)amino]methyl]-4,6-bis(1,1-dimethylethyl)phenol, dihydroxytoluene, and ethyl (triphenylphosphine)pyruvate were added sequentially in a molar ratio of 0.007:0.0105:0.0077:0.028 to a 150 mL lactic acid solution. The mixture was heated to 55 °C and stirred under reflux for 5 hours. Excess solvent was then evaporated to obtain 16.22 g of a pale yellow liquid. This liquid was then added to the lactic acid solution to prepare a 25 wt% solution of 64.91 g, which was used as composition solution 3.

[0045] Example 1

[0046] The condensation reaction and lactide synthesis reaction are as follows: 300g of L-lactic acid aqueous solution (87wt%) was added to a 1L prepolymerization reactor, the system temperature was raised to 150℃, the pressure was 50hpa, and dehydration was carried out for 30min, with the water content being less than 1%; then, 15.7g of the prepared amine-based metal catalyst composition 1 solution (the content of stannous octoate in this composition is about 0.4wt% of the reaction raw material lactic acid aqueous solution) was added, the reaction system was heated to 170℃, the system pressure was 30hpa, and the reaction was carried out for 60min. The molecular weight distribution of the lactic acid oligomer obtained by the prepolymerization reaction was about 1500, and the distribution coefficient was 1.31; finally, the prepolymerization reaction solution was taken out and placed in a depolymerization reactor, the temperature was raised to 150℃, the system pressure was 25hpa, and the reaction was carried out for 1h to obtain crude lactide with an optical purity of 99.5%, an acid value of 80.4ppm, and a color of 32hazen.

[0047] Example 2

[0048] The condensation reaction and lactide synthesis reaction are as follows: 300g of L-lactic acid aqueous solution (87wt%) was added to a 1L prepolymerization reactor, the system temperature was raised to 150℃, the pressure was 50hpa, and dehydration was carried out for 30min, with the water content being less than 1%; then, 27.8g of the prepared amine-based metal catalyst composition 2 solution (the content of stannous octoate in this composition is about 0.4wt% of the reaction raw material lactic acid aqueous solution) was added, the temperature was kept constant, the system pressure was 50hpa, and the reaction was carried out for 3h. The molecular weight distribution of the lactic acid oligomer obtained by the prepolymerization reaction was about 1500, and the distribution coefficient was 1.4; finally, the prepolymerization reaction solution was taken out and placed in a depolymerization reactor, the temperature was raised to 200℃, the system pressure was 10hpa, and the reaction was carried out for 30min to obtain crude lactide with an optical purity of 99.5%, an acid value of 137ppm, and a color of 24hazen.

[0049] Example 3

[0050] The condensation reaction and lactide synthesis reaction are as follows: 300g of L-lactic acid aqueous solution (87wt%) was added to a 1L prepolymerization reactor, the system temperature was raised to 150℃, the pressure was 50hpa, and dehydration was carried out for 30min, with the water content being less than 1%; then, 64.91g of the prepared amine-based metal catalyst composition 3 solution (the content of stannous octoate in this composition is about 0.4wt% of the reaction raw material lactic acid aqueous solution) was added, the reaction system was heated to 160℃, the system pressure was 30hpa, and the reaction was carried out for 0.5h. The molecular weight distribution of the lactic acid oligomer obtained by the prepolymerization reaction was about 1000, and the distribution coefficient was 1.04; finally, the prepolymerization reaction solution was taken out and placed in a depolymerization reactor, the temperature was raised to 160℃, the system pressure was 10hpa, and the reaction was carried out for 30min to obtain crude lactide with an optical purity of 99.2%, an acid value of 54ppm, and a color of 17hazen.

[0051] Example 4 (Application Test)

[0052] In the lactide synthesis reaction of Example 1 above, the reaction was stopped after 190g of lactide was distilled off from the reactants. 256g of lactic acid solution was added to the residue in the reactor and the reaction was repeated 10 times. The optical purity of the distilled lactide was then tested, and the crude lactide was found to have an optical purity of 97.4%, an acid value of 172ppm, and a color of 54hazen.

[0053] Comparative Example 1 (compared to Example 2)

[0054] The condensation reaction and lactide synthesis reaction are as follows: 300g of L-lactic acid aqueous solution (87wt%) was added to a 1L prepolymerization reactor, the system temperature was raised to 150℃, the pressure was 50hpa, and dehydration was carried out for 30min, with the water content being less than 1%; then, 4.86g of lactic acid solution (this lactic acid solution was prepared from 1.2g of stannous octoate to form a 25wt% lactic acid solution; the content of stannous octoate accounted for about 0.4wt% of the lactic acid aqueous solution of the reaction raw material) was added, the temperature was kept constant, the system pressure was 50hpa, and the reaction was carried out for 3h. The molecular weight distribution of the lactic acid oligomer obtained by the prepolymerization reaction was about 1500, and the distribution coefficient was 2.1; finally, the prepolymerization reaction solution was taken out and placed in a depolymerization reactor, the temperature was raised to 200℃, the system pressure was 10hpa, and the reaction was carried out for 30min to obtain crude lactide with an optical purity of 90.5%, an acid value of 372ppm, and a color of 52hazen.

[0055] Comparative Example 2 (Application Experiment)

[0056] In the lactide synthesis reaction of Comparative Example 1, the reaction was stopped after 190g of lactide was distilled off from the reactants. 256g of lactic acid solution was added to the residue in the reactor for reuse. After 10 cycles, the optical purity of the distilled lactide was tested. The crude lactide obtained had an optical purity of 87.3%, an acid value of 753ppm, and a color of 172hazen.

[0057] The above embodiments are merely illustrative of the principles and effects of the present invention and are not intended to limit the invention. Any person skilled in the art can modify or alter the above embodiments without departing from the spirit and scope of the present invention. Therefore, all equivalent modifications or alterations made by those skilled in the art without departing from the spirit and technical concept disclosed in the present invention should still be covered by the claims of the present invention.

Claims

1. An amine-based metal catalyst composition, characterized in that, The amine-based metal catalyst composition comprises the following components: stannous octoate, 2-[[bis(2-methoxyethyl)amino]methyl]-4,6-bis(1,1-dimethylethyl)phenol, dihydroxytoluene, and ethyl (triphenylphosphine)pyruvate; The molar ratio of each component is stannous octoate: 2-[[bis(2-methoxyethyl)amino]methyl]-4,6-bis(1,1-dimethylethyl)phenol: dihydroxytoluene: ethyl (triphenylphosphine)pyruvate = 1:(1.1-3):(1.1-4):(1.1-6).

2. The amine-based metal catalyst composition according to claim 1, characterized in that, The amine-based metal catalyst composition is prepared by the following steps: adding stannous octoate, 2-[[bis(2-methoxyethyl)amino]methyl]-4,6-bis(1,1-dimethylethyl)phenol, dihydroxytoluene, and ethyl (triphenylphosphine)pyruvate to a reactor, introducing a solvent under inert gas protection, stirring the mixture and refluxing, and removing excess solvent to obtain the amine-based metal catalyst composition.

3. The amine-based metal catalyst composition according to claim 2, characterized in that, The solvent is selected from acetone, toluene, tetrahydrofuran, or ethylene glycol dimethyl ether.

4. The amine-based metal catalyst composition according to claim 3, characterized in that, The solvent is acetone.

5. The amine-based metal catalyst composition according to claim 1, characterized in that, The molar ratio of stannous octoate, 2-[[bis(2-methoxyethyl)amino]methyl]-4,6-bis(1,1-dimethylethyl)phenol, dihydroxytoluene, and ethyl (triphenylphosphine)pyruvate is 1:(1.1-1.5):(1.1-2):(1.1-4).

6. The amine-based metal catalyst composition according to claim 2, characterized in that, The reaction temperature of the mixture during stirring is 40–60°C, and the reaction time is 1–6 h.

7. The amine-based metal catalyst composition according to claim 6, characterized in that, The reaction temperature for the mixture is 50–55°C, and the reaction time is 1–2 hours.

8. A method for catalytic polycondensation of lactic acid monomers and / or synthesis of lactide, characterized in that, The catalyst includes the amine-metal catalyst composition according to any one of claims 1 to 7.

9. The method according to claim 8, characterized in that, During the lactic acid monomer polycondensation stage, the reaction temperature is 140–170℃, the reaction pressure is 30–50 hPa, and the reaction time is 0.5–3 h.

10. The method according to claim 8, characterized in that, During the lactide synthesis stage, the reaction temperature is 140–200℃, the reaction pressure is 10–25 hPa, and the reaction time is 0.5–1 h.

11. The method according to claim 8, characterized in that, During the lactide synthesis stage, the reaction temperature is 150℃~160℃.

12. The method according to any one of claims 8 to 11, characterized in that, The amount of the amine metal catalyst composition added in the catalytic lactic acid monomer condensation and / or lactide synthesis stage is 0.1% to 1%, based on the mass of stannous octoate in the amine metal catalyst composition as 87 wt% of the lactic acid aqueous solution added in the reaction.

13. The method according to claim 12, characterized in that, The amount of the amine metal catalyst composition added in the catalytic lactic acid monomer condensation and / or lactide synthesis stage is 0.4% to 1%, based on the mass of stannous octoate in the amine metal catalyst composition as 87 wt% of the lactic acid aqueous solution added in the reaction.

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