CN115677648B - Synthesis method of high optical purity lactide - Google Patents

Abstract

The invention discloses a method for synthesizing high optical purity lactide, which comprises the following steps: s1, dissolving substituted aniline in hydrochloric acid solution to obtain mixed solution, adding calcium oxide into the mixed solution, uniformly mixing, adding ammonium persulfate for oxidation polymerization reaction, adding alkali liquor for neutralization after the reaction is finished, performing solid-liquid separation, and collecting solid phase to obtain a calcium doped polyaniline catalyst; s2, mixing lactic acid and a prepolymerization catalyst for polymerization reaction, adding the calcium doped polyaniline catalyst prepared in the step S1 and a catalytic auxiliary agent for cracking reaction, and collecting fractions to obtain lactide. The invention innovatively adopts calcium with weaker catalytic performance to prepare the calcium doped polyaniline catalyst, and combines the calcium doped polyaniline catalyst with a catalytic auxiliary agent to synthesize lactide, so that not only can the high optical purity (up to 99.7%) of the lactide be realized, but also the calcium element is nontoxic, belongs to a large amount of elements existing in organisms, has better biocompatibility, and is suitable for medical materials.

Description

Synthesis method of high optical purity lactide

Technical Field

The invention belongs to the technical field of chemical production, and particularly relates to a method for synthesizing high-optical-purity lactide.

Background

High molecular weight PLA is a low gloss hard thermoplastic polymer with properties similar to polystyrene. PLA is considered to be one of the most valuable biodegradable polymers in the future due to its low toxicity, good biocompatibility and harmless degradation products such as carbon dioxide and water.

Lactide is an intermediate for industrial production of polylactic acid, and the yield and purity of the lactide limit the cost and quality of the downstream product polylactic acid. Efficient synthesis of lactide with high chemical and optical purity under conditions suitable for industrial production is a great challenge. In recent years, many studies on lactide synthesis have been made, and a novel catalyst has been developed to promote the cleavage of an oligolactic acid to lactide, and currently, commonly used catalysts include an organometallic compound, an inorganic catalyst, an organic catalyst, a composite catalyst, and the like. The catalyst adopted at the present stage mostly contains transition metal ions such as Sn (II), zn (II) and the like, and the metal ions have high catalytic activity, but are mostly toxic metals, have poor biocompatibility and limit the application range, so that the development of a nontoxic catalytic system for synthesizing lactide with high optical purity has very important significance.

Calcium belongs to alkaline earth metal, is not heavy metal, is the metal element with the highest content in human body, and has weak catalytic activity, and no related report about calcium is found in the lactide catalyst.

Disclosure of Invention

Aiming at the problem of poor biocompatibility caused by the use of toxic metal in the catalyst for lactide synthesis, the invention provides a method for synthesizing high optical purity lactide, which innovatively adopts polyaniline-supported calcium as the catalyst and is combined with a catalytic auxiliary agent, so that the high optical purity of lactide can be realized, and the biocompatibility of calcium is better, thereby being suitable for medical materials.

In order to achieve the above object, the present invention provides a method for synthesizing lactide of high optical purity, comprising the steps of:

s1, dissolving substituted aniline in hydrochloric acid solution to obtain mixed solution, adding calcium oxide into the mixed solution, uniformly mixing, adding ammonium persulfate solution for oxidation polymerization reaction, adding alkali liquor for neutralization after the reaction is finished, performing solid-liquid separation, and collecting solid phase to obtain a calcium doped polyaniline catalyst;

s2, mixing lactic acid and a prepolymerization catalyst for polymerization reaction, adding the calcium doped polyaniline catalyst prepared in the step S1 and a catalytic auxiliary agent for cracking reaction, and collecting fractions to obtain lactide.

Specifically, in the step S1, the substituted aniline is one or more of p-methylaniline, p-methoxyaniline, p-ethylaniline, p-phenylenediamine, 2-methoxy-p-phenylenediamine, 2-ethoxy-p-phenylenediamine, 2-isopropoxy-p-phenylenediamine, p-fluoroaniline and 2-fluoroaniline, wherein p-methoxyaniline is preferred, and the excellent electron donating property and coordination property of the substituent in the aniline can well improve the activity of the catalyst. And/or

The concentration of the substituted aniline in the mixed solution is 0.1-0.3 mol/L, preferably 0.25mol/L, and the concentration has the best effect.

Specifically, in step S1, the concentration of hydrochloric acid in the mixed solution is 1 to 3mol/L, preferably 2 mol/L.

Specifically, in the step S1, the molar ratio of the calcium oxide to the substituted aniline is 1 (3-7), preferably 1:5, and the catalyst activity is optimal when the ratio is used for preparing the catalyst.

Specifically, in step S1, the concentration of the ammonium persulfate solution is 0.1 to 0.3mol/L, preferably 0.25mol/L.

Specifically, in the step S1, the loading amount of the calcium element in the calcium-doped polyaniline material catalyst is 5.1-9.7 wt%, and the calcium element exists in the form of calcium sulfate dihydrate.

Preferably, in step S2, the prepolymerized catalyst is calcium oxide, and the amount is 0.1 to 0.5% by mass, preferably 0.4% by mass of lactic acid.

The dosage of the calcium-doped polyaniline material catalyst is 0.1-0.5% of the mass of lactic acid, preferably 0.3%. The use of this amount of calcium catalyst can maximize the optical purity of the product and provide a high cost performance.

Preferably, in step S2, the catalyst promoter is (S) -1,1 '-binaphthyl-2, 2' -dicarboxylic acid or (R) -1,1 '-binaphthyl-2, 2' -dicarboxylic acid, preferably (S) -1,1 '-binaphthyl-2, 2' -dicarboxylic acid, and lactide of high optical purity can be obtained using this catalyst promoter.

The mass ratio of the catalyst auxiliary agent to the prepolymerized catalyst is 1 (0.2-1.4), preferably 1:1, the optimum catalytic effect can be obtained by the use amount, and higher optical purity can be obtained.

In a preferred embodiment of the present invention, the method for synthesizing lactide of high optical purity comprises the steps of:

s1, dissolving p-methoxy aniline in hydrochloric acid to obtain a mixed solution, adding calcium oxide into the mixed solution, uniformly mixing, adding ammonium persulfate solution, uniformly stirring, standing for 3-5 h, adding sodium hydroxide solution for neutralization after the reaction is finished, then carrying out solid-liquid separation, and collecting a solid phase to obtain the calcium doped polyaniline catalyst;

s2, mixing lactic acid and calcium oxide for polymerization reaction, adding the calcium doped polyaniline catalyst prepared in the step S1 and (S) -1,1 '-binaphthyl-2, 2' -dicarboxylic acid for cracking reaction, and collecting fractions to obtain lactide;

in the step S1, the concentration of hydrochloric acid in the mixed solution is 1-3 mol/L, and the concentration of p-methoxyaniline is 0.1-0.3 mol/L; the molar ratio of the calcium oxide to the p-methoxyaniline is 1 (3-7); the concentration of the ammonium persulfate solution is 0.1-0.3 mol/L, and the volume ratio of the ammonium persulfate solution to the p-methoxy aniline hydrochloric acid solution is 1 (0.5-2.5);

in the step S2, the dosage of the calcium oxide is 0.1 to 0.5 percent of the mass of the lactic acid; the mass ratio of the added (S) -1,1 '-binaphthyl-2, 2' -dicarboxylic acid to the prepolymerized catalyst calcium oxide is 1 (0.2-1.4).

Through the technical scheme, the invention has the following beneficial effects:

1. the invention innovatively adopts calcium with weaker catalytic performance to prepare the calcium doped polyaniline catalyst, and combines the calcium doped polyaniline catalyst with a catalytic auxiliary agent to synthesize lactide, so that not only can the high optical purity (up to 99.7%) of the lactide be realized, but also the calcium element is nontoxic, belongs to a large amount of elements existing in organisms, has better biocompatibility, and is suitable for medical materials.

2. The method has the advantages of simple process for preparing the catalyst, obvious improvement of yield, higher optical purity of the product and obvious cost reduction by using low-cost metal calcium as the catalyst.

Drawings

FIG. 1 is a Scanning Electron Microscope (SEM) image of a catalyst prepared in example 1 of the present invention;

FIG. 2 is a Transmission Electron Microscope (TEM) image of the catalyst prepared in example 1 of the present invention;

FIG. 3 is a high resolution transmission electron microscope (HR-TEM) image of the catalyst according to example 1 of the present invention;

FIG. 4 is an electron diffraction pattern of calcium sulfate crystals in the catalyst prepared in example 1 of the present invention;

FIG. 5 is an energy dispersive X-ray spectroscopy (EDX) diagram of the catalyst prepared in example 1 of the present invention;

FIG. 6 is an infrared absorption spectrum (IR) diagram of a catalyst prepared in example 1 of the present invention;

FIG. 7 is an X-ray diffraction (XRD) pattern of the catalyst prepared in example 1 of the present invention.

Detailed Description

The following describes specific embodiments of the present invention in detail with reference to examples. It should be understood that the detailed description and specific examples, while indicating and illustrating the invention, are not intended to limit the invention.

In the following examples, the sources and purities of the drugs used are as follows:

para-methoxyaniline (CAS number: 94-9), purchased from Annaiji, 99% purity

Hydrochloric acid (CAS number: 7647-01-0), purchased from national drug group, concentration 36% -38%

Calcium oxide (CAS number 1305-78-8), purchased from Annaiji, purity 98%

Ammonium persulfate (CAS number: 7727-54-0), purchased from Annaiji, purity 97%

Sodium hydroxide (CAS number 1310-73-2), purchased from Annaiji, 97% purity

Ethanol (CAS number 64-17-5), purchased from Annaiji, 99.5% purity

Ethyl acetate (CAS number 141-78-6), purchased from Annaiji, 99.8% purity

L-lactic acid (CAS number: 79-4), purchased from Annaiji, purity 90%

Toluene (CAS number: 108-88-3), available from the national drug group, purity 99.5%

Malonic acid (CAS number 141-82-2), purchased from Annaiji, purity 98%

Oxalic acid (CAS number 144-62-7), purchased from Annaiji, purity 98%

Phthalic acid (CAS number 88-99-3), purchased from Annaiji, 99% purity

Difluoro malonic acid (CAS number 1514-85-8), purchased from Annaiji, purity 95%

2-Butylmaleic acid (CAS number 534-59-8), purchased from Annaiji, 97% purity

2-Benzylmalonic acid (CAS number 616-75-1), purchased from Anazadirachtin, 97% purity

(S) -1,1 '-binaphthyl-2, 2' -dicarboxylic acid (CAS number 18531-96-9), purchased from Angustifolia, 97% purity

(R) -1,1 '-binaphthyl-2, 2' -dicarboxylic acid (CAS number: 80703-23-7), purchased from Angustifolia, 97% purity

(S) -2,2' -dihydroxy- [1,1' -binaphthyl ] -3,3' -dialdehyde (CAS number: 141779-46-6), purchased from Angustifolia, 97% purity

S- (-) -1,1 '-binaphthyl-2, 2' -bisdiphenylphosphine (CAS number: 76189-56-5), purchased from Anagliptin, purity 98%

(R) - (+) -1,1 '-binaphthyl-2, 2' -diamine (CAS number: 18741-85-0), purchased from Anagliptin, 99% purity

Trans-1, 2-cyclohexyl-dicarboxylic acid dimethyl ester (CAS number 3205-35-4), purchased from Annaiji, 97% purity

Trans-1, 4-cyclohexanedicarboxylic acid (CAS number 619-82-9), purchased from Annaiji, 97% purity

Cis-4-cyclohexene-1, 2-dicarboxylic acid (CAS number 2305-26-2), purchased from Annaiji, purity 98%

Example 1

The synthesis method of the lactide with high optical purity comprises the following steps:

s1, synthesizing a calcium doped polyaniline catalyst: adding calcium oxide (5 mmol) into 100mL of p-methoxyaniline hydrochloric acid solution (wherein the hydrochloric acid concentration is 2mol/L and the p-methoxyaniline concentration is 0.25 mol/L), slowly dropwise adding 100mL of ammonium persulfate aqueous solution (the concentration is 0.25 mol/L), stirring uniformly under air, standing for 4h, neutralizing with 1mol/L sodium hydroxide aqueous solution, washing the obtained black precipitate with water and ethanol for three times respectively, and vacuum drying the black precipitate at 70 ℃ to obtain the calcium-doped poly-p-methoxyaniline catalyst. Scanning Electron Microscope (SEM) images thereof show (fig. 1), the calcium-doped poly-p-methoxyaniline catalyst is a micron-sized block; transmission Electron Microscope (TEM) images show (fig. 2, 3) that the black spots on the nanometer scale (i.e. calcium compounds) are uniformly distributed in the carrier (i.e. polyaniline); under a high resolution transmission electron microscope (HR-TEM), a lattice plane with a spacing d=0.27 nm can be observed (fig. 3), which shows that in the calcium doped poly-p-methoxyaniline catalyst, calcium is present as calcium sulfate dihydrate, the presence of calcium sulfate dihydrate crystals also being reflected by its electron diffraction pattern (fig. 4); it can be demonstrated by energy dispersive X-ray spectroscopy (EDX) that calcium sulfate dihydrate was successfully supported on a polyaniline support (fig. 5), both catalysts not being present as simple physical blends; the characteristic spectrum of the polyaniline skeleton can be seen from the infrared absorption spectrum (figure 6) of the catalyst; from the X-ray diffraction pattern (fig. 7) it can be seen that the characteristic peaks of calcium sulfate dihydrate and polyaniline skeleton in the calcium doped poly-p-methoxyaniline catalyst, the broad peak at 2θ=25.3 corresponds to the (110) face of polyaniline, 2θ=11.5, 20.7, 23.4, 29.1, 31.1, 33.3, 40.7, 43.4, 47.8, 56.7, 68.7 and CaSO as given in JCPDS document (33-0311) ₄ ·2H ₂ And (3) conforming to O.

S2, synthesizing lactide: in a 250mL four-necked flask, 50g of L-lactic acid and 200mg of calcium oxide are firstly added to react for 4 hours at 150 ℃ under the condition of 40kPa to obtain the low polylactic acid (the process is a lactic acid prepolymerization stage), then 150mg of prepared calcium-doped poly-p-methoxyaniline catalyst and 200mg of (S) -1,1 '-binaphthyl-2, 2' -dicarboxylic acid are added to a reaction system to carry out cracking reaction for 3 hours at 240 ℃ under the condition of 1kPa, lactide is distilled off, and ethyl acetate is used for recrystallization, so that the yield is 78.3%. The optical rotation of the toluene solution was measured by polarimeter, and the optical purity was 99.7% compared with the standard sample (optical purity greater than 99.9%).

Example 2

Other conditions are the same as in example 1, and the properties of materials synthesized by using different substituted anilines in preparing the calcium-doped polyaniline catalyst are examined, and the experimental results are shown in table 1.

TABLE 1 Properties of materials synthesized using different substituted anilines

From the above results, it was found that the use of each substituted aniline gave a good catalytic effect, and the yield of the final lactide was 31.1% or more and the optical purity was 87.2% or more, and among them, the material synthesized using p-methoxyaniline (example 1) had the best catalytic effect (No. 3), and the yield of lactide was as high as 78.3% and the optical purity was as high as 99.7%.

Example 3

Other conditions were the same as in example 1, and the catalytic effect of using different hydrochloric acid concentrations in preparing the calcium-doped poly-p-methoxyaniline catalyst was examined, and the experimental results are shown in table 2.

TABLE 2 catalytic Effect Using different hydrochloric acid concentrations

From the above results, it was found that a catalyst prepared using hydrochloric acid can achieve a good catalytic effect, and the optical purity of lactide exceeds 99.0% at a concentration (1 to 3 mol/L) within the scope of the present invention, wherein the catalytic effect is best (No. 4) when the hydrochloric acid concentration is 2mol/L (example 1), and the yield of lactide can be as high as 78.3% and the optical purity as high as 99.7%.

Example 4

Other conditions were the same as in example 1, and the effect of different concentrations of p-methoxyaniline in the preparation of the calcium-doped poly-p-methoxyaniline catalyst was examined, and the experimental results are shown in table 3.

TABLE 3 catalytic Effect Using different para-methoxyaniline concentrations

From the above results, it was found that, in the concentration range of the substituted aniline of the present invention (0.1 to 0.3 mol/L), the lactide yield was 59.2% or more and the optical purity was 97.0% or more, and among them, the catalytic effect was best (No. 4) when the p-methoxyaniline concentration was 0.25mol/L (example 1), the lactide yield was as high as 78.3% and the optical purity was as high as 99.7%.

Example 5

Other conditions were the same as in example 1, and the effect of using different molar ratios of calcium oxide to p-methoxyaniline on the reaction was examined in the preparation of the calcium-doped poly-p-methoxyaniline catalyst, and the experimental results are shown in table 4.

TABLE 4 catalytic Effect Using different molar ratios of calcium oxide to substituted anilines

From the above results, it is understood that, within the scope of the present invention (molar ratio of calcium oxide to substituted aniline is 1 (3 to 7)), the lactide yield is 41.1% or more, the optical purity is 80.5% or more, and when the molar ratio of calcium oxide to substituted aniline is 1:5 (example 1) shows the best catalytic effect (No. 3), and the yield of lactide can reach as high as 78.3% and the optical purity can reach as high as 99.7%.

Example 6

Other conditions were the same as in example 1, and the effect of using different ammonium persulfate aqueous solution concentrations in preparing the calcium-doped poly-p-methoxyaniline catalyst was examined, and the experimental results are shown in table 5.

TABLE 5 catalytic Effect Using different ammonium persulfate aqueous solution concentrations

From the above results, it was found that, in the range of the present invention (the concentration of ammonium persulfate solution is 0.1 to 0.3 mol/L), the lactide yield is 71.1% or more, the optical purity is 97.5% or more, and the catalytic effect is best (No. 4) when the concentration of ammonium persulfate aqueous solution is 0.25mol/L (example 1), the lactide yield can be as high as 78.3%, and the optical purity can be as high as 99.7%.

Example 7

Other conditions are the same as in example 1, and the volume ratio effect of using different ammonium persulfate aqueous solutions and substituted aniline hydrochloric acid solutions in preparing the calcium-doped polyaniline material is examined, and the experimental results are shown in table 6.

TABLE 6 examination of the effect of using different volume ratio of aqueous ammonium persulfate solution to substituted aniline hydrochloric acid solution

From the above results, it is understood that the yield of lactide is 51.1% or more and the optical purity is 90.6% or more within the scope of the present invention (the volume ratio of the ammonium persulfate aqueous solution to the substituted aniline hydrochloric acid solution is 1 (0.5 to 2.5)), and the catalyst preparation effect (No. 2) is best when the volume ratio of the ammonium persulfate aqueous solution to the substituted aniline hydrochloric acid solution is 1:1 (example 1), and the yield of lactide can be as high as 78.3% and the optical purity can be as high as 99.7%.

Example 8

Other conditions were the same as in example 1, and the effect of the amount of the catalyst (i.e., calcium oxide) used in the stage of the prepolymerization of lactic acid on the catalytic effect was examined, and the experimental results are shown in Table 7.

TABLE 7 examination of catalytic Effect of catalyst amount in lactic acid Pre-polymerization stage

The above results indicate that, within the scope of the present invention (the amount of the prepolymerized catalyst is 0.1 to 0.5% by mass of lactic acid), the yield of lactide is 60.3% or more, the optical purity is 96.6% or more, and the effect is optimal when the amount of the prepolymerized catalyst reaches 0.4% (example 1). The yield (number 4) cannot be improved by increasing the amount.

Example 9

Other conditions are the same as in example 1, the effect of the amount of the calcium-doped poly-p-methoxyaniline catalyst used in the cracking stage on the catalytic effect is examined, and the experimental results are shown in table 8.

TABLE 8 influence of different amounts of calcium-doped Poly-p-Methoxyaniline catalyst used in the cleavage stage on the catalytic Effect

The above results demonstrate that, within the scope of the present invention (0.1 to 0.5% of calcium-doped polyaniline material by mass of lactic acid), the yield of lactide is 50.3% or more, the optical purity is 97.5% or more, and when the amount of calcium-doped poly-p-methoxyaniline catalyst reaches 0.3% (example 1), the effect is optimal (No. 3), and the yield cannot be improved by further increasing the amount.

Example 10

Other conditions were the same as in example 1, and the effect of the catalyst promoter on the catalytic effect was examined at different stages of cracking, and the experimental results are shown in Table 9.

TABLE 9 examination of the catalytic effect of different catalytic promoters in the cracking stage

From the above results, it was found that (S) -1,1 '-binaphthyl-2, 2' -dicarboxylic acid (example 1) had the best catalytic effect, and lactide (No. 7, 99.7%) and (R) -1,1 '-binaphthyl-2, 2' -dicarboxylic acid was obtained in a relatively high purity (No. 8, 99.2%). With other diacids, the effect may be rather lower than in the blank experiment (No. 14).

Example 11

Other conditions were the same as in example 1, and the effect of the amount of the catalyst auxiliary used in the cracking stage on the catalytic effect was examined, and the experimental results are shown in Table 10.

TABLE 10 examination of the effect of catalyst promoter usage on catalytic Effect in cracking stage

From the above results, it is understood that within the scope of the present invention (mass ratio of the catalyst auxiliary to the prepolymerized catalyst is 1 (0.2 to 1.4)), the yield of the synthesized lactide is 55.1% or more, the optical purity is 87.2% or more, and when the mass ratio of the catalyst auxiliary to the prepolymerized catalyst is 1: the best catalytic effect (example 1) was found to give lactide of higher purity (No. 5, 99.7%).

The preferred embodiments of the present invention have been described in detail above with reference to the examples, but the present invention is not limited to the specific details of the above embodiments, and various simple modifications can be made to the technical solutions of the present invention within the scope of the technical concept of the present invention, and all the simple modifications belong to the protection scope of the present invention.

In addition, the specific features described in the above embodiments may be combined in any suitable manner, and in order to avoid unnecessary repetition, various possible combinations are not described further.

Moreover, any combination of the various embodiments of the invention can be made without departing from the spirit of the invention, which should also be considered as disclosed herein.

Claims (5)

1. The synthesis method of the lactide with high optical purity is characterized by comprising the following steps:

s1, dissolving substituted aniline in hydrochloric acid solution to obtain mixed solution, adding calcium oxide into the mixed solution, uniformly mixing, adding ammonium persulfate solution for oxidation polymerization reaction, adding alkali liquor for neutralization after the reaction is finished, performing solid-liquid separation, and collecting solid phase to obtain a calcium doped polyaniline catalyst;

s2, mixing lactic acid and a prepolymerization catalyst for polymerization reaction, adding the calcium-doped polyaniline catalyst prepared in the step S1 and a catalytic auxiliary agent for cracking reaction, and collecting fractions to obtain lactide;

in the step S1, the substituted aniline is one or more of para-methylaniline, para-methoxyaniline, para-ethylaniline, p-phenylenediamine, 2-methoxy-p-phenylenediamine, 2-ethoxy-p-phenylenediamine, 2-isopropoxy-p-phenylenediamine, para-fluoroaniline and 2-fluoroaniline; in the mixed solution, the concentration of the substituted aniline is 0.1-0.3 mol/L; the concentration of hydrochloric acid in the mixed solution is 1-3 mol/L; the molar ratio of the calcium oxide to the substituted aniline is 1 (3-7); the concentration of the ammonium persulfate solution is 0.1-0.3 mol/L; the volume ratio of the ammonium persulfate solution to the p-methoxy aniline hydrochloric acid solution is 1 (0.5-2.5);

in the step S2, the catalyst auxiliary agent is (S) -1,1 '-binaphthyl-2, 2' -dicarboxylic acid or (R) -1,1 '-binaphthyl-2, 2' -dicarboxylic acid; the mass ratio of the catalyst auxiliary agent to the prepolymerized catalyst is 1 (0.2-1.4).

2. The synthesis method according to claim 1, wherein in the step S1, the calcium-doped polyaniline catalyst has a loading of 5.1 to 9.7wt% of calcium element, and the calcium element is present in the form of calcium sulfate dihydrate.

3. The synthesis method according to claim 1, wherein in the step S2, the pre-polymerization catalyst is calcium oxide, and the amount of the pre-polymerization catalyst is 0.1-0.5% of the mass of lactic acid.

4. The synthesis method according to claim 1, wherein in the step S2, the amount of the calcium-doped polyaniline catalyst is 0.1 to 0.5% by mass of the lactic acid.

5. The synthesis method according to claim 1, comprising the steps of:

s1, dissolving p-methoxy aniline in hydrochloric acid to obtain a mixed solution, adding calcium oxide into the mixed solution, uniformly mixing, adding ammonium persulfate solution, uniformly stirring, standing for 3-5 hours, adding sodium hydroxide solution for neutralization after the reaction is finished, then carrying out solid-liquid separation, and collecting a solid phase to obtain the calcium doped polyaniline catalyst;

s2, mixing lactic acid and calcium oxide for polymerization reaction, adding the calcium doped polyaniline catalyst prepared in the step S1 and (S) -1,1 '-binaphthyl-2, 2' -dicarboxylic acid for cracking reaction, and collecting fractions to obtain lactide;

in the step S1, the concentration of hydrochloric acid in the mixed solution is 1-3 mol/L, and the concentration of p-methoxyaniline is 0.1-0.3 mol/L; the molar ratio of the calcium oxide to the p-methoxyaniline is 1 (3-7); the concentration of the ammonium persulfate solution is 0.1-0.3 mol/L, and the volume ratio of the ammonium persulfate solution to the p-methoxy aniline hydrochloric acid solution is 1 (0.5-2.5);

in the step S2, the consumption of the calcium oxide is 0.1-0.5% of the mass of the lactic acid; the mass ratio of the added (S) -1,1 '-binaphthyl-2, 2' -dicarboxylic acid to the prepolymerized catalyst calcium oxide is 1 (0.2-1.4).