CN114763404A - Method for preparing polylactic acid from fruit peel - Google Patents

Abstract

The invention provides a method for preparing polylactic acid from fruit peel, which sequentially comprises the processes of sample preparation, saccharide extraction, lactic acid preparation and polymerization. Furthermore, the method does not need to rely on enzyme for hydrolysis, but uses alkali liquor for hydrolysis, so that the glucose in the peel can be better extracted. Compared with starch-based materials, the yield of lactic acid obtained by fermenting and degrading the pericarp is higher. In addition, the lactic acid polymerization process obtained through optimized design can ensure that the prepared polylactic acid has high purity.

Description

Method for preparing polylactic acid from fruit peel

Technical Field

The invention belongs to the field of kitchen garbage recycling, and particularly relates to a method for preparing polylactic acid from fruit peel.

Background

Polylactic acid, also known as polylactide, is a novel biodegradable material. Polylactic acid has good thermal stability and good solvent resistance and can be processed in various ways, such as extrusion, spinning, biaxial stretching, and injection blow molding. The product made of polylactic acid can be biodegraded, and has good biocompatibility, glossiness, transparency, hand feeling and heat resistance, and certain bacteria resistance, flame retardance and ultraviolet resistance, so the polylactic acid has wide application, can be used as packaging materials, fibers, non-woven fabrics and the like, and is mainly used in the fields of clothing, industry (building, agriculture, forestry and paper making), medical sanitation and the like at present.

The production of polylactic acid takes lactic acid as raw material, and most of the traditional lactic acid fermentation adopts starchiness raw material, such as corn, cassava and the like. In recent years, waste is becoming a benefit of the present invention, and there has been much interest in changing kitchen waste into a usable material such as bio-alcohol, and among them, changing kitchen waste into polylactic acid is one of the ways to effectively use kitchen waste. At present, potato skins and the like in kitchen waste are fully developed into polylactic acid materials, and another major type of peel of kitchen waste is rarely reported to be developed into polylactic acid.

The existing technology for preparing polylactic acid from potato skins generally comprises the steps of adding water and a proper amount of alpha-amylase for size mixing, cooking, transferring the cooked soybean to a fermentation tank, fermenting, and polymerizing lactic acid to obtain the polylactic acid. However, this method firstly relies on amylase to hydrolyze, the potato peel contains a large amount of starch and can be hydrolyzed by amylase, while the peel contains a large amount of sugars such as cellulose, hemicellulose and pectin, which are not suitable for hydrolyzing by amylase, and the peel contains many more and more complex components, so that the hydrolysis by adding only a single enzyme cannot hydrolyze all the sugars in the peel completely, which results in low sugar extraction rate. And the subsequent fermentation and degradation of lactic acid to obtain lactic acid in low rate. Finally, impurities are introduced by enzyme hydrolysis, so that the product purity is not high.

Therefore, the existing method for preparing polylactic acid from potato peels cannot be used in the peels. Therefore, it would be of great practical significance to develop a method for developing the pericarp into polylactic acid material.

Disclosure of Invention

In order to overcome the defects of the prior art, the invention provides a method for preparing polylactic acid from fruit peel by designing a method for extracting saccharides from the fruit peel and preparing the polylactic acid.

The method is realized by the following technical scheme:

a method for preparing polylactic acid from pericarp comprises the steps of sample preparation, saccharide extraction, lactic acid preparation and polymerization, and specifically comprises the following steps:

s1, sample preparation: pretreating the peel, wherein the pretreatment process comprises drying a sample;

s2, extracting saccharides: hydrolyzing the dried pericarp in step S1 with alkali solution, and filtering to obtain glucose solution;

s3, preparation of lactic acid: preparing a culture medium by using the glucose solution obtained in the step S2, inoculating the single-purity cultured lactobacillus into the culture medium and incubating; inoculating a starter in a culture medium, and fermenting to produce lactic acid;

s4, prepolymer formation: distilling the lactic acid obtained in the step S3, removing water, putting into a pre-polymerization reactor, and polymerizing to obtain a prepolymer;

S5, polymerization process: and (4) performing polymerization reaction on the prepolymer obtained in the step S4 to obtain polylactic acid.

Further, the polymerization process is condensation polymerization or ring-opening polymerization.

Further, the condensation polymerization process comprises:

s511: in a first reactor with a catalyst, carrying out azeotropic dehydration on lactic acid and an organic solvent to form a polymer; recovering the polymer into a second reactor, and performing azeotropic dehydration to obtain a remaining solution;

s512: concentrating the remaining solution, and adding an extracting agent to obtain a mixed solution;

s513: removing the catalyst by filtration or extraction, pouring the mixed solution into an organic solvent, and separating out crystals; and carrying out suction filtration, collection, washing and reduced pressure drying on the crystals to obtain the polylactic acid.

Further, in the step S511, the polymer is recycled to the reactor through the molecular sieve.

Further, the ring-opening polymerization process comprises:

s521: injecting the prepolymer obtained in the step S4 into a first reactor to obtain crude L-lactide, and extracting and purifying the crude L-lactide;

s522: taking the second reactor, and removing oxygen in the second reactor; mixing the purified L-lactide obtained in the step S521, tin octylate and lauryl alcohol to form a mixed solution, sealing the mixed solution in a second reactor, and stirring and heating the mixed solution in a nitrogen atmosphere;

S523: keeping the mixed solution at the same temperature, reducing the pressure in the second reactor, and carrying out reduced pressure distillation; after the distillation of the mixed liquid is stopped, the poly (L-lactide) is discharged in the form of chains from the bottom of the container.

Preferably, in step S2, the lye is NaOH.

Preferably, in step S2, the hydrolysis time is 45 minutes. If the hydrolysis time is less than 45 minutes, the efficiency is low and complete hydrolysis to glucose is not possible.

Preferably, in step S3, the fermentation time is 72 h. If the time is shorter than 72h, the fermentation is not complete, and the fermentation time is 72h, so that better lactic acid fermentation yield can be obtained.

Preferably, in step S513, the organic solvent is methanol.

Preferably, the reactor in the ring-opening polymerization process is equipped with a dean-stark trap.

Compared with the existing method for preparing polylactic acid by using kitchen garbage, the method has the following advantages:

1. the yield of lactic acid obtained by fermenting and degrading the peel is high relative to starchiness raw materials such as potato peel and the like;

2. the hydrolysis is carried out without depending on enzyme, and the complete hydrolysis degree and the utilization rate of the saccharides in the peel are high;

3. the polylactic acid obtained by lactic acid polymerization has high purity.

Drawings

In order to more clearly illustrate the technical solutions in the embodiments of the present application, the drawings needed to be used in the description of the embodiments are briefly introduced below, and it is obvious that the drawings in the following description are only some embodiments of the present application, and it is obvious for those skilled in the art to obtain other drawings based on these drawings without creative efforts.

FIG. 1 is a schematic diagram of the synthesis of polylactic acid (PLA);

FIG. 2 is a schematic view of the structure of polylactic acid;

FIG. 3 is a schematic view of the process for preparing polylactic acid from pericarp;

FIG. 4 is a graph of the effect of different fermentation times on lactic acid yield;

FIG. 5 is a NMR spectrum of polylactic acid;

Detailed Description

The technical solutions in the embodiments of the present invention will be described clearly and completely with reference to the drawings in the embodiments of the present invention, and it is obvious that the described embodiments are only a part of the embodiments of the present invention, and not all of the embodiments. All other embodiments, which can be obtained by a person skilled in the art without making any creative effort based on the embodiments in the present invention, belong to the protection scope of the present invention.

As shown in fig. 1, polylactic acid (PLA) can be made from two monomeric polymers, one lactide and one lactic acid. If the monomer is lactide, polylactic acid is formed by ring-opening polymerization, i.e. process a in fig. 1; if the monomer is lactic acid, the polylactic acid is formed by condensation polymerization, i.e., the B process in fig. 1.

The specific reaction conditions required for the synthesis of polylactic acid by two different routes are different. The basic process of the condensation polymerization of lactic acid comprises: the lactic acid is distilled under reduced pressure at 130 ℃ for 2 to 3 hours, and then under the action of a catalyst, the lactic acid is linked together by water loss to form oligomers (generally with a weight average molecular weight of less than 1000g mol- ^-1) The water produced by the condensation reaction is removed by high temperature and vacuum, which is beneficial to the formation of oligomers. A catalyst (e.g., tin octoate) and a suitable solvent (e.g., diphenyl ether) are added and an additional 30-40 hours at 130 c are added to convert the oligomer to a polymer. The process can be carried out in a closed system and the water formed from this reaction stage can be removed by molecular sieves. The polymer can be isolated as such or dissolved and precipitated for further purification.

For the ring-opening condensation reaction of lactide, the ring-opening condensation reaction generally gives polylactic acid having better monodispersity and higher molecular weight, and the polymerization conditions are broader, compared with condensation polymerization. Commercially, a typical catalyst used in ring-opening polymerization is tin octylate. In addition, there are many different types of catalysts used, including mainly: enzyme catalysis, organic catalysts, metal-based catalysts. Suitable for commercial polylactic acid products, such as cups and bottles, the molecular weight M is generally greater than 100000g mol-1, whereas ring-opening polymerization of lactide generally yields molecular weights greater than 100000g mol^-1The polymer of (1).

The lactic acid molecule contains an asymmetric carbon atom. There are two different three-dimensional arrangements (R or S) of atoms around the carbon. When two lactic acid molecules are combined to form lactide (a dimer), three different monomers, L-lactide, D-lactide and meso-lactide, are produced. Five polylactic acids can be polymerized from these lactides as shown in fig. 2. Wherein L represents meso-lactide, M represents L-lactide, and N represents D-lactide; p1 represents isotactic polylactic acid, which is crystalline at normal temperature and has a melting point of 180 ℃; p2 represents stereoblock polylactic acid, which is crystal at normal temperature and has a melting point of more than 200 ℃; p3 represents an atactic polylactic acid, amorphous at room temperature; p4 represents an isosteric polylactic acid, amorphous form at room temperature; p5 represents a syndiotactic polylactic acid, and it is crystalline at room temperature and has a melting point of 152 ℃.

The ratio of monomers also affects some key properties of the polymer, such as crystallinity, melting temperature, and ease of processing. Crystallinity is a very important property that affects many properties of polymers, including hardness, tensile strength, stiffness, and melting point. Some mechanical properties of polylactic acid are shown in table 1. These values indicate that polylactic acid prepared from L-lactide monomer has stronger toughness than D, L-polylactic acid prepared from racemic lactide. Commercial polylactic acid resins are usually produced from L-lactide, and the resulting polymer, L-polylactic acid (L-PLA), is a semi-crystalline material with high melting temperature and glass transition temperature.

TABLE 1 Effect of stereochemistry and crystallinity on mechanical Properties

The mechanical properties of polylactic acid can also be improved by the formation of composite materials, such as blends with corn starch. Table 2 summarizes the mechanical properties of some of the composites. The greater the corn starch content in the composite, the lower the thermal degradation onset temperature. In addition, the L-polylactic acid/corn starch composite material has higher water leaching rate (5-7% of water is increased compared with L-polylactic acid). Additives may also be added with polycaprolactam, polyethylene glycol, glycerin and lauryl alcohol. The results show that the tensile strength and Young's modulus are reduced with the addition of the blend of L-polylactic acid and polyvinyl alcohol. Polylactic acid/PHB blend is a typical biopolymer blend with the best properties of both polymers. In addition, the polylactic acid/PHB blends improve the mechanical properties of both pure polymers. Blending of poly (butadiene-terephthalate) (PBAT) and polylactic acid has a positive effect on the elongation and strength of the polylactic acid and makes the viscosity of the polylactic acid more constant, thus allowing a wider processing temperature window. Referring to table 2, the mechanical properties of the composite materials synthesized from L-polylactic acid (L-PLA), Corn Starch (CS), High Amylose Corn Starch (HACS), Polyhydroxybutyrate (PHB), Polycaprolactone (PCL), polyvinyl alcohol (PVA), poly (butylene adipate-co-terephthalate) (PBAT) are listed in table 2.

TABLE 2 mechanical properties of the composite materials

As shown in table 3, the pericarp contains a large amount of carbohydrates including sugar, cellulose, hemicellulose and pectin. The preparation of polylactic acid with peel as material includes extracting saccharide from peel, microbial fermentation to convert saccharide into lactic acid, purification, concentration and final condensation polymerization or ring opening polymerization to obtain corresponding polylactic acid. The focus of the present invention is on how to extract sugars from the peel and convert them to lactic acid.

TABLE 3 Water and sugar content of some fruits (per 100g raw fruit)

The invention provides a method for preparing polylactic acid from pericarp, which sequentially comprises the processes of sample preparation, saccharide extraction, lactic acid preparation and polymerization, and can be seen in figure 3, and the method comprises the following specific steps:

pretreating the peel, wherein the pretreatment process comprises drying a sample; hydrolyzing the dried pericarp with alkali solution under heating and stirring, filtering to obtain glucose solution, wherein the alkali solution can hydrolyze ester bond on polysaccharide, and degrading to hydrolyze polysaccharide in pericarp into glucose.

Preparing a culture medium by using the obtained glucose solution, inoculating the single-purity cultured lactobacillus into the culture medium and incubating; inoculating a starter in a culture medium, and fermenting to produce lactic acid; distilling the obtained lactic acid, removing water, putting the lactic acid into a prepolymer reactor, and polymerizing to obtain a prepolymer; then carrying out polymerization reaction to obtain the polylactic acid.

The polymerization process may further include condensation polymerization or ring-opening polymerization.

The condensation polymerization process comprises:

in a first reactor with a catalyst, lactic acid is reacted with diphenyl ether in a ratio of 2: 3 to form a polymer; the polymer was recovered by molecular sieve into the reactor and azeotropically dehydrated leaving a solution. The molecular sieve can improve the overall efficiency and can be recycled. Concentrating the remaining solution to half of the original volume, and adding chloroform or ethyl acetate to obtain a mixed solution; removing the catalyst by filtration or extraction, pouring the mixed solution into an organic solvent, and separating out crystals; and carrying out suction filtration and collection on the crystals, washing the crystals with methanol, and drying the crystals under reduced pressure to obtain the polylactic acid.

The process of ring-opening polymerization comprises:

injecting the obtained prepolymer into a first reactor to obtain crude L-lactide, and extracting and purifying the crude L-lactide; removing oxygen from the reactor with nitrogen under vacuum; taking the purified L-lactide, tin octylate and lauryl alcohol to form a mixed solution, sealing the mixed solution in a second reactor, and stirring and heating the mixed solution in a nitrogen atmosphere; keeping the mixed solution at the same temperature, reducing the pressure in the second reactor, and carrying out reduced pressure distillation; after the distillation of the mixed liquid is stopped, the poly (L-lactide) is discharged in the form of chains from the bottom of the vessel.

Example 1

Peeling fruits, washing peels with clear water, and removing dirt particles. The peel was then cut into small pieces and dried at 60 ℃ for 24 hours. The moisture content of the peel was calculated by measuring the weight of the peel before and after drying, and used to calculate the amount of reagents required for the subsequent fermentation process. The water content is preferably 3% or less, and an error is large when the water content is more than 3%. The peel was then stirred into a powder with a stirrer.

Weighing dry peel powder, hydrolyzing the dry peel powder with sodium hydroxide under heating, cooling to room temperature, and neutralizing with acid to obtain a neutralized pH value of 5-7; filtering after neutralization to obtain a glucose solution; the single cultured lactic acid bacteria were inoculated into MRS broth pH 5.5 and incubated at 37 ℃ for 24 hours. 15% MRS broth was prepared with glucose extracted from fruit. Inoculating 10% of LAB standard starter, incubating in a shake flask controlled by 37 deg.C, and fermenting for 72 hr to produce lactic acid. Inoculating 10% LAB standard starter in the MRS culture medium to produce lactic acid; 40.2g L-lactic acid was azeotropically dehydrated in 400ml of diphenyl ether in the presence of 0.1g of methanesulfonic acid (catalyst) for 2h at 140 ℃ using a reaction vessel equipped with a dean-Stark trap. After removing the distilled water from the dean-stark trap, a test tube containing 40g of molecular sieve was installed above the reactor, and the distilled solvent was recovered into the reactor through the molecular sieve. Azeotropic dehydration is carried out at 130 ℃ for 20-40 h. The water content of the solvent after passing through the molecular sieve is below 3 ppm. After the reaction mixture was concentrated to about half volume, 300ml of chloroform was added. The catalyst is removed by filtration or extraction and the resulting mixture is poured into 900 ml of methanol. The precipitated crystals were collected by suction filtration, washed with methanol and dried under reduced pressure. The yield of the polylactic acid white powder is 80-85%.

Example 2

Compared with the embodiment 1, the difference of the embodiment is that the condensation process is ring-opening condensation, and the specific steps are as follows:

0.5g oxazolidine was added and in a 25mL round bottom flask, L-lactic acid (1.60g of 50 wt% lactic acid) was added followed by 10mL solvent (toluene or o-xylene) and a magnetic stir bar. A solvent reflux trap was installed on top of the round bottom flask, an arrangement that ensured the reflux of the lighter solvent phase and water could be recycled. At the start of the reaction, the flask was immersed in a preheated, stirred, temperature controlled oil bath. The bath is generally kept at 140 ℃ (or 130 ℃) for the reaction in toluene, and the reaction in o-xylene is 170 ℃ slightly above the respective boiling point, ensuring that the solvent can be refluxed. The mixture was continuously stirred under reflux of the solvent, and the reaction was carried out for 3 hours to obtain L-lactide.

In a thick-walled cylindrical stainless steel polymerization vessel equipped with a stirrer, 216g (1.5mol) of L-lactide, 0.01% by mass (calculated with respect to the amount of lactide) of tin octylate and 0.03% by mass (calculated with respect to the amount of lactide) of lauryl alcohol were sealed. The polymerization vessel was deoxygenated with nitrogen under vacuum for 2 hours. The mixture was heated with stirring at 200 ℃ for 3 hours under a nitrogen atmosphere. The polymerization vessel was gradually pumped to a lower pressure while maintaining the mixture at the same temperature. After 1 hour from the start of the extraction, the distillation of the monomers and low molecular weight volatiles ceased. The vessel was filled with nitrogen and the polymer was discharged from the bottom of the vessel as a chain. The chains were pelletized to obtain poly (L-lactide) in 96% yield. The NMR spectrum of polylactic acid is shown in FIG. 5.

Example 3: the glucose concentration obtained by hydrolyzing banana peels with different qualities by treating with NaOH with different concentrations is based on the embodiment 1, and the banana peels are selected as the peels of the embodiment.

5 grams of dried pericarp was weighed separately with an analytical balance and placed in three different beakers. The dried banana peels were subjected to alkaline pretreatment hydrolysis with 1%, 2% and 3% sodium hydroxide. The weighed samples in the beaker were then labeled and mixed with 1%, 2%, 3% NaOH, respectively, to start the hydrolysis. Then a magnetic stirrer is placed into the beaker to be fully stirred. The temperature of the three hotplates was set at 80 ℃ and the speed was adjusted to about 700rpm for 45 minutes of treatment. After all solutions in the beaker were cooled to room temperature, the initial pH of all samples was recorded. 1M HCl solution was then added dropwise to each sample until the sample was neutralized.

The contents of the solution were filtered into a test tube using filter paper, and then subjected to glucose content analysis. Weighing 1g of the filtered sample and diluting it with distilled water in a 50ml volumetric flask, in order to dilute the filtered sample to colorless color, since colored samples cannot be detected well in the uv-vis spectrum, since this would affect the concentration results obtained. The absorbance values were obtained in a UV-Vis spectrometer at 265 nm. The absorbance should be a linear function of concentration, and under ideal conditions, the concentration of a substance is proportional to the absorbance of the solution, according to lambert-beer's law.

The results of the extraction of glucose concentration in banana peel are shown in table 4.

TABLE 4 glucose concentration obtained by hydrolyzing banana peels of different qualities with NaOH of different concentrations

Example 4: glucose concentration obtained by hydrolyzing apple peels with different qualities through NaOH with different concentrations

The present embodiment is different from embodiment 3 in that: the peel of this embodiment is apple peel. The results of the extraction of the glucose concentration in the apple peel are shown in table 5.

TABLE 5 glucose concentrations obtained by hydrolysis of apple peels of different qualities treated with NaOH of different concentrations

Example 5: glucose concentration obtained by hydrolyzing mango peels with different qualities through NaOH with different concentrations

The present embodiment is different from embodiment 3 in that: the mango peel is selected as the peel in the embodiment. The results of the glucose concentration extraction in mango peel are shown in table 6.

TABLE 6 glucose concentrations obtained by hydrolysis of mango peel of different quality treated with NaOH of different concentrations

As can be seen from the experimental data in tables 4-6, under the condition of 3% NaOH concentration, the glucose concentration obtained after treatment is higher in both banana peel, apple peel and mango peel. Thus demonstrating that higher alkali concentrations can extract higher glucose concentrations.

Example 6: effect of different fermentation times on lactic acid yield

On the basis of the embodiment 1, 5 strains of lactobacillus, streptococcus lactis, lactobacillus delbrueckii, lactobacillus plantarum and pediococcus cerevisiae are selected, and the fermentation time of the lactobacillus is respectively set to be 36h, 48h, 72h and 96 h. 25ml of the medium of the solid residue after fermentation was transferred to a 100ml flask, and the content of lactic acid in the fermentation broth was measured. One milliliter of phenolphthalein indicator (0.5% in 5% alcohol) was added to the flask. Titration with 0.25M NaOH gave a pink surface. The titratable acidity was calculated as% W/V lactate with 1M NaOH per ml equaling 90.08mg lactate. The acidity of the titration was then calculated. The result is shown in FIG. 4. As can be seen from FIG. 4, the lactic acid yield was generally high after 72 hours of fermentation.

In conclusion, the invention provides a method for preparing polylactic acid from fruit peel, which sequentially comprises the processes of sample preparation, saccharide extraction, lactic acid preparation and polymerization. Compared with the existing method for extracting polylactic acid from starchy raw materials such as potato peels and the like, the yield of lactic acid obtained by fermenting and degrading the peel is high; the hydrolysis does not need to depend on enzyme, and the complete hydrolysis degree of the saccharides in the peel is high; finally, the polylactic acid obtained in the lactic acid polymerization process of the method has high purity.

The above description is only for the purpose of illustrating the preferred embodiments of the present invention and is not to be construed as limiting the invention, but rather as the subject matter of the invention is to be construed in all aspects and as broadly as possible, and all changes, equivalents and modifications that fall within the true spirit and scope of the invention are therefore intended to be embraced therein.

Claims (10)

1. A method for preparing polylactic acid from pericarp comprises the processes of sample preparation, saccharide extraction, lactic acid preparation and polymerization, and is characterized in that:

s1, sample preparation: pretreating the peel, wherein the pretreatment process comprises drying the sample;

s2, saccharide extraction: hydrolyzing the dried pericarp in the step S1 with alkali liquor, and filtering to obtain glucose solution;

s3, lactic acid preparation: preparing a culture medium with the glucose solution obtained in the step S2, inoculating a single-purity cultured lactic acid bacterium into the culture medium and incubating; inoculating a starter in the culture medium, and fermenting to produce lactic acid;

s4, prepolymer formation: distilling the lactic acid obtained in the step S3, removing water, putting the lactic acid into a prepolymer reactor, and polymerizing to obtain a prepolymer;

s5, polymerization process: and (4) carrying out polymerization reaction on the prepolymer obtained in the step S4 to obtain polylactic acid.

2. The method as claimed in claim 1, wherein the polymerization process is condensation polymerization or ring-opening polymerization.

3. The method as claimed in claim 2, wherein the condensation polymerization process comprises:

s511: azeotropically dehydrating the lactic acid with an organic solvent in a first reactor in the presence of a catalyst to form a polymer; recovering the polymer into a second reactor, and performing azeotropic dehydration to obtain a remaining solution;

s512: concentrating the remaining solution, and adding an extracting agent to obtain a mixed solution;

s513: removing the catalyst by filtration or extraction, and pouring the mixed solution into an organic solvent to separate out crystals; and carrying out suction filtration, collection, washing and reduced pressure drying on the crystals to obtain the polylactic acid.

4. The method of claim 3, wherein in step S511, the polymer is recycled back to the second reactor through molecular sieves.

5. The method of claim 2, wherein the ring-opening polymerization process comprises:

S521: injecting the prepolymer obtained in the step S4 into a first reactor to obtain crude L-lactide, and extracting and purifying the crude L-lactide;

s522: taking a second reactor, and removing oxygen in the second reactor; mixing the purified L-lactide obtained in the step S521, tin octylate and lauryl alcohol to form a mixed solution, sealing the mixed solution in the second reactor, and stirring and heating the mixed solution in a nitrogen atmosphere;

s523: keeping the mixed solution at the same temperature, reducing the pressure in the second reactor, and carrying out reduced pressure distillation; after the distillation of the mixed liquid is stopped, the poly (L-lactide) is discharged in the form of chains from the bottom of the container.

6. The method of claim 1, wherein in step S2, the alkali solution is NaOH.

7. The method of claim 1, wherein the hydrolysis time in step S2 is 45 minutes.

8. The method of claim 1, wherein the fermentation time of step S3 is 72 hours.

9. The method as claimed in claim 3, wherein the organic solvent in step S513 is methanol.

10. The method as claimed in claim 3, wherein said reactor is equipped with a dean-stark trap.

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