Preparation method of polylactic acid

Abstract

The invention relates to a preparation method of polylactic acid. Lactide is subjected to a polymerization reaction under the action of a catalyst, polylactic acid is prepared, the structural formula of the catalyst is shown in the formula (1), in the formula (1), R1, R2 and R3 are independently selected from-H or C1-C10 alkyl groups, x and y meet the condition that the algebraic sum of positive and negative valences in the structural formula is 0, and M is selected from metal elements of the IA group, the IIA group, the IIIA group, the IVA group, the IB group or the IIB group. According to the invention, by adding the catalyst with the synergistic catalysis effect of ring-opening polymerization and racemization of lactide, the ring-opening polymerization reaction of lactide is effectively promoted, and the racemization reaction of lactide is controlled, so that the ratio of L-lactic acid to D-lactic acid units in a polylactic acid product is controlled, the melting point of polylactic acid is adjusted, the production cost is reduced, and the product quality is improved. The limitation of raw materials is eliminated, so that the prepared polylactic acid product can be suitable for more application fields.

Description

Technical Field

[0001] The present invention relates to the technical field of polylactic acid, in particular to a method for preparing polylactic acid. Background Art

[0002] Different application scenarios have different requirements for the melting point of polylactic acid (PLA). The melting point of polylactic acid is related to the regularity of the molecular chain segments, and the regularity of the polylactic acid molecular segments is mainly affected by the optical purity, that is, it is related to the chemical stereocomposition and the ratio of L-lactic acid and D-lactic acid isomer units.

[0003] In existing industrial or laboratory processes for preparing polylactic acid, the melting point of the polylactic acid product is mainly adjusted by changing the ratio of L-lactide to D-lactide or meso-lactide in the polymer raw materials. However, the existing preparation process is limited by the production capacity and cost of D-lactide and meso-lactide. Meso-lactide is derived from a side reaction in lactide production and is limited in quantity. D-lactide requires the construction of separate production lines for D-lactic acid fermentation and D-lactide synthesis, which has a high production cost. Summary of the Invention

[0004] Based on this, it is necessary to provide a method for preparing polylactic acid to address the above-mentioned problems, using a catalyst with synergistic catalytic effects of lactide ring-opening polymerization and lactide racemization to achieve lactide racemization during the ring-opening polymerization process, effectively adjust the melting point of polylactic acid prepared based on a single lactide raw material, reduce production costs, and eliminate raw material restrictions.

[0005] A method for preparing polylactic acid comprises polymerizing lactide in the presence of a catalyst to obtain polylactic acid, wherein the catalyst has a structural formula as shown in formula (1). In formula (1), R1, R2 and R3 are each independently selected from -H or C1 to C 10 wherein x and y satisfy the algebraic sum of positive and negative valences in the structural formula to be 0, and M is selected from metal elements of Group IA, Group IIA, Group IIIA, Group IVA, Group IB or Group IIB.

[0006] In one embodiment, said R1 and said R2 are each independently selected from -H, -CH3, -C2H5, -C3H7 or -C4H9;

[0007] And / or, said R3 is selected from -H.

[0008] In one embodiment, the M is selected from Sn, Zn or Mg.

[0009] In one embodiment, x=1-3, y=1-3.

[0010] In one embodiment, the molar amount of the catalyst is 0.1‰ to 5‰ of the molar amount of the lactide.

[0011] In one embodiment, when the ratio of the molar amount of the catalyst to the molar amount of lactide increases by 0.1‰, the melting point of the obtained polylactic acid decreases by 1°C to 3°C.

[0012] In one embodiment, when the ratio of the molar amount of the catalyst to the molar amount of the lactide is 0.3‰ to 1.5‰, the melting point of the prepared polylactic acid is 140°C to 170°C.

[0013] In one embodiment, when the ratio of the molar amount of the catalyst to the molar amount of the lactide is 1.5‰ to 2.5‰, the melting point of the prepared polylactic acid is 125°C to 140°C.

[0014] In one embodiment, when the ratio of the molar amount of the catalyst to the molar amount of the lactide is less than 0.3‰, the melting point of the obtained polylactic acid is greater than 170°C.

[0015] In one embodiment, when the ratio of the molar amount of the catalyst to the molar amount of the lactide is greater than 2.5‰, the obtained polylactic acid is an amorphous polymer.

[0016] In one embodiment, lactide, a catalyst as shown in formula (1), and an initiator are mixed to carry out polymerization reaction, and polylactic acid is obtained by devolatilization.

[0017] In one embodiment, the polymerization reaction temperature is 150° C. to 200° C., and the time is 0.5 h to 6 h.

[0018] The above-mentioned method for preparing polylactic acid effectively promotes the ring-opening polymerization reaction of lactide by adding a catalyst with synergistic catalytic effects on the ring-opening polymerization and racemization of lactide, while controlling the racemization reaction of lactide, thereby controlling the ratio of L-lactic acid to D-lactic acid units in the polylactic acid product, achieving the adjustment of the melting point of polylactic acid, reducing production costs, eliminating raw material restrictions, and making the obtained polylactic acid product suitable for more application fields. DETAILED DESCRIPTION

[0019] To facilitate understanding of the present invention, the present invention will be described in more detail below. However, it should be understood that the present invention can be implemented in many different forms and is not limited to the embodiments or examples described herein. On the contrary, the purpose of providing these embodiments or examples is to make the understanding of the disclosure of the present invention more thorough and comprehensive.

[0020] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as those generally understood by those skilled in the art of the technical field of the present invention. The terms used herein in the specification of the present invention are only for the purpose of describing specific embodiments or embodiments and are not intended to limit the present invention. The optional scope of the term "and / or" used herein includes any one of two or more related listed items, and also includes any and all combinations of related listed items, including any two related listed items, any more related listed items, or the combination of all related listed items.

[0021] The present invention provides a method for preparing polylactic acid, wherein lactide is polymerized under the action of a catalyst to prepare polylactic acid, wherein the structural formula of the catalyst is as shown in formula (1). In formula (1), R1, R2 and R3 are each independently selected from -H or C1 to C 10 wherein x and y satisfy the algebraic sum of positive and negative valences in the structural formula to be 0, and M is selected from metal elements of Group IA, Group IIA, Group IIIA, Group IVA, Group IB or Group IIB.

[0022] The above-mentioned method for preparing polylactic acid effectively promotes the ring-opening polymerization reaction of lactide by adding a catalyst with synergistic catalytic effects on the ring-opening polymerization and racemization of lactide, while controlling the racemization reaction of lactide, thereby controlling the ratio of L-lactic acid to D-lactic acid units in the polylactic acid product, achieving the adjustment of the melting point of polylactic acid, reducing production costs, eliminating raw material restrictions, and making the obtained polylactic acid product suitable for more application fields.

[0023] In one embodiment, said R1 and said R2 are each independently selected from -H, -CH3, -C2H5, -C3H7 or -C4H9.

[0024] In one embodiment, said R3 is selected from -H.

[0025] In order to further increase the rates of the lactide ring-opening polymerization reaction and the lactide racemization reaction, the M is preferably Sn, Zn or Mg.

[0026] In one embodiment, x=1-3, y=1-3.

[0027] In one embodiment, the lactide is selected from L-lactide or D-lactide, and polylactic acid with different melting points can be prepared using a single lactide raw material.

[0028] In order to further control the racemization degree of lactide and adjust the melting point of the obtained polylactic acid, the molar amount of the catalyst is 0.1‰ to 5‰ of the molar amount of the lactide.

[0029] In one embodiment, when the ratio of the molar amount of the catalyst to the molar amount of lactide increases by 0.1‰, the melting point of the prepared polylactic acid decreases by 1°C to 3°C.

[0030] In one embodiment, when the ratio of the molar amount of the catalyst to the molar amount of the lactide is 0.3‰ to 1.5‰, the melting point of the obtained polylactic acid is 140°C to 170°C, and the polylactic acid can be used in the fields of film, extrusion or thermoforming.

[0031] In one embodiment, when the ratio of the molar amount of the catalyst to the molar amount of the lactide is 1.5‰ to 2.5‰, the melting point of the obtained polylactic acid is 125°C to 140°C, and the polylactic acid can be used in the fields of films, non-woven fabrics, etc.

[0032] In one embodiment, when the ratio of the molar amount of the catalyst to the molar amount of the lactide is less than 0.3‰, the melting point of the obtained polylactic acid is greater than 170°C, and the polylactic acid can be used in fields with high heat resistance requirements such as spinning and injection molding.

[0033] In one embodiment, when the ratio of the molar amount of the catalyst to the molar amount of the lactide is greater than 2.5‰, the obtained polylactic acid is an amorphous polymer without crystallinity and can be used in medical materials such as surgical sutures and bone screws.

[0034] It should be noted that by regulating the molar ratio of the catalyst to lactide, polylactic acid with a variety of different melting points can be achieved, such as melting points of 135°C to 145°C, 150°C to 180°C, etc., which can be formulated according to actual needs.

[0035] In one embodiment, lactide, a catalyst represented by formula (1), and an initiator are mixed to carry out polymerization reaction, and polylactic acid is obtained by devolatilization.

[0036] In one embodiment, the polymerization reaction temperature is 150° C. to 200° C., and the time is 0.5 h to 6 h.

[0037] Preferably, the polymerization reaction temperature is 160° C. to 180° C., and the time is 1 hour to 3 hours.

[0038] In one embodiment, the initiator is an alcohol compound, which is selected from at least one of a monohydric alcohol, a dihydric alcohol and a polyhydric alcohol. The present invention is not limited to this. For example, it can be selected from at least one of octanol, dodecanol, benzyl alcohol, ethylene glycol, butanediol, glycerol and pentaerythritol. The amount of the initiator is 0.1‰ to 5‰ of the molar amount of lactide, more preferably 0.5‰ to 2‰.

[0039] In one embodiment, the lactide is preheated to completely melt the lactide before the polymerization reaction with the initiator and the catalyst represented by formula (1) and then the initiator and the catalyst are added to carry out polymerization.

[0040] Specifically, the temperature of the preheat treatment is 100°C to 150°C, more preferably 120°C to 140°C.

[0041] The devolatilization treatment of the polymer in the present invention adopts existing methods, which can be achieved by a flash tank, a falling strip devolatilizer, a scraper devolatilizer or a venting extruder, etc. The present invention is not limited to this.

[0042] It should be noted that the catalyst represented by formula (1) in the present invention can be an existing compound or can be prepared in-house. The preparation methods include but are not limited to the following two methods:

[0043] Method 1: In a protective atmosphere, an amino acid, a catalyst, and an alcohol compound are subjected to a first mixing reaction in a solvent to obtain a first intermediate, wherein the first alcohol compound includes at least one of R1-OH and R2-OH;

[0044] The first intermediate and the compound containing the metal element M are subjected to a second mixing reaction. After the reaction is completed, the catalyst is concentrated and crystallized to obtain a catalyst as shown in formula (1), wherein R1 and R2 are each independently selected from C1 to C 10 Alkyl, R3 is selected from -H or C1~C 10 of alkyl.

[0045] In method 1, preferably, the molar ratio of the amino acid, the first alcohol compound, and the catalyst is 1:(100-500):(0.005-0.05).

[0046] It can be understood that in the first alcohol compound, R1 and R2 can be the same or different. When R1 and R2 are the same, the first alcohol compound is R1-OH, and the structural formula of the first intermediate obtained is

[0047] Preferably, in the first mixed reaction system, the concentration of the amino acid is 0.02 mol / L to 0.2 mol / L.

[0048] Preferably, the amino acid includes at least one of glycine, alanine, 2-aminobutyric acid, 2-aminopentanoic acid, 2-aminohexanoic acid, 2-aminoheptanoic acid, 2-aminooctanoic acid, 2-aminononanoic acid, 2-aminodecanoic acid, 2-aminoundecanoic acid, and 2-aminododecanoic acid, more preferably glycine.

[0049] The catalyst includes a ruthenium catalyst and an iron catalyst.

[0050] Preferably, the ruthenium catalyst is selected from 1-hydroxytetraphenyl-cyclopentadienyl (tetraphenyl-2,4-cyclopentadien-1-one)-μ-hydrogen tetracarbonyl diruthenium (II).

[0051] Preferably, the iron catalyst is selected from the compound shown in formula (2),

[0052] Preferably, the temperature of the first mixing reaction is 60° C. to 120° C., and the time is 18 h to 48 h.

[0053] Preferably, the molar ratio of the first intermediate to the compound containing the metal element M is 1:1 to 3:1.

[0054] Preferably, the compound containing the metal element M includes an oxide containing the metal element M and / or a hydroxide containing the metal element M.

[0055] Preferably, the temperature of the second mixing reaction is 50° C. to 90° C., and the time is 4 h to 6 h.

[0056] Method 2: An amino acid and a compound containing a metal element M are subjected to a third mixed reaction in water. After the reaction is completed, the catalyst is concentrated and crystallized to obtain a catalyst as shown in formula (1), wherein R1 and R2 are both selected from -H, and R3 is selected from -H or C1~C 10 of alkyl.

[0057] In method 1, the molar ratio of the amino acid to the compound containing the metal element M is 1:1 to 3:1.

[0058] Preferably, in the third mixed reaction system, the concentration of the amino acid is 0.2 mol / L to 1 mol / L.

[0059] Preferably, the amino acid includes at least one of glycine, alanine, 2-aminobutyric acid, 2-aminopentanoic acid, 2-aminohexanoic acid, 2-aminoheptanoic acid, 2-aminooctanoic acid, 2-aminononanoic acid, 2-aminodecanoic acid, 2-aminoundecanoic acid, and 2-aminododecanoic acid, more preferably glycine.

[0060] Preferably, the compound containing the metal element M includes an oxide containing the metal element M and / or a hydroxide containing the metal element M;

[0061] Preferably, the temperature of the third mixing reaction is 50° C. to 90° C., and the time is 4 h to 6 h.

[0062] Hereinafter, the preparation method of the polylactic acid will be further described through the following specific examples.

[0063] Example

[0064] 5 kg of lactide was placed in a 10 L titanium alloy reactor, preheated to 140°C, and stirred until the lactide was completely melted. A catalyst and 4.0 g of benzyl alcohol were added, and the temperature was raised to 180°C, with stirring for 2 hours. After the reaction, the polymer melt was transferred to a vented twin-screw extruder for devolatilization to remove unreacted lactide. The extruded melt was cooled and pelletized to produce polylactic acid particles. Lactide and catalyst were added according to Table 1.

[0065] The polylactic acid particles of all examples were subjected to melting point tests using a DSC differential scanning calorimeter (TA DSC250). The test temperature range was 25°C to 200°C, and the heating / cooling rate was 10°C / min. The test results are shown in Table 1.

[0066] Table 1

[0067]

[0068]

[0069] Catalysts A to G were prepared by the following methods:

[0070] Catalyst A

[0071] S1. Add 20 mmol of glycine and 0.2 mmol of 1-hydroxytetraphenyl-cyclopentadienyl (tetraphenyl-2,4-cyclopentadien-1-one)-μ-hydrogen tetracarbonyl diruthenium (II) into a 1 L three-necked flask;

[0072] S2, vacuum argon replacement 3 times through double-row tube;

[0073] S3. Add 500 mL of ethanol under argon purge and stir rapidly at room temperature for 1 minute;

[0074] S4, stirring the reaction in an 80°C oil bath for 18 h to synthesize 20 mmol of the first intermediate, the structural formula of which is

[0075] S5. Add 10 mmol of Sn(OH)2 and continue stirring the reaction for 5 h at 80°C until the solution becomes clear and transparent.

[0076] S6. After the reaction is completed, the solution is cooled to room temperature, and the solution is concentrated to 5 mL by vacuum rotary evaporation at 50°C. After cooling, the solution is crystallized, filtered, and dried to obtain a catalyst for preparing polylactic acid, which has the structural formula:

[0077] Catalyst B

[0078] S1. Add 30 mmol of glycine and 0.2 mmol of 1-hydroxytetraphenyl-cyclopentadienyl (tetraphenyl-2,4-cyclopentadien-1-one)-μ-hydrogen tetracarbonyl diruthenium (II) into a 1 L three-necked flask;

[0079] S2, vacuum argon replacement 3 times through double-row tube;

[0080] S3. Add 500 mL of ethanol under argon purge and stir rapidly at room temperature for 1 minute;

[0081] S4, stirring the reaction in an 80°C oil bath for 18 h to synthesize 30 mmol of the first intermediate, the structural formula of which is

[0082] S5. Add 10 mmol of MgO and continue stirring the reaction for 5 h at 80°C until the solution becomes clear and transparent.

[0083] S6. After the reaction is completed, the solution is cooled to room temperature, and the solution is concentrated to 5 mL by vacuum rotary evaporation at 50°C. After cooling, the solution is crystallized, filtered, and dried to obtain a catalyst for preparing polylactic acid, which has the structural formula:

[0084] Catalyst C

[0085] S1. Add 20 mmol of glycine and 0.2 mmol of 1-hydroxytetraphenyl-cyclopentadienyl (tetraphenyl-2,4-cyclopentadien-1-one)-μ-hydrogen tetracarbonyl diruthenium (II) into a 1 L three-necked flask;

[0086] S2, vacuum argon replacement 3 times through double-row tube;

[0087] S3. Add 500 mL of ethanol under argon purge and stir rapidly at room temperature for 1 minute;

[0088] S4, stirring the reaction in an 80°C oil bath for 18 h to synthesize 20 mmol of the first intermediate, the structural formula of which is

[0089] S5. Add 10 mmol of Zn(OH)2 and continue stirring the reaction for 6 h at a reaction temperature of 80°C until the solution becomes clear and transparent.

[0090] S6. After the reaction is completed, the solution is cooled to room temperature, and the solution is concentrated to 5 mL by vacuum rotary evaporation at 50°C. After cooling, the solution is crystallized, filtered, and dried to obtain a catalyst for preparing polylactic acid, which has the structural formula:

[0091] Catalyst D

[0092] S1. Add 20 mmol of glycine and 0.2 mmol of 1-hydroxytetraphenyl-cyclopentadienyl (tetraphenyl-2,4-cyclopentadien-1-one)-μ-hydrogen tetracarbonyl diruthenium (II) into a 1 L three-necked flask;

[0093] S2, vacuum argon replacement 3 times through double-row tube;

[0094] S3. Add 250 mL of ethanol and 250 mL of n-propanol under argon purge and stir rapidly at room temperature for 1 minute;

[0095] S4, stirring the reaction in an oil bath at 90°C for 20 h, synthesizing 20 mmol of the first intermediate, the structural formula of which is

[0096] S5. Add 10 mmol of Sn(OH)2 and continue stirring the reaction for 5 h at 80°C until the solution becomes clear and transparent.

[0097] S6. After the reaction is completed, the solution is cooled to room temperature, and the solution is concentrated to 5 mL by vacuum rotary evaporation at 50°C. After cooling, the solution is crystallized, filtered, and dried to obtain a catalyst for preparing polylactic acid, which has the structural formula:

[0098] Catalyst E

[0099] S1. Add 20 mmol of glycine and 1 mmol of iron catalyst (structural formula shown in Formula 2) into a 1 L three-necked flask;

[0100] S2, vacuum argon replacement 3 times through double-row tube;

[0101] S3. Add 400 mL of methanol under argon purge and stir rapidly at room temperature for 1 minute;

[0102] S4, stirring the reaction in an oil bath at 70°C for 18 h to synthesize 20 mmol of the first intermediate, the structural formula of which is

[0103] S5. Add 10 mmol of Sn(OH)2 and continue stirring the reaction for 5 h at 70°C until the solution becomes clear and transparent.

[0104] S6. After the reaction is completed, the solution is cooled to room temperature, and the solution is concentrated to 5 mL by vacuum rotary evaporation at 30°C. After cooling, the solution is crystallized, filtered, and dried to obtain a catalyst for preparing polylactic acid, the structural formula of which is

[0105] Catalyst F

[0106] S1. Add 20 mmol of alanine and 0.8 mmol of iron catalyst (structural formula shown in Formula 2) into a 1 L three-necked flask;

[0107] S2, vacuum argon replacement 3 times through double-row tube;

[0108] S3. Add 200 mL of butanol under argon purge and stir rapidly at room temperature for 1 minute;

[0109] S4, stirring the reaction in an oil bath at 120°C for 36 hours to synthesize 20 mmol of the first intermediate, the structural formula of which is

[0110] S5. Add 10 mmol of Sn(OH)2 and continue stirring the reaction for 6 h at 90°C until the solution becomes clear and transparent.

[0111] S6. After the reaction is completed, the solution is cooled to room temperature, and the solution is concentrated to 5 mL by vacuum rotary evaporation at 80°C. After cooling, the solution is crystallized, filtered, and dried to obtain a catalyst for preparing polylactic acid, whose structural formula is

[0112] Catalyst G

[0113] 30 mmol of glycine, 15 mmol of Mg(OH)2 and 100 mL of water were placed in a 1 L three-necked flask and stirred in an 80 °C oil bath for 5 h until the solution became clear.

[0114] After the reaction, the mixture was cooled to room temperature, and the solution was concentrated to 5 mL by vacuum rotary evaporation at 65°C. After cooling, the mixture was crystallized, filtered, and dried to obtain a catalyst for preparing polylactic acid, the structural formula of which is:

[0115] Comparative Example

[0116] Lactide was placed in a 10L titanium alloy reactor, preheated to 140°C, and stirred until the lactide was completely melted. Stannous octoate and 4.0g of benzyl alcohol were then added, the temperature was raised to 180°C, and the reaction was stirred for 2 hours. After the reaction, the polymer melt was transferred to a vented twin-screw extruder for devolatilization to remove unreacted lactide. The extruded melt was cooled and pelletized to produce polylactic acid particles. Lactide and stannous octoate were added according to Table 2.

[0117] The melting point of the polylactic acid particles of all comparative examples was tested using a DSC differential scanning calorimeter (TA DSC250). The test temperature range was 25°C to 200°C, and the heating / cooling rate was 10°C / min. The test results are shown in Table 2.

[0118] Table 2

[0119]

[0120] As can be seen from Tables 1 and 2, in all the examples, based on a single lactide racemic monomer, different polylactic acid melting points can be achieved by changing the amount of catalyst added provided by the present invention, while in all the comparative examples, the melting point of the polylactic acid obtained is mainly affected by the ratio of L-lactide to D-lactide in the raw material. Therefore, the catalyst used in the polylactic acid preparation method provided by the present invention can promote the ring-opening polymerization reaction of lactide while controlling the racemization reaction of lactide, thereby controlling the ratio of L-lactic acid to D-lactic acid units in the obtained polylactic acid product, achieving the adjustment of the polylactic acid melting point, reducing production costs, eliminating the limitation of raw materials, and the obtained polylactic acid product is suitable for more application fields.

[0121] The technical features of the above-mentioned embodiments can be combined arbitrarily. In order to make the description concise, not all possible combinations of the technical features in the above-mentioned embodiments are described. However, as long as there is no contradiction in the combination of these technical features, they should be considered to be within the scope of this specification.

[0122] The above-described embodiments merely illustrate several implementations of the present invention, and while their descriptions are relatively specific and detailed, they should not be construed as limiting the scope of the patent. It should be noted that a person skilled in the art would be able to make numerous variations and improvements without departing from the spirit of the present invention, all of which fall within the scope of protection of the present invention. Therefore, the scope of protection of the patent for this invention shall be determined by the appended claims.

Claims

1. A method for preparing polylactic acid, characterized in that: Lactide is polymerized under the action of a catalyst to produce polylactic acid, wherein the structural formula of the catalyst is as shown in formula (1). In formula (1), R1, R2 and R3 are each independently selected from -H or C1 to C 10 wherein x and y satisfy the algebraic sum of positive and negative valences in the structural formula to be 0, and M is selected from metal elements of Group IA, Group IIA, Group IIIA, Group IVA, Group IB or Group IIB.

2. The method for preparing polylactic acid according to claim 1, wherein The R1 and the R2 are each independently selected from -H, -CH3, -C2H5, -C3H7 or -C4H9; And / or, said R3 is selected from -H.

3. The method for preparing polylactic acid according to claim 1, wherein The M is selected from Sn, Zn or Mg.

4. The method for preparing polylactic acid according to claim 1, wherein x=1~3, y=1~3.

5. The method for preparing polylactic acid according to claim 1, wherein: The molar amount of the catalyst is 0.1‰ to 5‰ of the molar amount of the lactide.

6. The method for preparing polylactic acid according to claim 1, wherein: When the ratio of the molar amount of the catalyst to the molar amount of lactide increases by 0.1‰, the melting point of the prepared polylactic acid decreases by 1°C to 3°C.

7. The method for preparing polylactic acid according to claim 6, characterized in that: When the ratio of the molar amount of the catalyst to the molar amount of the lactide is 0.3‰ to 1.5‰, the melting point of the obtained polylactic acid is 140°C to 170°C; Alternatively, when the ratio of the molar amount of the catalyst to the molar amount of the lactide is 1.5‰ to 2.5‰, the melting point of the prepared polylactic acid is 125°C to 140°C.

8. The method for preparing polylactic acid according to claim 6, wherein: When the ratio of the molar amount of the catalyst to the molar amount of the lactide is less than 0.3‰, the melting point of the obtained polylactic acid is greater than 170°C; Alternatively, when the ratio of the molar amount of the catalyst to the molar amount of the lactide is greater than 2.5‰, the obtained polylactic acid is an amorphous polymer.

9. The method for preparing polylactic acid according to claim 1, wherein: Lactide, a catalyst as shown in formula (1) and an initiator are mixed to carry out polymerization reaction, and polylactic acid is obtained by devolatilization treatment.

10. The method for preparing polylactic acid according to claim 1 or 9, characterized in that: The polymerization reaction temperature is 150° C. to 200° C., and the reaction time is 0.5 h to 6 h.

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