PRODUCTION METHOD FOR PREPARING POLYLACTIC ACID BY MEANS OF RING-OPENING POLYMERIZATION METHOD, AND PREPOLYMER MIXTURE AND POLYLACTIC ACID

Abstract

The present invention relates to the technical field of the production of polylactic acid, and in particular to a production method for preparing polylactic acid by means of a ring-opening polymerization method, and a prepolymer mixture and the polylactic acid. The production method comprises: (1) enabling an initiator, a catalyst and a monomer I to be in contact in a production device to undergo a ring-opening polymerization reaction, so as to generate a prepolymer mixture containing a polylactic acid prepolymer; and (2) enabling the prepolymer mixture and a monomer II to be in contact with one another to undergo a reaction, so as to generate a high molecular weight polylactic acid. The monomer I and the monomer II are the same or are different, and each independently comprises lactide. The production method provided by the present invention can reduce the fluctuation in the feeding quality of the initiator and the catalyst, and can improve the production stability during the production process.

Claims

1. A production method for preparing polylactic acid by a ring-opening polymerization method, comprising the following steps: (1) contacting an initiator, a catalyst with a monomer I in a production device, and subjecting the same to a ring-opening polymerization reaction, to generate a prepolymer mixture containing a polylactic acid prepolymer; and (2) contacting the prepolymer mixture with a monomer II, and subjecting the same to a reaction, to generate a high-molecular weight polylactic acid; preferably, the polylactic acid has a number average molecular weight of more than or equal to 45000; the monomer I and the monomer II are identical or different, and each independently comprises lactide; preferably, the monomer I and the monomer II are identical in step (1) and step (2).

2. The production method according to claim 1, wherein, in step (1), when the monomer I is at 100% conversion, the polylactic acid prepolymer in the prepolymer mixture has a theoretical number average molecular weight of 1000-5000, preferably 2000-5000.

3. The production method according to claim 1, wherein, in step (1), a mass ratio of the initiator to the monomer I is 1.2:98.8 to 15.8:84.2, based on a total mass of the initiator and the monomer I being 100.

4. The production method according to claim 1, wherein, in step (2), a mass ratio of the prepolymer mixture to the monomer II is 0.5:99.5 to 10:90, preferably 1:99 to 5:95, based on a total mass of the prepolymer mixture and the monomer II being 100.

5. The production method according to claim 1, wherein the catalyst is selected from an organometallic compound and/or an organic base, preferably selected from an organometallic compound; preferably, the organometallic compound is selected from one or more of an organotin compound, an organoaluminum compound and an organozinc compound; preferably, the organic base is an organic guanidine.

6. The production method according to claim 1, wherein, in step (1), a ratio of a mass of the catalyst to a total mass of the initiator and the monomer I is 0.1:100 to 10:100; preferably, the catalyst is an organometallic compound, and in the prepolymer mixture, a content of the catalyst (calculated based on the corresponding metal) is 300-10000 ppm, more preferably 600-4000 ppm.

7. The production method according to claim 1, wherein, in the reaction system of the prepolymer mixture and the monomer II in step (2), a content of the catalyst is less than or equal to 0.12%, preferably less than or equal to 0.02%; preferably, the catalyst is an organometallic compound, and in the reaction system of the prepolymer mixture and the monomer II in step (2), a content of the catalyst (calculated based on the corresponding metal) is 15-50 ppm, more preferably 20-40 ppm.

8. The production method according to any claim 1, wherein, for the monomer I and the monomer II, the lactide is selected from one or more of L-lactide, D-lactide and meso-lactide; preferably, the monomer I and the monomer II each independently comprises a second monomer, and the second monomer is selected from a cyclic lactone and/or an epoxide, more preferably selected from a cyclic lactone, and further preferably selected from caprolactone and/or glycolide.

9. The production method according to claim 1, wherein the initiator is selected from one or more of hydroxyl-containing compounds, preferably selected from one or more of alcohol compounds.

10. The production method according to claim 1, wherein, in the reaction system of step (1), a residual rate of the monomer I is 2-20%; and/or in the reaction system of step (2), a conversion rate is 90-98%, preferably 94-96%.

11. The production method according to claim 1, wherein step (1) is carried out at a reaction temperature of 150-220° C., preferably 170-200° C.; step (2) is carried out at a reaction temperature of 170-220° C., preferably 175-200° C.

12. The production method according to claim 1, wherein step (1) is carried out in a manner of a batch production, a semi-continuous production or a continuous production; and/or step (2) is carried out in a manner of a batch production, and variation in the number average molecular weight of the prepared polylactic acid is less than or equal to 2%; or step (2) is carried out in a manner of a semi-continuous production, and variation in the number average molecular weight of the prepared polylactic acid is less than or equal to 2%; or step (2) is carried out in a manner of a continuous production, and variation in the number average molecular weight of the prepared polylactic acid is less than or equal to 5%, preferably less than or equal to 2%; preferably, step (1) and step (2) are both carried out in a manner of a continuous production; more preferably, a tubular reactor is used in step (1) and step (2).

13. A prepolymer mixture prepared by the production method according to claim 1, comprising a catalyst, an unreacted monomer I and a polylactic acid prepolymer.

14. The prepolymer mixture according to claim 13, wherein the prepolymer mixture has a viscosity of 10-500 cp at 180° C.; and/or in the prepolymer mixture, a content of the polylactic acid prepolymer is 80-98 wt %, and a content of the unreacted monomer I is 2-20 wt %, based on a total mass of the polylactic acid prepolymer and the unreacted monomer I being 100 wt %; and/or a mass of the catalyst is 0.09-9.1% of a total mass of the polylactic acid prepolymer and the unreacted monomer I; and/or the catalyst is an organometallic compound, and in the prepolymer mixture, a content of the catalyst (calculated based on the corresponding metal) is 300-10000 ppm, more preferably 600-4000 ppm; and/or the polylactic acid prepolymer has a number average molecular weight of 800-5000.

15. A high-molecular weight polylactic acid prepared by the production method according to any claim 1; preferably, the polylactic acid has a number average molecular weight (Mn) of more than or equal to 45000, and more preferably, has a number average molecular weight (Mn) of more than or equal to 60000; preferably, the polylactic acid has a polydispersity index (PDI) of 1.65-2.2; preferably, the polylactic acid is not a block copolymer).

Description

DETAILED DESCRIPTION

[0094] In order to understand the technical features and contents of the present application thoroughly, the preferred embodiments of the present application will be described in more detail below. Although the preferred embodiments of the present application are described in examples, it should be understood that the present application may be achieved in various forms and should not be limited by the embodiments set forth herein.

[0095] <Source of Raw Materials>

[0096] L-lactide and D-lactide, purchased from Corbion, industrial grade;

[0097] trimethylolpropane (TMP), purchased from Aladdin Reagent Co. Ltd., reagent grade;

[0098] ethylene glycol, purchased from Aladdin Reagent Co. Ltd., reagent grade;

[0099] isodecanol, purchased from Beijing Innochem Science & Technology Co., Ltd., reagent grade;

[0100] 1,4-butanediol, purchased from Markor, industrial grade;

[0101] 1,6-hexanediol, purchased from Yuanli, was industrial grade.

[0102] The other raw materials and reagents were all purchased from Aladdin Reagent Co. Ltd. and were of reagent grade.

[0103] <Test Method>

[0104] The conversion rate was measured by .sup.1H NMR; then, the monomer content in the system was calculated according to the measured conversion rate and the feed mass ratio of raw materials in the reaction process.

[0105] The actual number average molecular weight of the polylactic acid prepolymer was calculated based on the feed ratio of raw materials and the conversion rate.

[0106] The metal (for example, Sn element) content in the catalyst was detected by ICP.

[0107] The viscosity of the prepolymer mixture was measured by a Cone/Plate Viscometer from Brookfield.

[0108] The number average molecular weight (Mn) and PDI (polydispersity index, used to describe the molecular weight distribution of the polymer) of the high-molecular weight polylactic acid were measured by GPC, in which dichloromethane was used as the mobile phase and polystyrene was used as the reference standard.

[0109] Unless otherwise specified, the moisture contents of monomers are all less than or equal to 50 ppm.

[0110] During the production process, all production devices need to be purged with nitrogen to remove the air in the devices before put into use.

Example 1

[0111] Production of a Prepolymer Mixture:

[0112] 59.1 kg of 1,6-hexanediol, 941 kg of L-lactide and 6.8 kg of stannous octoate were added into a 1500 L stainless steel reactor for a ring-opening polymerization reaction. The ring-opening polymerization was performed at 180° C. with stirring for 45 min, to obtain a prepolymer mixture A containing a polylactic acid prepolymer. When the monomer is at 100% conversion, the theoretical number average molecular weight of the polylactic acid prepolymer is 2000. In the reaction system, the conversion rate was 97.1%, and the content of the L-lactide monomer in the prepolymer mixture A was calculated to be 2.7 wt %; the obtained polylactic acid prepolymer in the prepolymer mixture A had a number average molecular weight of 1945, a viscosity of 55 cp at 180° C., and an Sn content of 0.2% (2000 ppm).

[0113] In this production process, the feed mass ratio of 1,6-hexanediol was: 59.1 kg/(59.1 kg+941 kg)=5.9:100. The feed mass ratio of stannous octoate was: 6.8 kg/(59.1 kg+941 kg)=0.68:100. Under such feed mass ratios of the raw materials, the initiator and the catalyst can be fed and weighed accurately.

[0114] Production of a High-Molecular Weight Polylactic Acid:

[0115] 20.0 kg of the prepared prepolymer mixture A and 980 kg of L-lactide were added into a 1500 L stainless steel reactor for a reaction, and the reaction was performed at 180° C. with stirring for 4 h, to obtain a high-molecular weight polylactic acid product. The prepared polylactic acid had a Mn of 68640 and a PDI of 1.71. In the system, the residual rate of L-lactide monomer was 4.5%, and the Sn content was 40 ppm.

[0116] In this production process, the feed mass ratio of the prepolymer mixture A was: 20.0 kg/(20.0 kg+980 kg)=2.0:100. Under such feed mass ratio of the raw material, the prepolymer mixture can be fed and weighed accurately.

[0117] Then, the production process of polylactic acid was performed 4 times using the prepolymer mixture A, and 5 batches of polylactic acid products were obtained in total. The results of the 5 batches of polylactic acid produced are shown in Table 1.

TABLE-US-00001 TABLE 1 Results of the 5 batches of polylactic acid produced Production Batch Residual Rate of Monomer % Mn of PLA PDI Batch 1 4.5 68640 1.71 Batch 2 5.3 68295 1.72 Batch 3 4.4 69051 1.71 Batch 4 5.4 68229 1.72 Batch 5 4.4 68774 1.71

[0118] The variation range of the number average molecular weight of the obtained polylactic acid was −0.5%˜+0.7% of its average value.

Example 2

[0119] Production of a Prepolymer Mixture:

[0120] 59.1 kg of 1,6-hexanediol, 941 kg of L-lactide and 6.8 kg of stannous octoate were added into a 1500 L stainless steel reactor for a ring-opening polymerization reaction. The ring-opening polymerization was performed at 180° C. with stirring for 45 min, to obtain a prepolymer mixture A containing a polylactic acid prepolymer. When the monomer is at 100% conversion, the theoretical number average molecular weight of the polylactic acid prepolymer is 2000. In the reaction system, the conversion rate was 97.1%, and the content of the L-lactide monomer in the prepolymer mixture A was calculated to be 2.7 wt %; the obtained polylactic acid prepolymer in the prepolymer mixture A had a number average molecular weight of 1945, a viscosity of 55 cp at 180° C., and an Sn content of 0.2% (2000 ppm).

[0121] In this production process, the feed mass ratio of 1,6-hexanediol was: 59.1 kg/(59.1 kg+941 kg)=5.9:100. The feed mass ratio of stannous octoate was: 6.8 kg/(59.1 kg+941 kg)=0.68:100. Under such feed mass ratios of the raw materials, the initiator and the catalyst can be fed and weighed accurately.

[0122] Production of a High-Molecular Weight Polylactic Acid:

[0123] 13.3 kg of the prepared prepolymer mixture A, 900.0 kg of L-lactide and 86.7 kg of D-lactide were added into a 1500 L stainless steel reactor for a reaction, and the reaction was performed at 170° C. with stirring for 9 h, to obtain a high-molecular weight polylactic acid product. The prepared polylactic acid had a Mn of 97143 and a PDI of 1.93. In the system, the residual rate of the monomers was 4.5%, and the Sn content was 26.7 ppm.

[0124] In this production process, the feed mass ratio of the prepolymer mixture A was: 13.3 kg/(13.3 kg+986.7 kg)=1.3:100. Under such feed mass ratio of the raw material, the prepolymer mixture can be fed and weighed accurately.

[0125] Then, the production process of polylactic acid was performed 4 times using the prepolymer mixture A, and 5 batches of polylactic acid products were obtained in total. The results of the 5 batches of polylactic acid produced are shown in Table 2.

TABLE-US-00002 TABLE 2 Results of the 5 batches of polylactic acid produced Production Batch Residual Rate of Monomer % Mn of PLA PDI Batch 1 4.5 97143 1.93 Batch 2 5.3 96770 1.94 Batch 3 4.4 97992 1.91 Batch 4 5.4 97554 1.93 Batch 5 4.4 96991 1.94

[0126] The variation range of the number average molecular weight of the obtained polylactic acid was −0.5%˜+0.7% of its average value.

Example 3

[0127] Production of a Prepolymer Mixture:

[0128] Reaction materials were continuously added into a first plug flow tubular reactor and the product was continuously collected, and each reaction material flow rate was as follows: the flow rate of trimethylolpropane (TMP) was 11.2 kg/h, the flow rate of L-lactide was 88.8 kg/h, and the flow rate of stannous octoate was 1.36 kg/h; reaction materials were subjected to a ring-opening polymerization reaction at 160° C. for a reaction duration of 45 min, to obtain a prepolymer mixture B containing a polylactic acid prepolymer. When the monomer is at 100% conversion, the theoretical number average molecular weight of the polylactic acid prepolymer is 1200. In the reaction system, the conversion rate was 93.5%, and the content of the L-lactide monomer in the prepolymer mixture B was calculated to be 5.8 wt %; the obtained polylactic acid prepolymer in the prepolymer mixture B had a number average molecular weight of 1131, a viscosity of 38 cp at 180° C., and an Sn content of 0.4% (4000 ppm). The prepared product was stored in a storage tank.

[0129] In the production process, the feed mass ratio of trimethylolpropane was: 11.2 kg/(11.2 kg+88.8 kg)=11.2:100. The feed mass ratio of stannous octoate was: 1.36 kg/(11.2 kg+88.8 kg)=1.36:100. Under such feed mass ratios of the raw materials, the initiator and the catalyst can be fed and weighed accurately.

[0130] Production of a High-Molecular Weight Polylactic Acid:

[0131] Reaction materials were continuously added into a second plug flow tubular reactor and the product was continuously collected, and each reaction material flow rate was as follows: the flow rate of the prepolymer mixture B was 1.0 kg/h and the flow rate of L-lactide was 99.0 kg/h; a polymerization reaction was performed at 200° C. for a reaction duration of 4 h, to obtain a high-molecular weight polylactic acid product. The Sn content was 40 ppm in the system.

[0132] In this production process, the feed mass ratio of the prepolymer mixture B was: 1.0 kg/(1.0 kg+99.0 kg)=1.0:100. Under such feed mass ratio of the raw material, the prepolymer mixture can be fed and weighed accurately.

[0133] During the production process, a product sample was taken every 6 hours, and after sampling 5 times, 5 batches of polylactic acid products were obtained in total. The results of the 5 batches of polylactic acid produced are shown in Table 3.

TABLE-US-00003 TABLE 3 Results of the 5 batches of polylactic acid produced Production Batch Residual Rate of Monomer % Mn of PLA PDI Batch 1 4 80600 1.85 Batch 2 4.9 77664 1.86 Batch 3 3.9 83349 1.84 Batch 4 4 79836 1.83 Batch 5 4.1 81791 1.85

[0134] The variation range of the number average molecular weight of the obtained polylactic acid was −3.7%˜+3.3% of its average value.

Example 4

[0135] Production of a Prepolymer Mixture:

[0136] Reaction materials were continuously added into a first plug flow tubular reactor and the product was continuously collected, and each reaction material flow rate was as follows: the flow rate of ethylene glycol was 6.2 kg/h, the flow rate of L-lactide was 93.8 kg/h, and the flow rate of stannous octoate was 2.73 kg/h; reaction materials were subjected to a ring-opening polymerization reaction at 150° C. for a reaction duration of 45 min, to obtain a prepolymer mixture C containing a polylactic acid prepolymer. When the monomer is at 100% conversion, the theoretical number average molecular weight of the polylactic acid prepolymer is 1000. In the reaction system, the conversion rate was 97.0%, and the content of the L-lactide monomer in the prepolymer mixture C was calculated to be 2.8 wt %; the obtained polylactic acid prepolymer in the prepolymer mixture C had a number average molecular weight of 972, a viscosity of 10 cp at 180° C., and an Sn content of 0.8% (8000 ppm). The prepared product was stored in a storage tank.

[0137] In the production process, the feed mass ratio of ethylene glycol was: 6.2 kg/(6.2 kg+93.8 kg)=6.2:100. The feed mass ratio of stannous octoate was: 2.73 kg/(6.2 kg+93.8 kg)=2.73:100. Under such feed mass ratios of the raw materials, the initiator and the catalyst can be fed and weighed accurately.

[0138] Production of a High-Molecular Weight Polylactic Acid:

[0139] Reaction materials were continuously added into a second plug flow tubular reactor and the product was continuously collected, and each reaction material flow rate was as follows: the flow rate of the prepolymer mixture C was 0.5 kg/h and the flow rate of L-lactide was 99.5 kg/h; a polymerization reaction was performed at 175° C. for a reaction duration of 5.5 h, to obtain a high-molecular weight polylactic acid product. The Sn content was 40 ppm in the system.

[0140] In this production process, the feed mass ratio of the prepolymer mixture C was: 0.5 kg/(0.5 kg+99.5 kg)=0.5:100. Under such feed mass ratio of the raw material, the prepolymer mixture can be fed and weighed accurately.

[0141] During the production process, a product sample was taken every 6 hours, and after sampling 5 times, 5 batches of polylactic acid products were obtained in total. The results of the 5 batches of polylactic acid produced are shown in Table 4.

TABLE-US-00004 TABLE 4 Results of the 5 batches of polylactic acid produced Production Batch Residual Rate of Monomer % Mn of PLA PDI Batch 1 5 121178 2.05 Batch 2 5.7 117379 2.06 Batch 3 4.5 125536 2.04 Batch 4 5.3 119080 2.05 Batch 5 5.1 124340 2.06

[0142] The variation range of the number average molecular weight of the obtained polylactic acid was −3.4%˜+3.3% of its average value.

Example 5

[0143] Production of a Prepolymer Mixture:

[0144] 23.6 kg of 1,6-hexanediol, 898 kg of L-lactide, 78 kg of D-lactide and 6.8 kg of stannous octoate were added into a 1500 L stainless steel reactor for a ring-opening polymerization reaction; the ring-opening polymerization was performed at 220° C. with stirring for 45 min, to obtain a prepolymer mixture D containing a polylactic acid prepolymer. When the monomer is at 100% conversion, the theoretical number average molecular weight of the polylactic acid prepolymer is 5000. In the reaction system, the conversion rate was 97.0%, and the content of the monomers in the prepolymer mixture D was calculated to be 2.9 wt %; the obtained polylactic acid prepolymer in the prepolymer mixture D had a number average molecular weight of 4854, a viscosity of 218 cp at 180° C., and an Sn content of 0.2% (2000 ppm).

[0145] In this production process, the feed mass ratio of 1,6-hexanediol was: 23.6 kg/(23.6 kg+976 kg)=2.36:100. The feed mass ratio of stannous octoate was: 6.8 kg/(23.6 kg+976 kg)=0.68:100.

[0146] Under such feed mass ratios of the raw materials, the initiator and the catalyst can be fed and weighed accurately.

[0147] Production of a High-Molecular Weight Polylactic Acid:

[0148] 20.0 kg of the prepared prepolymer mixture D, 902 kg of L-lactide and 78 kg of D-lactide were added into a 1500 L stainless steel reactor for a reaction, and the reaction was performed at 220° C. with stirring for 3 h, to obtain a high-molecular weight polylactic acid. The prepared polylactic acid product had a Mn of 145731 and a PDI of 2.17. In the system, the residual rate of the lactide monomers was 3%, and the Sn content was 40 ppm.

[0149] In this production process, the feed mass ratio of the prepolymer mixture D was: 20.0 kg/(20.0 kg+980 kg)=2.0:100. Under such feed mass ratio of the raw material, the prepolymer mixture can be fed and weighed accurately.

[0150] Then, the production process of polylactic acid was performed 4 times using the prepolymer mixture A, and 5 batches of polylactic acid products were obtained in total. The results of the 5 batches of polylactic acid produced are shown in Table 5.

TABLE-US-00005 TABLE 5 Results of the 5 batches of polylactic acid produced Production Batch Residual Rate of Monomer % Mn of PLA PDI Batch 1 3 145731 2.17 Batch 2 3.3 143399 2.18 Batch 3 2.8 147943 2.17 Batch 4 3.1 144207 2.16 Batch 5 2.9 146200 2.17

[0151] The variation range of the number average molecular weight of the obtained polylactic acid was −1.4%˜+1.7% of its average value.

Example 6

[0152] Production of a Prepolymer Mixture:

[0153] 45.1 kg of 1,4-butanediol, 225 kg of L-lactide and 5.5 kg of stannous octoate were added into a 1500 L stainless steel reactor, heated to 170° C. and stirred to perform a ring-opening polymerization reaction, then added with 730 kg of L-lactide within 30 min, and continued to react at 170° C. for 50 min after completing the feeding; a prepolymer mixture E containing a polylactic acid prepolymer was obtained after the reaction was completed. When the monomer is at 100% conversion, the theoretical number average molecular weight of the polylactic acid prepolymer is 2000. In the reaction system, the conversion rate was 96.5%, and the content of the monomers in the prepolymer mixture E was calculated to be 3.3 wt %; the obtained polylactic acid prepolymer in the prepolymer mixture E had a number average molecular weight of 1933, a viscosity of 22 cp at 180° C., and an Sn content of 0.16% (1600 ppm).

[0154] In this production process, the feed mass ratio of 1,4-butanediol was: 45.1 kg/(45.1 kg+955 kg)=4.5:100. The feed mass ratio of stannous octoate was: 5.5 kg/(45.1 kg+955 kg)=0.55:100. Under such feed mass ratios of the raw materials, the initiator and the catalyst can be fed and weighed accurately.

[0155] Production of a High-Molecular Weight Polylactic Acid:

[0156] Reaction materials were continuously added into a plug flow tubular reactor and the product was continuously collected, and each reaction material flow rate was as follows: the flow rate of the prepolymer mixture E was 2.5 kg/h and the flow rate of L-lactide was 97.5 kg/h; a polymerization reaction was performed at 180° C. for a reaction duration of 4 h, to obtain a high-molecular weight polylactic acid product. The Sn content was 40 ppm in the system.

[0157] In this production process, the feed mass ratio of the prepolymer mixture E was: 2.5 kg/(2.5 kg+97.5 kg)=2.5:100. Under such feed mass ratio of the raw material, the prepolymer mixture can be fed and weighed accurately.

[0158] During the production process, a product sample was taken every 6 hours, and after sampling 5 times, 5 batches of polylactic acid products were obtained in total. The results of the 5 batches of polylactic acid produced are shown in Table 6.

TABLE-US-00006 TABLE 6 Results of the 5 batches of polylactic acid produced Production Batch Residual Rate of Monomer % Mn of PLA PDI Batch 1 5.5 63817 1.68 Batch 2 6.1 62987 1.69 Batch 3 5.9 64774 1.68 Batch 4 5.7 63366 1.68 Batch 5 5.3 64015 1.69

[0159] The variation range of the number average molecular weight of the obtained polylactic acid was −1.3%˜+1.5% of its average value.

Example 7

[0160] Production of a Prepolymer Mixture:

[0161] 31.6 kg of isodecanol, 968 kg of L-lactide and 1.0 kg of stannous octoate were added into a 1500 L stainless steel reactor for a ring-opening polymerization reaction; the reaction was performed at 190° C. with stirring for 60 min, to obtain a prepolymer mixture F containing a polylactic acid prepolymer. When the monomer is at 100% conversion, the theoretical number average molecular weight of the polylactic acid prepolymer is 5000. In the reaction system, the conversion rate was 97.5%, and the content of the monomer in the prepolymer mixture F was calculated to be 2.4 wt %; the obtained polylactic acid prepolymer in the prepolymer mixture F had a number average molecular weight of 4879, a viscosity of 459 cp at 180° C., and an Sn content of 300 ppm.

[0162] In this production process, the feed mass ratio of isodecanol was: 31.6 kg/(31.6 kg+968 kg)=3.16:100. The feed mass ratio of stannous octoate was: 1.0 kg/(31.6 kg+968 kg)=0.1:100. Under such feed mass ratios of the raw materials, the initiator and the catalyst can be fed and weighed accurately.

[0163] Production of a High-Molecular Weight Polylactic Acid:

[0164] Reaction materials were continuously added into a plug flow tubular reactor and the product was continuously collected, and each reaction material flow rate was as follows: the flow rate of the prepolymer mixture F was 10 kg/h and the flow rate of L-lactide was 90 kg/h; reaction materials were subjected to a polymerization reaction at 180° C. for a reaction duration of 5 h, to obtain a high-molecular weight polylactic acid. The Sn content was 30 ppm in the system.

[0165] In this production process, the feed mass ratio of the prepolymer mixture F was: 10 kg/(10 kg+90 kg)=10:100. Under such feed mass ratio of the raw material, the prepolymer mixture can be fed and weighed accurately.

[0166] During the production process, a product sample was taken every 6 hours, and after sampling 5 times, 5 batches of polylactic acid were obtained in total. The results of the 5 batches of polylactic acid produced are shown in Table 7.

TABLE-US-00007 TABLE 7 Results of the 5 batches of polylactic acid produced Production Batch Residual Rate of Monomer % Mn of PLA PDI Batch 1 6.5 46004 1.65 Batch 2 6.9 45573 1.66 Batch 3 6.1 46420 1.65 Batch 4 6.7 45871 1.65 Batch 5 6.4 46114 1.66

[0167] The variation range of the number average molecular weight of the obtained polylactic acid was −0.9%˜+0.9% of its average value.

Comparative Example 1 (One-Step Polymerization)

[0168] Production of a High-Molecular Weight Polylactic Acid:

[0169] 1.18 kg of 1,6-hexanediol, 999 kg of L-lactide and 0.14 kg of stannous octoate were added into a 1500 L stainless steel reactor for a ring-opening polymerization reaction, and the reaction was performed at 180° C. with stirring for 4 h, to obtain a polylactic acid product. After the reaction was completed, the Sn content was 40 ppm in the system.

[0170] In the production process, the feed mass ratio of 1,6-hexanediol was: 1.18 kg/(1.18 kg+999 kg)=0.118:100. The feed mass ratio of stannous octoate was: 0.14 kg/(1.18 kg+999 kg)=0.014:100. Under such feed mass ratios of the raw materials, the initiator and the catalyst cannot be controlled in terms of the feeding and weighing accuracy, and the raw materials will also be affected in uniformity during the mixing process.

[0171] The production process was performed 4 times, and 5 batches of polylactic acid products were obtained in total. The results of the 5 batches of polylactic acid produced are shown in Table 8.

TABLE-US-00008 TABLE 8 Results of the 5 batches of polylactic acid produced Production Batch Residual Rate of Monomer % Mn of PLA PDI Batch 1 4.7 68430 1.75 Batch 2 4.3 62333 1.7 Batch 3 4.8 73106 1.82 Batch 4 4.4 65131 1.72 Batch 5 5.3 69171 1.8

[0172] The variation range of the number average molecular weight of the obtained polylactic acid was −7.8%˜+8.1% of its average value.

Comparative Example 2 (One-Step Polymerization)

[0173] Production of a High-Molecular Weight Polylactic Acid:

[0174] Reaction materials were continuously added into a plug flow tubular reactor and the product was continuously collected, and each reaction material flow rate was as follows: the flow rate of trimethylolpropane (TMP) was 0.11 kg/h, the flow rate of L-lactide was 99.89 kg/h, and the flow rate of stannous octoate was 0.014 kg/h. Reaction materials were subjected to a ring-opening polymerization reaction at 200° C. for a reaction duration of 3 h, to obtain a polylactic acid product. The Sn content was 40 ppm in the system.

[0175] In the production process, the feed mass ratio of trimethylolpropane was: 0.11 kg/(0.11 kg+99.89 kg)=0.11:100. The feed mass ratio of stannous octoate was: 0.014 kg/(0.11 kg+99.89 kg)=0.014:100. Under such feed mass ratios of the raw materials, the initiator and the catalyst cannot be controlled in terms of the feeding and weighing accuracy, and the raw materials will also be affected in uniformity during the mixing process.

[0176] During the production process, a product sample was taken every 6 hours, and after sampling 5 times, 5 batches of polylactic acid products were obtained in total. The results of the 5 batches of polylactic acid produced are shown in Table 9.

TABLE-US-00009 TABLE 9 Results of the 5 batches of polylactic acid produced Production Batch Residual Rate of Monomer % Mn of PLA PDI Batch 1 4 80541 1.84 Batch 2 4.8 69439 1.8 Batch 3 3.7 88602 1.89 Batch 4 4.1 77361 1.83 Batch 5 4.2 80971 1.84

[0177] The variation range of the number average molecular weight of the obtained polylactic acid was −12.5%˜+11.6% of its average value.

Comparative Example 3 (One-Step Polymerization)

[0178] Production of a High-Molecular Weight Polylactic Acid:

[0179] Reaction materials were continuously added into a plug flow tubular reactor and the product was continuously collected, and each reaction material flow rate was as follows: the flow rate of ethylene glycol was 0.03 kg/h, the flow rate of L-lactide was 99.97 kg/h, and the flow rate of stannous octoate was 0.014 kg/h. Reaction materials were subjected to a ring-opening polymerization reaction at 175° C. for a reaction duration of 5.5 h, to obtain a polylactic acid product. The Sn content was 40 ppm in the system.

[0180] In the production process, the feed mass ratio of ethylene glycol was: 0.03 kg/(0.03 kg+99.97 kg)=0.03:100. The feed mass ratio of stannous octoate was: 0.014 kg/(0.03 kg+99.97 kg)=0.014:100. Under such feed mass ratios of the raw materials, the initiator and the catalyst cannot be controlled in terms of the feeding and weighing accuracy, and the raw materials will also be affected in uniformity during the mixing process.

[0181] During the production process, a product sample was taken every 6 hours, and after sampling 5 times, 5 batches of polylactic acid products were obtained in total. The results of the 5 batches of polylactic acid produced are shown in Table 10.

TABLE-US-00010 TABLE 10 Results of the 5 batches of polylactic acid produced Production Batch Residual Rate of Monomer % Mn of PLA PDI Batch 1 5 118361 2.1 Batch 2 7.2 90149 2.05 Batch 3 4.6 136628 2.15 Batch 4 6.3 109457 2.09 Batch 5 5.7 125051 2.12

[0182] The variation range of the number average molecular weight of the obtained polylactic acid was −22.2%˜+17.9% of its average value.

Comparative Example 4 (One-Step Polymerization)

[0183] Production of a High-Molecular Weight Polylactic Acid:

[0184] 0.47 kg of 1,6-hexanediol, 919.5 kg of L-lactide, 80 kg of D-lactide and 0.14 kg of stannous octoate were added into a 1500 L stainless steel reactor for a ring-opening polymerization reaction. The reaction was performed at 220° C. with stirring for 3 h, to obtain a polylactic acid product. The Sn content was 40 ppm in the system.

[0185] In the production process, the feed mass ratio of 1,6-hexanediol was: 0.47 kg/(0.47 kg+999.5 kg)=0.047:100. The feed mass ratio of stannous octoate was: 0.14 kg/(0.47 kg+999.5 kg)=0.014:100. Under such feed mass ratios of the raw materials, the initiator and the catalyst cannot be controlled in terms of the feeding and weighing accuracy, and the raw materials will also be affected in uniformity during the mixing process.

[0186] The production process was performed 4 times, and 5 batches of polylactic acid products were obtained in total. The results of the 5 batches of polylactic acid produced are shown in Table 11.

TABLE-US-00011 TABLE 11 Results of the 5 batches of polylactic acid produced Production Batch Residual Rate of Monomer % Mn of PLA PDI Batch 1 3.1 145021 2.17 Batch 2 3.7 126448 2.12 Batch 3 2.5 155490 2.21 Batch 4 3.1 138741 2.15 Batch 5 3 150992 2.2

[0187] The variation range of the number average molecular weight of the obtained polylactic acid was −11.8%˜+8.5% of its average value.

Comparative Example 5 (One-Step Polymerization)

[0188] Production of a High-Molecular Weight Polylactic Acid:

[0189] Reaction materials were continuously added into a plug flow tubular reactor and the product was continuously collected, and each reaction material flow rate was as follows: the flow rate of 1,4-butanediol was 0.11 kg/h, the flow rate of L-lactide was 99.89 kg/h, and the flow rate of stannous octoate was 0.014 kg/h. Reaction materials were subjected to a ring-opening polymerization reaction at 180° C. for a reaction duration of 4 h, to obtain a polylactic acid product. The Sn content was 40 ppm in the system.

[0190] In the production process, the feed mass ratio of 1,4-butanediol was: 0.11 kg/(0.11 kg+99.89 kg)=0.11:100. The feed mass ratio of stannous octoate was: 0.014 kg/(0.11 kg+99.89 kg)=0.014:100. Under such feed mass ratios of the raw materials, the initiator and the catalyst cannot be controlled in terms of the feeding and weighing accuracy, and the raw materials will also be affected in uniformity during the mixing process.

[0191] During the production process, a product sample was taken every 6 hours, and after sampling 5 times, 5 batches of polylactic acid products were obtained in total. The results of the 5 batches of polylactic acid produced are shown in Table 12.

TABLE-US-00012 TABLE 12 Results of the 5 batches of polylactic acid produced Production Batch Residual Rate of Monomer % Mn of PLA PDI Batch 1 5.5 63635 1.71 Batch 2 7.5 57007 1.67 Batch 3 5 72470 1.81 Batch 4 5.8 60092 1.69 Batch 5 5.3 67749 1.76

[0192] The variation range of the number average molecular weight of the obtained polylactic acid was −11.2%˜+12.9% of its average value.

Comparative Example 6 (One-Step Polymerization)

[0193] Production of a High-Molecular Weight Polylactic Acid:

[0194] Reaction materials were continuously added into a plug flow tubular reactor and the product was continuously collected, and each reaction material flow rate was as follows: the flow rate of isodecanol was 0.32 kg/h, the flow rate of L-lactide was 99.68 kg/h, and the flow rate of stannous octoate was 0.010 kg/h. Reaction materials were subjected to a ring-opening polymerization reaction at 180° C. for a reaction duration of 5 h, to obtain a polylactic acid product. The Sn content was 30 ppm in the system.

[0195] In the production process, the feed mass ratio of isodecanol was: 0.32 kg/(0.32 kg+99.68 kg)=0.32:100. The feed mass ratio of stannous octoate was: 0.010 kg/(0.32 kg+99.68 kg)=0.010:100. Under such feed mass ratios of the raw materials, the initiator and the catalyst cannot be controlled in terms of the feeding and weighing accuracy, and the raw materials will also be affected in uniformity during the mixing process.

[0196] During the production process, a product sample was taken every 6 hours, and after sampling 5 times, 5 batches of polylactic acid products were obtained in total. The results of the 5 batches of polylactic acid produced are shown in Table 13.

TABLE-US-00013 TABLE 13 Results of the 5 batches of polylactic acid produced Production Batch Residual Rate of Monomer % Mn of PLA PDI Batch 1 6.5 46214 1.65 Batch 2 8.9 42438 1.63 Batch 3 5.5 50280 1.66 Batch 4 6.6 43711 1.64 Batch 5 6.4 48820 1.65

[0197] The variation range of the number average molecular weight of the obtained polylactic acid was −8.3%˜+8.6% of its average value.

[0198] By comparing Comparative Example 1 with Example 1 and comparing Comparative Examples 2-6 with Examples 3-7, it can be found that the Examples have lower requirements on the weighing accuracy of raw materials in the production process, have more stable Mn with smaller variation ranges, and facilitate to the production practice. In the present application, the production process of two-step monomer feed and polymerization and stepwise molecular weight amplification is used, improving the production stability.

[0199] By comparing Comparative Example 1 with Example 1 and Example 2, it can be found that the production method of the present application can more easily and flexibly produce polylactic acid products with different molecular weight.

[0200] Various embodiments of the present application have been described above, and the above descriptions are not exhaustive but illustrative, and the present application is not limited by the disclosed embodiments. A plurality of modifications and variations are obvious to those skilled in the art without departing from the scope and spirit of various embodiments.