Method for selectively depolymerizing polylactic acid to chiral lactide

Abstract

The invention discloses a method for selectively depolymerizing polylactic acid into chiral lactide, and belongs to the technical field of polylactic acid depolymerization. According to the invention, the problems that the existing conventional polylactic acid depolymerization means generally lacks stereoselectivity and a depolymerization product with high optical purity is difficult to obtain are solved. In the presence of a solvent and under the heating condition, polylactic acid matched with chirality is depolymerized into lactide with high optical purity under the catalysis of a chiral depolymerization catalyst, and polylactic acid not matched with chirality is not depolymerized, so that the selective depolymerization process of polylactic acid is realized. Different from the traditional asymmetric catalysis field, the method focuses on recognition of small molecules, realizes closed-loop depolymerization of polylactic acid to chiral monomers, realizes selective recognition and depolymerization of chiral polymers, and broadens the research range of asymmetric catalysis.

Description

Technical Field

[0001] The present invention relates to a method for selectively depolymerizing polylactic acid into chiral lactide, belonging to the technical field of polylactic acid depolymerization. Background Art

[0002] Achieving precise stereoselective control through asymmetric catalysis is an important direction in the field of chemistry. At present, strategies based on asymmetric catalysis have successfully realized the efficient synthesis of chiral small molecules and the preparation of stereoregular polymers. Chiral small molecules can be prepared from racemic or prochiral small molecule substrates through asymmetric catalysis. By precisely constructing chiral centers through kinetic resolution, stereoselective polymerization and other approaches, stereoregular polymers with specific functions can be synthesized. These methods show significant application value in the fields of drug synthesis, chiral materials, etc. However, there is no systematic solution to how to efficiently recycle chiral monomers or specific configuration fragments by selectively depolymerizing chiral polymers.

[0003] Achieving the selective depolymerization of polymers is of great significance. On the one hand, chirality is a basic property of biological macromolecules. Selectively degrading biological macromolecules such as DNA, RNA, proteins, etc. into chiral small molecules that can be absorbed by organisms is one of the important bases of life metabolism. In organisms, the key to this asymmetric transformation is the biomolecular catalyst enzyme. Achieving stereoselective control of polymer depolymerization is of great value for understanding the chiral recognition and transfer of biological macromolecules such as enzymes. On the other hand, the technical defects of existing methods for selectively depolymerizing chiral polymers limit the recycling of chiral materials and the development needs of green chemistry. Traditional chemical degradation means usually lack stereoselectivity and it is difficult to obtain depolymerization products with high optical purity, which will cause waste of resources and environmental pollution.

[0004] In summary, developing polymer selective depolymerization technology based on asymmetric catalysis mechanism to achieve efficient recycling and reuse of chiral units has become a key problem to be urgently solved. As the most representative polymer containing chiral sites, it is very necessary to achieve the selective depolymerization of polylactic acid into chiral lactide. Summary of the Invention

[0005] Aiming at the problem that existing conventional polylactic acid depolymerization means usually lack stereoselectivity and it is difficult to obtain depolymerization products with high optical purity, the present invention provides a method for selectively depolymerizing polylactic acid into chiral lactide.

[0006] The technical solution of the present invention:

[0007] One of the purposes of the present invention is to provide a method for selectively depolymerizing polylactic acid into chiral lactide. Specifically, the method is as follows: in the presence of a solvent and under heating conditions, a chiral depolymerization catalyst catalyzes the depolymerization of polylactic acid to obtain lactide with high optical purity.

[0008] The catalyst is R,R-Cat. with R configuration and / or S,S-Cat. with S configuration;

[0009] The structural formula of R,R-Cat. is:

[0010]

[0011] The structural formula of S,S-Cat. is:

[0012]

[0013] Further defined, the polylactic acid is one or a mixture of more than one of isotactic L-configured polylactic acid, isotactic D-configured polylactic acid, stereocomplex polylactic acid, polylactic acid diblock copolymer, and polylactic acid multiblock copolymer.

[0014] Further defined, the number average molecular weight of the polylactic acid is 10 3 ~10 6 g / mol.

[0015] Further defined, the molar equivalent ratio of the chiral depolymerization catalyst to the repeating unit of polylactic acid is (0.001 - 10):1.

[0016] Further defined, the solvent is one or a mixture of more than one of MeCN, DMSO, DMF, DMI, toluene, chlorobenzene, fluorobenzene, tetrahydrofuran, chloroform, and dichloromethane.

[0017] Even further defined, the solvent is MeCN.

[0018] Further defined, the concentration of the repeating unit of polylactic acid in the solvent is 0.01 - 10 mol / L.

[0019] Further defined, the heating temperature is 50 - 260 °C and the time is 1 - 48 h.

[0020] Further defined, the yield of the high optical purity lactide obtained is greater than 90%.

[0021] Further defined, the optical purity of the lactide obtained is greater than 90%.

[0022] Beneficial effects:

[0023] Under the conditions of the presence of a solvent and heating, the present invention uses a chiral depolymerization catalyst to catalyze the depolymerization of polylactic acid with a matching chirality to a high optical purity lactide, while the polylactic acid with a non-matching chirality does not depolymerize, thereby realizing a selective depolymerization process of polylactic acid. Compared with the prior art, the present application has the following beneficial effects:

[0024] (1) Different from the traditional asymmetric catalysis field that focuses on the recognition of small molecules, the present invention realizes the selective recognition and depolymerization of chiral polymers, broadening the research scope of asymmetric catalysis.

[0025] (2) The present invention realizes the selective depolymerization process of polylactic acid for the first time. The selective depolymerization matches the chiral polylactic acid and retains the unmatched chiral polylactic acid, thereby realizing the closed-loop depolymerization of polylactic acid to chiral monomers. The realization of this process benefits from the specific selection of the catalyst and the reaction solvent. The chiral cavity of the catalyst can directionally recognize and activate the hydroxyl chain end of polylactic acid, while polar aprotic solvents such as acetonitrile act on the catalyst and the polymer through high polarity, dielectric constant and weak coordination ability, further enhancing the chiral recognition effect of the catalyst on the polymer and inducing a decrease in the Gibbs free energy of the depolymerization process to promote the closed-loop backbiting. Through the synergistic effect of the active center of the metal catalyst and the microenvironment of the polar aprotic solvent above, the selective control of the depolymerization process of chiral polymers is realized.

[0026] (3) The reaction conditions of the present invention are mild, the process is efficient, and lactide monomers are obtained by depolymerization with high yield and high optical purity. Description of the Drawings

[0027] Figure 1 It is the chiral HPLC test spectrum of the lactide obtained by depolymerization in Example 1 and the corresponding peak analysis results;

[0028] Figure 2 It is the GPC spectrum of the remaining undepolymerized polylactic acid in Example 1. Detailed Embodiments

[0029] To make the above objects, features and advantages of the present invention more obvious and understandable, the following detailed description of the specific embodiments of the present invention is made in conjunction with the embodiments of the specification.

[0030] Many specific details are set forth in the following description in order to fully understand the present invention, but the present invention can also be implemented in other ways different from those described herein. Those skilled in the art can make similar promotions without departing from the connotation of the present invention. Therefore, the present invention is not limited by the specific embodiments disclosed below.

[0031] Secondly, the so-called "one embodiment" or "embodiment" herein refers to a specific feature, structure or characteristic that can be included in at least one implementation manner of the present invention. The appearances of "in one embodiment" in different places in this specification do not all refer to the same embodiment, nor are they separate or selectively exclusive embodiments from other embodiments.

[0032] Unless otherwise specified, the experimental methods used in the following examples are all conventional methods. The materials, reagents, methods, and instruments used are all conventional materials, reagents, methods, and instruments in this field without special instructions. Those skilled in the art can obtain them through commercial channels, and the purity of the solid and liquid reagents used is analytical pure.

[0033] Example 1:

[0034] Depolymerization of stereocomplex poly(lactic acid) catalyzed by R,R-Cat.

[0035] The specific structural formula is as follows:

[0036]

[0037] The specific operation process is as follows:

[0038] Weigh stereocomplex poly(lactic acid) (PLLA:PDLA = 50:50, molecular weight 15100 g / mol, molecular weight distribution 1.15) (36 mg, 0.5 mmol) and R,R-Cat. (10 mg, 10 μmol, 2 mol% relative to the lactic acid repeat unit) and add them to a 5 mL Schlenk flask. Then add acetonitrile solvent (1 mL, lactic acid repeat unit concentration 0.5 mol / L), and stir and react at 140 °C.

[0039] The obtained product was subjected to NMR and liquid phase analysis, and the conversion rate, the contents of meso-lactide, L-configured lactide, D-configured lactide in the product lactide, and the stereoregularity of the unreacted poly(lactic acid) at different reaction times are shown in the following table:

[0040] Entry t (min) Conv. (%) meso-LA (%) <![CDATA[L-LA:D-LA d > <![CDATA[P m > 1 20 12 <1 / 0.99 2 30 18 <1 / 0.99 3 40 23 <1 3.2:96.8 0.99 4 50 28 <1 5.0:95.0 0.99 5 65 36 <1 3.7:96.3 0.99 6 75 39 <1 4.3:95.7 0.99 7 90 41 <1 3.8:96.2 0.99 8 120 45 <1 4.5:95.5 0.99 9 150 46 <1 5.8:94.2 0.99 10 240 50 1 7.7:92.3 0.99 11 360 52 2 8.8:91.2 0.99

[0041] From the above data, it can be seen that before 50% conversion, the content of D-configured lactide in the obtained product lactide is as high as 92.3, indicating that only D-configured poly(lactic acid) undergoes depolymerization ( Figure 1 ). While PLLA hardly undergoes depolymerization ( Figure 2 ), so the reaction hardly proceeds after 50% conversion. The consistently high stereoregularity of poly(lactic acid) further indicates that there are no side reactions such as transesterification during the reaction, and the structure of PLLA is retained.

[0042] Example 2:

[0043] Depolymerization of stereocomplex poly(lactic acid) catalyzed by S,S-Cat.

[0044] The specific structural formula is as follows:

[0045]

[0046] The specific operation process is as follows:

[0047] Weigh stereocomplex polylactic acid (PLLA:PDLA = 50:50, molecular weight 15100 g / mol, molecular weight distribution 1.15) (36 mg, 0.5 mmol) and S,S-Cat. (10 mg, 10 μmol, 2 mol% relative to the polylactic acid repeating unit) and add them to a 5 mL Schlenk flask. Then add acetonitrile solvent (1 mL, polylactic acid repeating unit concentration 0.5 mol / L), and stir and react at 140 °C.

[0048] The obtained product was subjected to NMR and liquid phase analysis, and the conversion rate at different reaction times, the contents of meso-lactide, L-configured lactide, D-configured lactide in the product lactide, and the stereoregularity of the unreacted polylactic acid are shown in the following table:

[0049] Entry t (min) Conv. (%) meso-LA (%) <![CDATA[L-LA:D-LA d > <![CDATA[P m > 1 240 50 1 93.4:6.6 0.99

[0050] When using S,S-Cat. for depolymerization, only PLLA depolymerizes at this time, while PDLA is retained.

[0051] Example 3:

[0052] The specific structural formula is as follows:

[0053]

[0054] The specific operation process is as follows:

[0055] Weigh stereocomplex polylactic acid (PLLA:PDLA = 50:50, molecular weight 45500 g / mol, molecular weight distribution 1.16) (36 mg, 0.5 mmol) and S,S-Cat. (10 mg, 10 μmol, 2 mol% relative to the polylactic acid repeating unit) and add them to a 5 mL Schlenk flask. Then add acetonitrile solvent (1 mL, polylactic acid repeating unit concentration 0.5 mol / L), and stir and react at 140 °C.

[0056] The obtained product was subjected to NMR and liquid phase analysis, and the conversion rate at different reaction times, the contents of meso-lactide, L-configured lactide, D-configured lactide in the product lactide, and the stereoregularity of the unreacted polylactic acid are shown in the following table:

[0057] Entry t (h) Conv. (%) meso-LA (%) <![CDATA[L-LA:D-LA d > <![CDATA[P m > 1 8 50 1 93.5:6.5 0.99

[0058] It can be seen that this depolymerization method is also applicable to polylactic acids with different molecular weights.

[0059] Example 4:

[0060] The specific structural formula is as follows:

[0061]

[0062] The specific operation process is as follows:

[0063] Weigh stereocomplex polylactic acid (PLLA:PDLA = 50:50, molecular weight 15100 g / mol, molecular weight distribution 1.15) (36 mg, 0.5 mmol) and S,S-Cat. (10 mg, 10 μmol, 2 mol% relative to the polylactic acid repeating unit) and add them to a 5 mL Schlenk flask. Then add acetonitrile solvent (1 mL, polylactic acid repeating unit concentration 0.5 mol / L), and stir and react at 120 °C.

[0064] Perform NMR and liquid phase analysis on the obtained product, and the conversion rate, the contents of meso-lactide, L-configured lactide, D-configured lactide in the product lactide, and the stereoregularity of the unreacted polylactic acid at different reaction times are as follows:

[0065] Entry t (h) Conv. (%) meso-LA (%) <![CDATA[L-LA:D-LA d > <![CDATA[P m > 1 6 50 1 93.1:6.9 0.99

[0066] It can be seen therefrom that good depolymerization selectivity can be maintained at different temperatures.

[0067] Example 5:

[0068] The specific structural formula is as follows:

[0069]

[0070] The specific operation process is as follows:

[0071] Weigh stereocomplex polylactic acid (PLLA:PDLA = 50:50, molecular weight 15100 g / mol, molecular weight distribution 1.15) (36 mg, 0.5 mmol) and S,S-Cat. (10 mg, 10 μmol, 2 mol% relative to the polylactic acid repeating unit) and add them to a 5 mL Schlenk flask. Then add DMF solvent (1 mL, polylactic acid repeating unit concentration 0.5 mol / L), and stir and react at 140 °C.

[0072] Perform NMR and liquid phase analysis on the obtained product, and the conversion rate, the contents of meso-lactide, L-configured lactide, D-configured lactide in the product lactide, and the stereoregularity of the unreacted polylactic acid at different reaction times are as follows:

[0073] Entry t (h) Conv. (%) meso-LA (%) <![CDATA[L-LA:D-LA d > <![CDATA[P m > 1 5 50 1 92.2:7.8 0.99

[0074] Comparative Example 1:

[0075] Depolymerization under solvent-free conditions

[0076] The specific structural formula is as follows:

[0077]

[0078] The specific operation process is as follows:

[0079] Weigh stereocomplex poly(lactic acid) (PLLA:PDLA = 50:50, molecular weight 15100 g / mol, molecular weight distribution 1.15) (36 mg, 0.5 mmol) and R,R-Cat. (10 mg, 10 μmol, 2 mol% relative to the lactic acid repeat unit), and add them to a 5 mL Schlenk flask. Stir and react at 140 °C without additional solvent.

[0080] After reacting for 4 hours, NMR analysis was carried out, and the conversion rate of lactide monomer was <5%. The reaction could not proceed smoothly under solvent-free conditions.

[0081] Comparative Example 2:

[0082] Depolymerization under toluene solvent conditions

[0083] The specific structural formula is as follows:

[0084]

[0085] The specific operation process is as follows:

[0086] Weigh stereocomplex poly(lactic acid) (PLLA:PDLA = 50:50, molecular weight 15100 g / mol, molecular weight distribution 1.15) (36 mg, 0.5 mmol) and R,R-Cat. (10 mg, 10 μmol, 2 mol% relative to the lactic acid repeat unit), and add them to a 5 mL Schlenk flask. Then add toluene solvent (1 mL, lactic acid repeat unit concentration 0.5 mol / L), and stir and react at 140 °C.

[0087] Example 6:

[0088] The specific structural formula is as follows:

[0089]

[0090] The specific operation process is as follows:

[0091] Weigh stereocomplex polylactic acid (PLLA-b-PDLA, molecular weight 14300 g / mol, molecular weight distribution 1.18) (36 mg, 0.5 mmol) and R,R-Cat. (10 mg, 10 μmol, 2 mol% relative to the polylactic acid repeating unit) and add them to a 5 mL Schlenk flask. Then add acetonitrile solvent (1 mL, polylactic acid repeating unit concentration 0.5 mol / L) and stir the reaction at 140 °C.

[0092] The obtained product was analyzed by NMR and liquid chromatography to obtain the conversion rate at different reaction times, the contents of meso-lactide, L-lactide, D-lactide in the product lactide, and the stereoregularity of the unreacted polylactic acid as follows:

[0093] Entry t (h) Conv. (%) meso-LA (%) <![CDATA[L-LA:D-LA d > <![CDATA[P m > 1 4 47 2 7.8:92.2 0.99

[0094] For the polylactic acid block copolymer, the selective depolymerization process can also be achieved.

[0095] Example 7:

[0096] The specific structural formula is as follows:

[0097]

[0098] The specific operation procedure is as follows:

[0099] Weigh PLLA (molecular weight 14900 g / mol, molecular weight distribution 1.14) (36 mg, 0.5 mmol) and S,S-Cat. (10 mg, 10 μmol, 2 mol% relative to the polylactic acid repeating unit) and add them to a 5 mL Schlenk flask. Then add acetonitrile solvent (1 mL, polylactic acid repeating unit concentration 0.5 mol / L) and stir the reaction at 140 °C.

[0100] Weigh PLLA (molecular weight 14900 g / mol, molecular weight distribution 1.14) (36 mg, 0.5 mmol) and R,R-Cat. (10 mg, 10 μmol, 2 mol% relative to the polylactic acid repeating unit) and add them to a 5 mL Schlenk flask. Then add acetonitrile solvent (1 mL, polylactic acid repeating unit concentration 0.5 mol / L) and stir the reaction at 140 °C.

[0101] The obtained product was analyzed by NMR and liquid chromatography to obtain the conversion rate at different reaction times, the contents of meso-lactide, L-lactide, D-lactide in the product lactide, and the stereoregularity of the unreacted polylactic acid as follows:

[0102] Entry Cat. t (h) Conv. (%) meso-LA (%) <![CDATA[L-LA:D-LA d > <![CDATA[P m > 1 S,S-Cat. 2 93 2 7.8:92.2 / R,R-Cat. 2 5 <1 / 0.99

[0103] For PLLA, S,S-Cat. can selectively recognize and rapidly depolymerize, while no depolymerization occurs under the catalysis of R,R-Cat.

[0104] Example 8:

[0105] The specific structural formula is as follows:

[0106]

[0107] The specific operation process is as follows:

[0108] Weigh PDLA (molecular weight 16400 g / mol, molecular weight distribution 1.19) (36 mg, 0.5 mmol) and S,S-Cat. (10 mg, 10 μmol, 2 mol% relative to the polylactic acid repeating unit) and add them into a 5 mL Schlenk flask. Then add acetonitrile solvent (1 mL, polylactic acid repeating unit concentration 0.5 mol / L), and stir and react at 140 °C.

[0109] Weigh PDLA (molecular weight 16400 g / mol, molecular weight distribution 1.19) (36 mg, 0.5 mmol) and R,R-Cat. (10 mg, 10 μmol, 2 mol% relative to the polylactic acid repeating unit) and add them into a 5 mL Schlenk flask. Then add acetonitrile solvent (1 mL, polylactic acid repeating unit concentration 0.5 mol / L), and stir and react at 140 °C.

[0110] The obtained products were subjected to NMR and liquid phase analysis, and the conversion rate, the contents of meso-lactide, L-configured lactide, D-configured lactide in the product lactide, and the stereoregularity of the unreacted polylactic acid at different reaction times were obtained as follows:

[0111] Entry Cat. t (h) Conv. (%) meso-LA (%) <![CDATA[L-LA:D-LA d > <![CDATA[P m > 1 S,S-Cat. 2 5 <1 / / R,R-Cat. 2 90 1 1.0:99.0 0.99

[0112] For PLLA, R,R-Cat. can selectively recognize and rapidly depolymerize, while no depolymerization occurs under the catalysis of S,S-Cat.

[0113] Although the present invention has been disclosed above with preferred embodiments, it is not intended to limit the present invention. Any person familiar with this technology can make various modifications and decorations without departing from the spirit and scope of the present invention. Therefore, the protection scope of the present invention should be defined by the claims.

Claims

1. A method for selectively depolymerizing polylactic acid to chiral lactide, characterized in that, In the presence of a solvent and under heating conditions, a chiral depolymerization catalyst catalyzes the depolymerization of polylactic acid to obtain lactide with high optical purity. The catalyst is R,R-Cat. with the R configuration and / or S,S-Cat. with the S configuration; The structural formula of R,R-Cat. is: The structural formula of S,S-Cat. is:

2. The method according to claim 1, wherein The polylactic acid is one or a mixture of more than one of isotactic L-configured polylactic acid, isotactic D-configured polylactic acid, stereocomplex polylactic acid, polylactic acid diblock copolymer, and polylactic acid multiblock copolymer.

3. The method according to claim 1, wherein The number-average molecular weight of polylactic acid is 10 3 to 10 6 g / mol.

4. The method according to claim 1, wherein The molar equivalent ratio of the chiral depolymerization catalyst to the repeating unit of polylactic acid is (0.001 - 10):

1.

5. The method according to claim 1, wherein The solvent is one or a mixture of more than one of MeCN, DMSO, DMF, DMI, toluene, chlorobenzene, fluorobenzene, tetrahydrofuran, chloroform, and dichloromethane.

6. The method according to claim 5, wherein The solvent is MeCN.

7. The method according to claim 1, wherein The concentration of the repeating unit of polylactic acid in the solvent is 0.01 - 10 mol / L.

8. The method according to claim 1, wherein The heating temperature is 50 - 260 °C and the time is 1 - 48 h.

9. The method according to claim 1, characterized in that The yield of the obtained lactide with high optical purity is greater than 90%.

10. Lactide with high optical purity prepared by the method according to any one of claims 1-9, characterized in that, The optical purity is greater than 90%.

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