Method for preparing polylactic acid by using organic base thiourea double catalytic system

Abstract

The application discloses a method for preparing polylactic acid by using an organic base-sulfourea dual catalytic system, and comprises the following steps: under the catalysis of the organic base-sulfourea dual catalytic system, ring-opening polymerization of lactide is carried out under the action of an initiator to obtain the polylactic acid; and the sulfourea is disulfourea or trisulfourea. The sulfourea catalyst is easy to synthesize, the organic base-sulfourea dual catalytic system formed by modular combination of the organic base and the sulfourea has a high decomposition temperature, and therefore has good high-temperature resistance, so that the controllable ring-opening polymerization of lactide in a solution can be realized, and the controllable ring-opening polymerization of lactide at high temperature can also be realized; the catalyst used in polymerization is a hydrogen bond bifunctional catalyst, and the controllable polymerization effect that cannot be achieved when only alkali is used for catalysis can be achieved, so that the polylactic acid has a narrow molecular weight distribution; by adjusting the ratio of lactide, the initiator, the organic base and the sulfourea, the catalytic activity can be further regulated, and the polylactic acid with a high molecular weight and a controllable structure can be obtained.

Description

Technical Field

[0001] This invention relates to the field of polymer materials technology, specifically to a method for preparing polylactic acid using an organic base thiourea dual-catalytic system. Background Technology

[0002] Polylactic acid (PLA) is a biodegradable and environmentally friendly material that can be synthesized through controlled ring-opening polymerization. Its monomer, lactide, has a wide range of biological sources, and the synthesized polymer exhibits good biodegradability and biocompatibility, as well as thermal and mechanical properties comparable to polyolefins, attracting widespread attention and research.

[0003] In the ring-opening polymerization of lactide, early catalysts used were metal catalysts, such as CN1814644, CN1814645, US5235031, US5357034, US4045418, US4057537, and US3736646. While metal catalysis offers good selectivity and stability, it is costly, time-consuming, and prone to metal contamination, hindering the application of polylactic acid (PLA) in biomedicine, microelectronics, food packaging materials, and cosmetics. Compared to metal catalysts, organic catalysts offer unique advantages. In terms of sustainability, most organic catalysts are commercially available and inexpensive, some can be synthesized in one or two steps, and their reaction conditions are milder (room temperature, common organic solvents, short reaction time). The resulting polymers exhibit narrow dispersion, high molecular weight, and high chain end fidelity, and can be easily removed or recovered from the polymer. Therefore, organocatalyzed ring-opening polymerization has become an attractive and practical alternative to traditional organometallic and enzyme-catalyzed ring-opening polymerization.

[0004] In the development of organic catalysis systems, strong base catalysis systems such as organophosphorus compounds, guanidine compounds, and N-heterocyclic carbene compounds have gradually emerged (Dirauf, M et al. Progress in Polymer Science 2022, 129, 101547.). However, under these strong base catalysis, the ring-opening polymerization of lactide is prone to a large number of side reactions, the polymerization reaction is uncontrollable, and phenomena such as low molecular weight and high dispersion occur. In order to solve the above problems, the organothiourea dual catalysis system has emerged, that is, adding thiourea co-catalyst to the organic strong base catalysis system, thereby forming anion-cation pairs to regulate the polymerization reaction.

[0005] In 2005, Professor Hedrick et al. first proposed an organothiourea dual-catalytic system, which has been widely adopted due to its high ring-opening selectivity (Hedrick, JL et al., Journal of the American Chemical Society 2005, 127, 13798-13799.). This type of catalyst consists of a set of hydrogen-bonding groups (thiourea) and a base-assisted catalyst, influencing the ROP of lactones by simultaneously activating both the monomer and the initiator (Thomas, C. et al., Green Chemistry 2014, 16, 1687-1699.). Studies have found that this hydrogen-bonded catalyst exhibits good catalytic performance at room temperature, effectively improving the controllability of cyclic ester ring-opening polymerization. However, bulk polymerization is generally used in industrial production, which requires high temperatures; for example, the bulk ring-opening polymerization of lactide requires 180°C. Existing bulk polymerization methods often employ high-temperature resistant metal catalysts, but these catalysts are characterized by high toxicity, difficulty in removal, and discoloration, hindering their application in bulk polymerization. Compared to metal catalysts, organic catalysts are less toxic and easier to remove. However, organic catalysts are prone to decomposition at high temperatures, causing the polymerization system to turn black or yellow, thus limiting their application in bulk polymerization. (Mezzasalma, L. et al., European Polymer Journal 2017, 95, 628-634.) Summary of the Invention

[0006] To address the aforementioned technical problems, this invention provides a method for preparing polylactic acid using an organic-base thiourea dual-catalytic system. This method provides an effective catalyst capable of achieving controlled ring-opening polymerization of lactide to synthesize polylactic acid at high temperatures. A high-temperature resistant thiourea catalyst is added to the organic-base catalytic system to form a highly stable organic-base thiourea dual-catalytic system at high temperatures, thereby achieving controlled ring-opening polymerization of lactide to obtain polylactic acid with narrow distribution, high molecular weight, and controllable structure.

[0007] The above-mentioned objective of this invention is achieved through the following technical solution:

[0008] A method for preparing polylactic acid using an organic base and thiourea dual catalytic system, comprising the following steps in the presence of an initiator: under the catalysis of an organic base and thiourea dual catalytic system, lactide undergoes ring-opening polymerization in the presence of an initiator to obtain the polylactic acid;

[0009] The thiourea is either dithiourea or trithiourea;

[0010] The structural formula of the dithiourea is shown in Formula I or Formula II:

[0011]

[0012] The structural formula of the trithiourea is shown in Formula III, Formula IV or Formula V:

[0013]

[0014]

[0015] The method for preparing polylactic acid using an organic-base thiourea dual-catalytic system provided by this invention involves first deprotonating dithiourea or trithiourea to form thiourea anions. These thiourea anions act as a bifunctional catalyst; the deprotonated nitrogen anions activate the initiator, while the remaining NH4+ ions simultaneously activate the lactide monomer through hydrogen bonding, achieving synergistic catalysis of both the initiator and monomer. Subsequently, the activated initiator nucleophilically attacks the carbonyl carbon of the monomer, resulting in ring-opening of the lactide. This cycle continues, leading to chain growth. Furthermore, due to the presence of more intramolecular hydrogen bonds in polythiourea, it exhibits better thermal stability, enabling controlled ring-opening polymerization of lactide at high temperatures.

[0016] The organic-base-thiourea dual-catalytic system formed by combining an organic base and thiourea has a high decomposition temperature and therefore good high-temperature resistance. It can realize both the controlled ring-opening polymerization of lactide in solution and the controlled ring-opening polymerization of lactide in bulk at high temperatures, thus expanding the application range of the organic-base-thiourea catalytic system.

[0017] The organic-base thiourea dual-catalytic system of the present invention is a binary catalytic system. The catalyst used for polymerization is a hydrogen-bonded bifunctional catalyst, which can achieve a controllable polymerization effect that cannot be achieved by base catalysis alone, thereby resulting in a narrow molecular weight distribution of polylactic acid. By adjusting the ratio of lactide, initiator, organic base and thiourea, the catalytic activity can be further controlled to obtain high molecular weight polylactic acid with controllable structure.

[0018] Furthermore, the thiourea is prepared by the following method:

[0019] 3,5-bis(trifluoromethyl)phenyl isothiocyanate and 1,3-cyclohexanediamine were added to an organic solvent, and the mixture was synthesized in one pot and then recrystallized to obtain thiourea formula I (1,1'-(cyclohexane-1,3-diyl)bis(3-(3,5-bis(trifluoromethyl)phenyl)thiourea)).

[0020] Preferably, the organic solvent is tetrahydrofuran.

[0021] In a specific embodiment, tetrahydrofuran was used as a solvent, and 2 equivalents of 3,5-bis(trifluoromethyl)phenyl isothiocyanate and 1 equivalent of 1,3-cyclohexanediamine were added. The mixture was synthesized in a one-pot process and then recrystallized to obtain thiourea formula I.

[0022] 3,5-bis(trifluoromethyl)phenyl isothiocyanate and 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide hydrochloride were added to an organic solvent. After the reaction was completed, trifluoroacetic acid was added to obtain the first product. After alkalization with sodium bicarbonate, cyclohexyl isothiocyanate was added. After the reaction was completed, recrystallization gave thiourea formula II (1-(3,5-bis(trifluoromethyl)phenyl)-3-(3-(3-cyclohexylthioureido)propyl)thiourea).

[0023] Preferably, the organic solvent is dichloromethane.

[0024] In a specific embodiment, dichloromethane is used as a solvent, and 1 equivalent of 3,5-bis(trifluoromethyl)phenyl isothiocyanate and 1 equivalent of 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide hydrochloride are added. After the reaction is completed, trifluoroacetic acid is added to obtain the first step product. After alkalization with sodium bicarbonate, 1 equivalent of cyclohexyl isothiocyanate is added. After the reaction is completed, recrystallization is performed to obtain thiourea formula II.

[0025] Cyclohexyl isothiocyanate and triethylenetetramine were added to an organic solvent and synthesized in a one-pot method. Recrystallization yielded thiourea formula III (1,1',1”-(nitrilotris(ethane-2,1-diyl))tris(3-cyclohexylthiourea)).

[0026] Preferably, the organic solvent is dichloromethane.

[0027] In a specific embodiment, dichloromethane was used as a solvent, and 3 equivalents of cyclohexyl isothiocyanate and 1 equivalent of triethylenetetramine were added. The mixture was synthesized in a one-pot process and then recrystallized to obtain thiourea type III.

[0028] Isopropyl isothiocyanate and triethylenetetramine were added to an organic solvent and synthesized in a one-pot method. Recrystallization yielded thiourea formula IV (1,1',1”-(nitrilotris(ethane-2,1-diyl))tris(3-isopropylthiourea)).

[0029] Preferably, the organic solvent is dichloromethane.

[0030] In a specific embodiment, dichloromethane was used as a solvent, and 3 equivalents of isopropyl isothiocyanate and 1 equivalent of triethylenetetramine were added. The mixture was synthesized in a one-pot process and then recrystallized to obtain thiourea type IV.

[0031] phenyl isothiocyanate and triethylenetetramine were added to an organic solvent and synthesized in a one-pot process. Recrystallization yielded thiourea V(1,1',1”-(nitrilotris(ethane-2,1-diyl))tris(3-phenylthiourea)).

[0032] Preferably, the organic solvent is dichloromethane.

[0033] In a specific embodiment, dichloromethane was used as a solvent, and 3 equivalents of phenyl isothiocyanate and 1 equivalent of triethylenetetramine were added. The mixture was synthesized in a one-pot process and then recrystallized to obtain thiourea formula V.

[0034] Furthermore, the reaction conditions for the one-pot synthesis method are: reaction at room temperature for 3-4 hours under an inert gas environment.

[0035] The thiourea catalyst provided by this invention is easy to synthesize, and the catalyst salt obtained by modular combination with organic base has a high decomposition temperature, can withstand high temperature, and can realize the controllable ring-opening polymerization of lactide at high temperature.

[0036] The catalyst salt is prepared by mixing an organic base with thiourea in a good solvent, reacting for 10-15 minutes, and then removing the good solvent to obtain the catalyst salt.

[0037] Specifically, the good solvent is dichloromethane, trichloromethane, or tetrahydrofuran.

[0038] Further, the organic base is 1,5,7-triazabicyclo[4.4.0]dec-5-ene (TBD), 4-dimethylaminopyridine (DMAP), 1,8-diazabispirocyclo[5.4.0]undec-7-ene (DBU) or sparteine ​​((-)-sparteine).

[0039] Furthermore, the lactide is L-lactide or racemic lactide (DL-lactide), preferably L-lactide.

[0040] Furthermore, the initiator is benzyl alcohol or phenylethanol.

[0041] Furthermore, the molar ratio of lactide, initiator, organic base and thiourea is 50:1:1:(1-5).

[0042] This invention proposes a favorable ratio of organic base to thiourea, which can better regulate the polymerization process of lactide.

[0043] Preferably, the molar ratio of lactide, initiator, organic base and thiourea is 50:1:1:2.

[0044] Furthermore, the specific steps of the bulk polymerization method are as follows: lactide, organic base and thiourea are heated to melt in an inert gas or vacuum environment, and an initiator is added to carry out the polymerization reaction to obtain polylactic acid.

[0045] Furthermore, the inert gas used in the bulk polymerization method is preferably nitrogen or argon.

[0046] Furthermore, the polymerization reaction includes a step of dissolving the reaction product in a good solvent and then purifying it in the precipitating solvent.

[0047] Furthermore, the good solvent used in the bulk polymerization method is dichloromethane, trichloromethane, or tetrahydrofuran.

[0048] Furthermore, the precipitation solvent in the bulk polymerization method is preferably methanol.

[0049] Furthermore, the ring-opening polymerization method is solution polymerization, and the reaction temperature is 25–60°C.

[0050] Furthermore, the specific steps of the solution polymerization method are as follows: dissolving lactide, initiator, organic base and thiourea in an organic solvent, and carrying out the polymerization reaction in an inert gas environment to obtain polylactic acid.

[0051] Furthermore, the initial reaction concentration of lactide in solution polymerization can be 1–10 mol / L.

[0052] Furthermore, the organic solvent used in the solution polymerization method is tetrahydrofuran, toluene, dichloromethane, dichloroethane, or xylene, preferably tetrahydrofuran, toluene, or dichloromethane, and more preferably toluene.

[0053] Furthermore, the inert gas used in solution polymerization is preferably nitrogen or argon.

[0054] Furthermore, the polymerization reaction is followed by a step of purifying the reaction product in a precipitation solvent.

[0055] Furthermore, the precipitating solvent in the solution polymerization method is preferably methanol.

[0056] Furthermore, the ring-opening polymerization method is bulk polymerization, and the reaction temperature is 60–180°C.

[0057] Although the solution polymerization and bulk polymerization methods provided by this invention do not prepare catalyst salts separately, the reaction system consists of a modular combination of thiourea and organic base to form an organic base-thiourea dual catalytic system that catalyzes the reaction together, rather than thiourea and organic base acting alone.

[0058] Furthermore, the preparation method also includes a step of terminating the reaction with a weak acid.

[0059] Furthermore, the weak acid is preferably benzoic acid.

[0060] The beneficial effects of this invention are:

[0061] (1) The thiourea catalyst of the present invention is easy to synthesize. The organic base thiourea dual catalytic system formed by modular combination of organic base and thiourea has a high decomposition temperature and therefore has good high temperature resistance. It can realize the controllable ring-opening polymerization of lactide in solution and the bulk controllable ring-opening polymerization of lactide at high temperature, thus expanding the application range of the organic base thiourea catalytic system.

[0062] (2) The organic base thiourea dual catalytic system of the present invention is a binary catalytic system. The catalyst used for polymerization is a hydrogen-bonded bifunctional catalyst, which can achieve a controllable polymerization effect that cannot be achieved by catalysis alone, thereby making the molecular weight distribution of polylactic acid narrower. By adjusting the ratio of lactide, initiator, organic base and thiourea, the catalytic activity can be further controlled to obtain polylactic acid with high molecular weight and controllable structure. Attached Figure Description

[0063] Figure 1 This is a schematic diagram illustrating the principle of preparing polylactic acid using the organic base thiourea dual-catalytic system of this invention.

[0064] Figure 2 This refers to the polylactic acid prepared by the ring-opening polymerization of lactide using an organic-base thiourea dual-catalyst system formed by the combination of TBD and thiourea I in Example 1 of this invention. 1 H NMR spectrum.

[0065] Figure 3 This is the GPC spectrum of polylactic acid prepared by the ring-opening polymerization of lactide using an organic base thiourea dual-catalyst system formed by combining TBD and thiourea I in Example 1 of the present invention.

[0066] Figure 4 The thiourea I in Example 1 of this invention 1 H NMR spectrum.

[0067] Figure 5 This refers to the polylactic acid prepared by the ring-opening polymerization of lactide using an organic-base thiourea dual-catalyst system formed by the combination of TBD and thiourea II in Example 2 of this invention. 1 H NMR spectrum.

[0068] Figure 6 For example, thiourea II in Example 2 of this invention 1 H NMR spectrum.

[0069] Figure 7 This refers to the polylactic acid prepared by the ring-opening polymerization of lactide using an organic-base thiourea dual-catalyst system formed by the combination of TBD and thiourea III in Example 3 of this invention. 1 H NMR spectrum.

[0070] Figure 8 The thiourea III in Example 3 of this invention 1 H NMR spectrum.

[0071] Figure 9 This refers to the polylactic acid prepared by the ring-opening polymerization of lactide using an organic-base thiourea dual-catalyst system formed by the combination of TBD and thiourea IV in Example 4 of this invention. 1 H NMR spectrum.

[0072] Figure 10 For example, thiourea IV in Example 4 of this invention 1 H NMR spectrum.

[0073] Figure 11 This refers to the polylactic acid prepared by the ring-opening polymerization of lactide using an organic-base thiourea dual-catalyst system formed by the combination of TBD and thiourea V in Example 5 of this invention. 1 H NMR spectrum.

[0074] Figure 12 For example, thiourea V in Example 5 of this invention 1 H NMR spectrum. Detailed Implementation

[0075] The present invention will be further described below with reference to the accompanying drawings and specific embodiments, so that those skilled in the art can better understand and implement the present invention. However, the embodiments described are not intended to limit the present invention.

[0076] This invention provides a method for preparing polylactic acid using an organic base and thiourea dual-catalytic system, comprising the following steps: under the catalysis of an organic base and thiourea dual-catalytic system, lactide undergoes ring-opening polymerization in the presence of an initiator to obtain the polylactic acid; wherein the thiourea is a dithiourea or a trithiourea.

[0077] The structural formula of the dithiourea is shown in Formula I or Formula II:

[0078]

[0079] The structural formula of the trithiourea is shown in Formula III, Formula IV or Formula V:

[0080]

[0081]

[0082] The principle of this invention is as follows Figure 1As shown, in dithiourea or trithiourea, TBD first deprotonates the dithiourea or trithiourea to form a thiourea anion (A). A acts as a bifunctional catalyst; its deprotonated nitrogen anion activates the initiator BnOH, while the remaining NH groups simultaneously activate the lactide monomer through hydrogen bonding, achieving synergistic catalysis of the initiator and monomer. Subsequently, the activated initiator nucleophilically attacks the carbonyl carbon of the monomer, achieving ring-opening of the lactide, and this cycle continues, resulting in chain growth. Furthermore, because polythiourea exhibits more intramolecular hydrogen bonding, it possesses better thermal stability, enabling controlled ring-opening polymerization of lactide at high temperatures.

[0083] The reaction conversion rate in the examples is determined by 1 The molecular weight and dispersity of polylactic acid (PLA) were determined by ¹H NMR using a Bruker DRX300 NMR spectrometer and d-CHCl₃ and d-DMSO as solvents. The theoretical molecular weight of PLA was calculated based on conversion efficiency, and the actual molecular weight was determined by... 1 The results were obtained by integrating the H NMR spectrum.

[0084] The reaction temperature for bulk polymerization in Examples 1-20 was 180°C, and the reaction temperature for solution polymerization in Examples 21-38 was 90°C.

[0085] In the example, the reaction conditions for the one-pot synthesis of thiourea were as follows: reaction at room temperature for 3 hours under nitrogen atmosphere.

[0086] Example 1

[0087] L-lactide (0.432 g, 3 mmol), TBD (0.0084 g, 0.06 mmol), and thiourea I (0.078 g, 0.12 mmol) were added to a reaction flask and heated to melt under Ar protection. Then, benzyl alcohol (6.23 μL, 0.06 mmol) was added, and the reaction was allowed to proceed for 1 hour ([M]0:[I]0:[B]:[TU] = 50:1:1:2). The reaction was terminated by adding a DCM solution of benzoic acid, which dissolved the reactants. The solution was then poured into ice-cold methanol, the precipitate was filtered, and dried to constant weight.

[0088] The thiourea I was prepared by means of the following method: using tetrahydrofuran as solvent, 2 equivalents of 3,5-bis(trifluoromethyl)phenyl isothiocyanate and 1 equivalent of 1,3-cyclohexanediamine were added, and the thiourea I was obtained by recrystallization after one-pot synthesis.

[0089] The polymer prepared is used 1 H NMR and GPC characterization. 1 H NMR spectrum and GPC spectrum as follows Figure 2 , 3 As shown, the thiourea I used1 H NMR spectrum as shown Figure 4 As shown, Figure 2 , 3 This indicates that Example 1 uses a combination of TBD and thiourea I to form an organic-base thiourea dual-catalytic system to catalyze the controlled ring-opening polymerization of lactide, resulting in polylactic acid with a narrow distribution, high molecular weight, and controllable structure. The number-average molecular weight (Mn) of the polylactic acid is 6800 g / mol, and the dispersion (D) is 1.22.

[0090] Example 2

[0091] L-lactide (0.432 g, 3 mmol), TBD (0.0084 g, 0.06 mmol), and thiourea II (0.058 g, 0.12 mmol) were added to a reaction flask and heated to melt under Ar protection. Then, benzyl alcohol (6.23 μL, 0.06 mmol) was added, and the reaction was allowed to proceed for 1 hour ([M]0:[I]0:[B]:[TU] = 50:1:1:2). The reaction was terminated by adding a DCM solution of benzoic acid, which dissolved the reactants. The solution was then poured into ice-cold methanol, the precipitate was filtered, and dried to constant weight.

[0092] The thiourea II was prepared by the following method: using dichloromethane as a solvent, 1 equivalent of 3,5-bis(trifluoromethyl)phenyl isothiocyanate and 1 equivalent of 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide hydrochloride were added. After the reaction was completed, trifluoroacetic acid was added to obtain the first step product. After alkalization with sodium bicarbonate, 1 equivalent of cyclohexyl isothiocyanate was added. After the reaction was completed, recrystallization was performed to obtain thiourea II.

[0093] The polymer prepared is used 1 H NMR and GPC characterization. 1 H NMR spectrum as shown Figure 5 As shown, the thiourea II used 1 HNMR spectrum as follows Figure 6 As shown, Figure 5 Characterization results showed that Example 2 prepared polylactic acid with narrow distribution, high molecular weight, and controllable structure by catalyzing the controlled ring-opening polymerization of lactide using a combination of TBD and thiourea II to form an organic base thiourea dual-catalytic system. The number average molecular weight Mn of the polylactic acid was 6900 g / mol, and the dispersity D was 1.22.

[0094] Example 3

[0095] L-lactide (0.432 g, 3 mmol), TBD (0.0084 g, 0.06 mmol), and thiourea III (0.103 g, 0.18 mmol) were added to a reaction flask and heated to melt under Ar protection. Then, benzyl alcohol (6.23 μL, 0.06 mmol) was added, and the reaction was allowed to proceed for 1 hour ([M]0:[I]0:[B]:[TU] = 50:1:1:2). The reaction was terminated by adding a DCM solution of benzoic acid, which dissolved the reactants. The solution was then poured into ice-cold methanol, the precipitate was filtered, and dried to constant weight.

[0096] The thiourea III was prepared by means of dichloromethane as solvent, adding 3 equivalents of cyclohexyl isothiocyanate and 1 equivalent of triethylenetetramine, and then recrystallizing to obtain thiourea III.

[0097] The polymer prepared is used 1 H NMR and GPC characterization. 1 H NMR spectrum as shown Figure 7 As shown, the thiourea III used 1 HNMR spectrum as follows Figure 8 As shown, Figure 7 Characterization results showed that Example 3 prepared polylactic acid with narrow distribution, high molecular weight, and controllable structure by catalyzing the controlled ring-opening polymerization of lactide using a combination of TBD and thiourea III to form an organic base thiourea dual-catalytic system. The number average molecular weight Mn of the polylactic acid was 7100 g / mol, and the dispersity D was 1.21.

[0098] Example 4

[0099] L-lactide (0.432 g, 3 mmol), TBD (0.0084 g, 0.06 mmol), and thiourea IV (0.068 g, 0.12 mmol) were added to a reaction flask and heated to melt under Ar protection. Then, benzyl alcohol (6.23 μL, 0.06 mmol) was added, and the reaction was allowed to proceed for 1 hour ([M]0:[I]0:[B]:[TU] = 50:1:1:2). The reaction was terminated by adding a DCM solution of benzoic acid, which dissolved the reactants. The solution was then poured into ice-cold methanol, the precipitate was filtered, and dried to constant weight.

[0100] The thiourea IV was prepared by means of dichloromethane as solvent, adding 3 equivalents of isopropyl isothiocyanate and 1 equivalent of triethylenetetramine, and then recrystallizing to obtain thiourea IV.

[0101] The polymer prepared is used 1 H NMR and GPC characterization. 1 H NMR spectrum as shown Figure 9 As shown, the thiourea IV used 1 HNMR spectrum as follows Figure 10 As shown, Figure 9 Characterization results showed that Example 4 used a combination of TBD and thiourea IV to form an organic base thiourea dual-catalytic system to catalyze the controlled ring-opening polymerization of lactide, which yielded polylactic acid with narrow distribution, high molecular weight, and controllable structure. The number-average molecular weight Mn of the polylactic acid was 7100 g / mol, and the dispersity D was 1.21.

[0102] Example 5

[0103] L-lactide (0.432 g, 3 mmol), TBD (0.0084 g, 0.06 mmol), and thiourea V (0.063 g, 0.12 mmol) were added to a reaction flask and heated to melt under Ar protection. Then, benzyl alcohol (6.23 μL, 0.06 mmol) was added, and the reaction was allowed to proceed for 1 hour ([M]0:[I]0:[B]:[TU] = 50:1:1:2). The reaction was terminated by adding a DCM solution of benzoic acid, and the reactants were dissolved. The solution was then poured into ice-cold methanol, the precipitate was filtered, and dried to constant weight.

[0104] The thiourea V was prepared by means of dichloromethane as solvent, adding 3 equivalents of phenyl isothiocyanate and 1 equivalent of triethylenetetramine, and then recrystallizing to obtain thiourea V.

[0105] The polymer prepared is used 1 H NMR and GPC characterization. 1 H NMR spectrum as shown Figure 11 As shown, the thiourea V used 1 H NMR spectrum as shown Figure 12 As shown, Figure 11 Characterization results showed that Example 5 used a combination of TBD and thiourea V to form an organic-base thiourea dual-catalytic system to catalyze the controlled ring-opening polymerization of lactide, which yielded polylactic acid with narrow distribution, high molecular weight, and controllable structure. The number-average molecular weight Mn of the polylactic acid was 7100 g / mol, and the dispersity D was 1.21.

[0106] Example 6

[0107] DL-lactide (0.432 g, 3 mmol), DBU (0.0091 g, 0.06 mmol), and thiourea I (0.078 g, 0.12 mmol) were added to a reaction flask and heated to melt under Ar protection. Then, benzyl alcohol (6.23 μL, 0.06 mmol) was added, and the reaction was allowed to proceed for 1 hour ([M]0:[I]0:[B]:[TU] = 50:1:1:2). The reaction was terminated by adding a DCM solution of benzoic acid, and the reactants were dissolved. The solution was then poured into ice-cold methanol, the precipitate was filtered, and dried to constant weight. The resulting polymer was then used... 1H NMR and GPC characterization showed that, in Example 6, a dual-catalytic system of organic base and thiourea I was formed to catalyze the controlled ring-opening polymerization of lactide, resulting in polylactic acid with narrow distribution, high molecular weight, and controllable structure.

[0108] Example 7

[0109] DL-lactide (0.432 g, 3 mmol), DBU (0.0091 g, 0.06 mmol), and thiourea II (0.058 g, 0.12 mmol) were added to a reaction flask and heated to melt under Ar protection. Then, benzyl alcohol (6.23 μL, 0.06 mmol) was added, and the reaction was allowed to proceed for 1 hour ([M]0:[I]0:[B]:[TU] = 50:1:1:2). The reaction was terminated by adding a DCM solution of benzoic acid, and the reactants were dissolved. The solution was then poured into ice-cold methanol, the precipitate was filtered, and dried to constant weight. The resulting polymer was then used... 1 H NMR and GPC characterization showed that the controlled ring-opening polymerization of lactide in Example 7, using a combination of DBU and thiourea II to form an organic base thiourea dual-catalyst system, yielded polylactic acid with narrow distribution, high molecular weight, and controllable structure.

[0110] Example 8

[0111] L-lactide (0.432 g, 3 mmol), DBU (0.0091 g, 0.06 mmol), and thiourea III (0.103 g, 0.18 mmol) were added to a reaction flask and heated to melt under Ar protection. Then, benzyl alcohol (6.23 μL, 0.06 mmol) was added, and the reaction was allowed to proceed for 1 hour ([M]0:[I]0:[B]:[TU] = 50:1:1:2). The reaction was terminated by adding a DCM solution of benzoic acid, and the reactants were dissolved. The solution was then poured into ice-cold methanol, the precipitate was filtered, and dried to constant weight. The resulting polymer was then used... 1 H NMR and GPC characterization showed that in Example 8, polylactic acid with narrow distribution, high molecular weight, and controllable structure was prepared by catalyzing the controlled ring-opening polymerization of lactide using an organic base thiourea dual-catalytic system formed by the combination of DBU and thiourea III.

[0112] Example 9

[0113] L-lactide (0.432 g, 3 mmol), DBU (0.0091 g, 0.06 mmol), and thiourea IV (0.068 g, 0.12 mmol) were added to a reaction flask and heated to melt under Ar protection. Then, benzyl alcohol (6.23 μL, 0.06 mmol) was added, and the reaction was allowed to proceed for 1 hour ([M]0:[I]0:[B]:[TU] = 50:1:1:2). The reaction was terminated by adding a DCM solution of benzoic acid, and the reactants were dissolved. The solution was then poured into ice-cold methanol, the precipitate was filtered, and dried to constant weight. The resulting polymer was then used... 1 H NMR and GPC characterization showed that, in Example 9, a dual-catalytic system of organic base and thiourea (DBU and thiourea IV) was used to catalyze the controlled ring-opening polymerization of lactide to prepare polylactic acid with narrow distribution, high molecular weight, and controllable structure.

[0114] Example 10

[0115] L-lactide (0.432 g, 3 mmol), DBU (0.0091 g, 0.06 mmol), and thiourea V (0.063 g, 0.12 mmol) were added to a reaction flask and heated to melt under Ar protection. Then, benzyl alcohol (6.23 μL, 0.06 mmol) was added, and the reaction was allowed to proceed for 1 hour ([M]0:[I]0:[B]:[TU] = 50:1:1:2). The reaction was terminated by adding a DCM solution of benzoic acid, and the reactants were dissolved. The solution was then poured into ice-cold methanol, the precipitate was filtered, and dried to constant weight. The resulting polymer was then used... 1 H NMR and GPC characterization showed that the controlled ring-opening polymerization of lactide in Example 10, using a combination of DBU and thiourea V to form an organic base thiourea dual-catalyst system, yielded polylactic acid with narrow distribution, high molecular weight, and controllable structure.

[0116] Example 11

[0117] L-lactide (0.432 g, 3 mmol), DMAP (0.0073 g, 0.06 mmol), and thiourea I (0.078 g, 0.12 mmol) were added to a reaction flask and heated to melt under Ar protection. Then, benzyl alcohol (6.23 μL, 0.06 mmol) was added, and the reaction was allowed to proceed for 1 hour ([M]0:[I]0:[B]:[TU] = 50:1:1:2). The reaction was terminated by adding a DCM solution of benzoic acid, and the reactants were dissolved. The solution was then poured into ice-cold methanol, the precipitate was filtered, and dried to constant weight. The resulting polymer was then used... 1 H NMR and GPC characterization showed that the controlled ring-opening polymerization of lactide in Example 11, using an organic base thiourea dual-catalyst system composed of DMAP and thiourea I, was used to prepare polylactic acid with narrow distribution, high molecular weight, and controllable structure.

[0118] Example 12

[0119] L-lactide (0.432 g, 3 mmol), DMAP (0.0073 g, 0.06 mmol), and thiourea II (0.058 g, 0.12 mmol) were added to a reaction flask and heated to melt under Ar protection. Then, benzyl alcohol (6.23 μL, 0.06 mmol) was added, and the reaction was allowed to proceed for 1 hour ([M]0:[I]0:[B]:[TU] = 50:1:1:2). The reaction was terminated by adding a DCM solution of benzoic acid, and the reactants were dissolved. The solution was then poured into ice-cold methanol, the precipitate was filtered, and dried to constant weight. The resulting polymer was then used... 1 H NMR and GPC characterization showed that the controlled ring-opening polymerization of lactide in Example 12, which was prepared by the combination of DMAP and thiourea II to form an organic base thiourea dual catalytic system, yielded polylactic acid with narrow distribution, high molecular weight and controllable structure.

[0120] Example 13

[0121] DL-lactide (0.432 g, 3 mmol), DMAP (0.0073 g, 0.06 mmol), and thiourea III (0.103 g, 0.18 mmol) were added to a reaction flask and heated to melt under Ar protection. Then, benzyl alcohol (6.23 μL, 0.06 mmol) was added, and the reaction was allowed to proceed for 1 hour ([M]0:[I]0:[B]:[TU] = 50:1:1:2). The reaction was terminated by adding a DCM solution of benzoic acid, and the reactants were dissolved. The solution was then poured into ice-cold methanol, the precipitate was filtered, and dried to constant weight. The resulting polymer was then used... 1 H NMR and GPC characterization showed that the controlled ring-opening polymerization of lactide in Example 13, which was prepared by the combination of DMAP and thiourea III to form an organic base thiourea dual catalytic system, yielded polylactic acid with narrow distribution, high molecular weight and controllable structure.

[0122] Example 14

[0123] DL-lactide (0.432 g, 3 mmol), DMAP (0.0073 g, 0.06 mmol), and thiourea IV (0.068 g, 0.12 mmol) were added to a reaction flask and heated to melt under Ar protection. Then, benzyl alcohol (6.23 μL, 0.06 mmol) was added, and the reaction was allowed to proceed for 1 hour ([M]0:[I]0:[B]:[TU] = 50:1:1:2). The reaction was terminated by adding a DCM solution of benzoic acid, and the reactants were dissolved. The solution was then poured into ice-cold methanol, the precipitate was filtered, and dried to constant weight. The resulting polymer was then used... 1H NMR and GPC characterization showed that the controlled ring-opening polymerization of lactide in Example 14, which was prepared by the combination of DMAP and thiourea IV to form an organic base thiourea dual catalytic system, yielded polylactic acid with narrow distribution, high molecular weight and controllable structure.

[0124] Example 15

[0125] L-lactide (0.432 g, 3 mmol), DMAP (0.0073 g, 0.06 mmol), and thiourea V (0.063 g, 0.12 mmol) were added to a reaction flask and heated to melt under Ar protection. Then, benzyl alcohol (6.23 μL, 0.06 mmol) was added, and the reaction was allowed to proceed for 1 hour ([M]0:[I]0:[B]:[TU] = 50:1:1:2). The reaction was terminated by adding a DCM solution of benzoic acid, and the reactants were dissolved. The solution was then poured into ice-cold methanol, the precipitate was filtered, and dried to constant weight. The resulting polymer was then used... 1 H NMR and GPC characterization showed that the controlled ring-opening polymerization of lactide in Example 15, using an organic base thiourea dual-catalyst system formed by the combination of DMAP and thiourea V, was used to prepare polylactic acid with narrow distribution, high molecular weight, and controllable structure.

[0126] Example 16

[0127] L-lactide (0.432 g, 3 mmol), (-)-sparteine ​​(0.014 g, 0.06 mmol), and thiourea I (0.078 g, 0.12 mmol) were added to a reaction flask and heated to melt under Ar protection. Then, benzyl alcohol (6.23 μL, 0.06 mmol) was added, and the reaction was allowed to proceed for 1 hour ([M]0:[I]0:[B]:[TU] = 50:1:1:2). The reaction was terminated by adding a DCM solution of benzoic acid, and the reactants were dissolved. The solution was then poured into ice-cold methanol, the precipitate was filtered, and dried to constant weight. The resulting polymer was then used... 1 H NMR and GPC characterization showed that the controlled ring-opening polymerization of lactide in Example 16, which was prepared by the combination of (-)-sparteine ​​and thiourea I to form an organic base thiourea dual catalytic system, yielded polylactic acid with narrow distribution, high molecular weight and controllable structure.

[0128] Example 17

[0129] L-lactide (0.432 g, 3 mmol), (-)-sparteine ​​(0.014 g, 0.06 mmol), and thiourea II (0.058 g, 0.12 mmol) were added to a reaction flask and heated to melt under Ar protection. Then, benzyl alcohol (6.23 μL, 0.06 mmol) was added, and the reaction was allowed to proceed for 1 hour ([M]0:[I]0:[B]:[TU] = 50:1:1:2). The reaction was terminated by adding a DCM solution of benzoic acid, and the reactants were dissolved. The solution was then poured into ice-cold methanol, the precipitate was filtered, and dried to constant weight. The resulting polymer was then used... 1 H NMR and GPC characterization showed that the controlled ring-opening polymerization of lactide in Example 17, which was prepared by the combination of (-)-sparteine ​​and thiourea II to form an organic base thiourea dual catalytic system, yielded polylactic acid with narrow distribution, high molecular weight and controllable structure.

[0130] Example 18

[0131] L-lactide (0.432 g, 3 mmol), (-)-sparteine ​​(0.014 g, 0.06 mmol), and thiourea III (0.103 g, 0.18 mmol) were added to a reaction flask and heated to melt under Ar protection. Then, benzyl alcohol (6.23 μL, 0.06 mmol) was added, and the reaction was allowed to proceed for 1 hour ([M]0:[I]0:[B]:[TU] = 50:1:1:2). The reaction was terminated by adding a DCM solution of benzoic acid, and the reactants were dissolved. The solution was then poured into ice-cold methanol, the precipitate was filtered, and dried to constant weight. The resulting polymer was then used... 1 H NMR and GPC characterization showed that Example 18 used an organic-base thiourea dual-catalyst system composed of (-)-sparteine ​​and thiourea III to catalyze the controlled ring-opening polymerization of lactide, resulting in polylactic acid with narrow distribution, high molecular weight, and controllable structure.

[0132] Example 19

[0133] L-lactide (0.432 g, 3 mmol), (-)-sparteine ​​(0.014 g, 0.06 mmol), and thiourea IV (0.068 g, 0.12 mmol) were added to a reaction flask and heated to melt under Ar protection. Then, benzyl alcohol (6.23 μL, 0.06 mmol) was added, and the reaction was allowed to proceed for 1 hour ([M]0:[I]0:[B]:[TU] = 50:1:1:2). The reaction was terminated by adding a DCM solution of benzoic acid, and the reactants were dissolved. The solution was then poured into ice-cold methanol, the precipitate was filtered, and dried to constant weight. The resulting polymer was then used... 1H NMR and GPC characterization showed that Example 19 used an organic-base thiourea dual-catalyst system composed of (-)-sparteine ​​and thiourea IV to catalyze the controlled ring-opening polymerization of lactide, resulting in polylactic acid with narrow distribution, high molecular weight, and controllable structure.

[0134] Example 20

[0135] DL-lactide (0.432 g, 3 mmol), (-)-sparteine ​​(0.014 g, 0.06 mmol), and thiourea V (0.063 g, 0.12 mmol) were added to a reaction flask and heated to melt under Ar protection. Then, benzyl alcohol (6.23 μL, 0.06 mmol) was added, and the reaction was allowed to proceed for 1 hour ([M]0:[I]0:[B]:[TU] = 50:1:1:2). The reaction was terminated by adding a DCM solution of benzoic acid, and the reactants were dissolved. The solution was then poured into ice-cold methanol, the precipitate was filtered, and dried to constant weight. The resulting polymer was then used... 1 H NMR and GPC characterization showed that the controllable ring-opening polymerization of lactide in Example 20, using an organic-base thiourea dual-catalyst system composed of (-)-sparteine ​​and thiourea V, was used to prepare polylactic acid with narrow distribution, high molecular weight, and controllable structure.

[0136] Example 21

[0137] L-lactide (0.432 g, 3 mmol), benzyl alcohol (6.23 μL, 0.06 mmol), TBD (0.0084 g, 0.06 mmol), and thiourea I (0.039 g, 0.06 mmol) were dissolved in 3 mL of DCM and reacted under nitrogen protection for 1 hour ([M]0:[I]0:[B]:[TU]=50:1:1:1). The reaction was terminated by adding formic acid. The reaction solution was poured into ice-cold methanol, the precipitate was filtered, and dried to constant weight, with a conversion rate of 98%. The obtained polymer was used... 1 Characterization by 1H NMR and GPC showed that the number-average molecular weight (Mn) of the polymer was 6900 g / mol and the dispersion (D) was 1.59. The characterization results indicate that in Example 21, a controlled ring-opening polymerization of lactide was prepared by using an organic-base thiourea dual-catalyst system formed by the combination of TBD and thiourea I to catalyze the polymerization of lactide. The resulting polymer had a narrow distribution, high molecular weight, and controllable structure.

[0138] Example 22

[0139] L-lactide (0.432 g, 3 mmol), benzyl alcohol (6.23 μL, 0.06 mmol), TBD (0.0084 g, 0.06 mmol), and thiourea I (0.078 g, 0.12 mmol) were dissolved in 3 mL of DCM and reacted under nitrogen protection for 1 hour ([M]0:[I]0:[B]:[TU]=50:1:1:2). The reaction was terminated by adding formic acid. The reaction solution was poured into ice-cold methanol, the precipitate was filtered, and dried to constant weight, with a conversion rate of 98%. The obtained polymer was used... 1 Characterization by 1H NMR and GPC showed that the number-average molecular weight (Mn) of the polymer was 6900 g / mol and the dispersion (D) was 1.23. The characterization results indicate that Example 22 prepared polylactic acid with narrow distribution, high molecular weight, and controllable structure by using an organic base thiourea dual-catalyst system formed by combining TBD and thiourea I to catalyze the controlled ring-opening polymerization of lactide.

[0140] Example 23

[0141] DL-lactide (0.432 g, 3 mmol), benzyl alcohol (6.23 μL, 0.06 mmol), TBD (0.0084 g, 0.06 mmol), and thiourea I (0.117 g, 0.18 mmol) were dissolved in 3 mL of DCM and reacted under nitrogen protection for 1 hour ([M]0:[I]0:[B]:[TU]=50:1:1:3). The reaction was terminated by adding formic acid. The reaction solution was poured into ice-cold methanol, the precipitate was filtered, and dried to constant weight, with a conversion rate of 98%. The obtained polymer was used... 1 Characterization by 1H NMR and GPC showed that the number-average molecular weight (Mn) of the polymer was 6900 g / mol and the dispersion (D) was 1.46. The characterization results indicate that Example 23 prepared polylactic acid with narrow distribution, high molecular weight, and controllable structure by using an organic base thiourea dual-catalyst system formed by combining TBD and thiourea I to catalyze the controlled ring-opening polymerization of lactide.

[0142] Example 24

[0143] DL-lactide (0.432 g, 3 mmol), benzyl alcohol (6.23 μL, 0.06 mmol), TBD (0.0084 g, 0.06 mmol), and thiourea II (0.029 g, 0.06 mmol) were dissolved in 3 mL of DCM and reacted under nitrogen protection for 1 hour ([M]0:[I]0:[B]:[TU] = 50:1:1:1). The reaction was terminated by adding formic acid, and the reaction solution was poured into ice-cold methanol. The precipitate was filtered and dried to constant weight. The polymer was then used... 1 H NMR and GPC characterization showed that the controlled ring-opening polymerization of lactide in Example 24, which was prepared by the combination of TBD and thiourea II to form an organic base thiourea dual catalytic system, yielded polylactic acid with narrow distribution, high molecular weight and controllable structure.

[0144] Example 25

[0145] L-lactide (0.432 g, 3 mmol), benzyl alcohol (6.23 μL, 0.06 mmol), TBD (0.0084 g, 0.06 mmol), and thiourea II (0.058 g, 0.12 mmol) were dissolved in 3 mL of DCM and reacted under nitrogen protection for 1 hour ([M]0:[I]0:[B]:[TU] = 50:1:1:2). The reaction was terminated by adding formic acid, and the reaction solution was poured into ice-cold methanol. The precipitate was filtered and dried to constant weight. The polymer was then used... 1 H NMR and GPC characterization showed that the controlled ring-opening polymerization of lactide in Example 25, which was prepared by the combination of TBD and thiourea II to form an organic base thiourea dual catalytic system, yielded polylactic acid with narrow distribution, high molecular weight and controllable structure.

[0146] Example 26

[0147] L-lactide (0.432 g, 3 mmol), benzyl alcohol (6.23 μL, 0.06 mmol), TBD (0.0084 g, 0.06 mmol), and thiourea II (0.088 g, 0.18 mmol) were dissolved in 3 mL of DCM and reacted under nitrogen protection for 1 hour ([M]0:[I]0:[B]:[TU] = 50:1:1:3). The reaction was terminated by adding formic acid, and the reaction solution was poured into ice-cold methanol. The precipitate was filtered and dried to constant weight. The polymer was then used... 1 H NMR and GPC characterization showed that the controlled ring-opening polymerization of lactide in Example 26, which was prepared by the combination of TBD and thiourea II to form an organic base thiourea dual catalytic system, yielded polylactic acid with narrow distribution, high molecular weight and controllable structure.

[0148] Example 27

[0149] L-lactide (0.432 g, 3 mmol), benzyl alcohol (6.23 μL, 0.06 mmol), TBD (0.0084 g, 0.06 mmol), and thiourea III (0.034 g, 0.06 mmol) were dissolved in 3 mL of DCM and reacted under nitrogen protection for 1 hour ([M]0:[I]0:[B]:[TU] = 50:1:1:1). The reaction was terminated by adding formic acid, and the reaction solution was poured into ice-cold methanol. The precipitate was filtered and dried to constant weight. The polymer was then used... 1 H NMR and GPC characterization showed that the controlled ring-opening polymerization of lactide in Example 27, which was prepared by the combination of TBD and thiourea III to form an organic base thiourea dual catalytic system, yielded polylactic acid with narrow distribution, high molecular weight and controllable structure.

[0150] Example 28

[0151] DL-lactide (0.432 g, 3 mmol), benzyl alcohol (6.23 μL, 0.06 mmol), TBD (0.0084 g, 0.06 mmol), and thiourea III (0.068 g, 0.12 mmol) were dissolved in 3 mL of DCM and reacted under nitrogen protection for 1 hour ([M]0:[I]0:[B]:[TU] = 50:1:1:2). The reaction was terminated by adding formic acid, and the reaction solution was poured into ice-cold methanol. The precipitate was filtered and dried to constant weight. The polymer was then used... 1 H NMR and GPC characterization showed that the controlled ring-opening polymerization of lactide in Example 28, which was prepared by the combination of TBD and thiourea III to form an organic base thiourea dual catalytic system, yielded polylactic acid with narrow distribution, high molecular weight and controllable structure.

[0152] Example 29

[0153] L-lactide (0.432 g, 3 mmol), benzyl alcohol (6.23 μL, 0.06 mmol), TBD (0.0084 g, 0.06 mmol), and thiourea III (0.103 g, 0.18 mmol) were dissolved in 3 mL of DCM and reacted under nitrogen protection for 1 hour ([M]0:[I]0:[B]:[TU] = 50:1:1:3). The reaction was terminated by adding formic acid, and the reaction solution was poured into ice-cold methanol. The precipitate was filtered and dried to constant weight. The polymer was then used... 1 H NMR and GPC characterization showed that Example 29 prepared polylactic acid with narrow distribution, high molecular weight, and controllable structure by using an organic base thiourea dual-catalyst system formed by the combination of TBD and thiourea III to catalyze the controlled ring-opening polymerization of lactide.

[0154] Example 30

[0155] L-lactide (0.432 g, 3 mmol), benzyl alcohol (6.23 μL, 0.06 mmol), TBD (0.0084 g, 0.06 mmol), and thiourea IV (0.034 g, 0.06 mmol) were dissolved in 3 mL of DCM and reacted under nitrogen protection for 1 hour ([M]0:[I]0:[B]:[TU] = 50:1:1:1). The reaction was terminated by adding formic acid. The reaction solution was poured into ice-cold methanol, the precipitate was filtered, and dried to constant weight, with a conversion rate of 99%. The obtained polymer was used... 1 Characterization by 1H NMR and GPC showed that the number-average molecular weight (Mn) of the polymer was 7100 g / mol and the dispersion (D) was 1.43. The characterization results indicate that in Example 30, a controlled ring-opening polymerization of lactide was prepared by using a dual-catalytic system of organic base and thiourea (TBD and thiourea IV) to catalyze the polymerization of lactide. The polymer had a narrow distribution, high molecular weight, and controllable structure.

[0156] Example 31

[0157] L-lactide (0.432 g, 3 mmol), benzyl alcohol (6.23 μL, 0.06 mmol), TBD (0.0084 g, 0.06 mmol), and thiourea IV (0.068 g, 0.12 mmol) were dissolved in 3 mL of DCM and reacted under nitrogen protection for 1 hour ([M]0:[I]0:[B]:[TU] = 50:1:1:2). The reaction was terminated by adding formic acid. The reaction solution was poured into ice-cold methanol, the precipitate was filtered, and dried to constant weight, with a conversion rate of 99%. The obtained polymer was used... 1 Characterization by 1H NMR and GPC showed that the number-average molecular weight (Mn) of the polymer was 7100 g / mol and the dispersion (D) was 1.24. The characterization results indicate that in Example 31, a controlled ring-opening polymerization of lactide was prepared by using a dual-catalytic system of organic base thiourea (TBD and thiourea IV) to catalyze the polymerization of lactide. The polymer had a narrow distribution, high molecular weight, and controllable structure.

[0158] Example 32

[0159] DL-lactide (0.432 g, 3 mmol), benzyl alcohol (6.23 μL, 0.06 mmol), TBD (0.0084 g, 0.06 mmol), and thiourea IV (0.102 g, 0.18 mmol) were dissolved in 3 mL of DCM and reacted under nitrogen protection for 1 hour ([M]0:[I]0:[B]:[TU] = 50:1:1:3). The reaction was terminated by adding formic acid. The reaction solution was poured into ice-cold methanol, the precipitate was filtered, and dried to constant weight, with a conversion rate of 99%. The obtained polymer was used... 1 Characterization by 1H NMR and GPC showed that the number-average molecular weight (Mn) of the polymer was 7100 g / mol and the dispersion (D) was 1.30. The characterization results indicate that Example 32 prepared polylactic acid with narrow distribution, high molecular weight, and controllable structure by using an organic-base thiourea dual-catalyst system formed by the combination of TBD and thiourea IV to catalyze the controlled ring-opening polymerization of lactide.

[0160] Example 33

[0161] L-lactide (0.432 g, 3 mmol), benzyl alcohol (6.23 μL, 0.06 mmol), TBD (0.0084 g, 0.06 mmol), and thiourea V (0.032 g, 0.06 mmol) were dissolved in 3 mL of DCM and reacted under nitrogen protection for 1 hour ([M]0:[I]0:[B]:[TU] = 50:1:1:1). The reaction was terminated by adding formic acid, and the reaction solution was poured into ice-cold methanol. The precipitate was filtered and dried to constant weight. The polymer was then used... 1H NMR and GPC characterization showed that the controlled ring-opening polymerization of lactide in Example 33, which was prepared by the combination of TBD and thiourea V to form an organic base thiourea dual catalytic system, yielded polylactic acid with narrow distribution, high molecular weight and controllable structure.

[0162] Example 34

[0163] L-lactide (0.432 g, 3 mmol), benzyl alcohol (6.23 μL, 0.06 mmol), TBD (0.0084 g, 0.06 mmol), and thiourea V (0.063 g, 0.12 mmol) were dissolved in 3 mL of DCM and reacted under nitrogen protection for 1 hour ([M]0:[I]0:[B]:[TU] = 50:1:1:2). The reaction was terminated by adding formic acid, and the reaction solution was poured into ice-cold methanol. The precipitate was filtered and dried to constant weight. The polymer was then used... 1 H NMR and GPC characterization showed that the controlled ring-opening polymerization of lactide in Example 334, which was prepared by the combination of TBD and thiourea V to form an organic base thiourea dual catalytic system, yielded polylactic acid with narrow distribution, high molecular weight and controllable structure.

[0164] Example 35

[0165] DL-lactide (0.432 g, 3 mmol), benzyl alcohol (6.23 μL, 0.06 mmol), TBD (0.0084 g, 0.06 mmol), and thiourea V (0.095 g, 0.18 mmol) were dissolved in 3 mL of DCM and reacted under nitrogen protection for 1 hour ([M]0:[I]0:[B]:[TU] = 50:1:1:3). The reaction was terminated by adding formic acid, and the reaction solution was poured into ice-cold methanol. The precipitate was filtered and dried to constant weight. The polymer was then used... 1 H NMR and GPC characterization showed that the controlled ring-opening polymerization of lactide in Example 35, which was prepared by the combination of TBD and thiourea V to form an organic base thiourea dual catalytic system, yielded polylactic acid with narrow distribution, high molecular weight and controllable structure.

[0166] Example 36

[0167] L-lactide (0.432 g, 3 mmol), benzyl alcohol (6.23 μL, 0.06 mmol), DBU (0.0091 g, 0.06 mmol), and thiourea IV (0.068 g, 0.12 mmol) were dissolved in 3 mL of DCM and reacted under nitrogen protection for 1 hour ([M]0:[I]0:[B]:[TU] = 50:1:1:2). The reaction was terminated by adding formic acid, and the reaction solution was poured into ice-cold methanol. The precipitate was filtered and dried to constant weight. The polymer was then used... 1H NMR and GPC characterization showed that the controlled ring-opening polymerization of lactide in Example 36, using a combination of DBU and thiourea IV to form an organic base thiourea dual-catalyst system, yielded polylactic acid with narrow distribution, high molecular weight, and controllable structure.

[0168] Example 37

[0169] L-lactide (0.432 g, 3 mmol), benzyl alcohol (6.23 μL, 0.06 mmol), DMAP (0.0073 g, 0.06 mmol), and thiourea IV (0.068 g, 0.12 mmol) were dissolved in 3 mL of DCM and reacted under nitrogen protection for 1 hour ([M]0:[I]0:[B]:[TU] = 50:1:1:2). The reaction was terminated by adding formic acid, and the reaction solution was poured into ice-cold methanol. The precipitate was filtered and dried to constant weight. The polymer was then used... 1 H NMR and GPC characterization showed that the controlled ring-opening polymerization of lactide in Example 37, using an organic base thiourea dual-catalyst system formed by the combination of DMAP and thiourea IV, yielded polylactic acid with narrow distribution, high molecular weight, and controllable structure.

[0170] Example 38

[0171] L-lactide (0.432 g, 3 mmol), benzyl alcohol (6.23 μL, 0.06 mmol), (-)-sparteine ​​(0.014 g, 0.06 mmol), and thiourea IV (0.068 g, 0.12 mmol) were dissolved in 3 mL of DCM and reacted under nitrogen protection for 1 hour ([M]0:[I]0:[B]:[TU] = 50:1:1:2). The reaction was terminated by adding formic acid, and the reaction solution was poured into ice-cold methanol. The precipitate was filtered and dried to constant weight. The polymer was then used... 1 H NMR and GPC characterization showed that Example 38 used an organic-base thiourea dual-catalyst system composed of (-)-sparteine ​​and thiourea IV to catalyze the controlled ring-opening polymerization of lactide, resulting in polylactic acid with narrow distribution, high molecular weight, and controllable structure.

[0172] The thiourea catalyst of the present invention is easy to synthesize. The organic-base thiourea dual catalytic system formed by the modular combination of organic base and thiourea has a high decomposition temperature and therefore has good high temperature resistance. It can realize the controlled ring-opening polymerization of lactide in solution and the bulk controlled ring-opening polymerization of lactide at high temperature, thus expanding the application range of the organic-base thiourea catalytic system.

[0173] The organic-base thiourea dual-catalytic system of the present invention is a binary catalytic system. The catalyst used for polymerization is a hydrogen-bonded bifunctional catalyst, which can achieve a controllable polymerization effect that cannot be achieved by base catalysis alone, thereby resulting in a narrow molecular weight distribution of polylactic acid. By adjusting the ratio of lactide, initiator, organic base and thiourea, the catalytic activity can be further controlled to obtain high molecular weight polylactic acid with controllable structure.

[0174] The above-described embodiments are merely preferred embodiments provided to fully illustrate the present invention, and the scope of protection of the present invention is not limited thereto. Equivalent substitutions or modifications made by those skilled in the art based on the present invention are all within the scope of protection of the present invention. The scope of protection of the present invention is defined by the claims.

Claims

1. A method for preparing polylactic acid using an organic base thiourea dual-catalytic system, characterized in that, Includes the following steps: Under the catalysis of an organic base and thiourea dual catalytic system, lactide undergoes ring-opening polymerization in the presence of an initiator to obtain polylactic acid; the organic base is 1,5,7-triazabicyclo[4.4.0]dec-5-ene, 4-dimethylaminopyridine, 1,8-diazabispirocyclo[5.4.0]undec-7-ene, or stigmine; the molar ratio of lactide, initiator, organic base, and thiourea is 50:1:1:(1~5); the ring-opening polymerization method is bulk polymerization, and the reaction temperature is 60~180 ℃; The thiourea is either dithiourea or trithiourea; The structural formula of the dithiourea is shown in Formula I or Formula II: Formula I; Formula II; The structural formula of the trithiourea is shown in Formula III, Formula IV or Formula V: Formula III; Formula IV; Formula V.

2. The method for preparing polylactic acid using the organic-base thiourea dual-catalytic system as described in claim 1, characterized in that, The lactide is either levolactone or racemic lactide.

3. The method for preparing polylactic acid using the organic-base thiourea dual-catalytic system as described in claim 1, characterized in that, The initiator is benzyl alcohol or phenylethanol.

4. The method for preparing polylactic acid using the organic-base thiourea dual-catalytic system as described in claim 1, characterized in that, The specific steps of the bulk polymerization method are as follows: lactide, organic base and thiourea are heated to melt in an inert gas or vacuum environment, and an initiator is added to carry out the polymerization reaction to obtain polylactic acid.

5. The method for preparing polylactic acid using the organic-base thiourea dual-catalytic system as described in claim 1, characterized in that, It also includes the step of terminating the reaction with a weak acid.

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