A method for preparing highly isotactic polylactide

Abstract

The present invention provides a method for preparing a high isotactic polylactide, and belongs to the technical field of polymer preparation. The method for preparing a high isotactic polylactide provided by the present invention comprises the following steps: a lactide monomer, a main catalyst, a co-catalyst and an organic solvent are mixed, and a ring-opening polymerization reaction is carried out to obtain the high isotactic polylactide. The present invention is based on the combination of an achiral bis(thio)urea catalyst and a co-catalyst, and lactide monomers (including DLA, LLA, rac‑LA) can be polymerized under relatively mild conditions to obtain high isotactic polylactide. At the same time, the bis(thio)urea catalyst in the present invention is cheap and easy to obtain, and is suitable for large-scale production of high isotactic polylactide.

Description

Technical Field

[0001] The invention relates to the technical field of polymer preparation, and in particular to a method for preparing highly isotactic polylactide. Background Art

[0002] Polylactide (PLA) is renewable, biodegradable and biocompatible. It has important applications in the fields of medical devices and food packaging. It has been widely studied by academic and industrial circles at home and abroad. It is currently the largest biodegradable polymer material in terms of industry scale and market application. PLA is usually prepared from lactide (LA) monomers through ring-opening polymerization. Due to the diversity of chiral configurations of lactide monomers, such as levorotatory lactide (LLA), dextrorotatory lactide (DLA), meso-lactide (meso-LA) and racemic lactide (rac-LA), polylactide also has multiple stereoregularities such as isotactic, syndiotactic and atactic. The stereoregularity of the polymer determines the mechanical properties, thermal stability and degradability of the polymer, so the synthesis of stereo-controllable PLA has important research value.

[0003] The synthesis of isotactic PLA is currently the most popular. Isotactic PLA can be prepared by catalyzing LLA or DLA, which requires that no diastereoisomerization occurs during the catalytic process. Isotactic PLA can also be synthesized by catalyzing the stereoselective polymerization of racemic rac-LA. The existing systems for catalytic synthesis of isotactic PLA include: achiral catalysts (such as nitrogen heterocyclic carbenes, tBu-P2, metal organic catalysts and cyclic super strong phosphazene bases, etc.) and chiral catalysts (such as phosphoric acid based on binaphthyl structure, cinchona alkali, chiral metal organic catalysts and amino acid derivatives, etc.). Among them, achiral catalysts generally need to be operated at ultra-low temperatures (such as -70°C) to achieve highly isotactic PLA (fully isotactic PLA). m Greater than 0.90) synthesis. Summary of the invention

[0004] The object of the present invention is to provide a method for preparing high isotactic polylactide. The method provided by the present invention can prepare high isotactic polylactide under relatively mild conditions.

[0005] In order to achieve the above-mentioned object of the invention, the present invention provides the following technical solutions:

[0006] The present invention provides a method for preparing highly isotactic polylactide, comprising the following steps:

[0007] The lactide monomer, the main catalyst, the co-catalyst and the organic solvent are mixed to carry out a ring-opening polymerization reaction to obtain the highly isotactic polylactide;

[0008] The main catalyst has a structure shown in Formula I:

[0009]

[0010] In the formula I, R is selected from

[0011] The X is selected from -S- or -O-;

[0012] The co-catalyst includes a first co-catalyst or a second co-catalyst;

[0013] The first co-catalyst is an alkali metal alkoxide;

[0014] The second co-catalyst includes an alkali agent and an alcohol compound; the alkali agent is KH, NaH, KN(SiMe3)2, NaN(SiMe3)2, LiN(SiMe3)2, n BuLi, MeLi, nitrogen heterocyclic carbene, nitrogen heterocyclic olefin or organic phosphine base, the nitrogen heterocyclic carbene has the structure shown in formula II, the nitrogen heterocyclic olefin has the structure shown in formula III or formula IV, and the organic phosphine base is t-Bu-P2, t-Bu-P4, TiBP, TTMPP, a compound with the structure shown in formula V or a compound with the structure shown in formula VI:

[0015]

[0016] In the formula II and formula III, R1 to R4 are independently selected from hydrogen, pentafluorophenyl, methyl, isopropyl, tert-butyl, phenyl or 2,4,6-trimethylphenyl;

[0017] In the formula IV and formula V, R1 to R6 are independently selected from hydrogen, pentafluorophenyl, methyl, isopropyl, tert-butyl, phenyl or 2,4,6-trimethylphenyl;

[0018] In the formula VI, R1 to R3 are independently selected from hydrogen, pentafluorophenyl, methyl, isopropyl, tert-butyl, n-butyl or phenyl;

[0019] Wherein, the ratio of the amount of the main catalyst to the amount of the alkali metal alkoxide or the alkaline reagent is ≥1.

[0020] Preferably, the main catalyst is any one of the following compounds:

[0021]

[0022]

[0023] Preferably, the alkali metal alkoxide includes sodium methoxide, potassium methoxide, sodium ethoxide or potassium ethoxide.

[0024] Preferably, the alcohol compound includes methanol, ethanol, propanol, n-butanol, isobutanol, tert-butanol, isopropanol, phenol, benzyl alcohol, phenylethyl alcohol, butanediol or glycerol.

[0025] Preferably, the lactide monomer includes L-lactide, D-lactide or racemic lactide.

[0026] Preferably, the molar ratio of the lactide monomer to the main catalyst is (10-2000): (1-100).

[0027] Preferably, the molar ratio of the lactide monomer, the main catalyst and the alcohol compound is (10-2000): (1-100): (1-100).

[0028] Preferably, the organic solvent comprises tetrahydrofuran, toluene, dichloromethane, acetonitrile or chloroform.

[0029] Preferably, the temperature of the ring-opening polymerization reaction is -80 to 100° C., and the time is 10 min to 48 h.

[0030] Preferably, the weight average molecular weight of the highly isotactic polylactide is 1.0 to 100 kg / mol, and the isotacticity P m It is 0.85~1.0.

[0031] The present invention provides a method for preparing highly isotactic polylactide, comprising the following steps: mixing a lactide monomer, a main catalyst, a co-catalyst and an organic solvent, and performing a ring-opening polymerization reaction to obtain the highly isotactic polylactide. The present invention is based on the combination of an achiral di(thio)urea catalyst and a co-catalyst, and can polymerize lactide monomers (including DLA, LLA, and rac-LA) under relatively mild conditions to obtain highly isotactic polylactide. Specifically, the co-catalyst in the present invention acts as a nucleophilic reagent and an initiator to achieve the ring opening of lactide; the two (thio)urea groups of the di(thio)urea catalyst are close to each other, and can cooperate with each other during the ring-opening polymerization reaction, activate the lactide monomer through multiple hydrogen bonds, and stabilize the chain end, so the catalytic conditions are mild, and at the same time, the active species formed by the di(thio)urea catalyst and the chain end have stereoselectivity for the monomer, and can obtain polylactide with a wide range of molecular weights, and has a high initiation efficiency; wherein, the di(thio)urea catalyst in the present invention The ratio of the amount of substance of the alkali metal alkoxide or the amount of substance of the alkali metal alkoxide or the alkali reagent must be ≥1, so that it can be ensured that one molecule of the di(thio)urea catalyst reacts with only one molecule of the alkali metal alkoxide or the alkali reagent to produce one active center. When the amount of the alkali metal alkoxide or the alkali reagent is greater than the amount of the di(thio)urea catalyst, two (thio)urea groups in one molecule will react with the alkali metal alkoxide or the alkali reagent to produce two active centers. Each active center catalyzes the ring-opening polymerization reaction separately, destroying the synergy between the two (thio)urea groups, which is not conducive to the smooth progress of the ring-opening polymerization reaction. At the same time, the di(thio)urea catalyst in the present invention is cheap and easy to obtain, and is suitable for large-scale production of high isotactic polylactide. BRIEF DESCRIPTION OF THE DRAWINGS

[0032] Figure 1 The homonuclear decoupling hydrogen spectrum of the polylactide prepared in Example 1;

[0033] Figure 2 The homonuclear decoupling hydrogen spectrum of the polylactide prepared in Example 2;

[0034] Figure 3 The homonuclear decoupling hydrogen spectrum of the polylactide prepared in Example 3;

[0035] Figure 4 The homonuclear decoupling hydrogen spectrum of the polylactide prepared in Example 4;

[0036] Figure 5 The homonuclear decoupling hydrogen spectrum of the polylactide prepared in Example 5;

[0037] Figure 6 The homonuclear decoupling hydrogen spectrum of the polylactide prepared in Example 6;

[0038] Figure 7 The homonuclear decoupling hydrogen spectrum of the polylactide prepared in Example 7;

[0039] Figure 8 The homonuclear decoupling hydrogen spectrum of the polylactide prepared in Example 8;

[0040] Fig. 9 This is the homonuclear decoupling hydrogen spectrum of the polylactide prepared in Example 9. DETAILED DESCRIPTION

[0041] The present invention provides a method for preparing highly isotactic polylactide, comprising the following steps:

[0042] The lactide monomer, the main catalyst, the co-catalyst and the organic solvent are mixed to carry out a ring-opening polymerization reaction to obtain the highly isotactic polylactide;

[0043] The main catalyst has a structure shown in Formula I:

[0044]

[0045] In the formula I, R is selected from

[0046] The X is selected from -S- or -O-;

[0047] The co-catalyst includes a first co-catalyst or a second co-catalyst;

[0048] The first co-catalyst is an alkali metal alkoxide;

[0049] The second co-catalyst includes an alkali agent and an alcohol compound; the alkali agent is KH, NaH, KN(SiMe3)2, NaN(SiMe3)2, LiN(SiMe3)2, n BuLi, MeLi, nitrogen heterocyclic carbene, nitrogen heterocyclic olefin or organic phosphine base, the nitrogen heterocyclic carbene has the structure shown in formula II, the nitrogen heterocyclic olefin has the structure shown in formula III or formula IV, and the organic phosphine base is t-Bu-P2, t-Bu-P4, TiBP, TTMPP, a compound with the structure shown in formula V or a compound with the structure shown in formula VI:

[0050]

[0051] In the formula II and formula III, R1 to R4 are independently selected from hydrogen, pentafluorophenyl, methyl, isopropyl, tert-butyl, phenyl or 2,4,6-trimethylphenyl;

[0052] In the formula IV and formula V, R1 to R6 are independently selected from hydrogen, pentafluorophenyl, methyl, isopropyl, tert-butyl, phenyl or 2,4,6-trimethylphenyl;

[0053] In the formula VI, R1 to R3 are independently selected from hydrogen, pentafluorophenyl, methyl, isopropyl, tert-butyl, n-butyl or phenyl;

[0054] Wherein, the ratio of the amount of the main catalyst to the amount of the alkali metal alkoxide or the alkaline reagent is ≥1.

[0055] The main catalyst in the present invention is an achiral di(thio)urea catalyst. Two (thio)urea groups of the di(thio)urea catalyst are close to each other and can cooperate with each other in the ring-opening polymerization process. The lactide monomer is activated through multiple hydrogen bonds and the chain end is stabilized. Therefore, the catalytic conditions are mild, the selectivity is high, and polylactide with a wide range of molecular weights can be obtained, and the initiation efficiency is high. The ratio of the amount of the di(thio)urea catalyst to the amount of the alkali metal alkoxide or the alkali reagent in the present invention must be greater than or equal to 1, so that one molecule of the di(thio)urea catalyst can react with only one molecule of the alkali metal alkoxide or the alkali reagent to generate one active center. When the amount of the alkali metal alkoxide or the alkali reagent is greater than the amount of the di(thio)urea catalyst, the two (thio)urea groups in one molecule will react with the alkali metal alkoxide or the alkali reagent to generate two active centers. Each active center catalyzes the ring-opening polymerization reaction separately, which destroys the synergy between the two (thio)urea groups and is not conducive to the smooth progress of the ring-opening polymerization reaction.When the ring-opening polymerization is carried out under the same temperature conditions, if the amount of the alkali metal alkoxide or the base reagent is greater than the amount of the di(thio)urea catalyst, the stereoregularity of the product is low; and if a non-chiral catalyst in the prior art (such as nitrogen heterocyclic carbene, tBu-P2, metal organic catalyst and cyclic super strong phosphazene base, etc.) is used, the stereoregularity of the product is even lower. For example, when nitrogen heterocyclic carbene or tBu-P2 is used in combination with alcohols, basically random polylactide is obtained under room temperature conditions (Lei Zhang et al., Organocatalytic Stereoselective Ring-Opening Polymerization of Lactide with DimericPhosphazene Bases, J.AM.CHEM.SOC.2007,129,12610-12611; Andrew P.Dove et al., Stereoselective polymerization of rac-and meso-lactide catalyzed bysterically encumbered N-heterocyclic carbenes, Chem. Commun., 2006, 2881-2883; Shaofeng Liu et.al, Stereoselective Ring-Opening Polymerization of rac-Lactide Using Organocatalytic Cyclic Trimeric Phosphazene Base, ACS Macro Lett. 2018, 7, 624-628), and the use of the non-chiral di(thio)urea catalyst in the present invention can obtain polylactide with higher stereoregularity. At the same time, the di(thio)urea catalyst in the present invention is cheap and easy to obtain, the synthesis steps are simple and the raw material price is low, which is suitable for large-scale production of high isotactic polylactide. The technical scheme of the present invention is described in detail below. In the present invention, unless otherwise specified, the raw materials used are commercially available products well known to those skilled in the art or prepared by methods well known to those skilled in the art.

[0056] In the present invention, the main catalyst is any one of the following compounds:

[0057]

[0058]

[0059] In the present invention, the first co-catalyst is an alkali metal alkoxide, and the alkali metal alkoxide preferably includes sodium methoxide, potassium methoxide, sodium ethoxide or potassium ethoxide, and more preferably is potassium methoxide.

[0060] In the present invention, the second co-catalyst includes an alkali agent and an alcohol compound. In the present invention, the alkali agent is KH, NaH, KN(SiMe3)2, NaN(SiMe3)2, LiN(SiMe3)2, n BuLi, MeLi, nitrogen heterocyclic carbene, nitrogen heterocyclic olefin or organic phosphine base, preferably KH, NaH, KN(SiMe3)2, NaN(SiMe3)2, LiN(SiMe3)2, n BuLi or MeLi. In the present invention, the nitrogen heterocyclic carbene has a structure shown in formula II, the nitrogen heterocyclic olefin has a structure shown in formula III or formula IV, and the organic phosphine base is t-Bu-P2, t-Bu-P4, TiBP, TTMPP, a compound having a structure shown in formula V, or a compound having a structure shown in formula VI. In the present invention, the structural formulas of t-Bu-P2, t-Bu-P4, TiBP and TTMPP are shown in the following order:

[0061]

[0062] In the present invention, the alcohol compound preferably includes methanol, ethanol, propanol, n-butanol, isobutanol, tert-butanol, isopropanol, phenol, benzyl alcohol, phenylethyl alcohol, butanediol or glycerol, more preferably methanol.

[0063] In the present invention, the lactide monomer preferably includes L-lactide, D-lactide or racemic lactide.

[0064] In the present invention, the ratio of the amount of the main catalyst to the amount of the alkali metal alkoxide or the alkaline agent is ≥1, preferably 1 to 5, and more preferably 1.

[0065] In the present invention, the molar ratio of the lactide monomer to the main catalyst is preferably (10-2000):(1-100), more preferably (100-1000):(1-10), further preferably (200-800):(1-5), and further preferably (400-600):(1-3).

[0066] In the present invention, the molar ratio of the lactide monomer, the main catalyst and the alcohol compound is preferably (10-2000): (1-100): (1-100), more preferably (30-1000): (1-10): (1-10), further preferably (80-500): (1-5): (1-5), and further preferably (100-200): (1-3): (1-3).

[0067] In the present invention, the organic solvent preferably includes tetrahydrofuran, toluene, dichloromethane, acetonitrile or chloroform, and more preferably tetrahydrofuran.

[0068] When the first cocatalyst is used, the present invention preferably dissolves the main catalyst in an organic solvent, mixes the obtained main catalyst solution with the first cocatalyst, then adds the lactide monomer to mix and adds the organic solvent to obtain a mixed reaction liquid, and performs a ring-opening polymerization reaction. In the present invention, the concentration of the main catalyst solution is preferably 0.0001-0.1 mol / L, more preferably 0.001-0.1 mol / L; the concentration of the lactide monomer solution in the mixed reaction liquid is preferably 0.1-1.0 mol / L, more preferably 0.3-0.5 mol / L. In the present invention, the temperature of the ring-opening polymerization reaction is preferably -80-100°C, more preferably -40-50°C, further preferably 0-35°C, and further preferably 15-25°C; the time is preferably 10min-48h, more preferably 20min-36h, further preferably 30min-32h, further preferably 1-20h, and further preferably 1.5-5h. In the present invention, the ring-opening polymerization reaction can be carried out in a dry Shrek bottle connected to a double Schlenk line to perform low-temperature ring-opening polymerization using an external temperature low-temperature bath, or in a glass reactor inside a glove box to perform room temperature ring-opening polymerization.

[0069] The present invention preferably takes out 0.1 mL of the reaction solution from the reaction system by a syringe or a pipette during the ring-opening polymerization reaction, and quickly quenches it into a vial containing 0.6 mL of undried CDCl3 (containing 250 ppm of BHT), and then 1 H NMR analysis of the quenched sample is used to determine the conversion rate of the lactide monomer. After the ring-opening polymerization reaction is completed, the present invention preferably immediately adds hexane to the resulting product system, stirs to fully precipitate the polymer, and then filters it, washes the filter cake with hexane, and then dries it in a vacuum oven at 50°C overnight to constant weight to obtain the highly isotactic polylactide. In the present invention, for polymer samples that need to be tested for homonuclear decoupling nuclear magnetic hydrogen spectrum, it is preferred to add dichloromethane for dissolution and then add hexane for precipitation, and repeat the dissolution-precipitation process three times to ensure that all lactide monomers and main catalysts are removed.

[0070] When the second co-catalyst is used, the present invention preferably dissolves the main catalyst in an organic solvent, mixes the obtained main catalyst solution with an alkaline reagent, sequentially adds an alcohol compound and a lactide monomer to the obtained mixture, and further adds an organic solvent to obtain a mixed reaction liquid, and performs a ring-opening polymerization reaction. In the present invention, the concentration of the main catalyst solution, the concentration of the lactide monomer in the mixed reaction liquid, the ring-opening polymerization reaction conditions, and the post-treatment method when the second co-catalyst is used are preferably consistent with the concentration of the main catalyst solution, the concentration of the lactide monomer in the mixed reaction liquid, the ring-opening polymerization reaction conditions, and the post-treatment method when the first co-catalyst is used, and will not be repeated here.

[0071] In the present invention, the weight average molecular weight of the polylactide is preferably 1.0 to 100 kg / mol, more preferably 10.0 to 75.0 kg / mol, and further preferably 14.0 to 51.0 kg / mol; the isotacticity P m (Lactide monomer is L-lactide, D-lactide or racemic lactide) is preferably 0.85 to 1.0, more preferably 0.90 to 0.93. The method provided by the present invention has a wide range of lactide monomer applications. As shown in the results of the embodiments, the polymerization of L-lactide or D-lactide can be achieved. During the polymerization process, the lactide monomer does not undergo diastereoisomerization. P can be obtained by polymerization under room temperature. m 1.0 and a weight average molecular weight of 14.4 to 16.0 kg / mol polylactide; at the same time, the stereoselective ring-opening polymerization of racemic lactide can be realized to synthesize highly isotactic lactide, and P can be obtained by polymerization under room temperature (25°C) m The polylactide has a molecular weight of 0.85 to 0.93 and a weight average molecular weight of 14.5 to 95.3 kg / mol.

[0072] The main catalyst of the present invention can be a commercially available product well known to those skilled in the art. Some of the main catalysts can be prepared by methods well known to those skilled in the art. The preparation method of the main catalyst of the present invention is described below using catalyst DiTU1 as an example. Specifically, the structural formula of the catalyst DiTU1 is as follows:

[0073]

[0074] N-methyl-2,2-diaminodiethylamine (2.234 g, 20 mmol) was dissolved in 60 mL of dichloromethane, and then the resulting solution was cooled to 0°C using an ice-salt bath, 3,5-bis(trifluoromethyl)phenylisothiocyanate (11.1 g, 41 mmol) was slowly added dropwise thereto, and the ice bath was stopped after the addition was complete. The reaction was carried out at room temperature (25°C) for 30 min. After the reaction was completed, the resulting product system was filtered, and the resulting solid material was washed with dichloromethane to obtain a white solid product, which was catalyst DiTU1, and its English name was 1,1'-((methylazanediyl)bis(ethane-2,1-diyl))bis(3-(3,5-bis(trifluoromethyl)phenyl)thiourea). The white solid product was placed in a vacuum oven, vacuum dried at 70°C overnight to remove residual solvent and water, and then transferred to a glove box for standby use. 1 H NMR (400MHz, DMSO-d6): δ2.33(s,3H,CH3),2.65(s,4H,CHCH2NH),3.63(s,4H,CHCH2NM e),7.56(s,2H,p-Ar-H),8.04(s,2H,CHNH),8.23(s,4H,o-Ar-H),10.15(s,2H,ArNH),. 13 C NMR (101MHz, DMSO-d6): δ116.15,121.83,123.57,142.36,130.52,180.70.

[0075] The technical solutions in the present invention will be described clearly and completely below in conjunction with the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, not all of them. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0076] Examples 1 to 8

[0077] The ring-opening polymerization reactions involved in Examples 1 to 8 were specifically carried out in a 25 mL dry Shrek bottle connected to a double Schlenk line to use an external temperature low temperature bath for low temperature ring-opening polymerization, or in a 20 mL glass reactor inside a glove box for room temperature ring-opening polymerization, and the specific steps are as follows:

[0078] The catalyst DiTU1 was dissolved in tetrahydrofuran (THF) to obtain a catalyst DiTU1 solution with a concentration of 0.1M, and the catalyst DiTU1 solution was mixed with an equivalent amount of an organic base KOMe, and after stirring for 2 minutes, the obtained mixed solution was added to a vial containing a lactide monomer, and then THF was added to make the concentration of the lactide monomer 0.5 mol / L, and a ring-opening polymerization reaction was carried out, and the molar ratio of the lactide monomer to the organic base KOMe met the ratio in Table 1; wherein, during the ring-opening polymerization reaction, 0.1 mL of the reaction solution was taken out from the reaction system by a pipette, and quickly quenched into a vial containing 0.6 mL of undried CDCl3 (containing 250 ppm of BHT), and then 1 H NMR analysis of the quenched samples was used to determine the conversion of lactide monomer;

[0079] After the ring-opening polymerization reaction was completed, hexane was immediately added to the obtained product system for precipitation, and the product was filtered after stirring for 1 hour, and the filter cake was washed with hexane and then dried in a vacuum oven at 50°C overnight to constant weight;

[0080] For polymer samples that need to be tested by homonuclear decoupling H-NMR, they need to be dissolved in dichloromethane and then precipitated in hexane. The dissolution-precipitation process should be repeated three times to ensure that all lactide monomers and catalyst DiTU1 are removed.

[0081] The structural formula of the catalyst DiTU1 is as follows:

[0082]

[0083] The types of raw materials, ratios, reaction conditions and test results involved in the ring-opening polymerization reactions in Examples 1 to 8 are shown in Table 1. Figures 1 to 8 The homonuclear decoupling hydrogen spectra (CDCl3, 500 MHz) of the polylactides prepared in Examples 1 to 8 are shown in Table 1 and Figures 1 to 8 It can be seen that the catalyst DiTU1 provided by the present invention combined with KOMe as a co-catalyst can catalyze the polymerization of lactide to obtain highly isotactic polylactide.

[0084] Table 1 Conditions and results of lactide polymerization catalyzed by dithiourea combined with organic base in Examples 1 to 8

[0085]

[0086] Note: a The reaction conditions were: initial molar ratio of DiTU1 to KOMe was [DiTU1]0 / [KOMe]0=1 / 1, THF, and room temperature (25°C). b pass 1H NMR was measured in CDCl3 using characteristic signal integration. c M was measured in THF at 35 °C by laser light scattering detection. w and d Using Same-core Decoupling 1 H NMR spectroscopy was measured in conjunction with the methine tetrad (δ 5.25-5.18 ppm). e The ring-opening polymerization (ROP) reaction was carried out at -30 °C.

[0087] Embodiments 9 to 15

[0088] The ring-opening polymerization reactions involved in Examples 9 to 15 were specifically carried out in a 25 mL dry Shrek bottle, which was connected to a double Schlenk line to use an external temperature low-temperature bath to carry out the low-temperature ring-opening polymerization reaction. The specific steps are as follows:

[0089] The catalyst DiTU1 was dissolved in THF to obtain a catalyst DiTU1 solution with a concentration of 0.1M, the catalyst DiTU1 solution was mixed with a base, and after stirring for 2 minutes, methanol was added, and then the obtained mixed solution was added to a vial containing a lactide monomer, and then THF was added to make the concentration of the lactide monomer 0.5 mol / L, and a ring-opening polymerization reaction was carried out, and the initial molar ratio of the lactide monomer, the catalyst DiTU1, the base and the methanol was 100:1:1:1; wherein, during the ring-opening polymerization reaction, 0.1 mL of the reaction solution was taken out from the reaction system by a pipette, and quickly quenched into a vial containing 0.6 mL of undried CDCl3 (containing 250 ppm of BHT), and then 1 H NMR analysis of the quenched samples was used to determine the conversion of lactide monomer;

[0090] After the ring-opening polymerization reaction was completed, hexane was immediately added to the obtained product system for precipitation, and the product was filtered after stirring for 1 hour, and the filter cake was washed with hexane and then dried in a vacuum oven at 50°C overnight to constant weight;

[0091] For polymer samples that need to be tested by homonuclear decoupling H-NMR, they need to be dissolved in dichloromethane and then precipitated in hexane. The dissolution-precipitation process should be repeated three times to ensure that all lactide monomers and catalyst DiTU1 are removed.

[0092] The types of raw materials, ratios, reaction conditions and test results involved in the ring-opening polymerization reactions in Examples 9 to 15 are shown in Table 2. Fig. 9 This is the homonuclear decoupling hydrogen spectrum (CDCl3, 500MHz) of the polylactide prepared in Example 9. From Table 2 and Fig. 9It can be seen that the catalyst DiTU1 provided by the present invention can be used in combination with other organic bases and alcohols as co-catalysts to catalyze the polymerization of lactide to obtain highly isotactic polylactide.

[0093] Table 2 Conditions and results of the polymerization of racemic lactide catalyzed by dithiourea combined with base and alcohol in Examples 9 to 15

[0094]

[0095] Note: a The reaction conditions were as follows: the initial molar ratio of rac-LA, RS-DiTU, base and methanol was [rac-LA]0 / [RS-DiTU]0 / [base]0 / [methanol]0=100 / 1 / 1 / 1, THF, -30°C. b pass 1 H NMR was measured in CDCl3 using characteristic signal integration. c M was measured in THF at 35 °C by laser light scattering detection. w and d Using Same-core Decoupling 1 H NMR spectroscopy was measured in conjunction with the methine tetrad (δ 5.25-5.18 ppm).

[0096] The above is only a preferred embodiment of the present invention. It should be pointed out that for ordinary technicians in this technical field, several improvements and modifications can be made without departing from the principle of the present invention. These improvements and modifications should also be regarded as the scope of protection of the present invention.

Claims

1. A method for preparing highly isotactic polylactide, comprising the following steps: The lactide monomer, the main catalyst, the co-catalyst and the organic solvent are mixed to carry out a ring-opening polymerization reaction to obtain the highly isotactic polylactide; The lactide monomer is racemic lactide; The main catalyst is the following compound: ； The co-catalyst includes a first co-catalyst or a second co-catalyst; The first co-catalyst is an alkali metal alkoxide; The second co-catalyst includes an alkali agent and an alcohol compound; the alkali agent is KH, NaH, KN(SiMe3)2, NaN(SiMe3)2, LiN(SiMe3)2, n BuLi, MeLi, nitrogen heterocyclic carbene, nitrogen heterocyclic olefin or organic phosphine base, the nitrogen heterocyclic carbene has the structure shown in formula II, the nitrogen heterocyclic olefin has the structure shown in formula III or formula IV, and the organic phosphine base is t -Bu-P2, t -Bu-P4, TiBP, TTMPP, a compound having a structure shown in Formula V or a compound having a structure shown in Formula VI: Formula II; Formula III; Formula IV; Formula V; Formula VI; In the formula II and formula III, R1 to R4 are independently selected from hydrogen, pentafluorophenyl, methyl, isopropyl, tert-butyl, phenyl or 2,4,6-trimethylphenyl; In the formula IV and formula V, R1 to R6 are independently selected from hydrogen, pentafluorophenyl, methyl, isopropyl, tert-butyl, phenyl or 2,4,6-trimethylphenyl; In the formula VI, R1 to R3 are independently selected from hydrogen, pentafluorophenyl, methyl, isopropyl, tert-butyl, n-butyl or phenyl; Wherein, the ratio of the amount of the main catalyst to the amount of the alkali metal alkoxide or the alkaline reagent is ≥1; The temperature of the ring-opening polymerization reaction is 25-50°C; The weight average molecular weight of the highly isotactic polylactide is 1.0-100 kg / mol, and the isotacticity is P m It is 0.85~0.

93.

2. The preparation method according to claim 1, characterized in that: The alkali metal alkoxide includes sodium methoxide, potassium methoxide, sodium ethoxide or potassium ethoxide.

3. The preparation method according to claim 1, characterized in that: The alcohol compound includes methanol, ethanol, propanol, n-butanol, isobutanol, tert-butanol, phenol, benzyl alcohol, phenylethyl alcohol, butanediol or glycerol.

4. The preparation method according to any one of claims 1 to 3, characterized in that The molar ratio of the lactide monomer to the main catalyst is (10-2000): (1-100).

5. The preparation method according to any one of claims 1 to 3, characterized in that The molar ratio of the lactide monomer, the main catalyst and the alcohol compound is (10-2000): (1-100): (1-100).

6. The preparation method according to claim 1, characterized in that: The organic solvent includes tetrahydrofuran, toluene, acetonitrile or chloroform.

7. The preparation method according to claim 1, characterized in that: The ring-opening polymerization reaction time is 10min~48h.

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