A ketimine zinc complex and a preparation method and application thereof

Abstract

The application provides a ketimine zinc complex and a preparation method and application thereof. The structural formula of the ketimine zinc complex is as follows: wherein R is any one of C2-C4 linear alkyl and derivatives thereof, C2-C4 branched alkyl and derivatives thereof. The ketimine zinc complex has the characteristics of easy synthesis (high yield), high catalytic activity for ring-opening polymerization of lactide to obtain high molecular weight PLA and high catalytic activity for ring-opening polymerization of lactide.

Description

Technical Field

[0001] This invention belongs to the field of coordination preparation and application technology, specifically relating to a zinc ketimide complex, a method for preparing the zinc ketimide complex, and the application of the zinc ketimide complex in the catalytic ring-opening polymerization of lactide. Background Technology

[0002] Polylactide (PLA) is a polymer with good biocompatibility, biodegradability, and physical and mechanical properties. PLA has good tensile strength and ductility, and can decompose into carbon dioxide and water in vivo, making it an ideal biomedical material. PLA can be used in drug delivery, surgical sutures, and artificial tissue materials, and has significant research value.

[0003] Currently, industrial production of PLA mainly involves the ring-opening polymerization of lactide catalyzed by stannous octoate. However, the polymers obtained from the ring-opening polymerization of lactide catalyzed by metal complex catalysts inevitably contain some metal residues. PLA obtained from the ring-opening polymerization of lactide catalyzed by stannous octoate will contain tin residues. Tin itself is toxic and harmful to human health, which limits the application of PLA in the food and pharmaceutical fields. Therefore, developing environmentally friendly metal complex catalysts to achieve green and environmentally friendly processes for the production of polylactide is of great significance.

[0004] Currently, some research has emerged on non-tin metal complex catalysts, such as Zn, Mg, and Al metal complex catalysts. However, these non-tin metal complex catalysts face numerous problems in their preparation and application in the catalytic ring-opening polymerization of lactide to produce PLA. The most significant issues include low yields requiring purification during preparation, small molecular weight of PLA produced from lactide ring-opening polymerization, and low activity in the catalytic production of PLA from lactide ring-opening polymerization. In existing non-tin metal complex catalysts, even the better-performing schemes typically achieve yields of only around 60%. When used to catalyze the ring-opening polymerization of lactide to produce PLA, the resulting PLA molecular weight rarely exceeds 100,000, and the molar ratio to lactide usually needs to be 1:100-1000 (e.g., the metal complex catalyst presented in "Syntheses, structures and catalysis of tetranuclear zinc N-alkoxide ketoiminate complexes for ring-opening polymerization of rac-lactide" published in Inorganic Chemistry Communications, 2020, Issue 119). Therefore, researchers in the field are dedicated to developing methods to improve the performance of non-tin metal complex catalysts.

[0005] However, improving the performance of metal complex catalysts is very difficult. The catalytic activity of metal complex catalysts is closely related to the ligands and metal ions used. The type of ligand, the site of the ligand substituent, the type of ligand substituent, the type of metal ion, and the coordination number of the metal ion all individually and in turn affect the preparation method, purification, yield, structure, and catalytic activity of the complex. Due to the diversity of influencing factors and the uncertainty of the relationships between them (for example, for a given class of metal complexes with different ligand substituents, the catalytic activity enhancement may have relationship A with the substituents, but when the coordination number of the metal complex is changed, the catalytic activity enhancement may have relationship B with the substituents that is completely opposite to or completely unrelated to relationship A; when the coordination number and the type of metal ion are changed, the catalytic activity enhancement may have relationship C with the substituents that is completely opposite to or completely unrelated to relationships A and B), it is difficult for researchers to simply infer and determine the factors that effectively enhance the performance of complexes.

[0006] In summary, it remains necessary to study metal complexes that are easy to synthesize (high yield), capable of catalyzing the ring-opening polymerization of lactide to obtain high molecular weight PLA, and highly active in catalyzing the ring-opening polymerization of lactide. Summary of the Invention

[0007] The purpose of this invention is to provide a metal complex that is easy to synthesize (high yield), can catalyze the ring-opening polymerization of lactide to obtain high molecular weight PLA, and has high catalytic activity for the ring-opening polymerization of lactide, as well as a method for preparing the metal complex and specific applications of the metal complex.

[0008] To achieve the above objectives, the present invention provides the following four technical solutions.

[0009] In a first aspect, the present invention provides a zinc ketimine complex having the structure shown in Formula I:

[0010]

[0011] In Formula I, R is any one of C2-C4 straight-chain alkyl groups and their derivatives, or C2-C4 branched alkyl groups and their derivatives.

[0012] According to a preferred embodiment of the first aspect, in Formula I, R is any one of a C2-C4 straight-chain alkyl group or a C2-C4 branched-chain alkyl group;

[0013] Furthermore, in formula I, R is one of -CH2CH2-, -CH(CH3)CH2-, -CH2CH(CH3)-, -CH2CH2CH2CH2-, and -CH(CH3)CH2CH2-;

[0014] Furthermore, in Formula I, R is any one of -CH2CH2-, -CH(CH3)CH2-, -(R)-CH2CH(CH3)-, -(S)-CH2CH(CH3)-, -CH2CH2CH2CH2-, -(R)-CH(CH3)CH2CH2-, and -(S)-CH(CH3)CH2CH2-. In this invention, R in -(R)-CH2CH(CH3)- and -(R)-CH(CH3)CH2CH2- refers to the chiral R-type.

[0015] In a second aspect, the present invention provides a method for preparing the zinc ketimine complex provided in the first aspect, wherein the method includes:

[0016] In a protective atmosphere, a ZnEt2 solution was added dropwise to a β-ketoimine ligand solution within a temperature range of 60℃-100℃ to carry out the reaction and obtain the ketoimine zinc complex.

[0017] The β-ketoimine ligand has the structure shown in Formula II:

[0018]

[0019] The R in Equation II is the same as the R in Equation I;

[0020] That is, in Formula II, R is any one of C2-C4 straight-chain alkyl and its derivatives, or C1-C4 branched alkyl and its derivatives; further, R is any one of C2-C4 straight-chain alkyl or C1-C4 branched alkyl; even further, R is one of -CH2CH2-, -CH(CH3)CH2-, -CH2CH(CH3)-, -CH2CH2CH2CH2-, -CH(CH3)CH2CH2-; even further, R is any one of -CH2CH2-, -CH(CH3)CH2-, -(R)-CH2CH(CH3)-, -(S)-CH2CH(CH3)-, -CH2CH2CH2CH2-, -(R)-CH(CH3)CH2CH2- and -(S)-CH(CH3)CH2CH2-.

[0021] According to a preferred embodiment of the second aspect, the molar ratio of ZnEt2 to the β-ketoimine ligand in the β-ketoimine ligand solution is 0.8-1.2:1 (e.g., 1:1).

[0022] According to a preferred embodiment of the second aspect, the solvent in the β-ketoimine ligand solution includes at least one of toluene, hexane, dichloromethane, and tetrahydrofuran.

[0023] According to a preferred embodiment of the second aspect, the solvent in the ZnEt2 solution includes at least one of toluene and hexane.

[0024] According to a preferred embodiment of the second aspect, the reaction time is 30-60 min.

[0025] Thirdly, the present invention provides the application of the zinc ketimine complex provided in the first aspect as a catalyst in the catalytic ring-opening polymerization of lactide.

[0026] According to a preferred embodiment of the third aspect, the application is the use of the zinc ketimine complex as a catalyst in the catalytic ring-opening polymerization of lactide to prepare PLA with a number average molecular weight of 150,000 or more.

[0027] Fourthly, the present invention provides a method for preparing PLA by ring-opening polymerization of lactide, wherein the method includes:

[0028] PLA was prepared by reacting a mixture of lactide monomer and the zinc ketimide complex provided in the first aspect.

[0029] According to a preferred embodiment of the fourth aspect, the lactide monomer and the zinc ketoimine complex provided in the first aspect are mixed at a molar ratio of 5000-50000:1.

[0030] Furthermore, the lactide monomer is mixed with the zinc ketimine complex provided in the first aspect at a molar ratio of 15000-50000:1;

[0031] Furthermore, the lactide monomer is mixed with the zinc ketimine complex provided in the first aspect at a molar ratio of 15,000-40,000:1.

[0032] According to the preferred embodiment of the fourth aspect, the reaction temperature is 80-180°C;

[0033] Furthermore, the reaction temperature is 130-180℃;

[0034] Furthermore, the reaction temperature is 130-150℃.

[0035] According to the preferred embodiment of the fourth aspect, the reaction continues until the powder is completely converted into a gel solid;

[0036] Furthermore, when lactide monomer and zinc ketimine complex are mixed at a molar ratio of 5000-50000:1, the reaction temperature is 80-180℃, and the reaction time is 5s-5min;

[0037] Furthermore, when lactide monomer and zinc ketimine complex are mixed at a molar ratio of 15000-50000:1, the reaction temperature is 130-180℃, and the reaction time is 5s-4min;

[0038] Furthermore, when lactide monomer and zinc ketimine complex are mixed at a molar ratio of 15000-40000:1, the reaction temperature is 130-150℃, and the reaction time is 5s-3min.

[0039] According to a preferred embodiment of the fourth aspect, the lactide monomer includes at least one of L-lactide and racemic lactide.

[0040] According to a preferred embodiment of the fourth aspect, the lactide monomer and the zinc ketoimine complex provided in the first aspect can be reacted under solvent conditions or under solvent-free conditions after mixing.

[0041] Furthermore, the lactide monomer is mixed with the zinc ketoimine complex provided in the first aspect and reacted under solvent-free conditions.

[0042] According to a preferred embodiment of the fourth aspect, the method for preparing PLA by ring-opening polymerization of lactide includes:

[0043] Racemic lactide is mixed with the zinc ketimide complex provided in the first aspect at a molar ratio of 15000-40000:1 and reacted at 130-150℃ to prepare PLA; this preferred technical solution can obtain PLA with a molecular weight greater than 150000 and a racemic lactide conversion rate of more than 80%.

[0044] Further, racemic lactide is mixed with the zinc ketimide complex provided in the first aspect at a molar ratio of 15000-20000:1 and reacted at 130-150°C to prepare PLA; this preferred technical solution can obtain PLA with a molecular weight greater than 150000 and a racemic lactide conversion rate of over 90%.

[0045] The ketimine zinc complex provided by this invention is a tetranuclear zinc complex with zinc as the coordinating metal and β-ketimines of the ligand skeleton having isopropyl substituents on the carbon atoms and C2-C4 alkyl groups or their derivatives as ligands. Compared with the prior art, it has the following advantages:

[0046] 1. The yield of metal complexes provided by existing technologies is generally low, usually reaching only about 60%; while the metal complexes provided by this invention, prepared using the preparation method provided by this invention, can achieve a yield of 99%, far exceeding the existing technologies, and can be used directly without purification, avoiding the economic losses caused by purifying catalysts.

[0047] 2. Existing metal complexes used in the catalytic ring-opening polymerization of lactide to prepare PLA require large quantities and long catalytic times, typically requiring a molar ratio of 1:100-1000 with lactide and a catalytic time of at least 3 minutes. In contrast, the metal complex provided by this invention significantly reduces the amount of lactide used in the catalytic ring-opening polymerization of lactide to prepare PLA and requires extremely short catalytic times, with a molar ratio of 1:15000-50000 with lactide and a catalytic time as short as 5 seconds, demonstrating catalytic activity far exceeding that of existing technologies.

[0048] 3. The metal complexes provided by the prior art, when used to catalyze the ring-opening polymerization of lactide to prepare PLA, result in PLA with relatively small molecular weights, usually not exceeding 100,000; while the metal complexes provided by the present invention, when used to catalyze the ring-opening polymerization of lactide to prepare PLA, result in PLA with significantly increased molecular weights, generally above 150,000, and exceeding 200,000 in preferred embodiments.

[0049] 4. The metal complex provided by this invention uses zinc metal as the coordinating metal. Zinc is inexpensive and readily available, and as one of the trace elements in the human body, it has good biocompatibility, making the polymerization process more green and environmentally friendly. The synthesized polyester material also has less biotoxicity, making it more widely used in biomedicine and other fields.

[0050] 5. The metal complex provided by this invention can directly catalyze the ring-opening polymerization of racemic lactide and L-lactide to obtain high molecular weight PLA with narrow molecular weight distribution under anhydrous, oxygen-free, and inert gas protection, either in bulk or in solution form, without the participation of a co-catalyst. Detailed Implementation

[0051] In order to provide a clearer understanding of the technical features, objectives and beneficial effects of the present invention, the technical solution of the present invention will now be described in detail below, but it should not be construed as limiting the scope of implementation of the present invention.

[0052] Examples 1-6 of this invention used β-ketoimine ligands 1a, 1b, 1c, 1d, 1e, and 1f to prepare zinc ketoimine complexes 2a, 2b, 2c, 2d, 2e, and 2f, respectively. The reaction formulas involved are shown in Formula III.

[0053]

[0054] Example 1

[0055] This embodiment provides a ketimine zinc complex 2a, with the structural formula [(CH3)2CHC(O)CHCN(CH2CH2O)CH(CH3)2]4Zn4, as shown in Formula III 2a. The preparation method includes:

[0056] In an argon atmosphere, 0.40 g (approximately 2.0 mmol) of β-ketoimine ligand 1a (structural formula see formula III) was dissolved in anhydrous and oxygen-free toluene to obtain a β-ketoimine ligand solution. The β-ketoimine ligand solution was heated to 60 °C, and then 1.0 mL of diethylzinc solution (concentration 2 M, solvent toluene, total diethylzinc 2.0 mmol) was added dropwise. The mixture was stirred at 60 °C for 30 min to obtain a colorless and clear solution. The solvent was then removed under vacuum to obtain a white powder, thus completing the preparation of ketoimine zinc complex 2a.

[0057] The products were characterized by nuclear magnetic resonance, elemental analysis, and high-resolution mass spectrometry.

[0058] Yield of ketimine zinc complex 2a: 0.52 g (99%).

[0059] 1 H-NMR (400MHz, CDCl3): δ=4.84(s,4H,C(O)CH=CN),3.97-3.91(m,4H,NCH2CH2O),3.74-3.69(m,4H,NCH2CH2O),3.6-3.55(m,4H,NCH2CH2 O), 3.18-3.11(m,4H,NCH2CH2O), 2.94(sept,J=6.8Hz,4H,Me2CHC(O)), 2.34(sept,J=6.8Hz,4H,Me2CHC(N)), 1.13-1.07(m,48H,Me2CH). 13 C NMR (100MHz, CDCl3): δ=192.2,181.3,86.9,63.7,50.4,39.7,31.2,21.7,21.4,21.1,20.9.

[0060] Elemental analysis: C 44 H 76 Theoretical content of N4O8Zn4 (%): C, 50.30; H, 7.29; N, 5.33; Measured content (%): C, 50.58; H, 7.317; N, 5.13.

[0061] MALDI-TOF HRMS:C 44 H 76 N4O8Zn4+H + The theoretical value is 1045.2902, and the measured value is 1045.2883.

[0062] Example 2

[0063] This embodiment provides a ketimine zinc complex 2b, with the structural formula [(CH3)2CHC(O)CHCN((S)-CH2CH(CH3)O)CH(CH3)2]4Zn4, as shown in Formula III for 2b. The preparation method includes:

[0064] In an argon atmosphere, 0.43 g (approximately 2.0 mmol) of β-ketoimine ligand 1b (structural formula see 1b in Formula III) was dissolved in anhydrous and oxygen-free toluene to obtain a β-ketoimine ligand solution. The β-ketoimine ligand solution was heated to 60 °C, and then 1.0 mL of diethylzinc solution (concentration 2 M, solvent toluene, total diethylzinc 2.0 mmol) was added dropwise. The mixture was stirred at 60 °C for 30 min to obtain a colorless and clear solution. The solvent was then removed under vacuum to obtain a white powder, thus completing the preparation of ketoimine zinc complex 2b.

[0065] The products were characterized by NMR and elemental analysis.

[0066] Yield of ketimine zinc complex 2b: 0.55 g (99%).

[0067] 1 H-NMR (400MHz, CDCl3): δ=4.84(s,2H,C(O)CH=CN), 4.82(s,2H,C(O)CH=CN), 3.99-3.91(m,2H,CH2CHMeO), 3.84-3.76(m,2H,NCH2CHMeO),3.75-3.69(m,2H,CH2CHMeO),3.41-3.27(m,4H,NCH2CHMeO),3.06(sept,J= 6.8Hz,2H,Me2CHC(O)),2.83(sept,J=6.8Hz,2H,Me2CHC(O)),2.56-2.45(m,2H,NCH2CHMeO),2.38-2.24(m ,4H,Me2CHC(N)),1.39(d,J=6Hz,6H,NCH2CHMeO),1.19-0.95(48H,Me2CH),0.84(d,J=6Hz,6H,NCH2CHMeO). 13 C NMR (100MHz, CDCl3): δ=191.2,190.4,181.1,180.7,87.5,87.0,70.4,68. 8,55.7,55.3,39.7,39.2,31.3,31.2,22.5,22.2,21.6,21.3,21.1,20.9.

[0068] Elemental analysis: C 48 H 84 N4O8Zn 44 Theoretical content (%): C, 52.09; H, 7.65; N, 5.06; Measured content (%): C, 52.57; H, 7.690; N, 4.76.

[0069] Example 3

[0070] This embodiment provides a ketimine zinc complex 2c, with the structural formula [(CH3)2CHC(O)CHCN((R)-CH(CH3)CH2CH2O)CH(CH3)2]4Zn4, as shown in Formula III for 2c. The preparation method includes:

[0071] In an argon atmosphere, 0.45 g (approximately 2.0 mmol) of β-ketoimine ligand 1c (structural formula see 1c in Formula III) was dissolved in anhydrous and oxygen-free toluene to obtain a β-ketoimine ligand solution. The β-ketoimine ligand solution was heated to 70 °C, and then 1.0 mL of diethylzinc solution (concentration 2 M, solvent toluene, total diethylzinc 2.0 mmol) was added dropwise. The mixture was stirred at 70 °C for 30 min to obtain a colorless and clear solution. The solvent was then removed under vacuum to obtain a white powder, thus completing the preparation of ketoimine zinc complex 2c.

[0072] The products were characterized by NMR and elemental analysis.

[0073] Yield of ketimine zinc complex 2c: 0.54 g (99%).

[0074] 1 H-NMR (400MHz, CDCl3): δ=4.75(s,4H,C(O)CH=CN), 3.79-3.70(m,4H,NCH(Me)CH2CH2), 3.53-3.23(m,8H,CHMeCH2CH2O), 2.99(sept,J=6. 8Hz,4H,Me2CHC(O)), 2.33(sept,J=6.8Hz,4H,Me2CHC(N)), 2.15-1.80(m,8H,NCH(Me)CH2CH2, 1.20-0.84(m,60H,Me2CH,NCH(Me)CH2CH2). 13 C NMR (100MHz, CDCl3): δ = 181.6, 87.3, 64.7, 48.5, 39.5, 33.3, 32.8, 30.1, 29.8, 21.4, 21.2, 20.2.

[0075] Elemental analysis: C 52 H 92 Theoretical content of N4O8Zn4 (%): C, 53.71; H, 7.98; N, 4.82; Measured content (%): C, 53.83; H, 8.069; N, 4.78.

[0076] Example 4

[0077] This embodiment provides a ketimine zinc complex 2d, with the molecular formula [(CH3)2CHC(O)CHCN(CH2CH2CH2CH2O)CH(CH3)2]4Zn4, and the structural formula is shown in Formula III for 2d. The preparation method includes:

[0078] In an argon atmosphere, 0.45 g (approximately 2.0 mmol) of β-ketoimine ligand 1d (structural formula see Formula III for 1d) was dissolved in anhydrous and oxygen-free toluene to obtain a β-ketoimine ligand solution. The β-ketoimine ligand solution was heated to 70 °C, and then 1.0 mL of diethylzinc solution (concentration 2 M, solvent toluene, total diethylzinc 2.0 mmol) was added dropwise. The mixture was stirred at 70 °C for 60 min to obtain a colorless and clear solution. The solvent was then removed under vacuum to obtain a white powder, thus completing the preparation of ketoimine zinc complex 2d.

[0079] The products were characterized by NMR and elemental analysis.

[0080] 2-day yield of ketimine zinc complex: 0.54 g (99%).

[0081] 1 H-NMR (400MHz, CD2Cl2): δ=4.79(s,4H,C(O)CH=CN),3.79-3.54(m,8H,NCH2CH2CH2CH2O),3.53-3.28(8H,NCH2CH2CH2CH2O),3.02(se pt,J=6.8Hz,4H,Me2CHC(O)), 2.33(sept,J=6.8Hz,4H,Me2CHC(N)), 2.10-1.38(m,16H,NCH2CH2CH2CH2O), 1.16-1.00(m,48H,Me2CH). 13 C NMR (100MHz, CD2Cl2): δ=190.9,182.3,87.8,65.0,48.8,39.8,32.9,30.5,28.7,26.3,25.4,21.4,21.3,20.2.

[0082] Elemental analysis: C 52 H 92 Theoretical content of N4O8Zn4 (%): C, 53.71; H, 7.98; N, 4.82; Measured content (%): C, 53.34; H, 8.008; N, 4.75.

[0083] Example 5

[0084] This embodiment provides a ketimine zinc complex 2e, with the molecular formula [(CH3)2CHC(O)CHCN(CH2CH(CH3)O)CH(CH3)2]4Zn4, and the structural formula is shown in Formula III as 2e. The difference from Example 2 is that the β-ketimine ligand used is a racemic β-ketimine ligand 1e (structural formula shown in Formula III as 1e), and the preparation method includes:

[0085] In an argon atmosphere, 0.43 g (approximately 2.0 mmol) of β-ketoimine ligand 1e (structural formula see 1e in Formula III) was dissolved in anhydrous and oxygen-free toluene to obtain a β-ketoimine ligand solution. The β-ketoimine ligand solution was heated to 60 °C, and then 1.0 mL of diethylzinc solution (concentration 2 M, solvent: toluene Tol, total diethylzinc 2.0 mmol) was added dropwise. The mixture was stirred at 60 °C for 30 min to obtain a colorless and clear solution. The solvent was then removed under vacuum to obtain a white powder, thus completing the preparation of ketoimine zinc complex 2e.

[0086] The product was characterized by nuclear magnetic resonance.

[0087] Yield of ketimine zinc complex 2e: 0.55 g (99%).

[0088] 1 H-NMR (400MHz, CDCl3): δ=4.89-4.79(m,4H,C(O)CH=CN)), 4.34-2.44(m,16H,CH2CHMeO,NC H2CHMeO,Me2CHC(O)), 2.4-2.23(m,4H,Me2CHC(N)), 1.46-0.77(m,60H,Me2CH,NCH2CHMeO).

[0089] Example 6

[0090] This embodiment provides a ketimine zinc complex 2f, with the molecular formula [(CH3)2CHC(O)CHCN(CH(CH3)CH2O)CH(CH3)2]4Zn4, and the structural formula is shown in Formula III for 2f. The preparation method includes:

[0091] In an argon atmosphere, 0.43 g (approximately 2.0 mmol) of β-ketoimine ligand 1f (structural formula see 1f in Formula III) was dissolved in anhydrous and oxygen-free toluene to obtain a β-ketoimine ligand solution. The β-ketoimine ligand solution was heated to 70 °C, and then 1.0 mL of diethylzinc solution (concentration 2 M, solvent toluene, total diethylzinc 2.0 mmol) was added dropwise. The mixture was stirred at 70 °C for 30 min to obtain a colorless and clear solution. The solvent was then removed under vacuum to obtain a white powder, thus completing the preparation of ketoimine zinc complex 2f.

[0092] The product was characterized by nuclear magnetic resonance.

[0093] Yield of ketimine zinc complex 2f: 0.55 g (99%).

[0094] 1H-NMR (400MHz, CDCl3): 4.88-4.63 (m, 4H, C(O)CH=CN), 4.42-3.21 (m, 12H, NCHMeCH2O), 3.14-2 .68(m,4H,Me2CHC(O)),2.35-2.15(m,4H,Me2CHC(N)),1.35-0.83(m,60H,NCHMeCH2O,Me2CH).

[0095] Example 7

[0096] This embodiment provides a ketimine zinc complex with the same structure as the ketimine zinc complex 2a provided in Example 1. The only difference from Example 1 is that hexane is used instead of toluene as the solvent in the β-ketimine ligand solution, and hexane is used instead of toluene as the solvent in the diethylzinc solution.

[0097] The products were characterized by nuclear magnetic resonance, elemental analysis, and high-resolution mass spectrometry.

[0098] The yield of the ketimine zinc complex was the same as in Example 1, reaching 99%.

[0099] The obtained ketimine zinc complex was not substantially different from the elemental analysis results and high-resolution mass spectrometry characterization results of the ketimine zinc complex 2a provided in Example 1.

[0100] Example 8

[0101] This embodiment provides a method for preparing PLA by ring-opening polymerization of lactide, wherein the method includes:

[0102] Under argon protection, racemic lactide monomer (3.0 mmol) and zinc ketimide complex 2a (0.00015 mmol) provided in Example 1 were accurately weighed. Under solvent-free conditions, the two (monomer to zinc ketimide complex molar ratio of 20000:1) were thoroughly stirred in the reactor and heated to 130°C. The reaction system was carefully observed until the solid powder was completely converted into a gel solid, at which point the reaction was stopped. The reaction time was recorded as t = 15 s, and the ring-opening polymerization of lactide to prepare PLA was completed.

[0103] Open the reactor lid, cool it in an ice-water bath, and add a small amount (2 ml) of CH2Cl2 to the reaction mixture until completely dissolved. This yields a sample for performance testing.

[0104] Take a small amount of sample and remove volatiles under vacuum, then... 1 ¹H NMR calculations showed a monomer conversion rate of conv. = 97%.

[0105] The remaining sample solution was subjected to vigorous stirring and a large amount (20 mL) of cold ethanol solution was added to precipitate the crude polymer. The polymer was then separated and washed with a small amount of cold ethanol to remove residual catalyst and oligomers. This washing operation was repeated three times. The polymer was vacuum dried at 60 °C to constant weight to obtain a thin film plastic polymer sample. The molecular weight Mn = 191505 g / mol and the molecular weight distribution PDI = 1.7 were determined by gel permeation chromatography (GPC). The stereoselectivity of the polymer was calculated by isosteric spectroscopy, and its isotacticity Pm = 0.43. See Table 1 for details.

[0106] Example 9

[0107] This embodiment provides a method for preparing PLA by ring-opening polymerization of lactide. The difference between this method and Example 7 is that the zinc ketimide complex 2b (0.0002 mmol) provided in Example 2 is weighed instead of the zinc ketimide complex 2a (0.00015 mmol) provided in Example 1, and the molar ratio of monomer to zinc ketimide complex is 15000:1 instead of 20000:1.

[0108] The reaction time t = 1.5 min, monomer conversion rate conv. = 96%, polymer molecular weight Mn = 220698 g / mol, polymer molecular weight distribution PDI = 1.61, and polymer isotacticity Pm = 0.46. See Table 1 for details.

[0109] Example 10

[0110] This embodiment provides a method for preparing PLA by ring-opening polymerization of lactide. The difference between this method and Example 7 is that: the lactide monomer L-lactide (3.0 mmol) is weighed instead of racemic lactide (3.0 mmol), the ketimine zinc complex 2b provided in Example 2 (0.0002 mmol) is weighed instead of the ketimine zinc complex 2a provided in Example 1 (0.00015 mmol), and the molar ratio of monomer to ketimine zinc complex is 15000:1 instead of 20000:1.

[0111] The reaction time t = 2.1 min, the monomer conversion rate conv. = 91%, the polymer molecular weight Mn = 169279 g / mol, and the polymer molecular weight distribution PDI = 1.89. See Table 1 for details.

[0112] Example 11

[0113] This embodiment provides a method for preparing PLA by ring-opening polymerization of lactide. The difference between this method and Example 7 is that the zinc ketimide complex 2c (0.0002 mmol) provided in Example 3 is weighed instead of the zinc ketimide complex 2a (0.00015 mmol) provided in Example 1, and the molar ratio of monomer to zinc ketimide complex is 15000:1 instead of 20000:1.

[0114] The reaction time t = 1.5 min, monomer conversion rate conv. = 96%, polymer molecular weight Mn = 154268 g / mol, polymer molecular weight distribution PDI = 1.5, and polymer isotacticity Pm = 0.58. See Table 1 for details.

[0115] Example 12

[0116] This embodiment provides a method for preparing PLA by ring-opening polymerization of lactide. The difference between this method and Example 7 is that: the lactide monomer L-lactide (3.0 mmol) is weighed instead of racemic lactide (3.0 mmol), the ketimine zinc complex 2c provided in Example 3 (0.0002 mmol) is weighed instead of the ketimine zinc complex 2a provided in Example 1 (0.00015 mmol), and the molar ratio of monomer to ketimine zinc complex is 15000:1 instead of 20000:1.

[0117] The reaction time t = 1.8 min, the monomer conversion rate conv. = 90%, the polymer molecular weight Mn = 93608 g / mol, and the polymer molecular weight distribution PDI = 1.55. See Table 1 for details.

[0118] Example 13

[0119] This embodiment provides a method for preparing PLA by ring-opening polymerization of lactide. The difference between this method and Example 7 is that the zinc ketimide complex 2d (0.00015 mmol) provided in Example 4 is weighed instead of the zinc ketimide complex 2a (0.00015 mmol) provided in Example 1.

[0120] The reaction time t = 5 s, monomer conversion rate conv. = 94%, polymer molecular weight Mn = 176190 g / mol, polymer molecular weight distribution PDI = 1.85, and polymer isotacticity Pm = 0.46. See Table 1 for details.

[0121] Example 14

[0122] This embodiment provides a method for preparing PLA by ring-opening polymerization of lactide. The difference between this method and Example 7 is that the zinc ketimide complex 2a (0.000075 mmol) provided in Example 1 is weighed instead of the zinc ketimide complex 2a (0.00015 mmol) provided in Example 1, and the molar ratio of monomer to zinc ketimide complex is 40000:1 instead of 20000:1.

[0123] The reaction time t = 3 min, the monomer conversion rate conv. = 83%, the polymer molecular weight Mn = 171704 g / mol, and the polymer molecular weight distribution PDI = 2.11. See Table 1 for details.

[0124] Example 15

[0125] This embodiment provides a method for preparing PLA by ring-opening polymerization of lactide. The difference between this method and Example 7 is that the zinc ketimide complex 2a (0.00006 mmol) provided in Example 1 is weighed instead of the zinc ketimide complex 2a (0.00015 mmol) provided in Example 1, and the molar ratio of the zinc ketimide complex to the monomer is 50000:1 instead of 20000:1.

[0126] The reaction time t = 4 min, the monomer conversion rate conv. = 61%, the polymer molecular weight Mn = 137103 g / mol, and the polymer molecular weight distribution PDI = 2.14. See Table 1 for details.

[0127] Example 16

[0128] This embodiment provides a method for preparing PLA by ring-opening polymerization of lactide, wherein the method includes:

[0129] Under argon protection, racemic lactide monomer (3.0 mmol) and zinc ketimide complex 2a provided in Example 1 (0.0006 mmol) were accurately weighed. The racemic lactide was dissolved in toluene to obtain a 1 M solution. The solution was then thoroughly stirred with zinc ketimide complex 2a (monomer to zinc ketimide complex molar ratio of 5000:1) in a reactor. The temperature was raised to 80 °C. The reaction was stopped when the reaction system was completely converted into a gel solid. The reaction time was recorded as t = 5 min, and the ring-opening polymerization of lactide to prepare PLA was completed.

[0130] The monomer conversion rate conv. = 90%, the polymer molecular weight Mn = 143523 g / mol, and the polymer molecular weight distribution PDI = 1.77 were determined using the same method as in Example 8. See Table 1 for details.

[0131] Table 1

[0132]

[0133]

[0134] Note:

[0135] 1 Monomer conversion rate through 1 H-NMR spectroscopy determination.

[0136] 2 The molecular weight of the polymer was determined by gel permeation chromatography (GPC) using polystyrene as a standard and tetrahydrofuran as the eluent.

[0137] 3 Solvent-free, bulk catalysis was carried out at 130-180℃, and the monomer used was racemic lactide.

[0138] 4 The degree of isotacticity of polymers is determined by isonuclear decoupling. 1 The methyl region was determined by H-NMR spectroscopy.

[0139] 5 The monomer used is L-lactide.

[0140] 6 The initial monomer concentration was 1M, and the monomer used was racemic lactide.

[0141] Table 1 shows that the ketimine zinc complex provided by this invention exhibits superior catalytic activity in the ring-opening polymerization of lactide. The lactide monomer conversion rate is as high as 97%, with a number-average molecular weight of 90,000-220,000 g / mol and a narrow molecular weight distribution. Compared with existing methods for catalyzing the ring-opening polymerization of lactide, the method provided by this invention uses less catalyst and has higher activity in the preparation of polylactide. Furthermore, the ketimine zinc complex provided by this invention has low toxicity and can catalyze lactide polymerization without the need for a co-catalyst. The catalyst synthesis process is simple, with high yield; high-purity catalyst (yield reaching 99%) can be obtained after removing the dry solvent, and can be used directly without purification, reducing the economic losses caused by catalyst purification.

[0142] Comparative Example

[0143] This comparative example provides a comparative experiment on the preparation of complexes using alternative ligands.

[0144] Specifically, tetranuclear zinc complexes were prepared by reacting diethylzinc with β-ketoimine ligands that differed from those used in Examples 1-6 only in that the C atom of the ligand skeleton was tert-butyl instead of isopropyl. The preparation method was similar to that in Examples 1-6. However, no purified tetranuclear zinc complexes could be obtained in any of these cases (i.e., the yield was too low to achieve purification).

[0145] Specifically, tetranuclear zinc complexes were prepared by reacting diethylzinc with β-ketoimine ligands that differed from those used in Examples 1-6 only in that the C atom of the ligand skeleton was not isopropyl but an aromatic phenyl substituent. The preparation method was similar to that in Examples 1-6. However, no purified tetranuclear zinc complexes could be obtained in any of these cases (i.e., the yield was too low to achieve purification).

[0146] Specifically, referring to the method in "Syntheses, structures and catalysis of tetranuclear zinc N-alkoxideketoiminate complexes for ring-opening polymerization of rac-lactide" published in Inorganic Chemistry Communications, 2020, Issue 119, tetranuclear zinc complexes were prepared by reacting a β-ketoimine ligand, which differs from the β-ketoimine ligand used in the ketoimine zinc complex provided in this invention only in that the C atom of the ligand backbone is methyl instead of isopropyl, with diethylzinc. Ultimately, tetranuclear zinc complexes could not be obtained in high yield, with the highest yield reaching only 61%. Furthermore, the catalytic activity of the zinc ketimine complex prepared therefrom in the preparation of PLA via the ring-opening polymerization of lactide is significantly worse than that of the zinc ketimine complex provided in this invention. When used to catalyze the ring-opening polymerization of racemic lactide at 130°C to prepare PLA, the molar ratio of the zinc ketimine complex to lactide obtained therefrom reaches 1:1000, the catalytic time is at least 3 minutes, and the molecular weight of the obtained polymer PLA is 15,000-91,000. In contrast, the molar ratio of the zinc ketimine complex to lactide provided in this invention is 1:15,000-20,000, the catalytic time is only 5 seconds to 1.5 minutes, and the molecular weight of the polymer PLA reaches over 150,000.

Claims

1. A zinc ketimine complex having the structure shown in Formula I: Formula I, In Formula I, R is any one of C2-C4 straight-chain alkyl or C2-C4 branched alkyl.

2. The zinc ketimine complex according to claim 1, wherein, In Formula I, R is one of -CH2CH2-, -CH(CH3)CH2-, -CH2CH(CH3)-, -CH2CH2CH2CH2-, and -CH(CH3)CH2CH2-.

3. The zinc ketimine complex according to claim 2, wherein, In formula I, R is -CH2CH2-, -CH(CH3)CH2-, -( R )-CH2CH(CH3)-、-( S )-CH2CH(CH3)-, -CH2CH2CH2CH2-, -( R )-CH(CH3)CH2CH2- and -( S Any one of )-CH(CH3)CH2CH2-.

4. The method for preparing the zinc ketimine complex according to any one of claims 1-3, wherein, The method includes: In a protective atmosphere, a ZnEt2 solution was added dropwise to a β-ketoimine ligand solution within a temperature range of 60℃-100℃ to carry out the reaction and obtain the ketoimine zinc complex. The β-ketoimine ligand has the structure shown in Formula II: Formula II The R in Equation II is the same as the R in Equation I.

5. The method according to claim 4, wherein, The molar ratio of ZnEt2 to β-ketoimine ligand in the solution is 0.8-1.2:

1.

6. The method according to claim 4, wherein, The solvent in the β-ketoimine ligand solution includes at least one of toluene, hexane, dichloromethane, and tetrahydrofuran; The solvent in the ZnEt2 solution includes at least one of toluene and hexane.

7. The method according to claim 4, wherein, The reaction time is 30-60 min.

8. The use of the zinc ketimine complex according to any one of claims 1-3 as a catalyst in the catalytic ring-opening polymerization of lactide.

9. The application according to claim 8, wherein, The application is the use of the zinc ketimine complex according to any one of claims 1-3 as a catalyst in the catalytic ring-opening polymerization of lactide to prepare PLA with a number average molecular weight of 150,000 or more.

10. A method for preparing PLA by ring-opening polymerization of lactide, wherein, The method includes: PLA is prepared by reacting a mixture of lactide monomer and the zinc ketimine complex according to any one of claims 1-3.

11. The method according to claim 10, wherein, The lactide monomer and the zinc ketimide complex were mixed at a molar ratio of 5000-50000:

1.

12. The method according to claim 11, wherein, The lactide monomer and the zinc ketimide complex were mixed at a molar ratio of 15,000-50,000:

1.

13. The method according to claim 12, wherein, The lactide monomer and the zinc ketimide complex were mixed at a molar ratio of 15,000-40,000:

1.

14. The method of claim 10, wherein, The reaction temperature is 80-180℃.

15. The method according to claim 14, wherein, The reaction temperature is 130-180℃.

16. The method according to claim 15, wherein, The reaction temperature is 130-150℃.

17. The method according to claim 10, wherein, The lactide monomer includes at least one of L-lactide and racemic lactide.

18. The method according to claim 10, wherein, Methods for preparing PLA by ring-opening polymerization of lactide include: PLA was prepared by mixing racemic lactide and zinc ketimide complex at a molar ratio of 15000-40000:1 and reacting at 130-150℃.

19. The method according to claim 18, wherein, PLA was prepared by mixing racemic lactide and zinc ketimide complex at a molar ratio of 15000-20000:1 and reacting at 130-150℃.

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