Ytterbium-based MOF heterogeneous catalyst for ring-opening copolymerization of caprolactone and polyethylene glycol and preparation method of ytterbium-based MOF heterogeneous catalyst

Abstract

The invention belongs to the technical field of heterogeneous catalysts for synthesis of polycaprolactone-polyethylene glycol copolymers, and particularly relates to a lanthanide MOF heterogeneous catalyst for ring-opening copolymerization of caprolactone and polyethylene glycol and a preparation method of the lanthanide MOF heterogeneous catalyst. At present, a lanthanide MOF material is used as a heterogeneous catalyst for catalyzing the ring-opening copolymerization reaction of caprolactone and polyethylene glycol, and no related technology report exists. Aiming at the problems, the invention provides the ytterbium-based MOF heterogeneous catalyst for ring-opening copolymerization of caprolactone and polyethylene glycol, the chemical formula of the ytterbium-based MOF heterogeneous catalyst is [Yb2 (mu3-O) (L) 2 (DMF) 3] n, the catalyst is a heterogeneous catalyst, the catalyst can be separated from a polymer system through a simple solid-liquid separation method after the reaction is finished, and the catalyst can be used as a catalyst for ring-opening copolymerization of caprolactone and polyethylene glycol. The ytterbium-based MOF heterogeneous catalyst has the advantages that the ytterbium-based MOF heterogeneous catalyst is used as a catalyst, the influence of catalyst residues on the safety of biomedical materials is greatly reduced, and in the ring-opening copolymerization reaction of caprolactone and polyethylene glycol, the PCEC copolymer with narrower molecular weight distribution and higher conversion rate can be obtained by adopting the ytterbium-based MOF heterogeneous catalyst.

Description

Technical Field

[0001] This invention belongs to the technical field of heterogeneous catalysts for the synthesis of polycaprolactone-polyethylene glycol copolymers, specifically relating to a ytterbium-based MOF heterogeneous catalyst for the ring-opening copolymerization of caprolactone and polyethylene glycol and its preparation method. Background Technology

[0002] In the fields of pharmaceuticals and biomaterials, aliphatic polyesters have become highly sought-after materials due to their excellent biocompatibility and biodegradability, and are widely used in various pharmaceutical products and bio-related applications. Among them, polycaprolactone (PCL), as a carrier material for drug controlled-release systems, has attracted much research attention in recent years. However, PCL itself has some inherent drawbacks. Its strong hydrophobicity means that its loading capacity for some hydrophilic drugs is limited; at the same time, its slow degradation rate makes it difficult to precisely control the drug release cycle. These shortcomings seriously limit the in-depth application of PCL in the field of drug delivery.

[0003] To improve these properties of PCL, researchers chemically modified it by introducing polyethylene glycol (PEG), successfully constructing an amphiphilic block copolymer, PCL-PEG. This modified material effectively overcomes some of the original defects of PCL, demonstrating broader application potential. PCL-PEG triblock copolymers are mainly divided into two structural types: ABA type (PCL-PEG-PCL, abbreviated as PCEC) and BAB type (PEG-PCL-PEG, abbreviated as PECE).

[0004] The synthesis of PCEC is relatively simple, typically using dihydroxyl-terminated polyethylene glycol as an initiator, and achieving one-step ring-opening polymerization of ε-caprolactone under the action of a catalyst. This method has many advantages, including simple process and precise control of molecular weight and block ratio. More importantly, PCEC has a high drug loading capacity for hydrophobic drugs, making it very suitable for long-acting sustained-release systems and demonstrating unique advantages in drug delivery.

[0005] In the synthesis of PECE, commonly used catalysts include stannous octoate, aluminum alkoxy, and rare earth alkoxy compounds. Among them, stannous octoate is the most widely used in industry. However, these homogeneous catalysts generally suffer from sensitivity to water and air, exhibiting poor stability. During the catalytic ring-opening polymerization of ε-caprolactone, they easily trigger side reactions, broadening the molecular weight distribution of the product and affecting the material's performance and quality. Of particular concern is that stannous octoate, as a homogeneous catalyst, remains in the product after the polymerization reaction, raising concerns about its potential biotoxicity and safety in pharmaceutical applications.

[0006] Studies have shown that, similar to alkoxyaluminum and alkoxyiron, rare earth alkoxy compounds, such as lanthanum, samarium, yttrium, and ytterbium, also exhibit certain catalytic activity in the ring-opening polymerization of cyclic esters such as lactide and ε-caprolactone. Numerous ytterbium complexes containing ytterbium-oxygen, ytterbium-nitrogen, ytterbium-carbon, and ytterbium-hydrogen bonds have been reported in the literature to initiate ε-caprolactone polymerization; however, the resulting polycaprolactones typically have a wide molecular weight distribution (PDI = 1.5–3.3), and the polymerization controllability is generally limited (DM Lyubov, AO Tolpygin, AA Trifonov, Rare-earth metal complexes as catalysts for ring-opening polymerization cyclic esters, Coordination Chemistry Reviews, 2019, 392, 83–145). Xie et al. reported that tris(2,6-di-tert-butyl-4-methylphenol) ytterbium complexes can be used to catalyze the synthesis of mPEG-PCL diblock copolymers (W. Xie; W. Zhu; Z. Shen. Synthesis, isothermal crystallization and micellization of mPEG-PCL diblock copolymers catalyzed by yttrium complex, 2007, 48, 6791–6798). However, such homogeneous catalysts are difficult to completely remove from polymer systems, and their residue problem seriously restricts the further development of biomedical polymer materials such as PCEC.

[0007] On the other hand, in cyclic ester ring-opening copolymerization, the reactivity ratio of different monomers directly affects the sequence structure, molecular weight distribution, and final properties of the copolymer, and is a key parameter for controlling the product composition. However, the reactivity ratio is influenced by multiple factors, including monomer structure, catalyst type, and reaction conditions, making precise control difficult. Traditional catalytic systems often suffer from insufficient activity and limited selectivity, making it difficult to achieve ideal sequence control and efficient polymerization. Therefore, developing heterogeneous catalytic systems that combine high activity, high selectivity, good stability, low toxicity, and easy separation is particularly urgent. Such systems are expected to improve reaction efficiency, enhance sequence controllability, and expand the performance design space of copolymers, which is of great significance for promoting the practical application of biodegradable polymer materials.

[0008] Metal-organic frameworks (MOFs), as novel porous coordination polymers, have shown great potential in the field of catalysis in recent years. They possess unique characteristics such as unsaturated metal active sites, functionalized organic ligands, high specific surface area, and tunable pore structures, exhibiting excellent catalytic activity in various organic transformation reactions, including oxidation, condensation, transesterification, alkylation, and polymerization. For example, zinc-containing polymolybdate MOFs have been proven to efficiently catalyze the ring-opening polymerization of caprolactone, successfully preparing high-molecular-weight polycaprolactone. Rare earth alkoxy compounds have also shown excellent performance in cyclic ester ring-opening polymerization reactions, possessing advantages such as high activity, controllability, and coordination flexibility. However, there are currently no reports in the literature on using rare earth metal-organic frameworks as heterogeneous catalysts for catalyzing the ring-opening polymerization of cyclic esters and the ring-opening copolymerization of caprolactone and polyethylene glycol.

[0009] In summary, developing a novel rare-earth MOF catalyst for the ring-opening block copolymerization of caprolactone and polyethylene glycol is of great practical significance for obtaining PCL-PEG-PCL (PCEC) copolymers with controllable composition and narrow molecular weight distribution, and for promoting the further development and application of this material in the biomedical field. Summary of the Invention

[0010] A problem with existing technologies is that there are currently no reported techniques using lanthanide MOF materials as heterogeneous catalysts for the ring-opening block copolymerization of caprolactone and polyethylene glycol. To address this issue, this invention provides a ytterbium-based MOF heterogeneous catalyst for the ring-opening copolymerization of caprolactone and polyethylene glycol, with the chemical formula [Yb2(μ3-O)(L)2(DMF)3]. n In the chemical formula, L is the organic ligand 4,4'-(carbonylbis(azadiyl))dibenzoate divalent anion, DMF is the solvent N,N-dimethylformamide, Yb is the metal central ion, μ3-O represents the bridging oxygen atom that simultaneously connects to three metal central ions, and n represents the infinite extension of the structural repeating unit Yb2(μ3-O)(L)2(DMF)3.

[0011] Preferably, the secondary structure unit crystal of the ytterbium-based MOF heterogeneous catalyst belongs to the triclinic crystal system, with space group [missing information]. .

[0012] Preferably, the cell parameters of the ytterbium-based MOF heterogeneous catalyst are a=13.719(1) Å, b=14.637(1) Å, c=18.811(1) Å, α=95.1(1)°, β=109.7(1)°, γ=96.4(1)°, cell volume is 3502.2(2) Å3, and Z=2.

[0013] Preferably, the preparation method of the ytterbium-based MOF heterogeneous catalyst for ring-opening copolymerization of caprolactone and polyethylene glycol includes the following steps:

[0014] Ytterbium salt was mixed uniformly with 4,4'-(carbonylbis(azadiyl))dibenzoic acid and N,N-dimethylformyl. Through a solvothermal reaction, ytterbium ions coordinated with organic ligands to assemble and form a three-dimensional porous crystal network structure [Yb2(μ3-O)(L)2(DMF)3]. n .

[0015] Preferably, the temperature of the solvothermal reaction is 40-100℃.

[0016] Preferably, after the solvothermal reaction is completed, the reaction system is cooled to room temperature, and the solid product is collected by solid-liquid separation. The obtained solid product is then passed through deionized water and ethanol in sequence, and then dried to obtain [Yb2(μ3-O)(L)2(DMF)3]. n .

[0017] Preferably, the ytterbium salt includes one or more of ytterbium nitrate, ytterbium chloride, ytterbium acetate, and their hydrates.

[0018] Preferably, the molar ratio of 4,4'-(carbonylbis(azadiyl))dibenzoic acid to ytterbium salt is 1:1 to 1:5.

[0019] A ring-opening copolymerization reaction of caprolactone and polyethylene glycol (PEG) is disclosed, employing the aforementioned ytterbium-based MOF heterogeneous catalyst as the heterogeneous catalyst for the reaction. The ring-opening copolymerization temperature of caprolactone and PEG is 120-180℃. The molar ratio of PEG to caprolactone is 1:50 to 1:200. The amount of ytterbium-based MOF heterogeneous catalyst is determined based on a molar ratio of ytterbium-based MOF heterogeneous catalyst to caprolactone of 1:1000 to 1:10000.

[0020] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0021] (1) This invention solves the problem of residual homogeneous catalysts in existing homogeneous catalysts: Currently, in the catalytic ring-opening copolymerization reaction of caprolactone and polyethylene glycol, commonly used homogeneous catalysts such as stannous octoate are sensitive to water and air, have poor stability, are prone to side reactions that broaden the molecular weight distribution of the product, and are difficult to completely remove from the polymer system. The residual catalyst may have potential biotoxicity, affecting the safety of the material in pharmaceutical applications. The ytterbium-based MOF heterogeneous catalyst prepared in this invention can effectively avoid this problem. Due to its heterogeneous characteristics, it can be separated from the polymer system by a simple solid-liquid separation method after the reaction, which greatly reduces the impact of catalyst residue on the safety of biomedical materials and provides a strong guarantee for the safe application of biomedical polymer materials such as PCEC in the pharmaceutical field;

[0022] (2) Currently, there are no reports on the use of ytterbium-based MOF materials as heterogeneous catalysts for the ring-opening copolymerization reaction of caprolactone and polyethylene glycol. This invention provides for the first time a ytterbium-based MOF heterogeneous catalyst that can be used for this reaction, filling a gap in this technical field and providing a new catalytic approach and method for this type of reaction.

[0023] (3) The ytterbium-based MOF heterogeneous catalyst of the present invention has a definite chemical formula [Yb2(μ3-O)(L)2(DMF)3] n The components have clear functions, and the secondary structural unit crystal belongs to the triclinic crystal system with well-defined cell parameters, including a = 13.719(1) Å, b = 14.637(1) Å, c = 18.811(1) Å, α = 95.1(1)°, β = 109.7(1)°, γ = 96.4(1)°, cell volume of 3502.2(2) ų, and Z = 2. The ytterbium-based MOF heterogeneous catalyst obtained in this invention has a definite crystal structure and chemical composition, belongs to the triclinic crystal system space group, has high structural stability, and is easy to separate and recover from the reaction system as a heterogeneous catalyst, which is beneficial to reducing catalyst residue in the product and may realize the recycling of the catalyst, which is in line with the development concept of green chemistry.

[0024] (4) The preparation method of this ytterbium-based MOF heterogeneous catalyst is simple and easy to operate. After the ytterbium salt is mixed evenly with 4,4'-(carbonylbis(azadiyl))dibenzoic acid and N,N-dimethylformamide, the ytterbium ion can be coordinated with the organic ligand through a solvothermal reaction to assemble and form a three-dimensional porous crystal network structure [Yb2(μ3-O)(L)2(DMF)3]. n The solvothermal reaction temperature is between 40-100℃, and the reaction conditions are relatively mild, making it easy to achieve industrial production. Furthermore, after the reaction, a pure catalyst product can be obtained through simple solid-liquid separation, washing, and drying steps, further improving the preparation efficiency.

[0025] (5) In the ring-opening copolymerization reaction of caprolactone and polyethylene glycol, the ytterbium-based MOF heterogeneous catalyst of the present invention can be used to obtain PCEC block copolymers with narrower molecular weight distribution and higher conversion rate. Attached Figure Description

[0026] Figure 1 This invention relates to the self-made 4,4'-(carbonylbis(azadiyl))dibenzoic acid. 1 H NMR spectrum.

[0027] Figure 2 : This is a diagram of the coordination environment of ytterbium ions in the ytterbium-based metal-organic framework catalyst obtained in Example 1 of this invention.

[0028] Figure 3 The structure of the ytterbium-based metal-organic framework catalyst obtained in Example 1 is a two-dimensional network structure diagram.

[0029] Figure 4 : This is the PXRD spectrum of the ytterbium-based metal-organic framework catalyst obtained in Example 1.

[0030] Figure 5 : is the TG curve of the ytterbium-based metal-organic framework catalyst obtained in Example 1.

[0031] Figure 6 : is the polycaprolactone-polyethylene glycol triblock copolymer obtained by application 1. 1 H NMR spectrum.

[0032] Figure 7 : This is a graph showing the molecular weight and molecular weight distribution of the polycaprolactone-polyethylene glycol copolymer obtained by application 1. Detailed Implementation

[0033] The present invention will be described in detail below with reference to embodiments. However, it should be understood that the following embodiments are merely illustrative examples of implementation of the present invention and are not intended to limit the scope of the present invention.

[0034] The 4,4'-(carbonylbis(azadiyl))dibenzoic acid (CAS: 1234-27-1) used in the following embodiments of the present invention can be commercially available or prepared according to the following method:

[0035] (1) Starting with 4-aminobenzoic acid (1.4 g, 10 mmol), it was added to a 100 mL three-necked flask with 15 mL of anhydrous ethanol. After dissolving by stirring in an ice-water bath, thionyl chloride (2 mL) was slowly added dropwise, and stirring was continued for 2 hours. Then, the mixture was heated under reflux for 6 hours to obtain a clear solution. After concentration, the product was dissolved in ethyl acetate, washed with saturated sodium bicarbonate aqueous solution (100 mL), dried over anhydrous sodium sulfate, and the solvent was removed by distillation to obtain a white solid ethyl 4-aminobenzoate (compound I).

[0036] (2) Subsequently, compound I (1.7 g, 10 mmol) was dissolved in 25 mL of tetrahydrofuran, and 1,1'-carbonyldiimidazole (0.9 g, 6 mmol) was added. The mixture was refluxed overnight under nitrogen protection. After cooling to room temperature, the solvent was removed by distillation. The resulting solid was dissolved in ethyl acetate and washed three times each with hydrochloric acid and saturated brine. The solid was dried over anhydrous sodium sulfate and the ethyl acetate was evaporated to obtain white crystals of diethyl 4,4'-(carbonylbis(azadiyl))dibenzoate (compound II).

[0037] (3) Finally, a methanol solution (55 mL) of compound II (3.6 g, 10 mmol) and potassium hydroxide (4.0 g, 72 mmol) was added to a round-bottom flask and refluxed for 6 hours with mechanical stirring, resulting in a white precipitate. After cooling, the solvent was evaporated, and the white oily residue was dissolved in 100 mL of deionized water. After stirring until clear, hydrochloric acid was slowly added dropwise under ice bath conditions to adjust the pH to 1-2. The white solid was collected by filtration, washed three times with distilled water, and dried under vacuum to obtain the target product 4,4'-(carbonylbis(azadiyl))dibenzoic acid, with a yield of approximately 83%.

[0038] The 4,4'-(carbonylbis(azadiyl))dibenzoic acid prepared by the above method was characterized by the following NMR spectrometry:

[0039] Instruction manual attached Figure 1 The nuclear magnetic resonance hydrogen spectrum of the self-made 4,4'-(carbonylbis(azadiyl))dibenzoic acid of this invention ( 1 ¹H NMR. The characteristic signal at δ=12.65 (single peak, 2H) corresponds to the active hydrogen atom on the carboxylic acid in 4,4'-(carbonylbis(azadiyl))dibenzoic acid; the characteristic signal at δ=9.18 (single peak, 2H) corresponds to the two hydrogen atoms on the urea functional group in 4,4'-(carbonylbis(azadiyl))dibenzoic acid; and the characteristic signal at δ=7.57~7.91 corresponds to the hydrogen atoms on the benzene ring in 4,4'-(carbonylbis(azadiyl))dibenzoic acid. The test conditions were: ¹H NMR (300 MHz, DMSO-d6), acquired using an AVANCE NEO 300 MHz NMR spectrometer from Bruker GmbH, Germany.

[0040] Example 1

[0041] A method for preparing a ytterbium-based MOF heterogeneous catalyst (also known as a ytterbium-based metal-organic framework material) for the ring-opening copolymerization of caprolactone and polyethylene glycol is as follows:

[0042] Ytterbium nitrate pentahydrate (44.9 mg, 0.1 mmol) and the organic ligand compound 4,4'-(carbonylbis(azadiyl))dibenzoic acid (15.0 mg, 0.05 mmol) were dissolved in 4 mL of N,N-dimethylformamide. The mixture was transferred to a 10 mL pressure-resistant single-necked glass flask and sealed. After sonication for 5 minutes to obtain a clear solution, the reaction system was subjected to a solvothermal reaction at 80 °C for 72 hours. Subsequently, the reaction system was slowly cooled to room temperature at a cooling rate of 5 °C per hour. After the reaction was completed, colorless transparent crystals were obtained. These crystals were then dried in air with deionized water and ethanol to finally obtain the ytterbium-based metal-organic framework catalyst (molecular formula: C). 39 H 41 N7O 14 Yb2) 33.3 mg (86% yield based on 4,4'-(carbonylbis(azadiyl))dibenzoic acid organic ligand).

[0043] The ytterbium-based metal-organic framework material obtained in Example 1 was characterized as follows:

[0044] (1) Crystal structure determination:

[0045] The crystal structure of the ytterbium-based metal-organic framework material obtained in Example 1 was determined using a Bruker Apex II CCD diffractometer at 293(2) K. Diffraction points were collected in ω-scan mode using graphite-monochromated MoKα rays (λ=0.71073 Å). The collected data were restored using the SAINT program and semi-empirical absorption correction was performed using the SADABS method. Structural analysis and refinement were performed using the SHELXS and SHELXL programs of the SHELXTL program, respectively. The coordinates and anisotropic parameters of all non-hydrogen atoms were obtained by correcting F2 using the full matrix least squares method. During the structural refinement process, all hydrogen atoms were theoretically fixed on the parent atom and given isotropic displacement parameters that were slightly larger than the parent atom displacement parameters (C–H, 1.2 or O–H, 1.5 times). Detailed crystal measurement data are shown in Table 1. (Instrument model: Bruker SMART APEX II, Bruker GmbH, Germany).

[0046] Table 1 , Analysis of the data in Table 1 shows that the asymmetric unit of the ytterbium-based metal-organic framework material obtained in Example 1 contains two crystallographically independent ytterbium(III) ions, two L2 anions with completely deprotonated carboxyl groups, one μ3-O2 anion, and three coordinated DMF molecules. (See attached specification) Figure 2 This is a coordination environment diagram of ytterbium ions in the ytterbium-based metal-organic framework material obtained in Example 1 of the present invention. (See attached specification.) Figure 2 As shown, both Yb1 and Yb2 are octahedral with a decahedral coordination configuration. Yb1 is coordinated with carboxyl oxygen atoms from four ligands (O1, O2, O5#1, O6, O7, and O10#2), an oxygen atom in one coordinated DMF (O13), and an oxygen atom in μ3-O2 (O14), with Yb–O bond lengths ranging from 2.203(5) to 2.486(4) Å. Yb2 is coordinated with carboxyl oxygen atoms from four ligands (O1#3, O4#1, O7, and O9#4), oxygen atoms in two coordinated DMFs (O11 and O12), and an oxygen atom in μ3-O2 (O14), with Yb–O bond lengths ranging from 2.278(4) to 2.348(4) Å.

[0047] Instruction manual attached Figure 3 The ytterbium-based metal-organic framework material obtained in Example 1 has a two-dimensional network structure. (See attached specification.) Figure 3 As shown, ytterbium ions form tetranuclear secondary unit structures through ligand carboxyl oxygen atoms (O1 and O7) and μ3-O2 oxygen atoms. Adjacent tetranuclear secondary unit structures are bridged by ligands, ultimately forming a two-dimensional bilayer structure.

[0048] (3) Characterization of phase purity:

[0049] Instruction manual attached Figure 4 This is the X-ray powder diffraction (PXRD) pattern of the ytterbium-based metal-organic framework material obtained in Example 1. The figure shows a series of sharp characteristic diffraction peaks at 2θ≈5.0°, 6.1°, 6.9°, 7.4°, and 9.9°, which are in high agreement with the theoretical spectrum obtained through single-crystal structure simulation. This result confirms that the ytterbium-based metal-organic framework material possesses high crystallinity and phase purity, providing a structural guarantee for its reliable application in the catalytic ring-opening copolymerization of ε-caprolactone and polyethylene glycol to prepare polycaprolactone-polyethylene glycol block copolymers. The tests were performed using a Rigaku D / Max-2500 diffractometer (Japan).

[0050] (4) Thermal stability characterization:

[0051] Instruction manual attached Figure 5The thermogravimetric (TG) curves of the ytterbium-based metal-organic framework material obtained in Example 1 are shown. The results indicate that this coordination polymer framework exhibits high thermal stability, showing no significant decomposition before 275°C, suggesting that it can maintain structural stability at higher temperatures, meeting the catalyst stability requirements of the ring-opening copolymerization reaction temperature between ε-caprolactone and polyethylene glycol. The tests were performed using a NETZSCH TG 209 F3 thermogravimetric analyzer (NETZSCH AG, Germany).

[0052] Example 2 is the same as Example 1, except that the same molar amount of ytterbium acetate tetrahydrate is used instead of ytterbium nitrate pentahydrate in Example 1. The yield of the ytterbium-based metal-organic framework material obtained in Example 2 is 68%.

[0053] Example 3 is the same as Example 1, except that the same molar amount of ytterbium chloride hexahydrate is used in Example 3 instead of ytterbium nitrate pentahydrate. The yield of the ytterbium-based metal-organic framework material obtained in Example 3 is 72%.

[0054] Example 4 is the same as Example 1, except that the solvothermal reaction temperature in Example 4 is 40°C. The yield of the ytterbium-based metal-organic framework material obtained in Example 4 is 41%.

[0055] Example 5 is the same as Example 1, except that the solvothermal reaction temperature in Example 5 is 100°C. The yield of the ytterbium-based metal-organic framework material obtained in Example 5 is 83%.

[0056] Example 6 is the same as Example 1, except that the solvothermal reaction time in Example 6 is 12 hours. The yield of the ytterbium-based metal-organic framework material obtained in Example 6 is 45%.

[0057] Example 7 is the same as Example 1, except that the solvothermal reaction time in Example 7 is 96 hours. The yield of the ytterbium-based metal-organic framework material obtained in Example 7 is 85%.

[0058] Example 8 is the same as Example 1, except that the amount of N,N-dimethylformamide used in Example 8 is 2 ml. The yield of the ytterbium-based metal-organic framework material obtained in Example 8 is 53%.

[0059] Example 9 is the same as Example 1, except that the amount of N,N-dimethylformamide used in Example 9 is 6 ml. The yield of the ytterbium-based metal-organic framework material obtained in Example 9 is 82%.

[0060] Example 10 is the same as Example 1, except that the amount of ytterbium nitrate pentahydrate used in Example 10 is 0.05 mmol. The yield of the ytterbium-based metal-organic framework material obtained in Example 10 is 54%.

[0061] Example 11 is the same as Example 1, except that the amount of ytterbium nitrate pentahydrate used in Example 10 is 0.25 mmol. The yield of the ytterbium-based metal-organic framework material obtained in Example 11 is 69%.

[0062] Comparative Example 1 is the same as Example 1, except that the same molar amount of zinc nitrate hexahydrate was used to replace ytterbium nitrate pentahydrate in Comparative Example 1. Comparative Example 1 yielded an amorphous powder, which could not be subjected to X-ray single-crystal diffraction analysis.

[0063] Comparative Example 2 is the same as Example 1, except that the same molar amount of copper nitrate trihydrate was used in Comparative Example 2 to replace the ytterbium nitrate pentahydrate in Example 1. Comparative Example 2 yielded a blue amorphous powder with an unknown structure, which could not be analyzed by X-ray single-crystal diffraction.

[0064] Comparative Example 3 is the same as Example 1, except that the same volume of deionized water was used in Comparative Example 3 to replace N,N-dimethylformamide in Example 1. Comparative Example 3 yielded an amorphous powder with an unknown structure, which could not be analyzed by X-ray single-crystal diffraction.

[0065] Comparative Example 4 is the same as Example 1, except that the same volume of anhydrous methanol was used in Comparative Example 4 instead of N,N-dimethylformamide in Example 1. Comparative Example 4 yielded an amorphous powder with an unknown structure, which could not be analyzed by X-ray single-crystal diffraction.

[0066] Specific applications

[0067] Application 1

[0068] The ytterbium-based MOF heterogeneous catalyst obtained in Example 1 was used as a catalyst for the ring-opening copolymerization reaction of ε-caprolactone and polyethylene glycol to prepare polycaprolactone-polyethylene glycol copolymer. The specific synthesis steps are as follows:

[0069] 3424.80 mg ε-caprolactone (CAS No.: 502-44-3) (30.00 mmol), 1800.00 mg polyethylene glycol 6000 (0.30 mmol), and 6.45 mg ytterbium-based MOF heterogeneous catalyst were added to a Schlenk reaction tube and reacted at 150 °C with magnetic stirring for 36 hours. After the reaction was completed, the mixture was cooled to room temperature, dissolved in dichloromethane, and petroleum ether was added to the filtrate to precipitate the copolymer. The copolymer was then dried under vacuum at 40 °C for 12 hours to obtain a white polycaprolactone-polyethylene glycol copolymer, denoted as PCEC-1, with a weight-average molecular weight of 13091 and a molecular weight distribution index of 1.07.

[0070] The polycaprolactone-polyethylene glycol copolymer obtained by application 1 was characterized as follows:

[0071] (1) Determination of ε-caprolactone conversion rate

[0072] The polymer after reaction was dissolved in deuterated chloroform, with tetramethylsilane as an internal standard. The content of ε-caprolactone was then determined by 1H NMR spectroscopy to obtain the corresponding conversion rate (instrument model: Bruker Avance 400 MHz).

[0073] Instruction manual attached Figure 6 The figure shows the 1H NMR spectrum of the polycaprolactone-polyethylene glycol triblock copolymer obtained by application 1. As can be seen from the figure, the signal at δ=3.65 is the characteristic peak of PEG methylene, and the peaks at δ=4.04, 2.31, 1.68 and 1.36 correspond to the -CH2- protons on the PCL segments, respectively.

[0074] (2) Molecular weight and molecular weight distribution index test:

[0075] The molecular weight and distribution of the polycaprolactone-polyethylene glycol copolymer obtained in Application 1 were determined by gel permeation chromatography (GPC). The determination conditions were as follows: polystyrene as the standard sample, tetrahydrofuran as the mobile phase, RID-10A differential refractive index detector, flow rate 0.6 mL / min, run time 40 min, injection volume 20 μL, and column temperature 30 °C. The tests were performed using a Shimadzu LC-20AD gel permeation chromatograph (Japan).

[0076] Instruction manual attached Figure 7 The graph shows the molecular weight and molecular weight distribution of the polycaprolactone-polyethylene glycol copolymer obtained in Example 1. As can be seen from the graph, the number-average molecular weight of PCEC-1 is 12235, the weight-average molecular weight is 13091, and the molecular weight distribution is relatively narrow, only 1.07. This strongly demonstrates that the ytterbium-based MOF heterogeneous catalyst obtained in Example 1 is a highly efficient and controllable "quasi-active" polymerization system. It not only has high catalytic efficiency, but more importantly, it achieves precise molecular-level control over the polymerization process, enabling the preparation of polymers with highly uniform molecular weights. This is usually the goal pursued by homogeneous metal-organic catalysts, and ytterbium-based metal-organic framework materials achieve this in a heterogeneous form, which is an important manifestation of their potential as a new generation of high-performance catalysts.

[0077] Application 2 is the same as Application 1, except that the reaction temperature for the ring-opening copolymerization of caprolactone and polyethylene glycol in Application 2 is 120℃, and the reaction time is 72 hours. The relevant test results of the polycaprolactone-polyethylene glycol copolymer (denoted as PCEC-2) obtained in Application 2 are shown in Table 2.

[0078] Application 3 is the same as Application 1, except that the reaction temperature for the ring-opening copolymerization of caprolactone and polyethylene glycol in Application 3 is 180℃, and the reaction time is 6 hours. The relevant test results of the polycaprolactone-polyethylene glycol copolymer (denoted as PCEC-3) obtained in Application 3 are shown in Table 2.

[0079] Application 4 is the same as Application 1, except that the amount of caprolactone used in Application 4 is 15.00 mmol. The relevant test results of the polycaprolactone-polyethylene glycol copolymer (PCEC-4) obtained in Application 4 are shown in Table 2.

[0080] Application 5 is the same as Application 1, except that the amount of caprolactone used in Application 5 is 60.00 mmol. The relevant test results of the polycaprolactone-polyethylene glycol copolymer (denoted as PCEC-5) obtained by Application 5 are shown in Table 2.

[0081] Application 6 is the same as Application 1, except that the amount of ytterbium-based metal-organic framework material used in Application 6 is 0.0012 mmol. The relevant test results of the polycaprolactone-polyethylene glycol copolymer (denoted as PCEC-6) obtained by Application 6 are shown in Table 2.

[0082] Application 7 is the same as Application 1, except that the amount of ytterbium-based metal-organic framework material used in Application 7 is 0.012 mmol. The relevant test results of the polycaprolactone-polyethylene glycol copolymer (denoted as PCEC-7) obtained in Application 7 are shown in Table 2.

[0083] Comparative Application 1 is the same as Application 1, except that the catalyst used for the ring-opening copolymerization of caprolactone and polyethylene glycol in Comparative Application 1 is ytterbium nitrate pentahydrate, and the molar amount of ytterbium nitrate pentahydrate is the same as that of the ytterbium-based MOF heterogeneous catalyst in Application 1. The relevant test results of the polycaprolactone-polyethylene glycol copolymer (denoted as PCEC-8) obtained in Comparative Application 1 are shown in Table 2.

[0084] Comparative Application 2 is similar to Application 1, except that the catalyst used for the ring-opening copolymerization of caprolactone and polyethylene glycol in Comparative Application 2 is 4,4'-(carbonylbis(aziridyl))benzoic acid. The molar amount of 4,4'-(carbonylbis(aziridyl))benzoic acid is the same as that of the ytterbium-based MOF heterogeneous catalyst in Application 1. The relevant test results of the polycaprolactone-polyethylene glycol copolymer (denoted as PCEC-9) obtained in Comparative Application 2 are shown in Table 2.

[0085] Comparative Application 3 is the same as Application 1, except that the catalyst used for the ring-opening copolymerization of caprolactone and polyethylene glycol in Comparative Application 3 is stannous octoate, and the molar amount of stannous octoate is the same as the molar amount of the ytterbium-based MOF heterogeneous catalyst in Application 1. The relevant test results of the polycaprolactone-polyethylene glycol copolymer (denoted as PCEC-10) obtained in Comparative Application 3 are shown in Table 2.

[0086] Comparative Application 4 is the same as Application 1, except that the catalyst used for the ring-opening copolymerization of caprolactone and polyethylene glycol in Comparative Application 4 is zinc lactate, and the molar amount of zinc lactate is the same as that of the ytterbium-based MOF heterogeneous catalyst in Application 1. The relevant test results of the polycaprolactone-polyethylene glycol copolymer (denoted as PCEC-11) obtained in Comparative Application 4 are shown in Table 2.

[0087] Comparative Application 5 is the same as Application 1, except that the catalyst used for the ring-opening copolymerization of caprolactone and polyethylene glycol in Comparative Application 5 is ytterbium isopropoxide (CAS: 6742-69-4, Aladdin Reagent (Shanghai) Co., Ltd.). The molar amount of ytterbium isopropoxide is the same as that of the ytterbium-based MOF heterogeneous catalyst in Application 1. The relevant test results of the polycaprolactone-polyethylene glycol copolymer (denoted as PCEC-12) obtained in Comparative Application 5 are shown in Table 2.

[0088] Comparative Application 6 is the same as Application 1, except that the catalyst used for the ring-opening copolymerization of caprolactone and polyethylene glycol in Comparative Application 6 is [Mo2O4(μ2-OH)2(Htrz)] (synthesized according to Example 1 described in Chinese Invention Patent CN114015070A). The molar amount of [Mo2O4(μ2-OH)2(Htrz)] is the same as the molar amount of the ytterbium-based MOF heterogeneous catalyst in Application 1. The relevant test results of the polycaprolactone-polyethylene glycol copolymer (denoted as PCEC-13) obtained in Comparative Application 6 are shown in Table 2.

[0089] Table 2 , The test data in Table 2 show that the molecular weight of the polycaprolactone-polyethylene glycol copolymer increases with increasing synthesis conditions (reaction time and temperature), and its dispersion is positively correlated with its molecular weight. The molecular weight of the polycaprolactone-polyethylene glycol copolymer is also related to the monomer structure and catalyst type. Among various catalyst types, the ytterbium-based MOF heterogeneous catalyst obtained in this invention exhibits the best catalytic effect. Using ytterbium nitrate pentahydrate or 4,4'-(carbonylbis(azadiyl))dibenzoic acid alone as a catalyst results in significantly lower activity and extremely low weight-average molecular weight of the obtained copolyester. Compared with traditional stannous octoate, zinc lactate, or ytterbium isopropoxide, the ytterbium-based MOF heterogeneous catalyst developed in this invention has significantly higher catalytic activity, effectively improving polymerization efficiency and product molecular weight. Simultaneously, it demonstrates excellent control over the monomer reactivity ratio during copolymerization, enabling more precise sequence control, thereby obtaining copolymers with narrower molecular weight distribution and more regular structures. The test results from Application 6 show that although the heterogeneous catalyst suitable for ring-opening homopolymerization still exhibits certain catalytic activity in ring-opening block copolymerization and achieves a high ε-caprolactone conversion rate, its ability to regulate the copolymerization reactivity ratio is relatively limited. The resulting copolymers have a wide molecular weight distribution, and the overall molecular weight of the polyester products is relatively low. This further illustrates that catalysts suitable for ε-caprolactone homopolymerization systems may not be well-suited to the requirements of ring-opening block copolymerization.

[0090] Based on the above-described preferred embodiments of the present invention, and through the foregoing description, those skilled in the art can make various changes and modifications without departing from the inventive concept. The technical scope of this invention is not limited to the contents of the specification, but must be determined according to the scope of the claims.

Claims

1. A ytterbium-based MOF heterogeneous catalyst for the ring-opening copolymerization of caprolactone and polyethylene glycol, characterized in that, The chemical formula is [Yb2(μ3-O)(L)2(DMF)3] n In the chemical formula, L is the organic ligand 4,4'-(carbonylbis(azadiyl))dibenzoate divalent anion, DMF is the solvent N,N-dimethylformamide, Yb is the metal central ion, μ3-O represents the bridging oxygen atom that simultaneously connects to three metal central ions, and n represents the infinite extension of the structural repeating unit Yb2(μ3-O)(L)2(DMF)3.

2. The ytterbium-based MOF heterogeneous catalyst for the ring-opening copolymerization of caprolactone and polyethylene glycol according to claim 1, characterized in that, The secondary structure unit crystal of the ytterbium-based MOF heterogeneous catalyst belongs to the triclinic crystal system, with space group . .

3. The ytterbium-based MOF heterogeneous catalyst for the ring-opening copolymerization of caprolactone and polyethylene glycol according to claim 2, characterized in that, The cell parameters of the ytterbium-based MOF heterogeneous catalyst are a = 13.719(1) Å, b = 14.637(1) Å, c = 18.811(1) Å, α = 95.1(1)°, β = 109.7(1)°, γ = 96.4(1)°, and the cell volume is 3502.2(2) Å. 3 Z=2.

4. A ytterbium-based MOF heterogeneous catalyst for the ring-opening copolymerization of caprolactone and polyethylene glycol according to any one of claims 1-3, characterized in that, The preparation method includes the following steps: Ytterbium salt was mixed with 4,4'-(carbonylbis(azadiyl))dibenzoic acid and N,N-dimethylformyl. Through a solvothermal reaction, ytterbium ions coordinated with organic ligands to assemble and form a three-dimensional porous crystal network structure [Yb2(μ3-O)(L)2(DMF)3]n.

5. The ytterbium-based MOF heterogeneous catalyst for the ring-opening copolymerization of caprolactone and polyethylene glycol according to claim 4, characterized in that, The temperature for solvothermal reactions is 40-100℃.

6. The ytterbium-based MOF heterogeneous catalyst for the ring-opening copolymerization of caprolactone and polyethylene glycol according to claim 4, characterized in that, After the solvothermal reaction is completed, the reaction system is cooled to room temperature, and the solid product is collected by solid-liquid separation. The obtained solid product is washed with deionized water and ethanol in sequence, and then dried to obtain [Yb2(μ3-O)(L)2(DMF)3]n.

7. The ytterbium-based MOF heterogeneous catalyst for the ring-opening copolymerization of caprolactone and polyethylene glycol according to claim 4, characterized in that, Ytterbium salts include one or more of ytterbium nitrate, ytterbium chloride, ytterbium acetate, and their hydrates.

8. The ytterbium-based MOF heterogeneous catalyst for the ring-opening copolymerization of caprolactone and polyethylene glycol according to claim 4, characterized in that, The molar ratio of 4,4'-(carbonylbis(azadiyl))dibenzoic acid to ytterbium salt is 1:1 to 1:

5.

9. A ring-opening copolymerization reaction of caprolactone and polyethylene glycol, characterized in that, The ytterbium-based MOF heterogeneous catalyst according to any one of claims 1-3 is used as the heterogeneous catalyst for the ring-opening copolymerization reaction of caprolactone and polyethylene glycol.

10. A ring-opening copolymerization reaction of caprolactone and polyethylene glycol, characterized in that, The amount of ytterbium-based MOF heterogeneous catalyst used is determined based on the molar ratio of ytterbium-based MOF heterogeneous catalyst to caprolactone being 1:1000 to 1:10000.

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