A method for preparing a cellulose grafting polycaprolactone copolymer by using DBU-based ionic liquid as catalyst

Abstract

The application discloses a method for preparing a cellulose grafting polylactone copolymer by using DBU-based ionic liquid as a catalyst, which simultaneously serves as a solvent and a catalyst for the cellulose grafting lactone copolymerization reaction. Under the condition of nitrogen protection, a certain amount of cellulose is dissolved in DBU-based ionic liquid, lactone monomers are added into the obtained cellulose solution, and the solution is reacted at a certain temperature for a period of time, and then is cooled to room temperature, purified and dried to obtain the grafting polymer. The preparation method is green and environmentally friendly, simple and easy to operate, the DBU-based ionic liquid is stable in nature, and the ionic liquid serves as a solvent for dissolving cellulose and a catalyst for the cellulose grafting lactone copolymerization reaction in the reaction, so that the cellulose grafting lactone copolymerization reaction is realized under homogeneous conditions, the grafting rate of the obtained copolymer is high, and the method provides a green catalytic method for preparing the cellulose grafting polylactone copolymer.

Description

Technical Field

[0001] This invention belongs to the field of polymer material synthesis technology, and relates to a method for preparing cellulose-grafted polylactone copolymers by DBU-based ionic liquid catalysis. Background Technology

[0002] Cellulose, as the most abundant and widely distributed natural polymer material, can serve as a green and environmentally friendly alternative to petroleum-based polymers in some fields. Furthermore, its molecular chain contains numerous modifiable hydroxyl groups, allowing for the creation of cellulose derivatives with varying chemical properties through modification of side-chain hydroxyl groups. However, due to the numerous hydrogen bonds inherent in cellulose, the intermolecular hydrogen bond network is too strong, making it insoluble in common organic solvents, thus severely limiting homogeneous modification of cellulose. In heterogeneous reactions, chemical reagents struggle to penetrate the crystalline regions of cellulose, resulting in random and uneven reaction sites throughout the process. Currently known cellulose dissolving systems include LiCl / polar solvent systems and N-methylmorpholine oxide, which can provide homogeneous conditions for cellulose modification under suitable circumstances, but also suffer from drawbacks such as high toxicity, difficulty in recycling, and instability.

[0003] Lactone monomers are widely available, and their polymers exhibit excellent biodegradability and bioabsorption, enabling recycling in nature and making them ideal green polymer materials. Grafting lactone polymers onto cellulose hydroxyl groups yields biodegradable cellulose materials. Cellulose-grafted polylactone copolymers obtained through homogeneous modification of cellulose have a wide range of applications. Grafting polylactic acid (PLA) onto cellulose produces modified cellulose that exhibits high biodegradability, making it a starting material for bioplastics and other industries, and possessing good potential as a drug delivery carrier (DOI: 10.1016 / j.colsurfb.2008.05.007).

[0004] Ionic liquids, as novel green solvents, are increasingly being used in the dissolution of cellulose due to their excellent properties such as tunable cation and anion balance and low vapor pressure. In current reports, imidazole-based ionic liquids are the most commonly used solvents for homogeneous cellulose modification. In the preparation of cellulose-grafted polylactone copolymers under existing homogeneous conditions, ionic liquids only serve as solvents and do not affect or participate in the cellulose grafting polymerization reaction. Under the homogeneous reaction conditions provided by ionic liquids, additional metals (stannous octoate, 2-ethyltin acetate, etc.) or non-metallic organic compounds (organic acids, DMAP, etc.) need to be added as catalysts for the cellulose grafting cyclic ester monomer reaction (grafting cellulose fibers with poly(ε-caprolactone) and poly(L-lactic acid) via ring-opening polymerization, DOI: 10.1021 / bm060178z; Surface grafting of microfibrillated cellulose with poly(ε-caprolactone) - Synthesis and characterization, DOI: 10.1016 / j.eurpolymj.2008.06.023; Synthesis of novel biocomposite materials by grafting polycaprolactone with microfibers - Effect of grafting length on mechanical properties, DOI: 10.1021 / am2001828;). This catalytic method requires the addition of a catalyst after cellulose dissolution, and the purification process involves the removal of both the ionic liquid and the catalyst, making the operation complex. Furthermore, metal catalysts are difficult to remove, which limits the application range of cellulose graft products to some extent. Ionic liquids, besides being green solvents, also possess other multifunctional properties. M. Lahcini's group reported the controlled ring-opening polymerization of CL catalyzed by imidazole ionic liquids in the presence of benzyl alcohol; Song's group reported the ring-opening polymerization of LA using a hydroxyl-functionalized ionic liquid (HEMIMB) as a catalyst without the addition of an initiator. These reports demonstrate that certain specific ionic liquids can initiate the ring-opening polymerization of cyclic ester monomers while simultaneously acting as an initiator for alcohol hydroxyl groups.

[0005] In existing research, ionic liquids mostly act as solvents in cellulose graft copolymerization and do not participate in the reaction. Based on the above research background, it is proposed that ionic liquids can act as both solvents and catalysts for the homogeneous reaction of cellulose and the copolymerization of cellulose grafted polylactones. This method can give full play to the multifunctionality of ionic liquids and has great potential in cellulose graft copolymerization. Summary of the Invention

[0006] The purpose of this invention is to provide a method for preparing cellulose-grafted polylactone copolymers using DBU-based ionic liquid catalysis. The DBU-based ionic liquid serves as both a solvent and a catalyst in the homogeneous cellulose reaction system to catalyze the preparation of cellulose-grafted polylactone copolymers.

[0007] To achieve the above objectives, the present invention adopts the following solution: A method for preparing cellulose-grafted polylactone copolymers catalyzed by a DBU-based ionic liquid, wherein the [DBU]-based ionic liquid dissolves cellulose and simultaneously catalyzes the homogeneous grafting of cyclic ester monomers onto cellulose, the specific steps of which are as follows: Cellulose was dissolved in a [DBU]-based ionic liquid under vacuum at 80-100°C to obtain a clear cellulose solution. A cyclic ester monomer was added to the cellulose solution, and the reaction was continued under vacuum at 80-100°C for 5-12 hours. After cooling to room temperature, the crude product obtained from the reaction was dissolved in chloroform and purified with a methanol solution acidified with hydrochloric acid to obtain a mixture of polyester and cellulose-grafted polyester. The cellulose-grafted polylactone was then separated by dissolving it in an organic solvent and dried under vacuum to obtain the target product.

[0008] The mass ratio of cellulose to [DBU]-based ionic liquid is 1:10; the molar ratio of cellulose to cyclic ester monomer is 1:5 to 1:50; and the cyclic ester monomer is caprolactone or lactide.

[0009] The cation of [DBU]-based ionic liquids is: The anions are four different acid radicals: formate [Fac], acetate [Aac], propionate [Pac], or lactate [Lac]. .

[0010] The chemical structural formula of cellulose is: One of the repeating units contains three -OH groups, and their reactivity order is C6-OH > C2-OH > C3-OH.

[0011] The chemical structural formula of the cellulose-grafted polylactone copolymer is: ,in Or the chemical structural formula of cellulose-grafted polylactone copolymer ,in .

[0012] The reaction process involved in this invention does not require the addition of an additional catalyst. The [DBU]-based ionic liquid acts as a green solvent for dissolving cellulose and also as a catalyst for the copolymerization of cellulose grafted cyclic ester monomers. The cellulose grafting polymerization reaction is a homogeneous system.

[0013] In summary, the synthesis method of this invention uses a [DBU]-based ionic liquid as both a solvent and a catalyst for cellulose grafting of cyclic ester monomers, eliminating the need for additional catalysts. Ionic liquids, as environmentally friendly solvents, possess numerous advantages, including low vapor pressure, good stability, excellent designability, and tunable anions and cations. They can provide favorable homogeneous reaction conditions for cellulose graft polymerization and also act as catalysts for the ring-opening grafting of cyclic ester monomers into cellulose graft polymers. This achieves both the dual functionality of ionic liquids and efficient graft polymerization. Attached Figure Description

[0014] Figure 1 This invention relates to a comparative diagram of ionic liquids, ionic liquid-mixed cellulose, and ionic liquid-dissolved cellulose.

[0015] Figure 2 The image shows the 1H NMR spectrum of cellulose-g-polycaprolactone prepared in Example 1.

[0016] Figure 3 This is a comparison of the infrared spectra of the copolymer and monomer prepared in Example 1 of the present invention.

[0017] Figure 4 This is a SEM image of the original cellulose that has not undergone graft copolymerization.

[0018] Figure 5 This is a SEM image of the poly(cellulose-g-polycaprolactone) prepared in Example 2.

[0019] Figure 6 This is a SEM image of the poly(cellulose-g-polycaprolactone) prepared in Example 3.

[0020] Figure 7 This is a combination diagram comparing the changes in XRD patterns of MCC-g-PCL and monomers obtained in Examples 1-4.

[0021] Figure 8 This is a comparison chart of the changes in DSC of MCC-g-PCL obtained in Examples 1-4.

[0022] Figure 9 The contact angle test diagrams are for MCC-g-PCL with different grafting parameters obtained in Examples 1-4.

[0023] Figure 10 The 1H NMR spectrum of the cellulose-g-polylactide prepared in Example 5.

[0024] Figure 11 The images show an infrared comparison between the MCC-g-PLA obtained in Examples 6 and 7 and the monomer.

[0025] Figure 12This is a composite XRD diagram comparing Examples 5 and 7 with the original cellulose. Detailed Implementation

[0026] The present invention will be further described below with reference to the accompanying drawings and specific embodiments.

[0027] Example 1 Under vacuum, cellulose and [DBU][Pac] ionic liquid were weighed at a mass ratio of 1:10. Cellulose was dissolved in [DBU][Pac] ionic liquid at 100°C to obtain a clear cellulose solution. Then, caprolactone (CL) was added to the clear cellulose solution at a molar ratio of cellulose to caprolactone of 1:10. Without adding any catalyst, the reaction was carried out under vacuum at 100°C for 11 hours. After cooling to room temperature, the crude product obtained from the reaction was dissolved in chloroform and purified with a methanol solution acidified with hydrochloric acid. The supernatant was poured off, and the remaining solvent was evaporated in a fume hood to obtain a mixed product of polycaprolactone and cellulose-grafted polycaprolactone. The mixed product was then dissolved in dichloromethane, stirred, filtered, and the undissolved product on the filter paper was dried in a vacuum drying oven at 60°C for 12 hours to obtain a cellulose-grafted polycaprolactone copolymer, which was named MCC-g-PCL1.

[0028] The synthetic route is as follows: ; Figure 1 The images show the ionic liquid [DBU][Pac] used in Example 1, the mixture of [DBU][Pac] and cellulose, and the actual photographs of [DBU][Pac] dissolving cellulose. This demonstrates that the ionic liquid can effectively dissolve cellulose at a certain temperature, providing homogeneous reaction conditions for subsequent graft copolymerization.

[0029] Figure 2 The 1H NMR spectrum of poly(cellulose-g-polycaprolactone) prepared in Example 1 1 (H-NMR, d6-DMSO) plot.

[0030] Figure 3 The image shows a comparison of the Fourier Transform Infrared (FTIR) spectra of poly(cellulose-g-polycaprolactone) and pure cellulose prepared in Example 1.

[0031] Depend on Figure 2 and Figure 3 The results showed that the polymer obtained in Example 1 was a graft copolymer of cellulose and polycaprolactone, rather than a mixture of homopolymers of the two.

[0032] Example 2 In a vacuum atmosphere, cellulose and [DBU][Pac] ionic liquid were prepared at a mass ratio of 1:10. Cellulose was dissolved in the ionic liquid at 100°C to obtain a clear cellulose solution. Then, caprolactone (CL) was added to the clear cellulose solution at a molar ratio of 1:20 (cellulose to caprolactone). Without adding any catalyst, the reaction was carried out in a vacuum atmosphere at 100°C for 11 hours. After cooling to room temperature, the crude product was dissolved in chloroform and purified with a methanol solution acidified with hydrochloric acid. The supernatant was discarded, and the remaining solvent was mixed in a fume hood to obtain a mixture of polycaprolactone and cellulose-grafted polycaprolactone. The mixture was then dissolved in dichloromethane, stirred, filtered, and the undissolved product on filter paper was dried in a vacuum drying oven at 60°C for 12 hours to obtain the corresponding cellulose-grafted polycaprolactone copolymer, named MCC-g-PCL2. The synthetic route and 1H NMR spectrum are the same as in Example 1.

[0033] Figure 4 The image shows the morphology of the original cellulose in SEM. Figure 5 This is a SEM image of the poly(cellulose-g-polycaprolactone) prepared in Experimental Example 2. Figure 4 It can be seen that the untreated raw cellulose exhibits a regular and very smooth rod-shaped structure in SEM; Figure 5 The graft copolymer modified with PCL side chains had a very rough surface and irregular protrusions in SEM, indicating that the structural characteristics of cellulose were changed after the cellulose side chains were modified with PCL, which destroyed the high crystallinity of the original cellulose. In addition, PCL itself has strong hydrophobicity, and the introduction of PCL side chains to modify the cellulose surface will improve the hydrophobicity of cellulose itself.

[0034] Example 3 Under vacuum, cellulose and [DBU][Pac] ionic liquid were prepared at a mass ratio of 1:10. The cellulose was dissolved in the ionic liquid at 100°C to obtain a clear cellulose solution. Then, caprolactone (CL) was added to the clear cellulose solution at a molar ratio of 1:30 (cellulose to caprolactone). Without adding any catalyst, the reaction was carried out under vacuum at 100°C for 11 hours. After cooling to room temperature, the crude product was dissolved in chloroform and purified with a methanol solution acidified with hydrochloric acid. The supernatant was discarded, and the remaining solvent was mixed in a fume hood to obtain a mixture of polycaprolactone and cellulose-grafted polycaprolactone. The mixture was then dissolved in dichloromethane, stirred, filtered, and the undissolved product on filter paper was dried overnight at 60°C in a vacuum drying oven to obtain the corresponding cellulose-grafted polycaprolactone copolymer, named MCC-g-PCL3. The synthetic route and 1H NMR spectrum are the same as in Example 1.

[0035] Figure 6 SEM image of poly(cellulose-g-polycaprolactone) prepared in Example 3; for comparison Figure 5 and Figure 6 The two groups of MCC-g-PCL surfaces with different grafting ratios show that the longer the PCL grafted side chains, the better the cellulose grafting effect, the rougher the surface of the copolymer, and the better the hydrophobicity of the copolymer.

[0036] Example 4 In a vacuum atmosphere, cellulose and [DBU][Fac] ionic liquid were prepared at a mass ratio of 1:10. Cellulose was dissolved in the ionic liquid at 100°C to obtain a clear cellulose solution. Then, caprolactone (CL) was added to the clear cellulose solution at a molar ratio of 1:40 (cellulose to caprolactone). Without adding any catalyst, the reaction was carried out in a vacuum atmosphere at 100°C for 11 hours. After cooling to room temperature, the crude product was dissolved in chloroform and purified with a methanol solution acidified with hydrochloric acid. The supernatant was discarded, and the remaining solvent was mixed in a fume hood to obtain a mixture of polycaprolactone and cellulose-grafted polycaprolactone. The mixture was then dissolved in dichloromethane, stirred, filtered, and the undissolved product on the filter paper was dried overnight at 60°C in a vacuum drying oven to obtain the corresponding cellulose-grafted polycaprolactone copolymer MCC-g-PCL4. The synthetic route and 1H NMR spectrum are the same as in Example 1. Figure 7 The image shows a comparison of the XRD changes of the cellulose-grafted polycaprolactone copolymers obtained in Examples 1-4 and the unmodified original cellulose. As can be seen from the image, after the polycaprolactone side chain is modified into cellulose, the high crystallinity of cellulose itself is significantly improved, and the graft copolymer exhibits a disordered and chaotic state inside.

[0037] Figure 8 The image shows a DSC comparison of the cellulose-grafted polycaprolactone copolymers obtained in Examples 1-4 and the unmodified original cellulose. The high crystallinity of the cellulose after [DBU][Pac] dissolution and PCL modification is destroyed, and it undergoes a glass transition at a given temperature, becoming easier to process.

[0038] Figure 9 The figures show the contact angle test results of MCC-g-PCL with different grafting parameters obtained in Examples 1-4. After grafting PCL side chains, the high hydrophilicity of the original cellulose is improved. With the increase of grafting parameters, the water contact angle of MCC-g-PCL can reach more than 90°, exhibiting hydrophobicity. Because PCL has a certain degree of hydrophobicity, grafting PCL onto cellulose can improve its own high hydrophilicity.

[0039] Example 5 In a vacuum atmosphere, cellulose and [DBU][Pac] ionic liquid were prepared at a mass ratio of 1:10. Cellulose was dissolved in the ionic liquid at 100°C to obtain a clear cellulose solution. Subsequently, lactide monomer was weighed and added to the cellulose solution at a molar ratio of cellulose to lactide of 1:5. Without the addition of an additional catalyst, the reaction was carried out under vacuum at 100°C for 9 hours. After cooling to room temperature, the mixture was washed with anhydrous ethanol to remove unreacted lactide LA monomer and ionic liquid, resulting in a mixture of MCC-g-PLA and PLA. The mixture was dissolved in acetone and precipitated with anhydrous methanol. After discarding the supernatant, the product was dried in a vacuum drying oven for 48 hours to finally obtain the pure grafted product MCC-g-PLA1.

[0040] The synthetic route is as follows: Figure 10 The 1H NMR spectrum of cellulose-g-polylactide prepared in Example 5 ( 1 The 1H-NMR (d6-DMSO) spectrum shows that the polymer obtained in Example 5 is a graft copolymer of cellulose and polylactide, rather than a mixture of homopolymers of the two.

[0041] Example 6 In a vacuum atmosphere, cellulose and [DBU][Pac] ionic liquid were prepared at a mass ratio of 1:10. Cellulose was dissolved in the ionic liquid at 100°C to obtain a clear cellulose solution. Then, lactide monomer was weighed at a molar ratio of cellulose to lactide of 1:10 and added to the cellulose solution. Without adding any catalyst, the reaction was carried out in a vacuum atmosphere at 100°C for 9 hours. After cooling to room temperature, the mixture was washed with anhydrous ethanol to remove unreacted LA monomer and ionic liquid, yielding a mixture of MCC-g-PLA and PLA. The mixture was dissolved in acetone and precipitated with anhydrous methanol. After discarding the supernatant, the product was dried in a vacuum drying oven for 48 hours, finally yielding a pure cellulose and polylactide graft copolymer, MCC-g-PLA2. The synthetic route and 1H NMR spectrum are the same as in Example 5.

[0042] Example 7 In a vacuum atmosphere, cellulose and [DBU][Pac] ionic liquid were prepared at a mass ratio of 1:10. Cellulose was dissolved in the ionic liquid at 100°C to obtain a clear cellulose solution. Then, lactide monomer was weighed at a molar ratio of cellulose to lactide of 1:15 and added to the cellulose solution. Without adding any catalyst, the reaction was carried out in a vacuum atmosphere at 100°C for 9 hours. After cooling to room temperature, the mixture was washed with anhydrous ethanol to remove unreacted LA monomer and ionic liquid, yielding a mixture of MCC-g-PLA and PLA. The mixture was dissolved in acetone and precipitated with anhydrous methanol. After discarding the supernatant, the product was dried in a vacuum drying oven for 48 hours to finally obtain the pure grafted product MCC-g-PLA3. The cellulose grafting synthesis route and 1H NMR spectrum are the same as in Example 5.

[0043] Figure 11 The infrared comparison images of MCC-g-PLA obtained in Examples 6 and 7 and the original cellulose prove that the obtained MCC-g-PLA is a pure substance rather than a blend of MCC and PLA. The images also show that the wavelengths are 2987-2815 cm⁻¹. -1 The interval represents the absorption vibration peak of -CH2- in the alkyl chain of cellulose. With the introduction of the side chain PLA, the peak at this position shows a split peak, and the higher the degree of PLA modification, the more obvious the split peak becomes.

[0044] Figure 12 The XRD comparison images of Examples 5 and 7 with the original cellulose show that after dissolution by [DBU][Pac] ionic liquid and modification by PLA side chains, the high crystallinity of cellulose is destroyed, and it is transformed into an amorphous product, which becomes easier to process.

Claims

1. A method for preparing cellulose-grafted polylactone copolymers catalyzed by DBU-based ionic liquids, characterized in that: Cellulose was dissolved in a [DBU]-based ionic liquid under vacuum and at 80-100°C to obtain a clear cellulose solution. A cyclic ester monomer was added to the cellulose solution, and the reaction was continued under vacuum and at 80-100°C for 5-12 hours. After cooling to room temperature, the crude product obtained from the reaction was dissolved in chloroform and purified with a methanol solution acidified with hydrochloric acid to obtain a mixed product of polyester and cellulose-grafted polylactone. The cellulose-grafted polylactone was then separated by dissolving it in an organic solvent and dried under vacuum to obtain the target product. The cation of the [DBU]-based ionic liquid is: The anion is: ; This method does not require the addition of an extra catalyst. The [DBU]-based ionic liquid acts as both a green solvent for dissolving cellulose and a catalyst for the copolymerization of cellulose-grafted cyclic ester monomers.

2. The method for preparing cellulose-grafted polylactone copolymers catalyzed by DBU-based ionic liquids according to claim 1, characterized in that: The mass ratio of cellulose to [DBU]-based ionic liquid is 1:

10.

3. The method for preparing cellulose-grafted polylactone copolymers catalyzed by DBU-based ionic liquids according to claim 1, characterized in that: The molar ratio of cellulose to cyclic ester monomer is 1:5 to 1:

50.

4. The method for preparing cellulose-grafted polylactone copolymers catalyzed by DBU-based ionic liquids according to claim 1, characterized in that: The cyclic ester monomer is caprolactone or lactide.

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