Unidirectional depolymerization, high-purity and high-yield epsilon-CL closed-loop recycling method

Abstract

The invention belongs to the technical field of chemical recycling, and particularly relates to a one-way depolymerization, high-purity and high-yield epsilon-CL closed-loop recycling method. According to the method, only trace acetylacetone metal salt is used as a catalyst, the epsilon-caprolactone monomer is recovered in a manner of in-situ catalytic degradation of PCL (polycaprolactone) under the assistance of heating and temperature rise in a low-vacuum environment, the depolymerization efficiency of 100% is realized, and the problems of low depolymerization yield, easiness in carbonization, long time, low purity of recovered products and the like in an existing method are solved. The product obtained by the method disclosed by the invention is pure, does not need complex post-treatment such as secondary purification, water removal and the like, has reaction activity of 100% repolymerization, can be directly used for repolymerization reaction to obtain regenerated PCL, and has the tensile strength of 31.3 MPa and the elongation at break of 831%. According to the method disclosed by the invention, polymer-monomer-polymer closed-loop circulation of PCL is perfectly realized by a mild depolymerization reaction process, and the possibility of industrial depolymerization-polymerization is expected to be realized.

Description

Technical Field

[0001] The invention belongs to the technical field of chemical cycle recovery, and in particular relates to a method for unidirectional depolymerization, high-purity and high-yield ε-CL closed-loop recovery and reuse. Background Art

[0002] At present, traditional petroleum-based disposable plastic products are difficult to degrade naturally after being discarded, which has brought tremendous pressure to the environment. Many researchers have devoted themselves to the research of degradable plastics, but their high cost, high carbon emissions, and difficulty in degradation under any natural environmental conditions have always been a problem that only treats the symptoms but not the root cause. In order to follow the principle of sustainable circulation, it is expected that plastic products can be depolymerized into reactive small molecules through chemical recycling, and the small molecules can be repolymerized to form a closed-loop recycling life cycle. Then, a new strategy is provided for the recycling and pollution problems of discarded plastic products, and it can effectively realize the recycling and utilization of high-value chemical resources and realize an energy-saving social model.

[0003] Polycaprolactone (PCL) is a non-toxic and harmless biopolyester with good biocompatibility, excellent shape memory and processability. It has broad prospects in the biomedical field, such as surgical sutures and medical dressings. However, PCL often needs to be prepared by ring-opening polymerization of ε-caprolactone (ε-CL), and the high cost of obtaining ε-CL makes it impossible to promote and use PCL on a large scale due to high cost and other issues. In addition, PCL products will be degraded by environmental influences after a period of use. When the molecular weight is lower than 30,000 g / mol, the mechanical properties deteriorate and cannot be used. At this time, if the ester bonds with lower bond energy in the lower molecular weight PCL can be effectively controlled to break in a controlled manner, and reactive ε-CL monomers or oligomers (dimers, trimers) can be efficiently generated, it can not only effectively recycle waste PCL products, avoid the high carbon emissions and difficulty in natural degradation of waste PCL, maximize the value of PCL, but also reduce the production cost of ε-CL and save chemical resources. The strategy of killing two birds with one stone shows the great potential of polycaprolactone for closed-loop recycling.

[0004] Su Jiafeng et al. from Qingdao University of Science and Technology respectively studied M nHigh-temperature thermal cracking of 2000-50000 g / mol PCL was performed. The higher the molecular weight of PCL, the longer the depolymerization time. It took up to 26 hours for 50000 g / mol PCL to achieve an 89% yield (Polymer Chemistry, 2022); Hui Zhang et al. successfully achieved catalytic depolymerization of PCL at 160°C under the action of ionic liquid [BMIm]Br-[BMIm]2[CuBr4] (3:1) to obtain 74.9% γ-CL (ACSSustainable Chemistry&Engineering, 2024); CN202311078227.3 used cyclometallated ruthenium nitroso complex as a catalyst, and achieved a depolymerization efficiency of 97% in 4 hours under the conditions of 0.01-0.02 MPa and 220°C. However, the above studies have the following limitations: only PCL with a molecular weight below 10,000 g / mol can achieve a high depolymerization efficiency, and most PCL degradation studies have problems such as low depolymerization yield and easy carbonization; the depolymerization efficiency of PCL is not high, the time required is long, and the purity of the recovered product is low. In addition to the caprolactone monomer, there are also small molecular acids, water and a large number of oligomers. These impurities seriously affect the repolymerization performance of ε-CL; the existing catalyst system is relatively complex. In addition to catalytic depolymerization, it will also cause the generated ε-CL monomers or oligomers (dimers, trimers) to undergo in situ polymerization, causing the depolymerization-polymerization reaction to enter a dead cycle, prolonging the depolymerization time and increasing energy consumption; the exploration of the repolymerization of the depolymerization products has not been realized, and whether the regenerated PCL of the secondary polymerization can have excellent mechanical properties has not been realized.

[0005] Based on this, it is of far-reaching significance to seek a simple and efficient in situ catalytic depolymerization method to quickly achieve 100% depolymerization efficiency and high-purity ε-CL recovery, and directly use the depolymerization recovery product to prepare regenerated PCL with excellent performance. Summary of the invention

[0006] In order to overcome the above-mentioned shortcomings and deficiencies of the prior art, the primary purpose of the present invention is to provide a method for unidirectional depolymerization, high purity, high yield ε-CL closed-loop recovery and reuse.

[0007] The method of the present invention utilizes heating and temperature-raising catalytic in-situ thermal cracking to achieve rapid and efficient 100% depolymerization of PCL. The final product has high purity and has 100% repolymerization reaction activity without the need for complex post-treatment such as secondary purification and water removal. It has the advantages of unidirectional high efficiency and high yield.

[0008] Another object of the present invention is to provide a depolymerization product recovered based on the above method.

[0009] Another object of the present invention is to provide a use of the above depolymerization product in repolymerization to form regenerated PCL.

[0010] Another object of the present invention is to provide a method for regenerating PCL based on the above-mentioned depolymerization product.

[0011] Another object of the present invention is to provide regenerated PCL prepared by the above method.

[0012] The purpose of the present invention is achieved through the following solutions:

[0013] A method for unidirectional depolymerization, high purity and high yield ε-CL closed-loop recycling and reuse, using acetylacetone metal salts [M(acac) n ] as a catalyst, and recovering the ε-caprolactone monomer by heating and assisting in-situ catalytic degradation of polycaprolactone (PCL); specifically, after mixing polycaprolactone with the catalyst, heating the polycaprolactone to depolymerize it and obtain the ε-caprolactone monomer.

[0014] Furthermore, the acetylacetonate metal salt may include at least one of nickel acetylacetonate, iron acetylacetonate, magnesium acetylacetonate, zinc acetylacetonate, copper acetylacetonate, and the like.

[0015] Furthermore, the amount of the catalyst can be a catalytic amount, such as 0.001-5% of the mass of PCL; in one embodiment, the amount of the catalyst is 0.125% of the mass of PCL; in another embodiment, the amount of the catalyst is 2% of the mass of PCL; in yet another embodiment, the amount of the catalyst is 5% of the mass of PCL.

[0016] The invention uses only a trace amount of acetylacetone metal salt as a catalyst, which has excellent catalytic activity and can achieve 100% depolymerization effect when the amount used is as low as 0.125% of the polyester mass.

[0017] Furthermore, the heating temperature may be 180-280°C.

[0018] Furthermore, the method of the present invention can achieve 100% depolymerization of PCL, and the depolymerization time can be controlled based on the amount of PCL added. Furthermore, the reaction time can be 0.5-5h.

[0019] Furthermore, the reaction is preferably carried out at a system pressure of 0.01-0.1 MPa.

[0020] Furthermore, during the reaction, the system is preferably continuously subjected to vacuum negative pressure treatment.

[0021] Furthermore, the molecular weight M of the PCL n It can be 1000-100000 g / mol.

[0022] Furthermore, the extracted small molecules are collected by cooling to obtain depolymerization products such as ε-CL or oligomers.

[0023] Furthermore, the cooling temperature may be 0-15°C.

[0024] The method of the present invention uses acetylacetone metal salts as catalysts and utilizes heating to assist catalytic thermal cracking to achieve a 100% depolymerization efficiency of polycaprolactone. The method of the present invention can cover the depolymerization of PCL with different molecular weights and can achieve excellent depolymerization effects, solving the problems of the existing methods, such as low PCL depolymerization yield, easy carbonization, long time required, low purity of the recovered product, and the presence of small molecular acids, water, and a large number of oligomers in addition to the caprolactone monomer.

[0025] The present invention also provides a depolymerization product recovered based on the above method. The product recovered by the depolymerization method of the present invention is pure, does not contain any impurities such as small molecule acids, does not require secondary purification, water removal and other complex post-treatments, has 100% repolymerization reaction activity, can be directly used for repolymerization, and the reaction obtains regenerated PCL.

[0026] The present invention also provides the use of the above depolymerization product in repolymerization to form regenerated PCL. The depolymerization product recovered by the depolymerization method of the present invention has 100% repolymerization reaction activity, and is directly used for repolymerization to form regenerated PCL. The reaction conditions are simple, the polymerization is fast, and the regenerated PCL obtained has excellent performance.

[0027] The present invention also provides a method for forming regenerated PCL based on the above-mentioned depolymerization product repolymerization, which specifically comprises the following steps: mixing the recovered depolymerization product with a catalyst, polymerizing by staged heating, and obtaining regenerated PCL.

[0028] Specifically, the staged temperature-raising polymerization refers to preheating the reaction first and then heating the reaction to obtain the regenerated PCL.

[0029] Furthermore, the preheating temperature may be 120-160° C. The preheating reaction time may be 5-16 hours.

[0030] Furthermore, the temperature of the temperature-raising reaction may be 180-250° C. The time of the temperature-raising reaction may be 0.5-3 h.

[0031] Furthermore, the temperature rising reaction can be terminated when an obvious pole climbing phenomenon occurs.

[0032] Furthermore, the catalyst can be a polymerization reaction catalyst conventionally used in the art, such as metal carboxylates and chlorides (such as stannous isooctanoate, alkyl aluminum complexes, ferric chloride, etc.), non-metallic catalysts imidazole heterocyclic carbenes, etc.; the amount of the catalyst used can be a catalytic amount, such as 0.1-2 mol.% of the depolymerization product.

[0033] Furthermore, the above-mentioned repolymerization reaction is preferably carried out under an inert atmosphere.

[0034] The repolymerization method of the present invention first slowly ring-opening polymerizes at a mild temperature to form a prepolymer with a lower molecular weight, and then increases the temperature to accelerate the ring-opening or condensation reaction to form a high-molecular PCL, directly achieving 100% repolymerization, and the formed regenerated PCL has excellent mechanical properties.

[0035] The present invention also provides the regenerated PCL prepared by the above method. The regenerated PCL obtained by repolymerizing the depolymerized product recovered by the depolymerization method of the present invention has mechanical properties comparable to those of commercial PCL, with a tensile strength of up to 31.3 MPa and an elongation at break of up to 831%.

[0036] This method is a unidirectional depolymerization, high-purity and high-yield ε-CL closed-loop recovery and reuse method. It is simple to operate and takes a short time. It has the characteristics of high efficiency, unidirectional depolymerization, and extremely high selectivity for ε-CL. It perfectly realizes the cycle of PCL from "polymer-monomer-polymer" with a mild depolymerization reaction process, and is expected to realize the possibility of industrial depolymerization-polymerization.

[0037] Compared with the prior art, the present invention has the following advantages:

[0038] (1) In the present invention, acetylacetone metal salt [M(acac) n ] as a catalyst, through the coordination of metal ions with the hydroxyl oxygen of PCL in the system, proton transfer removes the free acetylacetone compound to form an intermediate ligand, which nucleophilically attacks the adjacent carbonyl oxygen in the PCL molecular chain and coordinates to insert into the PCL molecular chain. Under the action of the acetylacetone ligand and the metal, the carbon-oxygen bond of the ester bond with lower bond energy is broken, releasing monomers or oligomers, and at the same time deprotonating caprolactone and M(acac) + Reduction elimination generates new ligands to continue the above process. Therefore, the method of the present invention can cover the depolymerization of PCL with different molecular weights, and can achieve excellent depolymerization effects. The lower the molecular weight of the applicable PCL, the higher its depolymerization efficiency.

[0039] (2) The acetylacetone metal salt catalyst used in the present invention is a directional, unidirectional depolymerization catalyst. There is no problem of repolymerization during the depolymerization process, and there is no dynamic equilibrium process of depolymerization-polymerization. Under the action of the catalyst of the present invention, a 100% depolymerization efficiency of PCL can be finally achieved. It has the characteristics of high efficiency, rapid depolymerization, short time, and simple operation, and is expected to achieve the promotion of industrial depolymerization. Secondly, the product recovered by the depolymerization method of the present invention has extremely high purity, does not contain any impurities such as small molecular acids, and is all ε-CL or a small amount of oligomers (dimers, trimers). The product obtained by depolymerization has a 100% repolymerization reaction activity without the need for complex post-treatment such as secondary purification and water removal.

[0040] (3) The present invention directly repolymerizes the depolymerized recovered product. The results show that the preparation of regenerated PCL with a number average molecular weight of up to 78873 g / mol can be achieved, the tensile strength can reach 31.3 MPa, and the elongation at break is 831%, which is comparable to commercial PCL. This perfectly realizes the cycle of PCL from "polymer-monomer-polymer" and maximizes the value of PCL. BRIEF DESCRIPTION OF THE DRAWINGS

[0041] In order to more clearly illustrate the technical solutions of the embodiments of the present invention, the drawings required for use in the embodiments are briefly introduced below. It should be understood that the following drawings only show certain embodiments of the present invention and therefore should not be regarded as limiting the scope. For ordinary technicians in this field, other related drawings can be obtained based on these drawings without creative work.

[0042] Figure 1 The figure is the hydrogen nuclear magnetic spectrum of the depolymerization recovery product of the present invention.

[0043] Figure 2-Figure 3 This is a GC-MS result diagram of the depolymerization recovery product of the present invention.

[0044] Figure 4 This is a comparative analysis of the mechanical properties of the regenerated PCL of the present invention and commercial PCL.

[0045] Figure 5 This is the GPC result diagram of the regenerated PCL of the present invention. DETAILED DESCRIPTION

[0046] The present invention is further described in detail below in conjunction with examples, but the embodiments of the present invention are not limited thereto. The materials involved in the following examples can be obtained from commercial channels unless otherwise specified. The methods described are conventional methods unless otherwise specified.

[0047] The polycaprolactone PCL (M n =80000 g / mol) was purchased from Hunan Juren Chemical New Material Technology Co., Ltd.

[0048] Depolymerization efficiency as described in the examples The calculation formula is shown in formula (1), liquid yield The calculation formula is shown in formula (2).

[0049]

[0050] Among them, m0 is the input mass of PCL before depolymerization, m1 is the residual mass after depolymerization, and m2 is the mass of liquid collected in the collection bottle in the distillation device (i.e., ε-CL). It should be emphasized that, except for the liquid mass, the rest of the depolymerization product is oligomer solid (dimer, trimer), without any other impurities.

[0051] Example 1

[0052] Take 50 parts by mass of PCL in a reactor, heat it to 220°C, add 0.125wt% of nickel acetylacetonate relative to the mass of PCL under stirring, and after mixing evenly, use an oil pump to reduce the pressure in the system to 0.1MPa-0.09MPa. The depolymerized small molecules are continuously extracted and cooled. After 18 hours, the depolymerization efficiency is calculated to be about 21.4%, and the liquid yield is 19.1%.

[0053] Example 2

[0054] Take 50 parts by mass of PCL in a reactor, heat it to 220°C, add 0.125wt% of ferric acetylacetonate relative to the mass of PCL under stirring, and after mixing evenly, use an oil pump to reduce the pressure in the system to 0.1MPa-0.09MPa. The depolymerized small molecules are continuously extracted and cooled. After 10 hours, the depolymerization efficiency is calculated to be about 99.8%, and the liquid yield is 81.2%.

[0055] Example 3

[0056] Take 50 parts by mass of PCL in a reactor, heat it to 220°C, add 0.125wt% of copper acetylacetonate relative to the mass of PCL under stirring, and after mixing evenly, use an oil pump to reduce the pressure in the system to 0.1MPa-0.09MPa. The depolymerized small molecules are continuously extracted and cooled. After 12 hours, the depolymerization efficiency is calculated to be about 30.1%, and the liquid yield is 28.5%.

[0057] Example 4

[0058] Take 50 parts by mass of PCL in a reactor, heat it to 220°C, add 0.125wt% of magnesium acetylacetonate relative to the mass of PCL under stirring, and after mixing evenly, use an oil pump to reduce the pressure in the system to 0.1MPa-0.09MPa. The depolymerized small molecules are continuously extracted and cooled. After 6 hours, the depolymerization efficiency is calculated to be about 100.0%, and the liquid yield is 90.9%.

[0059] Example 5

[0060] Take 50 parts by mass of PCL in a reactor, heat it to 220°C, add 0.125wt% of zinc acetylacetonate relative to the mass of PCL under stirring, and after mixing evenly, use an oil pump to reduce the pressure in the system to 0.1MPa-0.09MPa, and the depolymerized small molecules are continuously extracted and cooled. After 1.5h, the depolymerization efficiency is calculated to be about 100.0%, and the liquid yield is 97.0%.

[0061] Example 6

[0062] Take 50 parts by mass of PCL in a reactor, heat it to 220°C, add 2wt% of copper acetylacetonate relative to the mass of PCL under stirring, and after mixing evenly, use an oil pump to reduce the pressure in the system to 0.1MPa-0.09MPa. The depolymerized small molecules are continuously extracted and cooled. After 10 hours, the depolymerization efficiency is calculated to be about 70.1%, and the liquid yield is 65.5%.

[0063] Example 7

[0064] Take 50 parts by mass of PCL in a reactor, heat it to 220°C, add 5wt% of copper acetylacetonate relative to the mass of PCL under stirring, and after mixing evenly, use an oil pump to reduce the pressure in the system to 0.1MPa-0.09MPa. The depolymerized small molecules are continuously extracted and cooled. After 8 hours, the depolymerization efficiency is calculated to be about 99.3%, and the liquid yield is 81.4%.

[0065] Example 8

[0066] Take 50 parts by mass of PCL in a reactor, heat it to 220°C, add 5wt% of nickel acetylacetonate relative to the mass of PCL under stirring, and after mixing evenly, use an oil pump to reduce the pressure in the system to 0.1MPa-0.09MPa. The depolymerized small molecules are continuously extracted and cooled. After 12 hours, the depolymerization efficiency is calculated to be about 99.1%, and the liquid yield is 80.1%.

[0067] Example 9

[0068] Take 50 parts by mass of PCL in a reactor, heat it to 220°C, add 2wt% of ferric acetylacetonate relative to the mass of PCL under stirring, and after mixing evenly, use an oil pump to reduce the pressure in the system to 0.1MPa-0.09MPa. The depolymerized small molecules are continuously extracted and cooled. After 5 hours, the depolymerization efficiency is calculated to be about 99.7%, and the liquid yield is 85.5%.

[0069] Example 10

[0070] The product obtained by depolymerization of the above-mentioned Example 1-9 was thoroughly stirred and mixed with 0.125 mol% of stannous isooctanoate at room temperature and nitrogen atmosphere for 1 hour, and the temperature was raised to 140°C for ring-opening polymerization for 10 hours. Due to the increase in the molecular weight of PCL in the system, the melt viscosity increased and the magnet could not rotate. The mechanical stirring was replaced, and the temperature was continued to be raised to 220°C for another 1 hour. The polymerization stopped when obvious melt climbing phenomenon appeared.

[0071] Test Example

[0072] (1) The liquid products collected in Examples 1-9 were subjected to nuclear magnetic resonance analysis. The test results are as follows: Figure 1 shown. Figure 1 This is the H NMR spectrum of the depolymerized sample in Example 5. According to the comparative analysis of the H NMR spectra of the product and pure caprolactone (ε-CL), it can be seen that the main component of the product obtained by the depolymerization method of the present invention is caprolactone, with only a small amount of impurities, and it can be inferred that it is a dimer or trimer based on the chemical shift of the impurities. The results of other samples are similar and will not be described one by one.

[0073] Figure 2-Figure 3 This is the GC-MS result of the depolymerization recovery product in Example 5. Figure 3 Corresponding to Figure 2 The three peak positions in the figure indicate that the product obtained by depolymerization is pure. Figure 1 The infrared results of the other samples are similar and will not be elaborated one by one.

[0074] (2) The regenerated PCL prepared in Example 10 was subjected to a tensile performance test (according to GB / T 1040-2006, the tensile rate was set to 500 mm / min). The test results are as follows: Figure 4 shown. Figure 4 This is a comparative analysis of the mechanical properties of the regenerated PCL formed by direct repolymerization of the depolymerization product of Example 5 and the commercial PCL. Figure 5This is the GPC result of the regenerated PCL formed by direct repolymerization of the depolymerized product in Example 5. As can be seen from the figure, the regenerated PCL obtained by direct repolymerization of the ε-CL obtained by the depolymerization method of the present invention has mechanical properties comparable to those of commercial PCL, with a tensile strength of 31.3 MPa and an elongation at break of 831%. The results of other samples are similar and will not be described one by one.

[0075] (3) The depolymerization efficiency and reaction time of the above Examples 1-9 were statistically analyzed. The statistical results are shown in Table 1.

[0076] Table 1

[0077]

[0078] As shown in Table 1, the catalysts of different acetylacetone metal salts used in the method of the present invention all have excellent catalytic depolymerization effects, wherein ferric acetylacetonate, zinc acetylacetonate and magnesium acetylacetonate can achieve a depolymerization efficiency close to 100% at a usage amount of 0.125wt%, and zinc acetylacetonate can recover 97% of ε-CL in 1.5h. Therefore, the present invention provides a method for catalytic thermal cracking by heating and heating based on acetylacetone metal salts as catalysts, and realizes rapid and efficient 100% depolymerization of PCL. The final product has high purity, has 100% repolymerization reaction activity without complex post-treatment such as secondary purification and water removal, and has the advantages of unidirectional high efficiency and high yield.

[0079] The above embodiments are preferred implementation modes of the present invention, but the implementation modes of the present invention are not limited to the above embodiments. Any other changes, modifications, substitutions, combinations, and simplifications that do not deviate from the spirit and principles of the present invention should be equivalent replacement methods and are included in the protection scope of the present invention.

Claims

1. A method for unidirectional depolymerization, high purity, high yield ε-CL closed loop recovery and reuse, characterized in that Acetylacetone metal salts are used as catalysts to recover ε-caprolactone monomers by heating and temperature-assisted in-situ catalytic degradation of polycaprolactone.

2. The method for unidirectional depolymerization, high-purity, high-yield ε-CL closed-loop recycling and reuse according to claim 1, characterized in that: The acetylacetonate metal salt includes at least one of nickel acetylacetonate, iron acetylacetonate, magnesium acetylacetonate, zinc acetylacetonate and copper acetylacetonate.

3. The method for unidirectional depolymerization, high-purity, high-yield ε-CL closed-loop recycling and reuse according to claim 1, characterized in that: The amount of the catalyst is 0.001-5% of the mass of the polycaprolactone.

4. The method for unidirectional depolymerization, high-purity, high-yield ε-CL closed-loop recycling and reuse according to claim 1, characterized in that: The heating temperature is 180-280°C.

5. The method for unidirectional depolymerization, high-purity, high-yield ε-CL closed-loop recycling and reuse according to claim 1, characterized in that: The reaction is carried out at a system pressure of 0.01-0.1 MPa.

6. The method for unidirectional depolymerization, high-purity, high-yield ε-CL closed-loop recovery and reuse according to claim 1, characterized in that: The molecular weight M of the polycaprolactone n It is 1000-100000g / mol.

7. Recovering the depolymerization product obtained by the method according to any one of claims 1 to 6.

8. Use of the depolymerization product according to claim 7 in repolymerization to form regenerated PCL.

9. A method for regenerating PCL by repolymerizing the depolymerized product according to claim 7, characterized in that The specific steps include: The recovered depolymerization product is mixed with a catalyst, and polymerized in stages by increasing the temperature to obtain regenerated PCL.

10. The regenerated PCL prepared according to the method of claim 9.

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