A method for obtaining high yield and high purity epsilon-CL by thermal catalytic depolymerization of PCL

Abstract

The application discloses a method for obtaining high-yield and high-purity epsilon-caprolactone (epsilon-CL) through thermal catalytic depolymerization of PCL, which comprises the following steps: drying and impurity-removing pretreatment of hydroxyl-terminated PCL; mixing the pretreated PCL with a metal ion coordination catalyst, reducing viscosity at a temperature higher than the melting point of PCL by 10-100 DEG C, then performing thermal decomposition reaction under a pressure of 0.01-0.05 MPa, and simultaneously performing vacuum distillation to separate the generated epsilon-CL, caprolactone oligomer and water from the system. In the application, the reversible dynamic coordination metal coordination catalyst dissociates active centers such as ruthenium ions, iron ions, copper ions and cobalt ions at a lower temperature, the metal ions are coordinated with ester groups in the PCL main chain to reduce the breaking energy, and the breaking occurs under the attack of end hydroxyl groups; at this time, the metal ion coordination and the backbite of end hydroxyl groups synergize, the breaking continuously expands to the center, the PCL is gradually broken in a 'unzipping' depolymerization mode and is circularly formed into epsilon-CL.

Description

Technical Field

[0001] This invention belongs to the field of chemical recycling technology, specifically relating to a method for obtaining high-yield, high-purity ε-CL by thermocatalytic depolymerization of PCL. Background Technology

[0002] Most commercial plastics are non-biodegradable, meaning their waste remains permanently in the natural environment, causing serious ecological damage. Chemical recycling can reduce plastics to monomers and repolymerize them into the original materials without altering their properties or the economic value of the polymer. This offers a new solution to the environmental pollution caused by waste plastics.

[0003] Both domestically and internationally, plastic recycling primarily employs physical or chemical methods, resulting in low recycling efficiency and hindering efficient utilization of the recycled materials, while still posing potential plastic pollution. However, polyester materials such as polycaprolactone (PCL), polylactic acid (PLA), and polyethylene terephthalate (PET) possess low-energy ester bonds in their main chains. These ester bonds are easily broken under catalytic or heating conditions, forming low-molecular-weight polymers or depolymerizing to form monomers, thus providing the possibility for polymer depolymerization and recycling.

[0004] Polycaprolactone (PVC), a non-toxic, harmless, and 100% biodegradable polyester material, possesses excellent shape memory properties, low-temperature flexibility, and hydrolysis resistance. It shows great promise for applications in absorbable surgical sutures, facial fillers, tissue engineering, artificial skin, medical dressings, resin bandages, fracture fixation, and dental impression materials. Because its ring-opening polymerization is highly controllable and biodegradable, PVC possesses enormous potential for achieving closed-loop recycling.

[0005] Hoye et al. reversed the depolymerization of polycaprolactone (PCL) in the presence of stannous octanoate (Sn(Oct)2) to recover δ-CL, with a yield of only 82 wt% (Angewandte Chemie, 2022, 134(16)). Ding Songdong et al. of Sichuan University obtained low molecular weight polycarboxylate oligomers by catalytic depolymerization of PCL with boron trifluoride at room temperature (Polymer Degradation and Stability, 2009, 94(9): 1515-1519). Chinese patent CN111875576B depolymerized PCL (number average molecular weight ≈1000) with stannous benzoate in an anhydrous and oxygen-free environment, with a caprolactone recovery rate of 76.0 wt%; and depolymerized PCL (number average molecular weight ≈50000) with stannous chloride as a catalyst, with a caprolactone recovery rate of only 21.3 wt%. Chinese patent CN104140411B discloses a microwave-assisted depolymerization method for polycaprolactone polyol. This method uses diethanolamine as a depolymerization catalyst and reacts under conditions of 150–250°C, 1–2 MPa, and microwave radiation to recover caprolactone, achieving a caprolactone recovery rate of 72.3–81.8 wt%. In the aforementioned prior art, the recovery rate of polymerizable monomers from depolymerization is low, and the purity is insufficient. Furthermore, the research focuses on low molecular weight PCL, making industrial application difficult in practical situations. Summary of the Invention

[0006] To address the shortcomings and deficiencies of existing technologies, the present invention aims to provide a method for obtaining high-yield, high-purity ε-CL through thermocatalytic depolymerization of PCL. This invention employs a reversibly dynamically coordinated metal coordination catalyst, which can dissociate active centers such as ruthenium, iron, copper, and cobalt ions at relatively low temperatures. These metal ions then coordinate with the ester groups of the PCL backbone, reducing its cleavage energy. The attacked carbonyl oxygen in the backbone undergoes an elimination reaction, leading to chain scission of the macromolecule. The broken smaller molecular structures then cyclize to form caprolactone monomers, dimers, or trimers. These cyclized products can then undergo ring-opening polymerization again under catalysis to regenerate PCL. This novel strategy features high reaction efficiency and strong product selectivity, and it is of great significance for the chemical recycling and reuse of biodegradable polyesters.

[0007] The objective of this invention is achieved through the following technical solution:

[0008] A method for obtaining high-yield, high-purity ε-CL by thermocatalytic depolymerization of PCL includes the following steps:

[0009] (1) The hydroxyl-terminated PCL was pretreated by drying to remove impurities;

[0010] (2) The pretreated PCL in step (1) is mixed with the catalyst and the viscosity is reduced at a temperature 10 to 100°C above the melting point of PCL. Then, the pyrolysis reaction is carried out under a pressure of 0.01 to 0.05 MPa, and the generated ε-CL, caprolactone oligomer and water are separated from the reaction system by vacuum distillation at the same time.

[0011] The catalyst is a metal ion coordination catalyst.

[0012] The caprolactone oligomer is a caprolactone dimer or trimer.

[0013] Preferably, the hydroxyl-terminated PCL is PCL-6500 with a number average molecular weight of 50,000 or PCL600C with a number average molecular weight of 65,000, more preferably PCL600C.

[0014] Preferably, the drying and impurity removal pretreatment specifically involves drying and removing impurities at 40–60°C for 4–24 hours.

[0015] Preferably, the viscosity reduction is carried out at a temperature 40–60°C above the melting point of PCL.

[0016] Preferably, the viscosity reduction time is 1 to 5 hours, more preferably 3 to 4 hours.

[0017] Preferably, the amount of catalyst used in step (2) is 1-12 mol% of PCL, more preferably 8-12 mol%.

[0018] Preferably, the pyrolysis reaction is carried out under a pressure of 0.01 to 0.02 MPa.

[0019] Preferably, the pyrolysis reaction is carried out by heating to 180-300°C for 4-24 hours.

[0020] More preferably, the pyrolysis reaction is carried out by heating to 180-250°C for 4-18 hours, and even more preferably by heating to 180-220°C for 4-12 hours.

[0021] Preferably, the metal ion coordination catalyst is a complex of a main group metal element or a transition metal element with Lewis acidity as the metal ion ligand, and it is at least one of the following general structural formulas:

[0022]

[0023] In the formula, the metal ions include ruthenium ions, iron ions, copper ions, and cobalt ions;

[0024] The coordinating group in Formula I is a polysaccharide natural polymer. The carboxyl group formed by the oxidation of the methylene group connected to the hydroxyl group in the polysaccharide natural polymer structure coordinates with the metal ions to form the complex shown in Formula I.

[0025] The coordinating groups in Formulas II to IV are chelating ligands with coordination properties. When metal ions coordinate with atoms on the chelating ligands, a single-coordination site chelating complex as shown in Formula II or a multi-coordination site chelating complex as shown in Formula III is formed. When two different chelating ligands are introduced, a ternary complex as shown in Formula IV is formed.

[0026] Preferably, the polysaccharide natural polymer includes at least one of chitosan and cellulose.

[0027] Preferably, the chelating ligand with coordination properties includes at least one of 1,10-phenanthroline (phen), bipyridine (bipyridine (bpy) is a neutral ligand that forms an ionic complex after coordination with metal ions), and pyridine 3,4-dicarboxylate (Pydc).

[0028] Preferably, the metal ion coordination catalyst shown in Formula I includes compounds formed by coordinating iron ions and copper ions with carboxymethyl chitosan and carboxymethyl cellulose, respectively; the metal ion coordination catalysts shown in Formulas II to IV include Fe(phen)Cl3(H2O), Cu(phen)Cl2, Ru(phen)3(PF6)2, Co(phen)2Cl2(H2O), [Co(Pydc)(phen)2]NO3·H2O, Ru(bpy)3(PF6)2, and Ru(phen)(bpy)2(PF6)2.

[0029] Furthermore, the metal ion coordination catalysts shown in Formulas II to IV can be modified to form cyclic metallized nitroso complexes.

[0030] Preferably, the cyclic metallized nitroso complex is a cyclic metallized ruthenium nitroso complex [Ru(bpy)(ppy)(MeCN)NO](PF6)2.

[0031] Furthermore, the method also includes purifying and separating pure ε-CL from the isolated ε-CL, caprolactone oligomer, and water.

[0032] The mechanism of this invention is as follows:

[0033] By utilizing a metal coordination catalyst with controllable coordination bond strength, transition metal ions are ionized at appropriate temperatures to promote the hydroxyl group re-biting effect in PCL. The polarity of the carbonyl group in the PCL polymer chain is increased by the action of polar metal depolymerization molecules, followed by nucleophilic attack on the carbonyl carbon at the polymer chain end. The attacked carbonyl oxygen undergoes an elimination reaction, leading to chain scission of the macromolecule. Therefore, increasing the carbonyl polarity to reduce the ester bond energy or enhancing nucleophilic attack is key to accelerating the reaction in the depolymerization reaction. In this invention, the metal ions coordinate with the electron-rich oxygen atoms in the carbonyl group on the PCL backbone, thereby further increasing the electronegativity of the carbonyl carbon, reducing its cleavage energy, and making it more susceptible to nucleophilic attack by the hydroxyl oxygen, thus generating caprolactone molecules and new hydroxyl terminals. The general reaction formula is as follows:

[0034]

[0035] The reaction in which the terminal hydroxyl group of PCL attacks the adjacent ester group (i.e., the bite-back reaction) and the ring-opening growth reaction of caprolactone are reverse reactions. During the thermal decomposition of PCL, a "depolymerization-polymerization" equilibrium, i.e., the bite-back-ring-opening growth equilibrium, exists. Its reversibility makes chemical depolymerization possible. If the chemical depolymerization process is supplemented with methods such as vacuum distillation to separate the products in a timely manner, the reaction rate can be further increased. The synergistic effect of coordination and bite-back in the system makes the bite-back process highly controllable, thereby achieving directional depolymerization of the PCL terminal and realizing a gradual chain breakage in a "zipper-like" manner. The presence of water allows the active carbonyl group at the chain breakage site to form a carboxyl terminus. The formation of the carboxyl terminus both inhibits the bite-back of the hydroxyl terminus and hinders the cyclization of the short-chain structure. Therefore, the present invention effectively removes water molecules from the system, ensuring the continuous generation of monomers and avoiding premature termination of the depolymerization reaction.

[0036] Compared with the prior art, the present invention has the following advantages and beneficial effects:

[0037] (1) In this invention, the reversibly dynamically coordinated metal coordination catalyst dissociates active centers such as ruthenium, iron, copper, and cobalt ions at a relatively low temperature. These metal ions then coordinate with the ester groups of the PCL backbone, reducing its cleavage energy, and break under the attack of the terminal hydroxyl groups. At this time, the coordination of metal ions and the back-biting effect of terminal hydroxyl groups work synergistically, and the cleavage continues to extend towards the center. The polymer chain gradually breaks down and cyclizes to form caprolactone monomers in a "zipper-like" depolymerization process. This depolymerization process is highly controllable, achieving high selectivity in the PCL depolymerization process and effectively avoiding the formation of other impurities. In this invention, the PCL depolymerization yield can reach 97.86 wt%, of which the ε-CL content can reach 91.09 wt%.

[0038] (2) The present invention performs a pre-viscosity reduction treatment on PCL, which effectively promotes the dispersion of the catalyst in the polymer during the later depolymerization process, so that its active center can fully contact the ester group of the PCL main chain, greatly improving the catalytic efficiency and depolymerization rate. When depolymerizing and recovering PCL with a number average molecular weight of 50,000 and 65,000, the depolymerization reaction can still achieve extremely high depolymerization yield and depolymerization rate.

[0039] (3) In this invention, when the number of metal ions that can coordinate with carbonyl oxygen in the metal coordination catalyst is higher than the number of hydroxyl terminals in PCL, the excess metal ions coordinate with the ester groups on the main chain during the depolymerization process. Under high temperature, the ester groups break, reducing the molecular weight or undergoing ester exchange and forming new hydroxyl terminals. More hydroxyl terminal bite-back sites are generated, which further increases the depolymerization rate. The complete depolymerization of the low molecular weight polymer chain further releases the coordinating metal ions, which continuously accelerates the reaction. Therefore, this invention can achieve extremely high depolymerization yield in a very short time. Attached Figure Description

[0040] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0041] Figure 1 The image shows the 1H NMR spectrum of the depolymerized and recovered sample from Example 4.

[0042] Figure 2 This is a gas chromatography-mass spectrometry (GC-MS) spectrum of the depolymerized and recovered sample in Example 4.

[0043] Figure 3 This is a gas chromatography-mass spectrometry (GC-MS) spectrum of the depolymerized and recovered sample in Example 11.

[0044] Figure 4 This is a gas chromatography-mass spectrometry (GC-MS) spectrum of the depolymerized and recovered sample in Example 12. Detailed Implementation

[0045] The following embodiments are provided to further illustrate the present invention, but are not intended to limit the invention. It should be noted that the embodiments are only for further illustration and should not be construed as limiting the scope of protection of the present invention. Furthermore, it should be understood that after reading the teachings of this invention, those skilled in the art can make some non-essential improvements and adjustments based on the above description of the invention, and these equivalent forms also fall within the scope defined by the appended claims.

[0046] All raw materials involved in this invention can be purchased directly from the market. For process parameters not specifically specified, conventional techniques can be used as a reference.

[0047] The hydroxyl-terminated polycaprolactones used in the examples were divided into two categories: PCL-6500 (Mn=50000) was purchased from Hunan Juren Chemical New Material Technology Co., Ltd., and PCL600C (Mn=65000) was purchased from Shenzhen Guanghua Weiye Co., Ltd.; all reagents used were purchased from Shanghai Aladdin Biochemical Technology Co., Ltd. or Guangzhou Chemical Reagent Factory, and the metal ion coordination catalyst was prepared by the applicant.

[0048] Example 1

[0049] I. Catalyst Preparation

[0050] 100 mmol of copper chloride and 100 mmol of oxidized carboxymethyl cellulose were separately added to 100 ml of deionized water and stirred until completely dissolved. The two solutions were then mixed and stirred at room temperature for 3 hours. The reaction solution was then recrystallized to obtain yellow crystals, which were copper ion-coordinated carboxymethyl cellulose (Cu). 2+ -Carboxymethyl cellulose), whose structural formula is shown in general formula I.

[0051] II. Thermocatalytic depolymerization of PCL

[0052] (1) Place 10 mmol of PCL-6500 in a forced-air drying oven and dry at 40°C for 24 h for drying and impurity removal pretreatment;

[0053] (2) The pretreated PCL was mixed with 1 mmol of copper ion coordinated carboxymethyl cellulose and stirred at 110 °C for 3 h to reduce viscosity. Then, the mixture was stirred under reduced pressure of 0.01-0.02 MPa and 220 °C to carry out pyrolysis reaction. Simultaneously, vacuum distillation was carried out to separate the generated ε-CL, caprolactone oligomer and water from the reaction system. After 10 h, the depolymerization yield was about 80.03 wt%.

[0054] Example 2

[0055] I. Catalyst Preparation

[0056] 100 mmol of copper chloride and 100 mmol of carboxymethyl chitosan were separately added to 100 ml of deionized water and stirred until completely dissolved. The two solutions were then mixed and stirred at room temperature for 3 hours. The reaction solution was then recrystallized to obtain yellow crystals, which were copper ion-coordinated carboxymethyl chitosan (Cu... 2+ -Carboxymethyl chitosan), whose structural formula is shown in general formula I.

[0057] II. Thermocatalytic depolymerization of PCL

[0058] (1) Place 10 mmol of PCL-6500 in a forced-air drying oven and dry at 40°C for 24 h for drying and impurity removal pretreatment;

[0059] (2) The pretreated PCL was mixed with 1 mmol of copper ion coordinated carboxymethyl chitosan and stirred at 110 °C for 3 h to reduce viscosity. Then, the mixture was stirred under reduced pressure of 0.01-0.02 MPa and 220 °C to carry out pyrolysis reaction. Simultaneously, vacuum distillation was carried out to separate the generated ε-CL, caprolactone oligomer and water from the reaction system. After 10 h, the depolymerization yield was about 85.64 wt%.

[0060] Example 3

[0061] I. Catalyst Preparation

[0062] 100 mmol of ferric chloride hexahydrate and 100 mmol of oxidized carboxymethyl cellulose were separately added to 100 ml of deionized water and stirred until completely dissolved. The two solutions were then mixed and stirred at room temperature for 3 hours. The reaction solution was then recrystallized to obtain reddish-brown crystals, which were iron-coordinated carboxymethyl cellulose (Fe). 3+ -Carboxymethyl cellulose), whose structural formula is shown in general formula I.

[0063] II. Thermocatalytic depolymerization of PCL

[0064] (1) Place 10 mmol of PCL-6500 in a forced-air drying oven and dry at 40°C for 24 h for drying and impurity removal pretreatment;

[0065] (2) The pretreated PCL was mixed with 1 mmol of iron-coordinated carboxymethyl cellulose and stirred at 110 °C for 3 h to reduce viscosity. Then, the mixture was stirred at 220 °C under reduced pressure of oil pump to 0.01-0.02 MPa and continued to undergo pyrolysis reaction. Simultaneously, vacuum distillation was carried out to separate the generated ε-CL, caprolactone oligomer and water from the reaction system. After 12 h, the depolymerization yield was about 82.46 wt%.

[0066] Example 4

[0067] I. Catalyst Preparation

[0068] 100 mmol of ferric chloride hexahydrate and 100 mmol of carboxymethyl chitosan were separately added to 100 ml of deionized water and stirred until completely dissolved. The two solutions were then mixed and stirred at room temperature for 3 hours. The reaction solution was then recrystallized to obtain reddish-brown crystals, which were iron-coordinated carboxymethyl chitosan (Fe... 3+ -Carboxymethyl chitosan), whose structural formula is shown in general formula I.

[0069] II. Thermocatalytic depolymerization of PCL

[0070] (1) Place 10 mmol of PCL-6500 in a forced-air drying oven and dry at 40°C for 24 h for drying and impurity removal pretreatment;

[0071] (2) The pretreated PCL was mixed with 1 mmol of iron-coordinated carboxymethyl chitosan and stirred at 110°C for 3 h to reduce viscosity. Then, the mixture was stirred at 220°C under reduced pressure of oil pump to 0.01-0.02 MPa and continued to undergo pyrolysis reaction. Simultaneously, vacuum distillation was carried out to separate the generated ε-CL, caprolactone oligomer and water from the reaction system. After 11 h, the depolymerization yield was about 88.25 wt%.

[0072] The 1H NMR spectrum and GC-MS spectrum of the depolymerized and recovered sample in this embodiment are shown below. Figure 1 and Figure 2 As shown.

[0073] Figure 1 In the analysis of the NMR spectra of the recovered sample and pure ε-CL, it can be concluded that the main component of the recovered sample is ε-CL. In addition, based on the chemical shifts of the other impurities, it can be inferred that the impurities are caprolactone dimer and caprolactone trimer.

[0074] Figure 2 Based on the elution time and mass spectrometry data in the figure, the main component of the recovered sample is ε-CL. The content of ε-CL is calculated to be 81.09 wt% based on the integral area, the content of caprolactone dimer is 4.27 wt%, the content of caprolactone trimer is 9.53 wt%, and the content of other impurities is 5.11 wt%.

[0075] Example 5

[0076] I. Catalyst Preparation

[0077] 100 mmol of copper chloride and 100 mmol of 1,10-o-phenanthroline were added to 100 ml of anhydrous ethanol and stirred until completely dissolved. The two solutions were mixed and refluxed at 80 °C for 48 h. The reaction solution was then recrystallized to obtain yellow crystals, which were Cu(phen)Cl2, a complex of copper(II) and 1,10-o-phenanthroline, with the structural formula shown in general formula II.

[0078] II. Thermocatalytic depolymerization of PCL

[0079] (1) Place 10 mmol of PCL600C in a forced-air drying oven at 40°C for 24 h for drying and impurity removal pretreatment;

[0080] (2) The pretreated PCL was mixed with 1 mmol of Cu(phen)Cl2 and stirred at 110 °C for 3 h to reduce viscosity. Then, the mixture was stirred at 220 °C under reduced pressure of 0.01-0.02 MPa by an oil pump to carry out the pyrolysis reaction. Simultaneously, vacuum distillation was carried out to separate the generated ε-CL, caprolactone oligomer and water from the reaction system. After 8 h, the depolymerization yield was about 85.99 wt%.

[0081] Example 6

[0082] I. Catalyst Preparation

[0083] 100 mmol of ferric chloride hexahydrate and 100 mmol of 1,10-o-phenanthroline were added to 100 ml of anhydrous ethanol and stirred until completely dissolved. The two solutions were mixed and refluxed at 80 °C for 48 h. The reaction solution was then recrystallized to obtain reddish-brown crystals, which are Fe(phen)Cl3(H2O), a complex of iron(III) and 1,10-o-phenanthroline, with the structural formula shown in general formula II.

[0084] II. Thermocatalytic depolymerization of PCL

[0085] (1) Place 10 mmol of PCL600C in a forced-air drying oven at 40°C for 24 h for drying and impurity removal pretreatment;

[0086] (2) The pretreated PCL was mixed with 1 mmol of Fe(phen)Cl3(H2O) and stirred at 110℃ for 3 h to reduce viscosity. Then, the mixture was stirred under reduced pressure of 0.01-0.02 MPa and 220℃ to carry out pyrolysis reaction. Simultaneously, vacuum distillation was carried out to separate the generated ε-CL, caprolactone oligomer and water from the reaction system. After 9 h, the depolymerization yield was about 90.82 wt%.

[0087] Example 7

[0088] I. Catalyst Preparation

[0089] 100 mmol of cobalt chloride hexahydrate and 100 mmol of 1,10-o-phenanthroline were added to 100 ml of anhydrous ethanol and stirred until completely dissolved. The two solutions were mixed and refluxed at 80 °C for 48 h. The reaction solution was then recrystallized to obtain red crystals, which were the complex of cobalt(II) and 1,10-o-phenanthroline, Co(phen)Cl2(H2O), and its structural formula is shown in general formula II.

[0090] II. Thermocatalytic depolymerization of PCL

[0091] (1) Place 10 mmol of PCL600C in a forced-air drying oven at 40°C for 24 h for drying and impurity removal pretreatment;

[0092] (2) The pretreated PCL was mixed with 1 mmol of Co(phen)Cl2(H2O) and stirred at 110℃ for 3 h to reduce viscosity. Then, the mixture was stirred under reduced pressure of 0.01-0.02 MPa and 220℃ to carry out pyrolysis reaction. Simultaneously, vacuum distillation was carried out to separate the generated ε-CL, caprolactone oligomer and water from the reaction system. After 6 h, the depolymerization yield was about 91.75 wt%.

[0093] Example 8

[0094] I. Catalyst Preparation

[0095] 100 mmol of ruthenium chloride and 300 mmol of 1,10-o-phenanthroline were added to 100 ml of anhydrous ethanol and stirred until completely dissolved. The two solutions were mixed and refluxed at 80 °C for 48 h. The reaction solution was then precipitated with saturated NH4PF6 aqueous solution. The precipitate was washed with water and then recrystallized to obtain red crystals. These crystals were Ru(phen)3(PF6)2, a complex of ruthenium(II) and 1,10-o-phenanthroline, and their structural formula is shown in general formula III.

[0096] II. Thermocatalytic depolymerization of PCL

[0097] (1) Place 10 mmol of PCL600C in a forced-air drying oven at 40°C for 24 h for drying and impurity removal pretreatment;

[0098] (2) The pretreated PCL was mixed with 1 mmol of Ru(phen)3(PF6)2 and stirred at 110℃ for 3 h to reduce viscosity. Then, the mixture was stirred at 220℃ under reduced pressure of oil pump to 0.01-0.02 MPa and stirred to carry out pyrolysis reaction. Simultaneously, vacuum distillation was carried out to separate the generated ε-CL, caprolactone oligomer and water from the reaction system. After 8 h, the depolymerization yield was about 93.79 wt%.

[0099] Example 9

[0100] I. Catalyst Preparation

[0101] 100 mmol of ruthenium chloride and 300 mmol of bipyridine were added to 100 ml of anhydrous ethanol and stirred until completely dissolved. The two solutions were mixed and refluxed at 80 °C for 48 h. The reaction solution was then precipitated with saturated NH4PF6 aqueous solution. The precipitate was washed with water and then recrystallized to obtain red crystals. These crystals were Ru(bpy)3(PF6)2, a complex of ruthenium(II) and bipyridine, and its structural formula is shown in general formula III.

[0102] II. Thermocatalytic depolymerization of PCL

[0103] (1) Place 10 mmol of PCL600C in a forced-air drying oven at 40°C for 24 h for drying and impurity removal pretreatment;

[0104] (2) The pretreated PCL was mixed with 1 mmol of Ru(bpy)3(PF6)2 and stirred at 110 °C for 3 h to reduce viscosity. Then, the mixture was stirred under reduced pressure of 0.01-0.02 MPa and 220 °C to carry out the pyrolysis reaction. Simultaneously, vacuum distillation was carried out to separate the generated ε-CL, caprolactone oligomer and water from the reaction system. After 8 h, the depolymerization yield was about 94.01 wt%.

[0105] Example 10

[0106] I. Catalyst Preparation

[0107] 100 mmol of pyridine 3,4-dicarboxylate, 200 mmol of 1,10-phenanthroline, and 100 mmol of Co(NO3)2 were dissolved in 100 ml of anhydrous ethanol and mixed sequentially with stirring. The mixture was then refluxed at 80 °C for 48 h. The reaction solution was then recrystallized to obtain reddish-brown rectangular blocky crystals. These crystals are ternary complexes of cobalt(II) with 1,10-phenanthroline and pyridine 3,4-dicarboxylate [Co(Pydc)(phen)2]NO3·H2O, and their structural formula is shown in general formula IV.

[0108] II. Thermocatalytic depolymerization of PCL

[0109] (1) Place 10 mmol of PCL600C in a forced-air drying oven at 40°C for 24 h for drying and impurity removal pretreatment;

[0110] (2) The pretreated PCL was mixed with 1 mmol of [Co(Pydc)(phen)2]NO3·H2O and stirred at 110℃ for 3 h to reduce viscosity. Then, the mixture was stirred under reduced pressure of 0.01-0.02 MPa and 220℃ to carry out the pyrolysis reaction. Simultaneously, vacuum distillation was carried out to separate the generated ε-CL, caprolactone oligomer and water from the reaction system. After 5 h, the depolymerization yield was about 92.33 wt%.

[0111] Example 11

[0112] I. Catalyst Preparation

[0113] 100 mmol of ruthenium chloride, 100 mmol of 1,10-o-phenanthroline, and 200 mmol of bipyridine were added to 100 mL of anhydrous ethanol and stirred until completely dissolved. The three solutions were mixed and refluxed at 80 °C for 48 h. The reaction solution was then precipitated with a saturated NH4PF6 aqueous solution. The precipitate was washed with water and then recrystallized to obtain red crystals. These crystals were Ru(phen)(bpy)2(PF6)2, a ternary complex of ruthenium(II) with 1,10-o-phenanthroline and bipyridine, and its structural formula is shown in general formula IV.

[0114] II. Thermocatalytic depolymerization of PCL

[0115] (1) Place 10 mmol of PCL600C in a forced-air drying oven at 40°C for 24 h for drying and impurity removal pretreatment;

[0116] (2) The pretreated PCL was mixed with 1 mmol of Ru(phen)(bpy)2(PF6)2 and stirred at 110 °C for 3 h to reduce viscosity. Then, the mixture was stirred under reduced pressure of 0.01-0.02 MPa and 220 °C to carry out the pyrolysis reaction. Simultaneously, vacuum distillation was carried out to separate the generated ε-CL, caprolactone oligomer and water from the reaction system. After 6 h, the depolymerization yield was about 95.42 wt%.

[0117] The GC-MS spectrum of the depolymerization recovery sample in this embodiment is as follows: Figure 3 As shown in the figure, based on the elution time and mass spectrometry, the main component of the recovered sample is ε-CL. The content of ε-CL is calculated to be 88.45 wt% based on the integral area, the content of caprolactone dimer is 5.61 wt%, the content of caprolactone trimer is 4.00 wt%, and the content of other impurities is 1.94 wt%.

[0118] Example 12

[0119] I. Catalyst Preparation

[0120] (1) Preparation of cyclic ruthenium metallide complex: A mixture of 0.78 mmol of 2-phenylpyridine (ppy), 1.6 mmol of KPF6, 12 mL of acetonitrile and 0.30 mL of triethylamine was added to a 50 mL three-necked flask. The reaction system was deoxygenated for 10 min at room temperature, and then 0.5 mmol of [Ru(p-cymene)Cl2]2 was added. After stirring at 55 °C under argon for 16 h, the solvent was evaporated to obtain an orange-red solid. Then, 1.57 mmol of bipyridine and 12 mL of methanol were added to the reaction system. After reflux at 70 °C for 6 h, the solvent was evaporated to obtain an orange-red solid, which was the cyclic ruthenium metallide complex [Ru(bpy)2(ppy)](PF6)2.

[0121] (2) Preparation of cyclic ruthenium metallide nitroso complex: 0.13 mmol of cyclic ruthenium metallide complex was placed in 15 mL of anhydrous acetonitrile (MeCN) and 0.65 mmol of nitrosotetrafluoroborate was added. The mixture was stirred for 15 min at room temperature under argon atmosphere and then concentrated to about 5 mL. After precipitation with saturated NH4PF6 aqueous solution, the mixture was washed with water and then with ether. The mixture was then dried under vacuum at room temperature to obtain dark red crystals, which are the cyclic ruthenium metallide nitroso complex [Ru(bpy)(ppy)(MeCN)NO](PF6)2.

[0122] II. Thermocatalytic depolymerization of PCL

[0123] (1) Place 10 mmol of PCL600C in a forced-air drying oven at 40°C for 24 h for drying and impurity removal pretreatment;

[0124] (2) The pretreated PCL was mixed with 1 mmol of [Ru(bpy)(ppy)(MeCN)NO](PF6)2 and stirred at 110℃ for 3 h to reduce viscosity. Then, the mixture was stirred under reduced pressure of 0.01-0.02 MPa and 220℃ to carry out pyrolysis reaction. Simultaneously, vacuum distillation was carried out to separate the generated ε-CL, caprolactone oligomer and water from the reaction system. After 4 h, the depolymerization yield was about 97.86 wt%.

[0125] The GC-MS spectrum of the depolymerization recovery sample in this embodiment is as follows: Figure 4 As shown in the figure, based on the elution time and mass spectrometry, the main component of the recovered sample is ε-CL. The content of ε-CL is calculated to be 91.09 wt% based on the integral area, the content of caprolactone dimer is 1.79 wt%, the content of caprolactone trimer is 2.48 wt%, and the content of other impurities is 4.64 wt%.

[0126] The depolymerization yield and reaction time of Examples 1-12 above were statistically analyzed, and the statistical results are shown in Table 1.

[0127] Table 1. Statistics on the depolymerization effect of PCL in Examples 1-12

[0128]

[0129] As shown in Table 1, Fe in Examples 1-4 3+Higher depolymerization yields can be achieved when coordinated with chitosan, but the reaction time is slightly longer. Therefore, it is considered that polysaccharide groups are prone to carbonization at higher reaction temperatures, thus affecting the depolymerization of PCL. The multi-component coordination structures in Examples 5-12 effectively improve the depolymerization yield. The introduction of nitrosylation structures significantly increases the depolymerization rate while improving the yield, especially the cyclometalated ruthenium nitrosyl complex, which exhibits the highest depolymerization yield and fastest depolymerization rate, and also has the highest ε-CL content in its depolymerization products. In process exploration experiments, impurity removal (small molecules, water) and viscosity reduction treatments effectively improve the depolymerization yield and shorten the reaction time. When the catalyst has a stronger coordination ability, it can influence the polarity of the carbonyl oxygen to a greater extent when coordinating with the polycaprolactone backbone, effectively improving its depolymerization yield and selectivity for caprolactone during the depolymerization process, manifested as an increase in the ε-CL content in the product.

[0130] The above description, in conjunction with specific embodiments, provides a further detailed explanation of this application and should not be construed as limiting the specific implementation of this application to these descriptions. Those skilled in the art to which this application pertains can make several simple deductions or substitutions without departing from the concept of this application.

Claims

1. A method for obtaining ε-CL by thermocatalytic depolymerization of PCL, characterized in that, Includes the following steps: (1) The hydroxyl-terminated PCL is dried and purified before pretreatment; the hydroxyl-terminated PCL is PCL-6500 with a number average molecular weight of 50,000 or PCL600C with a number average molecular weight of 65,000. (2) The pretreated PCL in step (1) is mixed with the catalyst and the viscosity is reduced at a temperature 10 to 100°C above the melting point of PCL. Then, the pyrolysis reaction is carried out under a pressure of 0.01 to 0.05 MPa, and the generated ε-CL, caprolactone oligomer and water are separated from the reaction system by vacuum distillation at the same time. The catalyst is a metal ion coordination catalyst; The metal ion coordination catalyst is at least one of the following: compounds formed by coordinating iron ions and copper ions with carboxymethyl chitosan and carboxymethyl cellulose, respectively; Fe(phen)Cl3(H2O); Cu(phen)Cl2; Ru(phen)3(PF6)2; Co(phen)2Cl2(H2O); [Co(Pydc)(phen)2]NO3·H2O; Ru(bpy)3(PF6)2; Ru(phen)(bpy)2(PF6)2; and [Ru(bpy)(ppy)(MeCN)NO](PF6)2.

2. The method for obtaining ε-CL by thermocatalytic depolymerization of PCL according to claim 1, characterized in that, The drying and impurity removal pretreatment specifically involves drying and removing impurities at 40–60°C for 4–24 hours; the viscosity reduction time is 1–5 hours.

3. The method for obtaining ε-CL by thermocatalytic depolymerization of PCL according to claim 1, characterized in that, In step (2), the amount of catalyst used is 1-12 mol% of PCL.

4. The method for obtaining ε-CL by thermocatalytic depolymerization of PCL according to claim 1, characterized in that, The pyrolysis reaction is carried out by heating to 180-300℃ for 4-24 hours.

Images