Method for recycling waste PET and corresponding polymer

Abstract

The invention discloses a method for recycling waste PET (Polyethylene Terephthalate) and a corresponding polymer. The method comprises the following steps: directly mixing PET with 1, 4-butanediol, aliphatic polyester, polyethylene glycol, polytetrahydrofuran, an aliphatic polyester-nylon copolymer or PA66, inserting 1, 4-butanediol, aliphatic polyester, polyethylene glycol, polytetrahydrofuran, an aliphatic polyester-nylon copolymer or PA66 into a PET molecular chain through ester exchange under the catalysis condition of a multi-core metal catalyst to form a prepolymer, and carrying out heat treatment on the prepolymer to obtain the PET / PA66 composite material. The prepolymer is repolymerized under a reduced pressure condition to generate polymers such as PBT, PBAT, PEBAT and TPEE with higher additional values; according to the reaction mode provided by the invention, the waste PET can be directly subjected to chemical upgrading conversion, the conversion efficiency is high, the process is simple, the waste PET can be fully utilized, the cost can be obviously reduced, and the environmental pollution and the resource waste can be reduced.

Description

Technical Field

[0001] The present invention relates to the technical field of upgrading and recycling of waste PET, and in particular to a method for upgrading and recycling waste PET and polymers prepared by the method, typically biodegradable copolyesters and thermoplastic polyester elastomers. Background Art

[0002] Polyethylene terephthalate (PET) is widely used in food packaging and electronic appliances due to its low price, high transparency, light weight, good mechanical strength and processing performance. In 2023, the annual output of PET exceeded 82 million tons, and the annual output increased year by year. However, most PET products are disposable consumer products, and most of the discarded PET products are landfilled. Its inherent rigid benzene ring structure, high regularity and strong hydrophobicity make it difficult to degrade in nature for hundreds or even thousands of years, resulting in serious plastic pollution, which has seriously endangered the earth's environment and human life and health.

[0003] Chemical upgrading of plastic waste can produce new materials with high added value, which can not only effectively solve the problem of plastic pollution, but also turn waste into treasure and create greater economic benefits. Therefore, from the perspective of sustainable development, converting used plastic waste into other high value-added materials again can realize the two-way reuse of materials and energy in the plastic cycle, which will minimize the loss of materials and energy and the pollution caused by the use of plastics. Although there are currently existing technologies to achieve (Journal of Renewable Materials 2021, 9 (5), 867-880), the chemical upgrading of waste PET requires alcoholysis to an oligomer state, and then reacting it with other components, and the conversion process is complicated.

[0004] Chinese patent application CN115124702A discloses a degradable aromatic-aliphatic copolyester material and its preparation method and application, wherein the method comprises: (1) mixing aromatic dibasic acid, diol, aliphatic unit and catalyst to obtain a mixture containing ring-opening polymerization product and esterification product; (2) subjecting the mixture containing ring-opening polymerization product and esterification product to polycondensation reaction to obtain a degradable aromatic-aliphatic copolyester material. The method is aimed at the treatment of aromatics and is a method for degrading materials starting from monomers, and does not solve the problem of waste PET materials.

[0005] Chinese patent CN109776809B discloses an easily cross-linked biodegradable resin and a preparation method thereof, wherein the method comprises the following steps: firstly, glycolide is subjected to a ring-opening polymerization reaction to obtain polyglycolic acid; then, unsaturated aliphatic diacid or unsaturated aliphatic diacid anhydride and ethylene glycol are subjected to polycondensation to obtain polyunsaturated fatty acid ethylene glycol ester; finally, polyglycolic acid and polyunsaturated fatty acid ethylene glycol ester are prepared by melt mixing and solid phase reaction in the presence of an ester exchange reaction catalyst and a stabilizer, which also fails to solve the technical problem of waste PET.

[0006] Therefore, there is currently a lack of efficient catalytic processes and methods for directly converting waste PET into high value-added polymers.

[0007] The present invention aims at the above problems in the prior art and proposes a method for recycling waste PET and a corresponding polymer. Summary of the invention

[0008] In order to solve the above problems in the prior art, the main purpose of the present invention is to provide a method for recycling waste PET and corresponding polymers.

[0009] In a first aspect, the present invention provides a multi-nuclear metal catalyst, characterized in that it comprises:

[0010] (1) The general chemical formula of the metal catalyst is as shown in (I) and (II) below:

[0011]

[0012] R selection One or more of;

[0013] Among them, R1, R2, R3, R4, and R5 each independently represent hydrogen, halogen, alkyl, alkoxy, alkenyl, aryl, nitro, haloalkyl, silicon, and cycloalkyl, M each independently represents titanium ion, zirconium ion, or hafnium ion, and X each independently represents a coordinating solvent.

[0014] Preferably, R1, R2, R3, R4, and R5 are each independently hydrogen, fluorine, chlorine, bromine, iodine, methyl, ethyl, n-propyl, isopropyl, n-butyl, isobutyl, sec-butyl, tert-butyl, n-pentyl, sec-pentyl, isopentyl, cyclopentyl, n-hexyl, sec-hexyl, isohexyl, cyclohexyl, n-heptyl, cycloheptyl, methoxy, ethoxy, isopropoxy, tert-butyloxy, benzyl, phenyl, p-ethylphenyl, o-ethylphenyl, m-ethylphenyl, or silyl; and X each independently represents coordinated methanol, ethanol, isopropanol, or tetrahydrofuran.

[0015] Preferably, the preferred multi-nuclear metal catalyst is as shown in (IV) and (V) below:

[0016]

[0017] In the second aspect, the present invention provides a method for recycling waste PET, which method uses the multi-nuclear catalyst of the first aspect and comprises the following steps:

[0018] (1) mixing at least one of 1,4-butanediol, aliphatic polyester, polyethylene glycol, polytetrahydrofuran, aliphatic polyester-nylon copolymer or PA66 with PET, and reacting them under the catalysis of a multi-core metal catalyst to form a prepolymer, wherein the PET comprises at least one of waste PET or PET finished products, and the aliphatic polyester comprises a mixed system of any aliphatic diol, aliphatic dibasic acid or aliphatic dimethyl ester;

[0019] (2) subjecting the prepolymer to a polymerization reaction under reduced pressure, and removing volatile compounds in the reaction system to obtain polybutylene terephthalate (PBT), biodegradable aliphatic-aromatic copolyester (PBAT, PEBAT) and thermoplastic polyester elastomer (TPEE).

[0020] Preferably, in the step (1), at least one of an esterification reaction, an alcohol hydroxyl-ester group transesterification reaction, an ester group-ester group transesterification reaction and an acid carboxyl-ester group transesterification reaction occurs under the catalysis of the catalyst to form the prepolymer.

[0021] Preferably, in the step (1), at least one of 1,4-butanediol, aliphatic polyester, polyethylene glycol, polytetrahydrofuran, aliphatic polyester-nylon copolymer or PA66 undergoes an esterification reaction with PET under the catalysis of the catalyst, and the representative reaction is shown in formula (I):

[0022]

[0023] Preferably, in the step (2), the volatile compound is generated and removed by at least one of an alcohol hydroxyl group-ester group transesterification reaction, an ester group-ester group transesterification reaction, and an acid carboxyl group-ester group transesterification reaction.

[0024] Preferably, the mass ratio of the 1,4-butanediol, aliphatic polyester, polyethylene glycol, polytetrahydrofuran, aliphatic polyester-nylon copolymer or PA66 to the mass ratio of the PET is (1-99):(1-99).

[0025] Preferably, the mass ratio of the 1,4-butanediol, aliphatic polyester, polyethylene glycol, polytetrahydrofuran, aliphatic polyester-nylon copolymer or PA66 to the mass ratio of the PET is (10-99):(1-99).

[0026] Preferably, when the mass ratio of the repeating units of 1,4-butanediol, aliphatic polyester, polyethylene glycol, polytetrahydrofuran, aliphatic polyester-nylon copolymer or PA66 to PET is within the above range, copolyesters with different properties can be synthesized using the above reaction pathway proposed by the present invention.

[0027] Preferably, the amount of the multi-nuclear metal catalyst used is 0.01 to 20% of the mass of the reactants, calculated as a percentage by mass.

[0028] Preferably, the reaction temperature in step (1) is 100-280°C, and the reaction time is 1-24h; and the temperature for the polymerization reaction in step (2) is 150-300°C, the reaction time is 0.5-48h, and the reaction pressure is 1-1000Pa.

[0029] Preferably, the aliphatic diol is selected from at least one of ethylene glycol, 1,3-propylene glycol, 1,4-butanediol, 1,5-pentanediol, 1,6-hexanediol, 1,7-heptanediol, 1,8-octanediol, 1,9-nonanediol, 1,10-decanediol, diethylene glycol, polyethylene glycol, and polypropylene glycol; the aliphatic dibasic acid is selected from at least one of oxalic acid, malonic acid, succinic acid, glutaric acid, adipic acid, pimelic acid, suberic acid, azelaic acid, sebacic acid, and diglycolic acid; and the aliphatic polyester is selected from polyesters formed by copolymerization of the above-mentioned aliphatic diols and aliphatic dibasic acids.

[0030] In a third aspect, the present invention discloses a polymer, which is prepared according to the method of the second aspect.

[0031] Furthermore, the polymer is a high value-added polymer such as PBT, polyethylene terephthalate-1,4-cyclohexane dimethanol (PETG), biodegradable copolyester (PBAT, PEBAT) and thermoplastic polyester elastomer (TPEE).

[0032] In a fourth aspect, the present invention discloses an application of the multi-nuclear metal catalyst according to the first aspect, wherein the application is for recycling waste PET.

[0033] Furthermore, the multi-nuclear metal catalyst comprises:

[0034] (1) The general chemical formula of the metal catalyst is as shown in (I) and (II) below:

[0035]

[0036] R selection One or more of;

[0037] Among them, R1, R2, R3, R4, and R5 each independently represent hydrogen, halogen, alkyl, alkoxy, alkenyl, aryl, nitro, haloalkyl, silicon, and cycloalkyl, M each independently represents titanium ion, zirconium ion, or hafnium ion, and X each independently represents a coordinating solvent.

[0038] Preferably, R1, R2, R3, R4, and R5 are each independently hydrogen, fluorine, chlorine, bromine, iodine, methyl, ethyl, n-propyl, isopropyl, n-butyl, isobutyl, sec-butyl, tert-butyl, n-pentyl, sec-pentyl, isopentyl, cyclopentyl, n-hexyl, sec-hexyl, isohexyl, cyclohexyl, n-heptyl, cycloheptyl, methoxy, ethoxy, isopropoxy, tert-butyloxy, benzyl, phenyl, p-ethylphenyl, o-ethylphenyl, m-ethylphenyl, or silyl; and X each independently represents coordinated methanol, ethanol, isopropanol, or tetrahydrofuran.

[0039] Preferably, the preferred multi-nuclear metal catalyst is as shown in (IV) and (V) below:

[0040]

[0041] The specific application of the multi-nuclear metal catalyst comprises the following steps:

[0042] (1) mixing at least one of 1,4-butanediol, aliphatic polyester, polyethylene glycol, polytetrahydrofuran, aliphatic polyester-nylon copolymer or PA66 with PET, and reacting them under the catalysis of a multi-core metal catalyst to form a prepolymer, wherein the PET comprises at least one of waste PET or PET finished products, and the aliphatic polyester comprises a mixed system of any aliphatic diol, aliphatic dibasic acid or aliphatic dimethyl ester;

[0043] (2) subjecting the prepolymer to a polymerization reaction under reduced pressure, and removing volatile compounds in the reaction system to obtain polybutylene terephthalate (PBT), biodegradable aliphatic-aromatic copolyester (PBAT, PEBAT) and thermoplastic polyester elastomer (TPEE).

[0044] Preferably, in the step (1), at least one of an esterification reaction, an alcohol hydroxyl-ester group transesterification reaction, an ester group-ester group transesterification reaction and an acid carboxyl-ester group transesterification reaction occurs under the catalysis of the catalyst to form the prepolymer.

[0045] Preferably, in the step (1), at least one of 1,4-butanediol, aliphatic polyester, polyethylene glycol, polytetrahydrofuran, aliphatic polyester-nylon copolymer or PA66 undergoes an esterification reaction with PET under the catalysis of the catalyst, and the representative reaction is shown in formula (I):

[0046]

[0047] Preferably, in the step (2), the volatile compound is generated and removed by at least one of an alcohol hydroxyl group-ester group transesterification reaction, an ester group-ester group transesterification reaction, and an acid carboxyl group-ester group transesterification reaction.

[0048] Preferably, the mass ratio of the 1,4-butanediol, aliphatic polyester, polyethylene glycol, polytetrahydrofuran, aliphatic polyester-nylon copolymer or PA66 to the mass ratio of the PET is (1-99):(1-99).

[0049] Preferably, the mass ratio of the 1,4-butanediol, aliphatic polyester, polyethylene glycol, polytetrahydrofuran, aliphatic polyester-nylon copolymer or PA66 to the mass ratio of the PET is (10-99):(1-99).

[0050] Preferably, when the mass ratio of the repeating units of 1,4-butanediol, aliphatic polyester, polyethylene glycol, polytetrahydrofuran, aliphatic polyester-nylon copolymer or PA66 to PET is within the above range, copolyesters with different properties can be synthesized using the above reaction pathway proposed by the present invention.

[0051] Preferably, the amount of the multi-nuclear metal catalyst used is 0.01 to 20% of the mass of the reactants, calculated as a percentage by mass.

[0052] Preferably, the reaction temperature in step (1) is 100-280°C, and the reaction time is 1-24h; and the temperature for the polymerization reaction in step (2) is 150-300°C, the reaction time is 0.5-48h, and the reaction pressure is 1-1000Pa.

[0053] Preferably, the aliphatic diol is selected from at least one of ethylene glycol, 1,3-propylene glycol, 1,4-butanediol, 1,5-pentanediol, 1,6-hexanediol, 1,7-heptanediol, 1,8-octanediol, 1,9-nonanediol, 1,10-decanediol, diethylene glycol, polyethylene glycol, and polypropylene glycol; the aliphatic dibasic acid is selected from at least one of oxalic acid, malonic acid, succinic acid, glutaric acid, adipic acid, pimelic acid, suberic acid, azelaic acid, sebacic acid, and diglycolic acid; and the aliphatic polyester is selected from polyesters formed by copolymerization of the above-mentioned aliphatic diols and aliphatic dibasic acids.

[0054] Compared with the prior art, the beneficial effects of the present invention are:

[0055] (1) The technical solution of the present invention directly mixes PET with 1,4-butanediol, aliphatic polyester, polyethylene glycol, polytetrahydrofuran, aliphatic polyester-nylon copolymer or PA66. Under the condition of multi-core metal catalysis, the repeating units of 1,4-butanediol, aliphatic polyester, polyethylene glycol, polytetrahydrofuran, aliphatic polyester-nylon copolymer or PA66 are inserted into the PET molecular chain to form a prepolymer. Subsequently, the prepolymer is polymerized under reduced pressure to generate small molecule alcohols, acids or ester compounds, and the volatility of the small molecule alcohols, acids or ester compounds is utilized to remove them from the reaction system, and finally high value-added polymers such as polybutylene terephthalate (PBT), biodegradable aliphatic-aromatic copolyesters (PBAT, PEBAT) and thermoplastic polyester elastomers (TPEE) are generated. The reaction pathway provided by the present invention does not require alcoholysis of PET into original monomers or oligomers before reacting with other components, which greatly simplifies the chemical conversion steps, greatly simplifies the chemical conversion process of waste PET, can achieve 100% utilization of waste PET, and can significantly reduce the synthesis cost.

[0056] (2) The present invention uses waste PET to synthesize high value-added polymers such as polybutylene terephthalate (PBT), biodegradable aliphatic-aromatic copolyesters (PBAT, PEBAT) and thermoplastic polyester elastomers (TPEE), thereby achieving the upgrading and transformation of non-degradable PET and solving its recycling problem, which is of great significance to the solution of environmental pollution and resource recycling.

[0057] (3) In the upgrading and utilization method of the present invention, by selecting different types of compounds, the mechanical properties, heat resistance, degradation cycle and other corresponding comprehensive properties of the obtained high value-added polymers such as biodegradable aliphatic-aromatic copolyesters (PBAT, PEBAT) and thermoplastic polyester elastomers (TPEE) can be easily regulated.

[0058] (4) The upgrading and utilization method of the present invention can be directly carried out using existing polyester production equipment, which has low equipment requirements and is conducive to the rapid promotion of the process. BRIEF DESCRIPTION OF THE DRAWINGS

[0059] Figure 1 The tensile strength and elongation at break of the samples measured in Examples 3-6;

[0060] Figure 2 The tensile strength and elongation at break of the samples measured in Examples 7-11;

[0061] Figure 3 The tensile strength and elongation at break of the measured samples of Examples 12-15;

[0062] Figure 4 The PBT product of Example 11 H NMR spectrum;

[0063] Figure 5 The PETG product of Example 2 1 H NMR spectrum;

[0064] Figure 6 The PBAT product of Example 3 1 H NMR spectrum;

[0065] Figure 7 The PEBAT product of Example 10 1 H NMR spectrum;

[0066] Figure 8 The TPEE product of Example 14 1 H NMR spectrum. DETAILED DESCRIPTION

[0067] For a better understanding of the present invention, the following examples are provided to further illustrate the present invention, but the content of the present invention is not limited to the following examples.

[0068] The embodiments of the present invention all use waste PET as the raw material for upgrading and utilization. The source of the waste PET can be any PET product (such as PET bottles, etc.). Since waste PET is used as the raw material for upgrading and utilization, in the embodiments of the present invention, the waste PET material can be pre-treated before the feeding reaction. The pre-treatment method includes: washing, crushing and drying the waste PET material.

[0069] Example 1

[0070] Waste PET is upgraded to synthesize PBT.

[0071] The method for upgrading and utilizing waste PET disclosed in Example 1 comprises the following steps:

[0072] A certain amount of discarded PET mineral water bottle flakes, 1,4-butanediol and a multi-nuclear titanium metal catalyst (the amount of the multi-nuclear titanium metal catalyst is 1% of the total mass of the reactants) are added to a 100 mL round-bottom flask, heated to 220°C for reaction under normal pressure, and the ethylene glycol produced by the reaction is removed by nitrogen gas flow. The reaction time is 0.5 h. After 0.5 h, the reaction is vacuumed to 30 Pa, the temperature of the reaction system is maintained at 260°C, and then the polymerization reaction is carried out for 4 h. After the reaction is completed, PBT is obtained.

[0073] The corresponding PBT product of Example 1 1 H NMR spectrum Figure 4 shown.

[0074] Embodiment 2:

[0075] Waste PET is upgraded to synthesize PETG.

[0076] The method for upgrading and recycling waste PET disclosed in Example 2 comprises the following steps:

[0077] A certain amount of discarded PET mineral water bottle flakes, 1,4-cyclohexanedimethanol and multinuclear titanium metal catalyst 2 (the amount of multinuclear titanium metal catalyst 2 is 1% of the total mass of the reactants) are added to a 100 mL round-bottom flask, heated to 220°C for reaction under normal pressure, and the ethylene glycol produced by the reaction is removed by nitrogen gas flow. The reaction time is 0.5 h. After 0.5 h, the reaction is vacuumed to 30 Pa, the temperature of the reaction system is maintained at 260°C, and then the polymerization reaction is carried out for 4 h. After the reaction is completed, PETG is obtained.

[0078] Example 2 corresponding PETG product 1 H NMR spectrum Figure 5 shown.

[0079] Embodiment 3-6

[0080] The method for recycling waste PET disclosed in Examples 3-6 comprises the following steps:

[0081] A certain amount of discarded PET mineral water bottle flakes, 1,4-butanediol, polybutylene adipate (PBA) and a multi-core titanium metal catalyst (the amount of the multi-core titanium metal catalyst is 1% of the total mass of the reactants) are added to a 100 mL round-bottom flask, heated to 220°C for reaction under normal pressure, and the ethylene glycol produced by the reaction is removed by nitrogen gas flow. The reaction time is 1 hour. After 1 hour, the reaction is vacuumed to 30 Pa, the temperature of the reaction system is maintained at 260°C, and then the polymerization reaction is carried out for 4 hours. After the reaction is completed, PBAT is obtained.

[0082] Among the reaction raw materials of Examples 3-6, the mass ratios of PET mineral water bottle flakes, 1,4-butanediol and PBA are shown in Table 1 below, and the number average molecular weight of the PBAT product obtained in the steps of each embodiment is shown in Table 1 below.

[0083] Table 1. Raw material composition and product conditions of Examples 3 to 6

[0084]

[0085] Among them, the PBAT product corresponding to Example 3 1 H NMR spectrum Figure 6 shown.

[0086] Embodiment 7-11

[0087] The method for recycling waste PET disclosed in Examples 7-11 comprises the following steps:

[0088] A certain amount of discarded PET mineral water bottle flakes, PBA and a multi-nuclear metal catalyst (the amount of the multi-nuclear metal catalyst is 1% of the total mass of the reactants) were added to a 100 mL round-bottom flask, and heated to 220°C for reaction under normal pressure for 1 hour. After 1 hour, the reaction was vacuumed to 30 Pa, the temperature of the reaction system was maintained at 260°C, and then the polymerization reaction was carried out for 4 hours. After the reaction was completed, PEBAT was obtained.

[0089] Among the reaction raw materials of Examples 7-11, the mass ratios of PET mineral water bottle flakes and PBA are shown in Table 2 below, and the number average molecular weights of the PEBAT products obtained in the steps of each example are shown in Table 2 below.

[0090] Table 2. Raw material composition and product conditions of Examples 7-11

[0091]

[0092] Among them, the PEBAT product corresponding to Example 10 1 H NMR spectrum Figure 7 shown.

[0093] Examples 12-15

[0094] The method for recycling waste PET disclosed in Examples 12-15 comprises the following steps:

[0095] A certain amount of discarded PET mineral water bottle flakes, polytetrahydrofuran (PTMEG) and a multi-nuclear metal catalyst (the amount of the multi-nuclear metal catalyst is 1% of the total mass of the reactants) were added to a 100 mL round-bottom flask, heated to 220°C for reaction under normal pressure, and the ethylene glycol produced by the reaction was removed by nitrogen flow. The reaction time was 1 hour. After 1 hour, the reaction was vacuumed to 30 Pa, the temperature of the reaction system was maintained at 260°C, and then the polymerization reaction was carried out for 4 hours. After the reaction was completed, TPEE was obtained.

[0096] Among the reaction raw materials of Examples 12-15, the mass ratios of PET mineral water bottle flakes and PTMEG are shown in Table 3 below, and the number average molecular weights of the TPEE products obtained in the steps of each example are shown in Table 3 below.

[0097] Table 2. Raw material composition and product conditions of Examples 12-15

[0098]

[0099] Among them, the TPEE product corresponding to Example 14 1 H NMR spectrum Figure 8 shown.

[0100] Performance Testing

[0101] Mechanical properties test 1: The PBAT products prepared in Examples 3-6 were injection molded into corresponding test bars according to a unified process, and the tensile strength and elongation at break of the samples were measured according to the measurement method specified in ISO 527-2 Plastic Tensile Properties Test Method (tensile rate of 50 mm / min). The test results are as follows Figure 1 shown.

[0102] Mechanical properties test 2: The products prepared from PEBAT prepared in Examples 7-11 were injection molded into corresponding test bars according to a unified process, and the tensile strength and elongation at break of the samples were measured according to the measurement method specified in ISO 527-2 Plastic tensile properties test method (tensile rate of 50 mm / min). The test results are as follows Figure 2 shown.

[0103] Mechanical properties test 3: The TPEE products prepared in Examples 12-15 were injection molded into corresponding test bars according to a unified process, and the tensile strength and elongation at break of the samples were measured according to the measurement method specified in ISO 527-2 Plastic tensile properties test method (tensile rate of 50 mm / min). The test results are as follows Figure 3 shown.

[0104] It should be noted that, in this article, relational terms such as first and second, etc. are only used to distinguish one entity or operation from another entity or operation, and do not necessarily require or imply any such actual relationship or order between these entities or operations. Moreover, the terms "include", "comprise" or any other variants thereof are intended to cover non-exclusive inclusion, so that a process, method, article or device including a series of elements includes not only those elements, but also other elements not explicitly listed, or also includes elements inherent to such process, method, article or device.

[0105] Although embodiments of the present invention have been shown and described, it will be appreciated by those skilled in the art that various changes, modifications, substitutions and variations may be made to the embodiments without departing from the principles and spirit of the present invention, and that the scope of the present invention is limited by the appended claims and their equivalents.

Claims

1. A method for recycling waste PET, characterized in that: The steps include: (1) mixing at least one of 1,4-butanediol, aliphatic polyester, polyethylene glycol, polytetrahydrofuran, aliphatic polyester-nylon copolymer or PA66 with PET, and reacting them under the catalysis of a multi-core metal catalyst to form a prepolymer, wherein the PET comprises at least one of waste PET and / or finished PET products, and the aliphatic polyester comprises a mixed system of any aliphatic diol, aliphatic dibasic acid or aliphatic dimethyl ester; (2) subjecting the prepolymer to a polymerization reaction under reduced pressure, and removing volatile compounds in the reaction system to obtain a polymer to achieve the recycling of waste PET.

2. The method for recycling waste PET according to claim 1, characterized in that: The multi-nuclear metal catalyst has the following characteristics: (1) The general chemical formula of the metal catalyst is as shown in (I) and / or (II): R selects Me, Et, i Pr, n-Butyl, One or more of; Among them, R1, R2, R3, R4, and R5 each independently represent hydrogen, halogen, alkyl, alkoxy, alkenyl, aryl, nitro, haloalkyl, silicon, and cycloalkyl, M each independently represents titanium ion, zirconium ion, or hafnium ion, and X each independently represents a coordinating solvent.

3. A method for recycling waste PET according to claim 2, characterized in that: The R1, R2, R3, R4, and R5 are each independently hydrogen, fluorine, chlorine, bromine, iodine, methyl, ethyl, n-propyl, isopropyl, n-butyl, isobutyl, sec-butyl, tert-butyl, n-pentyl, sec-pentyl, isopentyl, cyclopentyl, n-hexyl, sec-hexyl, isohexyl, cyclohexyl, n-heptyl, cycloheptyl, methoxy, ethoxy, isopropoxy, tert-butyloxy, benzyl, phenyl, p-ethylphenyl, o-ethylphenyl, m-ethylphenyl, or silyl; and the Xs each independently represent coordinated methanol, ethanol, isopropanol, or tetrahydrofuran.

4. A method for recycling waste PET according to any one of claims 1 to 3, characterized in that: In the step (1), at least one of an esterification reaction, an alcohol hydroxyl group-ester group transesterification reaction, an ester group-ester group transesterification reaction and an acid carboxyl group-ester group transesterification reaction occurs under the catalysis of the catalyst to form the prepolymer.

5. A method for recycling waste PET according to any one of claims 1 to 3, characterized in that: In the step (2), the volatile compound is generated and removed by at least one of an alcohol hydroxyl group-ester group transesterification reaction, an ester group-ester group transesterification reaction, and an acid carboxyl group-ester group transesterification reaction.

6. A method for recycling waste PET according to any one of claims 1 to 3, characterized in that: The mass ratio of the 1,4-butanediol, aliphatic polyester, polyethylene glycol, polytetrahydrofuran, aliphatic polyester-nylon copolymer or PA66 to the PET is (10-99): (1-99).

7. A method for recycling waste PET according to any one of claims 1 to 3, characterized in that: In terms of mass percentage, the amount of the multi-nuclear metal catalyst used is 0.01 to 20% of the mass of the reactants.

8. A method for recycling waste PET according to any one of claims 1 to 3, characterized in that: The reaction temperature in step (1) is 100-280° C., and the reaction time is 1-24 h; and the polymerization reaction in step (2) is carried out at a temperature of 150-300° C., a reaction time of 0.5-48 h, and a reaction pressure of 1-1000 Pa.

9. A method for recycling waste PET according to any one of claims 1 to 3, characterized in that: The aliphatic diol is selected from at least one of ethylene glycol, 1,3-propylene glycol, 1,4-butanediol, 1,5-pentanediol, 1,6-hexanediol, 1,7-heptanediol, 1,8-octanediol, 1,9-nonanediol, 1,10-decanediol, diethylene glycol, polyethylene glycol, and polypropylene glycol; The aliphatic dibasic acid is selected from at least one of oxalic acid, malonic acid, succinic acid, glutaric acid, adipic acid, pimelic acid, suberic acid, azelaic acid, sebacic acid, and diglycolic acid; The aliphatic polyester is selected from polyesters formed by copolymerization of the above-mentioned aliphatic diols and aliphatic dibasic acids.

10. A polymer, characterized in that: The polymer is prepared according to the method according to any one of claims 1-9.

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