Method for depolymerizing polyamides

Abstract

A method for depolymerizing polyamides is provided. In embodiments, the method comprises mixing a polyamide with an alkali metal catalyst to depolymerize the polyamide to a product, wherein the alkali metal catalyst is of the formula MXR n Selected from those having the following characteristics, where M is an alkali metal, X is H, C, N, Si, or Sn, R is independently selected from hydrogen, alkyl, silyl, and aryl, and n is 0 to 2.

Description

[Technical Field]

[0001] Cross-references to related applications This application claims priority to U.S. Provisional Patent Application No. 63 / 466,775, filed on 16 May 2023, the entirety of which is incorporated herein by reference.

[0002] References concerning the rights of the government This invention was made with government support under grant numbers DE-SC0022290 and DE-FG02-03ER15457, awarded by the Department of Energy. The Government has certain rights in this invention. [Background technology]

[0003] Modern society relies heavily on plastic materials, as evidenced by the steady increase in their production. Plastics are extremely widespread, versatile, and low-cost polymer materials that have dramatically improved the quality of human life for over a century. Currently, plastics are produced worldwide at a rate of 450 million tons per year, and this amount is projected to double by 2045. One type of plastic commonly used in applications requiring materials and products that can withstand harsh mechanical and environmental conditions is engineering plastics such as polyamides.

[0004] Nylon 6 was one of the first synthetic fibers discovered and developed by Schrack in 1938. It is a thermoplastic polyamide produced industrially at a rate of 8.9 million tons per year by water-based ring-opening polymerization (ROP) of ε-caprolactam, with a market size projected to reach $21.5 billion by 2026. As a non-biodegradable plastic, nylon contributes significantly to marine and landfill pollution, reflecting its excellent chemical robustness and lack of effective recycling technologies. In fact, it is estimated that by 2050, plastic waste will outnumber fish in the ocean, and nylon 6 accounts for approximately 10% of marine plastic pollution as discarded or lost fishing nets (so-called "ghost nets"). This represents over 600,000 tons of abandoned fishing nets annually. [Overview of the Initiative]

[0005] A method for depolymerizing polyamides, such as nylon 6, using a specific alkali metal catalyst is provided. Further details are provided below. The following examples illustrate experimental results demonstrating that this method can yield polyamide monomers, such as ε-caprolactam, in remarkably high yields (e.g., about 100%) using mild conditions (e.g., low temperatures of 220°C and reaction times of less than a few hours, or even a few minutes) and small amounts of catalyst. Furthermore, this method can be carried out under high pressure, including under vacuum or an inert atmosphere. No solvents or other additives are required. This method can be carried out continuously over long periods without reducing the reaction rate. Alkali metal catalysts, such as alkali metal catalysts in reaction mixtures with polyamides, are also included in this disclosure.

[0006] In one embodiment, a method for depolymerizing a polyamide comprises mixing a polyamide with an alkali metal catalyst to depolymerize the polyamide to a product, wherein the alkali metal catalyst is of the formula MXR n The formula is selected from those having (wherein M is an alkali metal, X is H, C, N, Si or Sn, R is independently selected from hydrogen, alkyl, silyl and aryl, and n is from 0 to 2).

[0007] Other principal features and advantages of the present disclosure will become apparent to those skilled in the art upon consideration of the following drawings, detailed description, and appended claims.

Brief Description of the Drawings

[0008] Exemplary embodiments of the present disclosure will be described hereinafter with reference to the accompanying drawings.

[0009] Figs. 1A - 1D show examples of chemical recycling techniques for ε-caprolactam monomers of nylon 6, including the use of existing techniques (Figs. 1A - 1C) and the use of an exemplary embodiment of the present disclosure (Fig. 1D).

Mode for Carrying Out the Invention

[0010] A method for depolymerizing polyamide is provided. In one embodiment, the method includes mixing polyamide with a superbase alkali metal catalyst under conditions for depolymerizing the polyamide.

[0011] The polyamide depolymerized by the method of the present invention is a polymer composed of monomers covalently bonded in an extended chain via amide bond groups. The term "monomer" refers to a chemical reaction raw material substance incorporated to form an extended chain and an amide bond group during a polymerization reaction. The polyamide may be an aliphatic polyamide (i.e., not containing an aromatic ring) or an aromatic polyamide (i.e., containing an aromatic ring). The polyamide may be a homopolymer (i.e., formed from a single type of monomer) or a heteropolymer (i.e., formed from a plurality of types of monomers, for example, two types of monomers. "Heteropolymer" may sometimes be called a copolymer). The term "type" refers to a chemical formula. A single type means the same chemical formula, and different types mean different chemical formulas.

[0012] The polyamide may be formed by ring-opening polymerization of a cyclic amide (i.e., lactam). In such embodiments, the monomer is a cyclic amide and the polymerization reaction is ring-opening polymerization. Exemplary cyclic amides include 2-pyrrolidone, 2-piperidone, ε-caprolactam, enanthlactam, capryllactam, pelargonlactam, azacycloundecan-2-one, and azacyclotridecan-2-one.

[0013] The polyamide may be formed by condensation of an amine (including diamine) and an acid (including dibasic acid). The amine and acid may be provided as a single chemical compound such as 11-aminoundecanoic acid or ω-aminolauric acid. Alternatively, the amine and acid may be provided as two different chemical compounds such as hexamethylenediamine and adipic acid. In these embodiments, the monomer is a single chemical compound (having an amine and an acid group) or two chemical compounds (a diamine and a dibasic acid), and the polymerization reaction is condensation.

[0014] The polyamide may be identified by reference to the monomers (e.g., cyclic amide, diamine, dibasic acid) used to form the polyamide, recognizing that the chemical form of these monomers may be altered by subsequent polymerization reactions that provide the amide linkage groups of the polyamide.

[0015] Exemplary polyamides that are depolymerized by the method of the present invention include poly(2-pyrrolidinone) (nylon 4), poly(2-piperidone) (nylon 5), poly(hexano-6-lactam) (nylon 6), polyenanthamide (nylon 7), polycapryllactam (nylon 8), poly(9-aminononanoic acid) (nylon 9), poly(10-aminodecanoic acid) (nylon 10), poly(11-aminoundecanoic acid) (nylon 11), poly(dodecano-12-lactam) (nylon 12), poly[imino(1,6-dioxohexamethylene)iminohexamethylene] (nylon 66), and poly[imino(1,6-dioxohexamethylene)iminotetramethylene] (nylon 46).

[0016] The methods of the present invention can be used to depolymerize any of the disclosed polyamides. The depolymerization process decomposes the polyamide into a product. In embodiments, the depolymerization process provides monomers (e.g., cyclic amides) that form the polyamide, i.e., the product contains (or consists of) the monomers of the polyamide. Throughout this disclosure, the term “monomer” encompasses both a single type of monomer and several different types of monomers, depending on the particular polyamide. In such embodiments, this monomer can be recovered and used to reform the polyamide. This is in contrast to depolymerization processes that do not provide monomers and provide a product containing chemical compounds that cannot reform the polyamide without an additional step to convert the chemical compounds into monomers. However, the methods of the present invention encompass both types of depolymerization processes, i.e., processes that provide monomers of the polyamide and processes that provide different types of products.

[0017] In one embodiment, this method depolymerizes nylon 6 to produce ε-caprolactam. In another embodiment, this method depolymerizes nylon 4 to produce 2-pyrrolidone. In another embodiment, this method depolymerizes nylon 5 to produce 2-pyrrolidone. In another embodiment, this method depolymerizes nylon 7 to produce enantractam. In another embodiment, this method depolymerizes nylon 8 to produce capryllactam. In another embodiment, this method depolymerizes nylon 9 to produce pelargolactam. In another embodiment, this method depolymerizes nylon 10 to produce azacycloundecane-2-one. In another embodiment, this method depolymerizes nylon 11 to produce azacyclotridecane-2-one. In another embodiment, this method depolymerizes nylon 12 to produce ω-aminolauric acid.

[0018] A single type of polyamide may be used, or multiple different types of polyamides (i.e., a mixture of different types of polyamides) may be used.

[0019] The alkali metal catalyst used in the method of the present invention is formula I, MXR n This includes catalysts having (wherein M is an alkali metal, X is H, C, N, Si or Sn, R is independently selected from hydrogen, alkyl, silyl and aryl, and the value of n (which may be 0) depends on the selected X). In this disclosure, such alkali metal catalysts may be referred to by terms such as “superbase”. It is understood that Formula I encompasses embodiments in which X is an atom of the selected R group. For example, if X is C, this carbon may be an atom of the selected R group, e.g., a carbon of an alkyl group as defined below. In other words, X and R may together form an alkyl group, e.g., an unsubstituted linear alkyl group or a substituted branched alkyl group. This means that if X is C, an additional hydrogen may be bonded to the carbon. As another example, if X is N, this nitrogen may be a heteroatom of the selected R group, e.g., a nitrogen of a cycloalkyl group as defined below. In other words, X and R may together form a nitrogen-substituted cycloalkyl group. This means that if X is N, multiple atoms of the R group (e.g., two carbons) may be bonded to the nitrogen.

[0020] Formula I does not include alkali metal oxides, alkali metal hydroxides, alkali metal carbonates, and alkali metal carboxylates. Therefore, this catalyst is not any of these compounds.

[0021] Various alkali metals can be used in formula I and its related formulas. Exemplary alkali metals include Li, Na, K, Cs, and mixtures thereof.

[0022] In Formula I and its related formulas, “alkyl group” refers to a linear, branched, or cyclic alkyl group having, for example, 1 to 8, 1 to 6, 1 to 4, 1 to 3, or 1 to 2 carbon atoms. Cyclic alkyl groups are sometimes called cycloalkyl groups. Alkyl groups may be unsubstituted, meaning that the alkyl group does not contain heteroatoms. Unsubstituted alkyl groups include alkyl groups (e.g., benzyl) in which one or more bonds to carbon or hydrogen are substituted with bonds to an unsubstituted aromatic ring. Alkyl groups may be substituted, meaning that one or more bonds to carbon or hydrogen are substituted with bonds to non-hydrogen atoms and non-carbon atoms.

[0023] In formula I and related formulas, "aryl group" refers to a monocyclic aryl group having one aromatic ring (e.g., benzene) or a polycyclic group having multiple aromatic rings (e.g., two-ring, three-ring, etc.). Monocyclic aryl groups may be unsubstituted or substituted, as described above with respect to alkyl groups. With respect to polycyclic groups, adjacent aromatic rings may or may not be fused. The aromatic rings of polycyclic groups may be unsubstituted or substituted, as described above with respect to monocyclic aryl groups.

[0024] In formula I and its related formulas, “silyl group” refers to -SiR3, where R is independently selected from hydrogen, alkyl groups, and aryl groups (each of which is already defined herein), and “-” indicates a covalent bond, for example, a covalent bond to X in formula I and all its related formulas.

[0025] With respect to substituents of the groups described herein (as opposed to non-substituted groups), non-hydrogen atoms and non-carbon atoms include, for example, halogens, oxygen, sulfur, nitrogen, phosphorus, and silicon.

[0026] In embodiments, the alkali metal catalyst has formula I, where M is selected from Li, Na, K, Cs and mixtures thereof, X is selected from N and H, R is independently selected from hydrogen, alkyl and silyl, and n is 0 to 2, for example 0, 1, or 2. In embodiments, the silyl group is Si(CH3)3. In embodiments, the alkyl group is an unsubstituted linear alkyl group. In embodiments, the alkyl group is a substituted cycloalkyl group.

[0027] In this embodiment, the alkali metal catalyst is of formula IA, MNR n The formula is such that M is selected from Li, Na, K, Cs and mixtures thereof, R is independently selected from hydrogen, alkyl and silyl, and n is 1 to 2, for example 1, 2. In embodiments, the silyl group is Si(CH3)3. In embodiments, the alkyl group is an unsubstituted linear alkyl group. In embodiments, the alkyl group is a substituted cycloalkyl group.

[0028] In some embodiments, the alkali metal catalyst has the formula IB, MNH2, where M is selected from Li, Na, K, Cs, and mixtures thereof. In some such embodiments, M is Na. Examples of alkali metal catalysts represented by formula IB are shown in entries 9-14, 16-23, 26, and 27 of Table 1.

[0029] In some embodiments, the alkali metal catalyst has the formula IC, MNH, where M is selected from Li, Na, K, Cs, and mixtures thereof. In some such embodiments, M is Na. Examples of such alkali metal catalysts are shown in entries 15 and 25 of Table 1.

[0030] In embodiments, the alkali metal catalyst has the formula ID, where M is selected from Li, Na, K, Cs, and mixtures thereof, R is a substituted cycloalkyl group, and n is 1. In some such embodiments, M is Na. Examples of such alkali metal catalysts are shown in entries 33-38 of Table 1.

[0031] As mentioned above, examples of alkali metal catalysts are listed in Table 1, particularly in entries 6-30 and 33-38.

[0032] A single type of alkali metal catalyst may be used, or several different types of alkali metal catalysts may be used. The alkali metal catalyst used to catalyze depolymerization may include or consist of any of the disclosed alkali metal catalysts or combinations thereof.

[0033] The conditions used in the method of the present invention can refer to parameters such as temperature, time, atmosphere, and amount of alkali metal catalyst. Since the method of the present invention can be carried out using various types of reactor systems, including batch reactor systems, semi-batch reactor systems, continuous flow reactor systems, and extruder reactor systems (e.g., twin-screw extruder reactor systems), the conditions can also refer to a specific type of reactor system. These parameters can be adjusted to promote depolymerization and thus to achieve a desired (e.g., maximum) yield of monomer (or other depolymerization product).

[0034] Regarding temperature, the temperature can be within 30°C, below 20°C, or within 10°C of the polyamide's melting point. The temperature can be above the polyamide's melting point so that the polyamide becomes molten (liquid) during depolymerization. In embodiments, the temperature is 300°C or less, 280°C or less, 260°C or less, or 240°C or less. This includes ranges between any of these values, as well as the ranges of 200°C to 280°C, 200°C to 250°C, and 200°C to 245°C.

[0035] Regarding time, this can refer to the total time the polyamide and alkali metal catalyst are subjected to depolymerization. In embodiments, the time is less than 24 hours, less than 10 hours, less than 5 hours, or less than 2 hours. This includes the ranges of 1 minute to 10 hours and 10 minutes to 5 hours. In continuous flow reactor systems, flow rate, rather than time, is the relevant parameter. In embodiments, the flow rate is 5 sccm to 1500 sccm, but higher flow rates may be used. Reactor volume is another relevant parameter in continuous flow reactor systems. In embodiments, the reactor volume is 50 mL to 1000 L, but larger reactor volumes may be used.

[0036] Regarding the atmosphere, a vacuum, for example, 10 -3 Torr or less, 10 -2 Torr or less, 10 -1 A pressure of less than Torr may be used. However, this method is 10 -2 Torr~10 3 This method may be carried out at higher pressures, including in the Torr range. This method may also be carried out in an inert atmosphere (e.g., N2, dry air, argon) and at a pressure of about 1 atm (760 Torr).

[0037] Regarding the amount of alkali metal catalyst, it can be 20 mol% or less, 10 mol% or less, 8 mol% or less, or 6 mol% or less. This includes the ranges of 0.1 mol% to 5 mol% and 0.1 mol% to 2 mol%. The mol% is calculated based on the repeating units of the polyamide. For example, when using 1 g of nylon 6, the number of moles of repeating units = 0.00885 mol (the repeating units of nylon 6 are 113 g / mol). Therefore, the amount of catalyst used is 0.00885 mol × x mol%.

[0038] The methods of the present invention are generally carried out without the use of liquid media, such as solvents. This includes methods carried out without the use of water or vapor. For this reason, these methods are sometimes referred to as "solvent-free" methods.

[0039] The polyamide (its specific chemical type is described above) may be a “virgin” polyamide, which generally refers to a pure, as-synthesized polyamide that has not been further processed for use in a particular application (whether or not the virgin polyamide was used). Alternatively, the polyamide may be a “post-consumer” polyamide, which generally refers to a polyamide derived from a consumer product (whether or not the consumer product was actually used). The post-consumer polyamide may contain other components (e.g., other non-polyamide polymers such as polyolefins), and it may be considered a composite material with such other components and processed for use in a particular application (e.g., fishing nets, carpet fibers, clothing, medical gloves). In either embodiment, the form of the polyamide is not particularly limited. For example, the virgin polyamide may be in the form of a powder containing micron-sized particles. However, the method of the present invention can achieve high monomer yields from post-consumer polyamide that has been cleaved into fragments much larger than the particles of the virgin polyamide powder. In other words, grinding, such as cryogenic grinding, is not required to achieve high monomer yields from post-consumer polyamide using the method of the present invention. Other forms include granules, pellets, films, and fibers. Regardless of the source or form of the polyamide, before use in this method, the polyamide may be washed and dried by heating under vacuum, as described in the following examples.

[0040] When mixed to carry out the method of the present invention, the polyamide and alkali metal catalyst can be considered to form a reaction mixture containing each of these components. As stated above, the reaction mixture does not need to contain a liquid medium (e.g., a solvent). (This does not preclude the presence of liquid in the reaction mixture by using molten polyamide.) Similarly, the reaction mixture does not need to contain other additives. Therefore, the reaction mixture can be characterized as being free of liquid mediums (other than polyamide if the polyamide is in a molten state) and free of additives. This includes the fact that the reaction mixture does not contain one or more of the following: ionic liquids, water or vapor, ammonia, N,N-dimethylaminopyridine, acetic anhydride, phosphoric acid (or a salt thereof), boric acid (or a salt thereof), sulfonic acid (or a salt thereof), carboxylic acid (or a salt thereof), carbonate, alkali or alkaline earth oxide, alkali or alkaline earth hydroxide, alkali or alkaline earth carbonate, alkali or alkaline earth carboxylate, or alcohol. In embodiments, the reaction mixture consists of a polyamide and an alkali metal catalyst. These embodiments do not exclude the eventual presence of monomers provided by depolymerization, unreacted polyamide, and / or other depolymerization products (e.g., polyamide fragments) in the reaction mixture. These embodiments also do not exclude the presence of components or impurities that may be inherently present in the reaction mixture due to the specific synthetic techniques used to form the polyamide. These embodiments also do not exclude the presence of components that may be inherently present in the reaction mixture due to the source of the polyamide (e.g., used polyamide).

[0041] The method of the present invention can be carried out using various types of reactor systems, including batch reactor systems, semi-batch reactor systems, and continuous flow reactor systems. As described throughout, the method of the present invention can achieve continuous operation for a certain period of time while maintaining a high monomer yield (for example, by using a continuous flow of polyamide).

[0042] The method of the present invention can further include recovering the product and / or unreacted substances from the reaction mixture. This includes recovering and / or reusing the alkali metal catalyst (or its derivative) from the reaction mixture. A liquid medium is not required for the recovery of the alkali metal catalyst. The recovered alkali metal catalyst can be used to carry out the method one or more additional times (i.e., they can be reused). Similarly, the method of the present invention can further include recovering the depolymerization product (e.g., monomer) and using it for any desired purpose including the synthesis of new polymers containing new polyamides. The recovery of the depolymerization product can be carried out by evaporation or sublimation.

[0043] The method of the present invention can be characterized by the monomer yield. The yield is reported as (weight of monomer) / (starting weight of polyamide)×100%. The yield can be determined using 1 1H NMR as described in the following examples. The yield can be the initial yield obtained using a new (i.e., unused) alkali metal catalyst. The initial yield can be at least 70%, at least 75%, at least 80%, at least 85%, at least 90%, at least 95% or at least 98%. This includes ranges between any of these values, as well as the ranges of 70% - 100%, 80% - 100% and 80% - 95%. The yield can also be the yield obtained using the recovered alkali metal catalyst used one or more times (e.g., 1, 2, 3, 4 times, etc.). The yield from the reused / recovered alkali metal catalyst can be within ±50%, ±20%, ±10%, ±5% or ±2% of the initial yield. Any of the yield values in this paragraph can refer to a specific polyamide (e.g., nylon 6), a specific monomer (e.g., ε-caprolactam), a specific alkali metal catalyst (e.g., any of the catalysts in entries 6 - 30 and 33 - 38 of Table 1) and a method carried out under specific conditions (e.g., any of the methods in Tables 1 - 3).

[0044] As described above, the polyamides depolymerized by the method of the present invention may be provided in a mixture with a non-polyamide polymer (e.g., polyolefin). Other non-polyamide polymers include those used in the following examples. This method can depolymerize polyamides while leaving the other polymers intact. The term "intact" does not necessarily mean completely intact, as small amounts of the non-polyamide polymer may decompose. Therefore, the method of the present invention makes it possible to separate the polyamide from such mixtures. The following examples also show that the method of the present invention can be used to selectively depolymerize a particular type of polyamide (e.g., nylon 6) in a mixture with other different types of polyamides (e.g., nylon 12, nylon 66) and to separate different types of polyamides from each other.

[0045] This disclosure encompasses any of the alkali metal catalysts described herein, as well as any of the disclosed polyamides and reaction mixtures containing (or comprising) such catalysts. [Examples]

[0046] Preface

[0047] This application teaches a novel method for efficiently depolymerizing nylon 6 to ε-caprolactam using abundant and low-cost metal-organic and inorganic alkali-metal superbases, with quantitative conversion, short reaction time (e.g., around 10 minutes), low temperature (e.g., around 220°C), and low catalyst amount (e.g., around 0.2 wt%). These catalysts perform equally well under vacuum and inert atmospheres. In this example, the performance of these “superbase” catalysts is compared with other catalysts used in existing techniques, such as K2CO3, Na2CO3, and KOH, under identical reaction conditions. The results clearly show that these superbase catalysts exhibit significantly higher conversion rates and require lower reaction temperatures than catalysts used in existing techniques. Furthermore, the superbases can be used in continuous operation without reducing the reaction rate. The resulting ε-caprolactam can be repolymerized to the original nylon 6, demonstrating that this method is an alternative to current methods for the depolymerization of nylon 6. This discovery demonstrates that superbase catalysts offer an efficient and sustainable technology for the reuse of nylon 6 in a wide range of industrial applications.

[0048] experiment

[0049] General methods and materials

[0050] Method: All depolymerization reactions were carried out by mixing the polymer with the appropriate catalyst in a cylindrical 50 mL Schlenk tube in an MBraun glove box filled with N2. Heating was supplied by a customized aluminum heating block with holes for the Schlenk tubes.

[0051] General materials: All alkali metal catalysts were purchased commercially and used without purification. Pure nylon 6 powder with an average particle size of 15-20 μm and a molecular weight of 11930 g / mol (measured by GPC) was obtained from Goodfellow Inc. The nylon 6 powder was washed overnight with a 1M KOH solution, filtered, washed with H2O, and dried under high vacuum at 100°C for at least 24 hours before use. Nylon 6 pellets were purchased from Sigma Aldrich and used as is. Fishing nets were purchased from Amazon.

[0052] Physical and analytical methods

[0053] NMR spectra were recorded using a Varian Bruker Avance III HD system equipped with a TXO Prodigy probe (500 MHz). 1 The chemical shift (δ) of the 1H-NMR spectrum was referenced to the internal solvent. Gel permeation chromatography (GPC) was used to analyze the molecular weight and molecular weight dispersion index of synthetic nylon 6.

[0054] General depolymerization procedure

[0055] In some experiments, the following procedure (1) was used. Inside a glove box, a 50 mL oven-dried Schlenk tube was filled with a magnetic stirrer, nylon 6 polymer, and pulverized catalyst. The container was tightly sealed and stirred at room temperature for about 5 minutes to thoroughly mix the polymer and catalyst. The Schlenk tube was then 10 -3The reactor was evacuated to Torr, sealed, and heated to the specified temperature with slow magnetic stirring (50-100 rpm) for the specified time. Since it takes an average of 2.5 minutes for the reaction to reach the depolymerization temperature and for the polymer to begin dissolving, the reaction time was recorded 2.5 minutes after the reaction tube was placed on the heating block. During the reaction, the product sublimated from the high-temperature reaction zone and deposited as a crystalline layer on the cold wall of the reactor. After cooling to room temperature, the soluble portion of the reaction mixture was dissolved in 3-4 mL of deuterated solvent, and mesitylene was added as an internal standard. Samples of this solution were taken for NMR analysis. The yield was 1 The signal integrals of ε-caprolactam and mesitylene were determined by comparing them using 1H-NMR. In other experiments, the following procedure (2) was used. Nylon 6 stored in air was packed into a three-necked round-bottom flask equipped with an overhead stirrer and a cold trap. Flask sizes ranged from 3 liters to 12 liters. The flask was heated under reduced pressure (100-500 mTorr) until all nylon 6 dissolved. Once melted, the reactor was purged with argon gas and the catalyst was introduced. The catalyst can also be introduced before the nylon 6 dissolves. The reactor was sealed and a vacuum was applied to the reaction mixture. Rapid bubbling occurred in the reaction flask, indicating product formation and evaporation, while the product was simultaneously collected in the cold trap. Once the reaction was complete, the cold trap was returned to room temperature and the product was physically removed from the cold trap. The yield was determined by weighing the collected product, and the purity of caprolactam was evaluated using NMR spectroscopy. To further increase the purity of caprolactam, additional vacuum distillation of the contents of the cold trap may be performed at a temperature range of 120–150°C.

[0056] Reaction procedure and NMR spectrum shown in Table 1

[0057] Table 1, Entry 1 [ka]

[0058] Exactly 100 mg of nylon 6 powder (0.89 mmol) and 6.1 mg of K2CO3 (5 mol%, 6.1 wt%) were reacted according to a general procedure, and the reaction mixture was heated at 240°C for 6.0 hours. Exactly 18.5 mg of mesitylene (0.154 mmol) was used as an internal standard. ε-caprolactam was obtained in 29% yield.

[0059] Table 1, Entry 2 [ka]

[0060] Exactly 100 mg of nylon 6 powder (0.89 mmol) and 4.7 mg of Na2CO3 (5 mol%, 4.7 wt%) were reacted according to a general procedure, and the reaction mixture was heated at 240°C for 6.0 hours. Exactly 22.9 mg of mesitylene (0.191 mmol) was used as an internal standard. ε-caprolactam was obtained in 23% yield.

[0061] Table 1, Entry 3 [ka]

[0062] Exactly 100 mg of nylon 6 powder (0.89 mmol) and 4.7 mg of KOH (5 mol%, 2.5 wt%) were reacted according to a general procedure, and the reaction mixture was heated at 240°C for 6.0 hours. Exactly 19.6 mg of mesitylene (0.163 mmol) was used as an internal standard. ε-caprolactam was obtained in a yield of 3%.

[0063] Table 1, Entry 4 [ka]

[0064] Exactly 100 mg of nylon 6 powder (0.89 mmol) and 1.8 mg of NaOH (5 mol%, 1.8 wt%) were reacted according to a general procedure, and the reaction mixture was heated at 240°C for 6 hours. Exactly 26.3 mg of mesitylene (0.218 mmol) was used as an internal standard. ε-caprolactam was obtained in a yield of 3%.

[0065] Table 1, Entry 5 [ka]

[0066] Exactly 100 mg of nylon 6 powder (0.89 mmol), 1.8 mg of NaOH (5 mol%, 1.8 wt%), and 2.5 mg of KOH (5 mol%, 2.5 wt%) were reacted according to a general procedure, and the reaction mixture was heated at 240 °C for 6 hours. Exactly 22.8 mg of mesitylene (0.190 mmol) was used as an internal standard. ε-caprolactam was obtained in a yield of 3%.

[0067] Table 1, Entry 6 [ka]

[0068] Exactly 99.8 mg of nylon 6 powder (0.89 mmol) and 8.8 mg of KN(Si(CH3)3)2 (5 mol%, 8.8 wt%) were reacted according to a general procedure, and the reaction mixture was heated at 240°C for 2.0 hours. Exactly 23.2 mg of mesitylene (0.193 mmol) was used as an internal standard. ε-caprolactam was obtained in 99.8% yield.

[0069] Table 1, Entry 7 [ka]

[0070] Exactly 100 mg of nylon 6 powder (0.89 mmol) and 8.0 mg of NaN(Si(CH3)3)2 (5 mol%, 8.0 wt%) were reacted according to a general procedure, and the reaction mixture was heated at 240°C for 2.0 hours. Exactly 22.2 mg of mesitylene (0.185 mmol) was used as an internal standard. ε-caprolactam was obtained in 94% yield.

[0071] Table 1, Entry 8 [ka]

[0072] Exactly 100 mg of nylon 6 powder (0.89 mmol) and 7.5 mg of LiN(Si(CH3)3)2 (5 mol%, 7.5 wt%) were reacted according to a general procedure, and the reaction mixture was heated at 240°C for 2.0 hours. Exactly 31.8 mg of mesitylene (0.265 mmol) was used as an internal standard. ε-caprolactam was obtained in 99% yield.

[0073] Table 1, Entry 9 [ka]

[0074] Exactly 100 mg of nylon 6 powder (0.89 mmol) and 0.3 mg of NaNH2 (1 mol%, 0.3 wt%) were reacted according to a general procedure, and the reaction mixture was heated at 240°C for 1.0 hour. Exactly 24.0 mg of mesitylene (0.200 mmol) was used as an internal standard. ε-caprolactam was obtained in 81% yield.

[0075] Table 1, Entry 10 [ka]

[0076] Exactly 100 mg of nylon 6 powder (0.89 mmol) and 1.7 mg of NaNH2 (5 mol%, 1.7 wt%) were reacted according to a general procedure, and the reaction mixture was heated at 240°C for 1.0 hour. Exactly 22.5 mg of mesitylene (0.187 mmol) was used as an internal standard. ε-caprolactam was obtained in 85% yield.

[0077] Table 1, Entry 11 [ka]

[0078] Exactly 100 mg of nylon 6 powder (0.89 mmol) and 3.4 mg of NaNH2 (10 mol%, 3.4 wt%) were reacted according to a general procedure, and the reaction mixture was heated at 240°C for 1 hour. Exactly 19.1 mg of mesitylene (0.159 mmol) was used as an internal standard. ε-caprolactam was obtained in 93% yield.

[0079] Table 1, Entry 12 [ka]

[0080] Exactly 100 mg of nylon 6 powder (0.89 mmol) and 6.8 mg of NaNH2 (20 mol%, 6.8 wt%) were reacted according to a general procedure, and the reaction mixture was heated at 240°C for 1 hour. Exactly 29.0 mg of mesitylene (0.241 mmol) was used as an internal standard. ε-caprolactam was obtained in 76% yield.

[0081] Table 1, Entry 13 [ka]

[0082] Exactly 100 mg of nylon 6 powder (0.89 mmol) and 1.7 mg of NaNH2 (5 mol%, 1.7 wt%) were reacted according to a general procedure without applying static vacuum, and the reaction mixture was heated at 240°C for 1.0 hour. Exactly 23.2 mg of mesitylene (0.193 mmol) was used as an internal standard. ε-caprolactam was obtained in 85% yield.

[0083] Table 1, Entry 14 [ka]

[0084] Exactly 100 mg of nylon 6 powder (0.89 mmol) and 1.7 mg of NaNH2 (5 mol%, 1.7 wt%) were reacted according to a general procedure, and the reaction mixture was heated at 280°C for 10 minutes. Exactly 20.0 mg of mesitylene (0.166 mmol) was used as an internal standard. ε-caprolactam was obtained in 93% yield.

[0085] Table 1, Entry 15 [ka]

[0086] Exactly 100 mg of nylon 6 powder (0.89 mmol) and 4.0 mg of NaH (20 mol%, 4.0 wt%) were reacted according to a general procedure, and the reaction mixture was heated at 240°C for 1 hour. Exactly 21.5 mg of mesitylene (0.179 mmol) was used as an internal standard. ε-caprolactam was obtained in 83% yield.

[0087] Table 1, Entry 16 [ka]

[0088] Exactly 100 mg of nylon 6 powder (0.89 mmol) and 2.0 mg of LiNH2 (10 mol%, 2.0 wt%) were reacted according to a general procedure, and the reaction mixture was heated at 240°C for 2 hours. Exactly 26.6 mg of mesitylene (0.221 mmol) was used as an internal standard. ε-caprolactam was obtained in 97% yield.

[0089] Table 1, Entry 17 [ka]

[0090] Exactly 100 mg of nylon 6 powder (0.89 mmol) and 1.0 mg of LiNH2 (5 mol%, 1.0 wt%) were reacted according to a general procedure, and the reaction mixture was heated at 240°C for 0.5 hours. Exactly 21.5 mg of mesitylene (0.179 mmol) was used as an internal standard. ε-caprolactam was obtained in 91% yield.

[0091] Table 1, Entry 18 [ka]

[0092] Exactly 100 mg of nylon 6 powder (0.89 mmol) and 1.0 mg of LiNH2 (5 mol%, 1.0 wt%) were reacted according to a general procedure, and the reaction mixture was heated at 240°C for 1 hour. Exactly 28.0 mg of mesitylene (0.233 mmol) was used as an internal standard. ε-caprolactam was obtained in 94% yield.

[0093] Table 1, Entry 19 [ka]

[0094] Exactly 100 mg of nylon 6 powder (0.89 mmol) and 1.0 mg of LiNH2 (5 mol%, 1.0 wt%) were reacted according to a general procedure without applying static vacuum, and the reaction mixture was heated at 240°C for 1 hour. Exactly 22.7 mg of mesitylene (0.189 mmol) was used as an internal standard. ε-caprolactam was obtained in 87% yield.

[0095] Table 1, Entry 20 [ka]

[0096] Exactly 100 mg of nylon 6 powder (0.89 mmol) and 1.0 mg of LiNH2 (5 mol%, 1.0 wt%) were reacted according to a general procedure, and the reaction mixture was heated at 280°C for 10 minutes. Exactly 16.7 mg of mesitylene (0.139 mmol) was used as an internal standard. ε-caprolactam was obtained in 99% yield.

[0097] Table 1, Entry 21 [ka]

[0098] Exactly 100 mg of nylon 6 powder (0.89 mmol) and 0.2 mg of LiNH2 (1 mol%, 0.2 wt%) were reacted according to a general procedure, and the reaction mixture was heated at 240°C for 2 hours. Exactly 25.3 mg of mesitylene (0.211 mmol) was used as an internal standard. ε-caprolactam was obtained in 86% yield.

[0099] Table 1, Entry 22 [ka]

[0100] Exactly 100 mg of nylon 6 powder (0.89 mmol) and 0.2 mg of LiNH2 (1 mol%, 0.2 wt%) were reacted according to a general procedure, and the reaction mixture was heated at 240°C for 4 hours. Exactly 22.2 mg of mesitylene (0.185 mmol) was used as an internal standard. ε-caprolactam was obtained in 95% yield.

[0101] Table 1, Entry 23 [ka]

[0102] Exactly 100 mg of nylon 6 powder (0.89 mmol) and 1.0 mg of LiNH2 (5 mol%, 1.0 wt%) were reacted according to a general procedure, and the reaction mixture was heated at 220°C for 4 hours. Exactly 23.8 mg of mesitylene (0.198 mmol) was used as an internal standard. ε-caprolactam was obtained in 87% yield.

[0103] Table 1, Entry 24 [ka]

[0104] Exactly 100 mg of nylon 6 powder (0.89 mmol) and 1.5 mg of LiN(Si(CH3)3)2 (1 mol%, 1.5 wt%) were reacted according to a general procedure, and the reaction mixture was heated at 240°C for 6 hours. Exactly 35.2 mg of mesitylene (0.293 mmol) was used as an internal standard. ε-caprolactam was obtained in 62% yield.

[0105] Table 1, Entry 25 [ka]

[0106] Exactly 100 mg of nylon 6 powder (0.89 mmol) and 0.3 mg of LiH (5 mol%, 0.3 wt%) were reacted according to a general procedure, and the reaction mixture was heated at 240°C for 1 hour. Exactly 23.5 mg of mesitylene (0.196 mmol) was used as an internal standard. ε-caprolactam was obtained in 86% yield.

[0107] Table 1, Entry 26 [ka]

[0108] 1.95 g of NaNH2 (5.65 mol%, 1.95 wt%) was added to 100 g of molten nylon 6 pellets with a moisture content of approximately 1% under an Ar gas stream. The reactor was sealed and a vacuum was applied to the reaction mixture. The reaction mixture was heated at a temperature range of 260-270°C for 45 minutes, during which time 86 g of product was collected in a cold trap. The purity of ε-caprolactam was 95.6%.

[0109] Table 1, Entry 27 [ka]

[0110] 1.75 g of NaNH2 (5 mol%, 1.75 wt%) was added to 100 g of molten colored nylon 6 fishing net with a moisture content of 1.75% under an Ar gas stream. The reactor was sealed and a vacuum was applied to the reaction mixture. The reaction mixture was heated at a temperature range of 240-250°C for 50 minutes, during which time 90.57 g of product was collected in a cold trap. The purity of ε-caprolactam was 95.2%. The collected fraction was heated at 140°C for 10 minutes. -1 Vacuum distillation using Torr yielded 4.6% caprolactam dimer. The collected white ε-caprolactam had a purity of 99%.

[0111] Table 1, Entry 28 [ka]

[0112] Exactly 100 mg of nylon 6 powder (0.89 mmol) and 2.5 mg of LiNMe2 (5 mol%, 2.5 wt%) were reacted according to a general procedure, and the reaction mixture was heated at 240°C for 1 hour. Exactly 22.0 mg of mesitylene (0.183 mmol) was used as an internal standard. ε-caprolactam was obtained in 85% yield.

[0113] Table 1, Entry 29 [ka]

[0114] Exactly 100 mg of nylon 6 powder (0.89 mmol) and 10.6 mg of LiCH(Si(CH3)3)2 (5 mol%, 10.6 wt%) were reacted according to a general procedure, and the reaction mixture was heated at 240°C for 2 hours. Exactly 19.1 mg of mesitylene (0.159 mmol) was used as an internal standard. ε-caprolactam was obtained in 81% yield.

[0115] Table 1, Entry 30 [ka]

[0116] Precisely 0.5 mL of nBuLi solution (1.6 M in hexane) was diluted with dry pentane (17.6 mL) to prepare a stock solution of 0.0442 M. Precisely 100 mg of nylon 6 powder (0.89 mmol) was mixed with 1 mL of the above nBuLi stock solution (5 mol%, 0.0442 mmol), and the hexane and pentane were subsequently removed under reduced pressure for 3 hours. Following a general procedure, the mixture was heated at 240 °C for 2 hours. Precisely 22.0 mg of mesitylene (0.183 mmol) was used as an internal standard. ε-caprolactam was obtained in 77% yield.

[0117] Table 1, Entry 31 [ka]

[0118] 11.7 mg of sodium acetylide suspension (18 wt% slurry in xylene, light mineral oil, Sigma-Aldrich) was precisely added to 100 mg of nylon 6 powder (0.89 mmol), and the solvent was subsequently removed under reduced pressure for 72 hours. Following general procedures, the mixture was heated at 240°C for 2 hours. Caprolactam was not obtained.

[0119] Table 1, Entry 32 [ka]

[0120] Exactly 100 mg of nylon 6 powder (0.89 mmol) and 6.3 mg of Cp * Li (5 mol%, 6.3 wt%) was reacted according to a general procedure, and the reaction mixture was heated at 240°C for 2 hours. Exactly 26.3 mg of mesitylene (0.185 mmol) was used as an internal standard. ε-caprolactam was obtained in a yield of 1.2%.

[0121] Table 1, Entry 33 [ka]

[0122] Exactly 100 mg of nylon 6 powder (0.89 mmol) and 3.2 mg of LiNC6H 11 O(3 mol%, 3.2 wt%) was reacted according to a general procedure, and the reaction mixture was heated at 240°C for 1 hour. Exactly 17.0 mg of mesitylene (0.141 mmol) was used as an internal standard. ε-caprolactam was obtained in 93% yield.

[0123] Table 1, Entry 34 [ka]

[0124] Exactly 100 mg of nylon 6 powder (0.89 mmol) and 1.2 mg of LiNC6H 11 O(1.1 mol%, 1.2 wt%) was reacted according to a general procedure, and the reaction mixture was heated at 240°C for 1 hour. Exactly 23.70 mg of mesitylene (0.197 mmol) was used as an internal standard. ε-caprolactam was obtained in 89% yield.

[0125] Table 1, Entry 35 [ka]

[0126] Exactly 100 mg of nylon 6 powder (0.89 mmol) and 3.6 mg of NaNC6H 11 O(3 mol%, 3.6 wt%) was reacted according to a general procedure, and the reaction mixture was heated at 240°C for 1 hour. Exactly 19.0 mg of mesitylene (0.158 mmol) was used as an internal standard. ε-caprolactam was obtained in 97% yield.

[0127] Table 1, Entry 36 [ka]

[0128] Exactly 100 mg of nylon 6 powder (0.89 mmol) and 1.2 mg of NaNC6H 11 O(1 mol%, 1.2 wt%) was reacted according to a general procedure, and the reaction mixture was heated at 240°C for 1 hour. Exactly 17.3 mg of mesitylene (0.143 mmol) was used as an internal standard. ε-caprolactam was obtained in 94% yield.

[0129] Table 1, Entry 37 [ka]

[0130] Exactly 100 mg of nylon 6 powder (0.89 mmol) and 4.0 mg of KNC6H 11 O(3 mol%, 4 wt%) was reacted according to a general procedure, and the reaction mixture was heated at 240°C for 1 hour. Exactly 25.5 mg of mesitylene (0.212 mmol) was used as an internal standard. ε-caprolactam was obtained in 94% yield.

[0131] Table 1, Entry 38 [ka]

[0132] Exactly 100 mg of nylon 6 powder (0.89 mmol) and 1.4 mg of KNC6H 11 O(3 mol%, 4 wt%) was reacted according to a general procedure, and the reaction mixture was heated at 240°C for 1 hour. Exactly 17.5 mg of mesitylene (0.145 mmol) was used as an internal standard. ε-caprolactam was obtained in 87% yield.

[0133] Table 2, Entry 1 [ka]

[0134] Exactly 60 mg of nylon 6 powder (0.45 mmol), 50 mg of nylon 12 powder, and 3.6 mg of NaNC6H 11 O(5 mol%, 6 wt%) was reacted according to a general procedure, and the reaction mixture was heated at 240°C for 3 hours. Exactly 18.0 mg of mesitylene (0.149 mmol) was used as an internal standard. ε-caprolactam was obtained in 95% yield.

[0135] Table 2, Entry 2 [ka]

[0136] Exactly 50 mg of nylon 6 powder (0.45 mmol), 50 mg of nylon 12 powder, and 0.87 mg of NaNH2 (5 mol%, 1.74 wt%) were reacted according to a general procedure, and the reaction mixture was heated at 240°C for 3 hours. Exactly 25.0 mg of mesitylene (0.208 mmol) was used as an internal standard. ε-caprolactam was obtained in 73% yield.

[0137] Table 2, Entry 3 [ka]

[0138] Exactly 50 mg of nylon 6 powder (0.45 mmol), 50 mg of nylon 66 powder, and 3 mg of NaNC6H 11 O(5 mol%, 6 wt%) was reacted according to a general procedure, and the reaction mixture was heated at 260°C for 6 hours. Exactly 24.0 mg of mesitylene (0.199 mmol) was used as an internal standard. ε-caprolactam was obtained in 81% yield.

[0139] Table 2, Entry 4 [ka]

[0140] Exactly 50 mg of nylon 6 powder (0.45 mmol), 50 mg of PE (polyethylene homopolymer) powder, and 3 mg of NaNC6H 11 O(5 mol%, 6 wt%) was reacted according to a general procedure, and the reaction mixture was heated at 240°C for 3 hours. Exactly 19.0 mg of mesitylene (0.158 mmol) was used as an internal standard. ε-caprolactam was obtained in 88% yield.

[0141] Table 2, Entry 5 [ka]

[0142] Exactly 50 mg of nylon 6 powder (0.45 mmol), 16.67 mg of PE (isotactic polypropylene) powder, and 0.87 mg of NaNH2 (5 mol%, 1.74 wt%) were reacted according to a general procedure, and the reaction mixture was heated at 280°C for 3 hours. Exactly 19.0 mg of mesitylene (0.158 mmol) was used as an internal standard. ε-caprolactam was obtained in 84% yield.

[0143] Table 2, Entry 6 [ka]

[0144] Exactly 50 mg of nylon 6 powder (0.45 mmol), 50 mg of i-PP powder, and 3 mg of NaNC6H 11 O(5 mol%, 6 wt%) was reacted according to a general procedure, and the reaction mixture was heated at 240°C for 3 hours. Exactly 19.0 mg of mesitylene (0.158 mmol) was used as an internal standard. ε-caprolactam was obtained in 85% yield.

[0145] Table 2, Entry 7 [ka]

[0146] Exactly 50 mg of nylon 6 powder (0.45 mmol), 50 mg of i-PP powder, and 0.87 mg of NaNH2 (5 mol%, 1.74 wt%) were reacted according to a general procedure, and the reaction mixture was heated at 240°C for 3 hours. Exactly 23.0 mg of mesitylene (0.191 mmol) was used as an internal standard. ε-caprolactam was obtained in 90% yield.

[0147] Table 2, Entry 8 [ka]

[0148] Exactly 50 mg of nylon 6 powder (0.45 mmol), 50 mg of PECO (polyethylene-co-1-octene), and 3 mg of NaNC6H 11 O(5 mol%, 6 wt%) was reacted according to a general procedure, and the reaction mixture was heated at 240°C for 3 hours. Exactly 20.0 mg of mesitylene (0.149 mmol) was used as an internal standard. ε-caprolactam was obtained in 78% yield.

[0149] Table 2, Entry 9 [ka]

[0150] Exactly 71 mg of nylon 6 powder (0.45 mmol), 30 mg of PET (polyethylene terephthalate) powder, and 1:5.55 of NaNC6H 11 O:27 mg (5.7 mol%) of caprolactam mixture was reacted according to a general procedure, and the reaction mixture was heated at 240°C for 6 hours. Exactly 21.6 mg of mesitylene (0.179 mmol) was used as an internal standard. NaNC6H 11 After subtracting additional caprolactam from the O:caprolactam mixture, ε-caprolactam was obtained in 75% yield.

[0151] Table 2, Entry 10 [ka]

[0152] Exactly 50 mg of nylon 6 powder (0.45 mmol), 5 mg of PET powder, and 0.87 mg of NaNH2 (5 mol%, 1.74 wt%) were reacted according to a general procedure, and the reaction mixture was heated at 280°C for 3 hours. Exactly 21.0 mg of mesitylene (0.175 mmol) was used as an internal standard. ε-caprolactam was obtained in 52% yield.

[0153] Table 2, Entry 11 [ka]

[0154] Exactly 100 mg of nylon 6 powder (0.45 mmol), 20 mg of PEG (polyethylene glycol) powder, and 2 mg of NaNH2 (5.8 mol%, 2 wt%) were reacted according to a general procedure, and the reaction mixture was heated at 280°C for 20 minutes. Exactly 19.3 mg of mesitylene (0.160 mmol) was used as an internal standard. ε-caprolactam was obtained in 81% yield.

[0155] Table 2, Entry 12 [ka]

[0156] Exactly 50 mg of nylon 6 powder (0.45 mmol), 50 mg of Kevlar® fiber taken from Kevlar® gloves, and 3 mg of NaNC6H 11 O(5 mol%, 6 wt%) was reacted according to a general procedure, and the reaction mixture was heated at 240°C for 3 hours. Exactly 25.0 mg of mesitylene (0.208 mmol) was used as an internal standard. ε-caprolactam was obtained in 66% yield.

[0157] Table 2, Entry 13 [ka]

[0158] Exactly 100 mg of nylon 6 powder (0.88 mmol), 33 mg of PTFE strip taken from Teflon® tape, and 1.73 mg of NaNH2 (5 mol%, 1.73 wt%) were reacted according to a general procedure, and the reaction mixture was heated at 245°C for 90 minutes. Exactly 0.0206 mg of mesitylene (0.171 mmol) was used as an internal standard. ε-caprolactam was obtained in 89% yield.

[0159] Table 2, Entry 14 [ka]

[0160] Exactly 50 mg of nylon 6 powder (0.45 mmol), 5 mg of spandex, and 0.87 mg of NaNH2 (5 mol%, 1.74 wt%) were reacted according to a general procedure, and the reaction mixture was heated at 240°C for 3 hours. Exactly 23.0 mg of mesitylene (0.191 mmol) was used as an internal standard. ε-caprolactam was obtained in 66% yield.

[0161] Table 2, Entry 15 [ka]

[0162] Exactly 90 mg of nylon 6 powder (0.79 mmol), 10 mg of spandex, and 5.4 mg of NaNC6H 11 O(5 mol%, 6 wt%) was reacted according to a general procedure, and the reaction mixture was heated at 240°C for 2 hours. Exactly 33.2 mg of mesitylene (0.276 mmol) was used as an internal standard. ε-caprolactam was obtained in 87% yield.

[0163] Experimental procedure for sequential depolymerization reactions Before Run 1: In a glove box filled with argon, 13.26 mg of LiNC6H was added to a 100 mL glass Schlenk reactor that had been dried in an oven. 11 250 mg of nylon 6 powder mixed with oxygen was added. The reactor was sealed and the polymer and catalyst were stirred at room temperature for 5 minutes to mix thoroughly. Then the reactor was turned over 10 -3The reactor was evacuated to Torr, tightly sealed, and heated to 240°C for 20 minutes (Run 1). The reactor was cooled and returned to the glove box, and the caprolactam product was carefully scraped from the cold walls of the reactor and placed in vials. The vials were weighed to determine the monomer recovery yield. Runs 2-3: After removing the product from the previous run, 250 mg of fresh polymer was added to the bottom of the glass reactor. The reactor was then cooled for 10 minutes. -3 The reactor was evacuated to Torr, tightly sealed, and heated to 240°C for 20 minutes (Run 2). The above procedure was repeated for Run 3 (30 minutes). Final Run (Run 4): After removing the monomer product from Run 3, the reactor was removed from the glove box without adding any polymer. The reactor was then heated to 10°C. -3 The reactor was evacuated to Torr, tightly sealed, and heated at 240°C for 6 hours to ensure conversion of the remaining polymer. The reactor was then opened to air, and exactly 41.4 mg of mesitylene was added as an internal standard to determine the yield of the final run. 1 The determination was made based on 1H NMR analysis.

[0164] Results and Discussion

[0165] This example uses the general structure MXR nWe report experimental results demonstrating that metal-organic and inorganic alkali-metal superbases (M=Li, Na, K; X=H, C, N; R=alkyl, silyl, aryl, H) can efficiently depolymerize nylon 6 to ε-caprolactam in quantitative yield (e.g., up to 99%), short reaction time (e.g., minimum 10 minutes), low temperature (e.g., minimum 220°C), and with small catalytic amounts (e.g., minimum 0.2 wt%), performing equally well under vacuum and inert atmospheres such as N2 (Table 1). The results obtained using the superbase catalysts are compared under the same reaction conditions with results obtained using other catalysts used in existing techniques, such as K2CO3, Na2CO3, KOH, NaOH, and NaOH-KOH eutectic mixtures. The comparison shows that these superbase catalysts exhibit instantaneous activity and provide significantly faster conversion rates at lower temperatures. These superbase catalysts have the advantage of being suitable for continuous operation, allowing for a continuous supply of nylon 6 during the reaction without experiencing a decrease in reaction rate. Furthermore, the ε-caprolactam produced by the reaction can be successfully polymerized back into the original nylon 6.

[0166] In the initial stages of the study, the performance of K2CO3, Na2CO3, KOH, NaOH, and NaOH-KOH eutectic mixtures was investigated under mild reaction conditions. However, in all cases, a significant amount of the starting material nylon-6 remained unreacted, resulting in ε-caprolactam yields of only 29%, 23%, and 3%, 3%, and 3%, respectively. Note that the yields remained low even when the reaction time was increased to 6 hours and the catalyst amount was tripled compared to previously reported amounts (Table 1, entries 1-5).

[0167] After confirming that catalysts used in existing technologies exhibited low activity under mild conditions, the activity of several ultrabasic catalysts was tested. Surprisingly (considering the relatively small ionic radii of alkali metal ions compared to, for example, lanthanide ions), the use of the MN(Si(CH3)3)2(M=K,Na,Li) catalyst significantly improved the reaction rate, selectively producing ε-caprolactam in yields of 99%, 84%, and 99%, respectively, at 240°C for 2 hours (Table 1, entries 6-8). Despite obtaining quantitative ε-caprolactam yields, the weight of the catalyst used was relatively high (ranging from 7.5 to 8.8 wt%). Therefore, we investigated whether further enhancing the basicity of the catalyst would improve the overall performance of the nylon 6 depolymerization reaction.

[0168] To achieve this, we tested ultrabasic amide MNH2 and inorganic MH (M=Na,Li) catalysts. These catalysts are commercially available in ton quantities, are cost-effective, and have simple ligand structures (NH2 or H). They are also lightweight, significantly reducing the packing weight required to obtain optimal results. However, these catalysts also have relatively low solubility and high melting points (limiting mass transfer). Nevertheless, the results in Table 1 show that the catalysts exhibit surprisingly high activity. More specifically, entries 9–14 in Table 1 show systematic studies investigating the effects of catalyst (NaNH2) amount, reaction time, and temperature on the yield of ε-caprolactam. Note that in the depolymerization of nylon 6 with the NaNH2 catalyst, a low catalyst weight of 0.3 wt% was required (Table 1, entry 9), and ε-caprolactam was obtained in 81% yield after 1 hour. Increasing the catalyst amount from 0.3 wt% to 1.7% and 3.4% increased the yields to 85% and 93%, respectively (Table 1, entries 10 and 11). Further increasing the catalyst amount to 6.9 wt% decreased the yield to 76% (Table 1, entry 12). The reaction can proceed similarly under static vacuum or under an inert atmosphere of approximately 1 atm (N2 or Ar, Table 1, entry 13). Note that raising the reaction temperature to 280°C yielded a 93% ε-caprolactam yield after 10 minutes (Table 1, entry 14). Interestingly, using NaH as a catalyst yielded ε-caprolactam in 83% yield after 1 hour (Table 1, entry 15).

[0169] Surprisingly, the use of lithium amide (LiNH2), a smaller metal ion analog of sodium amide, further improved the conversion rate and yield. This is surprising, as catalytic activity for depolymerization is expected to be roughly proportional to the ionic radius of the metal. The superior performance of LiNH2 was also unexpected, as its melting point is 390°C, higher than the reaction temperature and 180°C higher than that of NaNH2 (mp=210°C). For example, using 2 wt% LiNH2 yielded a 97% ε-caprolactam yield after 2 hours at 240°C (Table 1, entry 16). Furthermore, the reaction was essentially complete even with a smaller catalyst amount (1 wt%) and a shorter reaction time (30 minutes) (Table 1, entry 17). Increasing the reaction time to 1 hour slightly increased the yield to 94% (Table 1, entry 18). Switching to an inert gas atmosphere (N2 or Ar) yielded ε-caprolactam in 87% yield (Table 1, entry 19). Impressively, raising the reaction temperature to 280°C resulted in a 99% ε-caprolactam yield after only 10 minutes (Table 1, entry 20). Further reducing the catalyst amount to 0.2 wt% produced 86% ε-caprolactam after 2 hours without significantly affecting the reaction yield (Table 1, entry 21). Increasing the reaction time to 4 hours increased the yield to 95% (Table 1, entry 22). Notably, the reaction proceeded smoothly at 220°C, the melting point of nylon 6, yielding a high yield of 87% after 4 hours (Table 1, entry 23). Note that at this temperature, nylon 6 exhibits semi-solid properties, hindering efficient mass transfer and mixing. Despite these challenges, the process remained efficient and yielded excellent results. In contrast, when LiN(Si(CH3)3)2 was used with the same 1 mol% catalyst amount and under the same conditions, the yield was significantly lower, reaching only 62% even after increasing the reaction time to 6 hours (Table 1, Entry 24). This further emphasizes the importance of catalyst basicity. When 0.3 wt% of LiH, the lightest ultrabase, was used as the catalyst, a yield of 86% was observed after 1 hour (Table 1, Entry 25).

[0170] The catalytic process was also carried out on a large scale and demonstrated to be suitable for untreated nylon 6 pellets and untreated colored nylon 6 fishing nets. Therefore, when 100 g of untreated nylon 6 pellets stored in air were reacted with 5 mol% (1.7 wt%) NaNH2 at 250–260°C for 45 minutes, an 82% caprolactam yield was obtained (Table 1, entry 26). Interestingly, when 100 g of untreated colored nylon 6 fishing nets stored in air with a moisture content of 1.75 wt% was used under similar reaction conditions, an isolation yield mass of 90.57 g was collected. The purity of ε-caprolactam was 95.2%. The collected fraction was heated at 140°C for 10 minutes. -1 Vacuum distillation with Torr yielded 99% pure ε-caprolactam in 88% yield (Table 1, entry 27). Additional experiments were conducted to investigate the substituent effect on alkali metals. When 5 mol% LiNMe2 was used, a yield of 85% was observed after 1 hour at 240°C (Table 1, entry 28), which is 9% lower than the reaction with LiNH2 under the same conditions (Table 1, entry 18) and 14% lower than with LiN(Si(CH3)3)2 (Table 1, entry 8). Surprisingly, when the organolithium reagent LiCH(Si(CH3)3)2 was used, a yield of 81% was obtained after 2 hours, indicating that catalytic activity is not limited to N-containing catalysts only (Table 1, entry 29). This result was unexpected, as organolithium reagents are known to nucleophilically add to carbonyl groups to produce organically substituted compounds such as oxygenates, and therefore were expected to inactivate rather than catalyze the depolymerization of nylon 6. In other words, given their high reactivity and low selectivity, organolithium reagents were expected to be inactivated by side reactions such as nucleophilic substitution. LiC4H9(nBuLi) was also found to be catalytically active, yielding a 77% yield after 2 hours (Table 1, entry 30). However, not all organometallic compounds showed catalytic activity. Therefore, NaCCH or Cp * When Li was used, CPL yields of 0% and 1.2% were obtained after 2 hours (Table 1, entries 31-32). Interestingly, the general formula is MNC6H 11When a catalyst having a cyclic caprolactam ligand on a metal O(M=Li,Na,K) was tested, an exceptionally high yield was observed. While several metal caprolactamates have been used as nylon 6 polymerization catalysts in the conversion of caprolactam to nylon 6, in this experiment, MNC6H 11 It was surprising to see that O(M=Li,Na,K) could efficiently perform the exact reverse operation, namely converting nylon 6 back to caprolactam. Specifically, entries 33-38 in Table 1 show catalytic amounts (1-3 mol%) of MNC6H 11 Using O(M=Li,Na,K), a caprolactam yield of 87-97% can be obtained after 1 hour at 240°C.

[0171] [Table 1A] [Table 1B]

[0172] Next, as shown in Equation 2, experiments were conducted to investigate whether this catalyst could depolymerize nylon 6 in the presence of other polymers, such as polyamide 12 (nylon 12), polyamide 66 (nylon 66), polyethylene (PE), polyethylene-co-1-octene (PECO), isotactic polypropylene (i-PP), polyethylene terephthalate (PET), polyethylene glycol (PEG), poly(p-phenylene terephthalamide) (Kevlar®), polytetrafluoroethylene (PTFE), and polyurethane-polyurea copolymer (spandex). Surprisingly, when nylon 6 was mixed with other polyamides such as nylon 12 and nylon 66, high selectivity and activity for converting nylon 6 to caprolactam were maintained despite the presence of potentially competing amide groups in the mixture. Therefore, a 1:1 mixture of nylon 6 and nylon 12 with 5 mol% NaNC6H 11When heated in the presence of 0 or NaNH2, caprolactam was obtained in yields of 95% and 73%, respectively (Table 2, entries 1-2). When a 1:1 mixture of nylon 6 and nylon 66 was used, a caprolactam yield of 81% was obtained (Table 2, entry 3). When quantitative conversion of nylon 6 to caprolactam was observed, the other polyamides could be recovered without alteration. This provides a unique solvent-free method for chemically separating different polyamides.

[0173] This process was also suitable for various types of plastic mixtures. For example, when nylon 6 was mixed with polyolefins such as polyethylene (PE), isotactic polypropylene (i-PP), or polyethylene-co-1-octene (PECO), caprolactam was quantitatively recovered in yields of 78–90% (Table 2, entries 4–8), with the unaltered polyolefin remaining at the bottom of the reactor. This process was also remarkably compatible with oxygen-rich polymers such as polyesters and polyethers. For polyethylene terephthalate (PET), caprolactam yields ranging from 52–75% were obtained (Table 2, entries 9–10), and a yield of 81% was observed with polyethylene glycol (PEG) (Table 2, entry 11).

[0174] This process also worked in the presence of used high-melting-point polymers such as Kevlar® fibers taken from gloves and Teflon® tape (PTFE). In the case of Kevlar®, a modest caprolactam yield of 66% was observed due to mass transfer problems and lack of stirring (Table 2, entry 12). In the case of Teflon®, a caprolactam yield of 89% was observed (Table 2, entry 13). Interestingly, this process could also work in the presence of polyurethanes such as spandex, a polymer commonly mixed with nylon 6, yielding caprolactam yields of 66–87% (Table 2, entries 14–15).

[0175] [Table 2]

[0176] Finally, to test the recyclability of the alkali ultrabase catalyst, we used the catalyst LiNC6H 11 Using O, a multi-batch simulated continuous depolymerization experiment was performed in a larger reactor (100 mL flask). Before each run, the caprolactam collected from the previous run was removed, weighed, and the same catalyst LiNC6H was used. 11 250 mg of new nylon 6 was added to a reactor packed with oxygen. The result was catalyst LiNC6H 11 The results showed that O maintained a high level of activity throughout each run. The final (fourth) run was performed without adding new nylon, and the overall yield of nylon 6 was 79.3%. The results are shown in Table 3 below.

[0177] [Table 3]

[0178] The term “exemplary” is used herein to mean that it serves as an example, instance, or illustration. Aspects or designs described herein as “exemplary” are not necessarily construed to be preferable or more favorable than other aspects or designs. Furthermore, for the purposes of this disclosure, and unless otherwise specified, “a” or “an” means “one or more.”

[0179] Unless already included, all numerical values ​​of parameters in this disclosure are preceded by the term “approximately,” meaning they are approximate. This includes variations inherent in the measurement of the relevant parameters, as will be understood by those skilled in the art. This includes the exact values ​​of the disclosed numerical values ​​and the values ​​rounded to the disclosed numerical values.

[0180] The foregoing description of exemplary embodiments of the disclosure is provided for illustrative and descriptive purposes only. It is not intended to be exhaustive or to limit the disclosure to the exact form disclosed, and modifications and alterations are possible in light of the above teachings or may be derived from the practice of the disclosure. The embodiments are selected and described as practical applications of the disclosure, to illustrate the principles of the disclosure and to enable those skilled in the art to utilize the disclosure in various embodiments and with various modifications to suit specific intended uses. The scope of the disclosure is intended to be defined by the claims and equivalents thereof attached herein.

Claims

1. A method for depolymerizing a polyamide, the method comprising mixing a polyamide with an alkali metal catalyst for depolymerizing the polyamide to a product, wherein the alkali metal catalyst is of the formula MXR ｎ A method selected from those having (wherein M is an alkali metal, X is H, C, N, Si or Sn, R is independently selected from hydrogen, alkyl, silyl and aryl, and n is 0 to 2).

2. The method according to claim 1, wherein M is selected from Li, Na, K, Cs and mixtures thereof.

3. The method according to claim 1, wherein M is selected from Li, Na, K, Cs and mixtures thereof, X is N or H, and R is independently selected from hydrogen, alkyl and silyl.

4. The method according to claim 1, wherein M is selected from Li, Na, K, Cs and mixtures thereof, X is N, R is hydrogen, and n is 2.

5. The method according to claim 4, wherein M is Na.

6. The method according to claim 1, wherein M is selected from Li, Na, K, Cs and mixtures thereof, X is H, and n is 0.

7. The method according to claim 6, wherein M is Na.

8. The method according to claim 1, wherein M is selected from Li, Na, K, Cs and mixtures thereof, X is N, and R is independently selected from hydrogen, alkyl and silyl.

9. The method according to claim 8, wherein R is alkyl, alkyl is a substituted cycloalkyl, and n is 1.

10. The method according to claim 9, wherein M is Na.

11. The method according to claim 1, wherein the polyamide is a polymerization product of a monomer selected from 2-pyrrolidone, 2-piperidone, ε-caprolactam, enantractam, capryllactam, pelargolactam, azacycloundecane-2-one, azacyclotridecane-2-one, and combinations thereof.

12. The method according to claim 1, wherein the polyamide is selected from poly(2-pyrrolidine) (nylon 4), poly(2-piperidone) (nylon 5), poly(hexano-6-lactam) (nylon 6), polyenanthamide (nylon 7), polycapryllactam (nylon 8), poly(9-aminononanoic acid) (nylon 9), poly(10-aminodecanoic acid) (nylon 10), poly(11-aminoundecanoic acid) (nylon 11), poly(dodecano-12-lactam) (nylon 12), poly[imino(1,6-dioxohexamethylene)iminohexamethylene] (nylon 66), poly[imino(1,6-dioxohexamethylene)iminotetramethylene] (nylon 46), and combinations thereof.

13. The method according to claim 1, wherein the product comprises a monomer that forms the polyamide.

14. The method according to claim 13, wherein the monomer is a cyclic amide.

15. The method according to claim 1, wherein the method is carried out in the absence of a liquid medium.

16. The method according to claim 1, wherein a polyamide and an alkali metal catalyst form a reaction mixture consisting of a polyamide and an alkali catalyst.

17. The method according to claim 1, wherein the method is carried out using a continuous flow reactor system.

18. The method according to claim 1, further comprising recovering a product from a reaction mixture comprising a polyamide and an alkali metal catalyst, wherein the product comprises monomers that form the polyamide.

19. The above method involves a temperature of 240°C or lower, a time of 4 hours or less, an amount of alkali metal catalyst of 10 mol% or less, and 10 －３ The method according to claim 3, wherein the method is carried out using a pressure below Torr, and the method provides a yield of at least 90% of the product.

20. The method according to claim 19, wherein the polyamide comprises poly(hexano-6-lactam)(nylon 6).

21. The method according to claim 20, wherein the product comprises a monomer that forms the polyamide.

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