Chemical recycling of nylon-6 to epsilon-caprolactam via depolymerization catalysts

Abstract

Provided are methods for depolymerizing a polyamide, which may comprise combining a polyamide and an alkaline earth metal catalyst to depolymerize the polyamide to a product, wherein the alkaline earth metal catalyst comprises an alkaline earth metal and ligands bound to the alkaline earth metal, wherein the ligands are independently selected from an oxygen-containing ligand other than an oxygen atom and other than hydroxide; hydrogen (H); a carbon-containing ligand other than a carbonate and other than a carboxylate; a nitrogen-containing ligand; a halogen; and an alkali metal.

Description

Atty. Dkt. No. 00100-0413 -PCTCHEMICAL RECYCLING OF NYLON-6 TO EPSILON-CAPROLACTAM VIA DEPOLYMERIZATION CATALYSTS CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] The present application claims priority to U.S. provisional patent application number 63 / 720,257 that was filed November 14, 2024, the entire contents of which are incorporated herein by reference.REFERENCE TO GOVERNMENT RIGHTS

[0002] This invention was made with government support under grant number DE-SC0022290 awarded by the Department of Energy'. The government has certain rights in the invention.BACKGROUND

[0003] Modem society is heavily dependent on plastics-based materials, as evident by the steady growth in their production. Plastics are extremely ubiquitous, versatile, and low-cost polymeric materials that have dramatically enhanced the quality of human life for more than a century. Currently, plastics are being produced worldwide at an annual rate of 450 million tons year, which is projected to double by 2045. One class of commonly used plastics for applications requiring the material / product to withstand harsh mechanical and environmental conditions are engineering plastics such as polyamides.

[0004] Nylon-6 was one of the first synthetic fibers discovered and developed by Schlack in 1938. Nylon-6 is a thermoplastic polyamide produced industrially by water-assisted ring-opening polymerization (ROP) of E-caprolactam on an 8.9 million tons annual scale, with the market size expected to reach $21.5 billion in 2026. Nylons as non-degradable plastics substantially contribute to the pollution in oceans and landfills, reflecting their superior chemical robustness and the paucity of effective recycling technologies. Indeed, it is estimated that by 2050, plastic waste will outweigh fish in the ocean, with Nylon-6 contributing ca. 10% of ocean plastic pollution as discarded or lost fishing nets (so-called "ghost nets"). This amounts to more than 600.000 tons of abandoned fishing nets per year.Atty. Dkt. No. 00100-0413 -PCTSUMMARY

[0005] Provided are methods for depolymerizing polyamides, e.g., depolymerizing Nylon-6 to e-caprolactam, using certain alkaline earth metal catalysts comprising an alkaline earth metal, e.g., Mg, Ca. Magnesium and calcium are the 8thand 5thmost abundant metals in the Earth's crust, respectively, and are also readily available in seawater. This high abundance, coupled with their ease of extraction, renders the present catalysts sustainable and cost-effective. The Example, below, describes experimental results demonstrating that illustrative such catalysts achieve quantitative conversions under mild temperatures (e.g., 240 °C) and with low catalyst loadings (e.g., 5 mol%). Experimental results also show that the methods are compatible with virgin, post-consumer, and mixed plastic waste blends, including Nylon-6 mixed with a wide variety of other polymers, while providing high yields of pure e-caprolactam monomer. The resulting e-caprolactam obtained by using the present methods to depolymerize Nylon-6 can then be repolymerized back into high-quality7Nylon-6, providing a sustainable alternative to conventional Nylon-6 depolymerization methods. These results demonstrate the ability of the disclosed methods and alkaline earth metal catalysts to transform Nylon-6 recycling and support broad industrial applications. The Example, below, describes additional experimental results that demonstrate an unexpected synergy7between the alkaline earth metal species within the alkaline earth metal catalysts and alkali metal species. The alkali metal species may be present due to a polyamide pretreatment, present as an additive in a reaction mixture comprising the polyamide and the alkaline earth metal catalyst, or a combination thereof.

[0006] An embodiment 1 is a method for depolymerizing a polyamide, the method comprising combining a polyamide and an alkaline earth metal cataly st to depolymerize the polyamide to a product, wherein the alkaline earth metal catalyst comprises an alkaline earth metal and ligands bound to the alkaline earth metal, wherein the ligands are independently selected from an oxygen-containing ligand other than an oxygen atom and other than hydroxide; hydrogen (H); a carbon-containing ligand other than a carbonate and other than a carboxylate; a nitrogen-containing ligand; a halogen; and an alkali metal.

[0007] An embodiment 2 is according to embodiment 1, wherein the alkaline earth metal catalyst is selected from those having formula MXmRnMo’, wherein M is the alkaline earth metal; X is the halogen; each R is independently selected from an oxygen atom (O) of the oxy gen-containing ligand, H, a carbon atom (C) of the carbon-containing ligand, and aAtty. Dkt. No. 00100-0413 -PCTnitrogen atom (N) of the nitrogen-containing ligand; M' is the alkali metal; m is 0 or 1; n is 2, 3, or 1; and o is 0 or 1; and further wherein if mis 0 and o is 0, then n is 2; if m is 1, then n is 1 and o is 0; and if o is 1 , then m is 0 and n is 3.

[0008] An embodiment 3 is according to any of embodiments 1-2, wherein the alkaline earth metal is Mg or Ca.

[0009] An embodiment 4 is according to any of embodiments 1-3, wherein the oxy gencontaining ligand is an alkoxy or an aryloxy; the carbon-containing ligand is an alkyl; and the nitrogen-containing ligand is an amido, an amidate, or a nitrogen-substituted cycloalkyl.

[0010] An embodiment 5 is according to embodiment 2, wherein the formula is MR2.

[0011] An embodiment 6 is according to embodiment 5, wherein each R is an independently selected alkoxy.

[0012] An embodiment 7 is according to embodiment 6, wherein each R is ethoxy.

[0013] An embodiment 8 is according to embodiment 5, wherein each R is an independently selected amido.

[0014] An embodiment 9 is according to embodiment 8, wherein each R is N(Si(CH3)s)2.

[0015] An embodiment 10 is according to embodiment 5, wherein each R is an independently selected alkyl.

[0016] An embodiment 11 is according to embodiment 10, wherein each R is independently selected from n-butyl and sec-buty l.

[0017] An embodiment 12 is according to embodiment 5, wherein each R is H.

[0018] An embodiment 13 is according to any of embodiments 1-12, wherein the polyamide is poly(hexano-6-lactam) (Nylon-6), poly(2-pyrrolidinone) (Nylon-4), or a combination thereof.

[0019] An embodiment 14 is according to any of embodiments 1-13, wherein the product comprises a monomer from which the polyamide was formed.

[0020] An embodiment 15 is according to embodiment 14, wherein the monomer is a cyclic amide.Atty. Dkt. No. 00100-0413 -PCT

[0021] An embodiment 16 is according to any of embodiments 1-15, wherein the polyamide and the alkaline earth metal catalyst are combined in the presence of an alkali metal species.

[0022] An embodiment 17 is according to embodiment 16, wherein the alkali metal species is present due to pretreating the polyamide with the alkali metal species prior to combining the polyamide and the alkaline earth metal catalyst; the alkali metal species is an additive combined with the polyamide and the alkaline earth metal catalyst; or combinations thereof.

[0023] An embodiment 18 is according to any of embodiments 1-17, wherein the polyamide is Nylon-6; the alkaline earth metal is Mg or Ca; and the product comprises s-caprolactam.

[0024] An embodiment 19 is according to embodiment 18, wherein the polyamide and the alkaline earth metal catalyst are combined in the presence of an alkali metal species.

[0025] An embodiment 20 is according to embodiment 19, wherein the formula is MR2 and wherein each R is an independently selected alkoxy or aryloxy.

[0026] An embodiment 21 is according to any of embodiments 1-20, wherein the polyamide is Nylon-6 and the product comprises s-caprolactam, and further wherein the Nylon-6 is provided as a blend with a different type of polymer and the different type of polymer remains intact after the method.

[0027] An embodiment 2 is according to any of embodiments 1-21, wherein the polyamide is Nylon-6 and the product comprises s-caprolactam, and the method further comprises adding additional Nylon-6 an additional time or continuously without adding additional alkaline earth metal catalyst to produce additional s-caprolactam.

[0028] Other principal features and advantages of the disclosure will become apparent to those skilled in the art upon review of the following drawings, the detailed description, and the appended claims.DETAILED DESCRIPTION

[0029] A method for depolymerizing a polyamide comprises combining a polyamide and an alkaline earth metal catalyst, which are each further described below, under conditions to depolymerize the polyamide. The polyamide to be depolymerized by the present methods is aAtty. Dkt. No. 00100-0413 -PCTpolymer composed of monomers covalently bound into an extended chain via amide linking groups. The term '’monomer" refers to the chemical reactant that is incorporated to form the extended chain and the amide linking groups during a polymerization reaction. The polyamide may be an aliphatic polyamide (i.e., not comprising aromatic rings) or an aromatic polyamide (i.e., comprising aromatic rings). The polyamide may be a homopolymer (i.e., formed from a single type of monomer) or a heteropolymer (i.e., formed from more than one type of monomer, e.g., two; "heteropolymers" may be referred to as copolymers). The term “type” refers to a chemical formula such that a single type means the same chemical formula and a different ty pe means different chemical formulas.

[0030] The polyamide may be one which is 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. Illustrative cyclic amides include 2-pyrrolidone, 2-piperidone, s-caprolactam, enantholactam, capryllactam, pel argol actam, azacycloundecan-2-one, and azacyclotri decan-2-one.

[0031] The polyamide may be one which is formed from condensation of an amine (including a diamine) with an acid (including a diacid). The amine and the acid may be provided on a single chemical compound such as 11 -aminoundecanoic acid or co -aminolauric acid. Alternatively, the amine and the acid may be provided on two different chemical compounds such as hexamethylene diamine and adipic acid. In these embodiments, the monomer is the single chemical compound (with the amine and acid groups) or the two chemical compounds (the diamine and the diacid), and the polymerization reaction is condensation.

[0032] The polyamide may be identified by reference to the monomers (e.g., cyclic amide, diamine, diacid) used to form the polyamide, recognizing that the chemical form of these monomers may be modified by the ensuing polymerization reaction that provides the amide linking groups of the polyamide.

[0033] Illustrative polyamides to be depolymerized in the present methods include poly(2-pyrrolidinone) (Nylon-4), poly(2-piperidone) (Nylon-5), poly(hexano-6-lactam) (Nylon-6), polyenanthamide (Nylon-7), poly capryllactam (Nylon-8), poly(9-aminononanoic acid (Nylon-9), poly(10-aminodecanoic acid) (Nylon-10), poly(ll -aminoundecanoic acid) (Nylon-11), poly(dodecano- 12-1 actam) (Nylon-12), poly[imino(l,6-dioxo hexamethylene)imino hexamethylene] (Nylon-66), and poly [imino(l,6-di oxohexamethylene)Atty. Dkt. No. 00100-0413 -PCTiminotetramethylene] (Nylon-46). In embodiments, the polyamide to be depolymerized is Nylon-6, Nylon-4, or a combination thereof.

[0034] The present methods may be used to depolymerize any of the disclosed polyamides. The depolymerization process deconstructs the polyamide into a product. In embodiments, the depolymerization process provides a monomer (e.g., a cyclic amide) from which the polyamide was formed, i.e., the product comprises (or consists of) the monomer of the polyamide. Throughout this disclosure, the terms “a monomer” and “monomer” encompass both a single type of monomer and multiple, different t pes of monomers, i.e., depending upon the particular polyamide. In such embodiments, this monomer may be recovered and used to reform the polyamide. This is by contrast to depolymerization processes which do not provide a monomer, but rather a product comprising a chemical compound which cannot reform the polyamide absent additional steps to convert the chemical compound to the monomer. However, the present methods encompass both types of depolymerization processes, i.e., those that provide the monomer of the polyamide and those that provide a different t pe of product.

[0035] In embodiments, the methods depolymerize Nylon-6 to produce s-caprolactam. In embodiments, the methods depolymerize Nylon-4 to produce 2-pyrrolidone. In embodiments, the methods depolymerize Nylon-5 to produce 6-Valerolactam. In embodiments, the methods depolymerize Nylon-7 to produce enantholactam. In embodiments, the methods depolymerize Nylon-8 to produce capryllactam. In embodiments, the methods depolymerize Nylon-9 to produce pelargolactam. In embodiments, the methods depolymerize Nylon- 10 to produce azacycloundecan-2-one. In embodiments, the methods depolymerize Nylon- 11 to produce w-undecalactam . In embodiments, the methods depolymerize Nylon- 12 to produce laurolactam.

[0036] A single type of polyamide or multiple, different types of polyamides may be used (i.e., a blend of different types of polyamides). In embodiments, the polyamide is a blend comprising different types of polyamides and the present method selectively depolymerizes certain of the polyamides. For example, in embodiments, the polyamide is a blend comprising Nylon-6, Nylon-4, or a combination thereof, and a different type of polyamide (e.g., Nylon-66, Nylon-46, Nylon- 12, or combinations thereof) and the present methods depolymerize the Nylon-6 / Nylon-4 to the corresponding monomers (e-caprolactam / 2-pyrrolidone) without depolymerizing Nylon-66 / Nylon-46 / Ny Ion- 12. By “selective depolymerization” andAtty. Dkt. No. 00100-0413 -PCT“without"’ it is meant that any yield of the Nylon-66 / Nylon-46 / Nylon-12 monomer(s) (or other polyamide monomer) is less than the detection limit of the 'H NMR system used to determine yield as described in the Example, below. This includes measuring a monomer yield viaXH NMR that is less than 5%.

[0037] The alkaline earth metal catalysts used in the present methods comprise (or consist of) an alkaline earth metal (M) and ligands (L) bound (e.g., covalently) thereto. The alkaline earth metal catalysts include organometallic compounds in which the ligands are independently selected from hydrogen (H), a carbon atom (C) of a carbon-containing ligand, or both; metal-organic compounds in which the ligands are independently selected from a nitrogen atom (N) of a nitrogen-containing ligand, an oxygen atom (O) of an oxy gencontaining ligand, or both; mixed organometallic and metal-organic compounds in which the ligands are independently selected from each of the corresponding ligands for these compounds; Grignard reagents in which at least one ligand is a halogen and at least one ligand is selected from the ligands corresponding to the organometallic and metal-organic compounds; and “-ate” complexes in which at least one ligand is selected from an alkali metal and another ligand(s) is independently selected from the corresponding ligands for the organometallic and metal-organic compounds in this paragraph. In such -ate complexes, the alkali metal ligand can be a counterion associated with the alkaline earth metal. Thus, a bound ligand may be bound via an electrostatic association with the alkaline earth metal.

[0038] Regarding the organometallic compounds, the metal-organic compounds, the mixed organometallic and metal-organic compounds, the Grignard reagents, and the -ate complexes described above, these are distinguished from oxides of the alkaline earth metal, hydroxides of the alkaline earth metal, carbonates of the alkaline earth metal, and carboxylates of the alkaline earth metal. Such compounds are excluded from the present alkaline earth metal catalysts. In embodiments, the Grignard reagents may be excluded from the present alkaline earth metal catalysts.

[0039] Thus, the alkaline earth metal catalysts encompass those having Formula I, MXmRnMo’, wherein M is an alkaline earth metal; X is a halogen; each R is independently selected from H, C, N. or O; M’ is an alkali metal; m is 0 or 1; n is 1. 2, or 3; and o is 0 or 1. In Formula I, if m is 1, then n is 1 and o is 0; if o is 1, then m is 0 and n is 3; and if m is 0 and o is 0, then n is 2. Regarding R in Formula I, as noted above, the carbon (C) refers to a carbon atom of a carbon-containing ligand bound to the alkaline earth metal via the carbon atom,Atty. Dkt. No. 00100-0413 -PCTe.g., alkyl. Similarly, the nitrogen (N) refers to a nitrogen atom of a nitrogen-containing ligand bound to the alkaline earth metal via the nitrogen atom, e.g., an amido, an amidate, a nitrogen-substituted cycloalkyl. (The term amido is analogous to ’‘amine’’ but more accurately reflects the deprotonated or metalated form of the ligand bound to the alkaline earth metal via the nitrogen atom. The term amidate is analogous to “amide” but more accurately reflects the deprotonated and metalated form of the ligand bound to the alkaline earth metal via the nitrogen atom.) Similarly, the oxygen (O) refers to an oxygen atom of an oxy gen-containing ligand bound to the alkaline earth metal via the oxygen atom, e.g., an alkoxy, an aryloxy. If multiple R groups are present, they may be the same or different.

[0040] The alkaline earth metal catalysts include those having Formula IA, MR2, wherein M and R are as defined above. In some embodiments of Formula IA, each R is H. In some embodiments of Formula I A, each R is C, wherein C is part of independently selected carbon-containing ligands such as alkyds (e.g.. n-butyl, sec-butyl). In some embodiments of Formula IA, each R is N, wherein N is part of independently selected nitrogen-containing ligands such as amidos (e.g., -N(Si(CHs)3)2) or nitrogen-substituted cycloalkyls (e.g., caprolactamato). In some embodiments of Formula IA, each R is O, wherein O is part of independently selected oxygen-containing ligands such as an alkoxy (e.g., ethoxy).

[0041] The alkaline earth metal catalysts include those having Formula IB, MXR, wherein M, X, and R are as defined above. In some embodiments of Formula IB, R is an alkyl (e.g., methyl).

[0042] The alkaline earth metal catalysts include those having Formula IC, R3MM’, wherein M, R, and M’ are as defined above.

[0043] In Formula I and its related formulas, various alkaline earth metals may be used, e.g., Mg, Ca. In embodiments, the alkaline earth metal in Formula I and its related formulas is Mg. Similarly, various halogens may be used, e.g., Br. Similarly, various alkali metals may be used, e.g.. Li, Na. K.

[0044] In Formula I and its related formulas, “alkyl” refers to a linear, branched, or cyclic alkyl group in which the number of carbons may range from, e.g., 1 to 8. 1 to 6, 1 to 4. 1 to 3, or 1 to 2. A cyclic alkyl group may be referred to as a cycloalkyl group. The alkyl group may be unsubstituted, by which it is meant the alkyl group contains no heteroatoms. An unsubstituted alkyl group encompasses an alkyl group in which one or more bonds to a carbon(s) or hydrogen(s) are replaced by a bond to an unsubstituted aromatic ring, e.g.Atty. Dkt. No. 00100-0413 -PCTbenzyl. The alkyl group may be substituted, by which it is meant an unsubstituted alkyl group in which one or more bonds to a carbon(s) or hydrogen(s) are replaced by a bond to nonhydrogen and non-carbon atoms.

[0045] In Formula I and its related formulas, "aryl” refers to a monocyclic aryl group having one aromatic ring (e.g., benzene) or a polycyclic group having more than one aromatic ring (e.g., two, three, etc. rings). Monocyclic ary l groups may be unsubstituted or substituted as described above with respect to alkyl groups. Regarding poly cyclic groups, neighboring aromatic rings may be fused or unfused. The aromatic rings of a polycyclic group may be unsubstituted or substituted as described above with respect to monocyclic aryl groups.

[0046] In Formula I and its related formulas, "silyl group” refers to -SiRs, wherein R is independently selected from hydrogen, alkyl groups, and ar l groups (each of which has been defined herein) and denotes the bond, e.g., to a nitrogen atom in R of Formula I and all its related formulas.

[0047] In Formula I and its related formulas, "amido” refers to -NR2, wherein each R is independently selected from hydrogen, alkyl groups, aryl groups, and silyl groups (each of which has been defined herein) and “-” denotes the bond to the alkaline earth metal in Formula I and all its related formulas.

[0048] In Formula I and its related formulas, "amidate” refers to -N(R)C(O)R, wherein each R is independently selected from hydrogen, alkyl groups, and ary l groups (each of which has been defined herein) and denotes the bond to the alkaline earth metal in Formula I and all its related formulas.

[0049] In Formula I and its related formulas, "nitrogen-substituted cycloalkyl” refers to a cycloalkyd group (which has been defined herein) that contains N as a heteroatom. Other substituents may' be included, e.g., a carbon -hydrogen bond may be replaced by a carbonoxygen bond as in caprolactamato (-NCeHnO), wherein denotes the bond to the alkaline earth metal in Formula I and all its related formulas.

[0050] In Formula I and its related formulas, “alkoxy” refers to -OR, wherein R is an alkyl group (which has been defined herein) and "-” denotes the bond to the alkaline earth metal in Formula I and all its related formulas.Atty. Dkt. No. 00100-0413 -PCT

[0051] In Formula I and its related formulas, “aryloxy” refers to -OAr, wherein Ar is an aryl group (which has been defined herein) anddenotes the bond to the alkaline earth metal in Formula I and all its related formulas.

[0052] Regarding substituents in the groups of described herein (as opposed to unsubstituted groups), non-hydrogen and non-carbon atoms include, e.g., halogen; oxygen; sulfur; nitrogen; phosphorus; and silicon.

[0053] Illustrative alkaline earth metal catalysts are listed in Table 1, particularly, entries 4-11.

[0054] A single type of alkaline earth metal catalyst or multiple, different types of alkaline earth metal catalysts may be used. The alkaline earth metal catalyst being used to catalyze depolymerization may comprise (or consist of) any of the disclosed alkaline earth metal catalysts or a combination thereof.

[0055] The conditions being used in the present methods may refer to parameters such as the temperature, period of time, atmosphere, and alkaline earth metal catalyst loading. The present methods may be conducted using a variety of 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 may also refer to a specific type of reactor system. These parameters may be adjusted in order to promote depolymerization and thus, to achieve a desired (e.g., maximum) yield of monomer (or other depolymerization product).

[0056] Regarding temperature, the temperature may be within 30 °C, within 20 °C, or within 10 °C of the melting temperature of the polyamide. The temperature may be at or above the melting temperature of the polyamide such that it is in its molten (liquid state) during the depolymerization. In embodiments, the temperature is no more than 300 °C, no more than 280 °C, no more than 260 °C, or no more than 240 °C. This includes a range between any of these values and from 200 °C to 280 °C, from 200 °C to 250 °C, and from 200 °C to 245 °C.

[0057] Regarding time, this may refer to a total period of time over which the polyamide and the alkaline earth metal catalyst are subjected to the other depolymerization conditions. In embodiments, the period of time is less than 24 hours, less than 10 hours, less than 5 hours, or less than 2 hours. This includes a range between any of these values and from 1 minute to 10 hours and 1 hour to 6 hours. For continuous flow reactor systems, flow rate, rather thanAtty. Dkt. No. 00100-0413 -PCTtime, is a relevant parameter. In embodiments, the flow rate is from 5 seem to 1500 seem, although higher flow rates may be used. Reactor volume is another relevant parameter for continuous flow reactor systems. In embodiments, the reactor volume is from 50 mL to 1000 L, although greater reactor volumes may be used.

[0058] Regarding the atmosphere, a vacuum may be used, e.g., a pressure of 1 O'3Torr or less, IO'2Torr or less, I0'1Torr or less. However, the methods may be conducted at higher pressures, including in a range of from 10'2Torr to 103Torr. The methods may be conducted under an inert atmosphere (e.g., N2, argon) and a pressure of about 1 atm (760 Torr). As demonstrated in Table 1, entry 13, the present methods may exclude the use of air as an atmosphere.

[0059] Regarding alkaline earth metal catalyst loading, the loading may be no more than 20 mol%, no more than 10 mol%, no more than 8 mol%, or no more than 6 mol%. This includes a range of from 0.1 mol% to 5 mol% and from 1 mol% to 5 mol%. Mol% is calculated based on the number of mol of repeating unit of the polyamide. For example, if 1 g of Nylon-6 is used, the number of mol of repeating unit = 0.00885 mol (repeating unit of Nylon-6 is 113 g / mol). Thus, the amount of catalyst used is 0.00885 mol * x mol%.

[0060] The present methods are generally conducted without using any liquid medium, e.g., solvent. This includes the methods being conducted without using water or steam. As such, the methods may be referred to as being “solvent-free” and the like.

[0061] The conditions being used in the present methods may further refer to whether or not the polyamide has been subjected to an alkali metal pretreatment step prior to depolymerization. The alkali metal pretreatment comprises exposing the polyamide to an alkali metal species including an alkali metal hydroxide, an alkali metal (bi)carbonate, and an alkali metal alkoxide. Various alkali metal hydroxides may be used, e.g., NaOH, KOH, LiOH. Various alkali metal (bi)carbonates may be used, e.g., Na2CO3, KHCO3. Various alkali metal alkoxides may be used, e.g., sodium methoxide (NaOMe), potassium tert-butoxide (KOtBu).The alkali metal species may be provided as an aqueous solution and the exposure may be carried out by washing the polyamide with the aqueous solution for a period of time followed by drying as described in the Example, below. Due to the synergistic effects of the alkaline earth metal and the alkali metal as described herein, the specific conditions of the alkali metal pretreatment step may be selected to achieve a desired (e.g., maximum) yield of monomer (or other depolymerization product). These conditions may include ensuring thatAtty. Dkt. No. 00100-0413 -PCTa certain amount of alkali metal is present during depolymerization, including up to one alkali metal per polyamide chain. For example, if Nylon-6 has about 100 repeating s-caprolactam units, the conditions may include ensuring that up to about 1 mol% alkali metal is present during depolymerization. However, smaller amounts of alkali metal may be used. In embodiments, the polyamide has been subjected to an alkali metal pretreatment step and the present methods may include such an alkali metal pretreatment step. However, as the alkali metal species may be present as a separate additive (as described below), in embodiments, the present methods do not include an alkali metal pretreatment step and the polyamide has not been subjected to such an alkali metal pretreatment step. (See entries 5 and 6 of Table 2.)

[0062] The conditions being used in the present methods may further refer to whether or not an alkali metal additive is present along with the polyamide and the alkaline earth metal catalyst during depolymerization. The alkali metal additive may be any of the alkali metal species described above with respect to alkali metal pretreatment, including any of the alkali metal hydroxides, alkali metal (bi)carbonates, and alkali metal alkoxides. The alkali metal species may be added to the polyamide and the alkaline earth metal catalyst in a desired amount, e.g., from 0.1 to 15 mol% alkali metal species. Due to the synergistic effects of the alkaline earth metal and the alkali metal as described herein, the specific amount may be selected to achieve a desired (e.g., maximum) yield of monomer (or other depolymerization product). In embodiments, an alkali metal additive is included during depolymerization. However, as the alkali metal species may be present due to a pretreatment step (as described above), in embodiments, the present methods do not include an alkali metal additive during depolymerization.

[0063] The polyamide (the specific chemical type of which has been described above) may be “virgin” polyamide, which generally refers to pure, as-synthesized polyamide that has not been further processed for use in a particular application (whether the virgin polyamide has been used or not). Alternatively, the polyamide may be “post-consumer” polyamide, which generally refers to a polyamide derived from a consumer product (whether that consumer product has actually been used or not). Post-consumer polyamide may include other components (e.g., other non-polyamide polymers such as polyolefins) such that it may be considered to be a composite with such other components and may have been processed for use in a particular application (e.g., fishing nets, carpet fibers, clothing, medical gloves). Blends of different types of polyamide polymers have been described above. Illustrative non-polyamide polymers include those listed in Table 3 of the Example, below, includingAtty. Dkt. No. 00100-0413 -PCTpolyolefins, polycarbonates, polyethers, polyesters, polyurethanes, etc. In either embodiment, the morphology of the polyamide is not particularly limited. For example, virgin polyamide may be in the form of a powder, including powder composed of micron-sized particles. However, the present methods are capable of achieving high monomer yields from postconsumer polyamide that has been chopped into pieces significantly larger than the particles of virgin polyamide powder. In other words, no milling, e.g., cryogenic milling, is required to achieve high monomer yields from post-consumer polyamide using the present methods. Other morphologies include granules, pellets, films, and fibers.

[0064] When combined for conducting the present methods, the polyamide and the alkaline earth metal catalyst may be considered to form a reaction mixture comprising (or consisting of) each of these components. As noted above, the reaction mixture need not comprise any liquid medium (e.g., solvent). (This does not preclude the presence of a liquid in the reaction mixture due to the use of molten polyamide.) Similarly, the reaction mixture need not comprise other additives (other than the alkali metal species / additive described above, if used). Thus, in such embodiments, the reaction mixture may be characterized as being free of a liquid medium (other than the polyamide if in its molten state) and free of an additive (other than the alkali metal species / additive described above, if used). Non-alkali metal additives which may be excluded from the reaction mixture in embodiments include the following: an ionic liquid; water or steam; ammonia; an amine; N,N-dimethylaminopyridine; acetic anhydride; a phosphoric acid (or salt thereof); a boric acid (or salt thereof); a sulfonic acid (or salt thereof); a carboxylic acid (or salt thereof); a carbonate; an alkaline earth oxide; an alkaline earth hydroxide; an alkaline earth carbonate; an alkaline earth carboxylate; an alcohol. It is noted that these non-alkali metal additives do not work to depolymerize polyamides using the method conditions described herein. In embodiments, the reaction mixture consists of the polyamide, the alkaline earth metal catalyst, and optionally, the alkali metal additive. These embodiments do not preclude the ultimate presence of monomers provided by the depolymerization, unreacted polyamide and / or other depolymerized product (e.g., polyamide fragments) in the reaction mixture. These embodiments also do not preclude the presence of components or impurities which may be inherently present in the reaction mixture due to the particular synthetic technique used to form the polyamide. These embodiments also do not preclude the presence of components which may be inherently present in the reaction mixture due to the source of the polyamide (e.g., post-consumer polyamide).Atty. Dkt. No. 00100-0413 -PCT

[0065] The present methods may be conducted using a variety' types of reactor systems, including batch reactor systems, semi-batch reactor systems, and continuous flow reactor systems. As noted throughout, the present methods are capable of achieving continuous operation for a period of time (e.g., by7using a continuous flow of polyamide) while maintaining high monomer yields.

[0066] The present methods may further comprise recovering any products and / or unreacted material from the reaction mixture. This includes recovering and / or recycling the alkaline earth metal catalysts (or a derivative thereof) from the reaction mixture. No liquid medium is required for recovery of the alkaline earth metal catalysts. The recovered alkaline earth metal catalysts may be used to conduct the methods one or more additional times (i.e., they may be recycled). Similarly, the present methods may further comprise recovering the depolymerized product, e.g., monomers, and using them for any desired purpose, including synthesizing a new polymer, including a new polyamide. Recovery of the depolymerized products may be by vaporization or sublimation.

[0067] The present methods may be characterized by a yield of monomer. The yield is reported as (weight of monomer) / (starting weight of polyamide)* 100%. The yield may be determined using1H NMR as described in the Example, below. The yield may be an initial yield obtained by using fresh (i.e., unused) alkaline earth metal catalyst. The initial yield may be at least 40%, at least 60%, at least 80%, at least 85%, at least 90%, at least 95%, or at least 98%. This includes a range between any of these values, as well as from 40% to 100%, from 50% to 100%, and from 80% to 95%. The yield may be a yield obtained by using recovered alkaline earth metal catalyst which has been used one or more times (e.g., 1, 2, 3. 4, etc.). The yield from a recycled / recovered alkaline earth metal catalyst may be within ±50%, ±20%, ±10%, ±5%, or ±2% of the initial yield. Any of the yield values in this paragraph may refer to a specific polyamide (e.g., Nylon-6), a specific monomer (e.g., s -caprolactam), a specific alkaline earth metal catalyst (e.g., any of those in entries 4-11 of Table 1.), and the method carried out under specific conditions (e.g.. any of those described in the Example below).

[0068] As noted above, the polyamide to be depolymerized by the present methods may be provided in a blend with non-polyamide polymers. The methods may depolymerize the polyamide while leaving the other polymer intact. The term “intact” has a meaning analogous to “without” and “selective depolymerization” as described above. That is, intact polymers are evidenced by a yield of the corresponding monomer(s) that is below the detection limit ofAtty. Dkt. No. 00100-0413 -PCTtheJH NMR system used to measure yield as described in the Example, below. This includes measuring a monomer yield viarH NMR that is less than 5%.Thus. the present methods allow for separation of a polyamide from such a blend. Blends of different types of polyamide polymers have been described above.

[0069] The present disclosure encompasses reaction mixtures comprising (or consisting of) any of the disclosed alkaline earth metal catalysts with any of the disclosed polyamides, including blends thereof with non-polyamide polymers.EXAMPLE

[0070] Introduction

[0071] This Example includes experimental results that demonstrate that alkaline earth metal catalysts can efficiently depolymerize Nylon-6 to E-caprolactam in quantitative yields, in short reaction times, at low temperatures, and at low catalyst loadings while functioning equally well under vacuum or an inert atmosphere such as N2 or Ar and without the need of using solvents. The alkaline earth metal catalysts can also operate in the presence of mixed plastic waste containing Nylon-6. The alkaline earth metal catalysts include: organometallic MgR2 (R = H or a carbon-based ligand); metal-organic MgR'2 (R1= nitrogen or oxygencontaining ligands or a mixture of thereof such as amidates. amidos, and alkoxy ligand); mixed organometallic and metal-organic MgRR'; and Grignard reagents such as RMgX or R’MgX (X = halogen). Furthermore, the E-caprolactam produced by the depolymerization reaction can be successfully polymerized back to pristine Nylon-6.

[0072] Experimental

[0073] General methods and materials

[0074] Methods: All depolymerization reactions were assembled by mixing the polymer and the appropriate catalyst in an Ar-fdled MBraun glovebox in cylindrical 100 mL Schlenk tubes. A customized aluminum heating block supplied heating with a fitted hole for the Schlenk tubes.

[0075] General materials: All catalysts were purchased from commercial sources and used directly without further purification. In some experiments, the Nylon-6 was subjected to a KOH pretreatment step in which Nylon-6 was washed with 1 M KOH solution overnight, filtered and washed with H2O, and dried under a vacuum at 100 °C before use. Pristine Nylon-6 powder with a mean particle size of 15-20 pm was obtained from Goodfellow Inc.Atty. Dkt. No. 00100-0413 -PCT

[0076] Physical and analytical methods

[0077] NMR spectra were recorded on a Varian Broker Avance III HD system equipped with a TXO Prodigy probe (500 MHz) spectrometer. Chemical shifts (5) for H-NMR were referenced to the internal solvent.

[0078] General procedure for depolymerization reactions

[0079] In a glove box, a 100 mL oven-dried Schlenk tube was charged with a magnetic stir bar, Nylon-6 polymer, and catalyst. The vessel was sealed, and the polymer and catalyst were thoroughly mixed by stirring at room temperature for approximately 30 seconds. The Schlenk tube was then connected to a vacuum line, subjected to reduced pressure (~10‘3torr), sealed, and heated to the specified temperature with magnetic stirring for the specified time.Reaction time was recorded starting 2.5 mln after the reaction tube was placed in the heating, block since it took an average of 2.5 min for the reaction to reach the depolymerization temperature and the polymer to begin melting. During the reaction, the products sublimed from the hot reaction zone and deposited as a cry stalline layer on the cold wall of the reactor. After cooling to room temperature, the soluble part of the reaction mixture was dissolved in 3-4 mL of deuterated solvent, and mesitylene was added as an internal standard. A sample of this solution was withdrawn for NMR analysis. Yields were determined by 'H-NMR, comparing the signal integrals of s-caprolactam and mesitylene.

[0080] Procedures for reactions in Table 1

[0081] Table 1, entry 1Nylon-6 (KOH treated) e-Caprolactam29%

[0082] Exactly 0.1 gr of KOH treated Nylon-6 powder and K2CO3 (5.0 mol%) w ere reacted according to the general procedure, heating the reaction mixture to 240 °C for 6 h. s-caprolactam was obtained in 29 % yield.

[0083] Table 1, entry 2Atty. Dkt. No. 00100-0413 -PCT6.0 hNylon-6 (KOH treated) s-Caprolactam23%

[0084] Exactly 0.1 gr of KOH treated Nylon-6 powder and Na2CO- (5.0 mol%) were reacted according to the general procedure, heating the reaction mixture to 240 °C for 6 h. e-caprolactam was obtained in 23 % yield.

[0085] Table 1, entry 3Nylon-6 (KOH treated) 1.0 he-Caprolactam1%

[0086] Exactly 0.1 gr of KOH treated Nylon-6 powder and KOH (5.0 mol%) were reacted according to the general procedure, heating the reaction mixture to 240 °C for 1 h. s-caprolactam was obtained in 1 % yield.

[0087] Table 1, entry 41.0 hNylon-6 (KOH treated) e-Caprolactam89%

[0088] Exactly 0.1 gr of KOH treated Nylon-6 powder and Mg(OEt)2 (5.04 mol%) were reacted according to the general procedure, heating the reaction mixture to 240 °C for 1 h. s-caprolactam was obtained in 89 % yield.

[0089] Table 1, entry 52.0 hNylon-6 (KOH treated) e-Caprolactam81%Atty. Dkt. No. 00100-0413 -PCT

[0090] Exactly 0.1 gr of KOH treated Nylon-6 powder and Mg[N(Si(CH3)3)2]2 (5.0 mol%) were reacted according to the general procedure, heating the reaction mixture to 240 °C for 2 h. e-caprolactam was obtained in 81 % yield.

[0091] Table 1, entry 62.0 hNylon-6 (KOH treated) e-Caprolactam45%

[0092] Exactly 0.1 gr of KOH treated Nylon-6 powder and Mg(nBu)2 in heptane (5 mol%) were mixed, heptane was removed under vacuum, and the mixture was reacted according to the general procedure, heating the reaction mixture at 240 °C for 2 h. s-caprolactam was obtained in 45 % yield.

[0093] Table 1, entry 72.0 hNylon-6 (KOH treated) e-Caprolactam53%

[0094] Exactly 0.1 gr of KOH treated Nylon-6 powder and Mg( / 7-Buty l-scc-buty I) in heptane (5 mol%) were mixed, heptane was removed under vacuum, and the mixture was reacted according to the general procedure, heating the reaction mixture at 240 °C for 2 h. 8-caprolactam was obtained in 53 % yield.

[0095] Table 1, entry 8Nylon-6 (KOH treated) E-Caprolactam26%

[0096] Exactly 0.1 gr of KOH treated Nylon-6 powder and MeMgBr (5.0 mol%) were reacted according to the general procedure, heating the reaction mixture to 240 °C for 24 h. s-caprolactam was obtained in 26 % yield.Atty. Dkt. No. 00100-0413 -PCT

[0097] Table 1, entry 92.0 hNylon-6 (KOH treated) s-Caprolactam26%

[0098] Exactly 0.1 gr of KOH treated Nylon-6 powder and CaH2 (5.0 mol%) were reacted according to the general procedure, heating the reaction mixture to 240 °C for 2 h. e-caprolactam was obtained in 26 % yield.

[0099] Table 1, entry’ 101.5 hNylon-6 (KOH treated) e-Caprolactam82%

[0100] Exactly 0.1 gr of KOH treated Nylon-6 powder and Mg(OEt)2 (2.5 mol%) were reacted according to the general procedure, heating the reaction mixture to 240 °C for 1.5 h. s-caprolactam was obtained in 82 % yield.

[0101] Table 1, entry 1115 minNylon-6 (KOH treated) ^-Caprolactam87%

[0102] Exactly 0.1 gr of KOH treated Nylon-6 powder and Mg(OEt)2 (5.0 mol%) were reacted according to the general procedure, heating the reaction mixture to 280 °C for 15 min. s-caprolactam was obtained in 87 % yield.

[0103] Table 1, entry 12Atty. Dkt. No. 00100-0413 -PCT3.0 hNylon-6 (KOH treated) s-Caprolactamnegligible

[0104] Exactly 0.1 gr of KOH treated Nylon-6 powder were reacted according to the general procedure, heating the reaction mixture to 280 °C for 3.0 h. s-caprolactam was obtained in negligible amount.

[0105] Table 1, entry 13Nylon-6 (KOH treated) s-Caprolactamnegligible

[0106] Exactly 0.1 gr of KOH treated Nylon-6 powder and Mg(OEt)2 (5.0 mol%) were reacted under an air Exactly 0.1 gr of KOH treated Nylon-6 powder and C5H1 IMgCl in diethyl ether (5.1 mol%) were mixed, diethyl ethe was removed under vacuum, and the mixture was reacted according to the general procedure, heating the reaction mixture at 240 °C for 2 h. s-caprolactam was obtained in 5 % yield, atmosphere, heating the reaction mixture to 240 °C for 2 h. s-caprolactam was obtained in negligible amount.

[0107] Table 1, entry 142 hNylon-6 (KOH treated) s-Caprolactam5%

[0108] Exactly 81 mg of KOH treated Nylon-6 powder and CsHi iMgCl in diethyl ether (5.1 mol%) were mixed, diethyl ether was removed under vacuum, and the mixture was reacted according to the general procedure, heating the reaction mixture at 240 °C for 2 h. s-caprolactam was obtained in 5 % yield.

[0109] Results and DiscussionAtty. Dkt. No. 00100-0413 -PCT

[0110] In the first set of experiments, the performance of the alkali metal compounds, K2CO3. Na2CO3. and KOH, were investigated under mild reaction conditions. However, in all cases, a significant amount of the starting Nylon-6 remained unreacted, resulting in yields of only 29%, 23%, and 3.3% of e-caprolactam, respectively. Note that the yield remained low despite increasing the reaction time to 6 hours and tripling the catalyst loading with respect to the loadings used (Table 1, entries 1-3).

[0111] Next, the activity of several commercially available Mg catalysts was tested. For example, oxygen-containing ligands such as alkoxy ligands are highly attractive due to their high accessibility, air stability, and low cost. Despite these advantages, however, due to the low activity of the basic catalysts above (K2CO3, Na2COs, and KOH), it was expected that the Mg catalysts would exhibit similar low activity7and yields. Surprisingly, using a commercially-available Mg(OEt)2 catalyst gave a significant improvement in the rate of the reaction, selectively yielding e-caprolactam in 89% yield in only 1 h at 240 °C (Table 1, entry 4). Similarly, Mg[N(Si(CHs)3)]2 was also found to be an effective catalyst, giving an 81% yield in 2 h (entry 5). Surprisingly, and in contrast to lanthanide-based catalysts (for which depolymerization rates ty pically increase with the basicity7of the ligand), substituting the alkoxy ligand with highly basic ligands, such as an alkyl ligand, unexpectedly resulted in lower yields (45% and 53%, respectively, entries 6, 7). Interestingly, Grignard reagents such as RMgX afforded up to 5% yield of caprolactam (entries 8 and 14). Other Group II metals, such as Ca, can be used for the disclosed depolymerization methods. Inexpensive, readily available CaH2 afforded 26% yield in 2 h at 240 °C of e-caprolactam (entry 9).

[0112] Additional experiments revealed that 5 mol% loading of Mg(OEt)2exhibited the highest catalytic efficiency among the various tested magnesium-based catalysts at 240 °C (Table 1, entry 4). When the catalyst loading was reduced to 2.5 mol% under optimized conditions, a comparable depolymerization yield of 84% was still obtained (Table 1, entry 10). Notably, increasing the reaction temperature to 280 °C under these conditions significantly enhanced the reaction rate, affording 87% of caprolactam within 15 minutes (Table 1, entry 11). Depolymerization at 280 °C in the absence of a catalyst (KOH pretreatment only) produced only trace amounts of E-caprolactam after 3 h, confirming that the reaction does not proceed without the Mg or Ca catalysts (Table 1, entry 12). These results also indicate that any residual potassium from the Nylon-6 washing process by itself does not contribute to catalysis under these conditions. The depolymerization of Nylon-6Atty. Dkt. No. 00100-0413 -PCTunder air atmosphere resulted into the formation of a negligible amount of product, highlighting the importance of the inert atmosphere for this process (Table 1. entry 13).

[0113] Table 1. Screening of catalysts in Equation l.aNylon-6 E-CaprolactamaGeneral depolymerization conditions: static vacuum (10‘3Torr), 100 mg of KOH treated virgin Nylon-6 powder in 50 mL Schlenk flask, unless otherw ise stated.bYields determined by1H NMR with a mesitylene internal standard.c100 mg of KOH treated virgin Nylon-6 powder in 100 mL Schlenk flask under static vacuum (10‘3Ton).d81 mg of KOH treated virgin Nylon-6 powder in 100 mL Schlenk flask under static vacuum (10‘3Torr).e1.4 M solution of CsHnMgCl in diethyl ether, n.d. =not detected.

[0114] Additional experiments were performed to examine the role of KOH pretreatment of Nylon-6 powder, which was used in the experiments conducted above and summarized in Table 1. The procedures for the additional experiments in which no KOH pretreatment was used are provided below and the results collected in Table 2, below.

[0115] Procedures for reactions in Table 2Atty. Dkt. No. 00100-0413 -PCT

[0116] Table 2, entry 11.5 hNylon-6 (non KOH treated) s-Caprolactam negligible

[0117] Exactly 0.1 gr of non KOH treated Nylon-6 powder and Mg(OEt)2 (5.0 mol%) were reacted according to the general procedure, heating the reaction mixture to 240 °C for 1.5 h. e-caprolactam was obtained in negligible amount.

[0118] Table 2, entry 2Nylon-6 (non KOH treated) s-Caprolactam14%

[0119] Exactly 0.082 gr of non KOH treated Nylon-6 powder and Mg(OEt)2 (13.5 mol%) were reacted according to the general procedure, heating the reaction mixture to 240 °C for 1.5 h. e-caprolactam was obtained in 14% yield.

[0120] Table 2, entry 31.0 hNylon-6 (non KOH treated) e-Caprolactam negligible

[0121] Exactly 0.082 gr of non KOH treated Nylon-6 powder and Mg(OEt)2 (6.5 mol%) were reacted according to the general procedure, heating the reaction mixture to 280 °C for 1.0 h. 8-caprolactam was obtained in negligible yield.

[0122] Table 2, entry 4Atty. Dkt. No. 00100-0413 -PCT1.0 hNylon-6 (non KOH treated) s-Caprolactam65%

[0123] Exactly 0.082 gr of non KOH treated Nylon-6 powder and Mg(OEt)2 (13.5 mol%) were reacted according to the general procedure, heating the reaction mixture to 280 °C for 1.0 h. 8-caprolactam was obtained in 65% yield.

[0124] Table 2, entry 5Nylon-6 7.2 mol% 1.0 h s-Caprolactam (non KOH treated) 62%

[0125] Exactly 0.082 gr of non KOH treated Nylon-6 powder, Mg(OEt)2 (6.3 mol%) and KOH (7.2 mol%) were reacted according to the general procedure, heating the reaction mixture to 240 °C for 1.0 h. e-caprolactam was obtained in 62% yield.

[0126] Table 2, entry 8Nylon-6 7.2 mol% 1.0 h s-Caprolactam (non KOH treated) 61%

[0127] Exactly 0.082 gr of non KOH treated Nylon-6 powder, Mg(OEt)2 (6.3 mol%) and KOH (7.2 mol%) were reacted according to the general procedure, heating the reaction mixture to 280 °C for 1.0 h. s-caprolactam was obtained in 61% yield.

[0128] Table 2. Screening of conditions in Equation 2.aNylon-6 (Non-KOH Treated) 8-Atty. Dkt. No. 00100-0413 -PCTaGeneral depolymerization conditions: static vacuum (10‘3Torr), 82 mg of non KOH treated virgin Nylon-6 powder in 100 mL Schlenk flask, unless otherwise stated.bYields determined by1H NMR with a mesitylene internal standard.c100 mg of non KOH treated virgin Nylon-6 powder was used.dMixture of Mg(OEt)2(6.3 mol%) and KOH (7.2 mol%) were used.

[0129] The results of Table 2 show that depolymerization of untreated Nylon-6 (i.e.. no KOH pretreatment) using 5.0 mol% of Mg(OEt)2 at 240 °C afforded only trace amounts of caprolactam (Table 2, entry 1; compare the results of Table 1, entry' 4 showing 89% yield under similar conditions but using KOH treated Nylon-6). Increasing the catalyst loading to 13.1 mol% did not markedly enhance the reaction, yielding merely 14%, monomer (Table 2. entry 2). Similarly, elevating the temperature to 280 °C with 6.5 mol% catalyst gave only trace quantities of caprolactam (Table 2, entry73). It was only after both increasing the amount of catalyst (13.5 mol% Mg(OEt)2) and increasing the temperature (280 °C) that improved depolymerization was achieved, yielding 65% caprolactam (Table 2, entry 4). These observations indicate that without the KOH pretreatment, low catalyst loadings are ineffective even at elevated temperatures, suggesting that the overall efficiency is strongly governed by the catalyst concentration. It is noted that in KOH treated Nylon-6, the KOH may react with terminal carboxylic acid groups (R(C=O)OH) to form potassium carboxylate moieties (R(C=O)OK). However, under the studied conditions, these potassium carboxylate moi eties by themselves are not catalytically active (Table 1, entry' 12).

[0130] As the results of Table 1 further demonstrate, however, catalytic activity is achieved when KOH pretreatment is combined with the IGMg catalysts. Specifically, rapid and nearly quantitative depolymerization was achieved even at low temperatures (240 °C)Atty. Dkt. No. 00100-0413 -PCTusing only 2.5 mol % Mg(OEt)2 (see Table 1, entry 10). These results highlight the unexpected and synergistic role of KOH pretreatment in the catalytic process and strongly suggest that the presence of some alkali metal species is required to depolymerize Nylon-6 under mild conditions and lower catalyst loading.

[0131] Encouraged by these observations, it was explored whether introducing a small quantity of KOH directly into the reaction mixture would suppress catalyst deactivation and enable depolymerization at lower Mg(OEt)2 loadings. As Table 2 shows, heating the untreated Nylon-6 with a mixture of 7.5 mol% KOH and 6.3 mol% Mg(OEt)2, did efficiently depolymerize the polymer chain to yield 62% s-caprolactam at 240 °C within 1 h (Table 2, entry 5). Increasing the temperature provided no further improvement (Table 2, entry 6), indicating that the results are due to the cooperative interaction between the base and Mg catalyst, rather than thermal effects. As noted above, the control experiments confirmed that neither KOH alone, nor Mg(OEt)2 at 6.3 mol% alone, promotes depolymerization under identical solvent-free conditions (Table 1, entry 12 and Table 2, entry 3, respectively). Such cooperative activation, in which two individually catalytically inactive components achieve efficient depolymerization together, reveals a previously unknown pathway for Nylon-6 depolymerization. Such synergistic base-metal catalysis highlights a powerful and conceptually simple strategy for designing sustainable, solvent-free depolymerization systems.

[0132] Finally, additional experiments were performed to evaluate whether selective depolymerization of Nylon-6 could be achieved in the presence of other polymers. The procedures for these additional experiments are provided below and the results collected in Table 3, below.

[0133] Procedures for reactions in Table 3

[0134] Table 3, entry 1>Atty. Dkt. No. 00100-0413 -PCT

[0135] Exactly 80 mg of KOH-treated Nylon-6 powder, 20 mg of KOH-treated Nylon-66 powder, and 5.1 mg of Mg(OEt)2 (5.1 wt.%) were reacted according to the general procedure, heating the reaction mixture to 240 °C for 3 h. s-caprolactam was obtained in 80% yield.

[0136] Table 3, entry 2Nylon-6 3.0 h s-Caprolactam (KOH treated) 93%

[0137] Exactly 80 mg of KOH-treated Nylon-6 pow der. 20 mg of i-pp (isotactic polypropylene) powder, and 5.1 mg of Mg(OEt)2 (5.1 wt.%) were reacted according to the general procedure, heating the reaction mixture to 240 °C for 3 h. s-caprolactam was obtained in 93% yield.

[0138] Table 3, entry 3Nylon-6 1.0 h s-Caprolactam (KOH treated) 85%

[0139] Exactly 80 mg of KOH-treated Nylon-6 powder, 20 mg of PC (polycarbonate) powder, and 5.1 mg of Mg(OEt)2 (5.1 wt.%) were reacted according to the general procedure, heating the reaction mixture to 240 °C for 1 h. e-caprolactam was obtained in 85% yield.

[0140] Table 3, entry 4Nylon-6 3.0 h E-Caprolactam(KOH treated) 95%

[0141] Exactly 90 mg of KOH-treated Nylon-6 powder, 10 mg of spandex pow der, and 5.1 mg of Mg(OEt)2 (5.1 wt.%) were reacted according to the general procedure, heating the reaction mixture to 240 °C for 3 h. s-caprolactam was obtained in 95% yield.

[0142] Table 3, entry 5Atty. Dkt. No. 00100-0413 -PCTNylon-6 10 Torr, 3.0 h e-Caprolactam (KOH treated) 30%

[0143] Exactly 90 mg of KOH-treated Nylon-6 powder, 10 mg of PET pow der, and 5.1 mg of Mg(OEt)2 (5.1 wt.%) were reacted according to the general procedure, heating the reaction mixture to 240 °C for 1.5 h and then raising the temperature to 280 °C for another 1.5 h. s-caprolactam was obtained in 30% yield.

[0144] Table 3, entry 6Nylon-6 2.0 h s-Caprolactam (KOH treated) 86%

[0145] Exactly 90 mg of KOH-treated Nylon-6 powder, 10 mg of PET powder, and 11 mg of Mg(OEt)2 (11 wt.%) were reacted according to the general procedure, heating the reaction mixture to 240 °C for 2.0 h. s-caprolactam was obtained in 86% yield.

[0146] Table 3, entry 7Nylon-6 2.0 h s-Caprolactam (KOH treated) 94%

[0147] Exactly 90 mg of KOH-treated Nylon-6 powder, 10 mg of PTFE powder, and 5.1 mg of Mg(OEt)2 (5.1 wt.%) were reacted according to the general procedure, heating the reaction mixture to 240 °C for 2.0 h. s-caprolactam was obtained in 94% yield.

[0148] Table 3, entry 8Atty. Dkt. No. 00100-0413 -PCT

[0149] Exactly 90 mg of KOH-treated Nylon-6 powder, 10 mg of Kevlar thread, and 11 mg of Mg(OEt)2 (11 wt.%) were reacted according to the general procedure, heating the reaction mixture to 240 °C for 3.0 h. 8-caprolactam was obtained in 30% yield.

[0150] Table 3. Depolymerization of KOH-treated Nylon-6 in the presence of other polymers, as shown in equation 3.aNylon-6 (KOH treated) s-CaprolactamAtty. Dkt. No. 00100-0413 -PCTaGeneral depolymerization conditions: static vacuum (10‘3Torr) and 100 mg of mixed plastic blend (Nylon-6 and polymers) in a 100 mL Schlenk flask, unless otherwise stated.hYields determined by!H NMR with a mesitylene internal standard.cKOH treated Nylon-66 powder was used.dA mixture of KOH treated Nylon-6 and polymer in a 4: 1 (80 mg:20 mg) weight ratio was used.eA plastic blend of KOH treated Nylon-6 and other polymers in a 9: 1 (90 mg: 10 mg) weight ratio was used.

[0151] As noted above, experiments were conducted to examine the ability of the catalytic system to selectively depolymerize Nylon-6 in the presence of various other polymers, including polyamide 66 (Nylon-66), isotactic polypropylene (i-PP), polycarbonate (PC), polytetrafluoroethylene (PTFE), polyethylene terephthalate (PET), a polyurethanepolyurea copolymer (Spandex), and poly(paraphenylene terephthalamide) (Kevlar), as summarized in Equation 3.

[0152] Remarkably, the catalytic system exhibited excellent activity' and selectivity' toward Nylon-6 depolymerization even in mixed-polymer systems containing Nylon-66 polyamide. Under these conditions, Nylon-6 was converted entirely to e-caprolactam in 80% yield, while Nylon-66 remained unaffected (Table 3, entry 1). This striking selectivity highlights the ability of the catalytic system to achieve efficient, solvent-free depolymerization and provides a simple chemical strategy’ for the selective recovery of Nylon-6 from mixed polyamide waste streams.

[0153] Encouragingly, the catalytic system also demonstrated excellent selectivity in the presence of more challenging polymers such as isotactic polypropylene (i-PP) and polycarbonate (PC). Under these conditions, Nylon-6 was selectively depolymerized to afford s-caprolactam in 85% and 93% yields, respectively, while the i-pp and PC components remained unreacted in the reactor (Table 3, entries 2-3).

[0154] To assess the robustness of the catalytic system under conditions resembling real-world waste streams, depolymerization was conducted on Nylon-6 in combination with polyurethane-polyurea copolymer (Spandex) and polyethylene terephthalate (PET), materials commonly found in textiles, packaging, and composite applications. The strong interfacial interactions and chemical heterogeneity in such mixtures typically hinder selective recycling. Remarkably, the catalyst maintained high selectivity for Nylon-6, converting it to s-caprolactam in 95% yield in the presence of Spandex (Table 3, entry 4). When PET was included as a co-component, caprolactam was recovered in up to 85% yield (Table 3, entriesAtty. Dkt. No. 00100-0413 -PCT5-6). However, at low catalyst loading (5.1 wt%), depolymerization efficiency was limited (= 30% yield) (Table 3. entry 5). Increasing the Mg(OEt)2 loading to 11 wt% mitigated this effect, enhancing e-caprolactam formation to 85% yield (Table 3, entry 6). These findings highlight the intrinsic challenge of selectively depolymerizing Nylon-6 in oxygen-rich polymer matrices and demonstrate that careful tuning of catalyst concentration can overcome inhibitory interactions, offering a practical route for chemical separation and recovery from mixed polymer wastes.

[0155] The system also proved effective with post-consumer, high-melting point polymers. When PTFE (Teflon tape) was present, Nylon-6 depolymerization proceeded efficiently, yielding 94% E-caprolactam (Table 3, entry 7). Mixtures containing fibrous materials such as Kevlar exhibited comparatively lower yield (up to 30%, Table 3, entry 8), likely due to mass-transfer limitations and restricted polymer-catalyst contact. Efficient recovery of e-caprolactam from such mixed streams provides a practical and sustainable pathway for chemical separation and monomer reclamation.

[0156] The reusability of the Mg(OEt)2catalyst was evaluated via successive depolymerization cycles under identical reaction conditions. A single batch containing 5.1 mg of Mg(OEt)2(5.0 mol%) and 100 mg of KOH treated Nylon-6 was heated at 250 °C in a 100 mL sealed reactor. After each run, the caprolactam was isolated, followed by the addition of a fresh 100 mg portion of Nylon-6 without introducing additional catalyst. Remarkably, the caprolactam yields remained nearly unchanged across these five consecutive runs, highlighting the high structural integrity and catalytic resilience of Mg(OEt)2under these conditions. A subsequent final run (6th), was performed without additional Nylon-6, enabling complete depolymerization of the residual polymer, affording a cumulative Nylon-6 conversion of 91%. These results clearly demonstrate that Mg(OEt)2maintains consistent catalytic performance over six cycles, highlighting its robustness and potential for scalable, continuous depolymerization processes (Table 4).

[0157] Table 4. Catalyst Recyclability Evaluation in Multi-Batch Continuous Depolymerization.Atty. Dkt. No. 00100-0413 -PCTaThe total yield was determined by combining the isolated yield with the NMR-denved yield of the residual product remaining in the reaction flask after the monomer was isolated in the 6thrun. NMR yield was determined by 'H NMR analysis using mesitylene as an internal standard.

[0158] Conclusions

[0159] In conclusion, this Example demonstrates that earth-abundant, sustainable, and low-cost alkaline earth metal catalysts provide a highly effective method for solventless Nylon-6 depolymerization to E-caprolactam. Compared to alkali metal bases like K2CO3, Na2COs, and KOH, the alkaline earth metal catalysts achieved quantitative conversions at notably lower temperatures (as low as 240 °C) and required minimal catalyst loadings (as low as 5 mol%). The present catalysts performed consistently under both vacuum and inert atmospheres, highlighting their versatility and practical viability . In addition, the results show that Nylon-6 can be efficiently and selectively depolymerized in mixed plastic wastes, including Nylon-66, i-PP, PC, Spandex, PET, PTFE, and Kevlar, yielding high-purity e-caprolactam while leaving co-polymers largely intact. This demonstrates both the robustness and chemoselectivity of the approach, offering a practical route for monomer recovery from mixed waste streams. Finally, for the first time, the results reveal an unexpected cooperative effect between the alkaline earth metal catalyst and alkali metal species that leads to solvent-free depolymerization of Nylon-6. Individually, neither component is catalytically active under mild conditions whereas their combination is highly catalytically active. This simple yet effective cooperative strategy provides a sustainable and scalable route for the chemical recycling of Nylon-6, highlighting the potential of synergistic base-metal catalysis in circular polymer chemistry. The resulting e-caprolactam can be readily repolymerized into high-quality7Nylon-6, offering a compelling, sustainable approach to Nylon-6 recycling with promising industrial applicability, positioning alkaline earth metal catalysts as a superior alternative for efficient polymer recycling technologies.

[0160] The word "illustrative" is used herein to mean serving as an example, instance, or illustration. Any aspect or design described herein as "illustrative" is not necessarily to be construed as preferred or advantageous over other aspects or designs. Further, for the purposes of this disclosure and unless otherwise specified, "a" or "an" means "one or more.”Atty. Dkt. No. 00100-0413 -PCT

[0161] If not already included, all numeric values of parameters in the present disclosure are proceeded by the term “about” which means approximately. This encompasses those variations inherent to the measurement of the relevant parameter as understood by those of ordinary skill in the art. This also encompasses the exact value of the disclosed numeric value and values that round to the disclosed numeric value.

[0162] The foregoing description of illustrative embodiments of the disclosure has been presented for purposes of illustration and of description. It is not intended to be exhaustive or to limit the disclosure to the precise form disclosed, and modifications and variations are possible in light of the above teachings or may be acquired from practice of the disclosure. The embodiments were chosen and described in order to explain the principles of the disclosure and as practical applications of the disclosure to enable one skilled in the art to utilize the disclosure in various embodiments and with various modifications as suited to the particular use contemplated. It is intended that the scope of the disclosure be defined by the claims appended hereto and their equivalents.

[0163] In recognition of the inherent nature of chemical synthesis and chemical processes, throughout the present disclosure, terms and phrases such as “absence,” “free,” “does not comprise,” etc. encompass, but do not require a perfect absence of the referenced entity.

[0164] The term “type” as used herein refers to chemical formula such that a single type means the same chemical formula and different type means different chemical formula. The terms “same” and “different” in reference to elements or groups as used herein also refers to chemical formula such that same means the same chemical formula and different means different chemical formula.

[0165] Terms such as “comprising” and the like may be replaced with terms such as “consisting” and the like.

Claims

Atty. Dkt. No. 00100-0413 -PCTWHAT IS CLAIMED IS:

1. A method for depolymerizing a polyamide, the method comprising combining a polyamide and an alkaline earth metal catalyst to depolymerize the polyamide to a product, wherein the alkaline earth metal catalyst comprises an alkaline earth metal and ligands bound to the alkaline earth metal, wherein the ligands are independently selected from an oxygencontaining ligand other than an oxygen atom and other than hydroxide; hydrogen (H); a carbon-containing ligand other than a carbonate and other than a carboxylate; a nitrogencontaining ligand; a halogen; and an alkali metal.

2. The method of claim 1, wherein the alkaline earth metal catalyst is selected from those having formula MXmRnMo’, wherein M is the alkaline earth metal; X is the halogen; each R is independently selected from an oxygen atom (O) of the oxy gen-containing ligand, H, a carbon atom (C) of the carbon-containing ligand, and a nitrogen atom (N) of the nitrogencontaining ligand; M' is the alkali metal; m is 0 or 1; n is 2. 3, or 1; and o is 0 or 1; and further wherein if m is 0 and o is 0, then n is 2; if m is 1, then n is 1 and o is 0; and if o is 1, then m is 0 and n is 3.

3. The method of claim 2. wherein the alkaline earth metal is Mg or Ca.

4. The method of claim 2, wherein the oxygen-containing ligand is an alkoxy or an aryloxy; the carbon-containing ligand is an alky l; and the nitrogen-containing ligand is an amido, an amidate. or a nitrogen-substituted cycloalkyl.

5. The method of claim 2, wherein the formula is MR2.

6. The method of claim 5. wherein each R is an independently selected alkoxy.

7. The method of claim 6, wherein each R is ethoxy.

8. The method of claim 5. wherein each R is an independently selected amido.

9. The method of claim 8, wherein each R is N(Si(CHs)3)2.

10. The method of claim 5, wherein each R is an independently selected alkyl.Atty. Dkt. No. 00100-0413 -PCT11. The method of claim 10, wherein each R is independently selected from n-butyl and sec-butyl.

12. The method of claim 5, wherein each R is H.

13. The method of claim 1. wherein the polyamide is poly(hexano-6-lactam) (Nylon-6), poly(2-pyrrolidinone) (Nylon-4), or a combination thereof.

14. The method of claim 1, wherein the product comprises a monomer from which the polyamide was formed.

15. The method of claim 14, wherein the monomer is a cyclic amide.

16. The method of claim 1. wherein the polyamide and the alkaline earth metal catalyst are combined in the presence of an alkali metal species.

17. The method of claim 16, wherein the alkali metal species is present due to pretreating the polyamide with the alkali metal species prior to combining the polyamide and the alkaline earth metal catalyst; the alkali metal species is an additive combined with the polyamide and the alkaline earth metal catalyst; or combinations thereof.

18. The method of claim 1. wherein the polyamide is Nylon-6; the alkaline earth metal is Mg or Ca; and the product comprises s-caprolactam.

19. The method of claim 18, wherein the polyamide and the alkaline earth metal catalyst are combined in the presence of an alkali metal species.

20. The method of claim 19, wherein the formula is MR2 and wherein each R is an independently selected alkoxy or aryloxy.

21. The method of claim 1, wherein the polyamide is Nylon-6 and the product comprises 8-caprolactam, and further wherein the Nylon-6 is provided as a blend with a different type of polymer and the different type of polymer remains intact after the method.

22. The method of claim 1, wherein the polyamide is Nylon-6 and the product comprises e-caprolactam, and the method further comprises adding additional Nylon-6 anAtty. Dkt. No. 00100-0413 -PCTadditional time or continuously without adding additional alkaline earth metal catalyst to produce additional s-caprolactam.

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