WO2024176223A1

WO2024176223A1 - 3d printing by irradiation of recyclable polymers containing reversible covalent bonds

Info

Publication number: WO2024176223A1
Application number: PCT/IL2024/050190
Authority: WO
Inventors: Hanna Dodiuk; Shlomo Magdassi; Natanel JARACH
Original assignee: Yissum Research Development Co of Hebrew University of Jerusalem; Shenkar College of Engineering and Design
Current assignee: Yissum Research Development Co of Hebrew University of Jerusalem; Shenkar College of Engineering and Design
Priority date: 2023-02-21
Filing date: 2024-02-20
Publication date: 2024-08-29
Anticipated expiration: 2025-08-21

Abstract

The invention generally concerns a method for forming fully recyclable thermoset polymers, and products obtained by the method.

Description

3D PRINTING BY IRRADIATION OF RECYCLABLE POLYMERS CONTAINING REVERSIBLE COVALENT BONDS

TECHNOLOGICAL FIELD

The invention generally contemplates processes of reversible manufacturing of thermoset polymers and products thereof.

BACKGROUND

While polymers are some of the most commonly used materials, their environmental impact is on the increase. Several approaches have been offered to decrease the negative impact polymers impose on the environment. One of these approaches has been to use polymers containing reversible covalent bonds (so called reversible covalent bond-based polymers, RCBPs, covalent adaptable networks, CANs, or simply Vitrimers). Among the reversible and dynamic bonds known, [2 + 2] and [4 + 4] cycloadditions are unique in having potential use in irradiation-based applications, such as photocuring of adhesives and stereolithography-based 3D printing. This potential results from the adduct formation under irradiation at specific wavelengths and dissociation under shorter ones. Unlike any other radiation-curing mechanisms, the complete dissociation of the adducts into their original components holds their potential for recyclability, followed by re-using in irradiation-based applications.

The most common systems are cinnamic acid and coumarin derivatives. Cinnamic acid's derivatives undergo [2 + 2] cycloaddition to form cyclobutanes under irradiation at k>260 nm and undergo a reverse reaction under k<260 nm. Anthracene is the most common moiety to undergo [4 + 4] cycloaddition, which occurs under irradiation at -350 nm. Whereas this is a longer wavelength than most [2 + 2] addition reactions, it is still considered a harmful wavelength and shorter than that used in most photopolymerization-based applications, yet requires a relatively long reaction time.

Despite their potential, [2 + 2] or [4 + 4] reactions suffer from two main disadvantages that prevent their use (especially in 3D printing and fast-cure adhesives that require short irradiation times): the reactions require irradiation at short wavelengths, and long reaction times (hours to days). Two approaches may be used to overcome these shortcomings: redshift and acceleration catalysis. Cycloaddition redshift catalysis is mainly based on photooxidation reagents, like ruthenium and iridium complexes and thioxanthone derivatives. Conductive and semi-conductive particles or salts like pyrylium were reported to achieve cycloreversion redshift. Both cycloaddition and cycloreversion were found to undergo a significant redshift when irradiated under a two-photons beam, which requires unique instruments and high- power lasers. Acceleration of [2 + 2] cycloaddition can be obtained using external factors, such as microwave irradiation, UV-flow reactors, and increased pressure during reaction. The addition of accelerators, such as Lewis acids and bases combinations or ion-containing solvents, is also useful.

REFERENCES

[1] N. Jarach, D. Golani, N. Naveh, H. Dodiuk, S. Kenig, Thermosets Based On Reversible Covalent Bonds (Vitrimers), in: H. Dodiuk (Ed.), Handb. Thermoset Plast., 4th ed., Elsevier, 2021: pp. 757-800.

[2] J. Zhao, J.L. Brosmer, Q. Tang, Z. Yang, K.N. Houk, P.L. Diaconescu, O. Kwon, Intramolecular Crossed [2 + 2] Photocycloaddition through Visible Light- Induced Energy Transfer, J. Am. Chem. Soc. 139 (2017) 9807-9810.

[3] S. Ha, Y. Lee, Y. Kwak, A. Mishra, E. Yu, B. Ryou, C.M. Park, Alkyne-Alkene [2 + 2] cycloaddition based on visible light photocatalysis, Nat. Commun. 11 (2020) 1- 12.

[4] M. Bednarek, P. Kubisa, Reversible Networks of Degradable Polyesters Containing Weak Covalent Bonds, Polym. Chem. 10 (2019) 1848-1872.

[5] D. Tunc, C. Le Coz, M. Alexandre, P. Desbois, P. Lecomte, S. Carlotti, Reversible Cross-linking of Aliphatic Polyamides Bearing Thermo- and Photoresponsive Cinnamoyl Moieties, Macromolecules. 47 (2014) 8247-8254.

[6] D.T. Van-Pham, M.T. Nguyen, K. Ohdomari, H. Nakanishi, T. Norisuye, Q. Tran-Cong-Miyata, Controlling the Nano -Deformation of Polymer By a Reversible Photo-Cross-Linking Reaction, Adv. Nat. Sci. Nanosci. Nanotechnol. 8 (2017).

[7] S. Radi, M. Kreimer, T. Griesser, A. Oesterreicher, A. Moser, W. Kern, S. Schlbgl, New Strategies Towards Reversible and Mendable Epoxy Based Materials Employing [4KS + 4KS] Photocycloaddition and Thermal Cycloreversion of Pendant Anthracene Groups, Polymer (Guildf). 80 (2015) 76-87. GENERAL DESCRIPTION

Recycling of polymers is a major current challenge in many fields. In an effort to improve recycling processes and increase sustainability of recycling materials, both in terms of ease and efficiency of conversion and in terms of the reusability of materials obtained from the conversion process, the inventors have developed a novel class of thermoset polymers that is fully recyclable into its original building blocks. The process used for the formation of the polymers and the processes used for their de-formation or recycling are based on finely controlled cycloaddition reactions, e.g., [4 + 4] or [2 + 2] cycloaddition reactions, and facile cycloreversion processes.

While conventional recycling of thermoset polymers via stimuli-triggered degradation typically increases thermoset circularity, the original polymer structure and properties are lost. This loss in architecture and properties leads to a decrease in reusability as the polymer performance is reduced as well. Processes of the invention avoid such a decrease in reusability by converting or recycling the thermoset polymer obtained by 3D printing back to its original components. As the radiation curing of the monomers and the reversion to the same monomers following recycling are both nearly stoichiometric, processes of the invention are superior to those known in the art.

In a first of its aspects the invention provides a thermoset polymer formed by 3D printing involving radiation curing of unsaturated monomers capable of undergoing radiation-mediated cycloaddition (e.g., light-mediated cycloaddition), the thermoset polymer being fully reversible to the unsaturated monomers under thermal or microwave-mediated conditions. Generally, the cycloaddition conditions for achieving polymers of the invention exclude or preferably do not involve irradiation at wavelengths under 250nm or below 300nm or at a wavelength between about 250nm and 280nm or between 200nm and 300nm. However, in some cases, cycloreversion may involve irradiation under such wavelengths.

The invention further provides a 3D printed object consisting or comprising a thermoset polymer, the thermoset polymer is capable of thermal or microwave- mediated cycloreversion to monomers forming same.

Polymers of the invention are thermoset polymers which may be formed into objects or patterns by printing (or deposition) and radiation curing of one or a plurality of unsaturated monomeric materials, which may be in a form of at least one monomer, oligomer or prepolymer of preselected properties (e.g., structure, molecular weight, etc). Unlike thermoset polymers known to be poorly recyclable, polymers of the invention are fully recyclable as the polymer is easily caused to revert to its original components, monomers. Thus, the term “fully reversible is meant to encompass complete or nearly complete cycloreversion of the polymer to the individual components or monomers used to form it. The percent conversion to the monomeric material may be between 85 and 100%, or between 85 and 95, 85 and 90, 90 and 100, 95 and 100, or may be 85, 86, 87, 88, 89, 90, 91, 92, 93, 94, 95, 96, 97, 98, 99 or 100% conversion. As such, the polymer and objects made therefrom may be regarded as truly recyclable, namely de-formed by reversing the covalent bonds formed by cycloaddition. The recycling of the thermoset polymer does not require any of the mechanical or harsh conditions used industrially for recycling of typical thermoset materials. An exemplary reversible reaction is depicted below, wherein two identical monomers (left) are combined under the cycloaddition conditions disclosed herein to provide the cycloaddition adduct (right), which may undergo cycloreversion to the monomeric building blocks.

It should be understood that, as disclosed herein, the purpose of the cycloaddition and cycloreversion steps is to form polymers of predesigned structures and properties that can be fully recycled under conditions that are safe, effective and which are industrially useful.

The invention thus additionally provides a recyclable material, the material comprising or consisting of a thermoset polymer in a form of a cycloaddition adduct, wherein said thermoset polymer is fully reversible upon thermal or microwave irradiation to monomers of said adduct.

The thermoset polymer of the invention is typically structured as a network of 4-membered and/or 8-membered ring structures formed of covalent bonds resulting from radiation-mediated cycloaddition of two same or different unsaturated or 7t-bond containing monomeric materials. Depending on the structure of the monomeric material(s), the number of n bonds (double or triple bonds) and the functionalities present, the cycloaddition reactions may proceed via a [2 + 2] and/or [4 + 4] cycloaddition to provide a cycloaddition adduct which may, in some cases, be of a predictable network or structure. As known in the art, cycloaddition reactions are pericyclic reactions in which two or more unsaturated bonds combine with a cyclic movement of electrons to form a ring structure, i.e., an adduct, with a net reduction in bond multiplicity. The cycloaddition reactions employed for forming the polymers of the invention are typically [2 + 2] and/or [4 + 4] cycloaddition reactions which may be homodimeric, wherein the two unsaturated monomers are identical, or heterodimeric where the two unsaturated monomers are different. Typically, the polymers are formed of a single type of monomer.

The cycloaddition reactions involved in forming the polymers of the invention are not [4 + 2] cycloaddition (or Diels Alder) reactions, as known in the art.

The [2 + 2] -cycloaddition reaction results in 4-membered ring structures which ring atoms depend on the atoms forming the unsaturated monomers undergoing cycloaddition. Typically, the 4-membered rings are carbocycles, though can also have non-carbon members, such as oxygen or nitrogen atoms. The [4 + 4] cycloaddition reaction is similarly a cycloaddition reaction in which two unsaturated monomers having each two double bonds interact to create an eight- membered ring. The ring may be a carbocycle or a heterocycle and typically contains one or two ring double bonds. In some cases, where the monomers can undergo both [2 + 2] and [4 + 4] cycloaddition reactions, the resulting polymer may comprise a network of different carbo- and/or heterocyclic rings structures, with some being 4-memebered and others 8-memebered.

As stated herein, polymers of the invention are formed into thermoset polymers by a method of 3D printing involving radiation-curing of the unsaturated monomers. The term “thermoset used in the context of the present invention uniquely refers to a polymer that is formed by radiation-curing or crosslinking of the monomers or prepolymers and which unlike typical thermosetting materials, which set irreversibly such that the monomers are joined together by irreversible covalent bonds, polymers of the invention are capable of undergoing reversion and re-curing upon cycles of thermo- or microwave-mediated reversion and radiation-mediated curing.

Polymers of the invention may independently of the process for their preparation be characterized by a unique repeating structural motif. The “repeating structural motif is a sub-structure that is repeated in the polymer, and as such is characteristic of the polymer structure. The structural motif typically represents the structure of the adduct formed by the cycloaddition reaction. In some cases, the motif is the sole repeating unit characterizing the polymer. In other cases, one or more repeating structural motifs may be present.

In some embodiments, polymers of the invention comprise a repeating structural motif selected from

designates a point of connectivity to another motif or an atom or a group of atoms in the polymer; each of Xi and X2, independently, is a heteroatom selected from N, O and S; and wherein each of R’ and R”, independently of the other, is selected from H, optionally substituted -C₁-C₅alkyl, optionally substituted -C₁-C₅heteroalkyl, optionally substituted -C₆-C₁₂aryl, optionally substituted -C₁-C₅alkylene-C₆-C₁₂aryl, optionally substituted -C₆-C₁₂arylene-C₁-C₅alkyl, optionally substituted -C₃-C₇heteroaryl (comprising one or more heteroatom selected from N, O and S), a polyamine, a polyol, or a polythiol.

In some embodiments, each of R’ and R” is different from H.

In some embodiments, each of R’ and R” is same or different and selected from optionally substituted -C₆-C₁₂aryl, optionally substituted -C₁-C₅alkylene-C₆-C₁₂aryl, optionally substituted -C₆-C₁₂arylene-C₁-C₅alky 1, and optionally substituted -C3- Cvheteroaryl (comprising one or more heteroatom selected from N, O and S).

In some embodiments, each of R’ and R” is same or different and selected from optionally substituted -C₆-C₁₂aryl. In some embodiments, each of Xi and X2 is same or different. In some embodiments, one or both of Xi and X2 is N atom or a NH group. In some embodiments, one or both of Xi and X2 is O atom.

In some embodiments, the polymer is of a structure comprising the repeating structural motif

wherein each of Xi and X2 is same or different and selected as herein, and wherein each of R’ and R” is same or different and selected as herein.

In some embodiments, the thermoset polymer has a structure comprising the motif:

, each of the 8-memebered rings

, wherein

designates a point of connectivity to another 8-memebered ring or an atom or a group of atoms in the polymer, as shown; each of Xi and X2, independently is a heteroatom selected from N, O and S; and wherein each of R’ and R”, independently of the other, is selected from H, optionally substituted -C₁-C₅alkyl, optionally substituted -C₁-C₅heteroalkyl, optionally substituted -C₆-C₁₂aryl, optionally substituted -C₁-C₅alkylene-C₆-C₁₂aryl, optionally substituted - C₆-C₁₂arylene-C₁-C₅alkyl, optionally substituted -C₃-C₇hetcroaryl (comprising one or more heteroatom selected from N, O and S), a polyamine, a polyol, or a polythiol.

In some embodiments, Xi and X2 are the same and R’ and R” are the same.

In some embodiments, in each of the structural motifs, each of Xi and X2 is N or O and the repeating structural motif is selected from

to another motif or an atom or a group of atoms in the polymer and wherein each of R’ and R” is as defined above.

In some embodiments, the polymer having the structural motif

wherein each of R’ and R” is as defined above. In some embodiments, the thermoset polymer has a structure:

, each of the 8-memebered rings is wherein ' designates a point of connectivity to another 8-

memebered ring or an atom or a group of atoms in the polymer, as shown; and wherein each of R’ and R”, independently of the other, is selected from H, optionally substituted -C₁-C₅alkyl, optionally substituted -C₁-C₅heteroalkyl, optionally substituted -C₆- C₁₂aryl, optionally substituted -C₁-C₅alkylene-C₆-C₁₂aryl, optionally substituted -C₆- C₁₂arylene-C₁-C₅alkyl, optionally substituted -C₃-C₇heteroaryl (comprising one or more heteroatom selected from N, O and S), a polyamine, a polyol, or a polythiol.

In some embodiments, in the polymer, each of R’ and R” is different from H.

In some embodiments, each of R’ and R” is same or different and selected from optionally substituted -C₆-C₁₂aryl, as defined herein.

The molecular weights of polymers of the invention may vary. Typically, the molecular weights of each monomer or prepolymer may range between 300 and 6000 Da.

The molecular weight between reactive groups (Me) of polymers of the invention is between 200 Da to 10 kDa.

In another aspect, the invention provides a thermoset polymer having a structural motif selected from:

designates a point of connectivity to another motif or an atom or a group

of atoms in the polymer; each of Xi and X2, independently is a heteroatom selected from N, O and S; and wherein each of R’ and R”, independently of the other, is selected from H, optionally substituted -C₁-C₅alkyl, optionally substituted -C₁-C₅heteroalkyl, optionally substituted -C₆-C₁₂aryl, optionally substituted -C₁-C₅alkylene-C₆-C₁₂aryl, optionally substituted - C₆-C₁₂arylene-C₁-C₅alkyl, optionally substituted -C₃-C₇heteroaryl (comprising one or more heteroatom selected from N, O and S), a polyamine, a polyol, or a polythiol.

In some embodiments, each of R’ and R” is different from H.

In some embodiments, each of R’ and R” is same or different and selected from optionally substituted -C₆-C₁₂aryl.

In some embodiments, each of Xi and X2 is same or different. In some embodiments, one or both of Xi and X2 is N atom or a NH group. In some embodiments, one or both of Xi and X2 is O atom.

In some embodiments, each of Xi and X2 is N and each of R’ and R” is a -C₆- C₁₀aryl or a substituted form thereof.

, wherein each of Xi and X2 is same or different and selected as herein, and wherein each of R’ and R” is same or different and selected as herein.

The invention further provides a thermoset polymer having a structure:

designates a point of connectivity to another 8-memebered ring or an

atom or a group of atoms in the polymer, as shown; each of Xi and X2, independently is a heteroatom selected from N, O and S; and wherein each of R’ and R”, independently of the other, is selected from H, optionally substituted -C₁-C₅alkyl, optionally substituted -C₁-C₅heteroalkyl, optionally substituted -C₆-C₁₂aryl, optionally substituted -C₁-C₅alkylene-C₆-C₁₂aryl, optionally substituted - C₆-C₁₂arylene-C₁-C₅alkyl, optionally substituted -C₃-C₇heteroaryl (comprising one or more heteroatom selected from N, O and S), a polyamine, a polyol, or a polythiol.

In some embodiments, Xi and X2 are the same and R’ and R” are the same.

The invention further provides a thermoset polymer having a structural motif

'Tu\rv' designates a point of connectivity to another motif or an atom or a group of atoms in the polymer, and wherein each of R’ and R”, independently of the other, is selected from H, optionally substituted -C₁-C₅alkyl, optionally substituted -C₁-C₅heteroalkyl, optionally substituted -C₆-C₁₂aryl, optionally substituted -C₁-C₅alkylene-C₆-C₁₂aryl, optionally substituted - C₆-C₁₂arylene-C₁-C₅alkyl, optionally substituted -C₃-C₇heteroaryl (comprising one or more heteroatom selected from N, O and S), a polyamine, a polyol, or a polythiol.

In some embodiments, each of R’ and R” is different from H.

The invention further provides a thermoset polymer having the structure:

'/vw' designates a point of connectivity to another 8-memebered ring or an atom or a group of atoms in the polymer, as shown; and wherein each of R’ and R”, independently of the other, is selected from H, optionally substituted -C₁-C₅alkyl, optionally substituted -C₁-C₅heteroalkyl, optionally substituted -C₆-C₁₂aryl, optionally substituted -C₁-C₅alkylene-C₆-C₁₂aryl, optionally substituted - C₆-C₁₂arylene-C₁-C₅alkyl, optionally substituted -C₃-C₇heteroaryl (comprising one or more heteroatom selected from N, O and S), a polyamine, a polyol, or a polythiol.

In some embodiments, R’ and R” are the same.

In some embodiments, in each of the polymers disclosed herein, each of R’ and R”, independently of the other, may be selected from optionally substituted -C₆- C₁₂aryl, wherein the -C₆-C₁₂aryl, as defined herein may be a phenyl ring, which may be bonded to another ring, such as biphenyl having a total of 12 ring carbon atoms, or fused to another ring, e.g., 1 -naphthyl or 2-naphthyl, having a total of 10 ring carbon atoms; or the phenyl ring may be substituted. Non-limiting examples for each of R' and R” include phenyl, benzyl, naphthyl, hydroxyphenyl (wherein the hydroxyl group is substituted at any position of the phenyl ring), a dihydroxyphenyl (wherein the two hydroxy groups are substituted at any of the phenyl ring carbon atoms, and may be ortho, meta or para to each other), a phenyl ring having two or more different substitutions (e.g., selected from alkoxy, hydroxy, methyl, or any substitution as disclosed herein), and other.

In some embodiments, in each of the polymers disclosed herein, each of R’ and

R’ ’ , independently of the other, may be an arylene of the structure

, wherein

' w designates a point of connectivity and wherein Ra is one or more substituents which may be selected from -H, -Ci-C₁₂alkyl, -C₆-C₁₀aryl, -OH, -OCi-C₁₂alkyl, -OC₆- C₁₀aryl, -COOH, -COOCi-C₁₂alkyl, -COOC₆-C₁₀aryl, and others.

In some embodiments, Ra represents a single substituent positioned ortho, metal or para to the atom of connectivity. In some embodiments, Ra designates two or more substituents. The two or more substituents may be same or different and may be positions 1,2, or 1,3, or 1,4 to each other. In some embodiments, each of R’ and R”, independently may be selected from

others.

In some embodiments, each of R’ and R”, independently, may be selected from:

of R’ and R” is the same and selected as above. In some embodiments, each of R’ and R” is selected from:

Polymers of the invention are formed from unsaturated monomers or prepolymers having one or more double bonds of the form X=C, wherein X is a carbon atom or a heteroatom such as oxygen, nitrogen or sulfur. The nature of X and the number of such double bonds in each of the unsaturated monomers will determine whether the polymer formed is structured of a network of carbocyclic, heterocyclic, 4- membered and/or 8-memebered rings. In some embodiments, the unsaturated monomer or prepolymer is formed by reacting a compound having at least one double bond in the form O=C, or O=C-C=C (wherein each atom further comprises a proper number of hydrogen or other atoms to complete valency) with a polyamine, a polyol, a polythiol (polymercaptane), a polycarboxylic acid, a polyester, a polyamide, or a polyimide to form a monomer or a prepolymer. The compound may be an aldehyde, an ester, or a carboxylic acid, such that the O=C constitutes a carboxyl group of the aldehyde, the ester, or the carboxylic acid. The monomer or prepolymer formed may thus comprise the double bond(s) of the precursor compounds.

In some embodiments, the monomer or prepolymer may be derived from cinnamaldehyde, coumaryl aldehyde, caffeic aldehyde, cinnamic acid, caffeic acid, 4- hydroxy cinnamic acid, coumaric acid, ferulic acid, 4-trans cinnamic acid, 3,4- dimethoxy cinnamic acid and others. In some embodiments, the monomer or prepolymer may be derived from a reaction between any one or more of cinnamaldehyde, coumaryl aldehyde, caffeic aldehyde, cinnamic acid, caffeic acid, 4- hydroxy cinnamic acid, and coumaric acid; and a polyamine, a polyol, a polycarboxylic acid, a poly thiol, etc. the reaction leading to the formation of the monomer or prepolymer may vary and is not limited to any one type of reaction.

In some embodiments, the monomer or prepolymer has the structure (Z- )N=CR’R”, or (Z-)N=C(RI)-C(R2)=CR’R”, wherein N represents a nitrogen atom of a polyamine Z; each of R’, R”, Ri, and R2, independently of the other, is H (provided that two geminal R groups are not both H) or a group selected from optionally substituted - C₁-C₅alkyl, optionally substituted -C₁-C₅heteroalkyl, optionally substituted -C₆- C₁₂aryl, optionally substituted -C₁-C₅alkylene-C₆-C₁₂aryl, optionally substituted -C₆- C₁₂arylene-C₁-C₅alkyl, optionally substituted -C₃-C₇heteroaryl (comprising one or more heteroatom selected from N, O and S), a polyamine, a polyol, and a polythiol.

The -C₁-C₅alkyl refers to a radical of a straight-chain or branched saturated hydrocarbon group having from 1 to 5 carbon atoms (inclusive). In some embodiments, the alkyl group has 1 to 4 carbon atoms, 1 to 3 carbon atoms, or has 1, 2, 3, 4, or 5 carbon atoms. Non-limiting examples of -C₁-C₅ alkyl groups include methyl, ethyl, propyl (e.g., n-propyl, isopropyl), butyl (e.g., n-butyl, tert-butyl, sec-butyl, isobutyl) and pentyl (e.g., n-pentyl, 3-pentanyl, amyl, neopentyl, 3-methyl-2-butanyl, tert-amyl). Unless otherwise specified, each instance of an alkyl group is independently unsubstituted or substituted with one or more substituents, as defined herein. In some embodiments, the -C₁-C₅alkyl group is an unsubstituted -C₁-C₅alkyl group.

The group -Ci-C₁₂alkyl similarly apply to an alkyl group having between 1 and 12 caron atoms, each may be defined, selected, optionally substituted above.

The group -C₁-C₅alkylene refers to an alkyl group, as defined, having two atoms of connectivity (namely having a methylene, -CH2-, group or a sequence of methylene groups).

The -Ci-Csheteroalkyl is an alkyl group, as defined, which further includes between 1 and 4 heteroatoms, e.g., N, O and S. The heteroatom may be inserted between adjacent carbon atoms to provide an interrupted alkyl, such that any carbon chain or segment separated by the heteroalkyl comprises at least one carbon atom (and additional hydrogen atoms).

The -C₆-C₁₂aryl refers to a radical of a monocyclic or polycyclic (fused or multicyclic) aromatic ring system having between 6 and 12 ring carbon atoms and alternating double bonds The aryl group may have 6 ring carbon atoms, e.g., phenyl, which may be bonded to another ring, such as biphenyl having a total of 12 ring carbon atoms, or fused to another ring, e.g., 1-naphthyl or 2-naphthyl, having a total of 10 ring carbon atoms. Non-limiting examples include phenyl, benzyl, naphthyl and others.

The group -C₁-C₅alkylene-C₆-C₁₂aryl is an alkyl group, as defined, that is substituted by an aryl group, as defined, wherein the point of attachment is on the alkyl (alkylene) end. Similarly, the group -C₆-C₁₂arylene-C₁-C₅alkyl is an aryl group that is substituted with an alkyl, wherein the point of attachment is on the aryl end. The -Cs-Cvheteroaryl refers to a radical of a 4 to 10 membered monocyclic or polycyclic ring system, typically aromatic ring system(s) comprising 3 to 7 carbon atoms and 1 to 4 ring heteroatoms (e.g., N, S or O). Non-limiting examples of heteroaryl ring systems include pyrrolyl, furanyl, thiophenyl, imidazolyl, pyrazolyl, oxazolyl, isoxazolyl, thiazolyl, isothiazolyl, triazolyl, oxadiazolyl, and others.

The polyamine is an oligomer or a polymer, typically a short polymer, having a monomer with an amine functionality. The amine may be incorporated in the backbone, may be provided at the end of each monomer or may be a pendent group. As such, the amine may be selected amongst primary amines, secondary amines or tertiary amines. In some cases, the amine may be charged and may be accompanied with a corresponding counter ion. The polyamine may be a polyalkyleneimine; a polyvinyl amine; a triamine such as diethylenetriamine, or bis(hexamethylene)triamine; a tetramine such as triethylenetetramine; a pentamine such as tetraethylenepentamine, or any higher homologue. Non-limiting examples of the polyamines include branched or linear polyethyleneimine (PEI), cadaverine, spermidine, spermine, triethylenetetramine (TETA), tris(2-aminoethyl)amine, and others.

The polythiol, also known as polymercaptan, is a polyfunctional material containing two or more thiol (SH) functionalities. Suitable polythiols may be any of those that are known in the art. Exemplary materials include polythiols having at least two thiol groups including polythiol having ether linkages (-O-), sulfide linkages (-S-), polysulfide linkages (-S-S-. . .), and others. Non-limiting examples of polythiols include ethylene glycol bis(thioglycolate), ethylene glycol bis(mercaptopropionate), trimethylolpropane tris(thioglycolate), trimethylolpropane tris(mercaptopropionate), pentaerythritol tetrakis (thioglycolate) and pentaerythritol tetrakis(mercapto propionate), and others.

The polycarboxylic acid is any polymer having two or more carboxylic acid moieties, including acid halides, esters, half-esters, salts, half-salts, anhydrides, and others.

The polyester encompasses a polymer containing two or more ester functionalities.

Any of aforementioned functionalities is “optionally substituted” , namely is substituted or unsubstituted. In certain embodiments, the alkyl, heteroalkyl, aryl, or the heteroaryl groups are substituted. The substitution may be any one or more substituents that provide a stable compound. The substituents may be selected amongst polar, a polar, electron withdrawing, hydrophilic, hydrophobic, etc, atom or a group of atoms. Non-limiting examples include a halogen, a cyano group, a nitro group, an azide group, an hydroxy group, an ether group, an amine group (primary, secondary, tertiary or quaternary), a thiol group, an ester group, a ketone, an acid group, an alkyl group, an aryl group, etc.

In some embodiments, the unsaturated monomer or prepolymer is Z-N=CR’R”, or Z-N=C(Ri)-C(Ri)=CR’R”, wherein Z-N represents a polyamine, e.g., polyethyleneimine (PEI) or triethylenetetramine (TETA), wherein Z is the poly amine and N is a nitrogen atom thereof. In some embodiments, the unsaturated monomer is PEI-N=CR’R”, PEI-N=C(RI)-C(R₂)=CR’R”, TETA-N=CR’R”, or TETA-N=C(Ri)- C(R₂)=CR’R”, wherein in each case, the N atom is an atom of the polyamine.

In some embodiments, the unsaturated monomer is a prepolymer formed of a polyamine such as PEI or TETA and O=CHR’, or O=CH-CH=CHR’ , wherein each of R’ and R” is different from H.

In some embodiments, the unsaturated monomer is a prepolymer formed of a polyamine such as PEI or TETA and 0=CH-CH=CH-C6-C₁₀aryl.

In some embodiments, the compound 0=CH-CH=CH-C6-C₁₀aryl represents cinnamaldehyde or coumaryl aldehyde or caffeic aldehyde.

In some embodiments, the compound H0-(C=0)-CH=CH-C6-C₁₀aryl represents cinnamic acid or a derivative such a caffeic acid, 4-hydroxy cinnamic acid, or coumaric acid.

In some embodiments, the unsaturated monomer is a prepolymer of a polyamine, as defined herein, and cinnamaldehyde or coumaryl aldehyde or caffeic aldehyde. In some embodiments, the unsaturated monomer is

integer between 1 and 50 and wherein each of R’, independently, may be same or different and may be selected as disclosed herein.

In some embodiments, each of R’ is same or different -C₆-C₁₀aryl, e.g., a substituted or an unsubstituted phenyl ring. In some embodiments, R’ is phenyl; 2-, 3- or 4-hydroxyphenyl; 2,3- 2,4 or 3,4-dihydroxyphenyl.

In some embodiments, the unsaturated monomer is of the structure:

designating the number of repeating units, optionally being between 1 and 50.

In some embodiments, the unsaturated monomer is

, wherein n is an integer between 1 and 50.

In some embodiments, the unsaturated monomer is any of:

wherein each of R’ is a -C₆-C₁₀aryl, e.g., a substituted or an unsubstituted phenyl ring. In some embodiments, R’ is phenyl; 2-, 3- or 4-hydroxyphenyl; or 2,3- 2,4 or 3,4- dihy droxypheny 1.

In some embodiments, the unsaturated monomer is any of:

Any of the aforementioned unsaturated monomers or prepolymers may be used to form a polymer or an object via [2 + 2] or [4 + 4] cycloaddition.

Monomers and prepolymers used in methods of the invention may be prepared by a variety of reaction methodologies. For example, where aldehydes are reacted with amines, the process may involve a Schiff-base, as known in the art. When an acid derivative is used, the reaction may proceed to esterification or amidation under acidic conditions, as known in the art.

In another aspect, the invention provides a prepolymer as disclosed herein.

In some embodiments, the prepolymer is for use in a method of cycloaddition.

In some embodiments, the polymer of the invention is formed of a polymer that

herein.

In some embodiments, the polymer of the invention is formed of a polymer that

herein. In some embodiments, the polymer of the invention is formed of a polymer that is a [2 + 2] cycloaddition adduct

In some embodiments, the polymer of the invention is formed of a polymer that is a [2 + 2] cycloaddition adduct

In some embodiments, the polymer of the invention is formed of a polymer that is a [4 + 4] cycloaddition adduct

In some embodiments, each of the structural motifs disclosed and defined herein may be bonded to an atom or a group of a polymer, wherein the group of the polymer is a repeating group having a structure selected from: ethylene diamine,

each of a, b and c, independently, is an integer between 1 and 50. In some embodiments, the number of groups designated by a, b and c, combined, is between 5 and 60.

In some embodiments, the monomer or prepolymer has the structure:

independently is as defined hereinabove, and wherein each of R’, being same or different may be selected as above.

In some embodiments, the monomer or prepolymer is

independently is as defined hereinabove.

In some embodiments, the monomer or prepolymer is:

, wherein each of a, b and c, independently is as defined hereinabove, and wherein each of R’, being same or different may be selected as above.

In some embodiments, the monomer or prepolymer is

, wherein each of a, b and c, independently is as defined hereinabove. In some embodiments, the monomer or prepolymer is:

, wherein each of R’, being same or different, is selected as defined herein.

In some embodiments, the monomer or prepolymer is

The invention further provides any one of the following compounds: (a)

, wherein each of a, b and c, independently is as defined hereinabove, and wherein each of R’, being same or different may be selected as above;

independently is as defined hereinabove;

(c)

, wherein each of a, b and c, independently is as defined hereinabove, and wherein each of R’, being same or different may be selected as above; T1

(d)

, wherein each of a, b and c, independently is as defined hereinabove;

(e)

different, is selected as defined herein;

integer between 1 and 50 and wherein each of R’, independently, may be same or different and may be selected as disclosed herein;

(h)

designating the number of repeating units, optionally being between 1 and 50;

(i)

an integer between 1 and 50; (J)

, wherein each of R’ is a -C₆-C₁₀aryl, e.g., a substituted or an unsubstituted phenyl ring;

(k)

(1)

(m)

(n)

, wherein each of R’ is -C₆-C₁₀aryl, e.g., a substituted or an unsubstituted phenyl ring;

(o)

In some embodiments, each of the aforementioned compounds is for use in a method of manufacturing or forming a polymeric material, a polymeric pattern or a polymeric object.

In some embodiments, each of the aforementioned compounds for use in a method of manufacturing or forming a polymeric material, a polymeric pattern or a polymeric object by 3D printing.

In some embodiments, each of the aforementioned compounds for use as a monomer in a method of cycloaddition.

The invention further provides a kit comprising at least one monomer as disclosed herein and instructions of use. The invention further provides a 3D printing method for manufacturing a chemically recyclable thermoset polymer, the method comprising radiation-curing unsaturated monomers capable of cycloaddition to form the chemically recyclable thermoset polymer, wherein the polymer is reversible to the unsaturated monomers under thermal or micro wave radiation.

Also provided is a 3D printing method for manufacturing a thermoset polymer, the method comprising radiation curing unsaturated monomers capable of photochemical [2+2] and/or [4+4] cycloaddition to form the thermoset polymer capable of cycloreversion under thermal conditions.

Also provided is a 3D printing method for manufacturing a chemically recyclable thermoset polymer, the method comprising radiation curing unsaturated monomers capable of photochemical cycloaddition under light having a wavelength between 360 and 460 nm to form the chemically recyclable thermoset polymer, wherein the polymer is reversible to the unsaturated monomers under thermal or microwave radiation.

Further provided is a 3D printing method for forming a thermoset object, the method comprising radiation curing at least one unsaturated monomer capable of undergoing [2 + 2] or [4 + 4] cycloaddition with another same or different unsaturated monomer, to form the thermoset object, wherein the object is capable of undergoing cycloreversion to the unsaturated monomers.

In some embodiments of methods of the invention, the at least one unsaturated monomer is a monomer or a prepolymer as disclosed herein.

In some embodiments of methods of the invention, the at least one unsaturated monomer is a monomer or a prepolymer having at least one double bond capable of undergoing [2+2] or [4+4] cycloaddition. In some embodiments, the at least one unsaturated monomer is a monomer or a prepolymer having two or more double bonds capable of undergoing [2+2] or [4+4] cycloaddition. In some embodiments, at least one of the double bonds is a bond of the form X=C, wherein X is N, O or S.

Methods of the invention typically involve printing or deposition of a monomeric material, as defined herein, and radiation curing same in a layer-by-layer fashion, or after the object or pattern has been formed, or simultaneously with the deposition of the monomeric material. The 3D methods which may be employed may be selected amongst those known in the art and may be chosen based on a variety of method- or object- specific factors. The printing method may involve a 3D deposition method or stereolithography. Non-limitedly, the 3D printing method may be Digital Light Processing (DLP), stereolithography (SLA), Direct Ink Write (DIW) combined with light irradiation, Polyjet printing, volumetric printing, two-photon polymerization printing, extrusion deposition combined with light irradiation and others. In some embodiments, the 3D printing method is DIW combined with light irradiation.

Irrespective of the particular 3D printing method used, the method comprises radiation curing of the monomeric material. The radiation curing" encompasses any radiation source capable of causing the monomeric material to solidify by crosslinking or curing. The radiation may be light radiation or thermal or IR radiation emitted from an IR source or due to friction forces. In some embodiments, the radiation curing is light curing using a light irradiation in the visible or UV regime.

In some embodiments, radiation curing is light curing, e.g., UV or visible light curing.

As a radiation source for radiation curing, a projection unit such as a DLP projector, a LED projector, an LCD projector, a laser source, an electron beam, or any other light emitting unit, e.g., low-pressure mercury lamps, medium-pressure mercury lamps, high-pressure lamps, fluorescent tubes, pulsed lamps, metal halide lamps (halogen lamps), or electron flash units may be used. In some embodiments, radiation curing may be achieved by exposure to a low-energy radiation, i.e., a radiation dose that is between 2 mW/cm² and -100 W/cm². In some embodiments, radiation curing is achievable under light irradiation with a light of a wavelength between 360 and 405 nm.

As stated herein, [2 + 2] or [4 + 4] cycloaddition reactions are limited by their inherent short wavelength activation and very long reaction times. Both limitations render 3D printing of monomers capable of undergoing cycloaddition nearly impossible. To overcome these limitations and enable 3D printing, the inventors have introduced a novel class of catalysts which under light irradiation at wavelengths between 360 and 460 nm effectively catalyze the cycloaddition to provide the cycloaddition adduct within seconds to few minutes. Thus, the monomeric material disclosed herein may be used in combination with a catalytic amount of a catalyst having light absorbance between 360 and 460 nm. The amount or concentration of catalyst used may be between 3 to 20 mol% (relative to the number of the unsaturated bonds). The concentration may be varied based on the reactivity of the unsaturated monomer or prepolymer. In some embodiments, a method of the invention comprises curing at least one unsaturated monomer capable of undergoing [2 + 2] or [4 + 4] cycloaddition with another same or different unsaturated monomer, in presence of at least one catalyst (e.g., a photocatalyst), to form the thermoset object.

In some embodiments, the method comprises providing an ink formulation comprising at least one unsaturated monomer capable of undergoing [2 + 2] or [4 + 4] cycloaddition with another same or different unsaturated monomer, at least one catalyst (e.g., a photocatalyst), and optionally a carrier or a solvent. While the carrier/solvent may be any organic or inorganic solvent, limited only by toxicity, in some embodiments, the carrier/solvent is an organic solvent such as benzyl alcohol, eugenol, DMSO, and 2-hexanone.

In some embodiments, curing comprises light irradiation of the ink formulation with a light of a wavelength between 360 and 460 nm, under a light intensity between 2 mW/cm² and -100 W/cm².

In some embodiments, curing comprises light irradiation of the ink formulation with a light of a wavelength between 360 and 400 nm, under a light intensity between 2 mW/cm² and -10 W/cm². In some embodiments, curing comprises light irradiation of the ink formulation with a light of a wavelength between 360 and 370 nm, under a light intensity between 2 mW/cm² and -5 W/cm². In some embodiments, curing comprises light irradiation of the ink formulation with a light of a wavelength of 365 nm, under a light intensity of about 2 mW/cm² to 10 mW/cm².

In some embodiments, the light intensity may be between 2 mW/cm² and -100 W/cm², or between 3 mW/cm² and -100 W/cm² or between 5 mW/cm² and -100

W/cm², or between 10 mW/cm² and -100 W/cm², or between 20 mW/cm² and -100

W/cm², or between 30 mW/cm² and -100 W/cm², or between 50 mW/cm² and -100

W/cm², or between 100 mW/cm² and -100 W/cm², or between 3 mW/cm² and -100 mW/cm², or between 10 mW/cm² and -100 mW/cm², or between 2 mW/cm² and -10 mW/cm², or between 3 mW/cm² and -5 W/cm², or between 3 mW/cm² and -5 mW/cm², or between 2 mW/cm² and -10 mW/cm², or at any light intensity between 2 mW/cm² and -100 W/cm², such that any light intensity within the stated range constitutes a separate and explicitly disclosed value.

With regard to the reaction times, for achieving proper and complete curing of the unsaturated monomers or prepolymers, the irradiation period is not required to be long. In fact, proper and effective curing may be achieved within 30 seconds to 2 minutes. In some embodiments, the irradiation time is between 30 sec and 2 min, or between 45 sec and 2 min, or between 1 and 2 min, or between 1.5 and 2 min.

In some embodiments, curing comprises light irradiation of the ink formulation with a light of a wavelength between 360 and 460 nm, under a light intensity between 3.2 mW/cm² and -100 W/cm², for a period of between 30 sec and 2 minutes.

In some embodiments, curing comprises:

(i) light irradiation of the ink formulation with a light of a wavelength between 360 and 400 nm, or a wavelength between 360 and 370 nm, or a wavelength of 365 nm, or any wavelength disclosed herein;

(ii) light intensity between 3.2 mW/cm² and -10 W/cm², or under a light intensity between 3.2 mW/cm² and -5 W/cm², or under a light intensity of 3.2 mW/cm², or under any light intensity as disclosed herein;

(iii) irradiation time between 30 sec and 2 min, or between 45 sec and 2 min, or between 1 and 2 min, or between 1.5 and 2 min, or any irradiation time as disclosed herein.

In some embodiments, the at least one catalyst is a catalyst having a light absorbance between 360 and 460 nm. The catalyst may be selected amongst (a) metal phthalocyanines, e.g., zinc(II)phthalocyanine (ZnPC), tin(II)phthalocyanine (SnPC), cobalt(II)phthalocyanine (CoPC), Eu(II)phthalocyanine, or any of their derivatives; and (b) catalysts of formula (I):

wherein

M is a metal atom; each of Xi, X2, X3 and X4, independently of the other, is a nitrogen or an oxygen atom; each of R_m and R_n, independently of the other, is a group selected from:

point of connectivity to Xi, X₂, X3 or X₄;

Rs is selected from

wherein a/vx/'is a point of connectivity to the carbonyl groups;

R₄ is H or a -Ci-C₅alkyl; or -CH=CH-(C=O)-OH; or -CH₂-CH=C-(C=O)-O-

Aryl-CH=CH-(C=O)-OH; and wherein each of Z, Zi and Z₂, independently, is absent or is a -C₁-C₅alkylene.

The metal M may be a transition metal as known in the art. The transition metal may be selected from scandium (Sc), titanium (Ti), vanadium (V), chromium (Cr), manganese (Mn), iron (Fe), cobalt (Co), nickel (Ni), copper (Cu), zinc (Zn), zirconium (Zr), molybdenum (Mo), technetium (Tc), ruthenium (Ru), rhodium (Rh), palladium (Pd), silver (Ag), cadmium (Cd), tungsten (W), rhenium (Re), osmium (Os), iridium (Ir), platinum (Pt), gold (Au), cerium (Ce), europium (Eu), gadolinium (Gd), and ytterbium (Yb).

In some embodiments, the metal M may be Sn, Zn, Cu, Eu, Co or Pd, each constituting a separate embodiment.

In some embodiments, the catalyst is a metal phthalocyanine, wherein the metal is Sn, Co, Cu, Zn, Eu and others. In some embodiments, the catalyst is selected from zinc(II)phthalocyanine (ZnPC), tin(II)phthalocyanine (SnPC), cobalt(II)phthalocyanine (CoPC), and Eu(II)phthalocyanine.

In some embodiments, the catalyst is a compound of formula (I), wherein each of Xi, X₂, X3 and X₄ is O atom or N atom. In some embodiments, the catalyst is a compound of formula (I), wherein each of Xi, X₂, X3 and X₄ is O atom. In some embodiments, the catalyst is a compound of formula (I), wherein each of Xi, X₂, X3 and X₄ is N atom. In some embodiments, R4 is H.

In some embodiments,

In some embodiments, the catalyst of formula (I) is a catalyst of formula (II):

wherein

M is a metal atom; each of X1, X2, X3 and X4, independently of the other is a nitrogen or an oxygen atom; each of R1 and R2, independently of the other, is a group selected from:

a point of connectivity to Xi, X2, X3 or X4;

R₄ is H or a -C₁-C₅alkyl; or -CH=CH-(C=O)-OH; or -CH₂-CH=C-(C=O)-O- Aryl-CH=CH-(C=O)-OH; and wherein each of Z, Zi and Z2, independently, is absent or is a -C₁-C₅alkylene.

In some embodiments, each of Xi, X2, X3 and X4 is the same and is either a nitrogen atom or an oxygen atom.

In some embodiments, each of Xi, X2, X3 and X4 is a nitrogen atom.

In some embodiments, each of R1 and R2, independently of the other, is a group selected from

In some embodiments, each of R1 and R2, independently of the other, is

defined above.

In some embodiments, the catalyst is a compound of formula (I), wherein each of Xi, X2, X3 and X4 is N atom and R1 and R2 are each

or a -C₁-C₅alkyl. In some embodiments, R3 is H.

In some embodiments, the catalyst is a compound of formula (II) having the structure

wherein M is a metal selected from Sn, Cu, Zn, Co, and Eu. In some embodiments, M is Sn and the catalyst is

In some embodiments, the catalyst is a compound of formula (I) or formula (II), wherein each of Xi, X2, X3 and X4 is O atom, and wherein each of R1 and R2, independently of the other, is a group selected from:

wherein

R3 is H or a -C₁-C₅alkyl; each of Z, Zi and Z2, independently, is absent or a -C₁-C₅alkylene; and wherein

is a point of connectivity to Xi, X2, X3 or X4.

In some embodiments, R1 and R2 are the same.

In some embodiments, each of R1 and R2 is a group selected from:

R3 is H or a -C₁-C₅alkyl; each of Z, Zi and Z2, independently, is a -C₁-C₅alkylene; and wherein 'is a point of connectivity to Xi, X2, X3 or X4.

In some embodiments, each of R1 and R2 is a group selected from:

, wherein R3 is H or CH3, and each of Z,

Zi and Z2, independently, is a methylene or an ethylene group.

In some embodiments, each

each of Zi and Z2 is methylene or ethylene. In some embodiments, each

each of Zi and Z2 is methylene or ethylene.

In some embodiments, the catalyst is of formula (I), having the structure (IV)

wherein M is a metal as selected herein.

In some embodiments, the catalyst

In some embodiments, in a compound of formula (

In some embodiments, the catalyst is of structure (V) and (VI)

wherein for each of (V) and (VI), independently, each of Ri, R2, Xi, X2, X3 and X4 are as defined herein.

It is to be noted that compounds such as (V) and (VI), which contain chiral centers, each chiral center may be in the (R) or (S) configuration, or may be a mixture thereof. Thus, the compounds provided herein may be enantiomeric ally pure, or be stereoisomeric or diastereomeric mixtures. A person of skill in the art may realize that under certain conditions, use of a compound in its, e.g., (R) form, may undergo epimerization to its (S) form.

In some embodiments, each of R1 and R2 is a group selected from:

In some embodiments, each of R1 and R2 is a group selected from:

, wherein R3 is H or CH3, and each of Z,

Zi and Z2, independently, is a methylene or an ethylene group.

In some embodiments, each

each of Zi and Z2 is methylene or ethylene.

In some embodiments, each

each of Zi and Z2 is methylene or ethylene.

In some embodiments, the catalyst is of formula (VII):

wherein M is any metal atom as disclosed herein.

In some embodiments, the M is Sn, Zn, Cu, Eu, Co or Pd, each constituting a separate embodiment.

In some embodiments, the catalyst is:

As used herein, for a group R1 or R2 having the structure

extending from an undefined position of the benzene ring structure, i.e., extending to

R3, or the amine group or the Z group, designates a bond that may be formed from any carbon ring atom. For example, in the structure

, groups R3 and Z may be positioned ortho, meta or para to each other. In some embodiments, the catalyst is selected from metal (2,2'-[(4- methylphenyl)imino]bisethylbisphthalate) (M(PA-MPIB)), e.g., tin (II) (2,2'-[(4- methylphenyl)imino]bisethylbisphthalate) (Sn(PA-MPIB)), and metal (bis-Ni,Ni'-(l,3- phenylene)diphthalamide) (Sn(MPDA-PA)), e.g., tin (II) (bis-Ni,Ni'-(l,3- phenylene)diphthalamide) (Sn(MPDA-PA)).

Also provided is a 3D printing method for manufacturing a chemically recyclable thermoset polymer, the method comprising light curing unsaturated monomers capable of photochemical cycloaddition under light having a wavelength between 360 and 460 nm to form the chemically recyclable thermoset polymer, wherein the polymer is cycloreversible to the unsaturated monomers under thermal or microwave radiation. For a schematic illustration of a printing process using DIW, see Fig. 7. DLP may be used just the same.

As noted herein, the cycloaddition reaction to form the polymer of the invention can proceed under a variety of radiation curing conditions. These conditions may include radiation curing in presence of at least one catalyst, e.g., a catalytic as defined herein, wherein the curing conditions may be one or more of the following:

(1) Light irradiation (light curing) at a wavelength between 360 and 460 nm,

(2) Light irradiation (light curing) at a wavelength between 360 and 460 nm, and further under thermal radiation, e.g., a temperature between room temperature (23-30°C) and 90°C, and

(3) Light irradiation wherein the radiation dose is between 2 mW/cm² and ~100 W/cm².

The conditions leading to cycloreversion are not same or equivalent to conditions associated with the radiation curing. The cycloreversion may be achievable under conditions including one or more of:

(1) Thermal radiation to a temperature between 90 and 200°C, as shown in Figs. 22A-B; and

(2) Microwave radiation.

The invention further provides use of a catalyst having a light absorbance between 360 and 460 nm in a method of [4 + 4] or [2 + 2] cycloaddition reaction.

Further provides is a catalyst for use in a method of [4 + 4] or [2 + 2] cycloaddition reaction, the catalyst being selected amongst metal phthalocyanines, e.g., zinc(II)phthalocyanine (ZnPC), tin(II)phthalocyanine (SnPC), cobalt(II)phthalocyanine (CoPC), Eu(II)phthalocyanine, and other derivatives of these formulations.

The invention further provides a material of formula (I):

wherein

M is a metal atom; each of Xi, X₂, X3 and X4, independently of the other, is a nitrogen or an oxygen atom; each of R_m and R_n, independently of the other, is a group selected from:

connectivity to the carbonyl groups;

R₄ is H or a -Ci-C₅alkyl; or -CH=CH-(C=O)-OH; or -CH₂-CH=C-(C=O)-O- Aryl-CH=CH-(C=O)-OH; and wherein each of Z, Zi and Z₂, independently, is absent or is a -C₁-C₅alkylene.

In some embodiments, the catalyst is a compound of formula (I), wherein each of Xi, X2, X3 and X4 is O atom or N atom. In some embodiments, the catalyst is a compound of formula (I), wherein each of Xi, X2, X3 and X4 is O atom. In some embodiments, the catalyst is a compound of formula (I), wherein each of Xi, X2, X3 and X4 is N atom.

In some embodiments, R4 is H.

In some embodiments,

In some embodiments, the catalyst of formula (I) is a catalyst of formula (II):

wherein

M is a metal atom; each of Xi, X2, X3 and X4, independently of the other is a nitrogen or an oxygen atom; each of R1 and R2, independently of the other, is a group selected from:

a point of connectivity to Xi, X2, X3 or X4;

R₄ is H or a -Ci-C₅alkyl; or -CH=CH-(C=O)-OH; or -CH₂-CH=C-(C=O)-O- Aryl-CH=CH-(C=O)-OH; and wherein each of Z, Zi and Z2, independently, is absent or is a -C₁-C₅alkylene.

In some embodiments, each of Xi, X2, X3 and X4 is a nitrogen atom.

In some embodiments, each of R1 and R2, independently of the other, is

defined above.

, wherein R3 is H or a -C₁-C₅alkyl. In some embodiments, R3 is H. In some embodiments, the catalyst is a compound of formula (II) having the structure

wherein

R3 is H or a -C₁-C₅alkyl; each of Z, Zi and Z2, independently, is absent or a -C₁-C₅alkylene; and wherein is a point of connectivity to Xi, X2, X3 or X4.

In some embodiments, R1 and R2 are the same.

In some embodiments, each of R1 and R2 is a group selected from:

R3 is H or a -C₁-C₅alkyl; each of Z, Zi and Z2, independently, is a -C₁-C₅alkylene; and wherein is a point of connectivity to Xi, X2, X3 or X4.

In some embodiments, each of R1 and R2 is a group selected from:

, wherein R3 is H or CH3, and each of Z,

Zi and Z2, independently, is a methylene or an ethylene group.

In some embodiments, each

each of Zi and Z2 is methylene or ethylene.

In some embodiments, each

each of Zi and Z2 is methylene or ethylene.

In some embodiments, the catalyst is of formula (I), having the structure (IV)

, wherein M is a metal as selected herein.

In some embodiments, the catalyst

In some embodiments, in a compound of formula (

In some embodiments, the catalyst is of structure (V) and (VI)

wherein each of Ri, R2, Xi, X2, X3 and X4 are as defined herein.

In some embodiments, each of R1 and R2 is a group selected from:

R3 is H or a -C₁-C₅alkyl; each of Z, Zi and Z2, independently, is a -C₁-C₅alkylene; and wherein

is a point of connectivity to Xi, X2, X3 or X4.

In some embodiments, each of R1 and R2 is a group selected from:

, wherein R3 is H or CH3, and each of Z,

Zi and Z2, independently, is a methylene or an ethylene group.

In some embodiments, each

each of Zi and Z2 is methylene or ethylene. In some embodiments, each

each of Zi and Z2 is methylene or ethylene.

In some embodiments, the catalyst is of formula (VII):

wherein M is any metal atom as disclosed herein.

In some embodiments, the catalyst is:

Furter provided is the catalyst herein designated Sn(MPDA-PA) and a catalyst herein designated Sn(PA-MPIB).

Generally, catalysts of the invention may be formed by reacting a metal source, such as a metal chloride (MClx) or any other metal salt or metal complex, e.g., M(N03)_x with a cyclic anhydride in presence of a diol or a diamine. The reaction may take place under high temperature , e.g., 100-200°C or under 150°C in presence of a solvent (such as xylene, methyl ethyl ketone, dioxane, toluene, or any other organic solvent that does not include hydroxyl or amine groups) and under acidic conditions (pH~5). The catalyst is separated as a solid precipitate and may thereafter be treated to obtain a high yield of the catalyst. In some cases, the ratio between the metal precursor, the anhydride and the hydroxyl or amine -containing component is 1:2:2 (M : Anhydride: diol or diamine).

Each of the embodiments disclosed herein, independently of any other embodiment, constitutes a separate embodiment which may be a limitation of any product or method disclosed herein.

BRIEF DESCRIPTION OF THE DRAWING

In order to better understand the subject matter that is disclosed herein and to exemplify how it may be carried out in practice, embodiments will now be described, by way of non-limiting example only, with reference to the accompanying drawings, in which:

Figs. 1A-F : Schematic presentation of R n(b i py )3 (A), ZnPC (B), SnPC (C), CoPC (D), Sn(PA-MPIB) (E), and Sn(MDPA-PA) (F).

Figs. 2A-B: Absorbance (A) and fluorescence (B) of Sn(PA-MPIB), CoPC, SnPC, ZnPC, Sn(MPDA-PA), and Ru(bipy)3. Tire tests were conducted using ethanol (EtOH) as a solvent with 1.25- 10^-11 [M] and 1.81- 10^-2 [M] for absorbance and fluorescence, respectively.

Figs. 3A-B: A schematic illustration of PEI-CA pre-polymer synthesis from CA and PEI (A) and the different possible curing reactions (B).

Fig- 4 : A schematic illustration of TETA-CA synthesis.

Figs. 5A-B: An example of tire changes of the absorbance spectra of PEI-CA with Sn(PA-MPIB) as a catalyst (A) and the conversion up to two min. after irradiation under 395 nm lamp (27 mW/cm²) using different catalysts (B).

Figs. 6A-B. 'H-NMR (400 MHz, CDCl₃) of a comparison of CA’s dimerization process using Sn(PA-MPIB) (A) and CoPC (B) before irradiation and after 12 min. The dimerization was obtained after irradiation for 12 min, under 395 nm lamp (27 mW/cm²).

Figs. 7A-B: Monomer synthesis, curing and printing processes. A schematic illustration of the printing and reprocessing processes: the monomers arc injected into a printed form using a (direct ink writing) DIW printer. Then, the printed material, now in the form of polymer (due to UV irradiation), is recycled back into a monomeric form using microwave irradiation (A). A schematic illustration of the tri-functional monomer TETA-CA (B).

RECTIFIED SHEET (RULE 91) ISA/EP Figs. 8A-D: (A) a schematic illustration of the printing process. (B) Complex viscosity of TETA-CA with Sn(PA-MPIB) (20 mol% of CA) as a function of temperature. C - recovery test of the formulation at 70 (C, D) different samples printed by DIW from 60 °C to 90 °C.

Figs. 9A-B: Recycling process during microwave irradiation. Formulation’s conversion as a function of microwave irradiation time (A) and picture (B) of postcured samples after different irradiation time intervals. Note that 0 min is correlated to the post-cured sample discussed in Fig. 10, and 10 min is correlated to the 1^st microwave cycle in Error! Reference source not found..

Figs. 10A-E: Changes in the polymeric system during the different processes. A, polymer’s conversion based on 'H-NMR and UV-Vis. B, a comparison of sample before recycling (virgin sample) (I) and after the 10^th recycling cycle (II). C, DMA tests of the TETA-CA’ s virgin and 10^th reprinted cycle specimens following ASTM-D638 type IV standard. The error bars, based on five samples, are represented as shades. Tensile tests of samples with 16 wt% BnOH (D) and after drying (E).

Figs. 11A-C: 'H-NMR in DMSO-6D of SnPC (A), powder XRD of the complex

(B) and an interpretation of the signals (C).

Figs. 12A-C: 'H-NMR in DMSO-6D of CoPC (A), powder XRD of the complex (B) and an interpretation of the signals (C).

Figs. 13A-B: ATR-IR of PA (blue), MPIB (pale orange), and Sn(PA-MPIB) (black) (A) and 'H-NMR of the complex in DMSO-6D (B).

Figs. 14A-C: ATR-IR of PA (blue), MPDA (pale orange), and Sn(MPDA-PA) (black) (A) and 'H-NMR (B) and XRD (C) of the complex.

Figs. 15A-C: A schematic illustration of PEI-CA pre-polymer synthesis from CA and PEI (A) and their structure analysis by ATR-IR (B) and 'H-NMR in CDC13

(C). For both: CA, PEI and the pre-polymer PEI-CA.

Figs. 16A-C: TETA composition analysis. GC-MS (A), 'H-NMR (B), and ATR-IR (C) of TETA used in this study.

Figs. 17A-D: TETA-CA synthesis. A schematic illustration of TETA-CA prepolymers synthesis from CA and TETA (A) and their structure analysis by ATR-IR (B) and 'H-NMR in DMSO-6D (C). As a reference, 'H-NMR in DMSO-6D (under the same conditions as TETA-CA is also included (D).

Figs. 18A-M: PEI-CA absorbance’s and fluorescence (285 nm excitation) changes after irradiation under 395 nm lamp (27 W\cm²): without a catalyst (A, B), and

RECTIFIED SHEET (RULE 91) ISA/EP with Ru(bipy)₃ (C, D), Sn(MPDA-PA) (E,F), ZnPC (G,H), SnPC (I, J), CoPC (K,L), and Sn(PA-MPIB) (M). The curing conversion (%) as a function of irradiation time (min) of the pre-polymer with and without the tested catalysts, where: PEI-CA neat, Ru(ipy)₃, Sn(MPDA-PA), ZnPC, SnPC, CoPC, and Sn(PA-MPIB).

Figs. 19A-C: Absorbance changes in CA before (yellow) and after 12 min irradiation under 395 nm lamp (27 W\cm2) for Sn(PA-MPIB) (A), CoPC (B), and Sn(MPDA-PA) (C). All samples were tested in concentration of 1.91- 10^-6 [M] in ethanol.

Figs. 20A-E: Cyclic voltammetry (CV) of PEI-CA (A), Sn(PA-MPIB) (B), SnPC (C), Sn(MPDA-PA) (D), and Ru(bipy)3 (E). The CV was measured using 0.2M of the material in CHCI3 with 0.2M TBABF4. The used electrodes were gold (working electrode, -5.1 eV), Ag\AgCl (reference electrode, -4.6 eV), and platinum (counter electrode).

Figs. 21A-B: TETA-CA’s thermal analysis. (A) Isothermal (70 °C) of TETA- CA with Sn(PA-MPIB) for four hours. Note that the first “endothermic” peak is a result of the heating profile and not a signature of the material. (B) A heat-cool-heat cycle of a TETA-CA with Sn(PA-MPIB) sample.

Figs. 22A-B: Thermal analysis of TETA-CA during microwave heating. The temperature dissipation of a TETA-CA sample after post-curing (A) and after irradiation for 10 min under microwave (B). Noted that the main cold areas are of the Petri dish.

Figs. 23A-D: Curing and recycling analysis. (A) ¹H -NMR (DMSO-6D, 400 MHz) of TETA-CA before printing and after the 1^st recycling for 10 min and 15 min in a microwave oven (216 W). (B) ¹H -NMR (400 MHz, DMSO-d6) of TETA-CA with Sn(PA-MPIB) before printing, after the first microwave irradiation cycle, and after the 10^th microwave irradiation cycle. (C) UV-Vis (ethanol) absorbance of TETA-CA with Sn(PA-MPIB) before printing, after printing, after post curing, 1^st microwave cycle, and 10^th microwave cycle. (D) Changes in the absorbance of a post-cured sample before (0 min) and after irradiation for different time intervals under 216 W microwave irradiation.

Fig. 24: TGA results for TETA-CA with 16 wt% benzyl alcohol.

Figs. 25A-C: PTTM-FerAc Synthesis. (A) Schematic illustration PTTM-FerAc synthesis. (B) ¹H-NMR of the product. (C) A schematic illustration of the cross-linking of PTTM-FerAc. Figs. 26A-B: CafAc500 Synthesis. (A) CafAc5000 synthesis’ illustration. (B) 6CafAc5000 synthesis’ illustration. (C) A schematic illustration of the cross-linking of CafAc5000 and 6CafAc5000.

DETAILED DESCRIPTION OF EMBODIMENTS

Results and Discussion

Catalysts’ synthesis and structural analysis.

Ruthenium(II)tris(2,2'-bipyridine) [Ru(bipy)s] is the most common catalyst in the field of [2 + 2] and [4 + 4] cycloaddition reactions. However, this bidentate complex still suffers from the same disadvantages discussed above: high light intensity requirements and long reaction rates, unfitting for fast-curing applications such as 3D printing and fast-curing adhesives (e.g., dental adhesives). Herein, five tetradentate catalysts were used and are discussed as alternative catalysts with improved activity (Error! Reference source not found.): zinc(II)phthalocyanine (ZnPC, Fig. IB), tin(II)phthalocyanine (SnPC. Fig. IB), cobalt(II)phthalocyanine (CoPC, Fig. 1C), tin (II) (2,2'-[(4-methylphenyl)imino] bisethylbisphthalate) (Sn(PA-MPIB), Fig. IE), and tin (II) (bis-Ni,Ni'-(l,3-phenylene)diphthalamide) (Sn(MPDA-PA), Fig. IF). ZnPC was commercially available and used without further processing. The other catalysts were synthesized, with the latter two being novel complexes (Error! Reference source not found. E-F). All five catalyst were identify using absorbance and fluorescence spectra, as shown in Fig. 2A and Fig. 2B respectively.

The following first compares three different core metals with the same ligand (phthalocyanine, which is known for its lack of toxicity and its light absorbance in the 360-405 nm range), and second, two additional tin-containing catalysts were compared to better understand the ligand effect on the catalysts’ reactivity. The chosen metals are cobalt, tin, and zinc, all transition metals with different electron densities and coordination abilities.

The first phthalocyanine-containing complex, SnPC, was synthesized from SnCh and 1,2-dicyanobenzene in a microwave oven, resulting in a turquoise product that was ground to a powder and characterized using ATR-IR, ¹ H-NMR, and powder XRD. Both ¹H-NMR spectrum and XRD results (Fig. 11A-C) point to a complete conversion, with XRD showing that some of the catalysts were obtained in the form of SnPcCh (with two additional chlorides from the original SnCh). It should be noted that as ATR-IR showed a major overlap between the different signals, it could not be used to identify the catalyst’s structure. CoPC was synthesized similarly to SnPC, with C0CI2 replacing SnCl₂. All three characterization methods show similar results, with a signal overlap in ATR-IR and a complete conversion based on ¹ H-NMR and XRD results (Fig. 12A-C). As shown in Fig. 12C, 37.4% of the crystals are in a form, 31.5% are in a form, 23.5% are in β form, and 7.6% are nanowires.

Sn(PA-MPIB) is a new complex synthesized in this study. The main difference between this complex and SnPC lies in the ligand: instead of phthalocyanine, a new ester containing tetra dentate ligand is presented. This complex was synthesized in a solution [with ethanol (EtOH) as a solvent], reacting SnCh, phthalic anhydride (PA), and 2,2'-[(4-methylphenyl)imino]bisethanol (MPIB), resulting in a crimson viscous liquid. Based on both ATR-IR and ¹H -NMR results, an 86.23% conversion was calculated. As will be discussed later, this catalyst showed the highest efficiency (Fig. 13): higher conversion at the short irradiation time: 76.0±0.3% after 2 min.

Sn(MPDA-PA) is also novel, with an amide-containing tetra-dentate ligand with less flexibility and higher electron density. Sn(MPDA-PA) was synthesized in a solution (toluene as a solvent) from SnCh, PA, and m- Phenylenediamine (MPDA). The resulting product was dried overnight under vacuum at 70 °C and then ground into a brown powder, after which its structure was analyzed using ATR-IR, ¹ H-NMR, and powder XRD. All three characterization methods showed a 99.7% conversion.

The absorbance of all six catalysts was measured to understand their reactivity under irradiation. As shown in Fig. 14A -C, up to -390 nm, the highest absorbance in the range of 360-405 nm was obtained by Ru(bipy)s, despite not being the most efficient catalyst.

Pre-polymer synthesis and structural analysis.

As a model for RCBPs, a pre-polymer with the potential to undergo [2 + 2] or [4 + 4] cycloaddition was synthesized from polyethyleneimine (PEI) and cinnamaldehyde (CA) (Fig. 3A-B). In the synthesis, it was found that adding all the tetradentate catalysts caused an acceleration in the imine formation, shortening the reaction from half an hour to two minutes only. It should be noted that the light intensity used to cure this polymer is extremely low compared to the literature, 27 mW/cm² and -100 W/cm², respectively. PEI-CA’ s chemical composition was identified using ATR- IR, ¹H -NMR, UV-Vis, and fluorescence. IR spectra (Fig. 15B) of the polymer showed the formation of imine groups (-1630 cm ¹) and the disappearance of aldehyde groups (-1700 cm ¹). ¹H-NMR (Fig. 15C) also demonstrated this phenomenon (following the changes in the signals at 3.34-3.5 ppm, -8 ppm, and -7 ppm). Based on these results, an 80.3% conversion was achieved.

To prove the printability of [2 + 2] and [4 + 4] cycloaddition reactions-based polymers using the above catalysts, a second type of per-polymer was synthesized. A tri-functional monomer (TETA-CA) was synthesized from triethylenetetramine (TETA) (Fig. 16) and cinnamaldehyde (CA) to obtain a cycloaddition-based printable material. TETA-CA was synthesized (Figs. 4, 17) using a 1:2.37 molar ratio of CA:TETA in the presence of 20 mol% (of CA) Sn(PA-MPIB), and BnOH (16 wt%) as a solvent. The CA:TETA ratio assumes that secondary amines are as half reactive as primary ones; thus, TETA functionality is 3. However, because the TETA that was used is a mixture, the average mixture functionality is 2.33. The amount of catalyst and solvent was chosen to modify the viscosity along with maintaining a high reaction rate; since each layer was irradiated for only a few seconds, to achieve a high reaction rate, a more significant amount of catalyst compared to previous studies by our group was used. The monomer’s structure was analyzed using ¹H-NMR and ATR-IR (Fig. 17B- D). Following CA-aldehyde integration signal in ¹ H-NMR (9.6 ppm, integration value: 1) normalized to benzene hydrogens integration signals (7.37 ppm, integration values: 3.23 for CA and 71.68 for TETA-CA), 95.5% conversion was calculated. Further details are given in the experimental section.

Catalysis’ analysis

The main problem of the latter pre-polymer is its potential to undergo several different reactions: [2 + 2] cycloaddition, [4 + 4] cycloaddition, [4 + 2] cycloaddition, and even amines oxidation into imines after long irradiation time (see Figs. 2B). As these products have similarities with the original polymer, it was difficult to determine the conversion based on “classical” IR and/or NMR methods. Thus, the conversion was calculated based on the changes in the absorbance signal at 280-290 nm (see an example in Figs. 4A) and the correlative fluorescence signal at 320-350 nm. Since the formation of the cycloadducts occurred only after irradiation, and [4 + 2] is a forbidden transformation under these conditions (it occurs while heating the polymer), it can be concluded that most of the products obtained were due to [2 + 2] or [4 + 4] cycloaddition reactions. The evaluation of PEI-CA’s conversion was based on the minimal concentration of catalyst that results in formation of insoluble polymer after -10 min irradiation. 1 mol% of the catalysts was found to be the minimal concentration for all but Sn(PA-MPIB), for which it was 0.75 mol%. The conversion over time following irradiation under 395 nm was evaluated and compared with the conversion of PEI-CA without a catalyst. As shown in Figs. 4B and Fig. 18, Sn(PA-MPIB) was the most efficient, even at reduced concentration (0.75 mol% compared to 1 mol% of the others). Moreover, all the tetra-dentate catalysts were found to be more efficient than the common Ru(bipy)s used as the reference in this study, despite the use of two orders of magnitude lower intensity than the ruthenium literature-reported systems.

In some photolithography-based processes, the polymer is only required in the first stage to reach such a conversion that the structure can be preserved. Then, only in the second stage, known as the post-curing, the irradiation continues, and the material reaches its full conversion. For most polymers, -60% conversion can be considered high enough for the first stage; thus, when using Sn(PA-MPIB) in 3D printing, 40 seconds may be enough.

The tetra-dentate catalysts were also studied in the field of CA dimerization/ using the same concentrations. As shown in Fig. 6, Fig. 19, after 12 min, only Sn(PA- MPIB) caused a dimerization, with [4 + 4] being the dominant product.

The major differences between CA-dimerization and PEI-CA’ s curing are the differences between imine and aldehydes and the geometrical proximity of the functional groups. These differences probably result in CoPC being more efficient for polymer curing than CA’s dimerization.

Printing.

A tri-functional monomer (TETA-CA, Fig. 4) was synthesized from triethylenetetramine (TETA) and cinnamaldehyde (CA) to obtain a cycloaddition-based printable material. The reaction conditions were chosen based on PEI-CA, including the involvement of a tetra-dentate tin-based catalyst named Sn(PA-MPIB) that catalyzed the [4 + 4] and [2 + 2] cycloadditions. Further information on the catalyst’s synthesis is given in the experimental section. The only change was the using toluene instead of ethanol, leading to an increase in the conversion from 86.2% to 90.4%.

TETA-CA (Figs. 4, 17A) was synthesized using a 1:2.37 molar ratio of CA:TETA in the presence of 20 mol% (of CA) Sn(PA-MPIB), and 16 wt% benzyl alcohol (BnOH) as a solvent. The CA:TETA ratio assumes that secondary amines are half as reactive as primary ones; thus, TETA functionality is 3. However, because the TETA that was used is a mixture of isomers, the average mixture functionality is 2.33 instead of 3. The amount of catalyst and solvent was chosen to modify the viscosity while maintaining a high reaction rate; since each layer was irradiated for only a few seconds, to achieve a high reaction rate, a more significant amount of catalyst compared to previous studies by our group was used. The monomer’ s structure was analyzed using 1H-NMR (Fig. 17C) and ATR-IR (Fig. 17B). Following CA-aldehyde integration signal in ¹H-NMR (9.6 ppm, integration value: 1) normalized to benzene hydrogens integration signals (7.37 ppm, integration values: 3.23 for CA and 71.68 for TETA- CA), 95.5% conversion was calculated. Further details are given in the experimental section.

TETA-CA monomers were printed using a DIW printer with a 365 nm (3.4 mW/cm²) UV array to facilitate the [4 + 4] cycloaddition reaction. The UV array was used continuously during printing to achieve sufficient conversion for the printed object to retain its shape. To compensate for the short irradiation time of each layer during printing, 20 mol% of CA moieties were used, resulting in a significant increase in the zero-shear viscosity, turning the liquid monomer into a soft, solid-like material, as the catalyst served as a physical cross-linker through coordination with the monomer's functional groups. In a DIW process, the printed ink should have low enough viscosity for extrusion during printing and sufficient shape-retaining properties after printing. As shown in Fig. 8A, below 60 °C, the viscosity was too high for extrusion in the DIW printer, leading to delamination due to poor adhesion after only three layers were printed. Above 70 °C, the viscosity became too low and the temperature too high, resulting in shape deformation due to the polymer continuing to flow until it reached room temperature.

In DIW, to retain the printed shape before the cross-linking is finished, an almost complete recovery needs to be achieved, i.e., the material needs to regain its initial viscosity value quickly after the extrusion. Thus, a recovery test was conducted to understand whether TETA-CA fits this requirement. The sample was first tested under a low frequency of 0.1 Hz, simulating the material’s state before the extrusion. Then, the frequency increased to 100 Hz, simulating the stresses developed during the printing’s extrusion. Finally, the frequency decreased again to 0.1 Hz, simulating the material’s conditions after leaving the printer’s die. As demonstrated in Fig. 8C, the material reached its original viscosity after a few seconds, resulting in shape retaining after the printing. This fast recovery may result from the reformation of both coordination bonds between the catalyst and the monomers and the TT-TC stacking between the monomers to themselves. Printing at 70 °C was tested using an STL of a hollow object containing a bridge. Due to the high recovery and curing that occurs while irradiating TETA-CA, this object was printed successfully (Fig. 8D). This is 50 °C lower than the temperature needed for printing DA-based RCBPs in DIW.

Whereas the printed objects retained their shape due to irradiation and inherited high viscosity, post-curing was still required to achieve high conversion. Therefore, an analysis of the curing conversion was conducted following the changes in the double bonds’ signals’ absorbance at 258 nm. As shown in Fig. 10A, after printing, 53.5+0.4% conversion was obtained, whereas, after the post-curing conducted under 365 nm for 20 min, 81.4+0.5% was obtained.

Reprocessability .

[4 + 4] and [2 + 2] adducts' cycloreversion require the use of UVC wavelengths (>260 nm in the case of cinnamaldehyde-based moieties). This spectrum is considered dangerous and not commonly used in additive manufacturing, so an alternative is required. Four and eight-members rings are only partially stable due to intrinsic stresses in the ring resulting from the bonds' angles. UVC causes the excitation of electrons, which destabilizes the bonds and thus causes the ring to open and reverse into a more stable form: the original components. Therefore, an alternative to UVC irradiation should cause changes in the ring structure, destabilizing it. Microwave irradiation (25- 38 mm) is known to cause quick heating by inducing rotations of polar molecules. It is assumed that these rotations should be enough to destabilize the cycloadducts so that cycloreversion will occur. To test this theory, post-cured samples were placed in a microwave oven using an intensity of 216 W and heated for 15 minutes. Measuring took place every 40 seconds for two minutes, then every two minutes up to 10 minutes and finally in 13 and 15 minutes. Each measurement consisted of both calculating the conversion based on UV-Vis of a small fracture from the sample and by taking a photo of the heated sample (Fig. 9B). As shown in Fig. 9A, the minimum conversion was obtained after 10 min heating, indicating a high reduction in the presence of the cycloadducts and the adducts’ transformation into the original monomers' unsaturated imine. As the minimum conversion was achieved after 10 min, this time was chosen for further study. Moreover, it seems the conversion increased again after ~10 min (15.1+0.4% after 15 min compared to 2.4+0.4% after 10 min), probably due to either cycloaddition occurring under the long-time microwave irradiation or some irreversible reaction of the double bonds that are demonstrated in the ' H-N R's HC=CH signals’ changes. This may also explain the need for double the amount of solvent, compared to the 10 min microwave irradiation’s result, to dissolve the resulted material.

To estimate whether the samples’ high temperature generated from the microwave irradiation is the cause of the rings’ destabilizing or the combination of heating and the irradiation itself, post-cured TETA-CA samples were placed in a microwave oven (above a glass Petri dish) and heated for 10 min. Using a thermal camera to understand the temperature dissipation after irradiation, it was found that after the microwave irradiation, the samples reached -180 °C. Thus, other samples were placed in an oven at 180 °C. Even after 20 min, the samples remained without any visible change, thus pointing to the irradiation being a key feature in the reversibility process. When the cycloadduct irradiates, rotations and vibrations of the relatively polar bonds (both the C-N and the double bonds) occur. Since an eight-members ring is only partially stable, these rotations cause an increase in the ring tension, which leads to breaking the bonds and undergoing a reverse reaction into the more stable original component. These results show that heating alone does not cause enough rotations in the ring; thus, the ring remains stable. While increasing the temperature above 180 °C may induce a cycloreversion, it would also be followed by undesirable reactions and processed: from the evaporation of the solvents (Fig. 24) to irreversible reactions between double bonds.

Microwave irradiation, though found useful, might also cause some irreversible reactions due to high temperatures. Thus, to understand the numbers of microwave cycles that can be conducted without any significant decrease in the monomers’ and polymer’s reprocessability, ten printing-recycling cycles were done. The first and tenth cycles’ conversions were calculated by both the changes in the absorbance and following the H-N R’s C-H double bonds (~6.6 ppm) normalized to the aromatic C- H bonds (~7.3 ppm), in comparison to precured monomers. It was found (Fig. 10A) that 2.4±0.4% conversion remained after the first cycle of 10 min microwave irradiation, whereas after the tenth 2.8+0.1% were found. This neglectable difference shows the system’s stability under these reversible conditions.

To further investigate the differences between virgin and 10^th recycled samples, dynamic mechanical analysis (DMA) tests were conducted using a dual cantilever bending mode. As shown in Fig. 10C, both samples display nearly identical glass transition temperatures (Tg) around 34 °C. Additionally, both samples exhibit almost identical storage and loss moduli, with the sample after ten reprinting cycles displaying slightly higher values at 25°C (E' = 637 MPa and E" = 123 MPa for the virgin sample compared to E' = 645 MPa and E" = 124 MPa for the 10th recycled sample). These negligible differences support the previous findings indicating the formulation’s stability during the recycling process.

The tensile test results of the virgin post-cured sample and the 10^th recycled sample also show a high level of similarity (Fig. 10D-E). During the 3D printing process, no BnOH evaporates, meaning that the samples still contain 16 wt% BnOH, resulting in a very ductile behavior (1425 ± 302% elongation and Young's modulus of 35.5 ± 4.6 MPa for the virgin samples, compared to 1372 ± 362% elongation and Young's modulus of 34.0 ± 3.4 MPa for the recycled ones). These results are similar to those of commercial elastomers used in 3D printing, such as NinjaFlex® polyurethane (600% elongation and Young's modulus of 12 MPa) and Stratasys' FDM TPU 92A (552% elongation and Young's modulus of 15.3 MPa). However, "wet" TETA-CA exhibited lower stress at yield. The samples were dried under vacuum due to the solvent's effect, which made the material less ductile but with higher Young's modulus and stress at yield (928 ± 49% elongation and Young's modulus of 1.06 ± 0.15 GPa for the virgin samples, compared to 920 ± 59% elongation and Young's modulus of 1.12 ± 0.20 GPa for the recycled ones).

Conclusions

RCBPs are novel types of polymers with thermosets-like structures and thermoplastics -like processibility. Among the different reversible bonds, [2 + 2] and\or [4 + 4] cycloaddition are unique reactions as both cycloadducts’ association and dissociation are radiation assisted. These cycloaddition-based-RCBPs have only limited application due to short wavelength requirements and relatively low reaction rate. Therefore, there is a profound need to present an efficient catalyst for cycloaddition under longer than 360 nm wavelengths with accelerated kinetics. Herein, five tetra- dentate based transition metals complexes were studied as potential catalysts for [2 + 2] and [4 + 4] cycloaddition. All five catalysts were more efficient than the commonly used bidentate ruthenium-based catalyst. Newly synthesized Sn(PA-MPIB) complex, was shown to be the most efficient one, displaying more than double efficiency compared to the ruthenium catalyst. The new tetra-dentate catalysts can be used for different applications. We showed that due to the red-shift effect and acceleration, it can be used for stereolithography-based 3D printing. Moreover, we showed that by using microwave oven, the reaction can be reversed without the need of UVC irradiation.

Methods

Materials

Polyethyleneimine (PEI, branched, Mw 800), 1,2-Dicyanobenzene, tin (II) chloride (SnCh), cobalt (II) chloride (C0CI2), zinc phthalocyanine (ZnPC, 96%), phthalic anhydride, 2,2'-[(4-methylphenyl)imino]bisethanol, and m-phenylenediamine were all supplied by Sigma-Aldrich Israel. Trans-Cinnamaldehyde (CA, 99%) was supplied by Rhenium, Israel. Ethanol (EtOH) 96% was supplied by Biolab-Chemicals Inc., Israel. All materials were used without further treatment. Triethanolamine (TETA) was supplied by Elgad, Inc. Israel. Further details about the TETA can be found in Fig. 16.

Catalysts Synthesis.

Tin (II) phthalocyanine (SnPC) and cobalt (II) phthalocyanine (CoPC) were synthesized using a microwave oven (MW2031W, Sauter, Groupe Brandt, France). Tin (II) chloride (SnCh) or cobalt (II) chloride (C0CI2) and 1,2-dicyanobenzene were reacted in a 1:4 molar ratio (metal salt: 1,2-dicyanobenzene). At first, the components were dry -blended until a homogenous powder mixture was obtained. Then, the mixture was heated under 500 W microwave irradiation for 2 min. To overcome the high exotherm, the microwave was stopped for 15 sec every 30 sec. SnPC analysis: ^!H NMR (500 MHz, DMSO-6D): 8 8.15 (m, J=5.7 Hz, 8H), 7.94 (m, J=5.7 Hz, 8H). UV-Vis (absorbance): X_max =665, 275, 237, 205 nm. CoPC analysis: ¹H NMR (400 MHz, DMSO-6D): 8 8.13 (s, 8H), 7.92 (s, 8H). UV-Vis (absorbance): X_max=665, 274, 238, 205 nm.

Tin (II) (2,2'-[(4-methylphenyl)imino]bisethylbisphthalate) (Sn(PA-MPIB)) was synthesized in a solution as followed: 2 g SnCh, 3.12 g PA, and 3.53 g MPIB (1 : 2 : 2 molar ratio) were dissolved in 50 ml of acetone using ultrasonication bath at room temperature. Then, the solution was heated using reflux to 70 °C and stirred with a magnetic stirrer. A few droplets of 37% HC1 solution were added to obtain a pH of 6. Immediately after the HC1 addition, the solution’s color has changed from pale white to red and then deep red. After an additional hour, the temperature was risen to 120 °C, followed by the addition of 50 ml ethanol (EtOH, 100 ml solvents overall). After an hour, the reflux was stopped, and additional heating was then used (same temperature) until all EtOH evaporated. After this step, a crimson high viscous liquid was obtained. The liquid was heated to 140 °C for one more hour and then was cooled to room temperature, obtaining a dark crimson highly viscous liquid. Analysis of the complex was as follows: IR (ATR-IR): 1698 cm ^-1. ¹H NMR (400 MHz, DMSO-6D): 8 7.74 (d, J=8.4 Hz, 4H), 7.59 (d, J=6.96 Hz, 4H), 7.24 (q, J=8.5, 4H), 7.00 (d, J=8.4 Hz, 4H), 4.21 (q, J=5.8 Hz, 8H), 3.49 (m, J=10.8 Hz, 8H), 2.66 (t, J=7.0, 6H). As shown in Fig. 13, the residential reagents (mostly MPIB) can be seen at: 1.5 ppm, 3.18 ppm, 3.74 ppm, and 4.37 ppm. UV-Vis (absorbance): k_max=263, 227, 200 nm.

Tin (II) (bis-Ni,Ni'-(l,3-phenylene)diphthalamide) (Sn(MPDA-PA)) was also synthesized in a solution. 2 g SnCl₂, 3.12 g PA, and 2.3 g MPDA (1 : 2 : 2 molar ratio) were dissolved in 200 ml toluene using am ultrasonication bath. The solution was stirred with a magnetic stirrer and heated to 140 °C for three hours in reflux. A few 37% HC1 solution droplets were added to obtain a pH 6, followed by an immediate color change to dark crimson. A brown solid was obtained after completely drying under a vacuum oven at 70 °C overnight and grinding into powder. The complex structure was analyzed as follows (Fig. 14): IR (ATR-IR): 1770 cm ¹, 1712 cm ¹. ¹H NMR (400 MHz, DMSO- 6D): 87.99 (m, J=5.6 Hz, 8H), 7.92 (m, J=1.0 Hz, 2H), 7.71 (t, J=2.4 Hz, 2H), 7.58 (m, J=11.9 Hz, 4H). UV-Vis (absorbance): λ_max=480, 335, 267, 228 nm.

Pre-Polymers Synthesis.

The first pre-polymer (PAI-CA) was synthesized following known aldehyde and polyethyleneimine (PEI) reaction. Following their equivalent weight calculation and the fact that PEI is consist of 25% primary amines, 50% secondary amines and 25% tertiary amines, PEI was reacted with cinnamaldehyde (CA) in a molar ratio of 1:15.71 (PEECA). Different catalysts’ ratios were tested: from 0.5 mol% (of the total reagents) to 1 mol%. At first, the chosen catalyst was dissolved and mixed in CA using 15 min ultrasonication bath (15 Hz, Elmasonic P, Elma Schmidbauer GmbH, Germany) at 60 °C and vortex mixing until reaching a homogenous mixture. The mixture was then added to a pre-heated 60 °C PEI during mixing. After 5 min at 60 °C, the mixture was put under 15 min ultrasonication at 60 °C. Structural analysis was as follows (Fig. 15): IR (ATR-IR): 3100 cm ¹, 1707 cm ¹, 1674 cm ¹, 1630 cm ¹. ¹H NMR (500 MHz, CDC1₃): 6 7.98, 7.55, 7.43, 6.87, 6.52, 3.59, 3.00, 2.56, 1.27. All broad peaks are due to polymeric molecular weight distribution. UV-VIS: _λmax = 280 nm. Fluorescence (excitation: 280 nm): _λmax = 320 nm.

The second pre-polymer (TETA-CA) was synthesized following the same process, using 1:2.37 molar ratio of CA:TETA and the same amount of catalyst. Structural analysis was as follows (Fig. 17): IR (Fig. 17B (ATR-IR): 3022 cm^-1, 2922 cm^-1, 2822 cm^-1, 2343 cm^-1, 1670 cm^-1, 1632 cm^-1, 1490 cm^-1, 1448 cm^-1, 1370 cm^-1, 1337 cm^-1, 1292 cm^-1, 1248 cm^-1, 1151 cm^-1, 1124 cm^-1, 1070 cm^-1, 973 cm^-1, 747 cm' ^l, 689 cm ¹. ¹ H NMR (400 MHz, DMSO-6D, Fig. 17C.): 8 8.04 (d, J=8.8 Hz, 7H), 7.73 (d, J=2.2 Hz, 4H), 7.57 (t, J=3.1 Hz, 12H), 7.45 (t, J-3.3 Hz, 6H), 7.34 (m, J=2.6 Hz, 24H), 7.04 (t, J=9.8 Hz, 8H), 6.90 (m, J=4.1 Hz, 8H), 6.59 (t, J=15.5 Hz, 2H), 6.02 (d, J=8.0 Hz, 3H), 3.87 (m, J=10.8 Hz, 8H), 3.62 (d, J=18.0 Hz, 4H), 3.53 (m, J=6.4 Hz, 2H), 3.32 (d, J=7.9 Hz, 2H), 3.20 (d, J=2.9 Hz, 2H), 2.90 (m, J=6.2 Hz, 2H), 2.75 (t, J=6.2 Hz, 2H), 2.52 (m, J=2.6 Hz, 7H), 2.45 (q, J=9.1 Hz, 7H+DMSO), 1.80 (s, 1H). UV-VIS: _λmax= 255 nm, Fluorescence (excitation: 255 nm):_λmax = 286 nm. TETA-CA’s conversion was calculated using CA-aldehyde integration signal in ¹H-NMR (9.6 ppm) normalized to benzene hydrogens integration signals (7.37 ppm), as shown in equation 1. Based on this calculation, 95.5% conversion was estimated.

PTTM-FerAc (Fig. 25A) was synthesized according to the Steglichtype esterification process. 0.03 mol of Pentaerythritol Tetrakis(3 -Mercaptopropionate) (PTTM), 0.12 mol of Ferulic acid (FerAc), 0.03 mol 1,3-Dicyclohexylcarbdiimide, 0.14 mol Triethylamine, and 1 mol 1 -methylimidazole were mixed in dioxane as a solvent (75 ml). The mixture was heated to 60 °C for 24 hours under reflux. After 24 hours, the reflux was opened, and the mixture was heated to 140 °C for 1 hour. Then, the mixture was filtered using a gravimetric funnel and washed three times with acetone. The liquid phase was concentrated at 140 °C for 1 hour, and then dried at 70 °C under vacuum overnight. The product was characterized with ¹H-NMR as follows (Fig. 25B): ¹H NMR (400 MHz, DMSO-6D): 8 9.04 (s, 4H), 7.51 (m, 4H), 6.90 (m, 12H), (m, 4H), 5.64 (d, J=6.23 Hz, 1H), 5.07 (d, J=3.98 Hz, 1H), 4.10, 3.81 (m, 12H), 2.70 (q, J=2.32 Hz, 8H), 1.44 (d, J=2.29 Hz, 12H), 1.04 (t, J=7.21 Hz, 2H). Several isomers of the Ferulic acid that has been used caused multiplets in some of the recorded signals. As demonstrated, there are still remnants of the DCU by-product, with ratio of 2:1 with relation to the PTTM-FerAc product. The schematic illustration of the pre-polymer cross-linking is demonstrated in Fig. 25C.

CafAc500 was synthesized from Caffeic acid and Jeffamine T5000. Two sets of pre-polymers were prepared: with molar ratio of 1:3 Jeffamine to caffeic acid (Fig. 26A) or 1:6 (Fig. 26B), labelled CafAc5000 and 6CafAc5000, respectively. The synthesis was carried out in methyl ethyl ketone (MEK) as a solvent at 140 °C with the addition of HC1 as a catalyst (pH ~ 6). All the ingredients were mixed and heated for 4 hours. After this time, no solvent was remained. The product, in the form of brown viscous liquid, was then dried under vacuum at 60 °C for three hours. The schematic illustration of the pre-polymer cross-linking is demonstrated in Fig. 26C.

Curing

The pre-polymer (PEI-CA) was cured under a 395 nm lamp (27 mW/cm², Integration Technology Ltd., UK) for different time periods: 0.5, 1, 2, 4, 6, 8, 10, 12, and 14 min. TETA-CA was cured during printing, followed by a post-curing process, mentioned below.

Structure, Conversion and Curing Analysis.

To analyze pre-polymer’s, cured polymer’s, and catalysts’ compositions IR spectroscopy, NMR, and X-ray diffraction (XRD) were used. IR was recorded using the ATR-IR method, on a Bruker Alpha-P machine (Brucker, USA), in the range of 400-4000 cm ¹. ¹H-NMR was tested using CDCl3 or DMSO-6D as solvents and was performed in a 500 and 400 MHz spectrometer (Ascend™ 500 Neo and Ascend™ 400 Neo by Brucker, USA) with tetramethylsilane (TMS) as an internal reference. XRD was recorded using a thin-film powder diffraction (k_Cu K_α = 1.5406 A, Shimadzu XRD- 6000, Shimadzu, Japan). Catalysts’ absorbance spectra were recorded using UV-Vis- NIR spectrophotometer (UV-1800, Shimadzu, Japan) between 800 nm to 200 nm with 1.2540 ¹¹ [M] concentration in ethanol (EtOH) in in 1 cm path length’s quartz cuvettes. Their fluorescence was recorded (Cary Eclipse Fluorescence Spectrometer, Agilent, US) in 1 cm path length’s quartz cuvettes between 300-600 nm, using EtOH as a solvent, following an excitation of 285 nm, using EtOH solution (6.9 mg/ml [gr/ml]).

Curing’s conversion was analyzed using ultraviolet-visible (UV-Vis) absorbance and fluorescence tests of a dissolved 1 cm diameter and 1 mm thickness cured discs. The samples were tested in EtOH solution (2.81 • 10’² [mg/ml]) with 1 cm path length’s quartz cuvettes. The same method was also used to measure cinnamaldehyde’s dimerization under the same irradiation conditions, though the concentration of cinnamaldehyde in EtOH for UV-Vis’ spectra was 1.91-10’⁶ [M].

The Polymers’ conversion were measured following the changes of absorbance in 280-288 nm - a known signal of unsaturated aldehyde and/or imine, which may be found in cinnamaldehyde or the pre-polymer respectively. Polymers’ conversions were also measured following the changes of these signals’ fluorescence. As was discussed, only neglectable changes was found between the two methods. The conversion was calculated as followed (equation (1)) assuming the width at half-height is equivalent, where Int₀ refers to intensity before irradiation and Int_t refers to intensity at a specific irradiation time:

Energy levels Calculations.

Molecular orbitals energies (H0M0\LUM0) of PEI-CA, Sn(PA-MPIB), SnPC, Sn(MPDA-PA), and Ru(bipy)s were measured by cyclic voltammetry (CV. VSP™, BioLogic Science Instruments, France) (Fig. 20). The test was conducted in 0.2 M CHCI3 solution with an addition of 0.2M Tetrabutylammonium tetrafluoroborate (TBABF4). The used electrodes were gold (working electrode, -5.1 eV), Ag\AgCl (reference electrode, -4.6 eV), and platinum (counter electrode) with The

estimation of HOMO and LUMO levels versus vacuum were according to oxidation and reduction offsets, following equations 2 and 3.

Thermal Analysis

Dynamic Mechanical Analysis (DMA) of printed samples was conducted by TA Instruments’ DMA Q800 V21.3 Build 96, TA Instruments, USA, from -12 °C to 80° C at a rate of 3 °C\min in dual cantilever bending mode with an amplitude displacement of 8 pm and a frequency of 1 Hz. The DMA’s samples were printed following the standard ISO 6721 and then dried for 24 hours (until the samples’ weight remained constant) under vacuum at 25 °C. To understand the pot-life of TETA-CA with the catalyst at 70 °C an isothermal differential scanning calorimetry (DSC) was also conducted for 240 minutes using TA Instruments’ DSC Q200 V24. l l Build 124, TA Instruments, USA (Fig. 21). A heat-cool-heat cycle of TETA-CA with Sn(PA- MPIB) was also conducted from -25 to 200 °C at 10 °C\min heating rate. Samples’ temperature dissipation was measured using a thermal camera (FLIR-E63900, FLIR, Sweden) (Fig. 22).

Mechanical Analysis

The samples’ tensile tests were conducted following ASTM D638-Type IV standard using a mechanical tester (INSTRON 4481, Instron, USA). All samples were dried before testing under vacuum for 24 hours at 25 °C until the samples’ weight remained constant.

Printing.

Printing of TETA-CA was conducted using Hyrel System 30M™ with a 365 nm UV array (3.2 mW\cm²). All models STL files’ G-codes were generated by Slic3r. The material was printed at 60, 70, 80, and 90 °C using the built-in heater of the KR2 Extruder™ head with 0.5 mm gauge nozzle. The stage itself was kept under room temperature. The printing parameters were as follows: Layer thickness (mm): 0.4, Infill density: 100%, Perimeter speed (mm\sec): 3, Infill speed (mm\sec): 3, Travel speed (mm\sec): 60. During the printing process, the sample was continuously irradiated at 100% intensity. Due to the low-intensity lamp and the fast printing (15 mm/sec), each layer was irradiated only for a few seconds. Thus, a post-curing was required, which was conducted using Asiga Flash Post Curing Unit (365-405 nm, 4.7 mW\cm²) for one hour. The samples’ conversion after printing and after post-curing was conducted following the changes in the UV absorbance around 258 nm. Recycling.

To achieve the recycling of the cured polymer, TETA-CA’s samples were heated in a microwave oven for different periods using 216 W intensity. To overcome overheating and degradation, every 30 sec, the oven stopped for 15 sec. The heating was carried out using Sauter’s MW2031W microwave (Sauter, China).

Conversion of the recycled materials was calculated in comparison to pre-print samples, following both the changes in the absorbance as discussed above, or by following the changes in the C-H double bonds (~6.6 ppm) normalized to the aromatic C-H bonds (~7.3 ppm) (equation 2). It was found that 2.4±0.4% conversion remained after the first microwave recycling and 2.8 ±0.1% after the 10^th. Compared to the 10- min cycle, 15-min irradiation under the same conditions caused a much higher double bonds’ conversion, as around 15% conversion was calculated.

Extended Data

Catalysts Synthesis

Pre-Polymer and Monomer Synthesis

Since the triethylenetetramine (TETA) that was used (Elgand Inc.) in this study is in fact a mixture, it was analyzed using GC-MS, ¹H -NMR, and ATR-IR and compared to a cleaner (97%) reference sample (supplied by Sigma- Aldrich). GC-MS results (Fig. 16A) showed that the mixture is consisting of three components: TETA (51 wt%), Nl-(2-(piperazin-l-yl)ethyl)ethane-l,2-diamine (27.4 wt%), and 1,4- Piperazinediethanamine (21.6 wt%). The latter also exist in small amounts in the reference sample. All three components are known TETA’s impurities, resulted from its synthesis. Another proof for this component’s ratio may be found in the sample’s

¹ H-NMR (Fig. 17C), following signals integrations. ¹ H-NMR: (400 MHz, DMSO-6D): 8 2.41 (t, J=4.8 Hz, 4H, Int: 1.1), 2.32 (m, J=4.0 Hz, 12H. Int: 5.6), 2.22 (m, J=3.1 Hz, 10H, Int: 4.15), 2.09 (m, J=5.9 Hz, 12H, Int: 2.2), 2.00 (t, J=6.47, 6H, Int: 1.2), 1.28 (s, 14H, Int: 7). IR(ATR-IR): 3255 cm’¹, 2926 cm’¹, 2810 cm’¹, 2061 cm’¹, 1588 cm’¹, 1452 cm ¹, 1310 cm ¹, 1133 cm ¹, 1096 cm ¹, 764 cm ¹. GC-MS retention time: TETA 4.9 min, Nl-(2-(piperazin-l-yl)ethyl)ethane-l,2-diamine 5.6 min, 1,4- Piperazinediethanamine 5.9 min. All other signals are colonna-related.

CafAc500 was synthesized from Caffeic acid and Jeffamine T5000. Two sets of pre-polymers were prepared: with molar ratio of 1 :3 Jeffamine to caffeic acid (Fig. 26A) or 1:6 (Fig. 26B), labelled CafAc5000 and 6CafAc5000, respectively. The synthesis was carried out in methyl ethyl ketone (MEK) as a solvent at 140 °C with the addition of HC1 as a catalyst (pH ~ 6). All the ingredients were mixed and heated for 4 hours. After this time, no solvent was remained. The product, in the form of brown viscous liquid, was then dried under vacuum at 60 °C for three hours. The schematic illustration of the pre-polymer cross-linking is demonstrated in Fig. 26C.

Catalysts’ Analysis

Printing and Recycling Analysis

To achieve the recycling of the cured polymer, TETA-CA’s samples were heated in a microwave oven for different periods using 216 W intensity. To overcome overheating and degradation, every 30 sec, the oven stopped for 15 sec. The heating was carried out using Sauter’s MW2031W microwave (Sauter, China). Conversion of the recycled materials was calculated in comparison to pre-print samples, following both the changes in the absorbance as discussed above, or by following the changes in the C-H double bonds (~6.6 ppm) normalized to the aromatic C-H bonds (~7.3 ppm) (equation 3) (Fig. 23). It was found that 2.4±0.4 % conversion remained after the first microwave recycling and 2.8±0.1 % after the 10^th. Compared to the 10-min cycle, 15-min irradiation under the same conditions caused a much higher double bonds’ conversion, as around 15 % conversion was calculated.

Claims

CLAIMS:

1. A 3D printing method for manufacturing a thermoset polymer, the method comprising radiation curing unsaturated monomers capable of photochemical [2+2] and/or [4+4] cycloaddition to form the thermoset polymer capable of cycloreversion under thermal conditions.

2. The method according to claim 1, wherein the unsaturated monomers being monomers or prepolymers having one or more double bonds, wherein at least one double bond is of the form X=C, wherein X is S, N or O.

3. The method according to claim 1 or 2, wherein the 3D printing method is a 3D deposition method or stereolithography.

4. The method according to claim 3, wherein the 3D printing method is Digital Light Processing (DLP), stereolithography (SLA), Direct Ink Write (DIW) combined with light irradiation, Polyjet printing, volumetric printing, two-photon polymerization printing, or extrusion deposition combined with light irradiation.

5. The method according to claim 3, wherein the 3D printing method is DIW, combined with light irradiation.

6. The method according to any one of claims 1 to 5, wherein the radiation curing is light curing using a light irradiation in the visible or UV regime.

7. The method according to any one of claims 1 to 6, for manufacturing a thermoset polymer, the method comprising radiation curing unsaturated monomers capable of photochemical [2+2] and/or [4+4] cycloaddition under light having a wavelength between 360 and 460 nm to form the thermoset polymer capable of cycloreversion under thermal conditions.

8. The method according to any one of claims 1 to 7, wherein the radiation curing comprises irradiation with a radiation dose between 2 mW/cm² and -100 W/cm².

9. The method according to any one of claims 1 to 8, wherein the radiation curing comprises light irradiation at a wavelength between 360 and 405 nm to provide a cycloaddition adduct within seconds to few minutes.

10. The method according to any one of claims 1 to 9, wherein the radiation curing comprises:

(a) light irradiation of the monomers with a light of a wavelength between 360 and 400 nm, or a wavelength between 360 and 370 nm, or a wavelength of 365 nm; and (b) light intensity between 3.2 mW/cm² and ~10 W/cm², or under a light intensity between 3.2 mW/cm² and ~5 W/cm², or under a light intensity of 3.2 mW/cm²; and

(c) irradiation time between 30 sec and 2 min, or between 45 sec and 2 min, or between 1 and 2 min, or between 1.5 and 2 min.

11. The method according to any one of claims 1 to 10, wherein the unsaturated monomers capable of photochemical cycloaddition are irradiated in presence of at least one catalyst.

12. The method according to claim 11, wherein the at least one catalyst is provided in an amount between 3 to 20 mol%.

13. The method according to claim 11, wherein the at least one catalyst having a light absorbance between 360 and 460 nm.

14. The method according to any one of claims 11 to 13, wherein the at least one catalyst is selected from:

(a) a metal phthalocyanine; and

(b) a catalyst of formula (I):

wherein

point of connectivity to Xi, X₂, X3 or X₄;

Rs is selected from

wherein a/vx/'is a point of connectivity to the carbonyl groups;

R₄ is H or a -Ci-C₅alkyl; or -CH=CH-(C=O)-OH; or -CH₂-CH=C-(C=O)-O-

15. The method according to claim 14, wherein the metal of said metal phthalocyanine and M, independently, is a transition metal.

16. The method according to claim 15, wherein the transition metal is selected from scandium (Sc), titanium (Ti), vanadium (V), chromium (Cr), manganese (Mn), iron (Fe), cobalt (Co), nickel (Ni), copper (Cu), zinc (Zn), zirconium (Zr), molybdenum (Mo), technetium (Tc), ruthenium (Ru), rhodium (Rh), palladium (Pd), silver (Ag), cadmium (Cd), tungsten (W), rhenium (Re), osmium (Os), iridium (Ir), platinum (Pt), gold (Au), cerium (Ce), europium (Eu), gadolinium (Gd), and ytterbium (Yb).

17. The method according to claim 16, wherein the metal is Sn, Zn, Cu, Eu, Co or Pd.

18. The method according to claim 14, wherein the catalyst is a metal phthalocyanine, wherein the metal is Sn, Co, Cu, Zn, or Eu.

19. The method according to any one of claims 14 to 18, wherein the catalyst is selected from zinc(II)phthalocyanine (ZnPC), tin(II)phthalocyanine (SnPC), cobalt(II)phthalocyanine (CoPC), and Eu(II)phthalocyanine.

20. The method according to claim 14, wherein the catalyst is a compound of formula (I), wherein each of Xi, X₂, X3 and X₄ is O or N.

21. The method according to claim 20, wherein the catalyst is a compound of formula (I), wherein each of Xi, X2, X3 and X4 is O atom.

22. The method according to claim 20, wherein the catalyst is a compound of formula (I), wherein each of Xi, X2, X3 and X4 is N atom.

23. The method according to claim 14, wherein R4 is H.

The method according to claim 14, wherein

H.

25. The method according to claim 14, wherein the catalyst of formula (I) is a catalyst of formula (II):

M is a metal atom; each of Xi, X2, X3 and X4, independently of the other, is a nitrogen or an oxygen atom; each of R1 and R2, independently of the other, is a group selected from:

is a point of connectivity to Xi, X2, X3 or X4;

R₄ is H or a -Ci-C₅alkyl; or -CH=CH-(C=O)-OH; or -CH₂-CH=C-(C=O)-O- Aryl-CH=CH-(C=O)-OH; and wherein each of Z, Z1 and Z2, independently, is absent or is a -C₁-C₅alkylene.

26. The method according to claim 25, wherein each of Xi, X2, X3 and X4 is the same and is N or 0.

27. The method according to claim 26, wherein each of Xi, X2, X3 and X4 is a nitrogen atom.

28. The method according to claim 25, wherein each of R1 and R2, independently of the other, is a group selected from

29. The method according to claim 28, wherein each of R1 and R2, independently of the other,

30. The method according to claim 14, wherein the catalyst is a compound of formula (I), wherein each of Xi, X2, X3 and X4 is N and R1 and R2 are each

, wherein R3 is H or a -C₁-C₅alkyl.

31. The method according to claim 30, wherein R3 is H.

32. The method according to claim 14, wherein the catalyst is a compound of formula (II) having the structure (

, wherein M is a metal selected from Sn, Cu, Zn, Co, and Eu.

33. The method according to claim 32, wherein M is Sn and the catalyst is

34. The method according to claim 14 or 25, wherein the catalyst is a compound of formula (I) or formula (II), wherein each of Xi, X2, X3 and X4 is O atom, and wherein each of R1 and R2, independently of the other, is a group selected from:

wherein

R3 is H or a -C₁-C₅alkyl; each of Z, Zi and Z2, independently, is absent or a -C₁-C₅alkylene; and wherein s a point of connectivity to Xi, X2, X3 or X4.

35. The method according to claim 34, wherein R1 and R2 are the same.

36. The method according to claim 35, wherein each of R1 and R2 is a group selected from:

37. The method according to claim 36, wherein each of R1 and R2 is a group selected from:

, wherein R3 is H or -CH3, and each of Z,

Zi and Z2, independently, is a methylene or an ethylene group.

38. The method according to claim 37, wherein each

, R3 is H or -CH3, and each of Zi and Z2 is methylene or ethylene.

39. The method according to claim 38, wherein each

R3 is -CH3, and each of Zi and Z2 is methylene or ethylene.

40. The method according to claim 14, wherein the catalyst of formula (I), having structure (IV)

wherein M is a transition metal.

41. The method according to claim 40, wherein the catalyst is

42. The method according to claim 14, wherein in a compound of formula (I) R3 is

43. The method according to claim 42, wherein the catalyst is of structure (V) and (VI)

wherein for each of (V) and (VI), independently, each of Ri, R2, Xi, X2, X3 and X4 are as defined in claim 14.

44. The method according to claim 43 , wherein each of R1 and R2 is a group selected from:

R3 is H or a -C₁-C₅alkyl; each of Z, Zi and Z2, independently, is a -C₁-C₅alkylene; and wherein -Arvais a point of connectivity to Xi, X2, X3 or X4.

45. The method according to claim 44, wherein each of R1 and R2 is a group selected from:

, wherein R3 is H or -CH3, and each of Z,

Z1 and Z2, independently, is a methylene or an ethylene group.

46. The method according to claim 45, wherein each

R3 is H or -CH3, and each of Zi and Z2 is methylene or ethylene.

47. The method according to claim 45, wherein each

R3 is -CH3, and each of Zi and Z2 is methylene or ethylene.

48. The method according to claim 14, wherein the catalyst is of formula (VII):

wherein M is a transition metal.

49. The method according to claim 48, wherein M is Sn, Zn, Cu, Eu, Co or Pd.

50. The method according to claim 48, wherein the catalyst is:

51. The method according to any one of claims 1 to 50, wherein the irradiation curing is achievable in presence of a metal (2,2'-[(4- methylphenyl)imino]bisethylbisphthalate) (M(PA-MPIB)).

52. The method according to claim 51, wherein the metal (2,2'-[(4- methylphenyl)imino]bisethylbisphthalate) (M(PA-MPIB)) is tin (II) (2,2'-[(4- methylphenyl)imino]bisethylbisphthalate) (Sn(PA-MPIB)).

53. The method according to any one of claims 1 to 50, wherein the irradiation curing is achievable in presence of a metal (bis-Nl,Nl'-(l,3-phenylene)diphthalamide) (Sn(MPDA-PA)).

54. The method according to claim 53, wherein the metal (bis-Nl,Nl'-(l,3- phenylene)diphthalamide) (Sn(MPDA-PA)) is tin (II) (bis-Nl,Nl'-(l,3- phenylene)diphthalamide) (Sn(MPDA-PA)).

55. The method according to any one of claims 1 to 54, carried out under conditions comprising:

-Light irradiation (light curing) at a wavelength between 360 and 460 nm,

-Light irradiation (light curing) at a wavelength between 360 and 460 nm, and further under thermal radiation, e.g., a temperature between room temperature (23- 30°C) and 90°C, or

Light irradiation wherein the radiation dose is between 2 mW/cm² and -100 W/cm².

56. The method according to any one of claims 1 to 55, further comprises cycloreversion of the thermoset polymer to the unsaturated monomers, the cycloreversion comprises thermal radiation to a temperature between 90 and 200°C, or microwave radiation.

57. A method of recycling a thermoset polymer obtained by a method according to any one of claims 1 to 56, the method comprising thermally treating the polymer at a temperature between 90 and 200°C, or under microwave radiation to transform or cyclorevert the thermoset polymer to unsaturated monomers made therefrom.

58. The method according to any one of the preceding claims, wherein the thermoset polymer having a structural motif selected from:

'^/vvv' designates a point of connectivity to another motif or an atom or a group of atoms in the polymer; each of Xi and X2, independently of the other, is a heteroatom selected from N, O and S; and wherein each of R’ and R”, independently of the other, is selected from H, optionally substituted -C₁-C₅alkyl, optionally substituted -C₁-C₅heteroalkyl, optionally substituted -C₆-C₁₂aryl, optionally substituted -C₁-C₅alkylene-C₆-C₁₂aryl, optionally substituted - C₆-C₁₂arylene-C₁-C₅alkyl, optionally substituted -C₃-C₇heteroaryl (comprising one or more heteroatom selected from N, O and S), a polyamine, a polyol, or a polythiol.

59. The method according to claim 58, wherein each of R’ and R” is different from H.

60. The method according to claim 58 or 59, wherein each of R’ and R” is same or different and selected from optionally substituted -C₆-C₁₂aryl, optionally substituted - C₁-C₅alkylene-C₆-C₁₂aryl, optionally substituted -C₆-C₁₂arylene-C₁-C₅alkyl, and optionally substituted -C₃-C₇heteroaryl comprising one or more heteroatom selected from N, O and S.

61. The method according to claim 60, wherein each of R’ and R” is same or different and selected from optionally substituted -C₆-C₁₂aryl.

62. The method according to claim 58, wherein each of Xi and X2 is same or different.

63. The method according to any one of claims 58 to 62, wherein one or both of Xi and X2 is N atom or a NH group.

64. The method according to any one of claims 58 to 62, wherein one or both of Xi and X2 is O atom.

65. The method according to any one of claims 58 to 62, wherein each of Xi and X2 is N and each of R’ and R” is a -C₆-C₁₀aryl or a substituted form thereof.

66. The method according to claim 58, wherein the polymer is of a structure comprising the repeating structural motif

wherein each of Xi and

X2 is same or different, and wherein each of R’ and R” is same or different.

67. The method according to claim 66, wherein the polymer having a structure:

designates a point of connectivity to another 8-memebered ring or an atom or a group of atoms in the polymer; each of Xi and X2, independently is a heteroatom selected from N, O and S; and wherein each of R’ and R”, independently of the other, is selected from H, optionally substituted -C₁-C₅alkyl, optionally substituted -C₁-C₅heteroalkyl, optionally substituted -C₆-C₁₂aryl, optionally substituted -C₁-C₅alkylene-C₆-C₁₂aryl, optionally substituted - C₆-C₁₂arylene-C₁-C₅alkyl, optionally substituted -C₃-C₇heteroaryl (comprising one or more heteroatom selected from N, O and S), a polyamine, a polyol, or a polythiol.

68. The method according to claim 67, wherein each of Xi and X2 is the same and R’ and R” are the same.

69. The method according to claim 67, wherein each of Xi and X2 is N or O.

70. The method according to claim 67, wherein each of Xi and X2 is N.

71. The method according to any one of claims 58 to 65, having a structural motif selected from:

designates a point of connectivity to another motif or an atom or a group of atoms in the polymer, and wherein each of R’ and R”, independently of the other, is selected from H, optionally substituted -C₁-C₅alkyl, optionally substituted -C₁-C₅heteroalkyl, optionally substituted -C₆-C₁₂aryl, optionally substituted -C₁-C₅alkylene-C₆-C₁₂aryl, optionally substituted - C₆-C₁₂arylene-C₁-C₅alkyl, optionally substituted -C₃-C₇heteroaryl (comprising one or more heteroatom selected from N, O and S), a polyamine, a polyol, or a polythiol.

72. The method according to claim 71, each of R’ and R” is different from H.

73. The method according to claim 71, wherein each of R’ and R” is same or different and selected from optionally substituted -C₆-C₁₂aryl, optionally substituted - C₁-C₅alkylene-C₆-C₁₂aryl, optionally substituted -C₆-C₁₂arylene-C₁-C₅alkyl, and optionally substituted -C₃-C₇heteroaryl (comprising one or more heteroatom selected from N, O and S).

74. The method according to claim 71, wherein each of R’ and R” is same or different and selected from optionally substituted -C₆-C₁₂aryl.

75. The method according to claim 71, wherein the polymer having the structure:

'^/ designates a point of connectivity to another 8-memebered ring or an atom or a group of atoms in the polymer; and wherein each of R’ and R”, independently of the other, is selected from H, optionally substituted -C₁-C₅alkyl, optionally substituted -C₁-C₅heteroalkyl, optionally substituted -C₆-C₁₂aryl, optionally substituted -C₁-C₅alkylene-C₆-C₁₂aryl, optionally substituted - C₆-C₁₂arylene-C₁-C₅alkyl, optionally substituted -C₃-C₇heteroaryl (comprising one or more heteroatom selected from N, O and S), a polyamine, a polyol, or a polythiol.

76. The method according to claim 75, wherein R’ and R” are the same.

77. The method according to any one of claims 58 to 76, wherein each of R’ and R”, independently of the other, is selected from optionally substituted -C₆-C₁₂aryl.

78. The method according to any one of claims 58 to 77, wherein each of R’ and

R”, independently of the other, is of structure .

, wherein designates

a point of connectivity and wherein Ra is one or more substituents selected from -H, -

79. The method according to claim 77, wherein Ra represents a single substituent positioned ortho, metal or para to the atom of connectivity.

80. The method according to claim 77, wherein Ra designates two or more substituents.

81. The method according to claim 77, wherein each of R’ and R”, independently

82. The method according to any one of claims 58 to 81, wherein each of R’ and

R”, independently, is selected from:

83. The method according to any one of claims 58 to 82, comprising a repeated

84. The method according to any one of claims 1 to 83, wherein the unsaturated monomers is or comprises one or more of:

(a)

wherein each of a, b and c, independently, is between 1 and 50, or the number of groups designated by a, b and c, combined, is between 5 and 60, and wherein each of R’, being same or different, is selected as in claim 58;

wherein each of a, b and c, independently, is between 1 and 50, or the number of groups designated by a, b and c, combined, is between 5 and 60;

(c)

, wherein each of a, b and c, independently, is between 1 and 50, or the number of groups designated by a, b and c, combined, is between 5 and 60, and wherein each of R’, being same or different, is as defied in claim 58;

(d)

, wherein each of a, b and c, independently, is between 1 and 50, or the number of groups designated by a, b and c, combined, is between 5 and 60; (e)

, g between 1 and 50 and wherein each of R’, independently, being same or different, is as defined in claim 58;

designating the number of repeating units, optionally being between 1 and 50;

between 1 and 50;

, wherein each of R’ is an optionally -C₆- C₁₀aryl, being optionally a substituted or an unsubstituted phenyl ring; (k)

wherein each of R’ is an optionally substituted -C₆-C₁₀aryl, being optionally a substituted or an unsubstituted phenyl ring;

(1)

, wherein each of R’ is an optionally substituted -C₆-C₁₀aryl, being optionally a substituted or an unsubstituted phenyl ring;

(m)

(o)

85. A thermoset polymer having a structural motif selected from:

86. The polymer according to claim 85, wherein each of R’ and R” is different from H.

87. The polymer according to claim 85 or 86, wherein each of R’ and R” is same or different and selected from optionally substituted -C₆-C₁₂aryl, optionally substituted - C₁-C₅alkylene-C₆-C₁₂aryl, optionally substituted -C₆-C₁₂arylene-C₁-C₅alkyl, and optionally substituted -C₃-C₇heteroaryl comprising one or more heteroatom selected from N, O and S.

88. The polymer according to claim 87, wherein each of R’ and R” is same or different and selected from optionally substituted -C₆-C₁₂aryl.

89. The polymer according to claim 85, wherein each of Xi and X2 is same or different.

90. The polymer according to any one of claims 85 to 89, wherein one or both of Xi and X2 is N atom or a NH group.

91. The polymer according to any one of claims 85 to 89, wherein one or both of

Xi and X2 is O atom.

92. The polymer according to any one of claims 85 to 89, wherein each of Xi and X2 is N and each of R’ and R” is a -C₆-C₁₀aryl or a substituted form thereof.

93. The polymer according to claim 85, wherein the polymer is of a structure comprising the repeating structural motif

wherein each of Xi and

94. The polymer according to claim 93 wherein the polymer having a structure:

95. A thermoset polymer having a structure:

96. The polymer according to claim 94 or 95, wherein each of Xi and X2 is the same and R’ and R” are the same.

97. The polymer according to claim 94 or 95, wherein each of Xi and X2 is N or O.

98. The polymer according to claim 94 or 95, wherein each of Xi and X2 is N.

99. The polymer according to any one of claims 85 to 92, having a structural motif selected from:

designates a point of connectivity to another motif or an atom or a group

of atoms in the polymer, and wherein each of R’ and R”, independently of the other, is selected from H, optionally substituted -C₁-C₅alkyl, optionally substituted -C₁-C₅heteroalkyl, optionally substituted -C₆-C₁₂aryl, optionally substituted -C₁-C₅alkylene-C₆-C₁₂aryl, optionally substituted - C₆-C₁₂arylene-C₁-C₅alkyl, optionally substituted -C₃-C₇heteroaryl (comprising one or more heteroatom selected from N, O and S), a polyamine, a polyol, or a polythiol.

100. The polymer according to claim 99, each of R’ and R” is different from H.

101. The polymer according to claim 99, wherein each of R’ and R” is same or different and selected from optionally substituted -C₆-C₁₂aryl, optionally substituted - C₁-C₅alkylene-C₆-C₁₂aryl, optionally substituted -C₆-C₁₂arylene-C₁-C₅alkyl, and optionally substituted -C₃-C₇heteroaryl (comprising one or more heteroatom selected from N, O and S).

102. The polymer according to claim 99, wherein each of R’ and R” is same or different and selected from optionally substituted -C₆-C₁₂aryl.

103. The polymer according to claim 99, wherein the polymer having the structure:

designates a point of connectivity to another 8-memebered ring or an

atom or a group of atoms in the polymer; and wherein each of R’ and R”, independently of the other, is selected from H, optionally substituted -C₁-C₅alkyl, optionally substituted -C₁-C₅heteroalkyl, optionally substituted -C₆-C₁₂aryl, optionally substituted -C₁-C₅alkylene-C₆-C₁₂aryl, optionally substituted - C₆-C₁₂arylene-C₁-C₅alkyl, optionally substituted -C₃-C₇heteroaryl (comprising one or more heteroatom selected from N, O and S), a polyamine, a polyol, or a polythiol.

104. The polymer according to claim 103, wherein R’ and R” are the same.

105. The polymer according to any one of claims 85 to 103, wherein each of R’ and R”, independently of the other, is selected from optionally substituted -C₆-C₁₂aryl.

106. The polymer according to any one of claims 85 to 105, wherein each of R’ and

R”, independently of the other, is of structure .0'^Ra , wherein '^/vvv' designates a point of connectivity and wherein Ra is one or more substituents selected from -H, - Ci-Ci₂alkyl, -C₆-C₁₀aryl, -OH, -OCi-Ci₂alkyl, -OC₆-C₁₀aryl, -COOH, -COOCi-

Ci₂alkyl, and -COOC₆-C₁₀aryl.

107. The polymer according to claim 106, wherein Ra represents a single substituent positioned ortho, metal or para to the atom of connectivity.

108. The polymer according to claim 106, wherein Ra designates two or more substituents.

109. The polymer according to claim 106, wherein each of R’ and R”, independently may be selected from

110. The polymer according to any one of claims 85 to 109, wherein each of R’ and

R”, independently, is selected from:

111. The polymer according to any one of claims 85 to 110, comprising a repeated

112. The polymer according to any one of claims 85 to 111, formed by a 3D printing method comprising radiation curing of unsaturated monomers undergoing radiation- mediated cycloaddition, wherein the polymer undergoes cycloreversion is fully reversible to the unsaturated monomers under thermal or microwave-mediated conditions.

113. The polymer according to claim 112, wherein the cycloaddition comprising or consisting one or both [2 + 2] and [4 + 4] cycloaddition and excludes [4+2] cycloaddition.

114. The polymer according to claim 112 or 113, wherein the unsaturated monomers may be same or different.

115. The polymer according to any one of claims 112 to 114, wherein the unsaturated monomer is one or more of:

independently, is between 1 and 50, or the number of groups designated by a, b and c, combined, is between 5 and 60, and wherein each of R’, being same or different, is selected as in claim 85;

(b)

independently, is between 1 and 50, or the number of groups designated by a, b and c, combined, is between 5 and 60;

(c)

, wherein each of a, b and c, independently, is between 1 and 50, or the number of groups designated by a, b and c, combined, is between 5 and 60, and wherein each of R’, being same or different, is as defied in claim 85;

(d)

each of a, b and c, independently, is between 1 and 50, or the number of groups designated by a, b and c, combined, is between 5 and 60;

(e)

integer between 1 and 50 and wherein each of R’, independently, being same or different, is as defined in claim 85;

designating the number of repeating units, optionally being between 1 and 50;

(i)

, wherein n is an integer between 1 and 50;

(J)

, wherein each of R’ is an optionally -C₆- C₁₀aryl, being optionally a substituted or an unsubstituted phenyl ring;

(m)

(o)

116. A compound selected from any one of:

(b)

(c)

(d)

each of a, b and c, independently, , is between 1 and 50, or the number of groups designated by a, b and c, combined, is between 5 and 60;

(e)

, wherein each of R’, being same or different, is as defined in claim 85;

(h)

designating the number of repeating units, optionally being between 1 and 50;

integer between 1 and 50;

(J)

(k)

(m)

(o)

117. A material of formula (I):

wherein

a point of connectivity to the carbonyl groups;

118. The material according to claim 117, wherein the metal and wherein M is a transition metal.

119. The material according to claim 118, wherein the transition metal is selected from scandium (Sc), titanium (Ti), vanadium (V), chromium (Cr), manganese (Mn), iron (Fe), cobalt (Co), nickel (Ni), copper (Cu), zinc (Zn), zirconium (Zr), molybdenum (Mo), technetium (Tc), ruthenium (Ru), rhodium (Rh), palladium (Pd), silver (Ag), cadmium (Cd), tungsten (W), rhenium (Re), osmium (Os), iridium (Ir), platinum (Pt), gold (Au), cerium (Ce), europium (Eu), gadolinium (Gd), and ytterbium (Yb).

120. The material according to claim 119, wherein the metal is Sn, Zn, Cu, Eu, Co or Pd.

121. The material according to claim 119, wherein the material is a metal phthalocyanine, wherein the metal is Sn, Co, Cu, Zn, or Eu.

122. The material according to claim 119, wherein the material is a compound of formula (I), wherein each of Xi, X2, X3 and X4 is O or N.

123. The material according to claim 122, wherein the material is a compound of formula (I), wherein each of Xi, X2, X3 and X4 is O atom.

124. The material according to claim 122, wherein the material is a compound of formula (I), wherein each of Xi, X2, X3 and X4 is N atom.

125. The material according to claim 117, wherein R4 is H.

126. The material according to claim 117, wherein

is H.

127. The material according to claim 117, wherein the material of formula (I) is a catalyst of formula (II):

128. The material according to claim 127, wherein each of Xi, X₂, X3 and X₄ is the same and is N or O.

129. The material according to claim 128, wherein each of Xi, X₂, X3 and X₄ is a nitrogen atom.

130. The material according to claim 127, wherein each of R1 and R₂, independently of the other, is a group selected from

131. The material according to claim 130, wherein each of R1 and R₂, independently of the other,

132. The material according to claim 117, wherein the material is a compound of formula (I), wherein each of Xi, X₂, X3 and X₄ is N and R1 and R₂ are each

133. The material according to claim 132, wherein R3 is H.

134. The material according to claim 117, wherein the material is a compound of formula (II) having the structure (

, wherein M is a metal selected from Sn, Cu, Zn, Co, and Eu.

135. The material according to claim 134, wherein M is Sn and the material is

136. The material according to claim 117 or 127, wherein the material is a compound of formula (I) or formula (II), wherein each of Xi, X2, X3 and X4 is O atom, and wherein each of R1 and R2, independently of the other, is a group selected from:

wherein

137. The material according to claim 136, wherein R1 and R2 are the same.

138. The material according to claim 137, wherein each of R1 and R2 is a group selected from:

139. The material according to claim 138, wherein each of R1 and R2 is a group selected from:

, wherein R3 is H or -CH3, and each of Z,

Zi and Z2, independently, is a methylene or an ethylene group.

140. The material according to claim 139, wherein each of R1 and R2 is

each of Zi and Z2 is methylene or ethylene.

141. The material according to claim 140, wherein each of R1 and R2 is

each of Zi and Z2 is methylene or ethylene.

142. The material according to claim 117, wherein the material of formula (I), having

, wherein M is a transition metal.

143. The material according to claim 142, wherein the material is

144. The material according to claim 117, wherein in a compound of formula (I) R3

145. The material according to claim 144, wherein the catalyst is of structure (V) and (VI)

wherein for each of (V) and (VI), independently, each of Ri, R2, Xi, X2, X3 and X4 are as defined in claim 117.

146. The material according to claim 145, wherein each of R1 and R2 is a group selected from:

147. The material according to claim 145, wherein each of R1 and R2 is a group selected from:

, wherein R3 is H or -CH3, and each of Z,

Zi and Z2, independently, is a methylene or an ethylene group.

148. The material according to claim 147, wherein each of R1 and R2 is

each of Zi and Z2 is methylene or ethylene.

149. The material according to claim 147, wherein each of R1 and R2 is

each of Zi and Z2 is methylene or ethylene.

150. The material according to claim 117, wherein the material is of formula (VII):

wherein M is a transition metal.

151. The material according to claim 150, wherein M is Sn, Zn, Cu, Eu, Co or Pd.

152. The material according to claim 150, wherein the catalyst is:

153. A method of recycling a thermoset polymer according to any one of claims 85 to 115, the method comprising thermally treating the polymer at a temperature between 90 and 200°C, or under microwave radiation to transform or cyclorevert the thermoset polymer to unsaturated monomers made therefrom.