Controlled orthogonal reactions in neat polymerizations

Abstract

Polymerization reactions in neat conditions have had a long history since the beginning of synthetic polymers. The absence of a solvent is attractive for many applications, but also poses challenges in carrying out multiple polymerization reactions simultaneously or sequentially. In this Minireview, we focus on recent development of controlled orthogonal reactions used in neat systems. We first overview the available functional groups, as well as their synthesis from neat polymerization reactions, categorized by the involved atoms such as carbon only, carbon & oxygen, carbon & nitrogen, and carbon & sulfur. We then summarize literature reports, mostly from the past three years, that utilized orthogonal reactions to synthesize polymers with novel chemical structures for emerging applications including 3D printing, multimaterials, optical materials, and photo-responsive materials. Last, we discuss future directions for this exciting field that will unlock opportunities for unprecedented chemical structures, microstructures, material processing techniques, and material properties. Consequently, we hope this Minireview guides the design of orthogonal reactions in neat polymerizations for the synthesis of next-generation structural and functional polymer materials.

---

Introduction

Neat polymerization is a technique where reactive monomers are converted into polymers without using any solvent. In fact, some of the first examples of commercialized polymers are synthesized by neat polymerization, such as vulcanized rubber and Bakelite.^1,2 The solventless polymerization processes enable molding of the monomers—most commonly in the form of liquids—into materials with a desired shape. Fabrication of polymer materials by neat polymerizations is often more advantageous than the ones by solvent-based processes. Solvent-based polymer processing is ubiquitous for thin film materials such as paints, coatings, and adhesives, but its implementation is limited when fabricating thick and bulky parts where the diffusion (evaporation) of solvent becomes difficult. Since solvent often is toxic and is discarded, neat polymerization fits the green chemistry principle by reducing wasteful and harmful byproducts. In recent years, neat polymerizations have been rejuvenated to develop materials for emerging applications. One example is materials used in 3D printing by stereolithography, where the free-radical mediated photopolymerization of (meth)acrylate monomers is conducted in solventless conditions.³ Neat polymerization typically relies on a singular polymerization chemistry controlled by applying heat and light. Such singular polymerization reaction often faces challenges in obtaining desired material properties and/or processing conditions.

In chemistry, the concept of orthogonal reactions was first introduced by Merrifield in 1977 as “a set of completely independent classes of protecting groups”.⁴ In an ideal orthogonal system, at least two sets of reactants can react within each set without interacting with the other set(s). In another word, each set of reactions can be controlled independently and can be carried out in any order. The ability to orthogonally operate reactions is derived from chemical selectivity, such as selective generation of a specific catalyst by employing an external stimulus such as light and heat. In some cases, the selectivity is attained by vastly different reaction kinetics, i.e. one reaction completes before another reaction occurs. Orthogonal reactions have profound impact in protecting groups in organic synthesis,^5,6 supramolecular chemistry,^7–9 peptide ligation, biorthogonal reaction systems, and many other fields.¹⁰ In the context of polymer science, orthogonal reactions have proven to be a robust tool for obtaining well-defined macromolecular structures, including reactions for polymer synthesis and reactions for polymer functionalization. In the past few years, the application of orthogonal reactions in polymer science has been reviewed by Boyer,¹¹ Reuther,¹² and Jia.¹³

The absence of solvent poses challenges in achieving chemical selectivity because of the limited degrees of freedom. For example, the concentration of the reactants is restrained in neat conditions; catalyst must be soluble in the monomers; the sets of monomers should form a homogenous mixture; the monomer mixtures should be sufficiently bench-stable to be processed. Nevertheless, successful systems have been developed, albeit scarce. For stereolithography 3D printing, “hybrid” resins were developed in the early 1990s, comprising both acrylate and epoxy monomers that orthogonally undergo free-radical and cationic polymerization reactions, respectively, forming interpenetrating polymer networks.¹⁴ The combination of two reactions offers combined advantages: (i) ultrafast photopolymerization kinetics from the acrylate and (ii) reduced volumetric shrinkage from the epoxy. These hybrid resins have been a commercial success. We are delighted to see many innovative studies in orthogonal neat polymerization systems for the synthesis of advanced materials, motivating us to summarize recent reports to provoke future directions in this exciting field of polymer chemistry.

In this Minireview, we focus on the recent study of orthogonal reactions in neat polymerization systems for enabling new materials, including 3D printing materials, multimaterials, optical materials, and photo-responsive materials. We prioritize highlighting work reported after the year 2020. We set the following boundary for the scope of this article (Fig. 1A):

1. The polymerization reactions must be conducted in neat conditions. No solvent.

2. The polymerization reactions are carried out in a controlled manner, either temporally or spatially. In another word, the reactions only occur at the practitioner's desired time, and in some cases, only occur at a desired location.

3. The polymerization reactions do not generate volatile small molecules as byproducts.

Fig. 1 (A) Boundaries of the review paper highlighting solvent-free, controlled polymerization reactions with no volatile byproducts, conducted either sequentially or simultaneously. (B) Functional groups that can be synthesized by neat polymerization reactions.

Summary of polymerization reactions in neat conditions

Understanding the landscape of covalent bonds is essential to designing orthogonal reactions for neat polymerization systems (Fig. 1B). To this end, we summarize twenty-four common reactions used in neat polymerization systems (Fig. 2). Arguably the most fundamental covalent bond in polymer chemistry is carbon–carbon (C–C) bond, which contributes to >60% of the entire volume of commodity polymers.¹⁵ The common syntheses of C–C bond involve reactions of alkene groups through radical, anionic, cationic, or metal-coordination pathways, such as the formation of polyethylene (PE), polypropylene (PP), polystyrene (PS), and polyvinyl chloride (PVC).¹⁶ Since free-radical polymerizations of electron-deficient vinyl compounds can be straightforwardly initiated and terminated, they are commonly used in orthogonal reactions in neat polymerizations. For example, neat polymerization of (meth)acrylates (1) alone is widely used in coatings, adhesives, and moldable resins.¹⁷ Orthogonal systems have been demonstrated by coupling the (meth)acrylate free-radical homopolymerization with reactions that are inert to free-radicals, such as nucleophilic reactions between alcohol and isocyanate (forming polyurethane), as well as epoxy-anhydride reactions.^18,19 Other useful homopolymerizations of alkene monomers include ring-opening metathesis polymerization (ROMP, 2), which utilizes ring-strained alkene monomers and ruthenium catalysts.^20,21 ROMP yields unsaturated bonds remaining in the polymer, allowing possibilities of orthogonal reactions for post-polymerization functionalizations.²² Cycloaddition reactions are also useful C–C forming reactions such as [2 + 2] reactions (3) that are often UV-mediated and covalently reversible.²³ Diels–Alder (4) reactions can also form C–C bonds through [4 + 2] cycloaddition of a conjugated diene with an alkene (dienophile). An ideal dienophile is highly electron-deficient, such as maleimide, which enables orthogonality with less electron-deficient (meth)acrylate monomers.²⁴ Because cycloaddition utilizes unsaturated bonds, radical mechanisms involving alkenes can pose issues, however using fast-reacting vinyl monomers such as (meth)acrylates or with nucleophilic mechanisms that do not hinder the cycloaddition can be successfully performed.^25,26

Fig. 2 Summary of common polymerization reactions conducted in neat conditions, categorized by characteristic atoms involved in the target functional groups. The reaction entry numbers are consistently indexed throughout the article. Abbreviations in the reaction scheme: “R.” = free-radical, “M” = metal catalyst, “C+” = cation, “C−” = anion, “Nu:” = nucleophile, “EWG” = electron withdrawing group.

Carbon–oxygen (C–O) bond is ubiquitous in polymer chemistry, notably, in the backbone structures of high-volume community polymers such as polyesters and polyethers.¹⁵ Commodity thermoplastic polyesters are synthesized by Fischer esterification or transesterification reactions; these syntheses are conducted without the use of solvent, yet they generate small-molecule byproducts. C–O bonds are also commonly formed by ring-opening, chain-growth polymerizations of cyclic ethers (5) and lactones (6).^27–29 Several other reaction systems result in step-growth polyesters, including nucleophile-mediated alcohol-anhydride (7), epoxy-carboxylic acid (8), and epoxy-anhydride (9) reactions.^30,31 These ring-opening reactions do not generate small-molecule byproducts, making them ideal for neat polymerizations, which have been demonstrated in orthogonal systems with free-radical reactions of (meth)acrylates and thiol–ene.^32–34 However, these C–O forming reactions generally are not orthogonal with other nucleophilic mechanisms.

Carbon–nitrogen (C–N) bonds are commonly found in Nylon and epoxy-amine resin.³⁵ Epoxy-amine neat polymerizations are typically self-catalyzing; a primary amine may react with two epoxy groups through a secondary amine intermediate (10). In high-performance epoxy-amine resins, cyanate esters are often added with the epoxy-amine, which undergoes cyclotrimerization (as an orthogonal reaction) to yield cyanurate rings (11).³⁶ It is worth noting that in recent years, imines (“Schiff base”) have become popular in the design of bulk polymers for their covalent reversibility, though the aldehyde-amine reaction generates water as a byproduct.³⁷ Being popularized as a “click chemistry”, the copper-catalyzed azide–alkyne Huisgen cycloaddition reaction also forms C–N bonds as a 1,2,3-triazole (12).³⁸ Amines can undergo nucleophilic Michael addition (13) with (meth)acrylates under mild conditions, demonstrated in orthogonal systems together with free-radical polymerization of acrylates.³⁹

C–N bonds also constitute a variety of functional groups, such as amide, urea, and urethane, which are important for neat polymerizations. Amines react with cyclic anhydrides spontaneously to give amic acids, which can further ring close to synthesize imides, albeit generating water as a byproduct.⁴⁰ Polyurethane is generally synthesized by alcohol-isocyanate (14) reactions in neat, catalyzed by nucleophile or Lewis acid.⁴¹ As an isocyanate-free route for polyurethanes, amine-cyclic carbonate (15) reactions gained popularity in the past decades.⁴² Amine-isocyanate reaction (16) is often conducted simultaneously or sequentially in the synthesis of urethanes to obtain poly(urethane-urea) copolymers.⁴³ Benzoxazine has gained popularity in forming high-performance thermosetting polymers since its ring-opening reaction (17) does not require a catalyst.⁴⁴ Similar to C–O formations, C–N formations are orthogonal to free-radical polymerizations but are challenging with other nucleophilic reactions.⁴⁵

Carbon–sulfur (C–S) bonds are emerging in neat polymerizations, in a large part due to the diverse reactivity of thiols. Considered as a “click chemistry”, thiyl radical efficiently reacts with alkenes (18) or alkynes (19) to form thioethers.⁴⁶ Deprotonated thiols act as nucleophiles to ring open epoxides (20) and undergo Michael addition with electron-deficient alkenes (21), in both cases also forming thioethers.³⁴ There are vast opportunities when combining C–O, C–S, and C–N bonds. For example, thiol-anhydride reactions yield thioester (22), which has recently been shown to be covalently reversible.^47,48 Thiourea can be synthesized by reacting amines with thiocyanates (23). Treating isocyanate with thiols, catalyzed by base, yields thiourethane (24) which has been referred as a “click reaction”.⁴⁹ Very recently, polythiourethane has gained interest with respect to reprocessibility and recyclability by taking advantage of its covalent reversibility.^50–55

It is worth noting that this list is not exhaustive, but serves as a guide for the following discussions, where the reactions are accordingly indexed.

Emerging applications of orthogonal neat polymerizations

3D printing

Orthogonal polymerization reactions have emerged as a robust approach in both direct ink writing (DIW) and vat photopolymerization (VPP), offering enhanced control in material processing and material properties. DIW is a 3D printing technique where viscous ink is extruded through a nozzle to create three-dimensional structures, optionally followed by photopolymerization to set the structure.^56,57 VPP, sometimes referred to as stereolithography (SLA), involves the use of light to selectively initiate photopolymerizable resins in a vat, mostly commonly done in a layer-by-layer process.⁵⁸ Compared to traditional manufacturing techniques, both DIW and VPP enable fabricating complex geometries that are difficult, if possible at all, to be molded.

Herein, the term “dual-cure” refers to a set of monomers that are polymerized by two distinct polymerization reactions.⁵⁹ For VPP, the first stage “cure” is a photopolymerization reaction that defines the shape. Free-radical photopolymerization of multifunctional (meth)acrylates excels at fast polymerization kinetics, but has severe limitations including high volumetric shrinkage,⁶⁰ limited reaction conversions (due to vitrification),⁶¹ poor material properties, and incapability of depolymerization and recycling.⁶² The subsequent “cure” is often mediated thermally, which sets the final chemical structure and material properties. In the following section, we summarize recent developments of using “dual-cure” to mitigate the abovementioned limitations of photopolymers.

Cooper et al. demonstrated a dual-cure reaction system to obtain blends of thermoplastic polymethacrylate and polyurethane.⁶³ The first-stage cure is a free-radical polymerization of methacrylates (1) that contain blocked isocyanates in the form of hindered urea. In the second stage, the blocked isocyanate is cleaved when annealing at 100 °C, which released isocyanate that reacts (by reaction 16 in reverse) with the available alcohols (14) or amines (16) to form polyurethane or polyurea, respectively (Fig. 3). The final polymer is a blend of linear polymethacrylate and linear polyurethane (or polyurea). Rolland et al. followed the same design of orthogonal polymerizations and implemented them in VPP 3D printing. They demonstrated attractive materials properties that are superior to methacrylate-only materials.⁶⁴ The second-stage heat cure also enhances isotropy of materials properties, since the heat cure was carried out uniformly. As a result, the printed parts exhibited performances akin to traditional rigid cast polyurethane, such as an Izod impact resistance of 75 J m⁻¹ and a heat deflection temperature (HDT) of 120 °C at 0.46 MPa.⁶⁵ They also demonstrated elastomeric materials with large elongation-at-break (dL/L₀ of 250–380%) and high tear strengths of 20–44 kN m⁻¹. Demonstrations of the elastomeric polyurethane materials include shoe soles, seat cushions, and protective sports equipment.

Fig. 3 Reaction scheme of “dual-cure” orthogonal polymerizations comprising a difunctional methacrylate synthesized from t-butyl aminoethyl methacrylate and isocyanate, and a diol. First, free-radical polymerization of the methacrylate groups forms a crosslinked polymer network. Second, when heated, the thermally labile hindered urea releases isocyanate, which reacts with the diol to form polyurethane, resulting in a blend of thermoplastic polymethacrylate and polyurethane.

Epoxy-related polymerization reactions have been demonstrated in dual-cure systems, including epoxy homopolymerization (5),^17,66–68 epoxy-acid (8),⁶⁹ epoxy-anhydride (9),^70–73 epoxy-amine (10),⁷⁴ and thiol-epoxy (20).⁷⁵ To this end, although many of the epoxy-related dual-cure systems have been demonstrated in neat polymerization, we focus on the ones that have been implemented in 3D printing. Particularly, one of the benefits of epoxy-acid and epoxy-anhydride chemistry is their formation of ester groups, which are covalently reversible through transesterification reactions. The covalent reversibility in network polymers has been demonstrated in Vitrimer and covalent adaptable network (CAN) systems to obtain attractive material functionalities including remoldability, repairability, and recyclablity.⁷⁶ Zinc acetylacetonate (Zn(AcAc)₂) is a common catalyst used in CANs to catalyze the transesterification reactions. Ramis et al. demonstrated the repairability of 3D printable parts synthesized by acrylate/epoxy-acid dual-cure reactions. Repairability of the polymer network was obtained given the ample amount of β-hydroxy ester bonds (Fig. 4).⁶⁹ Their recycled sample had a Young's modulus of 1510 MPa compared to the original 3D printed sample's modulus of 1320 MPa, likely due to the volatilization of small molecules. The recycled sample was made by pressing chopped sample in a hydraulic press at 9.25 MPa at 180 °C for 5 h; the sample saw negligible differences in T_g, storage modulus, and rubbery modulus compared to the pristine sample.

Fig. 4 Reaction scheme of “dual-cure” orthogonal polymerizations comprising a multifunctional acrylate and epoxy/carboxylic acid monomers. The initial crosslinked network is formed by free-radical acrylate polymerization, while the epoxy/acids remain unreacted. The second, epoxy-acid polymerization is mediated by heat to form the polyester network. In the presence of catalyst, the transesterification reactions between the hydroxy and ester groups enable repairability.

Ge et al. further demonstrated that acrylate monomers containing stoichiometric β-hydroxy groups can be reprocessable, even without introducing the polyester network.⁷⁷ They found that the dynamic network of their 3D printed samples reached dynamic equilibrium after 4 hours at 180 °C. They also found slight mechanical degradation after three cycles of recycling, where each cycle consisted of hot pressing at 500 MPa at 220 °C for 2 h. Wang et al. depolymerized their 3D-printed acrylate/epoxy-anhydride network using transesterification with ethylene glycol at 180 °C in 8 h.⁷² To re-enable photopolymerization, the resulting recycled oligomers were mixed with virgin resin in a ratio of 3 : 7. The reprinted sample decreased in mechanical properties compared to the virgin material: the storage modulus decreased from 2.03 GPa to 1.01 GPa; T_g decreased from 95 °C to 68 °C. In summary, the orthogonality of polymerizations between (meth)acrylate and epoxy-acid/anhydride reactions have been demonstrated as a robust platform to obtain reprocessable VPP 3D printed parts.

Several recent works have demonstrated other heat-mediated polymerizations in a dual-cure system. Wiggins et al. reported using a benzoxazine (BOX)-containing dimethacrylate, which undergo sequential free-radical photopolymerization (1) and ring-opening polymerization of BOX (17).⁷⁸ Monofunctional acrylate reactive diluent was involved to lower viscosity for VPP 3D printing. The printed part was subjected to thermal treatment (200 °C, 1 h) to polymerize BOX functional groups (17) and form the resulting interpenetrating polymer network. The aromatic linkages from BOX resulted in high-moduli materials. The fully cured polymer exhibited an increase in T_g (from 32 °C to 106 °C), storage modulus (from 3.3 GPa to 4.7 GPa), and rubbery modulus (10.0 to 43.3 MPa) compared to the green part, as the BOX polymerization drastically increased the crosslink density. Ge et al. leveraged a dual-cure system to fabricate moldable parts, where the secondary curing set the final shape and material properties. They used glycidyl methacrylate and poly(ethylene glycol) dimethacrylate in the first stage, where the part was remoldable. The final shape was set by using thermally triggered etherification of the epoxide (5).⁷⁹ The final polymers had T_gs of 67 °C to 105 °C and Young's modulus up to 1607 MPa with low volumetric shrinkage (0.58%). Griesser et al. employed polymerization of alkyne as the heat-cure in their dual-cure system.⁸⁰ Their bispropargyl monomer, 4,4′-(propane-2,2-diyl)bis((ethynyloxy)benzene), underwent polymerization by thermal rearrangement at 170 °C, forming heterocyclic alkene linkages. Aromatic propargyl resin is an alternative to epoxy resin with higher heat resistance possessing T_g's upwards of 300 °C. The heat-cured propargyl resin had high thermal resistance, showing T_gs of 172–220 °C.

Long et al. used ionic salts of polyamic acid (PAA) to achieve a VPP printed all-aromatic polyimide.⁸¹ The resin comprised ionically bonded 2-(dimethylamino)ethyl methacrylate (DMAEMA) and PAA. N-Methyl-2-pyrrolidone solvent was added to solubilize PAA, which was removed during the thermal post-print treatment at 400 °C. During the heating process, an imidization reaction took place to obtain all-aromatic polyimide, which had excellent chemical and thermal resistance. A potential shortcoming is high volumetric shrinkage (48%) from the first-stage organogel to the final parts.

Thiol-acrylate (simultaneous 1 and 18),^82–85 thiol–ene (18),⁸⁶ and thiol-Michael⁸⁴ (21) have also been used as the first-stage polymerizations in VPP. The resulting polymers are often soft due to the flexible thioether linkages, which can be attractive to fabricate elastomeric materials. Mengüç et al. developed an orthogonal reaction system of thiol–ene polymerization, followed by silicone condenstation reactions to obtain tough silicone double networks.⁸⁶ The printed parts exhibited low elastic modulus (E_100% < 700 kPa), large elongation-at-break (dL/L₀ up to ∼400%), and high toughness (U ∼ 1.4 MJ m⁻³). The silicone condensation reaction proceeded at room temperature in the presence of a tin catalyst in ∼16 hours, which demonstrated a sufficient pot life for the 3D printing process.

Multimaterials

An exciting field for polymer chemistry involves the fabrication of “multimaterials”—materials with spatially varied properties. While multimaterials have been demonstrated by dispensing methods (e.g., extrusion-based 3D printing, polyjet printing), VPP can be advantageous in high printing resolution, fine feature sizes, and absence of layer interfaces, collectively rendering the potential to deliver complexities that mimic biological systems.⁸⁷ It has been long perceived as an impossible challenge to obtain varied material properties from a single composition of polymerizable monomers. Recently, this challenge has been overcome by several orthogonal neat polymerization systems.

Work pioneered by Boydston et al.⁸⁸ and Hawker et al.⁸⁹ focused on using two light sources with varied wavelengths to independently control the reaction of acrylates (1) and epoxy (5). The occurrence of epoxy polymerization was binarily switched on or off, based on the generation of a photo-acid catalyst at a shorter wavelength; the reaction conversion of acrylate was precisely controlled based on the dosage of the longer wavelength light. In both cases, spatially resolved, gradient material properties were achieved, though the landscape of property disparity was limited to five orders of magnitude (Young's modulus). Schlögl et al. utilized two-wavelength 3D printing to make a multimaterial using 405 nm wavelength to cure acrylate and thiols (1, 18) and 365 nm wavelength to selectively initiate a 2 + 2 photocycloaddition (3) of the coumarin chromophores.⁹⁰ An increase in T_g of 17 °C was achieved as crosslinking density increased, and Young's modulus ranged from 39.9 ± 10.7 to 285.4 ± 103.6 MPa. In these two dual-cure systems, the unreacted monomers may limit material durability, causing the multimaterials to be only transient and less practically useful.

Tertiary reaction systems have been developed to mitigate the issues of unreacted monomers. Qi et al. pioneered an orthogonal system comprising acrylates, epoxies, and amines, all of which participated in sequential polymerization reactions of varied mechanisms.⁹¹ Following acrylate photopolymerization (1) at a controlled conversion, three concurrent reactions occurred: (i) the remaining acrylates underwent aza-Michael addition (13) with the amines; (ii) epoxy-amine reactions (10); (iii) base-mediated epoxy anionic homopolymerization (5) (Fig. 5A). Since the aza-Michael and epoxy-amine are competing reactions, a potential shortcoming of this resin system is that some acrylate may remain in the final material and may eventually react. An impressive range of physical properties was achieved, with T_g's ranging from 14 to 68 °C and Young's modulus ranging from glassy (1.2 GPa) to rubbery (1.4 MPa). The vast range of properties was visualized well in Fig. 5B, where a lattice with a gradient of properties was compressed in the X and Z dimensions, showing isotropic and anisotropic deformations, respectively.

Fig. 5 (A) Ternary acrylate, amine, epoxy system developed by Qi et al. High light doses react most of the (meth)acrylates, leaving epoxy amines to react in the heat curing stage. Low light doses leave acrylates to undergo Michael addition with amines, with remaining epoxy undergoing homopolymerizion with ROP. (B) 3D printed lattice structure with different regions of stiffness showing that pressing from a different axis (X vs. Z) can highlight different stiffness regions. From [X. Kuang, J. Wu, K. Chen, Z. Zhao, Z. Ding, F. Hu, D. Fang, H. J. Qi, Grayscale Digital Light Processing 3D Printing for Highly Functionally Graded Materials. Sci. Adv., 2019, 5(5), eaav5790. DOI: 10.1126/sciadv.aav5790]. © The Authors, some rights reserved; exclusive licensee AAAS. Distributed under a CC BY-NC 4.0 license https://creativecommons.org/licenses/by-nc/4.0/”. Reproduced from ref. 91 with permission from AAAS, copyright 2024.

Although not intended to fabricate multimaterials, Serra et al. demonstrated a stable intermediate material that comprises thiol, acrylates, and epoxy monomers.⁹² A combination of free-radical thiol-acrylate (1 & 18) and thiol-Michael addition (21) reactions was performed in the first stage. Heat was then applied; thiol-epoxy (20) and epoxy homopolymerization (5) underwent. Huang and Wallin et al. modified Serra's approach for the design of multimaterials. Their tertiary orthogonal reaction system comprises thiol, allyl ether, and epoxy monomers, which proceed through various controlled polymerizations sequentially. The thiol–ene photopolymerization (18) was carried out as the first step, for which both thiol and alkene conversions were controlled by light dosage (Fig. 6A). Subsequently, upon heating the remaining thiol reacted with epoxy (20). Last, the remaining epoxy underwent anionic homopolymerization (5).⁹³ In this study, the two heat-mediated reactions are kinetically orthogonal, i.e., epoxy homopolymerization only occurs after the completion of the thiol-epoxy reaction, which offers wider attainable material properties. Young's modulus had an impressive range of 400 kPa to 1.6 GPa with elongation at break going from 300% to 3%, respectively. While the softest 400 kPa region was not VPP 3D printable due to no UV exposure, a 160 MPa Young's modulus was achievable, allowing for an order of magnitude of Young's modulus in properties as demonstrated by a braille display with soft and hard regions. In addition, the unreacted allyl ether remained stable in the final materials due to their incapability of free-radical homopolymerization, which resulted in excellent long-term durability. The authors demonstrated that the material properties were stable under 1150 hours of equivalent solar irradiation.

Fig. 6 (A) A ternary allyl, thiol, and epoxy multimaterial system developed by Huang and Wallins et al. is shown. High light dosage reacts most of the thiol–ene functional groups leaving epoxy homopolymerization to produce a highly crosslinked system giving stiff regions. Using lower light dosage, unreacted thiols remain, which can react with the epoxy. The remaining epoxy then undergoes homopolymerization. (B) Stress versus strain curve highlights how the postcuring temperature changes the bulk polymer from being stiff to rubbery, with pictures of the two extremes below. Reproduced with permission from ref. 94. {W. H. Hu, M. Tenjimbayashi, S. Wang, Y. Nakamura, I. Watanabe, M. Naito, Postprogrammable Network Topology with Broad Gradients of Mechanical Properties for Reliable Polymer Material Engineering, Chem. Mater., 2021, 33(17), 6876–6884}. Copyright {2021} American Chemical Society.

In addition, multimaterials have been demonstrated by neat polymerizations conducted with thermal gradients. Naito et al. achieved three orders of magnitude difference in mechanical properties by combining epoxy-amine (10) reactions with thiol–ene (18) reactions using protected thiols (disulfide).⁹⁴ The epoxy-amine reactions were completed at 120 °C, followed by the sample being partially submerged in a sand bath at 190 °C to obtain a heat gradient, selectively cleaving the disulfides. Young's modulus changed from 10 MPa to 5.8 GPa when fully post cured, with excellent elongation-at-breaks of 50–70% (Fig. 6B). Similar to the work done by Huang and Wallin et al., the unreacted allyl groups remained after the initial cure as shown by X-Ray photoelectron spectroscopy.

Ionization radiation has also been demonstrated to initiate orthogonal reactions to fabricate multimaterials. Schmidt et al. produced a functionally graded multimaterial with an initial stage using epoxy-anhydride (9) with unsaturated norbornene, allyl, and alkene bonds being radiated during postcuring (1).⁹⁵ An unsaturated liquid rubber was added possessing allyl and alkene bonds to allow for further material gradients. Once the initial epoxy-anhydride was cured, different amounts of ionizing radiation were applied to give a maximum gradient range of 0.3 to 2.2 GPa Young's modulus.

Optical materials

Holographic materials are crucial for applications such as 3D displays, data storage, and anticounterfeiting.^96–98 To obtain holographic materials, orthogonal neat polymerizations are an important tool for fabricating permanent interference patterns. It is desirable to couple the precise spatial controllability of photopolymerization and the heat-mediated polymerization systems to achieve the desired controllable properties such as refractive index (RI) modulation. Photocuring is optimal to achieve RI modulation since the ease of spatially resolving light irradiation and light dosage. The first example of using orthogonal reactions is a 2011 patent reported by Cole et al. utilizing acrylate photopolymerizations (1) and alcohol-isocyanate reactions (14).⁹⁹ A UV light filter was employed to irradiate only at patterned areas (hundreds of nanometers in size). In the irradiated areas, the monomers are concentrated by the diffusion of additional monomers from the adjacent dark areas, producing spatially varied monomer concentrations that result in two distinctly different RIs (Fig. 7B). Last, the alcohol-isocyanate reaction takes place to fully cure the polymer and set the mechanical properties. Zhao et al. improved Cole's system by custom-synthesized (meth)acrylate monomers to achieve a high ΔRI of 0.046 (Fig. 7C).¹⁰⁰ Other orthogonal reactions include Zheng et al. using acrylate (1) and epoxy (5) to achieve RI differences.¹⁰¹

Fig. 7 (A) Dual-cure optical grating is shown using acrylates, alcohols, and isocyanates. Zhao et al. used high RI acrylates to modify this system, which undergoes alcohol isocyanate (14) in the first heat curing step, then acrylate (1) polymerization with UV exposure. A gradient is obtained with acrylate diffusing towards the UV exposure area. (B) Writing mechanism is shown using monomer diffusion gradients, along with UV light to form two regions with different refractive indexes. Reproduced (adapted) with permission from ref. 103 {Y. Hu, S. Mavila, M. Podgórski, J. E. Kowalski, R. R. McLeod, C. N. Bowman, Manipulating the Relative Rates of Reaction and Diffusion in a Holographic Photopolymer Based on Thiol–Ene Chemistry, Macromolecules 2022, 55(5), 1822–1833}. Copyright {2022} American Chemical Society. (C) Grated picture of a model house after recording a reflected hologram illumination shown in a dark room. Reprinted with permission from ref. 100 {B. Guo, M. Wang, D. Zhang, M. Sun, Y. Bi, Y. Zhao, High Refractive Index Monomers for Improving the Holographic Recording Performance of Two-Stage Photopolymers, ACS Appl. Mater. Interfaces, 2023, 15(20), 24827–24835}. Copyright {2023} American Chemical Society.

Work by Bowman et al. used thiol-Michael (21) and thiol–yne (19) reactions in order to mitigate the shrinkage issues found in the acrylate systems.¹⁰² First, multifunctional acrylates were reacted with excess thiol in an off-stoichiometric reaction. UV light grading was then applied, and diffusion of alkyne monomers was obtained to gain regioselective areas with differed refractive indexes. ΔRI up to 0.0036 was achieved with high diffractive efficiencies of 96% for holograms. Meanwhile, lower shrinkage than that in a typical acrylate system was achieved. In another work, Bowman et al. used different reactivity of primary vs. secondary thiols for thiol–ene (18) click chemistry for region selectivity, achieving a ΔRI of 0.028.¹⁰³ Very recently, Bowman et al. used two-stage curing with thiol–ene (18), first followed by breaking cross-links via disulfide cleavage, which subsequently reacted with the remaining allyl groups for additional crosslinking.¹⁰⁴ Although a lower ΔRI (0.0022) was achieved, the disulfide system was advantageous in showing nearly zero haze, which is critical for optical material applications.

Photo-responsive intermediate materials

In the abovementioned 3D printing and optical materials, photo-curing is carried out as the first stage. In some cases, photo-curing is preferably carried out as the second stage. Benefits include synthesizing bench-stable intermediate materials that are responsive to UV light. Stable intermediate materials are potentially attractive in applications such as moldable materials because the intermediate polymers can be free of small molecules. Therefore, issues of many small-molecule monomers can be avoided, such as toxicity, volatility, and regulatory limitations. Bowman et al. developed systems of off-stoichiometric thiol-Michael polymerization (21) followed by acrylate homopolymerization (1) and demonstrated their unique applications as shape memory polymers.¹⁰⁵ Recently, Ramis et al. demonstrated such a dual-cure system by an off-stoichiometric aza-Michael polymerization (13) to obtain a crosslinked stable intermediate material that contains unreacted acrylates (Fig. 8).¹⁰⁶ The excess acrylates were triggered by UV light to allow polymerization (1) to obtain a mechanically robust, stiff material.

Fig. 8 Dual-cure of acrylate and amines using aza-Michael addition in the first stage gives a stable partially cured material. The second stage involves photocuring to give the final material.

Ding et al. used thiol-Michael (21) addition with acrylates followed by acrylate homopolymerization (1) for CO₂ membranes.¹⁰⁷ This orthogonality allowed for spatially controlled permeability and stiffness by first curing the thiol-Micheal followed by spatially controlled UV irradiation.

Discussion and outlook

We desire precise, independent control of the occurrence and the termination of a neat polymerization reaction. Ideally, we want sufficient time to process the monomers into a device (i.e. “pot life”), and then the monomers react efficiently, quantitively, and safely. In a solution polymerization system, it is common to initiate polymerization by the addition of an initiator or catalyst. To terminate a polymerization, it is straightforward to add of inhibitor, or simply to isolate the polymer product out of the system. It is also common for a solution polymerization to tune reactivity by the concentration of monomers. Unfortunately, none of those options applies to a neat polymerization. Therefore, it is often a dilemma between the polymerization reactivity and the pot life, which we believe leads to the popularity of photo-mediated reactions since they have robust control by the application of incident light.

We argue that orthogonal reactions can provide unique opportunities for optimizing both reactivity and pot life. If the triggering mechanisms are independent, each reaction can be separately controlled to achieve balanced efficiency and safety. As we survey recent reports of orthogonal reactions, we identify a few opportunities for reaction combinations that appear reasonable but have not been reported yet. For one example, the ring-opening polymerization of lactone (6) is typically done in neat conditions, such as the synthesis of polycaprolactone.¹⁰⁸ We envision that free-radical polymerizations will be orthogonal, which may enable a wide design landscape of polymers that exhibit biodegradability similar to that of polycaprolactone. Another example has shown that the cyanate ester reaction (11) is orthogonal with free-radical acrylate polymerization.¹⁰⁹ We envision that cyanate ester can also be orthogonal to the diverse thiol-based polymerization reactions, which could help mitigate the shortcomings of thiol-based polymers due to the flexible thioether bonds. The library of useful combinations will continue to expand.

The ability to direct different reaction pathways from a single monomer composition is at the cutting edge of the field. The abovementioned work of multimaterials all rely on multiple, tandem reactions that are tuned by the degree of the first photopolymerization reaction. Recent work by Page et al. showed that reaction pathways could be directed by selective activation of catalysts.¹¹⁰ Two ROMP catalysts were employed to form cis-polycyclooctene (amorphous) in the dark but form trans-polycyclooctene (semi-crystalline) when exposed to light. As a result, patterned multimaterials were fabricated from a singular monomer composition. Considering the rich literature on the catalyst design for controlled free-radical polymerization, especially the recently developed photo-redox catalysts,¹¹¹ we envision that free-radical neat polymerization of (meth)acrylate will also be conducted in a pathway-selective manner. Multimaterials are exciting since they have the potential to redefine material design. Enabling the synthesis of multimaterials from commercially abundant monomer feedstocks will likely revolutionize the plastics industry.

Beyond chemical means of controlling neat polymerization reactions, we would like to discuss physical means. In most cases, neat polymerizations are conducted from homogenous monomer mixtures, where each component is soluble in each other. It is possible to design orthogonal reactions by physically limiting the solubility of a reactive component, which can only be reactive when it becomes soluble. Such a design has commercial success in creating a one-part mixture of epoxy-amine monomer mixture with a nearly infinitely long pot life. For example, dicyandiamide (DICY) is a latent curing agent for epoxy because it has poor solubility and a melting temperature of 130 °C, which is often used in the form of microparticles to a stable suspension in epoxy monomers.¹¹² Mortezaei et al. designed a dual-cure system comprising epoxy-amine-DICY, where the regular epoxy-amine reaction reached the gel point, and only upon heating the epoxy-DICY curing could occur.¹¹³ This two-stage curing is potentially useful to create room-temperature stable carbon fiber prepreg materials, which typically must be stored in a freezer to tame the reactivity of epoxy-amine reactions.

Ample opportunities exist by coupling covalently reversible chemistry and secondary curing. For example, Cooper et al. has shown that network polymers can be molecularly altered during the second-stage cure to unlock a wide range of material properties that are unattainable with a single chemistry (Fig. 3).⁶³ For another example, Moradi et al. and Serra et al. demonstrated two distinct relaxation behaviors were possible by combing two types of reversible bonds, introduced by dual-cure.^114,115 Since the development of dynamic covalent chemistry in polymer science has been one of the most popular research topics, it is reasonable to foresee other orthogonal chemistries being developed as a means to alter network structures and properties.

In conclusion, we summarize recent development of implementing orthogonal polymerization reactions in neat conditions for various emerging applications. We hope this Minireview provides useful insights into the molecular design of orthogonal systems for specific applications. We are excited to witness more innovations in this field to unfold.

Author contributions

Conceptualization: C. W.; writing – original draft: C. J. R., G. M. M., and C. W.; writing – review & editing: C. J. R., G. M. M., and C. W.; funding acquisition: C. W.; supervision: C. W.

Data availability

No primary research results, software or code have been included and no new data were generated or analysed as part of this review.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work is supported by the start-up fund provided by the Price College of Engineering at the University of Utah. C. W. and C. J. R. thank Meta Inc for a gift that was donated to the University of Utah.

References

1. B. Garvey Jr and W. White, Ind. Eng. Chem., 1933, 25, 1042–1046.
2. L. H. Baekeland, Ind. Eng. Chem., 1909, 1, 149–161.
3. J. Zhang and P. Xiao, Polym. Chem., 2018, 9, 1530–1540.
4. G. Barany and R. Merrifield, J. Am. Chem. Soc., 1977, 99, 7363–7365.
5. M. Schelhaas and H. Waldmann, Angew. Chem., Int. Ed. Engl., 1996, 35, 2056–2083.
6. A. Isidro-Llobet, M. Álvarez and F. Albericio, Chem. Rev., 2009, 109, 2455–2504.
7. E. Elacqua, D. S. Lye and M. Weck, Acc. Chem. Res., 2014, 47, 2405–2416.
8. C.-H. Wong and S. C. Zimmerman, Chem. Commun., 2013, 49, 1679–1695.
9. S.-L. Li, T. Xiao, C. Lin and L. Wang, Chem. Soc. Rev., 2012, 41, 5950–5968.
10. E. M. Sletten and C. R. Bertozzi, Acc. Chem. Res., 2011, 44, 666–676.
11. N. Corrigan, M. Ciftci, K. Jung and C. Boyer, Angew. Chem., Int. Ed., 2021, 60, 1748–1781.
12. A. Shahrokhinia, P. Biswas and J. F. Reuther, J. Polym. Sci., 2021, 59, 1748–1786.
13. S. Liu, K. T. Dicker and X. Jia, Chem. Commun., 2015, 51, 5218–5237.
14. P. Jacobs, SPIE, 1994, vol. 2102, pp. 41–52.
15. R. Geyer, J. R. Jambeck and K. L. Law, Sci. Adv., 2017, 3, e1700782.
16. K. S. Whiteley, T. G. Heggs, H. Koch, R. L. Mawer and W. Immel, in Ullmann's Encyclopedia of Industrial Chemistry, Wiley-VCH Verlag GmbH & Co. KGaA, Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim, Germany, 2000, pp. 1–103.
17. A. O. Konuray, F. Di Donato, M. Sangermano, J. Bonada Bo, X. Fernández Francos, M. À. Serra Albet and X. Ramis Juan, eXPRESS Polym. Lett., 2020, 14, 881–884.
18. X. Kuang, Z. Zhao, K. Chen, D. Fang, G. Kang and H. J. Qi, Macromol. Rapid Commun., 2018, 39, 1700809.
19. K. Studer, C. Decker, E. Beck and R. Schwalm, Eur. Polym. J., 2005, 41, 157–167.
20. L. Delaude and A. F. Noels, in Kirk-Othmer Encyclopedia of Chemical Technology, John Wiley & Sons, Inc., Hoboken, NJ, USA, 2005.
21. P. Atallah, K. B. Wagener and M. D. Schulz, Macromolecules, 2013, 46, 4735–4741.
22. A. Wolfberger, B. Rupp, W. Kern, T. Griesser and C. Slugovc, Macromol. Rapid Commun., 2011, 32, 518–522.
23. S. Poplata, A. Tröster, Y.-Q. Zou and T. Bach, Chem. Rev., 2016, 116, 9748–9815.
24. G. J. Berg, T. Gong, C. R. Fenoli and C. N. Bowman, Macromolecules, 2014, 47, 3473–3482.
25. C. García-Astrain, A. Gandini, D. Coelho, I. Mondragon, A. Retegi, A. Eceiza, M. A. Corcuera and N. Gabilondo, Eur. Polym. J., 2013, 49, 3998–4007.
26. Q. Tian, M. Z. Rong, M. Q. Zhang and Y. C. Yuan, Polym. Int., 2010, 59, 1339–1345.
27. C. Jerome and P. Lecomte, Adv. Drug Delivery Rev., 2008, 60, 1056–1076.
28. A.-L. Brocas, C. Mantzaridis, D. Tunc and S. Carlotti, Prog. Polym. Sci., 2013, 38, 845–873.
29. T. Eren, S. H. Küsefoğlu and R. Wool, J. Appl. Polym. Sci., 2003, 90, 197–202.
30. I. E. Dell'Erba and R. J. J. Williams, Polym. Eng. Sci., 2006, 46, 351–359.
31. Y. Tanaka and H. Kakiuchi, J. Appl. Polym. Sci., 1963, 7, 1063–1081.
32. O. Konuray, J. M. Morancho, X. Fernández-Francos, M. García-Alvarez and X. Ramis, Thermochim. Acta, 2021, 701, 178963.
33. C. Fu, J. Xu, M. Kokotovic and C. Boyer, ACS Macro Lett., 2016, 5, 444–449.
34. J. A. Carioscia, J. W. Stansbury and C. N. Bowman, Polymer, 2007, 48, 1526–1532.
35. G. G. Odian, Principles of polymerization, Wiley-Interscience, Hoboken, N.J, 4th edn, 2004.
36. I. Hamerton, Chemistry and technology of cyanate ester resins, Springer Science & Business Media, 2012.
37. M. E. Belowich and J. F. Stoddart, Chem. Soc. Rev., 2012, 41, 2003.
38. H. C. Kolb, M. G. Finn and K. B. Sharpless, Angew. Chem., Int. Ed., 2001, 40, 2004–2021.
39. Y. Liu, Y. Li, D. Keskin and L. Shi, Adv. Healthcare Mater., 2019, 8, 1801359.
40. D.-J. Liaw, K.-L. Wang, Y.-C. Huang, K.-R. Lee, J.-Y. Lai and C.-S. Ha, Prog. Polym. Sci., 2012, 37, 907–974.
41. F. M. De Souza, P. K. Kahol and R. K. Gupta, in ACS Symposium Series, ed. R. K. Gupta and P. K. Kahol, American Chemical Society, Washington, DC, 2021, vol. 1380, pp. 1–24.
42. H. Khatoon, S. Iqbal, M. Irfan, A. Darda and N. K. Rawat, Prog. Org. Coat., 2021, 154, 106124.
43. J. O. Akindoyo, M. D. H. Beg, S. Ghazali, M. R. Islam, N. Jeyaratnam and A. R. Yuvaraj, RSC Adv., 2016, 6, 114453–114482.
44. Y.-X. Wang and H. Ishida, Polymer, 1999, 40, 4563–4570.
45. M. Bajpai, V. Shukla and A. Kumar, Prog. Org. Coat., 2002, 44, 271–278.
46. C. E. Hoyle and C. N. Bowman, Angew. Chem., Int. Ed., 2010, 49, 1540–1573.
47. B. T. Worrell, S. Mavila, C. Wang, T. M. Kontour, C.-H. Lim, M. K. McBride, C. B. Musgrave, R. Shoemaker and C. N. Bowman, Polym. Chem., 2018, 9, 4523–4534.
48. M. Podgórski, S. Mavila, S. Huang, N. Spurgin, J. Sinha and C. N. Bowman, Angew. Chem., Int. Ed., 2020, 59, 9345–9349.
49. A. S. Goldmann, M. Glassner, A. J. Inglis and C. Barner-Kowollik, in Chemistry of Organo-Hybrids, ed. B. Charleux, C. Copéret and E. Lacôte, John Wiley & Sons, Inc., Hoboken, NJ, USA, 2015, pp. 395–465.
50. S. Huang, M. Podgórski, X. Han and C. N. Bowman, Polym. Chem., 2020, 11, 6879–6883.
51. F. Gamardella, X. Ramis, S. De la Flor and À. Serra, React. Funct. Polym., 2019, 134, 174–182.
52. Z. Wen, X. Han, B. D. Fairbanks, K. Yang and C. N. Bowman, Polymer, 2020, 202, 122715.
53. L. Li, X. Chen and J. M. Torkelson, Macromolecules, 2019, 52, 8207–8216.
54. C.-J. Fan, Z.-B. Wen, Z.-Y. Xu, Y. Xiao, D. Wu, K.-K. Yang and Y.-Z. Wang, Macromolecules, 2020, 53, 4284–4293.
55. F. Gamardella, S. De la Flor, X. Ramis and A. Serra, React. Funct. Polym., 2020, 151, 104574.
56. M. A. S. R. Saadi, A. Maguire, N. T. Pottackal, M. S. H. Thakur, M. Md. Ikram, A. J. Hart, P. M. Ajayan and M. M. Rahman, Adv. Mater., 2022, 34, 2108855.
57. H. Baniasadi, R. Abidnejad, M. Fazeli, J. Lipponen, J. Niskanen, E. Kontturi, J. Seppälä and O. J. Rojas, Adv. Colloid Interface Sci., 2024, 324, 103095.
58. F. Zhang, L. Zhu, Z. Li, S. Wang, J. Shi, W. Tang, N. Li and J. Yang, Addit. Manuf., 2021, 48, 102423.
59. C. E. Hoyle, in Radiation Curing of Polymeric Materials, American Chemical Society, 1990, vol. 417, pp. 1–16.
60. Y. Jian, Y. He, T. Jiang, C. Li, W. Yang and J. Nie, J. Polym. Sci., Part B: Polym. Phys., 2012, 50, 923–928.
61. D. A. Abu-elenain, S. H. Lewis and J. W. Stansbury, Dent. Mater., 2013, 29, 1173–1181.
62. V. S. D. Voet, J. Guit and K. Loos, Macromol. Rapid Commun., 2021, 42, 2000475.
63. S. Velankar, J. Pazos and S. L. Cooper, J. Appl. Polym. Sci., 1996, 62, 1361–1376.
64. J. Poelma and J. Rolland, Science, 2017, 358, 1384–1385.
65. Carbon Dual-Cure Materials, https://www.carbon3d.com/resources/whitepaper/carbon-dual-cure-materials, (accessed June 14, 2024).
66. K. Chen, X. Kuang, V. Li, G. Kang and H. J. Qi, Soft Matter, 2018, 14, 1879–1886.
67. R. Yu, X. Yang, Y. Zhang, X. Zhao, X. Wu, T. Zhao, Y. Zhao and W. Huang, ACS Appl. Mater. Interfaces, 2017, 9, 1820–1829.
68. G. Griffini, M. Invernizzi, M. Levi, G. Natale, G. Postiglione and S. Turri, Polymer, 2016, 91, 174–179.
69. J. Casado, O. Konuray, A. Roig, X. Fernández-Francos and X. Ramis, Eur. Polym. J., 2022, 173, 111256.
70. K. Deng, C. Zhang and K. (Kelvin) Fu, Composites, Part B, 2023, 257, 110671.
71. X. Kuang, Z. Zhao, K. Chen, D. Fang, G. Kang and H. J. Qi, Macromol. Rapid Commun., 2018, 39, 1700809.
72. Z. Chen, M. Yang, M. Ji, X. Kuang, H. J. Qi and T. Wang, Mater. Des., 2021, 197, 109189.
73. O. Konuray, A. Sola, J. Bonada, A. Tercjak, A. Fabregat-Sanjuan, X. Fernández-Francos and X. Ramis, Materials, 2021, 14, 4544.
74. A. M. Dlugaj, P. A. Arrabiyeh, M. Eckrich and D. May, ACS Appl. Polym. Mater., 2024, 6, 2902–2912.
75. A. Roig, X. Ramis, S. De la Flor and À. Serra, Eur. Polym. J., 2023, 200, 112499.
76. D. Montarnal, M. Capelot, F. Tournilhac and L. Leibler, Science, 2011, 334, 965–968.
77. B. Zhang, K. Kowsari, A. Serjouei, M. L. Dunn and Q. Ge, Nat. Commun., 2018, 9, 1831.
78. J. J. Weigand, C. I. Miller, A. P. Janisse, O. D. McNair, K. Kim and J. S. Wiggins, Polymer, 2020, 189, 122193.
79. B. Zhang, A. Serjouei, Y.-F. Zhang, J. Wu, H. Li, D. Wang, H. Y. Low and Q. Ge, Chem. Eng. J., 2021, 411, 128466.
80. K. Sommer, P. Rieger, S. M. Müller, R. Schwarz, G. Trimmel, M. Feuchter and T. Griesser, Adv. Eng. Mater., 2023, 25, 2200901.
81. J. Herzberger, V. Meenakshisundaram, C. B. Williams and T. E. Long, ACS Macro Lett., 2018, 7, 493–497.
82. E. Rossegger, R. Höller, D. Reisinger, J. Strasser, M. Fleisch, T. Griesser and S. Schlögl, Polym. Chem., 2021, 12, 639–644.
83. U. Shaukat, E. Rossegger and S. Schlögl, Polymer, 2021, 231, 124110.
84. Y. Jian, Y. He, Y. Sun, H. Yang, W. Yang and J. Nie, J. Mater. Chem. C, 2013, 1, 4481–4489.
85. D. Böcherer, Y. Li, C. Rein, S. Franco Corredor, P. Hou and D. Helmer, Adv. Funct. Mater., 2024, 2401516.
86. T. J. Wallin, L.-E. Simonsen, W. Pan, K. Wang, E. Giannelis, R. F. Shepherd and Y. Mengüç, Nat. Commun., 2020, 11, 4000.
87. A. Bandyopadhyay and B. Heer, Mater. Sci. Eng., R, 2018, 129, 1–16.
88. G. I. Peterson, J. J. Schwartz, D. Zhang, B. M. Weiss, M. A. Ganter, D. W. Storti and A. J. Boydston, ACS Appl. Mater. Interfaces, 2016, 8, 29037–29043.
89. N. D. Dolinski, Z. A. Page, E. B. Callaway, F. Eisenreich, R. V. Garcia, R. Chavez, D. P. Bothman, S. Hecht, F. W. Zok and C. J. Hawker, Adv. Mater., 2018, 30, 1800364.
90. E. Rossegger, J. Strasser, R. Höller, M. Fleisch, M. Berer and S. Schlögl, Macromol. Rapid Commun., 2023, 44, 2200586.
91. X. Kuang, J. Wu, K. Chen, Z. Zhao, Z. Ding, F. Hu, D. Fang and H. J. Qi, Sci. Adv., 2019, 5, eaav5790.
92. A. Roig, X. Ramis, S. De la Flor and À. Serra, Polymer, 2021, 230, 124073.
93. S. Huang, S. M. Adelmund, P. S. Pichumani, J. J. Schwartz, Y. Mengüç, M. Shusteff and T. J. Wallin, Matter, 2023, 6, 2419–2438.
94. W. H. Hu, M. Tenjimbayashi, S. Wang, Y. Nakamura, I. Watanabe and M. Naito, Chem. Mater., 2021, 33, 6876–6884.
95. W. Xia, S. B. Hurvitz, J. Y. Lee, S. E. Stapleton and D. F. Schmidt, ACS Appl. Polym. Mater., 2023, 5, 4849–4858.
96. P.-A. Blanche, A. Bablumian, R. Voorakaranam, C. Christenson, W. Lin, T. Gu, D. Flores, P. Wang, W.-Y. Hsieh, M. Kathaperumal, B. Rachwal, O. Siddiqui, J. Thomas, R. A. Norwood, M. Yamamoto and N. Peyghambarian, Nature, 2010, 468, 80–83.
97. E. L. Prime and D. H. Solomon, Angew. Chem., Int. Ed., 2010, 49, 3726–3736.
98. R. R. McLeod, A. J. Daiber, T. Honda, M. E. McDonald, T. L. Robertson, T. Slagle, S. L. Sochava and L. Hesselink, Appl. Opt., 2008, 47, 2696–2707.
99. M. C. Cole, F. R. Askham and W. L. Wilson, US Pat., US7704643B2, 2010.
100. B. Guo, M. Wang, D. Zhang, M. Sun, Y. Bi and Y. Zhao, ACS Appl. Mater. Interfaces, 2023, 15, 24827–24835.
101. Y. Liu, T. Shen, B. Kang, S. Li, Z. Zheng, H. Lv and J. Zheng, Opt. Laser Technol., 2022, 156, 108580.
102. H. Peng, C. Wang, W. Xi, B. A. Kowalski, T. Gong, X. Xie, W. Wang, D. P. Nair, R. R. McLeod and C. N. Bowman, Chem. Mater., 2014, 26, 6819–6826.
103. Y. Hu, S. Mavila, M. Podgórski, J. E. Kowalski, R. R. McLeod and C. N. Bowman, Macromolecules, 2022, 55, 1822–1833.
104. Y. Hu, S. M. Soars, B. E. Kirkpatrick, M. Podgorski, N. Bongiardina, B. D. Fairbanks, K. S. Anseth and C. N. Bowman, Macromolecules, 2023, 56, 9778–9786.
105. D. P. Nair, N. B. Cramer, J. C. Gaipa, M. K. McBride, E. M. Matherly, R. R. McLeod, R. Shandas and C. N. Bowman, Adv. Funct. Mater., 2012, 22, 1502–1510.
106. G. González, X. Fernández-Francos, À. Serra, M. Sangermano and X. Ramis, Polym. Chem., 2015, 6, 6987–6997.
107. A. K. Blevins, L. M. Cox, L. Hu, J. A. Drisko, H. Lin, C. N. Bowman, J. P. Killgore and Y. Ding, Macromolecules, 2021, 54, 44–52.
108. M. Labet and W. Thielemans, Chem. Soc. Rev., 2009, 38, 3484–3504.
109. M. S. Menyo and J. P. Rolland, US Pat., US11090859B2, 2021.
110. A. K. Rylski, H. L. Cater, K. S. Mason, M. J. Allen, A. J. Arrowood, B. D. Freeman, G. E. Sanoja and Z. A. Page, Science, 2022, 378, 211–215.
111. D. A. Corbin and G. M. Miyake, Chem. Rev., 2021, 122, 1830–1874.
112. T. Güthner and B. Hammer, J. Appl. Polym. Sci., 1993, 50, 1453–1459.
113. A. R. Pouladvand, M. Mortezaei, H. Fattahi and I. A. Amraei, Composites, Part A, 2020, 132, 105852.
114. S. Moradi, X. Fernández-Francos, O. Konuray and X. Ramis, Eur. Polym. J., 2023, 196, 112290.
115. A. Roig, X. Ramis, S. De la Flor and À. Serra, Eur. Polym. J., 2024, 206, 112782.

---

Footnote

† These authors contributed equally.

---