Onium salts as photocages and methods of using the same

Abstract

An onium photocage has a structure according to Formula (I), (II), or (III), wherein each of R1-R6, Z, A, X, and Y are as defined herein. The onium photocage can be particularly useful in facilitating photochemical reactions, for example to form polymers, provide three-dimensional additively manufactured articles, and for small molecule coupling reactions. Curable compositions and methods of using the onium photocages are therefore also described.

Description

ONIUM SALTS AS PHOTOCAGES AND METHODS OF USING THE SAMECROSS-REFERENCE TO RELATED APPLICATIONThis application claims priority to U.S. Provisional Patent Application No. 63 / 563,072, filed on March 8, 2024, the contents of which are hereby incorporated by reference in their entirety.FEDERAL RESEARCH STATEMENTThis invention was made with government support under Grant No. CHE2107877 awarded by the National Science Foundation. The government has certain rights in the invention.BACKGROUND

[0001] Light-driven chemistry has enabled numerous technologies, from lithography to produce microelectronics to emergent additive manufacturing in the fabrication of medical devices. These transformative discoveries rely on the spatiotemporal control offered by light in polymer synthesis. Light-induced radical polymerizations have dominated over ionic ones owing to their lower photoinitiator cost, excellent temporal control, and faster, application relevant, reaction speeds (e.g., completion in < 60 seconds). However, radical-based photopolymerizations are principally limited in scope to acrylics.

[0002] Anionic photopolymerizations can greatly diversify the portfolio of accessible polymers. However, a lack of temporal control has so far precluded advanced manufacturing of materials using anionic polymerization.

[0003] It would therefore be advantageous to provide new compositions and methods enabling temporal activation of addition reactions, including anionic polymerizations. It would be further advantageous to provide a system that uses a low intensity visible light without the need for additional sensitizing agents and that limits or precludes undesirable decarboxylation.SUMMARY

[0004] An aspect of the present disclosure is an onium salt having a structure according to Formula (I), (II), or (III),wherein in each of the foregoing Formulas (I) to (III): R1and R2are independently at each occurrence hydrogen, a substituted or unsubstituted Ci-6 alkyl group, a substituted or unsubstituted Ce-i2aryl group, a substituted or unsubstituted Ci-6 alkoxy group, a substituted or unsubstituted Ci-6 thioetheralkyl group, or a substituted or unsubstituted amino group; R3, R4, and R5are independently at each occurrence a hydrogen, a substituted or unsubstituted Ci-6 alkyl group, a substituted or unsubstituted C6-12 aryl group, or a vinyl group; R6is independently at each occurrence fluorine, a substituted or unsubstituted C1-6 alkyl group, or a substituted or unsubstituted C6-12 aryl group; X is oxygen or sulfur; Y is independently at each occurrence hydrogen, a halogen, a substituted or unsubstituted C1-6 alkyl group, or a substituted or unsubstituted C6-12 aryl group; Z is independently at each occurrence an ammonium or phosphonium cation, wherein the ammonium cation iswherein R’ is independently at each occurrence a substituted or unsubstituted C1-6 alkyl group or a substituted or unsubstituted C1-6 alkenyl group; and the phosphonium cation iswherein R” is independently at each occurrence a substituted or unsubstituted C1-6 alkyl group or a substituted or unsubstituted C6-12 aryl group; and A is a tetraphenyl borate counteranion.

[0005] Another aspect is a curable composition comprising: the onium salt; a first monomer comprising at least two hydroxyl groups, at least two thiol groups, or a combination thereof; and a second monomer comprising at least two (meth)acrylate groups, at least two (meth)acrylamide groups, at least two vinyl sulfone groups, at least two isothiocyanate groups, at least two isocyanate groups, at least two epoxide groups, at least two carbonate groups, at least two ester groups, at least two alkynyl groups, or a combination thereof.

[0006] Another aspect is a method of forming a polymer, the method comprising: irradiating the curable composition with light having a wavelength of 350 to 1000 nanometers(nm) to provide a polymer comprising repeating units derived from addition polymerization of the first monomer and the second monomer.

[0007] Another aspect is a method of forming an object by additive manufacturing, the method comprising: irradiating a first portion of the curable composition with light having a wavelength of 350 to 1000 nm to provide a first layer comprising a polymer comprising repeating units derived from addition polymerization of the first monomer and the second monomer on a substrate; and forming at least one additional layer on the first layer by irradiating a second portion of the curable composition with light having a wavelength of 350 to 1000 nm to provide the at least one additional layer comprising a polymer comprising repeating units derived from addition polymerization of the first monomer and the second monomer to provide the object.

[0008] A cured composition derived from the curable composition or obtained by the methods described herein represents another aspect of the present disclosure.

[0009] Another aspect is a method of providing a reaction product, the method comprising: irradiating a reaction mixture with light having a wavelength of 350 to 1000 nm to provide the reaction product, wherein the reaction mixture comprises: the onium salt; a first reactant comprising at least one hydroxyl group, at least one thiol group, or a combination thereof; and a second reactant comprising at least one (meth)acrylate group, at least one (meth)acrylamide group, at least one vinyl sulfone group, at least one isothiocyanate group, at least one isocyanate group, at least one epoxide group, at least one carbonate group, at least one ester group, at least one alkynyl group, or a combination thereof.

[0010] The above described and other features are exemplified by the following figures and detailed description.BRIEF DESCRIPTION OF THE DRAWINGS

[0011] The following figures represent exemplary embodiments.

[0012] FIG. 1A shows a general synthesis of ortho-nitrobenzyl photocages. Reaction conditions: (i) base (R = DMPP, TBA, etc.), toluene, room temperature, 3 hours; (ii) 2% aqueous hydrochloric acid (HC1), tetraphenyl borate (BPlu), room temperature, 2 hours.

[0013] FIG. IB shows spectral profiles for selected photocage absorptions in dilute acetonitrile (CH3CN) (8 millimolar (mM)) overlaid with LED emission profiles for ultraviolet (UV) 365 nanometers (nm) and violet (405 nm). Inset is a photograph of the oNB-DPMP photocage.

[0014] FIG. 2 shows generation of methyldiphenylphosphine oxide upon irradiation with 100 milliwatt per square centimeter (mW / cm2) 405 nm light monitored by phosphorus nuclear magnetic resonance (31P NMR) spectroscopy.

[0015] FIG. 3 A shows a schematic illustration of a transmission FTIR setup.

[0016] FIG. 3B shows photopolymerization kinetics shown by isocyanate (NCO) conversion (p) vs. time (s) for oNB photocages using a violet LED (Xmax = 405 nm) at an intensity of 20 mW / cm2.

[0017] FIG. 3C shows photopolymerization with a 405 nm LED at variable intensity for oNB-DPMP (0.5 mole percent (mol%)) with 2:1 HMDI (diisocyanate):PETMP (tetrathiol).

[0018] FIG. 3D shows temporal control of oNB-DPMP against commercially available guanidine-caged xanthone photocages and TMG photocages with light (20 mW / cm2, 405 nm) turned on at 300 seconds.

[0019] FIG. 4A is a chemical scheme illustrating hard and soft compositions.

[0020] FIG. 4B shows photopolymerizaton kinetics of the soft and hard poly(thio)urethane (PTU) resins shown as NCO conversion monitored by ATR-FTIR.

[0021] FIG. 4C shows tensile data of soft and hard printed dogbones.

[0022] FIG. 4D shows (i) the print file of the “Rolling Hull Knot”; (ii) hard knot printed with 25pm thick slices at a layer exposure of 8 s using violet light (80 mW / cm2, 405 nm); and (iii) scanning electron microscope (SEM) imaging of the hard printed knot in (ii).DETAILED DESCRIPTION

[0023] Anionic photopolymerizations can provide access to polymers including, for example, polysulfides, polyesters, polyamides, and poly(thio)urethanes. In these polymers, the heteroatomic backbones provide a handle for intermolecular interactions to improve mechanical performance (e.g., strength and toughness), while simultaneously enabling recyclability. Among them, polythiourethanes from thiol-isocyanate addition standout in reaction speed, exceptional thermomechanical and optical properties (high toughness, hardness, and refractive index), and ease of triggered dynamic bond exchange for reprocessability.

[0024] Described herein are onium photocages that provide unprecedented temporal activation of addition reactions (e.g., thiol-isocyanate addition providing polythiourethanes) in a matter of seconds upon light exposure. Other notable features of the present photoanionic system relative to prior systems include the use of low intensity visible light emitting diodes (LEDs) without sensitization, where bimolecular processes tend to reduce efficiency (e.g., diffusion limitations and back-electron transfer), along with efficient uncaging withoutdecarboxylation, where CO2 gas evolution causes termination from acidification and mechanical embrittlement from voids.

[0025] Accordingly, an aspect of the present disclosure is an onium photocage, also referred to herein as an onium salt. The onium photocage according to the present disclosure has a structure according to Formula (I), (II), or (III), whereinh occurrence hydrogen, a substituted or unsubstituted Ci-6 alkyl group, a substituted or unsubstituted Ce-i2aryl group, a substituted or unsubstituted Ci-6 alkoxy group, a substituted or unsubstituted Ci-6 thioetheralkyl group, or a substituted or unsubstituted amino group; R3, R4, and R5are independently at each occurrence a hydrogen, a substituted or unsubstituted Ci-6 alkyl group, a substituted or unsubstituted C6-12 aryl group, or a vinyl group; R6is independently at each occurrence fluorine, a substituted or unsubstituted C1-6 alkyl group, or a substituted or unsubstituted C6-12 aryl group; X is oxygen or sulfur; and Y is independently at each occurrence hydrogen, a halogen, a substituted or unsubstituted C1-6 alkyl group, or a substituted or unsubstituted C6-12 aryl group.

[0026] In each of the foregoing Formulas, Z is independently an ammonium cationic group or a phosphonium cationic group. In an aspect, Z can be an ammonium cationic group. The ammonium cationic group can be of the formulawherein R’ is independently at each occurrence a substituted or unsubstituted C1-6 alkyl group or a substituted or unsubstituted C1-6 alkenyl group. The curved lines in each of the foregoing structures indicate the point of attachment of the cationic group to the rest of the molecule. In an aspect, the ammonium cationic group is of the formulawherein R’ is independently at each occurrence a substituted or unsubstituted C1-6 alkyl group or a substituted or unsubstituted C1-6 alkenyl group, preferably a substituted or unsubstituted C1-6alkyl group, for example a C3-5 alkyl group. In a specific aspect, each occurrence of R’ is a C3-5 alkyl group, preferably a n-propyl group or a n-butyl group. In an aspect, Z can be a phosphonium cationic group. The phosphonium cationic group can be of the formulawherein R” is independently at each occurrence a substituted or unsubstituted C1-6 alkyl group or a substituted or unsubstituted C6-12 aryl group. The curved lines indicate the point of attachment of the cationic group to the rest of the molecule. In an aspect, each of the R” groups is a C1-6 alkyl group, for example a n-propyl group. In an aspect, two of the R” groups can be C1-6 alkyl groups and the remaining R” group can be a C6-12 aryl group. For example, two of the R” groups can be methyl groups and the remaining R” group can be a phenyl group. In an aspect, two of the R” groups can be C6-12 aryl groups and the remaining R” group can be a C1-6 alkyl group. For example, two of the R” groups can be phenyl groups and the remaining R” group can be a methyl group.

[0027] The A in each of Formulas (I), (II), and (III) can be a tetraphenyl borate counteranion. The tetraphenyl borate counteranion can be of the structurewherein R7is independently at each occurrence a halogen (e.g., Br, Cl, F, I), a substituted or unsubstituted C1-12 alkyl group, a substituted or unsubstituted C1-12 alkoxy group, a nitro group, a nitrile group, an amine of the formula -NR2 wherein R is independently at each occurrence a Ci- 12 alkyl group, an ester group, or an amide group, and n is independently at each occurrence an integer from 0 to 5 (e.g., 0, 1, 2, 3, 4, or 5). It will be understood that when n is zero, the phenyl ring is substituted with hydrogen atoms. It will be further understood that when n is greater than zero but less than five, the valence of the phenyl ring is filled with hydrogen. In an aspect n is 0 and the phenyl rings of the tetraphenyl borate are unsubstituted. In an aspect, at least one occurrence of n is at least 1 and thus at least one substituent can be present on a phenyl ring. In an aspect, the substituent can be an electron donating group, such as a substituted or unsubstituted C1-12 alkoxy group (e.g., -OR), an amine of the formula -NR2, an ester group (e.g., -O(C=O)R), or an amide group (e.g., -N(C=O)R), wherein R in each of the foregoing exemplaryformulas can be a substituted or unsubstituted C1-12 alkyl group or a substituted or unsubstituted C6-20 aryl group. In an aspect, the substituent can be an electron donating group, for example a fluorinated C1-12 alkyl group, a halogen (e.g., F, Br, Cl, I), a nitrile group (-CN), a nitro group (-NO2), an ester group (e.g., -(C=O)OR), or an amide group (e.g., -(C=0)NR2), wherein R in each of the foregoing exemplary formulas can be a substituted or unsubstituted C1-12 alkyl group or a substituted or unsubstituted C6-20 aryl group.

[0028] In a specific aspect, the tetraphenyl borate counteranion is of the structure

[0029] In another aspect, the tetraphenyl borate counteranion can comprise substituted C1-12 alkyl groups, including fluorinated C1-12 alkyl groups. For example, in an aspect, each occurrence of n can be 2, and each occurrence of R7can be a trifluoromethyl group, wherein the two trifluoromethyl groups on each phenyl ring are positioned meta to each other, and meta to the carbon of the phenyl ring that is bound to the boron. An exemplary structure is shown below

[0030] The onium salt of the present disclosure can exclude a counteranion comprising a carbonate linkage, a carbamate linkage, or a combination thereof.

[0031] In an aspect, the onium salt can have a structure according to Formula (I)wherein R1is independently at each occurrence hydrogen, a substituted or unsubstituted C1-6 alkyl group, a substituted or unsubstituted C6-12 aryl group, a substituted or unsubstituted C1-6 alkoxy group, a substituted or unsubstituted C1-6 thioetheralkyl group, or a substituted or unsubstituted amino group and R4is a hydrogen, a substituted or unsubstituted C1-6 alkyl group, a substituted or unsubstituted C6-12 aryl group, or a vinyl group. In an aspect, R1isindependently at each occurrence a substituted or unsubstituted Ci-6 alkoxy group. In an aspect, each occurrence of R1is an unsubstituted Ci-6 alkoxy group, for example a methoxy group. In an aspect, R4is a hydrogen, a substituted or unsubstituted Ci-6 alkyl group. In an aspect, R4is hydrogen. In an aspect, R4is a methyl group.

[0032] In an aspect, Z in Formula (I) can be an ammonium cation, for example an ammonium cation of the structurewherein R’ is independently at each occurrence a substituted or unsubstituted Ci-6 alkyl group, preferably a C3-5 alkyl group, more preferably a butyl group. The curved lines indicate the point of attachment of the cationic group to the rest of the molecule. In aspect, Z in Formula (I) can be a phosphonium cation, for example a phosphonium cation of the structurewherein R” is independently at each occurrence a substituted or unsubstituted C1-6 alkyl group or a substituted or unsubstituted C6-12 aryl group. The curved lines indicate the point of attachment of the cationic group to the rest of the molecule. In an aspect, each of the R” groups is a C1-6 alkyl group, for example a n-propyl group. In an aspect, two of the R” groups can be C1-6 alkyl groups and the remaining R” group can be a C6-12 aryl group. For example, two of the R” groups can be methyl groups and the remaining R” group can be a phenyl group. In an aspect, two of the R” groups can be C6-12 aryl groups and the remaining R” group can be a C1-6 alkyl group. For example, two of the R” groups can be phenyl groups and the remaining R” group can be a methyl group.

[0033] In a specific aspect, the tetraphenyl borate counteranion A can be of the structure

[0034] In a specific aspect, the onium salt can be according to Formula (I)wherein each occurrence of R1is a methoxy group; R4is hydrogen; and Z is a phosphonium cation of the structure

[0035] In an aspect, the onium salt can have a structure according to Formula (II)wherein R2, R3, X, Z, and A can be as described above. In an aspect, X is oxygen. In an aspect, R2is a substituted or unsubstituted amino group, for example a di(Ci-6 alkyl)amino group. In an aspect, R2can be a diethyl amino group. In an aspect R3can be hydrogen. Preferably, Z is a phosphonium cation of the structurewherein R” is independently at each occurrence a substituted or unsubstituted Ci-6 alkyl group or a substituted or unsubstituted C6-12 aryl group. In an aspect, each of the R” groups is a C1-6 alkyl group, for example a n-propyl group. In an aspect, two of the R” groups can be C1-6 alkyl groups and the remaining R” group can be a C6-12 aryl group. For example, two of the R” groups can be methyl groups and the remaining R” group can be a phenyl group. In an aspect, two of the R” groups can be C6-12 aryl groups and the remaining R” group can be a C1-6 alkyl group. For example, two of the R” groups can be phenyl groups and the remaining R” group can be a methyl group. In a specific aspect, Z in Formula (II) is a phosphonium cation of the structure

[0036] In an aspect, the onium salt can be according to Formula (III)wherein R4, R5, R6, Y, Z, and A are as defined herein. In an aspect, R4is hydrogen. In an aspect, each occurrence of R5is hydrogen or a substituted or unsubstituted Ci-6 alkyl group, preferably hydrogen. In an aspect, each occurrence of R6is fluorine. In an aspect, each occurrence of Y is hydrogen or a halogen (e.g., Cl, Br, or I). In an aspect, Y is hydrogen or bromine.

[0037] The onium salts described herein are particularly useful as onium photocages. A photocage as used herein refers to a molecule in which the functionality is masked by a photocleavable protecting group. Upon irradiation with a particular wavelength of light (e.g., visible light in the present disclosure), the photocleavable group can be removed, releasing the active compound. The released active compounds of the present disclosure are bases capable of catalyzing various reactions, specifically amines or phosphines capable of facilitating addition reactions, including anionic polymerization. Accordingly, the onium photocages of the present disclosure may be particularly useful in facilitating various photochemical reactions which utilize a base as a catalyst or initiator.

[0038] Another aspect of the present disclosure is thus related to the use of the onium photocages in a photochemical reaction. For example, a method of providing a reaction product can comprise exposing a reaction mixture to light having a wavelength of 350 to 1000 nm, preferably 375 to 750 nm, or 385 to 600 nm, or 390 to 500 nm, or 400 to 500 nm, or 350 to 500 nm, to provide the reaction product.

[0039] The reaction mixture comprises the onium salt of the present disclosure, a first reactant having at least one functional group, preferably a nucleophilic functional group, and a second reactant having at least one complementary functional group that is reactive towards the functional group of the first reactant. For example, the first reactant can have at least one hydroxyl group, at least one thiol group, or a combination thereof. The second reactant can have at least one (meth)acrylate group, at least one (meth)acrylamide group, at least one vinyl sulfone group, at least one isothiocyanate group, at least one isocyanate group, at least one epoxide group, at least one carbonate group, at least one ester group, at least one alkynyl group, or a combination thereof.

[0040] The onium salts of the present disclosure can also be particularly well-suited for use as caged-photobases in a photopolymerization reaction. Accordingly, a curable composition represents another aspect of the present disclosure.

[0041] A curable composition can include the onium salt as described herein, a first monomer, and a second monomer. The first monomer includes at least two hydroxyl groups, at least two thiol groups, or a combination thereof, hi an aspect, the first monomer includes at least two thiol groups. Non-limiting examples of the first monomer comprising at least two thiol groups can include trimethylolpropane tris(3 -mercaptopropionate); trimethylolpropane tris(2- mercaptoacetate); pentaerythritol tetrakis(2-mercaptoacetate); pentaerythritol tetrakis(3- mercaptopropionate); ethoxylated trimethylolpropane tri (3-mercaptopropionate), 2,2’- (ethylenedioxy)diethanethiol; 3,6-dioxa-l,8-octane-dithiol; 1,3-propanedithiol; 1,2- ethanedithiol; 1,4-butanedithiol; ; 1,5-pentanedithiol; 1,6-hexanedithiol; 1,9-nonanedithiol; xylene dithiol; thiobis(benzenethiol); 1,4-butanediol bis(thioglycolate); l,4-bis(3- mercaptobutylyloxy)butane; tris[2-(3-mercaptopropionyloxy)ethyl] isocyanurate; 3,4- ethylenedioxythiophene; 1,10-decanedithiol; tricyclo[5.2.1.02,6]decanedithiol; benzene-1,2- dithiol; trithiocyanuric acid; 1 -butanethiol; 1 -hexanethiol; 1 -heptanethiol; 1 -octanethiol; 1- nonanethiol; 1 -decanethiol; and 1 -octadecanethiol.

[0042] The second monomer includes at least two (meth)acrylate groups, at least two (meth)acrylamide groups, at least two vinyl sulfone groups, at least two isothiocyanate groups, at least two isocyanate groups, at least two epoxide groups, at least two carbonate groups, at least two ester groups, at least two alkynyl groups, or a combination thereof. For example, the second monomer can comprise at least two groups selected fromThe curved lines in the foregoing formulas represent a point of attachment to the rest of the molecule and x represents the number of functional groups per molecule and can be for example, at least 2, or at least 3, or 2 to 5, or 2 to 4, or 2 to 3. In an aspect x is 2 and the second monomer is a difunctional monomer.

[0043] In an aspect, the second monomer includes at least two isocyanate groups. Diisocyanates that may be useful can include, for example, aliphatic and cycloaliphatic diisocyanates having 3 to 100 carbon atoms linked in a straight chain or cyclized and having twoisocyanate reactive end groups. Non-limiting examples of suitable aliphatic isocyanates include ethylene diisocyanate, trimethylene diisocyanate, 1,6-hexamethylene diisocyanate (HDI), tetramethylene diisocyanate, octamethylene diisocyanate, nonamethylene diisocyanate, decamethylene diisocyanate, 1,6,11 -undecanetriisocyanate, 1,3,6-hexamethylene triisocyanate, bis(isocyanatoethyl)-carbonate, bis(isocyanatoethyl)ether. Other non-limiting examples of suitable aliphatic isocyanates include branched isocyanates such as trimethylhexane diisocyanate, trimethylhexamethylene diisocyanate (TMDI), 2,2'- dimethylpentane diisocyanate, 2,2,4-trimethylhexane diisocyanate, 2,4,4- trimethylhexamethylene diisocyanate, l,8-diisocyanato-4-(isocyanatomethyl)octane, 2,5,7- trimethyl-l,8-diisocyanato-5-(isocyanatomethyl) octane, 2-isocyanatopropyl 2,6- diisocyanatohexanoate, lysinediisocyanate methyl ester and lysinetriisocyanate methyl ester. Non-limiting examples of suitable cycloaliphatic isocyanates include dinuclear compounds bridged by an isopropylidene group or an alkylene group of 1 to 3 carbon atoms. Non-limiting examples of suitable cycloaliphatic isocyanates include l,l'-methylene-bis-(4- isocyanatocyclohexane), 4,4'-methylene-bis-(cyclohexyl isocyanate) or 4,4'- dicyclohexylmethane diisocyanate , 4,4'-isopropylidene-bis-(cyclohexyl isocyanate), 1,4- cyclohexyl diisocyanate (CHDI), 3-isocyanato methyl-3,5,5-trimethylcyclohexyl isocyanate (a branched isocyanate also known as isophorone diisocyanate or IPDI). In an aspect, the second monomer can comprise hexamethylene diisocyanate; isophorone diisocyanate; diisocyanatobutane; diisocyanatooctane; 1 ,3 ,5-tris(6-isocyanatohexyl)- 1,3,5 -triazinane-2, 4,6- trione; phenylene diisocyanate; xylylene diisocyanate; tolyene diisocyanate; cyclohexylene diisocyanate; toluene diisocyanate; methylenebis(phenyl isocyanate); propyl isocyanate; 1- pentyl isocyanate; hexyl isocyanate; octyl isocyanate; nonyl isocyanate; sec-butyl isocyanate; 2- ethylhexyl isocyanate; cyclopentyl isocyanate; and l-isocyanato-3 -methylbutane. Use of polyethylene glycol) or polypropylene glycol) diisocyanate terminated polymers are also mentioned (e.g., tolylene 2,4-diisocyanate terminated-poly(propylene glycol) having a number average molecular weight of 1,000 to 5,000 grams per mole).

[0044] The onium salt can be present in the curable composition in an amount of 0.005 to 5 mole percent (mol%) relative to the total moles of the first and second of monomers. In an aspect, the onium salt can be present in the curable composition in an amount of 0.1 to 1 mol%. The ratio of the first and send monomer can be selected based on the chemical identity of each monomer (e.g., the number of reactive groups per molecular) and the desired properties (e.g., mechanical properties) of the resulting material, guided by the present disclosure.

[0045] The curable composition described herein can be photopolymerized by irradiating the curable composition with light having a wavelength of 350 to 1000 nm, preferably 375 to750 nm, or 385 to 600 nm, or 390 to 500 nm, or 400 to 500 nm, or 350 to 500 nm, to provide a polymer comprising repeating units derived from nucleophilic addition between the first monomer and the second monomer.

[0046] The first and second monomers can be selected to provide a (polyester-sulfide) (e.g., comprising -O(C=O)-L-S- linkages, wherein L is a linking group, for example a Ci-6 alkylene linking group), a poly(amide-sulfide), a poly(sulfone-sulfide), a poly(urethane) (e.g., comprising a -NH(C=O)O- linkage), a poly(thiourethane) (e.g., comprising a -NH(C=O)S- linkage), a poly(dithiourethane) (e.g., comprising a -NH(C=S)S- linkage), a polycarbonatesulfide) (e.g., comprising a -O(C=O)O-L-S- linkage, wherein L is a linking group, for example a Ci-6 alkylene linking group), or a poly(ether-sulfide).

[0047] In a specific aspect, at least one of the first monomer or the second monomer can comprise more than two reactive groups, and the polymer is a crosslinked polymer. For example, a first monomer can comprise at least three, preferably at least four thiol groups, and the second monomer can comprise at least two isocyanate groups (e.g., a diisocyanate).

[0048] The curable composition can optionally further comprise a suitable solvent. When present, the solvent is selected to sufficiently dissolve or disperse the components of the curable composition. Suitable solvents can be selected based on the selected components and guided by the present disclosure. Exemplary solvents can include but are not limited to tetrahydrofuran, dimethylformamide, N-methylformamide, formamide, acetonitrile, dimethylacetamide, dimethylacetamide, propylene carbonate, ethylene carbonate, N- methylpyrrolidone, dimethylsulfoxide, and combinations thereof. In an aspect, the curable composition does not include a solvent.

[0049] In an aspect, the curable composition can optionally further comprise an additive, provided that the presence of the additive does not significantly adversely affect one or more desired properties of the curable composition, the light-based method of curing, or the final cured composition.

[0050] For example, an opaquing agent can be present. Opaquing agents can also be referred to as opacifiers. Suitable opaquing agents can be dyes or pigments. In an aspect, the opaquing agent can comprise an azo-dye. In an aspect, the opaquing agent can comprise Sudan I, Sudan IV, Sudan black B, or a combination thereof. In an aspect, the opaquing agent can be present in an amount of from 0.00001 to 10 weight percent (wt%), or from 0.00001 to 5 wt%, or from 0.001 wt% to 1 wt%, or from 0.01 to 0.5 wt%, each based on the total weight of the curable composition.

[0051] In an aspect, a stabilizer may be present. For example, suitable stabilizing additives can include tetramethylpiperidin-l-yl)oxyl (TEMPO). Phenol-containing compounds,particularly hindered phenol-containing compounds, can also be useful as stabilizers. The hindered phenol stabilizer is not particularly limited as long as it has an antioxidant function and provides stability to the curable composition. Phenol-containing compounds, particularly hindered phenol-containing compounds, can also be useful as stabilizers. The hindered phenol stabilizer is not particularly limited as long as it has an antioxidant function and provides stability to the curable composition. Examples of the hindered phenol stabilizer can include compounds having a hydroxyphenyl group substituted with a t-butyl group, such as 2,6- di-t-butylhydroxytoluene, 2,2'-methylenebis (4-methyl-6- t-butylphenol), 4,4'-butylidenebis (3- methyl-6-t-butylphenol), triethylene glycol-bis [(3-t-butyl-5-methyl-4-hydroxyphenyl) propionate], 1, 6-hexanediol bis [3- (3,5-di-t-butyl-4-hydroxyphenyl) propionate], pentaerythritol-tetrakis [3-(3,5-di-t-butyl-4-hydroxyphenyl) Propionate], 2,4-bis-(n-octylthio)-6- (4-hydroxy-3,5-di-t-butylanilino) 1,3,5-triazine, 2,2-thio-diethylenebis [3-(3,5-di-t-butyl-4- hydroxyphenyl) propionate], octadecyl-3-(3,5-di-t -butyl-4-hydroxyphenyl) propionate, 2,2'- thiobis (4-methyl-6-tert-butyl) phenol, 4,4'-thiobis(3-methyl-6-tert-butyl) phenol, N,N'- hexamethylenebis(3 , 5 -di-t-butyl-4-hydroxy-hydrocinnamamide), 3 ,5-di-t-butyl-4-hydroxy- benzylphosphonate-diethyl ester, and the like. When present, a stabilizing additive can be included in the curable composition in an amount of 0.00001 to 5 wt%, or 0.001 to 1 wt %, or 0.001 to 0.5 wt % relative to the total weight of the curable composition.

[0052] In an advantageous feature, no additional sensitizer is needed to facilitate the photochemical reaction in the presence of the onium salt according to the present disclosure. A sensitizer as used herein refers to a compound added for the purpose of promoting a reaction or polymerization or improving the reaction rate. The sensitizer can absorb a particular wavelength of light and interacting with the reaction initiator (e.g., the photobase released from the onium salt in the present application), for example by energy transfer, electron transfer, etc. from the sensitizer in an excited state. Sensitizers can also be referred to as “photosensitizers”, “cocatalysts” and “co-initiators. Thus in some aspects, a sensitizer is excluded from the curable composition comprising the onium salt according to the present disclosure. Exemplary sensitizers which can be excluded from the composition can include anthracene derivatives, anthraquinone derivatives, pyrene derivatives, perylene derivatives, carbazole derivatives, benzophenone derivatives, thioxanthone derivatives, xanthone derivatives, coumarin derivatives, phenothiazine derivatives, camphorquinone derivatives, acridine dyes, and the like. In an aspect, xanthones and derivatives thereof and thioxanthones and derivatives thereof can be excluded from the curable composition of the present disclosure. Other initiators or co-catalyst that are generally known in the art may also be excluded from the reaction mixtures and curable compositions of the present disclosure.

[0053] The curable composition can be used to provide a corresponding cured composition, for example by photoinitiating a chemical reaction of the first and second monomers in the presence of the onium salt and a suitable wavelength of light. The wavelength of light can be selected based on the chemical structure of the compound according to Formula (I) to (III), guided by the present disclosure.

[0054] The present inventors have also discovered that the use of the onium salts according to the present disclosure can enable the use of lower light intensities, which can be advantageous for certain applications. For example, the curable composition or reaction mixture comprising the onium salt can be exposed to the light at an intensity of 2000 mW / cm2or less. For example, within this range, the light intensity can be 1 to 2000 mW / cm2, or 1 to 500 mW / cm2, or 1 to 100 mW / cm2, or 1 to 50 mW / cm2. The use of low intensity light, particularly in the absence of a sensitizer, was unexpected.

[0055] The temperature of the reaction is not particularly limited as long as the reaction proceeds. For example, the method can be conducted at a temperature of 0 to 100 °C, for example 10 to 60 °C, or 20 to 40 °C.

[0056] The irradiation time can also depend on the light source, the intensity, and the chemical identities of the onium salt and the first and second monomers. In an advantageous feature, the irradiation time can be short, for example 5 minutes or less, or 1 minute or less, or 30 seconds or less, or 1 to 60 seconds, or 1 to 30 seconds, or 1 to 15 seconds, or 1 to 10 seconds, or 1 to 8 seconds, or 1 to 5 seconds. Such timescales (e.g., 15 seconds or less) can be particularly useful for applications of the disclosed method to lithography or 3D printing (additive manufacturing)

[0057] Thus another aspect of the present disclosure is an additive manufacturing method, preferably a light based additive manufacturing method, of making a three dimensional structure. The method can comprise irradiating a first portion of a curable composition comprising the onium salt according to the present disclosure, the first monomer, and the second monomer to induce polymerization of the first and second monomers to form a first layer on a substrate. The irradiating light has an energy that at least partially overlaps with the absorption band of the onium salt, thereby inducing polymerization of the first and second monomers. The light can be provided by a light source comprising a light emitting diode. The method further comprises forming at least one additional layer on the first layer by irradiating a second portion of the curable composition to induce polymerization to form the additional layer. The method steps of irradiating and forming the layers can be repeated until a desired number of layers are formed to provide the three-dimensional structure. In an aspect, each layer can comprise acrosslinked polymer matrix derived from the first and second monomers. In an aspect, adjacent layers may be chemically bonded at an interface between the layers.

[0058] This disclosure is further illustrated by the following examples, which are nonlimiting.EXAMPLES

[0059] Photocage design was constructed from four pillars: 1) ortho-nitrobenzyl (oNB) as an efficient visible light reactive scaffold, 2) tetraphenylborate counteranions as alternatives to carbamates and carboxylates to mitigate CO2 gas release, 3) onium cages to prevent spontaneous thiourethane formation, and 4) phosphine cargo to rapidly catalyze thiol-isocyanate addition. The general synthesis and characterization of photocages according to the present disclosure is shown in FIG. 1A and IB.

[0060] A range of ortho-nitrobenzyl (oNB)-caged photobases were easily synthesized via a three-step process: bromination of commercially available 4,5-dimethoxy-2-nitrobenzyl alcohol, nucleophilic substitution with the desired phosphine or amine and subsequent anion exchange of the Br- anion for the selected tetraphenylborate (BPIK). The uncaging efficiency for oNB-DPMP was calculated using phosphorus nuclear magnetic resonance (31P NMR) spectroscopy, as shown in FIG. 2. Release of the phosphine cargo was followed by facile oxidation to methyldiphenylphosphine oxide which was quantified via31P NMR following irradiation at 405 nanometers (nm) (100 mW / cm2) at various time points (0, 2.5, 5, 10 minutes). The appearance of a peak corresponding to the oxide formation (828.0) was integrated relative to a known concentration of triphenylphosphine as an internal standard (shown in FIG. 2). An uncaging quantum yield of 5.5 ± 0.3% was calculated from trials where < 10% of phosphine was released and competitive absorption from the free oNB species was minimized.

[0061] Formation of a base-isocyanate complex by one of the tertiary nitrogen’s in the guanidine is anticipated to result in the lack of temporal control, despite partial deactivation upon quatemization. Temporal control experiments were carried out using attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectroscopy, as shown in FIG. 3A. The 405 nm light was not turned on until 300 s to distinguish the stability of resins in the absence of photons (FIG. 3D). No polymerizations were observed over the period until the light was turned on for all the onium derivatives. Due to similar reactivity and ease of synthesis, oNB-DPMP was used for further characterization and printing (FIG. 3C and 3D). As comparative examples, three commercial guanidine caged photobases (OXA-NR3H) and 1,1,3,3-tetramethylguanidine (TMG) containing photocages (Cou-TMG, MNNPOC-TMG, NVOC-TMG, synthesized inhouse) were employed in photopolymerization of 2:1 hexamethylene diisocyanate (HMDI) topentaerythritol tetra(3-mercaptopropionate) (PETMP). Interestingly, polymerization was observed immediately after adding the TMG-photocages to the mixture of the monomers, most likely due to the activation via a base-isocyanate complex (FIG. 3D). The xanthone-based commercial photocages were not soluble in the monomer mixture and saw no polymerization upon irradiation (FIG. 3D). Additionally, an intensity effect was seen using oNB-DPMP where higher intensities of light increased the rate of isocyanate consumption going from 11.0 mM / s with 2 mW / cm2to 210 mM / s with 80 mW / cm2of 405 nm light (FIG. 3C).

[0062] To show the ability of the photobase to efficiently initiate thiourethane polymerization for a range of disparate properties two formulations were prepared and optimized for digital light process (DLP) printing: one that would provide a tough thermoplastic material (termed “hard resin”) and one with soft elastomeric properties (termed “soft resin”). The hard resin formulation included a molar ratio of 100:90:10 isophorone diisocyanate: trimethylolpropane tris(3 -mercaptopropionate): ethoxylated trimethylolpropane tri (3- mercaptopropionate) (IPDI:TMPMP:ETTMP), with 1.0 mol% oNB-DPMP relative to the total mols of monomer and 10 wt% acetonitrile. The soft resin formulation included a molar ratio of 100:90:10 tolylene 2,4-diisocyanate terminated-poly(propylene glycol), (Average Mn-2,300): 3,6-dioxa-l,8-octane-dithiol:trimethylolpropane tris(3-mercaptopropionate) (TDI- PPG:DODT:TMPMP), 2.0 mol% oNB-DPMP, 10 wt% acetonitrile, with 0.01 mol% octanoic acid and 0.06 wt% Sudan I as additives to prevent overcuring. ATR-FTIR spectroscopy was used to characterize reaction kinetics of the hard and soft polythiourethane (PTU) formulations, and results are shown in FIG. 4A. Both resins exhibited fast rates of polymerization upon irradiation with 405 nm light at 80 mW / cm2, reaching qualitative full conversion within in a minute and well within optimal times for 3D printing.

[0063] After both the hard and soft resin formulations were characterized via FTIR, optimization could be pursued for 3D printing. First, the gel point was determined for the soft resin formulation via photorheology. The soft resin demonstrated gelation at 6.87 ± 0.70 seconds (s) for a 100 micrometer (pm) thick sample irradiated with 80 mW / cm2405 nm light (intensity and light source used for all experiments). To determine the ideal slice time for the hard resin, a “resolution print” was performed at 100 pm slice thickness. As a result of this, 20 s was selected as the printing slice exposure time.

[0064] Once slice exposure times were determined, 100 micrometer (pm) slice thickness dogbones were printed for both resin formulations, dried in a vacuum oven to remove acetonitrile, and subsequently characterized via uniaxial tensile testing. The hard dogbones displayed plastic behavior with necking and a high average Young’s modulus (E) of 1520 ± 152 Megapascals (MPa) with high stress at break (CTB) 53.6 ± 4.2 MPa and low strain at break (SB)5.45 ± 0.3%. The soft dogbone presented soft elastomeric behavior with average E of 1.29 ± 0.1 MPa, SB of 217 ± 34.1%, and OB of 1.03 ± 0.1 MPa. Results are shown in FIG. 4B. The soft resin was also characterized for hysteresis and Strobl tests. A complex print, “Rolling Hull Knot”, was attempted for each resin formulations at both 100 and 25 pm layer thicknesses, shown in FIG. 4C. Both resin formulations utilized a fluorinated oil in the resin vat to prevent sticking of the printed parts to the vat film. Notably, the exposure time for 25 pm layer thick knots did not correspond to ! of the slice exposure time for the 100 pm gel time; instead, exposure times of 8 s (hard) and 3.5 s (soft) were used to achieve the higher-resolution knots. Furthermore, overcuring was a pervasive issue in the printing of the soft knot, so additives such as an opaquing agent (Sudan I) and acid (octanoic acid) were added to mitigate this and increase resolution. Following printing, knots were imaged using scanning electron microcopy (SEM) to characterize their resolution, shown in FIG. 4C.

[0065] Thus, the present inventors have synthesized several onium photocages that can efficiently catalyze anionic polymerization, for example to prepare thiourethanes, using mild visible light irradiation (e.g., 405 nm). To showcase the versatility of the photobases, oNB- DPMP was used for DLP vat polymerization of a complex knot structure using two different thiourethane resins of distinct mechanical properties. The present inventors have further shown that these photobases that can be used for anionic photoinitiation without the use of sensitization. These photocages can act as useful tools for the development of materials used for light driven additive manufacturing beyond the current scope. A significant advantage is therefore provided by the present disclosure.

[0066] Experimental details follow. All reagents were used as received unless otherwise noted. Methyldiphenylphosphine (99%), sodium tetraphenylborate (99.5+%, ACS), tripropylphosphine (min. 98%), sodium tetrafluoroborate (98%) were purchased from STREM Chemicals. Methyldiphenylphosphine oxide (98%), tributylamine (98.0+%) and 1, 1,3,3- tetramethylguanidine were purchased from TCI Chemicals. 4,5-Dimethoxy-2-nitrobenzyl alcohol was purchased from Combi-blocks. 4-Dimethylaminopyridine was purchased from Acros Organics. Polypropylene glycol), tolylene 2,4-diisocyanate terminated (Average Mn-2,300), pentaerythritol tetrakis(3-mercaptopropionate) (PETMP, 95%), trimethylolpropane tris(3-mercaptopropionate) (>95.0%), 2,2'-(ethylenedioxy)diethanethiol (95%), Sudan I (Dye content >95 %) were purchased from Sigma Aldrich. Phosphorus tribromide, dimethylphenylphosphine (99%), n-Octanoic Acid (98.0+%), isophorone diisocyanate (IPDI, 99+% mixture of isomers), hydrochloric acid (Certified ACS Plus), and acetonitrile (HPLC) were purchased from Fisher Chemical. Sodium tetrakis 3,5-bis(trifluoromethyl)phenyl boratewas purchased from Ambeed. Ethoxylated-trimethylolpropane tris(3-mercaptopropionate) (ETTMP 700) was obtained from Bruno Bock.

[0067] l-(Bromomethyl)-4,5-dimethoxy-2-nitrobenzene and 4-(bromomethyl)-7- (diethylamino)-2H-chromen-2-one were synthesized according to the following general procedure. Inside a flame dried 2-necked round bottom flask, alcohol derivative (10.00 mmol) was dissolved into 150 mL of dry dichloromethane (DCM) under nitrogen atmosphere. Phosphorus tribromide (12. 00 mmol) in 10 mL of dry DCM was added dropwise into the reaction at 0 °C. The reaction was brought to room temperature and stirred for overnight. Then the reaction was quenched with 50 mL of deionized water, poured into 150 mL of concentrated NaHCCh solution and extracted with 3*100 mL of DCM. The combined organic layers were dried over anhydrous MgSC>4 and filtered. The crude product was purified by passes through silica pad in DCM, concentrated under reduce pressure.

[0068] Formation of Phosphonium salt with Bromide Counter ion: Under nitrogen atmosphere, bromomethyl derivative (5.00 mmol) was dissolved into 20 mL of dry toluene. Then the phosphorus or amine base (5.5 mmol) in 5 mL of dry toluene was added dropwise into the reaction vessel, immediately product was crash out from the reaction mixture (except tributyl amine needs to stir at 60°C for 16 hours). To confirmed complete consumption of reactant, the reaction was further stirred for 3 hours. Then the precipitate of the salt was filtered, and washed several times with toluene followed by hexane and yield was more than 98%.

[0069] Exchange Counter ion: The phosphonium bromide salt (2.00 mmol) was firstly dissolved in 200 mL 2% HC1 (aq), and then NaBPh4 (other counter ion) solution (1.8 mmol) in 10 mL of water was slowly added. Yellowish precipitate appeared during the addition of counter ion. After 2 hours of reaction, the precipitate of the salt was filtered, and washed several times with water. The product was then dried and kept in the dark under vacuum to avoid potential oxidation. The yield of the products were >70%.

[0070] A DLP 3D Printer with Two Color Channel having two built in light emitting diodes centered at 365nm and 405nm with maximum light intensities of 18.3 and 92.0 mW / cm2respectively was used. The FWHM values are 17.31 nm and 20.38 nm respectively. The projector resolution is 1920 x 1080 pixels and the lateral resolution for this printer is 23 pm with vertical resolution controlled by layer thickness which can range from 10 pm to 300 pm. The build area is 44.54 mm x 25.06 mm. A transparent fluorinated polymer film (Teflon FEP film, McMaster-Carr, 127 pm thick or PDMS film, McMaster-Carr, 127 pm thick) coated with a thin layer of fluorinated oil (Krytox by Chemours GLP 101 Industrial Oil) is used as the base of the resin tank to provide a non-stick, high-temperature resistant, and flexible surface.

[0071] Real-Time Fourier Transform Infrared Spectroscopy (RT-FTIR): Transmission. Resin formulations were injected between two 1 mm thick glass microscope slides (cat. no. 12- 550-A3, Fisher Scientific) separated by ~ 100 m polyester plastic shims (cat. no. 9513K66, McMaster- Carr) to maintain a constant sample thickness over the course of the photopolymerization. Each sample was placed in a horizontal transmission accessory (A043- N / Q, Bruker) equipped on the FTIR spectrometer (INVENIO-R, Bruker), which was controlled using OPUS spectroscopy software. Spectra were collected using a liquid nitrogen cooled (LN- MCT-Mid) detector from 2000 to 7000 cm-1at a rate of 1 scan every 0.36 s. All samples were monitored under ambient conditions (atmospheric and room temperature and pressure). The functional group conversion upon light exposure was determined by monitoring the disappearance of the peak area centered at ~2270 cm-1corresponding to the N=C=O stretch. Each sample was tested in triplicate and at printer light intensities (80 mW / cm2for 405 nm LED).

[0072] Attenuated Total Reflectance (ATR). Violet light was provided by a Type B LED (serial no. LCS-0405-12-22, Mightex Systems) with an emission centered at 405 nm. These LEDs were used in combination with a current-adjustable driver (SLC-MA02-U, Mightex Systems) for intensity control, such that all intensities between experiments (printing, FTIR, and photorheology) could be matched as accurately as possible. Irradiation intensities were measured with a Thorlabs PM100D photometer equipped with a silicon-based photodiode power sensor (S120VC and S130C, Thorlabs).

[0073] Photorheometry experiments were completed using a Discovery Hybrid rheometer from TA Instruments (TA Instruments, DE, USA). The rheometer was equipped with a “UV Light Guide” accessory, a disposable 20 mm diameter quartz bottom plate, and a 20 mm diameter geometry aluminum upper parallel plate. A liquid light guide was used to illuminate the samples with the corresponding visible light using a 405 nm LED connected to a driver from which light intensity could be controlled remotely through software (Mightex). Each sample was tested six times at the printer light intensity. The rheometer was set to run for one data acquisition cycle with the following experimental parameters: fast oscillation step at 1% strain and 10 rad / s frequency for a total run time of 300 s. The gap height was set to 100 pm for each experiment to best match the photoFTIR spectroscopy experiments. Photorheology was performed under ambient conditions (no argon degassing and open to air) for all samples. The light was not turned on until 10s into the fast oscillation step. The storage modulus (G') and the loss modulus (G") were monitored in real time and the light was turned off following inflection of the data, as noted by eye. To determine the gel point (where storage and loss moduli cross), we used the “moduli cross” data analysis function in TRIOS to calculate this value for each trial.All six trials were averaged and subtracted by 10 (to account for the light off portion of the run) to get an average gel point value. Photorheology was only completed for the soft resin material.

[0074] Tensile testing was carried out using a Shimadzu Autograph AGS-X universal testing machine equipped with a 1 kN load cell. Samples were printed as an ASTM Standard D638 Type IV scaled by a factor of 0.3 (2.0 mm width, 13 mm gauge length, 1.3 mm thickness) Axial extension was carried out at 5 mm / min until fracture.

[0075] Hysteresis was carried out using a Shimadzu Autograph AGS-X universal testing machine equipped with a 100 N load cell. Samples were printed as an ASTM Standard D638 Type IV scaled by a factor of 0.3 (2.0 mm width, 13 mm gauge length, 1.3 mm thickness). Hysteresis experiments were carried out at 10 mm / min and involved cycling to 100% strain, returning to zero force for each cycle.

[0076] Step cycle tensile testing was carried out using a Shimadzu Autograph AGS-X universal testing machine equipped with a 100 N load cell. Samples were printed as an ASTM Standard D638 Type IV scaled by a factor of 0.3 (2.0 mm width, 13 mm gauge length, 1.3 mm thickness). Axial extension was carried out at 10 mm / min (50% / min) until fracture. Elastic recovery and plastic deformation were extrapolated from cyclic straining (5, 10, 25, 50, 75, 100, 200, 300%) and relaxing to 0.005 N. Elastic recovery was calculated by the percentage of sample recovery divided by the strain.

[0077] Scanning electron microscopy (SEM) studies were carried out on a FEI Quanta 650 SEM instrument at an operating voltage of 5.00 kV. SEM was used to examine the surface characteristics of the 3D printed complex knot. For improved imaging, the sample was sputtered with platinum using an EMS sputter coater (Electron Microscopy Science). Sputtering was conducted for 1.5 min at 40 mA to target a 5 nm thick platinum layer.

[0078] UV-vis spectra were obtained using a system from Ocean Insight. The system utilized a balanced deuterium-tungsten halogen light source (DH-2000-BAL) with a typical output of 194 pW (deuterium bulb) and 615 pW (tungsten bulb) through an SMA 905 connector, covering a range from 230 nm - 2.5 pm. Multimode fiber-optic cables with SMA connectors on both ends and a 600 pm core diameter (QP600-025-SR) connected the light source to the sample holder. For dilute solution measurements a qpod cuvette holder (QNW qpod 2e™) capable of magnetic stirring and peltier-driven temperature control from -30 °C to 105 °C was used. The sample holder was coupled through another multimode fiber to the spectrometer (QEPRO-ABS) having an entrance slit of 5 pm (INTSMA-005 Interchangeable Slit). The spectrometer measured in the range from 200-950 nm, at an optical resolution of 1.7 nm, using a back- thinned, TE cooled, 1024 x 58 element CCD array.

[0079] For all experiments aside from printing, a violet LED (LCS series, Mightex Systems) was used with an emission centered at -405 nm. This LED was used in combination with a current-adjustable driver (SLC-MA02-U, Mightex Systems) for intensity control, such that intensity between experiments could be matched. Light was delivered via a 3 mm liquid light guide (LLG3-4H, ThorLabs) or a 1.5 mm fiber optic cable (M93L01, ThorLabs). Irradiation intensity was measured with a ThorLabs PM100D photometer equipped with silicon- based photodiode power sensor (SI 20 VC and S130C, Thorlabs).

[0080] This disclosure further encompasses the following aspects.

[0081] Aspect 1: An onium salt having a structure according to Formula (I), (II), or (III),wherein in each of the foregoing Formulas (I) to (III), R1and R2are independently at each occurrence hydrogen, a substituted or unsubstituted Ci-6 alkyl group, a substituted or unsubstituted Ce-i2aryl group, a substituted or unsubstituted Ci-6 alkoxy group, a substituted or unsubstituted Ci-6 thioetheralkyl group, or a substituted or unsubstituted amino group; R3, R4, and R5are independently at each occurrence a hydrogen, a substituted or unsubstituted Ci-6 alkyl group, a substituted or unsubstituted Ce-12 aryl group, or a vinyl group; R6is independently at each occurrence fluorine, a substituted or unsubstituted Ci-6 alkyl group, or a substituted or unsubstituted Ce-i2 aryl group; X is oxygen or sulfur; Y is independently at each occurrence hydrogen, a halogen, a substituted or unsubstituted Ci-6 alkyl group, or a substituted or unsubstituted Ce-12 aryl group; Z is independently at each occurrence an ammonium or phosphonium cation, wherein the ammonium cation iswherein R’ is independently at each occurrence a substituted or unsubstituted Ci-6 alkyl group or a substituted or unsubstituted Ci-6 alkenyl group; and the phosphonium cation iswherein R” is independently at each occurrence a substituted or unsubstituted C1-6 alkyl group or a substituted or unsubstituted C6-12 aryl group; and A is a tetraphenyl borate counteranion.

[0082] Aspect 2: The onium salt of aspect 1, wherein the tetraphenyl borate counteranion is of the structurewherein R7is independently at each occurrence a halogen, a substituted or unsubstituted C1-12 alkyl group, a substituted or unsubstituted C1-12 alkoxy group, a nitro group, a nitrile group, an amine of the formula -NR2 wherein R is independently at each occurrence a C1-12 alkyl group, an ester group, or an amide group, and n is independently at each occurrence an integer from 0 to 5.

[0083] Aspect 3: The onium salt of aspect 1 or 2, wherein the tetraphenyl borate counteranion is of the structure

[0084] Aspect 4: The onium salt of any of aspects 1 to 3, wherein the onium salt has the structure according to Formula (I).

[0085] Aspect 5: The onium salt of aspect 4, wherein R1is independently at each occurrence a substituted or unsubstituted C1-6 alkoxy group, preferably a methoxy group; R4is hydrogen; and Z is an ammonium cation of the structurewherein R’ is independently at each occurrence a substituted or unsubstituted C1-6 alkyl group, preferably a C3-5 alkyl group; or a phosphonium cation of the structurewherein R” is independently at each occurrence a substituted or unsubstituted C1-6 alkyl group or a substituted or unsubstituted C6-12 aryl group.

[0086] Aspect 6: The onium salt of any of aspects 1 to 5, wherein the onium salt is according to Formula (I), wherein each occurrence of R1is a methoxy group; R4is hydrogen; and Z is a phosphonium cation of the structure

[0087] Aspect 7: The onium salt of aspect 6, wherein the tetraphenyl borate counteranion is of the structure

[0088] Aspect 8: The onium salt of any of aspects 1 to 3, wherein the onium salt has the structure according to Formula (II).

[0089] Aspect 9: The onium salt of aspect 8, wherein X is oxygen; R2is an amino group; R3is hydrogen; and Z is a phosphonium cation of the structurewherein R” is independently at each occurrence a substituted or unsubstituted Ci-6 alkyl group or a substituted or unsubstituted Ce-12 aryl group, preferably Z is a phosphonium cation of the structure

[0090] Aspect 10: The onium salt of aspect 8 or 9, wherein the tetraphenyl borate counteranion is of the structure ogo

[0091] Aspect 11 : The onium salt of any of aspects 1 to 3, wherein the onium salt has the structure according to Formula (III).

[0092] Aspect 12: A curable composition comprising: the onium salt according to any of aspect 1 to 11; a first monomer comprising at least two hydroxyl groups, at least two thiolgroups, or a combination thereof; and a second monomer comprising at least two (meth)acrylate groups, at least two (meth)acrylamide groups, at least two vinyl sulfone groups, at least two isothiocyanate groups, at least two isocyanate groups, at least two epoxide groups, at least two carbonate groups, at least two ester groups, at least two alkynyl groups, or a combination thereof.

[0093] Aspect 13: The curable composition of aspect 12, wherein the first monomer comprises at least two thiol groups; and the second monomer comprises at least two isocyanate groups.

[0094] Aspect 14: A method of forming a polymer, the method comprising: irradiating the curable composition of aspect 12 or 13 with light having a wavelength of 350 to 1000 nm to provide a polymer comprising repeating units derived from addition polymerization of the first monomer and the second monomer.

[0095] Aspect 15: The method of aspect 14, wherein the method is conducted in the absence of a sensitizer.

[0096] Aspect 16: The method of aspect 14 or 15, wherein at least one of the first monomer or the second monomer comprises more than two reactive groups, and the polymer is a crosslinked polymer network.

[0097] Aspect 17: A method of forming an object by additive manufacturing, the method comprising: irradiating a first portion of the curable composition of aspect 12 or 13 with light having a wavelength of 350 to 1000 nm to provide a first layer comprising a polymer comprising repeating units derived from addition polymerization of the first monomer and the second monomer on a substrate; and forming at least one additional layer on the first layer by irradiating a second portion of the curable composition with light having a wavelength of 350 to 1000 nm to provide the at least one additional layer comprising a polymer comprising repeating units derived from addition polymerization of the first monomer and the second monomer to provide the object.

[0098] Aspect 18: The method of aspect 17, comprising repeating the irradiating with additional portions of the curable composition to form additional layers comprising the polymer.

[0099] Aspect 19: The method of aspect 17 or 18, wherein the method comprises digital light processing, stereolithography, or a combination thereof.

[0100] Aspect 20: A cured composition derived from the curable composition of aspects 12 or 13 or obtained by the method of any of aspects 14 to 19.

[0101] Aspect 21 : The cured composition of aspect 20, wherein the cured composition comprises a polythiourethane.

[0102] Aspect 22: A method of providing a reaction product, the method comprising: irradiating a reaction mixture with light having a wavelength of 350 to 1000 nm to provide the reaction product, wherein the reaction mixture comprises: the onium salt according to any of aspects 1 to 11; a first reactant comprising at least one hydroxyl group, at least one thiol group, or a combination thereof; and a second reactant comprising at least one (meth)acrylate group, at least one (meth)acrylamide group, at least one vinyl sulfone group, at least one isothiocyanate group, at least one isocyanate group, at least one epoxide group, at least one carbonate group, at least one ester group, at least one alkynyl group, or a combination thereof.

[0103] The compositions, methods, and articles can alternatively comprise, consist of, or consist essentially of, any appropriate materials, steps, or components herein disclosed. The compositions, methods, and articles can additionally, or alternatively, be formulated so as to be devoid, or substantially free, of any materials (or species), steps, or components, that are otherwise not necessary to the achievement of the function or objectives of the compositions, methods, and articles.

[0104] All ranges disclosed herein are inclusive of the endpoints, and the endpoints are independently combinable with each other. “Combinations” is inclusive of blends, mixtures, alloys, reaction products, and the like. The terms “first,” “second,” and the like, do not denote any order, quantity, or importance, but rather are used to distinguish one element from another. The terms “a” and “an” and “the” do not denote a limitation of quantity, and are to be construed to cover both the singular and the plural, unless otherwise indicated herein or clearly contradicted by context. “Or” means “and / or” unless clearly stated otherwise. Reference throughout the specification to “an aspect” means that a particular element described in connection with the aspect is included in at least one aspect described herein, and may or may not be present in other aspects. The term “combination thereof’ as used herein includes one or more of the listed elements, and is open, allowing the presence of one or more like elements not named. In addition, it is to be understood that the described elements may be combined in any suitable manner in the various aspects.

[0105] Unless specified to the contrary herein, all test standards are the most recent standard in effect as of the filing date of this application, or, if priority is claimed, the filing date of the earliest priority application in which the test standard appears.

[0106] Unless defined otherwise, technical and scientific terms used herein have the same meaning as is commonly understood by one of skill in the art to which this application belongs. All cited patents, patent applications, and other references are incorporated herein by reference in their entirety. However, if a term in the present application contradicts or conflictswith a term in the incorporated reference, the term from the present application takes precedence over the conflicting term from the incorporated reference.

[0107] Compounds are described using standard nomenclature. For example, any position not substituted by any indicated group is understood to have its valency filled by a bond as indicated, or a hydrogen atom. A dashthat is not between two letters or symbols is used to indicate a point of attachment for a substituent. For example, -CHO is attached through carbon of the carbonyl group.

[0108] As used herein, the term “hydrocarbyl”, whether used by itself, or as a prefix, suffix, or fragment of another term, refers to a residue that contains only carbon and hydrogen. The residue can be aliphatic or aromatic, straight-chain, cyclic, bicyclic, branched, saturated, or unsaturated. It can also contain combinations of aliphatic, aromatic, straight chain, cyclic, bicyclic, branched, saturated, and unsaturated hydrocarbon moieties. However, when the hydrocarbyl residue is described as substituted, it may, optionally, contain heteroatoms over and above the carbon and hydrogen members of the substituent residue. Thus, when specifically described as substituted, the hydrocarbyl residue can also contain one or more carbonyl groups, amino groups, hydroxyl groups, or the like, or it can contain heteroatoms within the backbone of the hydrocarbyl residue. The term "alkyl" means a branched or straight chain, saturated aliphatic hydrocarbon group, e.g., methyl, ethyl, n-propyl, i-propyl, n-butyl, s-butyl, t-butyl, n- pentyl, s-pentyl, and n- and s-hexyl. “Alkenyl” means a straight or branched chain, monovalent hydrocarbon group having at least one carbon-carbon double bond (e.g., ethenyl (-HC=CH2)). “Alkoxy” means an alkyl group that is linked via an oxygen (i.e., alkyl-O-), for example methoxy, ethoxy, and sec-butyloxy groups. "Alkylene" means a straight or branched chain, saturated, divalent aliphatic hydrocarbon group (e.g., methylene (-CH2-) or, propylene (-(CH2)3-)). “Cycloalkylene” means a divalent cyclic alkylene group, -CnH2n-x, wherein x is the number of hydrogens replaced by cyclization(s). “Cycloalkenyl” means a monovalent group having one or more rings and one or more carbon-carbon double bonds in the ring, wherein all ring members are carbon (e.g., cyclopentyl and cyclohexyl). "Aryl" means an aromatic hydrocarbon group containing the specified number of carbon atoms, such as phenyl, tropone, indanyl, or naphthyl. “Arylene” means a divalent aryl group. “Alkylarylene” means an arylene group substituted with an alkyl group. “Arylalkylene” means an alkylene group substituted with an aryl group (e.g., benzyl). The prefix "halo" means a group or compound including one more of a fluoro, chloro, bromo, or iodo substituent. A combination of different halo atoms (e.g., bromo and fluoro), or only chloro atoms can be present. The prefix “hetero” means that the compound or group includes at least one ring member that is a heteroatom (e.g., 1, 2, or 3 heteroatom(s)), wherein the heteroatom(s) is each independently N, O, S, Si, or P. “Substituted”means that the compound or group is substituted with at least one (e.g., 1, 2, 3, or 4) substituents that can each independently be a C 1.9 alkoxy, a C1-9 haloalkoxy, a nitro (-NO2), a cyano (-CN), a C1-6 alkyl sulfonyl (-S(=O)2-alkyl), a C1-6 alkyl ester (-C(=O)-O-alkyl), a C6-12 aryl sulfonyl (- S(=O)2-aryl), a thiol (-SH), a thiocyano (-SCN), a tosyl (CH3C6H4SO2-), a C3-12 cycloalkyl, a C2- 12 alkenyl, a C5-12 cycloalkenyl, a C6-12 aryl, a C7-13 arylalkylene, a C4-12 heterocycloalkyl, and a C3-12 heteroaryl instead of hydrogen, provided that the substituted atom’s normal valence is not exceeded. The number of carbon atoms indicated in a group is exclusive of any substituents. For example -CH2CH2CN is a C2 alkyl group substituted with a nitrile.

[0109] While particular embodiments have been described, alternatives, modifications, variations, improvements, and substantial equivalents that are or may be presently unforeseen may arise to applicants or others skilled in the art. Accordingly, the appended claims as filed and as they may be amended are intended to embrace all such alternatives, modifications variations, improvements, and substantial equivalents.

Claims

CLAIMS1. An onium salt having a structure according to Formula (I), (II), or (III),wherein in each of the foregoing Formulas (I) to (III)R1and R2are independently at each occurrence hydrogen, a substituted or unsubstituted Ci-6 alkyl group, a substituted or unsubstituted Ce-12 aryl group, a substituted or unsubstituted Ci- 6 alkoxy group, a substituted or unsubstituted Ci-6 thioetheralkyl group, or a substituted or unsubstituted amino group;R3, R4, and R5are independently at each occurrence a hydrogen, a substituted or unsubstituted Ci-6 alkyl group, a substituted or unsubstituted Ce-12 aryl group, or a vinyl group;R6is independently at each occurrence fluorine, a substituted or unsubstituted Ci-6 alkyl group, or a substituted or unsubstituted Ce-12 aryl group;X is oxygen or sulfur;Y is independently at each occurrence hydrogen, a halogen, a substituted or unsubstituted C1-6 alkyl group, or a substituted or unsubstituted Ce-i2aryl group;Z is independently at each occurrence an ammonium or phosphonium cation, wherein the ammonium cation iswherein R’ is independently at each occurrence a substituted or unsubstituted C1-6 alkyl group or a substituted or unsubstituted C1-6 alkenyl group; and the phosphonium cation iswherein R” is independently at each occurrence a substituted or unsubstituted Ci-6 alkyl group or a substituted or unsubstituted C6-12 aryl group; and A is a tetraphenyl borate counteranion.

2. The onium salt of claim 1, wherein the tetraphenyl borate counteranion is of the structurewhereinR7is independently at each occurrence a halogen, a substituted or unsubstituted C1-12 alkyl group, a substituted or unsubstituted C1-12 alkoxy group, a nitro group, a nitrile group, an amine of the formula -NR2 wherein R is independently at each occurrence a C1-12 alkyl group, an ester group, or an amide group, and n is independently at each occurrence an integer from 0 to 5.

3. The onium salt of claim 1, wherein the tetraphenyl borate counteranion is of the structure4. The onium salt of claim 1, wherein the onium salt has the structure according to Formula (I).

5. The onium salt of claim 4, whereinR1is independently at each occurrence a substituted or unsubstituted Ci-6 alkoxy group, preferably a methoxy group;R4is hydrogen; andZ is an ammonium cation of the structurewherein R’ is independently at each occurrence a substituted or unsubstituted Ci-6 alkyl group, preferably a C3-5 alkyl group; ora phosphonium cation of the structurewherein R” is independently at each occurrence a substituted or unsubstituted Ci-6 alkyl group or a substituted or unsubstituted C6-12 aryl group.

6. The onium salt of claim 1, wherein the onium salt is according to Formula (I), wherein each occurrence of R1is a methoxy group;R4is hydrogen; andZ is a phosphonium cation of the structure7. The onium salt of claim 6, wherein the tetraphenyl borate counteranion is of the structure8. The onium salt of claim 1, wherein the onium salt has the structure according to Formula(II).

9. The onium salt of claim 8, whereinX is oxygen;R2is an amino group;R3is hydrogen; andZ is a phosphonium cation of the structurewherein R” is independently at each occurrence a substituted or unsubstituted C1-6 alkyl group or a substituted or unsubstituted C6-12 aryl group, preferably Z is a phosphonium cation of the structure10. The onium salt of claim 8, wherein the tetraphenyl borate counteranion is of the structure11. The onium salt of claim 1, wherein the onium salt has the structure according to Formula (III).

12. A curable composition comprising: the onium salt according to claim 1; a first monomer comprising at least two hydroxyl groups, at least two thiol groups, or a combination thereof; and a second monomer comprising at least two (meth)acrylate groups, at least two (meth)acrylamide groups, at least two vinyl sulfone groups, at least two isothiocyanate groups, at least two isocyanate groups, at least two epoxide groups, at least two carbonate groups, at least two ester groups, at least two alkynyl groups, or a combination thereof.

13. The curable composition of claim 12, wherein the first monomer comprises at least two thiol groups; and the second monomer comprises at least two isocyanate groups.

14. A method of forming a polymer, the method comprising: irradiating the curable composition of claim 12 with light having a wavelength of 350 to 1000 nm to provide a polymer comprising repeating units derived from addition polymerization of the first monomer and the second monomer.

15. The method of claim 14, wherein the method is conducted in the absence of a sensitizer.

16. The method of claim 14, wherein at least one of the first monomer or the second monomer comprises more than two reactive groups, and the polymer is a crosslinked polymer network.

17. A method of forming an object by additive manufacturing, the method comprising: irradiating a first portion of the curable composition of claim 12 with light having a wavelength of 350 to 1000 nm to provide a first layer comprising a polymer comprising repeating units derived from addition polymerization of the first monomer and the second monomer on a substrate; and forming at least one additional layer on the first layer by irradiating a second portion of the curable composition with light having a wavelength of 350 to 1000 nm to provide the at least one additional layer comprising a polymer comprising repeating units derived from addition polymerization of the first monomer and the second monomer to provide the object.

18. The method of claim 17, comprising repeating the irradiating with additional portions of the curable composition to form additional layers comprising the polymer.

19. The method of claim 17, wherein the method comprises digital light processing, stereolithography, or a combination thereof.

20. A cured composition derived from the curable composition of claim 12.

21. The cured composition of claim 20, wherein the cured composition comprises a polythiourethane .

22. A method of providing a reaction product, the method comprising: irradiating a reaction mixture with light having a wavelength of 350 to 1000 nm to provide the reaction product, wherein the reaction mixture comprises: the onium salt according to claim 1; a first reactant comprising at least one hydroxyl group, at least one thiol group, or a combination thereof; and a second reactant comprising at least one (meth)acrylate group, at least one (meth)acrylamide group, at least one vinyl sulfone group, at least one isothiocyanate group, at least one isocyanate group, at least one epoxide group, at least one carbonate group, at least one ester group, at least one alkynyl group, or a combination thereof.

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