Review Review on UV-Induced Cationic Frontal of

Muhammad Salman Malik 1 , Sandra Schlögl 1 , Markus Wolfahrt 1 and Marco Sangermano 2,*

1 Competence Center Leoben GmbH, Rossegerstrasse 12, 8700 Leoben, Austria; [email protected] (M.S.M.); [email protected] (S.S.); [email protected] (M.W.) 2 Dipartimento di Scienza Applicata e Tecnologia, Politecnico di Torino, C.so Duca degli Abruzzi 24, I-10129 Torino, Italy * Correspondence: [email protected]

 Received: 28 August 2020; Accepted: 18 September 2020; Published: 20 September 2020 

Abstract: (UV)-induced cationic frontal polymerization has emerged as a novel technique that allows rapid of various epoxy monomers upon UV irradiation within a few seconds. In the presence of a diaryliodonium salt photoinitiator together with a thermal initiator, the cationic ring opening polymerization of an is auto-accelerated in the form of a self-propagating front upon UV irradiation. This hot propagating front generates the required enthalpy to sustain curing reaction throughout the resin formulation without further need for UV irradiation. This unique reaction pathway makes the cationic frontal polymerization a promising route towards the efficient curing of epoxy-based thermosetting resins and related composite structures. This review represents a comprehensive overview of the mechanism and progress of UV-induced cationic frontal polymerization of epoxy monomers that have been reported so far in literature. At the same time, this review covers important aspects on the frontal polymerization of various epoxide monomers involving the chemistry of the initiators, the effect of appropriate sensitizers, diluents and fillers.

Keywords: photoinitiator; thermal initiator; UV ; ; epoxy monomers; frontal polymerization

1. Introduction In the past few decades, polymer resins cured via frontal polymerization technique have been the focus of research for applications such as nanocomposites, functionally graded polymers, sensory materials, hydrogels and fibre-reinforced composites [1–7]. By definition, monomers that undergo frontal polymerization are able to propagate directionally in a confined reaction zone over a period of time, requiring a localized stimulus [8]. Once triggered, the polymerization front propagates in a wave-like fashion to all portions of the sample using the enthalpy of polymerization, without further need for an external stimulus. The first successful frontal polymerization was reported by Russian scientists in 1972 using methyl methacrylate as the monomer and benzoyl peroxide as thermal initiator [9]. Later, it was successfully implemented at a pilot production plant in Chernogolovka and to a polymeric material synthesis factory in Dzerzhinsk in Russia [10–12]. Frontal polymerization requires an external stimulus to trigger the reacting front. Based on different stimuli, frontal polymerization is classified into three categories namely thermal frontal polymerization, photo-frontal polymerization and isothermal polymerization [8]. In particular, ultraviolet (UV)-induced frontal polymerization has attracted a great number of industrial sectors for various applications due to its low operational costs, ambient temperature processing,

Polymers 2020, 12, 2146; doi:10.3390/polym12092146 www.mdpi.com/journal/polymers Polymers 2020, 12, 2146 2 of 34

-lessPolymers 2020, formulation 12, x FOR PEER and REVIEW thefreedom to have tuneable final properties of the cured polymer2 of [ 1333]. UV-curing is very well known for applications such as printing inks and adhesives, , protective coatings,varnishes, which protective have been coatings, implemented which have on abeen vast im scaleplemented thanks toon rapid a vast curing scale thanks[13–17] .to Photoinduced rapid curing [13–17]. Photoinduced proceed bypolymerizations a chain reaction proceed mechanism by a involving chain reaction the propagation mechanism of aninvolving active centre the bypropagation the interaction of an with active a monomer. centre by the The interaction active centre with can a monomer. be a radical, The a cation active orcentre more can rarely be a an radical, anion. a cationWhile or photoinducedmore rarely an radicalanion. chain growth polymerization has been widely investigated and exploitedWhile from photoinduced an industrial radical point ofchain view, growth it was notpolyme untilrization the discovery has been of oniumwidely salt investigated photoinitiators and byexploited James V. from Crivello an industrial in the 1970s point that of theview, first it cationicwas not photoinduceduntil the discovery polymerization of onium salt was ph reportedotoinitiators [18]. Typically,by James onium V. Crivello salts, eitherin the triarylsulfonium1970s that the first or cationic diaryliodonium photoinduced salts, arepolymerization considered as was a photo-acid reported generator[18]. Typically, under UV-irradiation.onium salts, either The triarylsulfonium strong photo-acid or generateddiaryliodonium is able salts, to promote are considered cationic as ring- a openingphoto-acid polymerization generator under of an UV-irr epoxyadiation. resin (reaction The strong reported photo-acid in Scheme generated1)[ 18 is– 20able]. These to promote highly reactivecationic cationsring-opening attack poly adjacentmerization epoxide of an groups epoxy resin forming (reaction polyether reported in in the Scheme ring opening1) [18–20]. cationic These polymerizationhighly reactive (seecations reaction attack Scheme adjacent1)[ epoxide21,22]. grou Theps Lewis forming acid polyether generated in in the cationic ring opening polymerization cationic sustainspolymerization its reactivity (see reaction with the Scheme epoxy 1) monomers [21,22]. The for Lewis a long acid time generated without in cationic further polymerization UV irradiation needed,sustains as its opposed reactivity to freewith radical the epoxy polymerization monomers wherefor a long thecuring time without reaction further stops upon UV irradiation removal of UVneeded, irradiation as opposed [13]. Thisto free so-called radical “darkpolymerization curing” o whffersere the the advantage curing reaction of curing stops residual upon removal monomer of andUV thermalirradiation post [13]. treatment This so-called for complete “dark conversioncuring” offers in cationicthe advantage polymerization of curing residual [23]. Furthermore, monomer cationicand thermal photopolymerizable post treatment monomersfor complete such conversion as in are cationic less toxic polymerization and irritating [23]. in comparisonFurthermore, to frequentlycationic photopolymerizable used in free monomers radical photopolymerization. such as epoxies are less Epoxide toxic and monomers irritating curedin comparison via cationic to routefrequently have lowerused acrylates volumetric in free shrinkage radical andphotopol also provideymerization. better Epoxide matrix monomers adhesion ascured opposed via cationic to free radicalroute have photopolymerized lower volumetric monomers shrinkage [17 ,24and–26 also]. Therefore, provide better most ofmatrix the early adhesion investigations as opposed have to beenfree focusedradical onphotopolymerized UV curing of epoxide monomers monomers [17,24–26]. for inks, Ther coatingsefore, andmost adhesives of the early where investigations they provide have high polymerizationbeen focused on rates, UV insensitivity curing of epoxide to monomers and low energyfor inks, requirement coatings and [27 –adhesives31]. where they provide high polymerization rates, insensitivity to oxygen and low energy requirement [27–31].

SchemeScheme 1. 1.Reaction Reaction schemescheme forfor cationiccationic ringring openingopening frontalfrontal polymerization of oxiranes. (1) (1) Photo- Photo- inducedinduced cleavage cleavage of of iodonium iodonium saltsalt fromfrom aa singletsinglet (S1)(S1) andand/or/or triplettriplet (T1) excited state, that that generates generates cations,cations, radical radical cations cations and and free free radicals radicals ultimatelyultimately producingproducing a superacid; (2) Protonation of of oxirane oxirane monomer;monomer; (3)(3) Exothermic re reactionaction of of protonated protonated monomer monomer with with oxiranes oxiranes resulting resulting in a polymer in a polymer chain chaingrowth. growth.

However,However, the the main main limitation limitation of UV-induced of UV-induced crosslinking crosslinking reaction reaction is related is to related the low to penetration the low depthpenetration of UV light,depth which of limitsUV polymerizationlight, which techniques,limits polymerization even cationic photopolymerization,techniques, even cationic to thin filmsphotopolymerization, and adhesive applications to thin [ 31films,32]. Theand firstadhesive report onapplications photoactivated [31,32]. cationic The polymerizations first report on to achievephotoactivated self-sustained cationic frontal polymerizations polymerizations to achieve was reported self-sustained by Crivello frontal et al. [polymerizations33], who showed was that thereported local application by Crivello of heatet al. to [33], an irradiated who showed monomer that the sample local results application in polymerization of heat to an that irradiated occurs as amonomer front, propagating sample results rapidly in throughoutpolymerization the entire that oc reactioncurs as massa front, [33 –propagating35]. rapidly throughout the Inentire 2004, reaction Mariani mass et al. [33–35]. [36] proposed the idea to combine cationic photopolymerization with frontal polymerization,In 2004, Mariani as a novel et al. method [36] proposed to cure the epoxy idea monomers to combine via cationic UV irradiation. photopolymerization The cationic with ring frontal polymerization, as a novel method to cure epoxy monomers via UV irradiation. The cationic ring opening polymerization of epoxide monomer is a highly exothermic reaction producing 18–24 kcal/mol of energy [37]. It was shown that the frontal polymerization is started by the dissociation of

Polymers 2020, 12, 2146 3 of 34 opening polymerization of epoxide monomer is a highly exothermic reaction producing 18–24 kcal/mol Polymers 2020, 12, x FOR PEER REVIEW 3 of 33 of energy [37]. It was shown that the frontal polymerization is started by the dissociation of a radical thermal initiator (RTI) promoted by the heat released during surface UV-induced cationic ring-opening a radical thermal initiator (RTI) promoted by the heat released during surface UV-induced cationic polymerization. Subsequently, the carbon-centred radicals are oxidized to carbocations in the presence ring-opening polymerization. Subsequently, the carbon-centred radicals are oxidized to carbocations of the iodonium salt in a radical-induced cationic frontal polymerization (RICFP) mechanism [36]. in the presence of the iodonium salt in a radical-induced cationic frontal polymerization (RICFP) In the last few years, the term RICFP has been used to define curing of epoxy and its composites mechanism [36]. In the last few years, the term RICFP has been used to define curing of epoxy and specifically induced by UV light (Scheme2)[36,38–42]. its composites specifically induced by UV light (Scheme 2) [36,38–42].

SchemeScheme 2.2. Reaction scheme illustrating thethe photocleavage ofof anan iodoniumiodonium saltsalt photoinitiatorphotoinitiator togethertogether withwith disassociationdisassociation of aof thermal a thermal radical radical initiator initiato to producer to produce radical cations radical and cations free radicals and free respectively, radicals duringrespectively, frontal during polymerization frontal polymerization of an epoxy monomer of an (M).epoxy Extracted monomer from (M). Composites Extracted Part from B:Engineering, Composites 143,Part M. B: Sangermano,Engineering, A.143, D’Anna, M. Sangermano, C. Marro, N.A. Klikovits,D’Anna, C. R. Marro, Liska, UV-activatedN. Klikovits, frontalR. Liska, polymerization UV-activated offrontal glass fibre-reinforcedpolymerization epoxyof glass composites, fibre-reinforced 168–171, epoxy Copyright composites, (2018) 168–171, with permission Copyright from (2018) Elsevier. with permission from Elsevier. The purpose of the current review is to give a comprehensive overview of studies on RICFP that haveThe been purpose investigated of the in current the literature review so is far.to give Since a comprehensive epoxy-based monomers overview are of studies the most on widely RICFP used that componentshave been investigated for transport in the and literature construction so far. sectors, Since epoxy-based this review ismonomers also aimed are for the providing most widely a guide used forcomponents UV frontal for curing transport of various and construction epoxy monomers sectors, used this commercially. review is also A aimed few authors for providing have presented a guide reviewsfor UV frontal on UV-activated curing of variou cationics epoxy polymerization monomers [25 used,43,44 commercially.], but to the best A few of our authors knowledge, have presented a review onreviews the radical-induced on UV-activated cationic cationic frontal polymerization polymerization [25,43,44], of epoxide but monomersto the best has of notour beenknowledge, reported a soreview far. on the radical-induced cationic frontal polymerization of epoxide monomers has not been reported so far. 2. Chemistry of Cationic Photoinitiators Based on Diaryliodonium Salts 2. ChemistryIn the past, of organometallic-basedCationic Photoinitiators photoinitiators Based on Diaryliodonium have been developed Salts and employed for cationic photopolymerizationIn the past, organometallic-ba of . Somesed of photoinitiators the photoinitiators have included been developed manganese and decacarbonyl employed [ 45for], complexescationic photopolymerization of and titanium of epoxides. tetrachloride Some [of46 ],the and photoinitiators diazonium salts included [47,48 ].manganese However, thesedecacarbonyl photoinitiators [45], hadcomplexes certain drawbacksof ferrocene that and limited titanium their applicabilitytetrachloride for [46], epoxide and curing. diazonium The Lewis salts [47,48]. However, these photoinitiators had certain drawbacks that limited their applicability for epoxide curing. The Lewis acid generated from photodecomposition of a ferrocenium salt is weaker than a Brønsted acid produced from iodonium salts and required heating to complete the epoxy curing process. Moreover, ferrocenium salts suffer from poor solubility in various epoxy resins [17].

Polymers 2020, 12, 2146 4 of 34 acid generated from photodecomposition of a ferrocenium salt is weaker than a Brønsted acid produced Polymersfrom iodonium 2020, 12, x saltsFOR PEER and requiredREVIEW heating to complete the epoxy curing process. Moreover, ferrocenium4 of 33 salts suffer from poor solubility in various epoxy resins [17]. In contrast, aryldiazonium salts produce Innitrogen contrast, gas aryldiazonium in epoxy curing salts due produce to its inherentnitrogen thermal gas in epoxy instability curing [17 due]. On to theits inherent contrary, thermal epoxies instabilitycured with [17]. iodonium-based On the contrary, photoinitiators epoxies providecured with excellent iodonium-based adhesion to variousphotoinitiators substrates provide due to excellentpresence adhesion of hydroxyl to various groups substrates in the growing due to polymerpresence chain. of hydroxyl Iodonium groups salts in the are growing colourless polymer to pale chain.yellow Iodonium crystalline salts compounds are colourless having totuneable pale yellow reactivity crystalline for epoxides. compounds Figure having1 shows tuneable the structure reactivity of fora diaryliodonium epoxides. Figure photoinitiator. 1 shows the Longerstructure substituted of a diaryliodonium alkyl chains photoinitiator. (R groups in FigureLonger1) substituted can reduce alkyltoxicity chains and increase(R groups solubility in Figure of iodonium1) can reduce salts toxicity in epoxide and monomers. increase solubility In addition, of iodonium their reactivity salts canin epoxidebe increased monomers. by the additionIn addition, of long their wave reactivity sensitizers can and be radicalincreased generators by the thataddition also preventof long releasewave sensitizersof toxic gases and during radical photocleavage, generators that as also opposed preven tot most release frequently of toxic used gases reactive during sulfonium photocleavage, salts that as opposedgenerate to toxic most diphenyl frequently sulphide used gasreactive [17,49 sulfoniu,50]. Morem salts details that about generate sensitizers toxic diphenyl will be discussed sulphide in gas the [17,49,50].section “E ffMoreect of details sensitizers”. about sensitizers will be discussed in the section “Effect of sensitizers”.

FigureFigure 1. 1. ChemicalChemical structure structure of of a a diaryliodonium diaryliodonium salt salt photoinitiator. photoinitiator. Mt Mt represents represents a a metallic metallic element. element.

TheThe cationic cationic and and anionic anionic species species in inan aniodonium iodonium salt saltphotoinitiator photoinitiator have have individual individual roles rolesto play to duringplay during cationic cationic ring ringopening opening polymerization polymerization of epoxides of epoxides [51]. [51 To]. Tobegin begin with, with, light light absorption, absorption, quantumquantum yield, yield, photosensitivity photosensitivity and and rate rate of of photolysis photolysis of of the the iodonium iodonium salt salt is is controlled controlled by by the the cation. cation. InIn addition, addition, thermal thermal stability stability of of the the iodonium iodonium salt salt is also is also governed governed by the by type the typeof cation of cation present. present. The anion,The anion, on the on other the other hand, hand, controls controls the thecuring curing reaction reaction in inthe the absence absence of of UV UV light light and and therefore therefore determinesdetermines the the strength strength of of the the photogenerated photogenerated Brønst Brønsteded acid, acid, and and the the reactivity reactivity and and nature nature of of cation cation radicalsradicals generated generated during during photolysis photolysis of of the the iodonium iodonium salt. salt. The The rate rate and and extent extent of of chain chain growth growth of of the the epoxideepoxide monomer monomer during during cationic cationic poly polymerizationmerization is is also also influenced influenced by by the the nature nature of of the the anion anion [23,51]. [23,51]. Ideally,Ideally, the the anion anion should should be be highly highly non-nucleophil non-nucleophilicic to to prevent prevent a a recombination recombination with with cations cations during during photocleavagephotocleavage of of the the iodonium iodonium salt, salt, which which would would further further prevent prevent the the formation formation of of radical radical cations cations andand free free radicals radicals required required to to generate generate the the superacid superacid [51]. [51]. This This relative relative nucleophilicity nucleophilicity of of the the anion anion is is classifiedclassified into into three three groups groups of of anions anions based based on on their reactivity in cationic polymerization. Strongly Strongly nucleophilicnucleophilic anions anions such such as as I− I, −Cl, Cl−, Br−,− Br generate− generate weak weak Brønsted Brønsted acids acids and the and the pairs ionpairs collapse collapse to give to stablegive stable covalently covalently bonded bonded compounds. compounds. The second The second type typeof anions, of anions, including including HSO HSO4−, ClO4−, ClO4− etc.,4− etc.,are calledare called intermediate intermediate nucleophilic nucleophilic anions, anions, while while SbF SbF6−,6 −AsF, AsF6−,6 −PF, PF6−, 6−BF, BF4− 4are− are termed termed as as weakly weakly nucleophilicnucleophilic anions [51]. [51]. The latter type ofof anionsanions areare arrangedarranged in in the the order order of of decreasing decreasing reactivity reactivity in incationic cationic polymerization polymerization specifically specifically for for epoxide epoxid monomerse monomers [16]. [16]. These These weakly weakly nucleophilic nucleophilic anions anions used usedas photoinitiators as photoinitiators slow down slow the down polymer the chainpolyme terminationr chain processtermination during process cationic during polymerization. cationic polymerization.This means that This the ring means opening that the polymerization ring opening polymerization of epoxides proceeding of epoxides via cationicproceeding mechanism via cationic can mechanismbe seen as a can “living be seen polymerization” as a “living polymerization” i.e., the polymerization i.e., the polymerization proceeds until proceeds ring chain until equilibria ring chain has equilibriabeen established, has been even establ afterished, the even irradiation after the has irradiation ceased [52 has,53]. ceased [52,53]. ItIt is alsoalso important important that that the the emission emission spectrum spectrum of the of irradiation the irradiation source matchessource withmatches the absorptionwith the absorptionproperties ofproperties the iodonium of the iodonium salt photoinitiator salt photoinitiator so that cation so that and cati freeon and radicals free radicals are produced are produced during duringinitiation initiation [51]. The [51]. absorption The absorption wavelength wavelength of a diaryliodonium of a diaryliodonium salt is very salt crucial is very while crucial selecting while the selectingirradiation the source irradiation [17]. Diaryliodonium source [17]. saltsDiaryliodonium typically have salts absorption typically wavelengths have absorption below 300 wavelengths nm. Selected belowexamples 300 of nm. such Selected salts are examples diphenyliodonium of such salt hexafluorophosphates are diphenyliodonium (λmax = 227hexafluorophosphate nm), phenyl-p-octylphenyl (λmax = 227 nm), phenyl-p-octylphenyl iodonium hexafluoroantimonate (λmax = 240 nm), 4-[(2- hydroxytetradecyl)oxy]phenyl] phenyliodonium hexafluoroantimonate (λmax = 250 nm) [17]. In addition, a two-fold increase in light intensity doubles the rate of polymerization of epoxy containing monomers [52]. The cationic polymerization is also inhibited in the presence of bases, water and hydroxyl compounds, which may act as agents or inhibitors [31,51].

Polymers 2020, 12, 2146 5 of 34

iodonium hexafluoroantimonate (λmax = 240 nm), 4-[(2-hydroxytetradecyl)oxy]phenyl] phenyliodonium hexafluoroantimonate (λmax = 250 nm) [17]. In addition, a two-fold increase in light intensity doubles the rate of polymerization of epoxy containing monomers [52]. The cationic polymerization is also inhibited in the presence of bases, water and hydroxyl compounds, which may act as chain transfer agents or inhibitors [31,51].

3. Onium Salt Photoinitiators for Different Epoxy Monomers In early works on the chemistry of diaryliodonium salt photoinitiators, Crivello and Lam [19], studied the effect of different parameters on photodecomposition of these salts to yield cations and cation radicals. They showed that photodecomposition of 4,40-dimethyldiphenyliodonium hexafluoroarsenate is linearly dependant on irradiation time. However, at wavelengths higher than 300 nm, the photodecomposition of this iodonium salt was drastically reduced. It was also reported that temperature variations between 15–35 ◦C and oxygen environment had no significant effect on photodecomposition of the iodonium salt. In addition, there is a linear dependence of the rate of photolysis on the light intensity. Since only the nature of the cation affects photosensitivity, slight variations in absorption wavelengths gave negligible differences in the photodecomposition of methoxy, methyl, bromo and chloro substituted diphenyliodonium salts. The results also revealed that most of the diphenyliodonium salts absorb at the region of the small extinction coefficient tail of major absorption band, i.e., 237–238 nm [19]. Crivello et al. [20], performed photopolymerization of some cyclic monomers such as cyclohexene oxide, to demonstrate the effect of the nature of anion in cationic polymerization using a triphenylsulfonium salt photoinitiator. Using a high-power mercury lamp as UV source, triphenylsulfonium salts bearing each of the anions SbF6−, AsF6−, PF6−, BF4− were investigated with cyclohexene oxide as monomer, and the conversion vs. irradiation time was monitored. The results showed that conversion of the monomers was very high with triphenylsulfonium salts bearing SbF6− anion, followed by AsF6− (<70%), PF6− (<50%) and BF4− (<5%) for cyclohexene oxide within 15 min of irradiation [20]. It was concluded that the larger the size of the anion in a photoinitiator is, the more active the propagating cationic species will be in the ring opening polymerization [20]. In another study, Crivello et al. [27] also demonstrated the effect of photoinitiator concentration, wavelength and intensity of irradiating light, effect of temperature and water vapour on the photopolymerization of cyclohexene oxide. To sum up, triphenyl sulfonium salts added with 1.5% molar concentration and high UV intensity gave maximum cure rate within 20 s of irradiation. In addition, temperatures above 30 ◦C and amounts of water greater than 1–2% by weight increase the time to reach maximum epoxy curing and irradiation time, respectively [27]. In a study on photoinitiated cationic polymerization of various silicon-containing epoxies, Crivello and Lee [54] showed that diaryliodonium salt containing a SbF6− anion gives higher polymerization rate and exothermicity than PF6− in the cationic ring opening polymerization of silicon-containing cycloaliphatic epoxy [54]. In their study, Crivello et al. [49] investigated the properties of 4-alkoxy-substituted diaryliodonium salt photoinitiators in cationic polymerization of heterocyclic monomers. The general structure of the 4-alkoxy iodonium salt photoinitiator is shown in Figure2. The R 3 group in this structure was varied between C8 to C18 in order to characterize the effect on melting point, solubility, spectral characteristics, toxicology and photosensitivity of the iodonium salt photoinitiator. It was found that the melting point increases linearly with a higher number of carbon atoms and all photoinitiators having alkoxy substitution > C8 were soluble in toluene, styrene and some epoxidized organic compounds. The absorption wavelengths for all these photoinitiators varied between 245–247 nm only, i.e., in the short wavelength region of UV spectrum. This slight variation in absorption wavelength for substituted alkyl groups was also reported for diaryliodonium salt photoinitiators in a previous study [19]. The toxicity (lethal oral dose) for C8 to C18 was also lower in comparison to methoxy substituted salts. In addition, the alkoxy-substituted diaryliodonium salts gave higher UV cure rates in comparison to alkyl substituted diaryliodonium salts. By increasing the concentration of alkoxy- Polymers 2020, 12, 2146 6 of 34

substituted salt up to 3%, the photo efficiency could be improved which enabled a cure with even low power lamps. These salts could also be photosensitized with various species to further increase their photosensitivity [49]. Polymers 2020, 12, x FOR PEER REVIEW 6 of 33

FigureFigure 2.2. Chemical structure ofof a 4-alkoxy substituted photoinitiator.photoinitiator. Redrawn fromfrom J.J. V. CrivelloCrivello andand J.J. L.L. Lee,Lee, JournalJournal ofof PolymerPolymer ScienceScience PartPart A:A: PolymerPolymer Chemistry,Chemistry, 1989,1989, 27,27, 3951–3968,3951–3968, JohnJohn WileyWiley andand Sons,Sons, CopyrightCopyright 19891989 JohnJohn WileyWiley && Sons,Sons, Inc.Inc.

CastellanosCastellanos etet al.al. [[55]55] synthesizedsynthesized aa novelnovel diaryliodoniumdiaryliodonium saltsalt photoinitiatorphotoinitiator basedbased onon tetrakistetrakis (pentafluorophenyl)(pentafluorophenyl) borate borate anion anio andn performedand performed thin film thin cationic film polymerizationcationic polymerization with a cyclohexene with a oxidecyclohexene epoxy andoxide silicone epoxy epoxy and monomers. silicone epoxy The group monomers. reported The that group the rate reported of propagation that the of tetrakisrate of (pentafluorophenyl)propagation of tetrakis borate (pentafluorophenyl) anion is twice as highborate as anion the rate is oftwice propagation as high as of the hexafluoroantimonate rate of propagation anionof hexafluoroantimonate in the cationic polymerization anion in the of cationic dicyclohexene polymerization oxide epoxy of dicyclohexene and 2.5 times oxide higher epoxy in the and case 2.5 oftimes epoxy higher silicone in the monomer. case of Itepoxy was also silicone found mono experimentallymer. It was that also the found mechanism experimentally of photolysis that forthe tetrakismechanism (pentafluorophenyl) of photolysis for borate tetrakis anion (pentafluoro type photoinitiatorphenyl) borate and anion its relative type productsphotoinitiator is similar and toits hexafluoroantimonaterelative products is similar type iodoniumto hexafluoroantimonate salt. This novel type anion iodonium iodonium salt. salt This photoinitiator novel anion also iodonium proved tosalt be photoinitiator resistant to humidity also proved with to several be resistant days pot to humidity life in epoxy with silicone several monomer days pot [life55]. in epoxy silicone monomerIn the [55]. first study on radical-induced cationic frontal polymerization conducted by Mariani et al. [36], {4-[(2-hydroxytetradecyl)In the first study on oxy] radical-induced phenyl} phenyliodonium cationic frontal hexafluoroantimoniate polymerization conducted (HOPH) by as photoinitiatorMariani et al. was[36],tested {4-[(2-hydroxytetradecyl) in various concentrations oxy] withphenyl} bis-cycloaliphatic phenyliodonium epoxide hexafluoroantimoniate (CE) and dibenzoyl (HOPH) peroxide as (BPO)photoinitiator as thermal was initiator. tested Itin was various found concentrations that the minimum with ratio bis-cycloaliphatic of HOPH and BPOepoxide required (CE) forand a self-sustainingdibenzoyl peroxide front is(BPO) 1 mol as/mol. thermal With initiator. a constant It 1was mol found/mol HOPH that the/BPO minimum molar concentration, ratio of HOPH results and ofBPO RICFP required experiments for a self-sustaining proved that an front increase is 1 mol/mol. in HOPH With with respecta constant to CE 1 mol/mol concentration HOPH/BPO increased molar the frontalconcentration, velocity results up to a of molar RICFP ratio experiments of 2, while aproved maximum that frontan increase temperature in HOPH of 256 with◦C wasrespect observed to CE atconcentration a molar ratio increased of 1.35. Atthe a higherfrontal molarvelocity ratio up of to 3 mola molar/mol %ratio of HOPHof 2, while/CE, the a maximum frontfront temperaturetemperature decreasedof 256 °C towas 232 observed◦C. Results at afrom molar Di ratiofferential of 1.35. Scanning At a higher Calorimetry molar ratio (DSC) of plots 3 mol/mol showed % thatof HOPH/CE, both initiators the aremaximum necessary front to allow temperature curing also decr ineased the bulk to of232 sample °C. Results where from light penetrationDifferential isScanning hindered Calorimetry [36]. (DSC) plots showed that both initiators are necessary to allow curing also in the bulkIn the of studies sample conducted where light by penetration Bomze et al. is [38 hindered] on radical-induced [36]. cationic frontal polymerization of variousIn epoxies,the studies two conducted commercially by Bomze available et al. photoinitiators [38] on radical-induced were tested cationic for their frontal applicability polymerization in RICFP ofof bisphenolvarious epoxies, A diglycidyl two commerciall (BADGE).y available It was photoinitiators found that RICFP were with tested equimolar for their ratio applicability of a thermal in initiatorRICFP of and bisphenol diphenyliodonium A diglycidyl tetrakis ether (pentafluorophenyl)(BADGE). It was found borate that as RICFP photoinitiator with equimolar gave a successful ratio of a self-sustainingthermal initiator polymerization and diphenyliodonium front. Comparing tetrakis (pen thistafluorophenyl) photoinitiator with borate a triphenylsulfonium as photoinitiator gave salt, a itsuccessful was found thatself-sustaining the latter did notpolymerization generate a front front. even atComparing 5 wt% in the resin.this Thisphotoinitiator was explained with by thea lowertriphenylsulfonium redox potential salt, of sulfonium it was found salts that compared the latter to thedid diaryliodonium not generate a onesfront [38even]. In at another 5 wt% study,in the Bomzeresin. etThis al. [was39] investigated explained theby etheffect lower of variable redox concentration potential of of psulfonium-(octyloxyphenyl)phenyliodonium salts compared to the hexafluoroantimonatediaryliodonium ones (PAG)[38]. In on another storage study, stability, Bomze thermal et al. and [39] frontal investigated polymerization the effect properties of variable of a formulationconcentration consisting of p-(octyloxyphenyl)phenyliodonium of BADGE and tetraphenyl ethanediol hexafluoroantimonate (TPED). Results confirmed(PAG) on the storage good storagestability, stability thermal of and a formulation frontal polymerization of BADGE with properties 1 mol/mol of molar a formulation ratio of PAG consisting/TPED giving of BADGE only slight and variationtetraphenyl inviscosity ethanediol for 28(TPED). days ofResults storage. confirmed In addition, the the good maximum storage front stability velocity of a and formulation temperature of wereBADGE also with una 1ff mol/molected by molar the storage ratio of time. PAG/TPED It was giving also shown only slight that variation a formulation in viscosity of BADGE for 28 withdays 1of mol storage. % PAG In and addition, increasing the TPEDmaximum concentration front velocity reduces and the temperature maximum front were temperature. also unaffected In contrast, by the withstorage 2 mol time. % PAGIt was the also front shown temperature that a formulation reaches a of maximum BADGE with of 215 1 mol◦C up % toPAG 1 mol and % increasing TPED but TPED then decreasesconcentration with reduces increasing the concentrationmaximum front of TPED.temperat Theure. glass In transitioncontrast, with temperature 2 mol % (T PAGg) of the BADGE front temperature reaches a maximum of 215 °C up to 1 mol % TPED but then decreases with increasing concentration of TPED. The glass transition temperature (Tg) of BADGE formulation changes only slightly with the change in molar ratio of PAG:TPED. In particular, 168 and 160 °C were obtained at 2:1 and 1:1 molar ratio of PAG:TPED, respectively. It should be further noted that a higher Tg was achieved with frontally cured epoxy resins than with only photocured and thermally cured counterparts [39].

Polymers 2020, 12, 2146 7 of 34 formulation changes only slightly with the change in molar ratio of PAG:TPED. In particular, 168 and 160 ◦C were obtained at 2:1 and 1:1 molar ratio of PAG:TPED, respectively. It should be further noted that a higher Tg was achieved with frontally cured epoxy resins than with only photocured and thermally cured counterparts [39]. Klikovits et al. [56] synthesized novel photoinitiators including diphenyliodonium tetrakis(perfluoro- t-butyloxy)aluminate (I-Al) and tris(4-((4-acetylphenyl)thio)phenyl)sulfonium tetrakis(perfluoro- t-butyloxy)aluminate (S-Al) that exhibited a higher photo efficiency than the diphenyliodonium hexafluoroantimonate (I-Sb) and diphenyliodonium tetrakis(perfluorophenyl) borate (I-B) photoinitiators in the cationic ring opening polymerization of epoxide monomers. This weakly coordinating anion (WCA) based on the tetrakis(perfluoro-t-butyloxy)aluminate anion gave a stronger Brønsted acid than the conventional antimonate and borate anions. The novel photoinitiators I-Al and S-Al were tested with BADGE and 1,4-bis[(3-ethyl-3-oxetanylmethoxy) methyl] benzene (BOB) and compared with their respective SbF6 and (BC6F5)4− anions. It was found that both diphenyliodonium and triarylsulfonium salts based on Al anion exhibited higher polymerization rates and monomer conversion than their corresponding SbF6− and (BC6F5)4− anions [56]. Knaack et al. [40] also tested an epoxy formulation consisting of BADGE, I-Al and TPED for its effect on frontal polymerization properties. It was found that a minimum of 0.015 mol % of I-Al with 1 mol % TPED is sufficient to achieve a self-sustaining front. 1 Front velocity and temperature increase up to 1 mol % of I-Al reaching a maximum of 10 cm min− velocity with a front temperature of 210 ◦C. Comparing the front starting time of I-Al with I-Sb, the group showed that front starting time decreases with the content of I-Al whereas for I-Sb, no front was possible below 1 mol %. The onset temperature of thermal curing for the BADGE formulation with 0.015 mol % I-Al was only slightly lower than with 1 mol % I-Sb. Results from storage stability tests conducted on a formulation of BADGE with 0.015 and 1 mol % I-Al showed no significant change in viscosity and corresponding frontal velocity and frontal temperature for 28 days of storage. More than 99% conversion for BADGE was achieved with a formulation of 1 mol % I-Al, while with I-Sb a 97% conversion was obtained. The Tg of the cured formulation with 0.5 mol % I-Al was almost similar to the Tg for the BADGE formulation cured with 1 mol % I-Sb. The group also investigated the effect of variable mould depth on the frontal polymerization of BADGE using I-Al and I-Sb photoinitiators by measuring the minimum layer thickness of the cured polymer. The results showed a three times lower minimal layer thickness of the cured formulation containing I-Al compared to I-Sb at 1 mol % with increasing concentration of thermal radical initiator [40]. Wang et al. [57] reported a novel combination of a photoinitiator based on diaryliodonium salt and silanes that gave adjustable gel time of polymerization as well as excellent storage stability for bisepoxides. This unique formulation provided the advantage of multi-mechanism curing, which includes redox free radical, free radical and thermal/UV-induced cationic polymerization. They proved the positive influence of silane groups in cationic photo polymerization of epoxy, where Si-H groups where able to participate in the initiation and promotion of epoxy polymerization [57].

Effect of Sensitizers Sensitization is required in formulations containing a photoinitiator that is unable to absorb in the spectral region of the provided irradiation source. Upon absorption of light, the sensitizer is photoexcited and transfers the energy to the photoinitiator, which reacts by cleavage and release of reactive species. Different types of sensitizers have been classified by Fouassier [31] and Green [17]. Photosensitization of onium salts can be divided into three mechanisms: (1) oxidation of certain photolysable compounds by photoinitiator to form free radicals, (2) electron transfer between a photoexcited molecule and onium salt to give sensitizer radical cations and (3) electronically excited charge-transfer complex formed between an aromatic pyridinium and onium salt to generate aromatic radical cations [58–60]. Diaryliodonium salts cannot photo-decompose under visible light (400–800 nm) and require sensitization togenerateradicalsandcationsduringinitiation. DifferentorganicdyeswereinvestigatedbyCrivello et al.[21] Polymers 2020, 12, 2146 8 of 34

for their sensitization effect on 4,40-dimethyldiphenyliodonium hexafluoroarsenate. It was shown that sensitized photopolymerization of cyclohexene oxide with sensitizers benzoflavin and acridine orange is possible with visible light within few minutes of irradiation. 4,40-dimethyldiphenyliodonium hexafluoroarsenate sensitized with benzoflavin and acridine orange gives a photodecomposition of 60 and 46%, respectively, and there exists a maximum concentration limit for sensitizers that could effectively photolyse the diphenyliodonium salts. The λmax for benzoflavin and acridine orange was found to be 460 and 539 nm, respectively, in the cationic photopolymerization of cyclohexene oxide. Dye sensitized photopolymerization of cyclohexene oxide and epichlorohydrin was also compared using 4,40-di-tert-butyldiphenyliodonium hexafluoroarsenate as photoinitiator. A 35% conversion was achieved within 900 s of irradiation for cyclohexene oxide while due to sluggish reactivity of epichlorohydrin, conversion was achieved after 2.5 h of irradiation [21]. Crivello and Lam [54], investigated the photosensitivity of 2-chlorothioxanthone (λmax = 386 nm) using a diphenyliodonium-based photoinitiator in the cationic polymerization of -functional epoxy monomers. Significant increase in polymerization rate was observed using 0.05 mol % of this sensitizer with 0.5 mol % of the diphenyliodonoium salt [54]. In 1994, Yagci and Schnabel [61] demonstrated the photosensitivity of pyridinium and quinolinium salts in the cationic photopolymerization of epoxy and vinyl ether monomers. They showed that reactive cations for the chain initiation can be formed either by direct photolysis of the pyridinium or by electron transfer from photogenerated free radicals or from photoexcited sensitizers to pyridinium salts [61]. A detailed study on the pyridinium and benzyl salts sensitization mechanism was also published by Yagci and Endo [62]. Bi and Neckers [63] introduced a novel combination of xanthene dye and an aromatic amine combined with diaryliodonium salt photoinitiator that was able to initiate the cationic ring opening polymerization of various epoxide monomers under visible light (>520 nm). The xanthene dye absorbs visible light to produce free radicals, which are oxidized by the diaryliodonium salt and reduced by aromatic amine to form α-amino radical, that is further oxidized by iodonium salt to generate α-amino carbocation, ultimately initiating the cationic polymerization. The xanthene dye provided a range of absorption maxima in order to have adjustable photosensitivity of the epoxy and iodonium formulation. The results also showed different irradiation times for a range of epoxide monomers. For example, bisphenol A diglycidyl ether required <300 min of irradiation, while cyclohexene oxide only 10 min. However, this initiator system cannot polymerize bis-cycloaliphatic epoxy monomers [63]. Yagci et al. [58] introduced a ternary system consisting of an alkyl halide, a dimanganese decacarbonyl, and a diaryliodonium salt capable of performing cationic polymerization of bisepoxides under visible light (435.8 nm). Free radical-promoted cationic polymerization of cyclohexene oxide was demonstrated using this ternary system. They showed that the oxidation of carbon-centred radicals to yield reactive cations depends on the polarity of alkyl halide and the redox potentials of radicals and iodonium salt [58]. Chen et al. [64] reported high photosensitization efficiency of carbazole derivatives (λmax = 365 nm) in the cationic photopolymerization of bis-cycloaliphatic epoxy monomer using diaryliodonium salt photoinitiators. These electron transfer photosensitizers provided either singlet or triplet excited photosensitization depending on the substituent groups in the carbazole compounds [64]. In 2000, Hua et al. [65] showed a marked influence of N-vinylcarbazole as a photosensitizer and as an accelerator in the cationic ring opening polymerization of various cyclic and aliphatic epoxide monomers using diaryliodonium salt photoinitiators. This photosensitizer follows an electron transfer photosensitization mechanism that involves a redox reaction between the excited carbazole and the onium salt. At high concentration, the presence of a photoinitiated free-radical chain reaction results in the generation of a large number of cationic species capable of initiating both N-vinylcarbazole and epoxide ring-opening polymerizations [65]. In another study, Hua and Crivello [66] reported enhanced UV spectral absorption of the polymer of N-vinylcarbazole, proving its remarkable capability as photosensitizer in the cationic ring opening polymerization of epoxy monomers. Polymers 2020, 12, 2146 9 of 34

Crivello et al. [67] developed a novel sensitized cationic curing mechanism for multifunctional epoxy monomers by combining direct and free radical-induced decomposition of diaryliodonium salts. They showed that camphorquinone upon photoexcitation generates radical species by abstracting a from monomer that induces the free radical decomposition of the diaryliodonium salt. Different cyclic epoxide monomers were tested, and it was found that bis-cycloaliphatic epoxy was the most reactive epoxy, stabilizing the free radical and cationic species that are formed. The results showed that aryl ketones, 1,2-diketones and quinones are effective photosensitizers, allowing cationic curing of epoxies under visible and long-wavelength UV [67]. In early 2002, Hua et al. [68] introduced a novel electron transfer photosensitizer based on polynuclear aromatic hydrocarbons modified with hydroxymethyl groups. These hydroxyl-methylated compounds such as 9-anthracenemethanol, pyrene, 3-perylenemethanol were able to participate in the cationic ring opening polymerization of 4-vinylcyclohexene diepoxide through the activated monomer mechanism due to the presence of hydroxyl groups, hence increasing the polymerization rate. These hydroxyl groups enhance solubility of the photosensitizer in the epoxy monomer and also reduce toxicity of the photosensitizer by forming covalent bonds with terminating ether groups in the polymer chain [68]. Gomurashvili and Crivello [69] reported dual property of phenothiazines as photosensitizers as well as co-monomers in the cationic ring opening polymerization of various epoxide monomers. These polymerizable photosensitizers also possessed an absorption maximum between 254–333 nm and were able to speed up the cationic ring opening polymerization of various epoxides [70]. In another study, the photosensitivity of an electron transfer photosensitizer based on pyrene derivatives in the cationic ring opening polymerization of epoxide monomers was demonstrated by Crivello and Jiang [71]. A diaryliodonium salt photoinitiator was sensitized with pyrene derivative and was found to give higher polymerization rates and monomer conversion for cyclo-epoxy and bis-cycloaliphatic epoxy monomers than with non-sensitized formulations [71]. In 2003, Crivello and Jang [72] prepared several anthracene compounds as electron transfer photosensitizers for cationic ring opening polymerization of various epoxide monomers. These anthracene compounds possessed a range of absorption maximum between 217–403 nm, among which 9,10- dialkoxyanthracene possessed the highest photosensitization efficiency. It was shown that anthracene derivatives with electron-withdrawing groups are not effective photosensitizers [72]. Different pigments (red, white and blue) were found to decrease the photoreactivity in the cationic polymerization of a siloxane and cyclohexene oxide using a diaryliodonium salt as photoinitiator [73]. A novel approach to sensitize epoxide compounds was introduced by Crivello et al. [74], in which electron transferphotosensitizerssuch asN-vinylcarbazole,N-allylcarbazole and9-vinylanthracene, werecovalently bound directly to the structure of the a polymeric photoinitiator. These polymer-bound sensitizers were found to accelerate the rate of polymerization in the cationic ring opening photopolymerization of difunctional epoxies as well as increase the solubility in various organic [74]. In 2005, Crivello and Bulut [75] reported curcumin as a naturally occurring long-wavelength (340–535 nm) photosensitizer capable to perform cationic ring opening polymerization of various epoxides. This electron transfer photosensitizer was found to be soluble in various monomers and able to initiate upon exposure with LEDs and sunlight. Due to its slightly acidic nature it did not retard cationic photopolymerization [75]. Subsequently, in 2006, the group demonstrated the broad UV and visible light absorption property of curcumin in the cationic ring opening polymerization of bio-based epoxy monomers [76]. Photopolymerization of synthetic oxirane and oxetane monomers was successfully performed upon exposure with 355–460 nm using curcumin-sensitized diaryliodonium salt photoinitiator. In addition, glass fibres impregnated with curcumin sensitized linseed oil with epoxy groups and Veronica galamensis oil was successfully fabricated and photopolymerized under direct sunlight [76]. Crivello [77] performed photopolymerization of neopentyl glycol diglycidyl ether in the presence and absence of 9,10-di-n-butoxyanthracene as a photosensitizer. In the presence of the photosensitizer, Polymers 2020, 12, 2146 10 of 34 the polymerization begins slowly upon UV exposure, with smaller induction period in comparison to non-sensitized formulations. The polymerization is auto-accelerated, as the temperature of the sample increases. The temperature–time curves showed that the profiles of sensitized and non-sensitized formulations are very similar and both formulations reach essentially the same maximum temperature (159 vs. 156 ◦C) [77]. In another study, Crivello [78] investigated a three-component system consisting of camphorquinone as sensitizer, a diaryliodonium salt as photoinitiator, and benzoyl alcohol. The epoxy curing followed a free radical chain-promoted cationic polymerization scheme induced by visible light, in which the latter photoexcites camphorquinone. Benzoyl alcohol donates a to generate a radical that reacts with diaryliodonium salt, producing a Brønsted acid. Crivello also studied the various experimental parameters that affect free radical chain-promoted cationic ring opening polymerization of limonene dioxide epoxy using optical pyrometry (monitoring temperature vs. irradiation time). A combination of limonene dioxide, camphorquinone, methoxybenzyl alcohol and photoinitiator based on tetrakis (pentafluorophenyl) borate anion cured the epoxy under visible light within 15 s with a highest temperature of more than 165 ◦C. It was found that cyclohexene oxide and 1,2-epoxyoctane displayed higher reactivities than n-butyl glycidyl ether. Crivello also fabricated a flat composite structure composed of four layers of glass cloth impregnated with a formulation consisting of dicycloepoxycarboxylate, 2.5% 4-n-(octyloxyphenyl) phenyliodonoium hexafluoroantimonate, 1.5% camphorquinone, and 20% methoxybenzyl alcohol. The assembled composite was exposed to direct sunlight for 3 min, during which fast, highly exothermic polymerization took place giving a completely dry, rigid and clear crosslinked laminate [78]. Along the same lines, a study on free radical chain-promoted cationic polymerization induced by visible light for various epoxide monomers was also done using titanium complex free radical photoinitiator instead of camphorquinone [79]. Similarly, another three-component system based on ethyl-4-dimethyl aminobenzoate, camphorquinone and a diaryliodonium salt showed epoxy curing upon visible light irradiation [80]. Pure BADGE has a sluggish reactivity in cationic polymerization. Moreover, sensitization has to be compromised with monomer conversion to achieve a successful polymerization front. In this regard, Liu et al. [81] reported cationic polymerization of BADGE with an iodonium salt photoinitiator sensitized with dihydroanthracene in 1:1 molar ratio quantity. They used light with an irradiation wavelength of 385 nm and found that heating the formulation from 30 to 60 ◦C increases the epoxy conversion by 25%. In addition, reactive diluents such as n-hexanol and dodecyl vinyl ether or the addition of a free radical initiator can also increase the rate of polymerization of BADGE [81]. Telitel et al. [82], reported chromophores based on boron–dipyrromethene (BODIPY) and boranil derivatives that were found to sensitize iodonium salt photoinitiator giving maximum absorption wavelengths in the range 400–600 nm and different oxidation potentials. Cationic polymerization of a bis-cycloaliphatic epoxy formulation sensitized with boranil and BODIPY derivatives was achieved at absorption wavelengths of 457 nm and 473 nm, respectively, with more than 60% monomer conversion upon 200 s of irradiation [82]. Recently, carbazole derivatives have also gained attention for their broad near-UV and visible light absorption [83–85]. In one study, Xiao et al. [86] found that N-vinylcarbazole possesses tri-functional properties, i.e., as a photoinitiator, additive and a monomer, that performs curing under the household UV-light emitting diode (LED) wavelength of 392 nm. In the case of epoxy,combination of N-vinylcarbazole and iodonium salt upon UV-LED irradiation generates cations based on N-vinylcarbazole that subsequently react with epoxy monomers to initiate the cationic ring opening polymerization. The group reported about 70% of monomer conversion within 1 min of UV irradiation [86]. Bomze et al. [39], investigated the effect of 2-isopropylthioxanthanone (ITX) with a bis-4- (methoxybenzoyl) diethylgermanium (Ivocerin) radical photoinitiator and perylene on photofrontal polymerization of BADGE. They showed that an optimum concentration of ITX is crucial to prevent shielding effect during sensitization of the diaryliodonium salt. A concentration of 0.2 mol % ITX sensitizer allows curing greater than UV light wavelengths without affecting frontal parameters in the RICFP of BADGE [39]. In another study, sensitization of two diaryliodonium salt photoinitiators Polymers 2020, 12, 2146 11 of 34 was compared using ITX upon light exposure within a wavelength range of 400–500 nm to analyse whether photopolymerization of BADGE was possible in the visible light spectral region [56]. It was found that sensitization of diaryliodonium salt based on tetrakis (perfluoro-t-butyloxy)aluminate anion Polymers 2020, 12, x FOR PEER REVIEW 11 of 33 enabled the polymerization of a BADGE formulation upon light exposure at wavelengths between 400 andcompared 500 nm. to Athe higher formulation polymerization consisting rate of and a monomersensitized conversion diaryliodonium were foundhexafluoroantimonate compared to the formulationphotoinitiator consisting [56]. of a sensitized diaryliodonium hexafluoroantimonate photoinitiator [56]. In their studies, Sangerma Sangermanono et al. [87] [87] demonstrated the effect effect of multi walled carbon nanotubes (MWCNT) as as free free radical radical generator generator induced induced by by visible visible light light for forthe thecationic cationic polymerization polymerization of bis- of bis-cycloaliphaticcycloaliphatic epoxy epoxy using using diaryliodonium diaryliodonium salt. salt. The The mechanism mechanism involves involves a a cationic cationic polymerization polymerization of epoxides through visible light generated electrons and holes from MWCNTs (Figure3 3A),A), followedfollowed by electron transfer reactions withwith thethe iodoniumiodonium saltsalt (Figure(Figure3 3B)B) [ 87[87].].

Figure 3. ((AA)) Radical polymerization upon upon visible visible light light irradiation irradiation on on multi multi walled walled carbon carbon nanotubes. nanotubes. (B) Oxidation of these radicals followed by hydrogen hydrogen abstraction. Extracted Extracted from from Marco Marco Sangermano, Sangermano, Damian Rodriguez, Monica C. Gonzalez, Enzo La Laurenti,urenti, and Yusuf Yagci, MacromolecularMacromolecular Rapid Communications, 2018,2018, 39, 39, 1800250, 1800250, Copyright Copyright 2018 WILEY-VCH2018 WILEY-VCH VerlagGmbH Verlag &GmbH Co. KGaA, & Co. Weinheim, KGaA, withWeinheim, permission with frompermission John Wiley from andJohn Sons. Wiley and Sons.

The incorporationincorporation of photoredoxof photoredox catalysts catalysts have also ha beenve consideredalso been inconsidered the cationic polymerizationin the cationic of epoxiespolymerization where the of latter epoxies provides where the the advantage latter provides of cheap and the safe advantage irradiation of sourcecheapsuch and assafe LEDs, irradiation sunlight etc.source [88 –such92]. Inas addition, LEDs, sunlight several newetc. [88–92]. compounds In addition, have also several shown new broad compounds visible light have absorption also shown in the cationicbroad visible polymerization light absorption of epoxides, in the such cationic as azahelicenes polymerization [93], rubrene of epoxides, [94], N such-[2-(dimethylamino)ethyl]- as azahelicenes [93], 1,8-naphthalimiderubrene [94], [N95-[2-(dimethylamino)ethyl]-1,8-naphthalimide], spiroxanthene [96], polyoxometalate [97], nanoparticles[95], spiroxanthene and ferrocenium [96], saltspolyoxometalate [98], etc. [97], nanoparticles and ferrocenium salts [98], etc. Several photoinitiator photoinitiator systems systems based based on on charge charge transfer transfer complex complex have have also also been been reported reported in the in thecationic cationic polymerization polymerization of epoxides of epoxides [99–102]. [99–102 In]. Inone one study, study, Garra Garra et et al. al. [103] [103 ]reported reported a a charge transfer complex (CTC)(CTC) basedbased onon mixturemixture ofof NN-phenylglycine-phenylglycine andand iodoniumiodonium saltsalt (NPG-Iod) (NPG-Iod)CTCCTC that proved to be an efficient efficient photoinitiating system in cationic polymerization of bis-cycloaliphatic epoxy monomer, enablingenabling aa curingcuring underunder LED irradiation withwith aa wavelength ofof 405 nm. The resulting epoxy network was clear, andand thickerthicker compositescomposites (about(about 99 cm)cm) werewere fabricatedfabricated usingusing (NPF-Iod)(NPF-Iod)CTCCTC epoxy formulation [[103].103].

4. Chemistry of Thermal Radical Initiators Mariani etet al. al. [36 [36]] were were the firstthe tofirst study to study the eff ectthe of effect a thermal of a radical thermal initiator radical on initiator the photofrontal on the polymerizationphotofrontal polymerization of a bis-cycloaliphatic of a bis-cycloaliphatic epoxy (CE). They epoxy found (CE). that They dibenzoyl found peroxide that dibenzoyl (BPO) as peroxide thermal 1 radical(BPO) as initiator thermal gave radical a maximum initiator gave front a maximum velocity of front 6 cm velocity min− with of 6 cm a maximum min−1 with front a maximum temperature front oftemperature 300 ◦C at of 0.67 300 mol °C at % 0.67 BPO mol/CE. % They BPO/CE. demonstrated They demonstrated the importance the importance of the combined of the combined effect of botheffect photoinitiatorof both photoinitiator and thermal and thermal radical initiatorradical initiator at equimolar at equimolar ratio in ratio achieving in achieving maximum maximum epoxy conversionepoxy conversion [36]. A [36]. detailed A detailed investigation investigation on the structure–propertyon the structure–property relationship relationship of thermal of thermal radical initiatorradical initiator on the curingon the of curing epoxy of resins epoxy was resins done wa bys Bomzedone by et al.Bomze [38]. et They al. showed[38]. They that showed generation that ofgeneration thermal radicalsof thermal in cationicradicals polymerizationin cationic polymerization of epoxy monomers of epoxy ismonomers dependent is ondependent the substituted on the Rsubstituted groups in R carbon–carbongroups in carbon–carbon (C-C) labile (C‒ compoundsC) labile compounds (Figure4). (Figure They 4). also They found also thatfound apart that fromapart from bulky (e.g., phenyl) groups present in C‒C labile compounds, presence of oxygen atom in the R group is also necessary to give a highly reactive thermal radical initiator capable of generating stable radical species [38].

Polymers 2020, 12, 2146 12 of 34 bulky (e.g., phenyl) groups present in C-C labile compounds, presence of oxygen atom in the R group is also necessary to give a highly reactive thermal radical initiator capable of generating stable radicalPolymers species2020, 12, x [38 FOR]. PEER REVIEW 12 of 33

Figure 4.4. General chemical structure of a carbon–carbon labilelabile thermalthermal radicalradical initiator.initiator.

ZhouZhou etet al.al. [[104]104] testedtested aa thermalthermal initiatorinitiator basedbased onon boronboron trifluoridetrifluoride amineamine complexcomplex inin thethe cationiccationic UV-inducedUV-induced frontalfrontal polymerizationpolymerization ofof aa bis-cycloaliphaticbis-cycloaliphatic epoxyepoxy withwith aa triaryltriaryl sulfoniumsulfonium saltsalt photoinitiator.photoinitiator. FrontFront velocity velocity and and front front temperature temperature increased increased with risingwith concentrationrising concentration of the thermal of the initiatorthermal initiator (from 3 up(from to 53 up wt. to %). 5 wt. They %). They showed show thated athat propagating a propagating front front angle angle is formed, is formed, which which is directlyis directly proportional proportional to theto the concentration concentration of theof the thermal thermal initiator, initiator, and and it can it can be controlled be controlled also also by the by additionthe addition of an of inorganic an inorganic filler. filler. Thermal Thermal and and mechanical mechanical tests tests confirmed confirmed higher higher curing curing degree degree of the of epoxythe epoxy monomer monomer than than the the thermally thermally cured cured resin resin with with high high values values of of T gTgdirectly directly proportional proportional toto anan increasingincreasing thermalthermal initiatorinitiator concentration.concentration. TheyThey alsoalso showedshowed thatthat almostalmost completecomplete epoxyepoxy conversionconversion cancan bebe achievedachieved withwith aa highhigh concentrationconcentration ofof thermalthermal radicalradical initiatorinitiator withoutwithout thethe needneed forfor aa postpost curingcuring [[104].104]. Analogous to this study, Knaack et al. [[40]40] showedshowed inin thethe frontalfrontal curingcuring ofof aa BADGEBADGE formulationformulation thatthat thethe presencepresence ofof aa thermalthermal radicalradical initiatorinitiator reducesreduces thethe onsetonset temperaturetemperature ofof thermalthermal curingcuring fromfrom 190190 toto 110110 ◦°C.C. In addition, an increase in concentration of tetraphenyltetraphenyl ethanediol (TPED) ((>1>1 mol %) only slightly decreases the thermal onsetonset temperaturetemperature ofof thethe curingcuring [[40].40].

5.5. UV-Induced CationicCationic FrontalFrontal PolymerizationPolymerization ofof DiDifferentfferent EpoxiesEpoxies StudiesStudies on the the cationic cationic UV-induced UV-induced frontal frontal polyme polymerizationrization were were not reported not reported until untilin 2004, in when 2004, whenMariani Mariani et al. [36] et al. demonstrated [36] demonstrated the frontal the frontalpolymerization polymerization of a bis-cycloaliphatic of a bis-cycloaliphatic epoxy using epoxy a usingphoto aand photo thermal and thermal radical radical initiator. initiator. However, However, before before the theinception inception of ofthe the concept concept of of frontalfrontal photopolymerization,photopolymerization, thethe group group of of Crivello Crivello published published several several important important studies studies that that were were focused focused on cationicon cationic photopolymerization photopolymerization of various of various in-house in synthesized-house synthesized epoxy monomers epoxy monomers requiring prolongedrequiring UVprolonged irradiation UV of irradiation the entire formulation.of the entire These formulation. studies were These mostly studies focused were on mostly the reactivity focused of on epoxy the monomersreactivity of based epoxy on monomers their chemical based structures, on their chemical effect of structures, various functional effect of va groupsrious andfunctional experimental groups parametersand experimental such as parameters UV intensity, such temperature as UV intensity, and so temperature on with the and help so of on FT-IR with spectroscopythe help of FT-IR and photocalorimetryspectroscopy and [photocalorimetry105–114]. [105–114]. AmongAmong thethe various various epoxy epoxy monomers monomers investigated investigated in the in past,the past, particularly particularly the cyclo-aliphatic the cyclo-aliphatic epoxy monomerepoxy monomer 3,4-epoxycyclohexylmethyl-3 3,4-epoxycyclohexylmethyl-30,40-epoxycyclohexane′,4′-epoxycyclohexane has been employed has commerciallybeen employed under variouscommercially trade names. under Crivello various and trade Varlemann names. [115Crivello] investigated and Varlemann the reactivity [115] of 3,4-epoxycyclohexylmethyl-investigated the reactivity 3of0,4 3,4-epoxycyclohexylmethyl-30-epoxycyclohexane. They demonstrated′,4′-epoxycyclohexane. the retarding effectThey of demonstrated ester carbonyl andthe etherretarding groups effect present of withinester carbonyl 3,4-epoxycyclohexylmethyl-3 and ether groups present0,40-epoxycyclohexane within 3,4-epoxycyclohexylmethyl-3 in photoinitiated cationic′,4′-epoxycyclohexane polymerization. Dioxacarbeniumin photoinitiated ionscationic are producedpolymerization. as a result Dioxacarbenium of interaction ions with are oxonium produced ions as anda result possess of highinteraction steric with hindrance oxonium and ions low and reactivity. possess Based high steric on the hindrance structure and of 3,4-epoxycyclohexylmethyl-low reactivity. Based on the 3structure0,40-epoxycyclohexane, of 3,4-epoxycyclohexylmethyl-3 possible precursors′,4 were′-epoxycyclohexane, prepared and investigated possible precursors for their reactivitywere prepared using real-timeand investigated infrared spectroscopy.for their reactivity It was foundusing thatreal-time cyclohexene infrared oxide spectroscopy. and hydroxy It cyclohexene was found oxide that werecyclohexene the most oxide reactive and epoxideshydroxy followedcyclohexene by etheroxide andwere ester the most substituted reactive cyclohexene epoxides followed oxide [115 by– ether117]. Inand another ester substituted analogous cyclohexene study, Crivello oxide and [115–117]. Liu [118 In] another investigated analogous the reactivitystudy, Crivello of some and in-houseLiu [118] preparedinvestigated epoxy the alcohol reactivity monomers of some in in-house the cationic prepared ring opening epoxy polymerizationalcohol monomers using in real-time the cationic infrared ring opening polymerization using real-time infrared spectroscopy. They showed that these hydroxy epoxies undergo faster kinetics via the activated monomer mechanism than their corresponding methoxy monomers. Their study proved that monomers bearing both hydroxy and epoxycyclohexane groups are low viscosity transparent liquids that can accelerate the rate of cationic polymerization of various epoxy compounds, giving hyperbranched polymers [118].

Polymers 2020, 12, 2146 13 of 34 spectroscopy. They showed that these hydroxy epoxies undergo faster kinetics via the activated monomer mechanism than their corresponding methoxy monomers. Their study proved that monomers bearing both hydroxy and epoxycyclohexane groups are low viscosity transparent liquids that can Polymersaccelerate 2020 the, 12, x rate FOR of PEER cationic REVIEW polymerization of various epoxy compounds, giving hyperbranched13 of 33 polymers [118]. In 2003, Falk et al. [73] introduced optical pyrometry as a versatile tool for studying the effect of In 2003, Falk et al. [73] introduced optical pyrometry as a versatile tool for studying the monomer structure, initiator concentrations and experimental parameters such as light intensity and effect of monomer structure, initiator concentrations and experimental parameters such as light temperature, on the cationic photopolymerization of various epoxies (Figure 5). Optical pyrometry intensity and temperature, on the cationic photopolymerization of various epoxies (Figure5). Optical allowed monitoring of reaction temperatures during the cationic photopolymerization of various pyrometry allowed monitoring of reaction temperatures during the cationic photopolymerization epoxy monomers and was also combined with real-time FT-IR spectroscopy to give monomer of various epoxy monomers and was also combined with real-time FT-IR spectroscopy to give conversion profiles. Results showed that 4-vinylcyclohexene dioxide exhibited the highest monomer conversion profiles. Results showed that 4-vinylcyclohexene dioxide exhibited the highest photoreactivity followed by bis1,2(2-(3,4-epoxycyclohexylethyl))-1,1,2,2-tetramethyldisiloxane and photoreactivity followed by bis1,2(2-(3,4-epoxycyclohexylethyl))-1,1,2,2-tetramethyldisiloxane and 3,4-epoxycyclohexylmethyl-3′,4′-epoxycyclohexane (ERL 4221-E). Studies on the 3,4-epoxycyclohexylmethyl-3 ,4 -epoxycyclohexane (ERL 4221-E). Studies on the photopolymerization photopolymerization of PC-10000 0 showed that diaryliodonium-based photoinitiator gives the highest of PC-1000 showed that diaryliodonium-based photoinitiator gives the highest photoreactivity, with a photoreactivity, with a direct relation between photoinitiator concentration and maximum direct relation between photoinitiator concentration and maximum exothermic temperature up exothermic temperature up to concentration of 1 mol %. In addition, oxetanes were found to have to concentration of 1 mol %. In addition, oxetanes were found to have long induction periods, long induction periods, which decreased with increasing temperatures. More importantly, the group which decreased with increasing temperatures. More importantly, the group found that glass and found that glass and poly methyl methacrylate substrates provided successful polymerization due to poly methyl methacrylate substrates provided successful polymerization due to their low thermal their low thermal conductivity in comparison to aluminium substrate. A pronounced effect of conductivity in comparison to aluminium substrate. A pronounced effect of cyclohexene oxide (CHO) cyclohexene oxide (CHO) was observed with oxetane, where the former reduces induction time and was observed with oxetane, where the former reduces induction time and completes the cationic completes the cationic polymerization within 80 s of irradiation owing to its very high reactivity. In polymerization within 80 s of irradiation owing to its very high reactivity. In contrast, benzylic alcohols contrast, benzylic alcohols have also proven to be effective accelerators in the cationic polymerization have also proven to be effective accelerators in the cationic polymerization of ERL 4221-E [73]. of ERL 4221-E [73].

Figure 5. Apparatus used used by Crivello et al. for for stud studyingying photopolymerization photopolymerization of of various various monomers. monomers. Extracted from J. V. Crivello, B. Falk, M. R. Zonca,Zonca, JR., Journal of Polymer Science Part A: Polymer Chemistry, Vol.Vol. 42,42, 1630–1646, 1630–1646, Copyright Copyright 2004 2004 Wiley Wiley Periodicals, Periodicals, Inc., Inc., with with permission permission from Johnfrom WileyJohn Wileyand Sons. and Sons.

In another study, Crivello et al. [33] reported the frontal polymerization of various oxetanes, alkyl glycidyl ether and bis-cycloaliphatic epoxy monomers initiated by UV triggering using optical pyrometry. They observed frontal polymerization behaviour of oxetane monomers, which were triggered by UV and heating. However, due to high reactivity of cycloaliphatic epoxides, UV triggering was enough to complete the cationic polymerization of these monomers specifically. They found that 1,4-butanediol diglycidyl ether (BDGE) and neopentyl glycol diglycidyl ether (NDGE)

Polymers 2020, 12, 2146 14 of 34

In another study, Crivello et al. [33] reported the frontal polymerization of various oxetanes, alkyl glycidyl ether and bis-cycloaliphatic epoxy monomers initiated by UV triggering using optical pyrometry. They observed frontal polymerization behaviour of oxetane monomers, which were triggered by UV and heating. However, due to high reactivity of cycloaliphatic epoxides, UV triggering was enough to complete the cationic polymerization of these monomers specifically. They found that 1,4-butanediol diglycidyl ether (BDGE) and neopentyl glycol diglycidyl ether (NDGE) frontally polymerize under UV irradiation for 6 min, reaching a very high front temperature of 250 ◦C. The resultant polymer was darker due to decomposition occurring at such a high temperature. On the contrary, aryl glycidyl such as phenyl glycidyl ether and 4-cresyl glycidyl ether did not undergo frontal polymerization due to their inherent sluggish reactivities. These studies revealed that frontal polymerization is a characteristic of only those monomers that possess ether oxygen atoms, which are able to stabilize the protonated oxonium ion species formed during acid and monomer reaction. Due to the presence of aromatic groups in aryl glycidyl ether and 4-cresyl glycidyl ether, the oxonium ion interacts with the aromatic group, thereby reducing the stability of the protonated oxonium ion. In addition, because of similar basicity of ether oxygen and oxirane atoms in aliphatic and arylalkyl glycidyl ethers, frontal polymerization is not affected [33]. In 2004, Crivello et al. [119] studied the effect of monomer structures on the cationic photopolymerization of various epoxide and oxetane monomers. It was shown that an appreciable induction period is necessary for epoxy monomer to post cure, after irradiation is terminated. The photopolymerization of 4-vinyl-1-cyclohexene dioxide (VCHDO) reached a maximum front temperature of 270 ◦C while CHO reached up to 63 ◦C owing to the lower epoxy equivalent weight of VCHDO than CHO. With increasing chain length (1 to 5) it was shown that the maximum polymerization temperature decreased due to a decrease in the density of epoxycyclohexyl groups present in the silicone epoxy monomer. They also found that CHO and VCHDO comprised a higher reactivity with respect to benzyl glycidyl ether (BGE) and neopentylglycol diglycidyl ether (NPGE). Figures6 and7 show the increase in polymerization temperature and irradiation time with the addition of reactive diluents (CHO and VCHDO) in the cationic ring opening polymerization of BGE and NPGE. Figure7 also reveals that NPGE suffers from a lower reactivity in cationic polymerization due to longer induction period than BGE, which is a characteristic feature of this epoxy monomer. On the contrary, ERL 4221-E has a shorter induction time but a sluggish polymerization. The authors also described a possible effect of relative humidity on the cationic ring opening polymerization of epoxy monomers. In the presence of water, the epoxy protonation is taken over by water, subsequently generating a diol. Water is removed during the reaction to produce alcohols that ultimately react with protonated epoxides to form ether linkages via activated monomer mechanism. As a consequence, there is less steric hindrance for epoxide monomers to form longer linkages under high relative humidity as compared to the conventional epoxide linkages during cationic ring opening polymerization [119]. Polymers 2020, 12, x FOR PEER REVIEW 14 of 33 frontally polymerize under UV irradiation for 6 min, reaching a very high front temperature of 250 °C. The resultant polymer was darker due to decomposition occurring at such a high temperature. On the contrary, aryl glycidyl ethers such as phenyl glycidyl ether and 4-cresyl glycidyl ether did not undergo frontal polymerization due to their inherent sluggish reactivities. These studies revealed that frontal polymerization is a characteristic of only those monomers that possess ether oxygen atoms, which are able to stabilize the protonated oxonium ion species formed during acid and monomer reaction. Due to the presence of aromatic groups in aryl glycidyl ether and 4-cresyl glycidyl ether, the oxonium ion interacts with the aromatic group, thereby reducing the stability of the protonated oxonium ion. In addition, because of similar basicity of ether oxygen and oxirane atoms in aliphatic and arylalkyl glycidyl ethers, frontal polymerization is not affected [33]. In 2004, Crivello et al. [119] studied the effect of monomer structures on the cationic photopolymerization of various epoxide and oxetane monomers. It was shown that an appreciable induction period is necessary for epoxy monomer to post cure, after irradiation is terminated. The photopolymerization of 4-vinyl-1-cyclohexene dioxide (VCHDO) reached a maximum front temperature of 270 °C while CHO reached up to 63 °C owing to the lower epoxy equivalent weight of VCHDO than CHO. With increasing chain length (1 to 5) it was shown that the maximum polymerization temperature decreased due to a decrease in the density of epoxycyclohexyl groups present in the silicone epoxy monomer. They also found that CHO and VCHDO comprised a higher reactivity with respect to benzyl glycidyl ether (BGE) and neopentylglycol diglycidyl ether (NPGE). Figures 6 and 7 show the increase in polymerization temperature and irradiation time with the addition of reactive diluents (CHO and VCHDO) in the cationic ring opening polymerization of BGE and NPGE. Figure 7 also reveals that NPGE suffers from a lower reactivity in cationic polymerization due to longer induction period than BGE, which is a characteristic feature of this epoxy monomer. On the contrary, ERL 4221-E has a shorter induction time but a sluggish polymerization. The authors also described a possible effect of relative humidity on the cationic ring opening polymerization of epoxy monomers. In the presence of water, the epoxy protonation is taken over by water, subsequently generating a diol. Water is removed during the reaction to produce alcohols that ultimately react with protonated epoxides to form ether linkages via activated monomer mechanism. As a consequence, there is less Polymerssteric hindrance2020, 12, 2146 for epoxide monomers to form longer linkages under high relative humidity15 of as 34 compared to the conventional epoxide linkages during cationic ring opening polymerization [119].

Figure 6. EffectEffect of cyclohexene oxide (CHO) on the po polymerizationlymerization temperature and irradiation time in cationic photopolymerization of of benzyl benzyl glycidyl glycidyl ether ether (BGE). (BGE). Extracted Extracted from from J. J. V. V. Crivello, Crivello, B.B. Falk, Falk, M. R.R. ZoncaZonca,, JR.,JR., JournalJournal ofof AppliedApplied PolymerPolymer Science,Science, Vol.Vol. 92, 3303–3319, Copyright 2004 Wiley PolymersPeriodicals, 2020, 12, x Inc.,FOR withPEER permissionpermisREVIEWsion from John Wiley andand Sons.Sons. 15 of 33

Figure 7.7. EEffectffect ofof 4-vinyl-1-cyclohexene4-vinyl-1-cyclohexene dioxidedioxide (VCHDO)(VCHDO) on thethe polymerizationpolymerization temperaturetemperature and irradiationirradiation timetime in in cationic cationic photopolymerization photopolymerizatio of neopentylglycoln of neopentylglycol diglycidyl diglycidyl ether (NPGE). ether Extracted(NPGE). fromExtracted J. V. from Crivello, J. V. B.Crivello, Falk, M.B. Falk, R. Zonca, M. R. JR.,Zonc Journala, JR., Journal of Applied of Applied Polymer Polymer Science, Science, 92, 3303–3319, 92, 3303– Copyright3319, Copyright 2004Wiley 2004 Wiley Periodicals, Periodicals, Inc., with Inc., permission with permission fromJohn from Wiley John Wiley and Sons. and Sons.

InIn 2005,2005, Bulut Bulut and and Crivello Crivello [35 ][35] conducted conducted a detailed a detailed study study on the structure-reactivitieson the structure-reactivities of different of epoxydifferent monomers epoxy monomers used in cationic used in photopolymerization. cationic photopolymerization. They also studiedThey also three studied different three approaches different forapproaches accelerating for photopolymerizationaccelerating photopolymerization namely: (i) photopolymerization namely: (i) photopolymerizatio at elevatedn temperatures, at elevated (ii)temperatures, copolymerization (ii) copolymerization with reactive monomers, with reactive and monomers, (iii) the addition and (iii) of the free addition radical photoinitiators. of free radical First,photoinitiators. they showed First, that the they extended showed induction that period the inextended cationic photopolymerization induction period depends in cationic on the formationphotopolymerization of a hydronium depends ion crown on the ether formation species of between a hydronium epoxy monomerion crownand ether acid species proton. between In the caseepoxy of monomer alkyl glycidyl and acid ether, proton. the hydrogen In the case bond of establishedalkyl glycidyl between ether, the acid hydrogen proton and bond crown established ether is highlybetween stable acid andproton prevents and crown ring opening.ether is highly However, stable with and the prevents increase ring in opening. temperature, However, this induction with the periodincrease is in greatly temperature, reduced this and induction photopolymerization period is greatly proceeds reduced rapidly, and asphotopolymerization shown in Figure8. proceeds rapidly, as shown in Figure 8.

Figure 8. Effect of temperature on the induction time of the cationic photopolymerization of benzyl glycidyl ether. Reprinted (adapted) with permission from Umut Bulut and James V. Crivello, Investigation of the Reactivity of Epoxide Monomers in Photoinitiated Cationic Polymerization, Macromolecules, 2005, 38, 3584–3595. Copyright (2005) American Chemical Society.

Secondly, addition of reactive monomers as copolymerization species increased the reactivity of the epoxy monomer during photopolymerization. The authors reported positive effect of trimethylolpropane trivinyl ether, 1,4-cyclohexanedimethanol divinyl ether, and diethylene glycol divinyl ether and showed by the example of NPGE and CHO that a high difference in reactivities of monomers will accelerate photopolymerization. Addition of a free radical photoinitiator (Irgacure

Polymers 2020, 12, x FOR PEER REVIEW 15 of 33

Figure 7. Effect of 4-vinyl-1-cyclohexene dioxide (VCHDO) on the polymerization temperature and irradiation time in cationic photopolymerization of neopentylglycol diglycidyl ether (NPGE). Extracted from J. V. Crivello, B. Falk, M. R. Zonca, JR., Journal of Applied Polymer Science, 92, 3303– 3319, Copyright 2004 Wiley Periodicals, Inc., with permission from John Wiley and Sons.

In 2005, Bulut and Crivello [35] conducted a detailed study on the structure-reactivities of different epoxy monomers used in cationic photopolymerization. They also studied three different approaches for accelerating photopolymerization namely: (i) photopolymerization at elevated temperatures, (ii) copolymerization with reactive monomers, and (iii) the addition of free radical photoinitiators. First, they showed that the extended induction period in cationic photopolymerization depends on the formation of a hydronium ion crown ether species between epoxy monomer and acid proton. In the case of alkyl glycidyl ether, the hydrogen bond established between acid proton and crown ether is highly stable and prevents ring opening. However, with the Polymersincrease2020 in, temperature,12, 2146 this induction period is greatly reduced and photopolymerization proceeds16 of 34 rapidly, as shown in Figure 8.

Figure 8.8. EEffectffect ofof temperaturetemperature onon thethe inductioninduction timetime ofof thethe cationiccationic photopolymerizationphotopolymerization ofof benzylbenzyl glycidyl ether.ether. Reprinted Reprinted (adapted) (adapted) with with permi permissionssion from from Umut Umut Bulut Bulut and and James James V. V. Crivello, InvestigationInvestigation ofof thethe ReactivityReactivity ofof EpoxideEpoxide MonomersMonomers inin PhotoinitiatedPhotoinitiated CationicCationic Polymerization,Polymerization, Macromolecules, 2005,2005, 38,38, 3584–3595.3584–3595. CopyrightCopyright (2005) AmericanAmerican ChemicalChemical Society.Society.

Secondly, addition addition of of reactive reactive monomers monomers as as copolymerization copolymerization species species increased increased the the reactivity reactivity of the of epoxythe epoxy monomer monomer during photopolymerization.during photopolymerization. The authors The reported authors positive reported effect of trimethylolpropanepositive effect of trivinyltrimethylolpropane ether, 1,4-cyclohexanedimethanol trivinyl ether, 1,4-cyclohexan divinyl ether,edimethanol and diethylene divinyl glycol ether, divinyl and etherdiethylene and showed glycol bydivinyl the example ether and of showed NPGE and by the CHO example that a highof NPGE difference and CHO in reactivities that a high of difference monomers in will reactivities accelerate of photopolymerization.monomers will accelerate Addition photopolymerization. of a free radical photoinitiatorAddition of a (Irgacurefree radical 651) photoinitiator reduced the induction(Irgacure period during photopolymerization of NPGE and also proved to be an effective accelerator for various mono- and multifunctional glycidyl ethers [35]. Similar studies were conducted with an oxetane monomer (bis{[(1-ethyl(3-oxetanyl)]-methyl} ether) where the effect of temperature had a similar profile as in Figure8 for benzyl glycidyl ether [120]. In the same year, Falk et al. [121] published a study on the UV-activated thermal-induced frontal polymerization for various oxirane and oxetane monomers. The group reported frontal behaviour of the epoxy monomers given in Table1, along with their irradiation time, maximum front temperature after thermal initiation, intensity and front velocity. The formation of secondary oxonium ion species from alkyl glycidyl ether is able to portray a frontal behaviour. If sufficient energy is available in the form of heat to push secondary oxonium or tertiary oxonium ions to propagating oxonium ions in ring opening polymerization, then a frontal behaviour is observed [121]. Additionally, a similar explanation about the formation of a stabilized ether oxygen and proton bond was given, similar to the results reported by Crivello et al. [33]. Various mono- and polyfunctional alkyl glycidyl ether monomers were investigated for their reactivity in photoactivated cationic frontal polymerization by Crivello [122]. Auto-acceleration of the propagating front was achieved via thermal triggering, and it was reported that monomers exhibit frontal behaviour depending on the stability of the secondary oxonium ion intermediates through multiple hydrogen-bonding effects to the ether groups present in the molecule. Polymers 20202020,, 1212,, xx FORFOR PEERPEER REVIEWREVIEW 1616 ofof 3333

651) reduced the induction period during photopolymerization of NPGE and also proved to be an effective accelerator for various mono- and multifunctional glycidyl ethers [35]. Similar studies were conducted with an oxetane monomer (bis{[(1-ethyl(3-oxetanyl)]-methyl}-oxetanyl)]-methyl} ether)ether) wherewhere thethe effecteffect ofof temperaturetemperature hadhad aa similarsimilar profileprofile asas inin FiFigure 8 for benzyl glycidyl ether [120]. InIn thethe samesame year,year, FalkFalk etet al.al. [121][121] publishedpublished aa studystudy onon thethe UV-activatedUV-activated thermal-inducedthermal-induced frontalfrontal polymerization for various oxirane and oxetane monomers. The group reported frontalfrontal behaviourbehaviour ofof thethe epoxyepoxy monomersmonomers givengiven inin TableTable 1,1, alongalong withwith theirtheir irradiationirradiation time,time, maximummaximum frontfront temperaturetemperature after thermal initiation, intensity and front velocity.ty. TheThe formationformation ofof secondarysecondary oxoniumoxonium ionion speciesspecies fromfrom alkylalkyl glycidylglycidyl etherether isis ableable toto portrayportray aa frontafrontall behaviour.behaviour. IfIf sufficientsufficient energyenergy isis availableavailable inin thethe formform ofof heatheat toto pushpush secondarysecondary oxoniumoxonium oror tertiatertiaryry oxoniumoxonium ionsions toto propagatingpropagating oxoniumoxonium ionsions inin ringring openingopening polymerization,polymerization, thenthen aa frontalfrontal behaviourbehaviour isis observedobserved [121].[121]. Additionally,Additionally, aa similarsimilar explanation about the formation of a stabilized ether oxygen and proton bond was given, similar to thethe resultsresults reportedreported byby CrivelloCrivello etet al.al. [33].[33]. VariousVarious mono-mono- andand polyfunctionalpolyfunctional alkylalkyl glycidylglycidyl etherether monomers were investigated for their reactivity in photoactivated cationic frontal polymerization by Crivello [122]. Auto-acceleration of the propagating frontfront waswas achievedachieved viavia thermalthermal triggering,triggering, andand itit was reported that monomers exhibit frontal behaviour depending on the stability of the secondary Polymers 2020, 12, 2146 17 of 34 oxonium ion intermediates through multiple hydrogen-bonding effects to the ether groups present inin thethe molecule.molecule. Table 1. Different epoxy monomers displaying frontal behaviour during cationic photopolymerization coupledTable with thermal 1. Different initiation. Redrawnepoxy frommonomers Benjamin displaying Falk, Michael frontal R. Zonca, behaviour Jr., James V.during Crivello ,cationic Photoactivatedphotopolymerization Cationic Frontal coupled Polymerization, with thermal Macromolecular initiation. Redrawn Symposia, from Benjamin 2005, 226, Falk, 97–107, Michael R. CopyrightZonca, 2005 Jr., International James V. UnionCrivello, of Photoactivated Pure and Applied Cationic Chemistry, Frontal with Polymerization, permission from Macromolecular John Wiley andSymposia, Sons. 2005, 226, 97–107, Copyright 2005 Internationalional UnionUnion ofof PurePure andand AppliedApplied Chemistry,Chemistry, with permission from John Wiley and Sons. Irrad. Intensity Monomer T (oC) t (s) V (cm/min) 2 f 22 f foo Monomer Irrad.Time (min)Time (m(mJin)in)/cm IntensityIntensitymin) (mJ/cm(mJ/cm2min) Tff ((oC) tff (s)(s) VVff (cm/min)(cm/min)

OO OO 2.52.5 439439 180 35180 7.2 35 7.2

OOOO OO OO OO OO 66 413413 232 22232 11 22 11

OO O 55 483483 169 14169 18 14 18 OO O 88 575575 203 6203 20 6 20 O O

Benzyl glycidyl ether (BGE) forms a relatively lessless stablestable intermediateintermediate secondarysecondary oxoniumoxonium ionion BenzylBenzyl glycidyl glycidyl ether (BGE) ether forms(BGE) a forms relatively a relatively less stable less intermediate stable intermediate secondary secondary oxonium oxonium ion ion inin whichwhich thethe hydrogenhydrogen ionion hashas aa bidentatebidentate coordinaticoordination with oxygen in comparison to 3-cyclohexene- in whichin the which hydrogen the hydrogen ion has ion a bidentate has a bidentate coordination coordinati withon oxygen with oxygen in comparison in comparison to 3-cyclohexene- to 3-cyclohexene- 1,1-dimethanol diglycidyl ether (CHDMDGE) that forms tetradentate coordination. For this reason, 1,1-dimethanol1,1-dimethanol diglycidyl diglycidyl ether (CHDMDGE) ether (CHDMDGE) that forms that tetradentate forms tetradentate coordination. coordination. For this For reason, this reason, CHDMDGE exhibits a longer induction period than BGE in the cationic photopolymerization. In CHDMDGECHDMDGE exhibits aexhibits longer induction a longer periodinduction than period BGE in thethan cationic BGE in photopolymerization. the cationic photopolymerization. In addition, In addition, several epoxy monomers exhibited frontal behaviour due to their relatively stable severaladdition, epoxy monomers several exhibitedepoxy monomers frontal behaviour exhibited due frontal to their relativelybehaviour stable due secondaryto their relatively oxonium stable secondarysecondary oxoniumoxonium ionion speciesspecies inin coordinationcoordination withwith oxygenoxygen atoms.atoms. OneOne ofof thethe epoxiesepoxies isis triethylenetriethylene ion speciessecondary in coordination oxonium with ion oxygenspecies atoms.in coordination One of the with epoxies oxygen is triethylene atoms. One glycol of the diglycidyl epoxies is ether triethylene glycol diglycidyl ether (TEGDGE), which exhibited a hexadentate proton coordination and thus had (TEGDGE),glycol which diglycidyl exhibited ether a (TEGDGE), hexadentate which proton exhibited coordination a hexadentate and thus proton had acoordination longer induction and thus had a longer induction time of 200 s for UV irradiation, after which the frontal polymerization proceeded time ofa 200longer s for induction UV irradiation, time of 200 after s for whichUV irradiation, the frontal after polymerization which the frontal proceeded polymerization with very proceeded with very high temperatures. Similar longer inductioninduction periodsperiods andand hihigh photopolymerization high temperatures.with very high Similar temperatures. longer induction Similar periods longer andinduction high photopolymerization periods and high photopolymerization temperatures temperaturestemperatures werewere observedobserved forfor pentaerythritolpentaerythritol tetraglycidyltetraglycidyl etherether (PETGE),(PETGE), trimethylolpropanetrimethylolpropane were observedtemperatures for pentaerythritol were observed tetraglycidyl for pentaerythritol ether (PETGE), tetraglycidyl trimethylolpropane ether (PETGE), triglycidyl trimethylolpropane ether triglycidyltriglycidyl etherether (TMPTGE),(TMPTGE), trimethylolethanetrimethylolethane trigtriglycidyllycidyl etherether (TMETGE)(TMETGE) andand glycerolglycerol triglycidyltriglycidyl (TMPTGE),triglycidyl trimethylolethane ether (TMPTGE), triglycidyl trimethylolethane ether (TMETGE) triglycidyl and glycerolether (TMETGE) triglycidyl and ether glycerol (GTGE) triglycidyl ether (GTGE) arranged in decreasing photopolymerization temperature. Several di and arrangedether in decreasing (GTGE) photopolymerizationarranged in decreasing temperature. photopolymerization Several di and tetrafunctional temperature. alkyl Several glycidyl di and tetrafunctionaltetrafunctional alkylalkyl glycidylglycidyl ethersethers exhibitedexhibited fronfrontaltal behaviourbehaviour uponupon thermalthermal heating.heating. InIn particular,particular, ethers exhibited frontal behaviour upon thermal heating. In particular, TEGDGE and PETGE required TEGDGE and PETGE required temperatures of 69 °C and 78 °C, respectively, for thermally triggering temperatures of 69 ◦C and 78 ◦C, respectively, for thermally triggering a rapid and spontaneous frontal a rapid and spontaneous frontal polymerization [122]. polymerization [122]. From another study done by Crivello [77], on cationic photopolymerization of alkyl glycidyl From another study done by Crivello [77], on cationic photopolymerization of alkyl glycidyl ethers, a higher cross-link density with lower frontt velocityvelocity waswas achievedachieved withwith trimethylolpropanetrimethylolpropane ethers, a higher cross-link density with lower front velocity was achieved with trimethylolpropane triglycidyltriglycidyl etherether asas comparedcompared toto ethyleneethylene glycolglycol digldiglycidyl ether. The formation of highly cross-linked triglycidyl ether as compared to ethylene glycol diglycidyl ether. The formation of highly cross-linked structuresstructures withwith trimethylolpropanetrimethylolpropane glycidylglycidyl etherether hashas beenbeen attributedattributed toto aa lowerlower frontfront velocity.velocity. structures with trimethylolpropane glycidyl ether has been attributed to a lower front velocity. Crivello also performed photopolymerization frontal experiments to confirm the effect of hydronium ion species formation on the stability of various epoxy monomers in frontal polymerization. The results shown in Figures9 and 10 clearly indicated the stability of the hydrogen-ether bond given by di fference in the temperature for auto-acceleration (TMPTGE>NPGDGE in Figure9 and 1,4-cyclohexanedimethanol diglycidyl ether CHDMDGE> 1,6-hexanediol diglycidyl ether, HDDGE, in Figure 10). It was also shown by the example of neopentylglycol diglycidyl ether, that elevated temperatures can destabilize the secondary oxonium ion species and reduce induction periods [77]. In a study conducted by Bulut and Crivello [123], it was found that benzyl sulphide is an inhibiting species in the cationic photopolymerization of BADGE and other aryl diglycidyl ethers that can inhibit polymerization of these epoxies below the gel point. Polymers 2020, 12, x FOR PEER REVIEW 17 of 33

Crivello also performed photopolymerization frontal experiments to confirm the effect of hydronium ion species formation on the stability of various epoxy monomers in frontal polymerization. The results shown in Figures 9 and 10 clearly indicated the stability of the hydrogen-ether bond given by difference in the temperature for auto-acceleration (TMPTGE>NPGDGE in Figure 9 and 1,4- cyclohexanedimethanol diglycidyl ether CHDMDGE> 1,6-hexanediol diglycidyl ether, HDDGE, in Figure 10). It was also shown by the example of neopentylglycol diglycidyl ether, that elevated temperatures can destabilize the secondary oxonium ion species and reduce induction periods [77]. In a study conducted by Bulut and Crivello [123], it was found that benzyl sulphide is an inhibiting Polymersspecies 2020in ,the12, 2146cationic photopolymerization of BADGE and other aryl diglycidyl ethers that18 ofcan 34 inhibit polymerization of these epoxies below the gel point.

Figure 9.9. ResultsResults of of the photopolymerization followed by thermal triggering for NPGDGE and TMPTGE. Extracted from James V. Crivello,Crivello, CationicCationic photopolymerization of alkyl glycidyl ethers, Journal of PolymerPolymer Science Part A: Polymer Chemistry,Chemistry, 2006, 44, 3036–3052, Copyright 2006 Wiley Periodicals Inc., with permission from JohnJohn WileyWiley andand Sons.Sons.

Figure 10.10.Results Results of photopolymerizationof photopolymerization followed followed by thermal by thermal triggering triggering for HDDGE for and HDDGE CHDMDGE. and ExtractedCHDMDGE. from Extracted James V. from Crivello, James CationicV. Crivello, photopolymerization Cationic photopolymerization of alkyl glycidyl of alkyl ethers, glycidyl Journal ethers, of PolymerJournal of Science Polymer Part Science A: Polymer Part Chemistry,A: Polymer 2006, Chemistry, 44, 3036–3052, 2006, 44, Copyright 3036–3052, 2006 Copyright Wiley Periodicals 2006 Wiley Inc., withPeriodicals permission Inc., with from permission John Wiley from and Sons. John Wiley and Sons.

In 2008,2008, Crivello [124] [124] published work on the eeffectffect of temperature in the cationic frontal polymerization inducedinduced by by UV UV light light on various on va epoxiderious epoxide monomers. monomers. Among theseAmong epoxide these monomers, epoxide n -butyl glycidyl ether was UV irradiated for 200 s followed by cooling to 3 C, and resultantly, − ◦ there was no exothermic response in the temperature profile. However, a frontal polymerization was observed when the liquid film was briefly heated. The author attributed this phenomenon to the formation of a stable bidentate coordination that involves the formation of a cyclic oxonium ion, a pseudo five-membered ring by coordination with a proton. Similarly, for phenyl glycidyl ether, frontal polymerization was observed when a liquid film of the sample was heated after discontinuing UV irradiation in 100 s (Figure 11). The slow reactivity of n-butyl glycidyl ether as compared to phenyl glycidyl ether was attributed to the weaker bidentate coordination of formed by phenyl Polymers 2020, 12, x FOR PEER REVIEW 18 of 33

monomers, n-butyl glycidyl ether was UV irradiated for 200 s followed by cooling to -3 °C, and resultantly, there was no exothermic response in the temperature profile. However, a frontal polymerization was observed when the liquid film was briefly heated. The author attributed this phenomenon to the formation of a stable bidentate coordination that involves the formation of a cyclic oxonium ion, a pseudo five-membered ring by coordination with a proton. Similarly, for phenyl Polymersglycidyl2020 ether,, 12, 2146 frontal polymerization was observed when a liquid film of the sample was heated19 of 34 after discontinuing UV irradiation in 100 s (Figure 11). The slow reactivity of n-butyl glycidyl ether as compared to phenyl glycidyl ether was attributed to the weaker bidentate coordination of protons glycidyl ether with the acid proton. These results showed that continuous UV irradiation of epoxy formed by phenyl glycidyl ether with the acid proton. These results showed that continuous UV monomers can be replaced with auto-accelerated frontal polymerization if sufficient energy is available irradiation of epoxy monomers can be replaced with auto-accelerated frontal polymerization if to induce ring opening polymerization of the epoxides [124]. sufficient energy is available to induce ring opening polymerization of the epoxides [124].

FigureFigure 11.11. Temperature vs.vs. time time plot plot for frontal polymerizationpolymerization of glycidylglycidyl phenylphenyl etherether uponupon discontinuationdiscontinuation of of UV UV irradiation irradiatio andn subsequentand subsequent heating heating after 100 after s. Extracted 100 s. fromExtracted Effect offrom Temperature Effect of onTemperature the Cationic on Photopolymerization the Cationic Photopolymerization of Epoxides, James V.of Crivello,Epoxides, Journal James of MacromolecularV. Crivello, Journal Science, of PartMacromolecular A: Pure and AppliedScience, Chemistry,Part A: Pure 2008, and 45, Applied 591–598, Chemistry, Version of 2008, record 45, first 591–598, published: Version 19 June of record 2008, reprintedfirst published: by permission 19 ofJun Taylor 2008, & Francisreprinted Ltd., http:by //www.tandfonline.compermission of Taylor. & Francis Ltd., http://www.tandfonline.com. In 2015, Bomze et al. [38] investigated various epoxy monomers that may give a successful frontal polymerizationIn 2015, Bomze (RICFP). et al. Their [38] studies investigated showed vari thatous NPDGE, epoxy monomers HDDGE, CHDGE that may and give cyclo-aliphatic a successful epoxyfrontal (CE) polymerization generated successful (RICFP). frontsTheir withstudies appreciable showed that bubble NPDGE, formation, HDDGE, whereas CHDGE combination and cyclo- of BADGEaliphatic with epoxy tetraphenyl (CE) generated ethandiol andsuccessfulp-octyloxyphenyl fronts with phenyliodonium appreciable bubble hexafluoroantimonate formation, whereas gave acombination successful front of withBADGE a sluggish with reactivitytetraphenyl (2.7 cmethandiol/min) and and the p largest-octyloxyphenyl induction timephenyliodonium among other epoxies.hexafluoroantimonate NPDGE possessed gave aa highsuccessful reactivity front among with a other sluggish epoxy reactivity monomers, (2.7 andcm/min) this explains and the whylargest it hasinduction the lowest time induction among periodother followedepoxies. byNPDGE HDDGE, po CHDGEssessed anda high CE. Frontreactivity velocities among for CHDGEother epoxy and HDDGEmonomers, were and among this theexplains highest why reported it has withthe lowest 37.9 and induction 28.6 cm/min, period respectively followed [by38]. HDDGE,Bomze et CHDGE al. [39] alsoand investigatedCE. Front velocities RICFP of for the CHDGE same formulation and HDDGE using were BADGE among to the determine highest thermomechanicalreported with 37.9 andand intrinsic28.6 cm/min, properties respectively of the cured [38]. system.Bomze et Their al. [39] results also showed investigated that BADGE RICFP of formulations the same formulation consisting ofusing TPED BADGE and iodonium to determine salt photoinitiator thermomechanical can be and stored intrinsic at 50 ◦ propertiesC protected of from the cured light for system. a period Their of 28results days withoutshowed significantthat BADGE change formulations in the viscosity consisting and frontof TPED velocity and and iodonium maximum salt front photoinitiator temperatures. can Inbe comparisonstored at 50 to °C thermally protected cured from BADGE, light for RICFP a period of BADGE of 28 days gave without a very highsignificant Tg of 168change◦C, tensile in the strengthviscosity of and 64 MPafront and velocity impact and resistance maximum of 14.9 front kJ /temperatures.m2 [39]. In comparison to thermally cured BADGE,Sangermano RICFP of et BADGE al. [125] observedgave a very photoinduced high Tg of 168 cationic °C, tensile frontal strength polymerization of 64 MPa for Ampregand impact 26 epoxyresistance resin of using 14.9 iodoniumkJ/m2 [39]. salt photoinitiator and TPED as thermal initiator.They obtained very high curingSangermano degrees (indicated et al. [125] by a observed little or no photoinduced exothermic peak cationic in Figure frontal 12) polymerization for the cationic polymerizationfor Ampreg 26 ofepoxy this particularresin using epoxy iodonium resin. salt The photoinitiator frontal polymerization and TPED wasas thermal monitored initiator.They using a thermal obtained camera very andhigh the curing temperature degrees vs. (indicated time evolution by a little was reportedor no exothermic at different peak positions in Figure on the 12) mould for the (Figure cationic 13). Thepolymerization cationic polymerization of this particular of the epoxyepoxy resin resin. proceeded The frontal with polymerization a maximum temperature was monitored of 263 using◦C and a a frontal velocity of 4 cm/min [125]. Polymers 2020, 12, x FOR PEER REVIEW 19 of 33 Polymers 2020, 12, x FOR PEER REVIEW 19 of 33 thermal camera and the temperature vs. time evolution was reported at different positions on the mouldthermal (Figure camera 13). and The the cationictemperature polymerization vs. time evol ofution the epoxy was reported resin proceeded at different with positions a maximum on the Polymerstemperaturemould2020 (Figure, 12 of, 2146 263 13). °C The and cationic a frontal polymerization velocity of 4 cm/min of the [125]. epoxy resin proceeded with a maximum20 of 34 temperature of 263 °C and a frontal velocity of 4 cm/min [125].

Figure 12. DSC plot of a formulation of BADGE with 0.5:1.5 (w/w) photo and thermal initiator FigureindicatingFigure 12. 12. DSCa smallDSC plot plotflex of apointof formulation a formulationat 150 °C of (blue BADGE of line)BADGE with whereas 0.5:1.5 with 1.5:1.5 (w0.5:1.5/w) photo (g (reenw/w and) line) photo thermal and and 2:1 initiator thermal(w/w) indicating(red initiator line).

aExtractedindicating small flex from a point small Marco at flex 150 Sangermano,point◦C (blue at 150 line) °C Ippaziowhereas (blue line) Anto 1.5:1.5 whereasnazzo, (green Lorenzo1.5:1.5 line) (g and reenSisca 2:1 line)and (w/w andMassimiliana) (red 2:1 line). (w/w) Extracted (red Carello, line). fromPhotoinducedExtracted Marco from Sangermano, cationic Marco Sangermano, Ippaziofrontal Antonazzo,polymerization Ippazio Lorenzo Anto ofnazzo, Siscaepoxy–carbon Lorenzo and Massimiliana Sisca fibre and Carello,composites, Massimiliana Photoinduced Polymer Carello, cationicInternational,Photoinduced frontal 2019, polymerizationcationic 68, 1662–1665, frontal of epoxy–carbonpolymerizationCopyright 2019 fibre Sociof composites,etyepoxy–carbon of Chemical Polymer fibre Indust International,composites,ry, with permission 2019,Polymer 68, 1662–1665,fromInternational, John Wiley Copyright 2019, and 68,Sons. 2019 1662–1665, Society of Copyright Chemical Industry,2019 Soci withety of permission Chemical from Indust Johnry, Wileywith permission and Sons. from John Wiley and Sons.

(a) (b) (a) (b) Figure 13.13. (a) ThermalThermal imagesimages obtainedobtained during frontal polymerization of Ampreg 26 resin showingshowing progressiveFigure 13. ( temperaturea) Thermal images increase obtained in thethe frontduringfront temperature.temperature. frontal polymerization ( b) Temperature-time of Ampreg evolution26 resin showing of the propagatingprogressive front fronttemperature at at di differentfferent increase distances distances in the from fromfront the theirradiationtemperature. irradiation point ( bpoint) on Temperature-time the on mould. the mould. Extracted evolutionExtracted from Marco offrom the Sangermano,Marcopropagating Sangermano, Ippaziofront at Ippazio Antonazzo,different Antonazzo, distances Lorenzo fromLorenzo Sisca the and Siirradiationsca Massimiliana and Massimiliana point Carello, on the PhotoinducedCarello,mould. ExtractedPhotoinduced cationic from frontalcationicMarco polymerization Sangermano,frontal polymerization Ippazio of epoxy–carbon Antonazzo, of epoxy–carbon fibre Lorenzo composites, fibre Si scacomposites, Polymer and Massimiliana International, Polymer International, Carello, 2019, 68,Photoinduced 1662–1665, 2019, 68, Copyright1662–1665,cationic frontal 2019Copyright Society polymerization 2019 of Chemical Society of of Industry,epoxy–carbon Chemical with Industry permissionfibre ,composites, with permission from John Polymer Wiley from International, andJohn Sons. Wiley and 2019, Sons. 68, 1662–1665, Copyright 2019 Society of Chemical Industry, with permission from John Wiley and Sons. EEffectffect of DiluentsDiluents inin UV-InducedUV-Induced CationicCationic FrontalFrontal PolymerizationPolymerization ofof EpoxideEpoxide MonomersMonomers Effect of Diluents in UV-Induced Cationic Frontal Polymerization of Epoxide Monomers In 2007,2007, Crivello [[126]126] preparedprepared aa hybridhybrid photopolymerphotopolymer system consisting of a free radicallyradically polymerizableIn 2007, multifunctionalCrivellomultifunctional [126] prepared acrylate monomera hybridmonomer photopolymer and and a cationically a cationically system polymerizable consistingpolymerizable epoxide of a freeepoxide or radically oxetane or monomer.oxetanepolymerizable monomer. Irradiation multifunctional Irradiation of these compounds ofacrylate these compoundmonomer results in sand theresults formationa cationically in the of formation a free-standingpolymerizable of a free-standing polyacrylate epoxide or networkpolyacrylateoxetane filmmonomer. network containing Irradiation film quiescent containing of oxonium these quiescent compound ions ox alongoniums withresults ions the alongin unreacted the with formation the cyclic unreacted etherof a monomerfree-standing cyclic ether via freemonomerpolyacrylate radical via polymerization. networkfree radical film polymerization. Atcontaining the same quiescent time, At frontal the ox sameonium polymerization time, ions frontal along was withpolymerization observed the unreacted after was application cyclic observed ether of aaftermonomer point application source via of free heatof radicala to point the polymerization. film, source initiating of heat a cationic Atto thethe same ring-openingfilm, time,initiating frontal frontal a cationicpolymerization polymerization ring-opening was propagating observed frontal topolymerizationafter all portionsapplication of propagating the of irradiateda point tosource sampleall portions of (Figure heat of to 14the the). irradiated A film, 10:1 mixtureinitiating sample of a trimethylolpropane(Figure cationic 14). ring-opening A 10:1 mixture triglycidyl frontal of ethertrimethylolpropanepolymerization (TMPTGE) propagating and triglycidyl trimethylolpropane to ether all portions (TMPTGE) triacrylate of theand irradiated trimethylolpropane (TMPTA) sample gave a (Figure slow triacrylate frontal 14). A(TMPTA) polymerization 10:1 mixture gave aof velocitytrimethylolpropane above 50 ◦C triglycidyl with a highly ether cross-linked (TMPTGE) an epoxyd trimethylolpropane network, and it triacrylate was deduced (TMPTA) that frontal gave a polymerization requires a low concentration of acrylate monomer.

Polymers 2020, 12, x FOR PEER REVIEW 20 of 33 Polymers 2020, 12, x FOR PEER REVIEW 20 of 33 slowPolymers frontal2020 12 polymerization velocity above 50 °C with a highly cross-linked epoxy network, and it slow frontal, polymerization, 2146 velocity above 50 °C with a highly cross-linked epoxy network, 21and of 34it was deduced that frontal polymerization requires a low concentration of acrylate monomer. was deduced that frontal polymerization requires a low concentration of acrylate monomer.

Figure 14. Photo-induced cationic frontal polymerization of a mixture of 5:1 TMPTGE and HDODA Figure 14. Photo-induced cationic frontal polymerizatio polymerizationn of a mixture of 5:1 TMPTGE and HDODA propagating in 10 s interval in the presence of 2-phenyl-2,2-dimethoxy-acetophenone (DMPA). propagating inin 10 10 s intervals interval in the in presence the presence of 2-phenyl-2,2-dimethoxy-acetophenone of 2-phenyl-2,2-dimethoxy-acetophenone (DMPA). Extracted(DMPA). Extracted from James V. Crivello, Hybrid free radical/cationic frontal photopolymerizations, Journal Extractedfrom James from V. Crivello, James V. Hybrid Crivello, free Hybrid radical free/cationic radical/cationic frontal photopolymerizations, frontal photopolymerizations, Journal of Polymer Journal of Polymer Science Part A: Polymer Chemistry, 2007, 45, 4331–4340, Copyright 2007 Wiley Periodicals ofScience Polymer Part Science A: Polymer Part A: Chemistry,Polymer Chemistry, 2007, 45, 2007, 4331–4340, 45, 4331–4340, Copyright Copyright 2007 Wiley2007 Wiley Periodicals Periodicals Inc., Inc., with permission from John Wiley and Sons. withInc., with permission permission from from John John Wiley Wiley andSons. and Sons.

In contrast, 3,3-disubstituted oxetane monomers undergo rapid photoactivated cationic frontal In contrast, 3,3-disubstituted oxetane monomers undergo rapid photoactivatedphotoactivated cationic frontal polymerizations because of their lower inherent ring strain than epoxides and the higher basicity of polymerizations because of their lowe lowerr inherent ring strain than epoxidesepoxides and the higher basicity of the oxetane oxygen that results in the formation of stabilized tertiary oxonium ion intermediates. In the oxetane oxygenoxygen thatthat resultsresults inin thethe formation formation of of stabilized stabilized tertiary tertiary oxonium oxonium ion ion intermediates. intermediates. In In a a similar study, Crivello demonstrated that the addition of a multifunctional acrylate monomer in a asimilar similar study, study, Crivello Crivello demonstrated demonstrated that that the the addition addition of ofa a multifunctionalmultifunctional acrylateacrylate monomermonomer in a multifunctional alkyl glycidyl ether or oxetane can accelerate the cationic photopolymerization in the multifunctional alkyl glycidyl ether or oxetane can accelerate the cationic photopolymerization in the presence of a free radical photoinitiator. Figures 15 and 16 show a marked accelerating effect of presence ofof aa free free radical radical photoinitiator. photoinitiator. Figures Figures 15 and 15 16and show 16 show a marked a marked accelerating accelerating effect of effect TMPTA of TMPTA on the cationic polymerization of NPGDGE and ERL 4221-E [127,128]. TMPTAon the cationic on the polymerizationcationic polymerization of NPGDGE of NPGDGE and ERL and 4221-E ERL [127 4221-E,128]. [127,128].

Figure 15. Comparison of photopolymerization of NPGDGE and multifunctional acrylate TMPTA in Figure 15. Comparison of photopolymerization of NPGDGE and multifunctional acrylate TMPTA in the presence of 2-phenyl-2,2-dimethoxy-acetophenone (DMPA) against pure NPGDGE. Extracted the presencepresence ofof 2-phenyl-2,2-dimethoxy-acetophenone 2-phenyl-2,2-dimethoxy-acetophenone (DMPA) (DMPA) against against pure pu NPGDGE.re NPGDGE. Extracted Extracted from from James V. Crivello, Synergistic effects in hybrid free radical/cationic photopolymerizations, Jamesfrom James V. Crivello, V. Crivello, Synergistic Synergistic effects in effects hybrid in free hy radicalbrid free/cationic radical/cationic photopolymerizations, photopolymerizations, Journal of Journal of Polymer Science Part A: Polymer Chemistry, 2007, 45, 3759–3769, Copyright 2007 Wiley JournalPolymer of Science Polymer Part Science A: Polymer Part Chemistry,A: Polymer 2007, Chemistry, 45, 3759–3769, 2007, 45, Copyright 3759–3769, 2007 Copyright Wiley Periodicals 2007 Wiley Inc., Periodicals Inc., with permission from John Wiley and Sons. withPeriodicals permission Inc., with from permission John Wiley from and Sons.John Wiley and Sons.

Polymers 2020, 12, 2146 22 of 34 Polymers 2020, 12, x FOR PEER REVIEW 21 of 33

FigureFigure 16.16. Comparison of photopolymerization of NPGDGE and multifunctional acrylate TMPTA inin thethe presencepresence ofof 2-phenyl-2,2-dimethoxy-acetophenone2-phenyl-2,2-dimethoxy-acetophenone (DMPA) againstagainst purepure ERLERL 4221-E.4221-E. ExtractedExtracted fromfrom JamesJames V. V. Crivello, Crivello, Synergistic Synergistic effects effects in hybrid in hy freebrid radical free radical/cationic/cationic photopolymerizations, photopolymerizations, Journal ofJournal Polymer of Polymer Science Part Science A: Polymer Part A: Chemistry,Polymer Ch 2007,emistry, 45, 3759–3769, 2007, 45, 3759–3769, Copyright 2007Copyright Wiley Periodicals2007 Wiley Inc.,Periodicals with permission Inc., with frompermission John Wiley from andJohn Sons. Wiley and Sons.

InIn 2012,2012, RyuRyu etet al.al. [[129]129] studiedstudied cationiccationic frontalfrontal polymerizationpolymerization ofof variousvarious epoxyepoxy monomersmonomers inducedinduced byby UVUV irradiationirradiation andand followedfollowed byby thermalthermal triggering.triggering. Their approach involvedinvolved aa generationgeneration ofof supramolecular proton complexe complexess by by reversibly reversibly sequestering sequestering Brønsted Brønsted acids acids in in the the presence presence of of a acationic cationic photo- photo- and and redox redox initiator initiator system system that that coul couldd result result in in poor poor reactivi reactivityty for the initiation ofof polymerizationpolymerization atat roomroom temperature.temperature. FrontalFrontal polymerizationpolymerization waswas observedobserved forfor triethylenetriethylene glycolglycol diglycidyldiglycidyl etherether upon upon contact contact with with a hot a solderinghot soldering iron at iron a temperature at a temperature of 50–70 ◦ofC after50–70 UV °C irradiation after UV ofirradiation the sample of wasthe sample discontinued. was discontinued. They also showed They al thatso showed even in that thepresence even in the of a presence redox initiator of a redox and metalinitiator catalyst, and metal the cationic catalyst, polymerization the cationic polymeri of dipropylenezation glycolof dipropylene diglycidyl glycol ether isdiglycidyl accelerated ether upon is brieflyaccelerated heating upon the briefly solution. heating In addition, the solution. they alsoIn addition, investigated they also the effectinvestigated of addition the effect of a crown of addition ether thatof a formscrown a ether hydronium that forms ion complexa hydronium with Brømstedion complex acids. with It wasBrømsted found thatacids. in It the was presence found ofthat a crownin the ether,presence the formationof a crown of ether, a hydronium the formation ion species of a entangledhydronium in ion a crown species ether entangled reduces thein a reactivity crown ether of a cyclohexenereduces the oxidereactivity monomer of a andcyclohexene an iodonium oxide salt mo photoinitiatornomer and byan increasing iodonium the salt induction photoinitiator time [129 by]. increasingThe effect the induction of various time reactive [129]. diluents on the RICFP of a formulation of BADGE, iodonium salt photoinitiatorThe effect and of various TPED was reactive investigated diluents by on Knaack the RICFP et al. of [ 40a ].formulation They found of thatBADGE, (3-ethyloxetan-3-yl) iodonium salt methanolphotoinitiator (EOM) and gave TPED the was lowest investigated minimal layer by Knaack thickness et al. on [40]. a PTFE They mould found (Figure that (3-ethyloxetan-3-yl) 17). Other reactive diluentsmethanol also (EOM) gave lowergave minimalthe lowest layer minimal thickness layer in comparison thickness on to thea PTFE reactivity mould of BADGE (Figure in17). the Other order ofreactive NPDGE diluents>CHDGE also> gaveCE [40 lower]. In minimal a recent layer study th byickness Tran etin al.comparison [41], several to the epoxy reactivity monomers of BADGE were investigatedin the order for of theirNPDGE>CHDGE>CE effect on the RICFP [40]. of BADGE. In a recent They study used 20by mol Tran % ofet eachal. [41], of these several epoxies epoxy as diluentsmonomers along were with investigated 0.1 mol % offor the their novel effect I-Al on photoinitiator. the RICFP of BADGE. Among them, They oxetaneused 20 monomermol % of each (EOM) of gavethesethe epoxies highest as diluents reduction along in viscosity with 0.1 and mol therefore % of the thenovel highest I-Al ph frontalotoinitiator. velocity Among in the cationic them, oxetane frontal polymerizationmonomer (EOM) of BADGEgave the (Figure highest 18 reduction). From their in viscosity study, it wasand foundtherefore that the HDDGE highest and frontal then EOMvelocity gave in thethe highestcationic heat frontal of polymerization polymerization followed of BADGE by NPDGE (Figure and CE18). in From comparison their study, to pure it BADGE. was found From that the storageHDDGE and and loss then modulus EOM curvesgave the and highest IR spectroscopy heat of polymerization data obtained forfollowed each of by the NPDGE diluent, itand was CE seen in thatcomparison a high degree to pure of cross-linkingBADGE. From is achievedthe storage with and monomer loss modulus conversion curves of more and thanIR spectroscopy 95% [41]. data obtained for each of the diluent, it was seen that a high degree of cross-linking is achieved with monomer conversion of more than 95% [41].

Polymers 2020, 12, x FOR PEER REVIEW 22 of 33

Polymers 2020, 12, 2146 23 of 34

Polymers 2020, 12, x FOR PEER REVIEW 22 of 33

Figure 17. Increase in the reactivity shown by decrease in the minimal layer thickness of the cured part of a BADGE formulation consisting of 20 mol % EOM as the reactive diluent. The black dotted FigurecurveFigure in 17. theIncrease 17. figure Increase incorresponds the in reactivitythe reactivity toshown 0.1 shown mol by % decreaseby I-Al decrease + 20 in mol thein the minimal % minimalEOM. layer Extracted layer thickness thickness from of of thePatrick the cured cured Knaack, part of aNicolas BADGEpart Klikovits, of formulation a BADGE Anh formulation consisting Dung Tran, ofcons 20 istingDaniel mol %of EOM20Bomze, mol as % the RobertEOM reactive as Liska,thediluent. reactive Radical-induced Thediluent. black The dotted black cationic curvedotted frontal in the curve in the figure corresponds to 0.1 mol % I-Al + 20 mol % EOM. Extracted from Patrick Knaack, figurepolymerization corresponds in thin to 0.1 layers, mol % Journal I-Al + 20 of molPolyme % EOM.r Science Extracted Part A: from Polyme Patrickr Chemistry, Knaack, Nicolas 2019, Klikovits57, 1155–, Nicolas Klikovits, Anh Dung Tran, Daniel Bomze, Robert Liska, Radical-induced cationic frontal Anh1159, Dung Copyright Tran, Daniel2019 The Bomze, Authors. Robert Journal Liska, of Radical-inducedPolymer Science Part cationic A: Polymer frontal Chemistry polymerization Published in thin by polymerization in thin layers, Journal of Polymer Science Part A: Polymer Chemistry, 2019, 57, 1155– layers, Journal of Polymer Science Part A: Polymer Chemistry, 2019, 57, 1155–1159, Copyright 2019 Wiley1159, Periodicals, Copyright Inc., 2019 with The permissionAuthors. Journal from of JohnPolymer Wiley Science and Part Sons. A: Polymer Chemistry Published by The Authors.Wiley Periodicals,Journal ofInc., Polymer with permission Science Part from A: John Polymer Wiley Chemistry and Sons.Published by Wiley Periodicals, Inc., with permission from John Wiley and Sons.

Figure 18. Comparison of viscosity and front velocity of several epoxy monomers in the RICFP of Figure 18. Comparison of viscosity and front velocity of several epoxy monomers in the RICFP of FigureBADGE. 18. Comparison Reprinted from of viscosity Composites and Part front A, velocity132, Anh ofDung several Tran, epoxy Thomas monomers Koch, Patrick inthe Knaack, RICFP of BADGE. Reprinted from Composites Part A, 132, Anh Dung Tran, Thomas Koch, Patrick Knaack, BADGE.Robert Reprinted Liska, Radical-induced from Composites cationic Part frontal A, 132,polymerization Anh Dung for Tran, preparation Thomas of Koch,epoxy Patrickcomposites, Knaack, Robert105855, Liska, Copyright Radical-induced 2020 with permissioncationic frontal from Elsevier.polymerization for preparation of epoxy composites, Robert Liska, Radical-induced cationic frontal polymerization for preparation of epoxy composites, 105855, Copyright 2020 with permission from Elsevier. 105855, Copyright 2020 with permission from Elsevier. 6. Effect of Fillers in Epoxy Composites 6. Effect Effect ofThe Fillers fabrication in Epoxy of an Compositesepoxy composite cured via UV light was first demonstrated on glass fibre- reinforced epoxidized vegetable oils by Crivello et al. [130]. However, these composites were not The fabrication of an epoxy composite cured via UV light was first first demons demonstratedtrated on on glass glass fibre- fibre- frontally polymerized, and instead were irradiated for a prolonged exposure time (25 min) until the reinforced epoxidized vegetable oils by Crivello et al. [130]. However, these composites were not reinforcedepoxy cured epoxidized completely. vegetable Using a oils frontal by Crivellopolymerization et al. technique, [130]. However, Bomze et these al. [39] composites investigated were the not frontallyeffect polymerized, of mica powder and on the instead polymerization were irradiated heat consumption for a prolonged and light exposure shielding time effect (25 in the min)min) RICFP untiluntil the epoxyof cureda BADGE completely. formulation. Using Their a frontalstudies showed polymerization that up to technique, 20 (parts per Bomze hundre etd al.rubber, [[39]39] phr) investigated of mica the eeffectffect of mica powder on the polymerization heat consumption and light shielding eeffectffect in the RICFP of a BADGE formulation. Their studies showed that up to 20 (parts per hundre hundredd rubber, phr) of mica powder can be incorporated without a change in the viscosity of the formulation. The addition of PolymersPolymers2020 2020, 12, ,12 2146, x FOR PEER REVIEW 23 of24 33 of 34

powder can be incorporated without a change in the viscosity of the formulation. The addition of mica to a formulation of BADGE and sensitizer did not affect the light sensitivity of the formulation mica to a formulation of BADGE and sensitizer did not affect the light sensitivity of the formulation and,and, only only a slighta slight reduction reduction in in the the front front velocityvelocity was observed with with increasing increasing the the mica mica concentration concentration fromfrom 5 to 5 to 20 20 phr phr [39 [39].]. Similar Similar results results were were achievedachieved by Knaack Knaack et et al. al. [40], [40], with with frontal frontal polymerization polymerization experimentsexperiments on on a BADGEa BADGE formulation formulation containingcontaining zirconia filler. filler. The The frontal frontal velocity velocity decreased decreased from from 9 cm9 cm/min/min to to 2.8 2.8 cm cm/min/min with with increasing increasing zirconiazirconia content from from 0 0 to to 80 80 phr phr [40]. [40 ].Using Using the the same same formulationformulation of of BADGE BADGE andand photo photo and and thermal thermal radical radical initiator, initiator, Klikovits Klikovits et al. [131], et al. studied [131], studied the effect the effectof silica of silica nanoparticles nanoparticles on onfrontal frontal polymerization polymerization and and mechanical mechanical properties properties of ofa aBADGE-silica BADGE-silica nanocomposite.nanocomposite. Their Their studies studies showed showed thatthat silicasilica concentration of of up up to to 3 3phr phr can can give give a reproducible a reproducible front,front, whereas whereas 4 and4 and 5 5 phr phr required required a a high-UV-intensity high-UV-intensity floodlight floodlight due due to to limited limited light light penetration, penetration, in orderin order to propagateto propagate the the front. front. A A decrease decrease in in the the front front velocity velocity and and front front starting starting time time was was observed observed for silicafor concentrationssilica concentrations up to 3 up phr to attributed 3 phr attributed to the low to thermal the low conductivity thermal conductivity and insulating and propertiesinsulating of silica.properties The results of silica. from The dynamic results mechanicalfrom dynamic analysis mechanical showed analysis a slight showed variation a slight in the variation glass transition in the glass transition temperatures of the UV-cured epoxy nanocomposite between 132 and 141 °C [131]. temperatures of the UV-cured epoxy nanocomposite between 132 and 141 ◦C[131]. InIn 2018, 2018, Sangermano Sangermano et et al. al. [ 42[42]] demonstrated demonstrated the possibility to to UV UV cure cure a aglass glass fibre-reinforced fibre-reinforced epoxy composite consisting of two glass-fibre layers deposited at 0° and 90°. The weight ratio for epoxy composite consisting of two glass-fibre layers deposited at 0◦ and 90◦. The weight ratio for thermal to photoinitiator was set to 1:2, while the epoxy formulation consisted of a blend of bisphenol thermal to photoinitiator was set to 1:2, while the epoxy formulation consisted of a blend of bisphenol A, A, bisphenol F and 1,6-hexanedioldiglycidylether, which was cured within 10 s of UV exposure bisphenol F and 1,6-hexanedioldiglycidylether, which was cured within 10 s of UV exposure (Figure 19). (Figure 19). The glass transition temperature was slightly higher (105 °C) than the thermally cured The glass transition temperature was slightly higher (105 C) than the thermally cured epoxy (at 95 C). epoxy (at 95 °C). Their results also demonstrated the◦ good applicability of UV-cured epoxy◦ Their results also demonstrated the good applicability of UV-cured epoxy composites in comparison to composites in comparison to thermally cured epoxy, since both types of curing methods had a slight thermallyvariation cured in the epoxy, tensile since strength both typesof the of cured curing composite methods (367 had aMPa slight for variation UV-cured in and the tensile345 MPa strength for of thethermally cured cured composite epoxy) (367 [42]. MPa for UV-cured and 345 MPa for thermally cured epoxy) [42].

FigureFigure 19. 19.Progress Progress of of a a radical-induced radical-induced cationiccationic frontal polymerization of of a aglass glass fibre-reinforced fibre-reinforced epoxyepoxy composite composite after after 10 10 ss ofof UV irradiation. irradiation. Extrac Extractedted from from Composites Composites PartPart B: Engineering, B: Engineering, 143, M. 143, Sangermano, A. D’Anna, C. Marro, N. Klikovits, R. Liska, UV-activated frontal polymerization of M. Sangermano, A. D’Anna, C. Marro, N. Klikovits, R. Liska, UV-activated frontal polymerization of glass fibre-reinforced epoxy composites, 168–171, Copyright (2018) with permission from Elsevier. glass fibre-reinforced epoxy composites, 168–171, Copyright (2018) with permission from Elsevier.

InIn another another similar similar study,study, Sangermano et et al. al. [125] [125 prepared] prepared four four piles piles of a 2 of × 2 a twill 2 carbon2 twill fibre- carbon × fibre-reinforcedreinforced epoxy epoxy composite composite cured cured via via UV UV light light an andd compared compared its itsproperties properties to tothe the corresponding corresponding thermally cured carbon-reinforced epoxy. Due to a high thermal conductivity of the carbon fibres, thermally cured carbon-reinforced epoxy. Due to a high thermal conductivity of the carbon fibres, the front temperature and velocity was slightly higher for carbon fibre-reinforced epoxy in the front temperature and velocity was slightly higher for carbon fibre-reinforced epoxy in comparison comparison to UV-cured epoxy without carbon fibre. In particular, the glass transition temperature to UV-cured epoxy without carbon fibre. In particular, the glass transition temperature of UV-cured of UV-cured carbon fibre-reinforced epoxy was higher than the thermally cured counterpart, which carbon fibre-reinforced epoxy was higher than the thermally cured counterpart, which indicates a more rigid cross-linked structure. Moreover, the UV-cured epoxy composite had a tensile strength of 447 MPa in comparison to thermally cured with 715 MPa [125]. Polymers 2020, 12, x FOR PEER REVIEW 24 of 33 Polymers 2020, 12, x FOR PEER REVIEW 24 of 33

Polymersindicatesindicates2020 aa , moremore12, 2146 rigidrigid cross-linkedcross-linked structure.structure. MoMoreover, the UV-cured epoxy composite had a tensile25 of 34 strength of 447 MPa in comparison to thermallythermally curedcured withwith 715715 MPaMPa [125].[125]. Tran et al. [41] fabricated several filler composites using BADGE, 1 mol % of thermal initiator Tran et al. [41] fabricated several filler composites using BADGE, 1 mol % of thermal initiator TPED, 0.1 mol % photoinitiator I-Al, and 40 mol % of the diluent EOM. As shown in Figures 20 and TPED, 0.1 mol % photoinitiator I-Al, and 40 mol % of the diluent EOM. As shown in Figures 20 and 21, 21, a general decrease in the front velocity and frontalfrontal temperaturetemperature waswas observedobserved forfor allall fillersfillers a general decrease in the front velocity and frontal temperature was observed for all fillers attributed attributed to an increased heat uptake of the fillers.llers. TheThe authorsauthors reportedreported thatthat duedue toto thethe highhigh to an increased heat uptake of the fillers. The authors reported that due to the high reactivity of EOM, reactivity of EOM, it was possible to increase the mica content in the formulation to up to 13.2 vol%. it was possible to increase the mica content in the formulation to up to 13.2 vol%. From Figure 21, From Figure 21, it was reported that due to high thermal conductivity of graphite, aluminium and it was reported that due to high thermal conductivity of graphite, aluminium and short carbon fibre, short carbon fibre, the frontal temperature remained high at 220 °C, which sustained the front until the frontal temperature remained high at 220 ◦C, which sustained the front until complete curing was complete curing was achieved. In addition, these fillersfillers alsoalso gavegave aa lowlow voidvoid contentcontent ofof 2%2% inin achieved. In addition, these fillers also gave a low void content of 2% in comparison to mica having comparison to mica having 15% void content, which was attributed to its intrinsicallyinsically highhigh porosity.porosity. 15% void content, which was attributed to its intrinsically high porosity. Therefore, a higher glass Therefore, a higher glass transition temperature was observed for composites with graphite, transition temperature was observed for composites with graphite, aluminium and short carbon fibre aluminium and short carbon fibre in comparison to mica, which gave a lower glass transition in comparison to mica, which gave a lower glass transition temperature than its neat formulation [41]. temperaturetemperature thanthan itsits neatneat formulationformulation [41].[41].

FigureFigure 20.20. EffectEffect of different different fillersfillers onon the front veloci velocitytyty of of a a UV-curedUV-cured BADGEBADGE formulationformulation containingcontaining 11 molmol %% ofof thermalthermal initiatorinitiator TPED,TPED, 0.10.1 molmol %% photoinitiatorphotoinitiator I-Al, and 40 mol % of the diluent EOM. ReprintedReprinted fromfrom CompositesComposites PartPart A,A, 132,132, Anh Dung Dung Tran, Tran, ThomasThomas Koch,KochKoch,, Patrick Patrick Knaack, Knaack, Robert Robert Liska, Liska, Radical-inducedRadical-induced cationic cationic frontal frontal polymerizationtal polymerizationpolymerization for preparation forfor preparatpreparat of epoxyionion ofof composites, epoxyepoxy composites,composites, 105855, Copyright 105855,105855, 2020Copyright with permission 2020 with perm fromissionission Elsevier. fromfrom Elsevier.Elsevier.

Figure 21. Effect of different fillers on the frontal temperature of a UV-cured BADGE formulation FigureFigure 21.21. EEffectffect of didifferentfferent fillersfillers onon thethe frontalfrontal temperaturetemperature ofof aa UV-curedUV-cured BADGEBADGE formulationformulation containing 1 mol % of thermal initiator TPED, 0.1 mol % photoinitiator I-Al, and 40 mol % of the containingcontaining 11 mol mol % % of of thermal thermal initiator initiator TPED, TPED, 0.1 mol0.1 mol % photoinitiator % photoinitiator I-Al, andI-Al, 40 and mol 40 % ofmol the % diluent of the diluent EOM. Reprinted from Composites Part A, 132, Anh Dung Tran, Thomas Koch, Patrick Knaack, EOM.diluent Reprinted EOM. Reprinted from Composites from Comp Partosites A, 132,Part AnhA, 132, Dung Anh Tran, Dung Thomas Tran, Thomas Koch, Patrick Koch, Knaack,Patrick Knaack, Robert Robert Liska, Radical-induced cationic frontal polymerization for preparation of epoxy composites, Liska,Robert Radical-induced Liska, Radical-induced cationic cationic frontal polymerizationfrontal polymerization for preparation for preparation of epoxy of composites, epoxy composites, 105855, 105855, Copyright 2020 with permission from Elsevier. Copyright105855, Copyright 2020 with 2020 permission with permission from Elsevier. from Elsevier.

Polymers 2020, 12, 2146 26 of 34 Polymers 2020, 12, x FOR PEER REVIEW 25 of 33

TheThe groupgroup alsoalso investigatedinvestigated thethe frontalfrontal andand thermomechanicalthermomechanical propertiesproperties ofof aa wovenwoven carboncarbon fibrefibre epoxyepoxy compositecomposite curedcured viavia UVUV lightlight andand comparedcompared itit withwith itsits correspondingcorresponding anhydrideanhydride curedcured epoxyepoxy networks.networks. The RICFPRICFP ofof thethe BADGEBADGE formulationformulation gavegave slightlyslightly higherhigher epoxyepoxy conversionsconversions inin comparisoncomparison toto thermallythermally curedcured specimenspecimen basedbased onon DSCDSC andand FTIRFTIR results.results. ThoughThough therethere waswas nono significantsignificant didifferencefference betweenbetween thethe tensiletensile propertiesproperties ofof UV-curedUV-cured andand thermallythermally curedcured specimens,specimens, interlaminarinterlaminar shear shear strength strength values values were were slightly slightly different. different. The failure The failure mode of mode the UV-cured of the UV-cured specimen showsspecimen multi-shear shows multi-shear failure, with failure, single with shear single being sh theear main being failure the main mode failure in the mode thermally in the cured thermally ones (Figurecured ones 22)[ (Figure41]. 22) [41].

50 Thermal curing RICFP curing 40

30 (MPa) τ 20

10

0 0 0.51.01.52.0 displacment (mm)

FigureFigure 22.22. Comparison of interlaminar shear strength ofof thermally cured andand UV-cured epoxyepoxy carboncarbon fibre-reinforcedfibre-reinforced composites.composites. ReprintedReprinted fromfrom CompositesComposites Part A, 132, Anh Dung Tran, ThomasThomas Koch,Koch, PatrickPatrick Knaack,Knaack, Robert Robert Liska, Liska, Radical-induced Radical-induced cationic cationic frontal frontal polymerization polymerization for preparation for preparation of epoxy of composites,epoxy composites, 105855, 105855, Copyright Copyright 2020 with 2020 permission with permission from Elsevier. from Elsevier.

7. Conclusions and Future Perspectives 7. Conclusions and Future Perspectives With this review, we provide a useful toolbox for the UV-triggered cationic frontal polymerization With this review, we provide a useful toolbox for the UV-triggered cationic frontal of various epoxide monomers that employ an iodonium salt photoinitiator and a thermal radical polymerization of various epoxide monomers that employ an iodonium salt photoinitiator and a initiator. Key aspects influencing the polymerization kinetics and related thermo-mechanical properties thermal radical initiator. Key aspects influencing the polymerization kinetics and related thermo- are summarized in a comprehensive way. In particular, an overview of the chemistry of selected mechanical properties are summarized in a comprehensive way. In particular, an overview of the iodonium salts and thermal initiators is given, while studies on the cationic photopolymerization chemistry of selected iodonium salts and thermal initiators is given, while studies on the cationic of various epoxide monomers are discussed and arranged chronologically. In addition, the use of photopolymerization of various epoxide monomers are discussed and arranged chronologically. In alternativeaddition, the light use sources of alternative for initiating light sources the cationic for init polymerizationiating the cationic ishighlighted polymerization together is highlighted with the etogetherffect of sensitizers with the effect on cure of sensitizers kinetics and on frontcure kinetics parameters. and front The structure–reactivity parameters. The structure–reactivity relationship and erelationshipffect of reactive and diluentseffect of onreactive the polymerization diluents on the of polymerization various epoxide of monomersvarious epoxide is detailed. monomers In the is lastdetailed. section, In the the last transfer section, of the the transfer UV-triggered of the UV cationic-triggered frontal cationic polymerization frontal polymerization from neat from resins neat to polymer-basedresins to polymer-based composites composites is discussed. is discussed. Particular Part focusicular is on focus the eisff ecton the of various effect of fillers various and fillers fibres and on thefibres radical-induced on the radical-induced cationic frontal cationic polymerization frontal polymerization of epoxy-based of epoxy-based resin formulations. resin formulations. CationicCationic frontalfrontal polymerizationpolymerization ofof epoxideepoxide monomersmonomers hashas openedopened upup aa newnew pathpath towardstowards aa rapid,rapid, lowlow costcost andand energy-independentenergy-independent curingcuring ofof epoxy-basedepoxy-based resins.resins. RecentRecent studiesstudies byby KnaackKnaack etet al.al. [[40],40], predictpredict thethe potentialpotential applicationsapplications ofof cationiccationic frontalfrontal polymerization,polymerization, demonstratingdemonstrating excellentexcellent performanceperformance ofof frontally cured epoxy epoxy resins resins in in chemical chemical anchor anchor bolts bolts and and underwater underwater crack crack filling filling of ofvarious various materials. materials. However, However, the the implementation implementation of of this this curing curing technique technique in variousvarious industrialindustrial applicationsapplications is yet to to be be achieved achieved.. Various Various potential potential applications applications of frontal of frontal polymerization polymerization were were also alsocompiled compiled by Pojman by Pojman [8] in [ 8his] in review, his review, where where applications applications such as such hydrogels, as hydrogels, consolidation consolidation of stone, of stone,autoclave-less autoclave-less curing curingof large of composites large composites and cure-on-demand and cure-on-demand repair and repair adhesives and adhesiveswere discussed. were discussed.Robertson Robertsonet al. [132] et al.demonstrated [132] demonstrated a thermall a thermallyy initiated initiated frontal frontal ring ring opening opening metathesismetathesis polymerization (FROMP) of dicyclopentadiene, which was successfully used to fabricate a 900 cm2

Polymers 2020, 12, 2146 27 of 34 polymerization (FROMP) of dicyclopentadiene, which was successfully used to fabricate a 900 cm2 corrugated carbon fibre-reinforced polymer composite (FRPC) panel in only 5 min and by spending 750 J of energy. From the results of a study on the energy consumed versus time required to cure composites, the group calculated that the curing of a section of the Boeing 787 fuselage by FROMP reduces the energy consumption by ten orders of magnitude compared with autoclave processing technique [132]. In the production of composite materials, the product cycle time, total energy consumption, fibre–matrix adhesion and the quality of the cured composite are crucial parameters that are presently a subject of improvement via an intensive researching. Particularly for thermosetting resins such as epoxide monomers, maximum monomer conversion or high cross-link density and uniform distribution within the fibre/filler can be improved by employing frontal polymerization. This technique does not only improve the final product quality but also cuts down a lot in terms of input energy requirements for thermal curing and subsequent post curing, as well as eliminating the usage of toxic solvents for dissolution of the curing agents. Furthermore, complex part geometries that are either not fully accessible during a conventional curing process can now be easily accessed via frontal polymerization at ambient processing temperature and without the necessity of tools for curing under high-pressure. The kinetics of frontal polymerization allows the use of various other monomers that are particularly used to improve the mechanical properties of neat epoxy resins. This means that any functionalized monomer that do not act as radical scavengers during the initial photo cleavage of the iodonium salt and also has appreciable polarity to solubilize with epoxy monomers can be used to prepare epoxy blends of desired thermomechanical properties. Epoxidized elastomers are an interesting class of monomers that can be employed also in the fabrication of high tensile and impact strength composites, which were firstly reported by Crivello and Yang in 1994 [133]. Due to the development of various visible light absorbing photoinitiators, the epoxy photopolymerization technology also has a potential to be expanded towards green synthesis pathways using sunlight as light source. Combined with epoxidized vegetable oils, which were firstly studied by Crivello et al. [106,130], green composites can be literally fabricated using natural fibres and sunlight only, which may find applications in sailing boats and roofing surfaces for homes. The on-site damage repair of epoxy composites as well as epoxy-based electrical insulating materials using frontal polymerization is also a promising alternative to conventional repairing processes providing rapid, low-cost and uniform curing of the damaged materials. With the photoinduced frontal polymerization technique, defects within the composite part can be rapidly fixed by injecting a small volume of the epoxy-initiator mixed solution filling the cracks and then allowing it to cure frontally with a local UV irradiation. Frontal polymerization was also acknowledged by Pojman et al. [134] as a potential technique for in-space repair and fabrication. In fact, studies on the possibility of employing frontal polymerization for aeronautics and space materials were also reported with positive results from orbital experiments and terrestrial simulations in the past few decades [135,136]. Cationic photopolymerization has been demonstrated in the past among materials used in various space missions such as the STS-46 by NASA (National Aeronautics and Space Administration), where epoxy-functionalized were employed as a protective coating against atomic oxygen [137]. These materials were successfully used to fabricate composites and transparent coatings for the space shuttle providing excellent coating resistance to outer space debris and micrometeoroids. We anticipate that with the currently increasing space missions organized by various aerospace institutes around the globe, the photoinduced frontal polymerization technology will play an active role in providing an economic and engineered performance for the various epoxy/fibre composites used. With recent successful studies focused on the thermomechanical properties of fibre/filler-reinforced epoxy composites [39–42,125], we predict that radical-induced cationic frontal polymerization will prove to be a promising curing technology providing energy efficient process, reduced cycle time and excellent product performance. Polymers 2020, 12, 2146 28 of 34

Author Contributions: All authors have contributed to the preparation of this manuscript. Writing—original draft preparation, M.S.M.; writing—review and editing, S.S.; writing—review and editing, M.S.; supervision, M.W.; project administration, M.W; funding acquisition, M.W. All authors have read and agreed to the published version of the manuscript. Funding: This research was funded by the Austrian Research Promotion Agency (FFG), grant number 854178. Acknowledgments: This research work was performed at the Polymer Competence Center Leoben GmbH (PCCL, Austria) within the framework of the COMET program of the Federal Ministry for Transport, Innovation and Technology and Federal Ministry for Economy, Family and Youth with contributions by the Institute of Chemistry of Polymeric Materials (Montanuniversitaet Leoben, Austria), MAGNA Energy Storage Systems GmbH and FACC Operations GmbH. The PCCL is funded by the Austrian Government and the State Governments of Styria, Upper and Lower Austria (grant number: 854178). Conflicts of Interest: The authors declare no conflict of interest.

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