State of the Art in Dual-Curing Acrylate Systems

Review State of the Art in Dual-Curing Acrylate Systems

Osman Konuray 1,*, Xavier Fernández-Francos 1 ID , Xavier Ramis 1 ID and Àngels Serra 2 ID

1 Thermodynamics Laboratory, ETSEIB, Universitat Politècnica de Catalunya, Av. Diagonal 647, 08028 Barcelona, Spain; [email protected] (X.F.-F.); [email protected] (X.R.) 2 Department of Analytical and Organic Chemistry, Universitat Rovira i Virgili, C/Marcel·lí Domingo s/n, 43007 Tarragona, Spain; [email protected] * Correspondence: [email protected]

Received: 22 January 2018; Accepted: 7 February 2018; Published: 12 February 2018

Abstract: Acrylate chemistry has found widespread use in dual-curing systems over the years. Acrylates are cheap, easily handled and versatile monomers that can undergo facile chain-wise or step-wise polymerization reactions that are mostly of the “click” nature. Their dual-curing processes yield two distinct and temporally stable sets of material properties at each curing stage, thereby allowing process ﬂexibility. The review begins with an introduction to acrylate-based click chemistries behind dual-curing systems and relevant reaction mechanisms. It then provides an overview of reaction combinations that can be encountered in these systems. It ﬁnishes with a survey of recent and breakthrough research in acrylate dual-curing materials for shape memory polymers, optical materials, photolithography, protective coatings, structured surface topologies, and holographic materials.

Keywords: acrylates; methacrylates; dual-curing; click chemistry

1. Introduction A dual-curing process is defined as a combination of two curing reactions taking place simultaneously or sequentially [1]. It is a method to combine two otherwise distinct polymer networks. The resulting interpenetrating polymer network (IPN) exhibits superior properties compared to its individual parts. If a functional group participates in both curing reactions, grafted IPNs can be obtained. Sequential dual-curing facilitates ease in processing and handling as long as the intermediate materials (i.e., after the first curing reaction) are chemically stable and their properties tailorable. If the dual-curing process merely aims for improved final properties, control of the reaction sequence may be irrelevant. Sequential dual-curing methodology is developed from multi-stage (or B-stage) processing techniques. Conventionally, multi-stage processing refers to a one-pack adhesive formulation, usually of epoxide origin, that can be partially pre-cured after application to a substrate. The adhesive applied substrate can be transported and/or further processed (such as in a final assembly stage), and the final cure of adhesive could be initiated whenever desired by using heat or ultraviolet (UV) light, depending on formulation chemistry. The same technology finds use in processing of complex composite parts as well [2,3]. In contrast, two-pot systems do not feature such flexibility in processing as the final curing reaction starts once the initial components are mixed. Multi-stage processing is well known from the early times of crosslinked polymers, with the development of Bakelite in the early 20th century. Pre-polymer synthesis and their further crosslinking is a common processing technology [4–6]. In dual-curing processing, the extent of both curing reactions and the properties of the intermediate material can be easily controlled by changing the composition of the formulation, as well as the duration and temperature of cure [1]. In the case of sequential dual-curing, the intermediate material will have stable properties and will not cure unless the second reaction is triggered. In comparison with pre-polymer synthesis approaches, processing and compounding is much easier

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triggered. In comparison with pre-polymer synthesis approaches, processing and compounding is inmuch dual-curing easier in dual-curing formulations. formulations. The pre-polymer The pre-po is producedlymer is produced in-situ during in-situ the during ﬁrst curing the first stage curing of thestage initial of the mixture initial ofmixture low molecular of low molecular weight monomers, weight monomers, rather than rather by mixing than by of largermixing polymeric of larger components.polymeric components. This is of greatThis useis of when great low-viscosity use when low-viscosity formulations formulations are required are by required the application. by the However,application. the However, concept of the dual-curing concept goesof dual-curing far beyond goes such conceptsfar beyond and such makes concepts it possible and to makes develop it custom-tailoredpossible to develop materials custom-tailored with a processing materials with ﬂexibility a processing that is flexibility useful for that a numberis useful offor advanced, a number high-added-valueof advanced, high-added-value applications. Dual-curingapplications. systems Dual-curing facilitate systems solvent-free facilitate processing, solvent-free starting processing, from liquidstarting monomers, from liquid and monomers, mild processing and mild conditions, processing whichconditions, helps which minimize helps the minimize emission the of emission volatile organicof volatile compounds organic compounds (VOCs) to the(VOCs) atmosphere to the atmo andsphere reduce and processing reduce energyprocessing costs. energy An exemplary costs. An sequentialexemplary dual-curingsequential dual-curing process is schematized process is schematized in Scheme1 .in Scheme 1.

Scheme 1. A sequential dual-curing process. The monomers depicted as green and light blue Scheme 1. A sequential dual-curing process. The monomers depicted as green and light blue participate inparticipate the ﬁrst in curing the first reaction curing (RXN reaction 1) leaving(RXN 1) the leaving dark the blue dark monomers blue monome unreacted.rs unreacted. At a later At a stage later (RXNstage (RXN 2), the 2), dark the blue dark monomers blue monomers polymerize polymerize with the with available the available light blue light functional blue functional groups and groups also homopolymerize.and also homopolymerize. A grafted A IPN grafted is obtained. IPN is obtained.

Successful sequential dual-curing processing requires that (a) both polymerization reactions are Successful sequential dual-curing processing requires that (a) both polymerization reactions are selective and compatible so that no undesired inhibition or reactivity effects take place; and (b) they selective and compatible so that no undesired inhibition or reactivity effects take place; and (b) they can be triggered using different stimulus such as UV light or temperature, or else they have can be triggered using different stimulus such as UV light or temperature, or else they have sufficiently sufficiently different reaction rates so that they can be controlled from a kinetics point of view in different reaction rates so that they can be controlled from a kinetics point of view in terms of time terms of time and temperature. An important advantage of sequential dual-curing is that the and temperature. An important advantage of sequential dual-curing is that the properties of the properties of the final and intermediate stages can be custom-tailored, which is of key importance for final and intermediate stages can be custom-tailored, which is of key importance for their use in their use in advanced applications. advanced applications. The control of the curing sequence may not be relevant if such combination aims only at the The control of the curing sequence may not be relevant if such combination aims only at the enhancement of the properties of a given base material. For instance, post-polymerization of excess enhancement of the properties of a given base material. For instance, post-polymerization of excess groups in off-stoichiometric step-wise curing systems is used to enhance the thermal–mechanical groups in off-stoichiometric step-wise curing systems is used to enhance the thermal–mechanical properties of thermosets [7,8]. In other cases, the crosslinking reaction is triggered by means of UV properties of thermosets [7,8]. In other cases, the crosslinking reaction is triggered by means of UV light leading to fast cure in the outermost layers, but extended through-cure can take place more light leading to fast cure in the outermost layers, but extended through-cure can take place more slowly slowly and reach completion by means of a thermal polymerization process in the deepest layers [9]. and reach completion by means of a thermal polymerization process in the deepest layers [9]. Likewise, Likewise, for certain applications such as coatings that are cured by UV light, the use of dual-curing for certain applications such as coatings that are cured by UV light, the use of dual-curing formulations formulations together with the use of photoabsorbers can make it possible to achieve complete, or at together with the use of photoabsorbers can make it possible to achieve complete, or at least sufficient, least sufficient, cure in shadowed areas [10]. In the synthesis of IPNs [11–15], the control of the dual- cure in shadowed areas [10]. In the synthesis of IPNs [11–15], the control of the dual-curing process curing process plays a role in the morphology. Materials with particular morphological features can plays a role in the morphology. Materials with particular morphological features can be developed be developed depending on whether the polymerization processes take place in a simultaneous or dependingsequential way, on whether and whether the polymerization or not there is processes crosslinking take between place in the a simultaneous two networks. or sequential way, and whetherDual curable or not theresystems is crosslinking are also betweenavailable the in two which networks. the two curing reactions proceed simultaneously.Dual curable In systems such aresystems, also available the ultimate in which goal the twoof employing curing reactions a dual-cure proceed simultaneously.formulation is Inenhancing such systems, end-product the ultimate properties goal ofrather employing than an a intermediate dual-cure formulation processing is possibility. enhancing For end-product example, propertiesmixtures of rather multifunctional than an intermediate acrylate and processing epoxy mo possibility.nomers can For be example, crosslinked mixtures simultaneously of multifunctional under acrylateUV light and and epoxy with the monomers use of radical can be and crosslinked cationic photoinitiators. simultaneously Decker under UV and light coworkers and with [16] the showed use of radicalthe synthesis and cationic of IPNs photoinitiators. from a 50:50 mixture Decker and(by coworkersweight) of [1,6-he16] showedxanediol the diacrylate synthesis (HDDA) of IPNs from and aepoxidized 50:50 mixture soybean (by weight) oil (ESO) of under 1,6-hexanediol intense UV diacrylate irradiation. (HDDA) The plasticizing and epoxidized effect soybeanof ESO facilitated oil (ESO)

Polymers 2018, 10, 178 3 of 24 under intense UV irradiation. The plasticizing effect of ESO facilitated higher conversion of HDDA compared to a neat formulation. Such acrylate/epoxide IPNs are suitable as fast-drying protective coatings, composites and optical components. The use of photolatent and thermally-latent catalysts, for base- or acid-catalysed reactions or radical polymerization reactions [17–19] can contribute to the design of efficient one-pot dual-curing formulations. These formulations have good storage stability of the uncured system and stable intermediate properties. The use of blocked species can also be exploited for the design of dual-curing formulations [20]. Careful control of the exothermicity of polymerization is a general concern in composites processing for control and quality purposes [3,21]. It is also a factor to be taken into consideration in dual-curing processing schemes, especially in the case where the second polymerization process is thermally activated [22]. The intermediate and final properties of dual-curing materials strongly depend on the network structure at the end of each polymerization process. Some criteria for dual-curing formulation design or preliminary network structure analysis based on ideal network build-up models have been outlined recently [23,24]. However, recent studies [25] suggest the need for a deeper understanding of the effect of monomer feed ratio and monomer structure and functionality on the structure-property relationships in dual-cure processing. A large number of reactions can be used in dual-curing thermosetting formulations [1] but “click” type reactions are of special interest. The click concept was first coined by Sharpless et al. to refer to a variety of chemical reactions that are orthogonal, selective, and highly efficient [26,27]. Click reactions have the following characteristics: (a) high yields; (b) regio and stereospecificity; (c) insensitivity to oxygen or water; (d) mild, solvent-less, or aqueous reaction conditions; (e) orthogonality with other common organic reactions; and (f) applicability to a wide range of available starting compounds [28,29]. The click chemistry concept has found a particular interest in polymer chemistry leading to a “paradigm change”: A modular construction approach is used [27,30,31] rather than conventional synthetic approaches. Product purification is mostly unnecessary thanks to the selectivity and quantitative yield of click reactions. In an extensive review, Tunca reported the combination of two or more orthogonal click processes to synthesize complex architectures [32]. In the field of thermosets, click reactions allow the preparation of well-defined network structures. They can be carried out under mild and solvent-free conditions and therefore can be designed in an environmentally friendly manner. As various chemical functionalities show click behaviour and each has chemically and mechanistically distinct reaction pathways, they can be combined in dual curing methodologies. Some common click reactions reported for the preparation of dual-curable materials are outlined in Scheme2. Polymers 2018, 10, 178 4 of 24 Polymers 2018, 10, x FOR PEER REVIEW 4 of 25

Scheme 2. Reaction schemes of several click reactions.

2. Acrylate Reactions in Dual-Curing Systems Acrylic monomers have have been been used used widely widely in inpoly polymermer formulations formulations since since decades. decades. There There is a vast is a vastselection selection of commercially of commercially available, available, affordable affordable and and easily easily handled handled acrylates. acrylates. By By carefully carefully selecting acrylate monomermonomer structure structure and and the the functionality, functionality, polymers polymers can be can synthesized be synthesized for a plethora for a ofplethora industrial of applicationsindustrial applications such as industrial such as coatings industrial [13, 33coatings,34], adhesives [13,33,34], [35, 36adhesives], inks [37 ],[35,36], and biomaterials inks [37], [and38]. Acrylatebiomaterials groups [38]. readily Acrylate react groups with a varietyreadily ofreact substrates with a owingvariety to of the substrates carbon–carbon owing double to the bond carbon– and itscarbon neighbouring double bond electron and withdrawingits neighbouring carbonyl electron group, withdrawing which enhances carbonyl the reactivity group, which of the enhances double bond. the Therefore,reactivity of they the are double widely bond. used Therefore, in a number they of reactionsare widely [39 used–44] thatin a cannumber fit the of click reactions concept. [39–44] that can fitA the prominent click concept. click reaction of acrylates is the Michael addition. Michael addition [39] is deﬁned as the 1,4-additionA prominent (or click conjugate reaction addition) of acrylates of resonance-stabilized is the Michael addition. carbanions Michael to activatedaddition [39] double is defined bonds. Michael-typeas the 1,4-addition addition (or reactionsconjugate are addition) used widely of reso innance-stabilized dual curing processes, carbanions because to activated of the variety double of commerciallybonds. Michael-type available addition nucleophiles reactions (Michael are donors)used wi anddely activated in dual doublecuring bondprocesses, compounds because (Michael of the acceptors)variety of thatcommercially can be used available in such processes. nucleophiles (Michael donors) and activated double bond compoundsTypical (Michael Michael donorsacceptors) are that amines, can be thiols used and in such acetoacetates. processes. The group of Michael acceptors is moreTypical numerous Michael and donors includes are amines, acrylate thiols esters, and acrylamides,acetoacetates. maleimides,The group of alkylMichael methacrylates, acceptors is cyanoacrylatesmore numerous and and vinyl includes sulfones. acrylate Of allesters, these acrylamides, different acceptors, maleimides, acrylates alkyl havemethacrylates, a suitable combinationcyanoacrylates of and reactivity, vinyl versatility,sulfones. Of availability all these and different cost. Althoughacceptors, there acrylates are Michael have a acceptors suitable withcombination a higher of reactivity, reactivity, such versatility, as vinyl availability sulfones [24 and], there cost. is Although a vast selection there are of commerciallyMichael acceptors available with anda higher affordable reactivity, acrylates. such as In vinyl contrast, sulfones widely [24], available there is methacrylatesa vast selection have of commercially much lower reactivityavailable and due toaffordable the steric acrylates. effect of theIn contrast, methyl group widely [45 available] and are methacrylates therefore less usedhave asmuch Michael lower acceptors. reactivity due to the stericAs co-reactant effect of the in methyl Michael group additions, [45] andthe are mosttherefore common less used nucleophylic as Michael monomersacceptors. are thiols (thiol–MichaelAs co-reactant addition), in Michael amines additions, (aza-Michael the most addition), common and nucleophylic acetoacetates. monomers As these are polymerization thiols (thiol– routesMichael of addition), acrylates amines usually (aza-Michael ﬁt click criteria, addition), they and are acetoacetates. used widely As in these dual-curable polymerization formulations, routes inof combinationacrylates usually with other fit click click reactions,criteria, they or with are other used non-click widely reactions.in dual-curable formulations, in combinationA variety with of basic other and, click especially, reactions, nucleophilic or with other catalysts non-click can reactions. be used to trigger and control Michael additionsA variety [39,46 of]. basic Michael-type and, especially, (mostly nucleophilic thiol–Michael) catalysts reactions can can be beused combined to trigger in dual-curingand control Michael additions [39,46]. Michael-type (mostly thiol–Michael) reactions can be combined in dual- curing systems with a variety of other reactions such as radical-mediated polymerizations in a

Polymers 2018, 10, 178 5 of 24 systems with a variety of other reactions such as radical-mediated polymerizations in a controlled and sequential way. However, the complex dependence of thiol–Michael additions on the solvent, nature and concentration of catalyst, polarity and basic/nucleophilic character of the catalyst makes Polymers 2018, 10, x FOR PEER REVIEW 5 of 25 it difficult to optimize the kinetics and extension of the reaction and to minimize side reactions [47]. The predictioncontrolled and of the sequential optimum way. reaction However, conditions the comple basedx dependence on the pK of athiol–Michael(negative logarithm additions on of the acid dissociationsolvent, constant)nature and of concentration thiols and bases of catalyst, is not polari possiblety and due basic/nucleophilic to the concurrence character of base of andthe catalyst nucleophile mediatedmakes thiol–Michael it difficult to optimize reactions the kinetics [46]. The and use extension of strong of the bases reaction or nucleophilesand to minimize leads side toreactions a very fast reaction,[47]. butThe prediction makes preparation of the optimum difficult reaction [24, 46conditions]. Weak based nucleophiles on the pK allowa (negative the controllogarithm of of the the initial reactionacid rate, dissociation showing constant) an induction of thiols period and thatbases can is not be furtherpossible tuned due to by the addition concurrence of small of base quantities and of nucleophile mediated thiol–Michael reactions [46]. The use of strong bases or nucleophiles leads to a an acid (protonic species) [48]. The anionic polymerization of acrylates, which relies upon Michael very fast reaction, but makes preparation difficult [24,46]. Weak nucleophiles allow the control of the addition chemistry, is another side reaction that cannot be disregarded taking place during the curing initial reaction rate, showing an induction period that can be further tuned by addition of small of thiol/acrylatequantities of an systems acid (protonic [39]. Photobase species) [48]. generators The anionic can polymerization be used in Michael of acrylates, addition which reactions relies to designupon latent Michael dual-curing addition systemschemistry, [45 is, 49another,50] but side a radicalreaction inhibitor that cannot is neededbe disregarded to suppress taking the place radical mediatedduring side the reactionscuring of thiol/acrylate [49–51]. systems [39]. Photobase generators can be used in Michael addition Radical-mediatedreactions to design latent reactions dual-curing such assystems acrylate [45,49 homopolymerization,50] but a radical inhibito orr thiol–ene is needed to reaction suppress can be foundthe in radical dual-curing mediated systems side reactions [1]. These [49–51]. reactions, although not fitting the click concept, are also highly versatileRadical-mediated and can benefit reactions from such selective as acrylate activation homopolymerization by means of a or number thiol–ene of thermalreaction initiatorscan be or photoinitiatorsfound in dual-curing [52–57]. systems [1]. These reactions, although not fitting the click concept, are also highly versatile and can benefit from selective activation by means of a number of thermal initiators 2.1. Thiol–Acrylateor photoinitiators Reaction [52–57].

The2.1. Thiol–Acrylate most common Reaction and clickable co-reactants for acrylates are thiols. Acrylates react with thiols via two distinctThe most mechanisms: common and (a) clickable Radical co-reactants mediated for thiol–acrylate acrylates are reaction;thiols. Acrylates (b) Thiol–acrylate react with thiols Michael addition.via two The distinct radical mechanisms: mediated (a) route Radical is shownmediated in thiol–acrylate Scheme3. The reaction; thiyl (b) radical Thiol–acrylate is formed Michael either by UV irradiationaddition. The in radical the presence mediated of route photoinitiators, is shown in Sche or byme thermal3. The thiyl activation radical is [formed52]. Once either the by radical UV is generated,irradiation it is addedin the presence to the double of ph bondotoinitiators, of theacrylate or by thermal monomer activation to yield [52]. an Once acrylic the radical. radical Inis turn, this radicalgenerated, specie it is eitheradded propagatesto the double through bond of theacr reactionylate with monomer another to yield acrylate, an acrylic or abstracts radical. In a hydrogenturn, fromthis the radical thiol to specie yield either the thiol–acrylate propagates through reaction reaction product. with Inanother polymeric acrylate, systems, or abstracts a competition a hydrogen exists betweenfrom the the chain-growththiol to yield the homopolymerization thiol–acrylate reaction ofproduct. the acrylate In polymeric and the systems, step-growth a competition mechanism exists of a between the chain-growth homopolymerization of the acrylate and the step-growth mechanism of a thiol–ene polymerization. It has been shown previously by Cramer and Bowman [58] that the rate thiol–ene polymerization. It has been shown previously by Cramer and Bowman [58] that the rate constant for acrylate propagation is 1.5 times higher than that of hydrogen abstraction from the thiol. constant for acrylate propagation is 1.5 times higher than that of hydrogen abstraction from the thiol. BecauseBecause of this, of this, a radical a radical mediated mediated thiol–acrylate thiol–acrylate reaction cannot cannot be be considered considered click click since since thiols thiols remainremain partially partially unreacted. unreacted.

Scheme 3. Radical-mediated thiol–acrylate polymerization. The propagation kinetic constant (k1) was Scheme 3. Radical-mediated thiol–acrylate polymerization. The propagation kinetic constant (k1) was reported to be 1.5 higher than the hydrogen abstraction kinetic constant (k2) [58]. reported to be 1.5 higher than the hydrogen abstraction kinetic constant (k2)[58].

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The thiol–acrylatethiol–acrylate MichaelMichael additionaddition (thiol–Michael (thiol–Michael in in short) short) reaction reaction starts starts with with the the formation formation of aof thiolate a thiolate anion, anion, which which could could take take place place via via two two different different mechanisms, mechanisms, as shownas shown in Schemein Scheme4[59 4]. [59].

Scheme 4. Thiol–Michael reaction mechanism. Thiolate anion may form form via via two two distinct distinct mechanisms: mechanisms: Catalysis by base or nucleophile. (EWG representsrepresents an electron withdrawing group.)group.)

Depending on the pKa and nucleophilic strength of the catalyst, the formation of the thiolate can Depending on the pKa and nucleophilic strength of the catalyst, the formation of the thiolate follow either mechanism. When a strong base such as 1,5,7-triazabicyclo[4.4.0]dec-5-ene (TBD) is can follow either mechanism. When a strong base such as 1,5,7-triazabicyclo[4.4.0]dec-5-ene (TBD) is used, the base catalysed mechanism is favoured for the thiol–Michael reaction. On the other hand, used, the base catalysed mechanism is favoured for the thiol–Michael reaction. On the other hand, the reaction follows the nucleophilic path when strong nucleophiles are used as catalyst such as 4- the reaction follows the nucleophilic path when strong nucleophiles are used as catalyst such as (N,N-dimethylamino)pyridine (DMAP) [60,61] or tertiary phosphines [46]. Thiol–Michael reactions 4-(N,N-dimethylamino)pyridine (DMAP) [60,61] or tertiary phosphines [46]. Thiol–Michael reactions generally fit click criteria as the reaction can be carried out in an eco-friendly manner. There are no generally ﬁt click criteria as the reaction can be carried out in an eco-friendly manner. There are competing reactions within the mechanism, thereby allowing quantitative conversions of both no competing reactions within the mechanism, thereby allowing quantitative conversions of both reactants. As a result, this reaction has been combined with many other click reactions to develop reactants. As a result, this reaction has been combined with many other click reactions to develop dual-curing systems over the years. dual-curing systems over the years.

2.2. Aza-Michael Addition Acrylates cancan also also undergo undergo Michael Michael addition addition in the in presencethe presence of amines. of amines. The so-called The so-called aza-Michael aza- additionsMichael additions have the samehave mechanismthe same mechanism as thiol–Michael as thiol–Michael reactions (Scheme reactions5). Amines (Scheme as Michael 5). Amines donors as haveMichael the donors following have advantages the following in comparison advantages to thiolsin comparison or acetoacetates: to thiols (a) or many acetoacetates: suitable amines (a) many are commerciallysuitable amines available, are commercially including available, amine-terminated including hyperbranched amine-terminated polymers; hyperbranched (b) catalysts polymers; are not required(b) catalysts since are amines not required can act since as both amines nucleophiles can act as and both bases; nucleophiles (c) radicals and or otherbases; active (c) radicals species or are other not formedactive species during are the not ﬁrst formed stage of during curing; the (d) thefirst presence stage of of curing; tertiary (d) amines the presence formed duringof tertiary aza-Michael amines additionformed during overcomes aza-Michael the intrinsic addition oxygen overcomes inhibition th ofe acrylateintrinsic free-radical oxygen inhibition polymerizations of acrylate [62 ]free- and allowsradical thepolymerizations curing to be [62] performed and allows in non-inertthe curing environments to be performed and in (e) non-inert these tertiary environments amines and can act(e) these as co-initiator tertiary amines when can type act II as photoinitiators co-initiator wh (i.e.,en type photoinitiators II photoinitiators that require(i.e., photoinitiators a co-initiator that or synergistrequire a co-initiator to produce or initiating synergist radicals) to produce are initiating used. Due radicals) to the are low used. reactivity Due to of the formed low reactivity secondary of amines,formed secondary aza-Michael amines, reaction aza-Michael may not reach reaction completion may not but reach further completion crosslinking but further during crosslinking subsequent photoirradiationduring subsequent leads photoirrad to completeiation acrylate leads to conversion complete acrylate [42]. conversion [42].

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Scheme 5. Aza-Michael addition between a primary amine and an acrylate. The resulting secondary Scheme 5. Aza-Michael addition between a primary amin aminee and an acrylate. The resulting secondary amine may further undergo aza-Michael addition at a slower reaction rate. amine may further undergo aza-Michael additionaddition at a slower reactionreaction rate.rate. 2.3. Acetoacetate–Acrylate Reaction 2.3. Acetoacetate–Acrylate Reaction Acetoacetate–acrylate reaction [39] is a good example of Michael addition to acrylates. Acetoacetate–acrylate reaction reaction [39] [39] is a good example of Michael addition to to acrylates. Mechanistically, the reaction proceeds through an enolate anion, which is formed after the Mechanistically, the reaction proceeds through an enolate anion, which is formed after the deprotonation of the active methylene. The deprotonation is facilitated by the base catalyst (Scheme deprotonation ofof thethe activeactive methylene.methylene. The The deprotonation deprotonation is is facilitated facilitated by by the the base base catalyst catalyst (Scheme (Scheme6). 6). The enolate then adds to the acrylic double bond. The adduct is formed and the base is regenerated The6). The enolate enolate then then adds adds to to the the acrylic acrylic double double bond. bond The. The adduct adduct is is formed formed and and thethe basebase is regenerated once proton transfer occurs. The rate determining step of the reaction is the attack of the enolate to once proton transfer occurs. The rate determining step of the reaction is the attack of the enolate to the oncethe acrylate.proton transfer The reaction occurs. kinetics The ra teis determininghighly dependent step ofon the base reaction strength. is the The attack formed of theadduct enolate may to acrylate. The reaction kinetics is highly dependent on base strength. The formed adduct may undergo theundergo acrylate. a second The reaction Michael kinetics addition, is but highly at a slowerdependent rate. on base strength. The formed adduct may aundergo second a Michael second addition,Michael addition, but at a slower but at rate.a slower rate.

Scheme 6. Michael reaction mechanism. The Michael donor is deprotonated by the base catalyst. The formed anion attacks the acrylate double bond and regenerates the base. The adduct formed may Scheme 6. MichaelMichael reaction reaction mechanism. mechanism. The The Michael Michael donor donor is deprotonated is deprotonated by the by base the basecatalyst. catalyst. The further undergo a second but slower Michael addition. Theformed formed anion anion attacks attacks the theacrylate acrylate double double bond bond and and regenerates regenerates the the base. base. The The adduct adduct formed formed may further undergo a second but slower MichaelMichael addition.addition. 2.4. Radical Homopolymerization of Acrylates 2.4. Radical Homopolymerization of Acrylates 2.4. RadicalFor the Homopolymerization sake of completeness, of Acrylates we present the well-known radical mediated photopolymerization reactionFor the of sakeacrylates of completeness, in Scheme 7. we The present reaction the star well-knownts with the radical photolysis mediat of edthe photopolymerization initiator that yields For the sake of completeness, we present the well-known radical mediated photopolymerization reactiontwo equally of acrylates reactive in radicals. Scheme The 7. Thenext reaction step is the star initiationts with the of thephotolysis chain by of the the reaction initiator of that a radical yields reaction of acrylates in Scheme7. The reaction starts with the photolysis of the initiator that yields twowith equally an acrylate reactive monomer. radicals. The The chain next steppropagates is the initiation by reaction of thewith chain other by monomers. the reaction Termination of a radical two equally reactive radicals. The next step is the initiation of the chain by the reaction of a radical withoccurs an whenacrylate the monomer.propagating The polymer chain reactspropagates either bywith reaction another with growing other polymeric monomers. radical Termination or with with an acrylate monomer. The chain propagates by reaction with other monomers. Termination occursa primary when radical. the propagating polymer reacts either with another growing polymeric radical or with occurs when the propagating polymer reacts either with another growing polymeric radical or with a a primary radical. primary radical.

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SchemeScheme 7. 7. RadicalRadical mediated mediated photopolymerization photopolymerization of of acry acrylates.lates. I: I:Photoinitiator, Photoinitiator, R: R: Radical, Radical, M: M: Monomer,Monomer, P: P: Polymer Polymer chain. chain.

3.3. Acrylate-Based Acrylate-Based Dual-Curing Dual-Curing Systems Systems

3.1.3.1. Thiol–Acrylate Thiol–Acrylate and and Thiol–Epoxy Thiol–Epoxy Reactions Reactions AlthoughAlthough the the radical-mediated radical-mediated thiol–acrylate thiol–acrylate reaction reaction does does not not generally generally exhibit exhibit click click characteristics,characteristics, it it has has been been combined combined with with other other c clicklick reactions reactions to to obtain obtain materials materials with with significantly significantly lowlow soluble soluble content. content. Jian Jian et et al. al. [50] [50] obtained hybrid organic networks byby thiol–epoxy/thiol–acrylatethiol–epoxy/thiol–acrylate polymerizationpolymerization processes processes in in which which both both reactions reactions proceeded proceeded sequentially sequentially wi withoutthout clear clear separation. separation. TheThe curing curing was was triggered triggered by by irradiation irradiation with with UV UVlight light by the by use the of use a photobase of a photobase generator generator 1,5,7- triazabicyclo[4.4.0]dec-5-enyl1,5,7-triazabicyclo[4.4.0]dec-5-enyl tetraphenylborate tetraphenylborate (TBD·HBPh (TBD4·)HBPh capable4) capable of in situ of generating in situ generating a strong a basestrong and base free andradicals, free with radicals, isopropylthiolxanthone with isopropylthiolxanthone (ITX) as photosensitizer (ITX) as photosensitizer to extend the to wavelength extend the absorptionwavelength range absorption of the range catalyst. of the catalyst.The free The radicals free radicals facilitated facilitated both both thiol–acrylate thiol–acrylate radical radical polymerizationpolymerization as as well well as as acrylate acrylate homopolymeri homopolymerization,zation, leading leading to to materials materials with with unreacted unreacted thiol thiol groups.groups. However, asas the the photobase photobase generator generator also also liberated liberated a strong a strong base (TBD), base thiol–Michael(TBD), thiol–Michael addition additionalso took also place took and place compensated and compensated partly for partly the stoichiometric for the stoichiometric imbalance. imbalance. The researchers The researchers found that foundthiol–acrylate that thiol–acrylate reactions andreactions acrylate and homopolymerization acrylate homopolymerization were faster were and faster more and efficient more efficient than the thanthiol–epoxy the thiol–epoxy reactions reactions (>95% conversion (>95% conversion in a matter in a ofmatter seconds of seconds and minutes, and minutes, respectively), respectively), and the andthiol the conversion thiol conversion increased increased with an with increase an increase in the epoxy in the content. epoxy content. InIn a arecent recent paper paper [61], [61 the], the authors authors of this of thisreview review presented presented a novel a novelphotolatent photolatent dual-cure dual-cure thiol– acrylate–epoxythiol–acrylate–epoxy system. system. Under UV Under light, UV the light, photobase the photobase generator generator liberates a liberates strong base a strong that catalyses base that thecatalyses thiol–Michael the thiol–Michael reaction at room reaction temperature. at roomtemperature. To initiate the Tosecond initiate curing the reaction second between curing reaction epoxy andbetween remaining epoxy thiols, and remaining the system thiols, is heated the systemabove 80 is heated°C. The above use of 80a radical◦C. The inhibitor use of a ensured radical inhibitor that no radicalensured mediated that no acrylate radical mediatedhomopolymerization acrylate homopolymerization took place. The final took material place. properties The final suggested material potentialproperties in suggested application potential such as in applicationchemically suchresist asant chemically industrial resistant coatings, industrial adhesives, coatings, and electronic adhesives, materials.and electronic materials. ItIt is is also also reported reported in in the the literature literature the the preparation preparation of of hybrid hybrid polymer polymer networks networks (HPNs) (HPNs) from from one-potone-pot thiol–acrylate–epoxy thiol–acrylate–epoxy mixtures. mixtures. Jin Jin et et al. al. [63] [63 ]cured cured a a stoichiometric stoichiometric combination combination of of a a diacrylatediacrylate and and a a diepoxide diepoxide using using difunctional difunctional or or multifunctional multifunctional thiol thiol crosslinkers. crosslinkers. The The nucleophilic nucleophilic thiol–acrylatethiol–acrylate reactionreaction andand thiol–epoxy thiol–epoxy reaction reaction was was carried carried out simultaneouslyout simultaneously at 80 ◦ Cat in80 the °C presence in the presenceof 1,8-diazabicyclo[5,4,0]undec-7-ene of 1,8-diazabicyclo[5,4,0]undec-7-ene (DBU). Depending (DBU). Depending on the functionality on the functionality of the thiol, of the resulting thiol, resultingmaterials materials exhibited aexhibited wide range a ofwide properties range characterizedof properties by characterized differential scanning by differential calorimetry scanning (DSC), calorimetrydynamic mechanical (DSC), dynamic analysis mechanical (DMA) and analysis scanning (DMA) electron and microscopy scanning (SEM)electron methods. microscopy (SEM) methods.

Polymers 2018, 10, 178 9 of 24

3.2. Thiol–Acrylate and Thiol–Ene (or Thiol–Yne) Reactions Chatani et al. [64] used selective and sequential thiol–divinyl sulfone and thiol–Michael additions to create materials with independent network structures and two different glass transition temperatures showing multiple shape-memory effect. Both reactions were catalysed by nucleophilic phosphine catalysts. It was possible to separate the process into two reaction steps since the two thiol monomers used (a mercaptoacetoacetate and a mercaptopropionate) showed signiﬁcantly different reactivities. Chan et al. [65] showed the possibility of combining phosphine-catalysed Michael addition of thiols to acrylates followed by UV-induced radical thiol–yne addition in a controlled and sequential way. Thiol–Michael addition to acrylates and thiol–ene click reactions were later combined by Peng et al. [66] in a sequential way using a tertiary amine for the thiol–Michael addition and a radical photoinitiator for the thiol–ene reaction for the preparation of holograms. A similar approach was used for sequential thiol–Michael and thiol–yne systems [67]. Recently, a thiol–acrylate and thiol–ene dual-curing system was developed [68]. First stage of curing is a click thiol–Michael reaction catalysed by a novel set of tertiary amines comonomers with allyl functionality. These allyl moieties later undergo a radical thiol–ene reaction to yield fully crosslinked dual-networks.

3.3. Thiol–Acrylate and Acetoacetate–Acrylate Reactions Using acetoacetates to accompany thiols as Michael donors is a strategy to achieve sequentiality in dual-curing processes since the proton acidities of acetoacetate and thiol groups are disparate. Recently, our group [43] developed dual-curable thiol–acetoacetate–acrylate thermosets by using thiol–Michael addition to acrylates at room temperature followed by Michael addition of acetoacetates to acrylates at moderately elevated temperature. The different acidities of the protons on thiol and acetoacetate groups, and the favorable pKa of the base catalyst ensured two separate curing stages. Using a photobase generator, it was also possible to design latent curing systems out of the same mixtures. Similarly to other dual-curing systems, the ﬁnal material properties could be tailored by changing the structure of the monomers and the relative contribution of both Michael addition reactions, by only changing the monomer proportion.

3.4. Thiol–Acrylate and Thiol–Isocyanate Reactions Thiol–isocyanate coupling and thiol–Michael addition reactions were combined by Matsushima et al. [69] to synthesize materials with tunable mechanical properties. Thiolisocyanate co-catalysed by phosphine–acrylate was faster than the thiol–Michael addition at low-phosphine concentration. Therefore, the curing steps were easily separable. Although phosphines were not basic enough to initiate a thiol–isocyanate reaction, the phosphonium enolate generated via nucleophilic addition of the phosphine to acrylate double bond acts as a strong base that can deprotonate the thiol and produce the thiolate anion, which can then react with isocyanate groups. In the presence of excess acrylate groups, a ﬁnal photopolymerization step could be used to produce ternary networks.

3.5. Combination of Two Thiol–Michael Additions on Different Acceptors Dual curing systems can be designed based on a single reaction mechanism, but using monomers whose reaction kinetics have different temperature dependencies so that the two stages can be carried out at different curing temperatures. As an example, thiol–Michael addition was employed in a sequential curing process using two monomers with distinct electrophilic strengths: vinyl sulfones and acrylates [64]. The former reacted at room temperature, whereas the latter reacted at 90 ◦C. Acrylates are more reactive than methacrylates in Michael additions due to the inductive effect and steric hindrance of the methyl group in the methacrylate. Thiol–Michael reactions with acrylates and methacrylates have been be employed in dual curing formulations by choosing an adequate combination of catalysts: A conventional tertiary amine for the thiol–Michael addition to the acrylate Polymers 2018, 10, 178 10 of 24 in dark conditions and a photolatent base leading to the release of a stronger base for the thiol–Michael addition to the methacrylate [45]. These formulations were used in photo-patterning with spatial and temporal control.

3.6. Michael Addition Followed by Homopolymerization In order to obtain storable and processable polymers at the final of the first stage of curing and final networks with optimum properties for different applications, the preparation of dual materials from off-stoichiometric formulations has recently gained interest [59]. The monomer in excess should be able to undergo homopolymerization reaction, without any interference. The first stage is a self-limiting click reaction between two multifunctional monomers with an excess of one of the two monomers. The second stage reaction is a thermally or UV-induced polymerization of the excess of unreacted groups. The materials obtained after the first curing stage can be gelled or ungelled and loosely or tightly crosslinked at the end of the second curing stage, depending on the functionality and feed ratio of the monomers. The combination of step-growth (usually first stage) and chain-growth (usually second stage) polymerization reactions confers these systems a great versatility, and complete curing can be generally achieved at the end of the second curing step. Crosslinked polymer networks obtained during the first stage of curing via step-growth polymerization reactions have some remarkable features such as homogeneity, high capacity for mechanical energy absorption near its glass transition temperature, high gel point conversion and low polymerization-induced shrinkage stress compared to chain-growth polymers. In applications that require high Tg value and hardness at ambient temperature, the relatively low Tg values and crosslinking densities of the step-growth intermediate polymers can increase significantly after the second stage of curing via chain-growth homopolymerization. As Michael addition reactions are self-limiting, all Michael donor species will have reacted by the end of the first stage of a dual-curing scheme. The remaining Michael acceptor species, usually acrylates, can homopolymerize in the second curing stage. Moszner et al. [70,71] synthesized networks from multifunctional acetoacetates (as Michael donors) with multiacrylates (as Michael acceptors) using 1,5-diazabicyclo-[4.3.0]non-5-ene (DBN) as the base catalyst. Tailor made materials could be prepared in a two-step process: Michael addition followed by radiation curing of excess acrylate. An analysis of crosslinked polymers showed that the longer spacer between β-ketoester groups and the lower functionality and higher flexibility of the reaction components made it possible to reach higher degrees of conversion. The vinyl monomers in excess, which act as reactive solvent in the first curing stage, had a positive effect on the homogeneity, conversion and crosslinking density of the synthesized networks. During the second stage, the soft Michael network was converted into a hard crosslinked material with high Shore hardness. The prepared materials were interpenetrating polymer networks covalently bonded. To synthesize similar materials, Moy et al. [72] used Michael addition of acetoacetates to multi-acrylate followed by photopolymerization of acrylates that were in excess in the initial mixture. The Michael addition has been used to synthesize liquid oligomers that find use in UV-curable formulations [73]. This alleviates the need for toxic photoinitiators, which would be indispensable in conventional solvent-based formulations to ensure the complete cure of thick films [37]. Despite the promising results obtained by Moszner et al., few studies have deepened in the formation of networks based on acetoacetates and acrylates. Probably, the relatively high complexity of synthesized acetoacetates of different functionalities and structure along with the relatively lower reactivity of the second acidic proton in the methylene of the acetoacetate are the reasons for the scarce number of publications on this specific type of Michael addition [74]. Taking advantage of the behaviour associated with click reactions, Bowman’s group [41,75,76] has developed a novel two-stage reactive system using off-stoichiometric thiol–acrylate formulations, in a similar way to the one developed by Moszner et al. [70,71] with acetoacetate–acrylate formulations. In the first step, a polymer network is formed via base-catalysed thiol–Michael addition. In the Polymers 2018, 10, 178 11 of 24 Polymers 2018, 10, x FOR PEER REVIEW 11 of 25 addition.second step, In thethe excesssecond of step, acrylate the groupsexcess undergoof acrylate photoinitiated groups undergo free radical photoinitiated polymerization. free radical Since polymerization.acrylate groups reactSince in acrylate both stages groups of curing, react in this both ensures stages covalent of curing, bonding this ensures between covalent poly(thio-ether) bonding betweenand poly(acrylate)s poly(thio-ether) networks, and poly(acrylate)s rendering the netw finalorks, materials rendering suitable the forfinal certain materials applications suitable for in whichcertain extractableapplications materialin which isextractable not desirable. material The is not strategy desirable. of combiningThe strategy a of click combining reaction a withclick reactionhomopolymerization with homopolymerization yields two distinct yields and two independent distinct and sets independent of material sets properties of material at the properties end of both at thestages. end Inof general,both stages. after In the general, first stage after of the curing, first stage the materials of curing, can the be materials stored,shaped can be stored, and processed. shaped andLater, processed. their properties Later, their can beproperties dramatically can be increased dramatically through increased the second through curing the stage.second By curing controlling stage. Bythe monomercontrolling ratio the inmonomer the formulation ratio in and the theformulation functionality and and the rigidity functionality of the monomers, and rigidity materials of the withmonomers, a broad materials range of with applications, a broad suchrange as of shape applications, memory such polymers as shape and memory imprinting polymers and optical and imprintingmaterials can and be optical obtained, materials as will can be laterbe obtained, shown. as will be later shown. The replacement of thiols or acetoacetates by amines as Michael donors has been recently explored as a way to prepareprepare aa newnew familyfamily ofof poly(aminopoly(amino ester)-ester)-coco-poly(acrylate)-poly(acrylate) thermosets based on off-stoichiometric amine-acrylate formulations [42], [42], in a dual-curing solvent-free process at room temperature. The The first first stage stage of of curing is a self-limiting aza–Michael addition between amine and acrylate groups groups and and the the second second stage stage is isa radical a radical photopolymerization photopolymerization of the of acrylates the acrylates in excess. in excess. The Thedual-curing dual-curing procedure procedure is shown is shown in Scheme in Scheme 8. 8.

Scheme 8. Aza–Michael reaction followed by acrylate homopolymerizationhomopolymerization under UV light.light. Both reactions were carried out at room temperature. St Stageage 1 network contains unreacted acrylate groups that photopolymerize later in Stage 2.

Based onon thethe same same idea, idea, Retailleau Retailleau et al.et [40al.] [40] took took advantage advantage of the of different the different reactivities reactivities of primary of primaryand secondary and secondary amine groups amine to groups synthesize to synthesize polymeric polymeric networksvia networks three time-controlled via three time-controlled steps using stepsdouble using click double Michael click addition Michael and additi radicalon photopolymerization.and radical photopolymerization.

3.7. Michael Addition Followed by Methacrylate Homopolymerization Methacrylates are are less less reactive reactive in in Michael Michael addition additionss than than acrylates acrylates due due to tothe the inductive inductive effect effect of theofthe methyl methyl group group in the in methacrylate the methacrylate and to and the tosteric the hindrance steric hindrance caused causedby the methyl by the in methyl α position in α position[39,45]. Accordingly, [39,45]. Accordingly, in the presence in the of presence acrylates, of acrylates,methacrylate methacrylate groups remain groups practically remain practicallyunreacted afterunreacted Michael after addition Michael and addition participate and preferably participate in preferably the homopolymerization in the homopolymerization process. Recently, process. the authorsRecently, of the this authors review of [44] this have review presented [44] have an presentedefficient dual an efﬁcient curing scheme dual curing for a scheme set of mixtures for a set containingof mixtures different containing proportions different of proportions acrylates and of acrylatesmethacrylates andmethacrylates (Scheme 9). The (Scheme first curing9). The stage ﬁrst is acuring stoichiometric stage is a aza–Michael stoichiometric addition aza–Michael between addition acrylates between and an acrylates amine, followed and an amine, by photo-initiated followed by photo-initiatedradical homopolymerization radical homopolymerization of methacrylates of and methacrylates remaining acrylates. and remaining An analysis acrylates. of aza–Michael An analysis reaction kinetics confirmed that amines react selectively with acrylates, leaving methacrylates

Polymers 2018, 10, 178 12 of 24 of aza–Michael reaction kinetics confirmed that amines react selectively with acrylates, leaving methacrylatesPolymers 2018, unreacted 10, x FOR PEER after REVIEW the first curing stage. Depending on the weight percentage12 of 25 of the constituent polymeric networks, a wide range of final material properties can result. unreacted after the first curing stage. Depending on the weight percentage of the constituent Withpolymeric a similar networks, approach, a wide Klee range and of co-workersfinal material [77 properties] employed can aza–Michaelresult. addition of diamines to acryloxylethylWith a methacrylates similar approach, to synthesize Klee and co-workers crosslinkable [77] oligomersemployed aza–Michael for dental composites addition of thatdiamines exhibited low curingto acryloxylethyl shrinkage methacrylates upon photocuring. to synthesize crosslinkable oligomers for dental composites that Inexhibited a similar low manner,curing shrinkage dual-curing upon photocuring. acetoacetate-acrylate/methacrylate formulations have been preparedIn and a characterizedsimilar manner, by dual-curing our research acetoacetate-a team [78].crylate/methacrylate The first stage of curingformulations was a base-catalysedhave been Michaelprepared addition and betweencharacterized acetoacetate by our research and acrylate team [78]. groups The first and stage the of second curing stagewas a base-catalysed was methacrylate radicalMichael photopolymerization. addition between acetoacetate Both are ambientand acrylate temperature groups and reactions. the second Becausestage was methacrylates methacrylate are poorradical Michael photopolymerization. acceptors, they did Both not are react ambient with temperature the acetoacetate, reactions. thereby Because ensuring methacrylates staged are curing. poor Michael acceptors, they did not react with the acetoacetate, thereby ensuring staged curing. The The materials gelled after the Michael addition and the final properties could be tuned by changing materials gelled after the Michael addition and the final properties could be tuned by changing the the relativerelative contribution contribution of the Michael Michael and and radical radical homopolymerization homopolymerization reactions. reactions. The Thelatency latency of the of the formulationsformulations at the at the beginning beginning of of the the second second stage stage cancan be regulated regulated as as desired desired by byUV UV irradiation. irradiation.

Scheme 9. A room temperature dual-curing process for an amine–acrylate–methacrylate. Stage 1 Scheme 9. A room temperature dual-curing process for an amine–acrylate–methacrylate. Stage 1 (acrylate–amine) network contains unreacted acrylate and methacrylate groups. UV curing at the final (acrylate–amine) network contains unreacted acrylate and methacrylate groups. UV curing at the ﬁnal stage ensures all these unreacted acrylate or methacrylate groups to photopolymerize. stage ensures all these unreacted acrylate or methacrylate groups to photopolymerize.

Polymers 2018, 10, 178 13 of 24 Polymers 2018, 10, x FOR PEER REVIEW 13 of 25

3.8. Isocyanate–Thiol Followed by Acrylate Homopolymerization There areare otherother choices choices for for ﬁrst first stage stage reactions reaction followeds followed by curing by curing of homopolymerizable of homopolymerizable groups. Podggroups.órski Podgórski et al. [79 et] employedal. [79] employed a base-catalysed a base-catalysed thiol–isocyanate thiol–isocyanate click coupling click coupling with free with radical free methacrylateradical methacrylate homopolymerization homopolymerization in a two-stage in a two-stage process process for the fabricationfor the fabrication of well-deﬁned of well-defined surface patternssurface patterns as well as well functional as functional geometric geometric shapes. shapes. The second The second stage materialsstage materials were obtainedwere obtained upon UVupon irradiation UV irradiation and using and using a photoinitiator, a photoinitiator, and a and photoabsorber. a photoabsorber. They They used used a monomer a monomer with bothwith isocyanateboth isocyanate and methacrylate and methacrylate functionality functionality that was that capable was ofcapable participating of participating in the thiol–isocyanate in the thiol– reactionisocyanate (Scheme reaction 10 (Scheme) and was 10) able and to was further able crosslink to further at crosslink the second at curingthe second stage. curing stage.

Scheme 10. Thiol–isocyanateThiol–isocyanate click click reaction reaction using using a a monomer monomer that that wields both isocyanate and methacrylate functionalities.functionalities. The The resulting resulting ﬁrst-stage first-stage network network can can be furtherbe furthe crosslinkedr crosslinked at a laterat a stagelater stageusing using photo-induced photo-induced methacrylate methacrylate homopolymerization. homopolymerization. Scheme Scheme adapted adapted from [from79]. [79].

3.9. Diels–Alder Reaction Followed by Acrylate Homopolymerization The Diels–Alder Diels–Alder reaction, reaction, which which also alsohas a hasclick anature, click was nature, recently was combined recently with combined UV-induced with UV-inducedacrylate homopolymerization acrylate homopolymerization by Bowman and by Bowmanco-workers and [80]. co-workers From mixtures [80]. Fromof bismaleimides, mixtures of bismaleimides,acrylates and monomers acrylates andwith monomers furane moieties, with furane new thermosets moieties, new by dual thermosets curing bywere dual prepared. curing were The prepared.contribution The of contributionthe Diels–Alder of the reaction Diels–Alder to the reactionnetwork to structure the network depended structure on dependedthe temperature- on the temperature-dependentdependent equilibrium, equilibrium,because of the because reversible of the character reversible of characterthis reaction. of this Unreacted reaction. maleimide Unreacted maleimidemoieties could moieties become could incorporated become incorporated into the acry intolate the acrylatenetworknetwork while the while resulting the resulting material material could couldmaintain maintain its thermoreversible its thermoreversible behaviour behaviour thanks thanks to the tounderlying the underlying Diels–Alder Diels–Alder network. network.

3.10. Combination of Acrylate Homo Homopolymerizationpolymerization and Other Reactions Nowers and co-workers [[11]11] have investigated IPNs synthesized from a combination of radical mediated acrylate homopolymerizationhomopolymerization and cationic epoxy homopolymerization. They explored the impact of composition and reaction sequence on on conversion, conversion, rheological rheological and and mechanical mechanical properties. properties. The structure-property relationships they discovereddiscovered were nonlinear and complex. They found out that maximum maximum conversions conversions could could be be achieved achieved at inte at intermediatermediate compositions compositions due to due the todilution the dilution effect. effect.Elastic Elasticmodulus modulus increased increased with withincreasing increasing epoxy epoxy content, content, whereas whereas yield yield strain strain increased increased with increasing acrylate content. Interestingly, Interestingly, they foundfound out that the reaction sequence had a significant significant effect on finalfinal glassglass transitiontransition temperatures:temperatures: Higher Higher glass glass transition transition temperatures temperatures (T (gT)g were) were obtained obtained if epoxiesif epoxies were were homopolymerized homopolymerized before before acrylates. acrylates. Cationic epoxy polymerization has been combinedcombined with radical mediatedmediated (meth)acrylate(meth)acrylate homopolymerization also also by by Li et al. [81] [81] to yieldyield IPNs for 3D printing applications. In In their work, they used mixtures of difunctional (meth)acrylates and epoxides. Firstly, Firstly, they irradiated the mixtures with UV light until full (meth)acrylate conversion andand cationic initiation. Later, they investigated the effect of the dark curing time of the epoxy on the finalfinal properties. Interestingly, they found that the dark curingcuring time time had had a significanta significant impact impact on the on IPNs the formed.IPNs formed. Furthermore, Furthermore, the extent the of extent this variation of this wasvariation comparable was comparable to that of changingto that of monomerchanging monomer compositions. compositions. In another interesting paper, LarsenLarsen andand co-workersco-workers [[82]82] studystudy anan acrylate/epoxyacrylate/epoxy dual system that cancan yieldyield either either a softa soft (acrylate (acrylate based) based) or a hardor a (epoxyhard (epoxy based) based) material material depending depending on the emission on the rangeemission of therange light of used the tolight irradiate used theto monomers.irradiate the The monomers. compression The modulus compression of end modulus materials of can end be tailoredmaterials between can be <100tailored kPa between and >20 MPa.<100 ThekPa curingand >20 process MPa. isThe summarized curing process in Scheme is summarized 11. in Scheme 11.

Polymers 2018, 10, 178 14 of 24 Polymers 2018, 10, x FOR PEER REVIEW 14 of 25

SchemeScheme 11.11.(A ) Reaction(A) Reaction schemes schemes for photoinitiation for photoinitiation of radical mediated of radical acrylate homopolymerizationmediated acrylate andhomopolymerization cationic epoxy homopolymerization. and cationic epoxy ( Bhomopolymerization.) Normalized emission (B spectra) Normalized of the light emission sources spectra used for of initiatingthe light sources the reactions. used for Reprinted initiating (adapted) the reactions. with permission Reprinted from(adapted) [82]. Copyrightwith permission 2016 John from Wiley [82]. andCopyright Sons. 2016 John Wiley and Sons.

AnionicAnionic epoxyepoxy curingcuring waswas also combined withwith acrylateacrylate homopolymerization toto designdesign adhesiveadhesive formulationsformulations [[83].83]. Both Both UV UV and thermal curing was employed and the resultingresulting materialsmaterials werewere characterizedcharacterized withwith respectrespect toto adhesiveadhesive strengthstrength andand pendulumpendulum hardness.hardness. Both properties werewere foundfound toto increaseincrease withwith increasingincreasing epoxyepoxy content.content. PolyurethanePolyurethane chemistry also found found use use in in dual dual networks networks based based on on acrylates. acrylates. Bertoldo Bertoldo and and coco-workersworkers [84] [84 reported] reported the the thermally thermally induced induced polyuret polyurethanehane formation formation via via the the reaction reaction of of propoxylated propoxylated aromaticaromatic epoxyepoxy diacrylateddiacrylated oligomeroligomer andand allophanateallophanate modifiedmodified polyisocyanurate,polyisocyanurate, followedfollowed by photoinitiatedphotoinitiated acrylate acrylate homopolymerization homopolymerization to to produce produce nanocomposite nanocomposite laminates. laminates. They They found found out thatout finalthat final acrylate acrylate conversion conversion was an was increasing an increasing function function of thermal of thermal pre-treatment pre-treatment temperature. temperature.

4.4. The Comonomer ApproachApproach InIn somesome dual-curing dual-curing formulations, formulations, certain certain click reactionsclick reactions are used are to produceused to in situproduce comonomers in situ thatcomonomers later catalyse that thelater curing catalyse reaction. the curing Starting reaction. from Starting an initial from mixture an initial of amines, mixture acrylates of amines, and thiols,acrylates Bounds and thiols, et al. Bounds [85] prepared et al. [85] microﬂuidic prepared microfluidic devices using devices a cost-effective using a cost-effective and eco-friendly and eco- procedure.friendly procedure. Firstly, the Firstly, comonomer the comonomer is synthesized is synthesized in situ via aza–Michael in situ via aza–Michael addition of a diethylamineaddition of a todiethylamine pentaerithritol to pentaerithritol triacrylate. The triacrylate. formed The tertiary formed amines tertiary in turnamines catalyse in turn thiol–Michael catalyse thiol–Michael addition ofaddition trimethylolpropane of trimethylolpropane tris(3-mercaptopropionate) tris(3-mercaptopropionate) to the remaining to the remaining acrylate groups. acrylate The groups. resulting The materialsresulting exhibitedmaterials exhibited diverse and diverse tunable and properties. tunable properties. InIn aa recentrecent studystudy byby thethe authorsauthors ofof thisthis reviewreview [[68],68], aa similarsimilar comonomercomonomer approachapproach waswas employedemployed toto synthesizesynthesize softsoft poly(thioether)poly(thioether) thermosets using a two-steptwo-step clickclick curingcuring procedureprocedure atat room temperature (Scheme 12). The first stage of curing is a thiol–Michael addition between acrylates (or methacrylates) and thiols in excess, catalyzed by a set of pre-synthesized tertiary amine catalyst

Polymers 2018, 10, 178 15 of 24 room temperature (Scheme 12). The ﬁrst stage of curing is a thiol–Michael addition between acrylates (or methacrylates)Polymers 2018, 10, x andFOR PEER thiols REVIEW in excess, catalyzed by a set of pre-synthesized tertiary amine15 of catalyst 25 comonomers with allyl functionality. Subsequently, the pendant allyl groups of the comonomer get incorporatedcomonomers into with the polymerallyl functionality. network Subsequently, by undergoing the thiol–enependant allyl UV groups photopolymerization of the comonomer withget the incorporated into the polymer network by undergoing thiol–ene UV photopolymerization with the remaining thiols. The strong catalysis by the high concentration of tertiary amine groups facilitated remaining thiols. The strong catalysis by the high concentration of tertiary amine groups facilitated quantitative conversions at the end of the ﬁrst stage. All remaining functional groups are depleted quantitative conversions at the end of the first stage. All remaining functional groups are depleted afterafter a short a short period period of UVof UV irradiation. irradiation. FinalFinal materialsmaterials were were clear, clear, transparent, transparent, and and they they exhibited exhibited homogeneoushomogeneous network network structures structures with with tunable tunable viscoelasticviscoelastic properties. properties.

Scheme 12. Thiol–Michael reaction catalysed by comonomer with allyl functionality. At the second Scheme 12. Thiol–Michael reaction catalysed by comonomer with allyl functionality. At the second curing stage, all allyl moieties get incorporated into the final network via thiol–ene radical reaction. curing stage, all allyl moieties get incorporated into the ﬁnal network via thiol–ene radical reaction. 5. Polymer Modification/Functionalization 5. Polymer Modiﬁcation/Functionalization Combination of controlled/living polymerization methods and thiol–Michael reactions has also Combinationbeen employed of to controlled/livingsynthesize highly advanced polymerization polymer methods architectures and such thiol–Michael as branched reactions or star has also beenmacromolecules employed [59]. to synthesize As the thiol–Michael highly advanced reaction exhibits polymer click architectures characteristics, such it has as been branched combined or star macromoleculeswith techniques [59]. such As theas reversib thiol–Michaelle addition-fragmentation reaction exhibits clickchain characteristics, transfer (RAFT) it polymerization, has been combined withcobalt-catalysed techniques such chain as reversibletransfer (CCT), addition-fragmentation atom transfer radical chain(ATRP), transfer or ring-opening (RAFT) polymerization, metathesis cobalt-catalysed(ROMP) polymerizations. chain transfer Usually, (CCT), the thiol–Michael atom transfer reaction radical is used (ATRP), as a post-polymerization or ring-opening metathesis stage to modify/functionalize the polymer. Readers are directed to [59] for a review of related systems. (ROMP) polymerizations. Usually, the thiol–Michael reaction is used as a post-polymerization stage to modify/functionalize6. Organic–Inorganic the Hybrid polymer. Dual-Curing Readers areSystems directed to [59] for a review of related systems.

6. Organic–InorganicDual-cured processes Hybrid Dual-Curinghave been proposed Systems for the synthesis of organic–inorganic hybrid materials [86]. In that paper, the author presents a thorough review of dual-cure hybrid materials Dual-cureddeveloped for processesinteresting applications have been such proposed cultural forheritage the protection, synthesis hydrophobic of organic–inorganic or oil-repellent hybrid materialscoatings [86 for]. Intextiles, that paper,dental composites, the author and presents light-emitting a thorough coatings. review of dual-cure hybrid materials developedThe for usual interesting method applications to prepare such organic–inorga cultural heritagenic hybrid protection, systems hydrophobic is a photoinitiated or oil-repellent coatingspolymerization for textiles, followed dental composites,by a thermal treatment and light-emitting for promoting coatings. sol-gel reactions of suitable alkoxyde Theprecursors usual methodalready embedded to prepare in organic–inorganic the UV curable sy hybridstem. In systems doing isso, a it photoinitiated is possible to polymerizationexploit the followedadvantages by a thermalof this fast treatment and solvent-free for promoting radiation-induced sol-gel reactionspolymerization, of suitable with the alkoxyde sol-gel benefits, precursors leading to the formation of hybrid networks, generally in the form of thin coatings with superior already embedded in the UV curable system. In doing so, it is possible to exploit the advantages scratch endurance, improved abrasion, heat and radiation resistance, and desirable mechanical and of this fast and solvent-free radiation-induced polymerization, with the sol-gel beneﬁts, leading

Polymers 2018, 10, 178x FOR PEER REVIEW 16 of 25 24 optical properties [87–89]. Vinylether, epoxide, or acrylate groups have been used as organic monomers,to the formation since ofthey hybrid can undergo networks, fast generally photopolym in theerizations form of [90–92] thin coatings but even with UV-mediated superior scratch click reactionsendurance, has improved also been abrasion,applied [93,94]. heat and Organic radiation and resistance,inorganic precursors and desirable should mechanical be compatible and optical in the reactiveproperties mixture. [87–89 ].The Vinylether, characteristics epoxide, of orthe acrylate final material groups depend have been on usedthe formulation as organic monomers,(type and proportionssince they can of undergo organic fastmonomers, photopolymerizations initiator, inor [ganic90–92 ]precursors, but even UV-mediated amount of clickwater reactions for sol-gel has process,also been and applied coupling [93, 94agents)]. Organic and on and pH inorganic (which af precursorsfects the microstructure should be compatible of the inorganic in the reactivephase), andmixture. time Theof irradiation characteristics and schedule of the ﬁnal of materialthermal treatment. depend on the formulation (type and proportions of organic monomers, initiator, inorganic precursors, amount of water for sol-gel process, and coupling 7.agents) Applications and on pH (which affects the microstructure of the inorganic phase), and time of irradiation and schedule of thermal treatment. Acrylate dual-curing systems are advantageous as they enable two distinct and largely independent7. Applications sets of material properties at each curing stages. It is possible to have a stable polymer network with desirable physical properties while maintaining the capacity to react and achieve, once Acrylate dual-curing systems are advantageous as they enable two distinct and largely initiated with an appropriate stimulus, a second and final network with enhanced material independent sets of material properties at each curing stages. It is possible to have a stable polymer properties. network with desirable physical properties while maintaining the capacity to react and achieve, once Various applications are described in the literature where acrylate dual-curing reactions are initiated with an appropriate stimulus, a second and ﬁnal network with enhanced material properties. elegantly employed. The most prominent of these applications are shape memory polymers, optical Various applications are described in the literature where acrylate dual-curing reactions are elegantly materials, photolithography, protective coatings, structured surface topologies, and holographic employed. The most prominent of these applications are shape memory polymers, optical materials, materials. photolithography, protective coatings, structured surface topologies, and holographic materials.

7.1. Shape-Memory Systems Apart fromfrom thethe epoxy epoxy based based polymers polymers in literaturein literature for shape-memoryfor shape-memory systems systems [95–97 [95–97],], dual-curing dual- curingsystems systems are being are developedbeing developed over theover years the years using using acrylate acrylate based based monomers. monomers. The The group group led led by byBowman Bowman [64 ][64] developed developed a well-deﬁned a well-defined Triple Triple Shape MemoryShape Memory Polymer Polymer (TSMP) with(TSMP) well-separated with well- separatedglass transitions glass transitions and strong and interfaces strong interfaces where the where thiol− theMichael thiol−Michael addition addition reaction reaction was used was to formused tostep form growth step networksgrowth networks based on based selective on additionselective toaddition divinyl to sulfones divinyl and sulfones acrylates. and Theacrylates. developed The developedTSMP was TSMP proved was to beproved stable to in be its stable intermediate in its intermediate temporary temporary shape for anshape extended for an timeextended thanks time to thanksthese two to these distinct two networks, distinct networks, each holding each separate holding temporary separate temporary shapes (Figure shapes1). (Figure 1).

Figure 1. TripleTriple shape memory effect of a dual-curingdual-curing material based on selective thiol–Michael addition to divinyl sulfones and acrylates with two well-separatedwell-separated glass transitions.transitions. Adapted with permission from [[64].64]. Copyright: 2014, American Chemical Society.

For shape memory devices in biomedical applications, such as stents used in minimally invasive treatment of aneurysms or in suture anchors, a high modulus is is required required of prepared materials to be able to function effectively [[98].98]. Also, a switching temperature is required to be near physiologicalphysiological temperature. Nair and co-workers [41] developed a dual-cure shape-memory polymer system based

Polymers 2018, 10, 178 17 of 24 Polymers 2018, 10, x FOR PEER REVIEW 17 of 25

on off-stoichiometric thiol–acrylate formulations with a set of intermediate thermal–mechanical temperature. Nair and co-workers [41] developed a dual-cure shape-memory polymer system based on properties (Tg = 33 °C, 20 MPa in modulus and high strain) suitable for deployment of a biomedical off-stoichiometric thiol–acrylate formulations with a set of intermediate thermal–mechanical properties device, while◦ the activation of the second stage of the curing process, after deployment, led to a final (Tg = 33 C, 20 MPa in modulus and high strain) suitable for deployment of a biomedical device, while set of properties (high modulus, 1500 MPa at 38 °C) appropriate for the long-term success and the activation of the second stage of the curing process, after deployment, led to a ﬁnal set of properties function of the device. (high modulus, 1500 MPa at 38 ◦C) appropriate for the long-term success and function of the device.

7.2. Optical Materials Acrylate dual-curing systems have also been evaluated as low refractive index structures. Bowman and co-workers [[69]69] synthesized thiol–isocyanate–acrylate ternary networks formed by the combination of thiol–isocyanate coupling, thiol–Michaelthiol–Michael addition, and acrylate homopolymerization withwith excellentexcellent opticaloptical propertiesproperties (refractive(refractive indexindex aboutabout 1.55),1.55), enablingenabling extensive use as materials for optical lenses. lenses. The The same same research research group group obtained obtained similar similar remarkable remarkable refractive refractive index index [57] with [57] thiol– with thiol–isocyanate–eneisocyanate–ene ternary ternary networks networks prepared prepared by sequential by sequential and simultaneo and simultaneousus thiol–ene thiol–ene and thiol– and thiol–isocyanateisocyanate click reactions. click reactions.

7.3. Lithography and Photopatterning Nanoscale lithographiclithographic patterns patterns can can be be manufactured manufactured using using dual-curing dual-curing acrylates. acrylates. Nair Nair et al. et [ 41al.] used[41] used off-stoichiometric off-stoichiometric thiol–acrylate thiol–acrylate systems systems to obtain to obtain soft intermediatesoft intermediate materials materials that werethat usedwere toused take to the take imprint the imprint of a micron-size of a micron-size pattern pattern mould mould and then and irradiated then irradiated to obtain to obtain a rigid a and rigid highly and crosslinkedhighly crosslinked material material producing producing excellent negativesexcellent ofnegatives the master of pattern.the master Whereas pattern. the Whereas intermediate the ◦ materialintermediate had amaterialTg of − had10 Ca T andg of a− rubbery10 °C and modulus a rubbery (E rmodulus) of 0.5 MPa, (Er) theof 0.5 ﬁnal MPa, material the final had material a Tg of ◦ 195had aC T andg of an195E °Cr of and 200 an Mpa. Er of 200 Mpa. Using an elegant procedure, Bowman and co-workersco-workers [[45]45] successfully implemented different photocagedphotocaged catalysts in both photopatterning and kinetically controlled two-stage polymer network formationformation based on on thiol–Michael thiol–Michael addition addition to to ac acrylatesrylates and and methacrylates. methacrylates. They They demonstrated demonstrated the thecapability capability of ofphoto photo thiol–Michael thiol–Michael addition addition for for spatial spatial and and temporal control.control. Layer-by-layer photopatterningphotopatterning toto produceproduce complexcomplex 3D3D structures waswas alsoalso demonstrateddemonstrated in dual-curable systems combining thermo-reversiblethermo-reversible Diels–Alder Diels–Alder reaction reaction with with non-reversible non-reversible acrylate acrylate homopolymerization homopolymerization [80]. The[80]. unexposedThe unexposed material material could could easily easily be be removed removed by by heating heating to to produce produce depolymerization depolymerization ofof the Diels–Alder networknetwork followed followed by by washing washing with with a suitable a suitable solvent. solvent. The whole The photolithographywhole photolithography process isprocess depicted is depicted in Figure in2. Figure 2.

Figure 2.2. The liquid mixture of monomers is cast as a thin filmfilm and heated up toto 7070 ◦°C,C, at whichwhich temperaturetemperature thethe Diels–AlderDiels–Alder (DA)(DA) networknetwork isis formed,formed, leaving unreacted monomers.monomers. As the monomer conversion exceeds the Flory–StockmayerFlory–Stockmayer threshold for gel point, the filmfilm solidifiessolidifies (Stage(Stage 1).1). UsingUsing a photomask,photomask, the filmfilm isis UVUV irradiatedirradiated toto formform thethe polyacrylatepolyacrylate networknetwork (Stage 2). When the filmfilm is ◦ heated toto 120120 °CC the the thermoreversible thermoreversible DA DA network network depolymerizes. depolymerizes. The The resulting resulting liquid liquid monomers monomers can becan washed be washed away easily,away easily, leaving leaving intact the intact polyacrylate the polyacrylate network (Stagenetwork 3). Reprinted(Stage 3). withReprinted permission with frompermission [80]. Copyright: from [80]. 2014,Copyright: American 2014, Chemical American Society. Chemical Society.

7.4. Creating Surface Topology Wrinkled polymericpolymeric surfacessurfaces havinghaving structuredstructured topologiestopologies withwith large rangerange ofof length-scales are efﬁcientlyefficiently usedused asas coatingcoating defencedefence againstagainst barnacle fouling [[99]99] oror improvingimproving adhesiveadhesive propertiesproperties ofof materials [100]. To control wrinkle formation and their orientation, Kloxin and co-workers [54] used

Polymers 2018, 10, 178 18 of 24

Polymers 2018, 10, x FOR PEER REVIEW 18 of 25 materials [100]. To control wrinkle formation and their orientation, Kloxin and co-workers [54] used a facile two-stage polymerization strategy based on off-stoichiometric thiol–acrylate formulations. a facile two-stage polymerization strategy based on off-stoichiometric thiol–acrylate formulations. Acrylate-rich elastomers that contain photoinitiator and photoabsorber were mechanically stretched Acrylate-rich elastomers that contain photoinitiator and photoabsorber were mechanically stretched and irradiated with UV light. Since the photoabsorber confines the light-induced curing reaction and irradiated with UV light. Since the photoabsorber conﬁnes the light-induced curing reaction within within a thin skin layer at the surface, upon releasing the strain, the resulting cross-link density a thin skin layer at the surface, upon releasing the strain, the resulting cross-link density gradient (i.e., gradient (i.e., a modulus gradient) causes wrinkle formation (see Figure 3). a modulus gradient) causes wrinkle formation (see Figure3).

Figure 3. Wrinkle formation via photopatterning on a biaxially stretched specimen. (a) Photolithography Figure 3. Wrinkle formation via photopatterning on a biaxially stretched specimen. (a) guides the alignment of the wrinkles perpendicular to the low stress regions of the thiol−ene elastomer Photolithography guides the alignment of the wrinkles perpendicular to the low stress regions of the (inset: corresponding photomasks). (b) Photopolymerized sample showing alternating wrinkle thiol−ene elastomer (inset: corresponding photomasks). (b) Photopolymerized sample showing wavelengths, created by stepwise dosing (c) Employing two different photoinitiators allows for wrinkle alternating wrinkle wavelengths, created by stepwise dosing (c) Employing two different formation and shape memory. Wrinkles were formed using a procedure similar to (a). Reprinted photoinitiators allows for wrinkle formation and shape memory. Wrinkles were formed using a (adapted) with permission from [54]. Copyright: 2013, American Chemical Society. procedure similar to (a). Reprinted (adapted) with permission from [54]. Copyright: 2013, American Chemical Society. To obtain well-defined surface patterns and functional geometric shapes, Bowman’s research groupTo [79 obtain] synthesized well-defined thiol–isocyanate–methacrylate surface patterns and functional ternary geometric networks usingshapes, a dual-curingBowman’s research process. ◦ Atgroup stage [79] 1, synthesized uniform and thiol–isoc tough thiourethaneyanate–methacrylate networks ternary with a networTg = 40–70ks usingC werea dual-curing obtained process. with a fixedAt stage content 1, uniform of fully and tethered tough thiourethane methacrylate networks functionalities with a T thatg = 40–70 served °C as were reactive obtained sites with forstage a fixed 2 methacrylatecontent of fully UV-induced tethered homopolymerization.methacrylate functional Theities use that of suitable served photomasks as reactive and sites gradient for stage stage 2 2methacrylate curing and propertiesUV-induced across homopolymerization. the film thickness The made use it of possible suitable to photomasks mechanically and program gradient specific stage 3D2 curing shapes and or pre-definedproperties across surface the topographies film thickness that made could it bepossible erased to with mechanically temperature program or fixed specific by UV flood3D shapes curing. or pre-defined surface topographies that could be erased with temperature or fixed by UV flood curing. 7.5. Holography 7.5. HolographyHolography has evolved rapidly over the years in a number of applications such as art items, data storage,Holography optical has elements, evolved artificialrapidly over intelligence, the years optical in a number interconnects, of applications commerce, such medical as art practice, items, data storage, optical elements, artificial intelligence, optical interconnects, commerce, medical practice, night vision goggles and games. However, lack of suitable recording medium is still a

Polymers 2018, 10, 178 19 of 24 night vision goggles and games. However, lack of suitable recording medium is still a bottleneck in this technology. Compared to conventional holographic materials such as dichromated gelatine and silver halide emulsions, photopolymers perform better in recording and reading holograms in real time. Furthermore, these materials possess characteristics such as good light sensitivity, real-time image development, large dynamic range, good optical properties, format flexibility, and low cost. Earlier polymeric systems for holography suffered from low sensitivity due to low curing rates [101] but recently, superior systems are being developed. In the field of non-rewritable holograms, Peng et al. [66] produced rainbow holograms via a sequential orthogonal thiol–click dual curing process. Through an initial thiol–Michael click reaction an intermediate substrate was formed with thiol–allyl photopolymerizable species remaining. Controlling the molar ratio of thiol, acrylate and allyl monomers, intermediate materials with low Tg and modulus could be prepared to optimize diffusion and reaction rates as required for holographic recording (writing step). After holographic patterning within the crosslinked network was complete, the polymer network was UV irradiated so that the excess thiol and allyl functional groups react, resulting in a material with high modulus, high Tg, and with holographic fringes captured within the substrate. Compared to chain-growth polymerizations, the thiol–ene step-growth reaction exhibits 48% lower volumetric shrinkage, much higher gel point conversions, and better oxygen tolerance. Holograms with high fidelity, good optical clarity and high light sensitivity can be produced without the need for solvent processing and/or protective layers to prevent oxygen inhibition. The same group [67] implemented direct image patterning through the radical-mediated thiol–yne click reaction on self-supporting substrates that were formulated through the base-catalysed thiol–Michael addition followed by a photoinitiated thiol–yne. The holographic patterns attained desirable properties such as high diffraction efficiency, high refractive index modulation, high dynamic range, and low volume shrinkage.

8. Conclusions and Future Directions Acrylates will continue to be employed globally in polymer formulations in different ways. A vast selection of acrylate monomers and oligomers with different backbones and functionalities are commercially available. Their click behaviour has been exploited widely in Michael-type reactions with key coreactants such as thiols or amines. A major challenge of such reactions in dual-curable formulations is the runaway reaction kinetics of Michael reactions. Several researchers have investigated latent catalytic systems in an attempt to address this issue. Consequently, the design of environmentally benign catalytic systems with well-defined curing kinetics stands out as a promising research avenue. Such controlled kinetics will also help in obtaining stable intermediate materials that would cure fully at a later stage, when triggered. From a physical standpoint, the intermediate materials should provide ease in handling, manipulability and latent reactivity for further curing. This is an interesting research topic that could benefit from multi-disciplinary efforts by organic chemists, reaction engineers, material scientists, and the medical community as these materials also find use in medical settings (such as stents with shape-memory capability). The design of reactive catalysts (the comonomer approach) that impact both the reaction kinetics and rheological/mechanical properties of the final materials (since they get incorporated in the final polymer network) is another research area. Last but not the least, more and more research is being directed towards green processes for thermoset preparation. Recently, Acik et al. [102] synthesized acrylates using natural sugar-based monomers. Furthermore, the enzymatic synthesis of photoinitiators have been reported [103]. This avenue of research is particularly important as it could reduce the dependence on non-renewable resources. It is of interest to identify bio-based monomers that can be directly polymerized without need for rigorous purification. For this end, plant oils have been examined thoroughly by Llevot [104] as a bio-based feedstock to synthesize polymers such as polyepoxides, polyurethanes, and unsaturated polyesters. It would be beneficial to investigate the viability of similar approaches to synthesize bio-based polyacrylates. Polymers 2018, 10, 178 20 of 24

Acknowledgments: The authors would like to thank MINECO (MAT2017-82849-C2-1-R and MAT2017- 82849-C2-2-R) and Generalitat de Catalunya (2014-SGR-67 and Serra Húnter programme) for the financial support. Author Contributions: Osman Konuray, Xavier Fernández-Francos, Xavier Ramis and Àngels Serra contributed in the compiling of the content and Osman Konuray wrote the paper. Conflicts of Interest: The authors declare no conflict of interest.

References

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