State of the Art in Dual-Curing Acrylate Systems

24 Pages·2017·3.19 MB·English

polymers
Review
State of the Art in Dual-Curing Acrylate Systems
OsmanKonuray1,*,XavierFernández-Francos1 ID,XavierRamis1 ID andÀngelsSerra2 ID
1 ThermodynamicsLaboratory,ETSEIB,UniversitatPolitècnicadeCatalunya,Av.Diagonal647,
08028Barcelona,Spain;[email protected](X.F.-F.);[email protected](X.R.)
2 DepartmentofAnalyticalandOrganicChemistry,UniversitatRoviraiVirgili,C/Marcel·líDomingos/n,
43007Tarragona,Spain;[email protected]
* Correspondence:[email protected]
Received:22January2018;Accepted:7February2018;Published:12February2018
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
processesyieldtwodistinctandtemporallystablesetsofmaterialpropertiesateachcuringstage,
therebyallowingprocessflexibility. Thereviewbeginswithanintroductiontoacrylate-basedclick
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, opticalmaterials, photolithography, protectivecoatings, structuredsurfacetopologies,
andholographicmaterials.
Keywords: acrylates;methacrylates;dual-curing;clickchemistry
1. Introduction
A dual-curing process is defined as a combination of two curing reactions taking place
simultaneouslyorsequentially[1]. Itisamethodtocombinetwootherwisedistinctpolymernetworks.
Theresultinginterpenetratingpolymernetwork(IPN)exhibitssuperiorpropertiescomparedtoits
individual parts. If a functional group participates in both curing reactions, grafted IPNs can be
obtained. Sequentialdual-curingfacilitateseaseinprocessingandhandlingaslongastheintermediate
materials(i.e.,afterthefirstcuringreaction)arechemicallystableandtheirpropertiestailorable. Ifthe
dual-curingprocessmerelyaimsforimprovedfinalproperties,controlofthereactionsequencemaybe
irrelevant. Sequentialdual-curingmethodologyisdevelopedfrommulti-stage(orB-stage)processing
techniques. Conventionally,multi-stageprocessingreferstoaone-packadhesiveformulation,usually
ofepoxideorigin,thatcanbepartiallypre-curedafterapplicationtoasubstrate. Theadhesiveapplied
substratecanbetransportedand/orfurtherprocessed(suchasinafinalassemblystage),andthefinal
cureofadhesivecouldbeinitiatedwheneverdesiredbyusingheatorultraviolet(UV)light,depending
onformulationchemistry. Thesametechnologyfindsuseinprocessingofcomplexcompositepartsas
well[2,3]. Incontrast,two-potsystemsdonotfeaturesuchflexibilityinprocessingasthefinalcuring
reaction starts once the initial components are mixed. Multi-stage processing is well known from
theearlytimesofcrosslinkedpolymers,withthedevelopmentofBakeliteintheearly20thcentury.
Pre-polymersynthesisandtheirfurthercrosslinkingisacommonprocessingtechnology[4–6].
In dual-curing processing, the extent of both curing reactions and the properties of the
intermediatematerialcanbeeasilycontrolledbychangingthecompositionoftheformulation,aswell
asthedurationandtemperatureofcure[1]. Inthecaseofsequentialdual-curing,theintermediate
material will have stable properties and will not cure unless the second reaction is triggered.
Incomparisonwithpre-polymersynthesisapproaches,processingandcompoundingismucheasier
Polymers2018,10,178;doi:10.3390/polym10020178 www.mdpi.com/journal/polymers
PPoollyymmeerrss 22001188,, 1100,, 1x7 F8OR PEER REVIEW 22 ooff 2245
triggered. In comparison with pre-polymer synthesis approaches, processing and compounding is
imnudcuha el-acsuierrin ing dfouraml-uculartiinogn fso.rTmhuelaptrioen-pso. Tlyhme eprreis-pporloydmuecre ids pinr-osdituuceddu irnin-sgittuh deufirrisntgc tuhrei nfigrsstt caugreinogf
tshtaegien iotifa lthme ixintuitriael omf lioxwturme oolfe clouwla rmwoeliegchutlamr owneoimghetr sm, roantohmeretrhs,a nrabthyemr tixhianng boyf lmarigxeinrgp oolfy lmaregreicr
cpoomlypmoenreicn tcso.mThpiosniesnotsf. gTrehaist uiss eofw ghreenatl ouwse- vwishcoensi tlyowfo-rvmisucolastiitoyn fsoarmreurleaqtiuoinresd arbey rtehqeuairpepdl icbayt itohne.
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hoifg ahd-vadandceedd-v, haliugeh-aapdpdleicda-tvioanluse. Dapupalli-ccautrioinngs.s Dysuteaml-csufraicnigli tsaytsetesomlvs efnact-iflriteaetep rsooclveessnitn-fgr,eset aprrtoincgesfsrionmg,
lsitqaurtiidngm foronmom lieqrus,ida nmdomnoimldeprsr,o acnesds imngildc opnrodciteisosnins,gw cohnicdhitihoenlps,s wmhiinchim hiezlepsth meienmimisiszieo tnheo femvoilsastiiolne
oorfg vaonliacticloe morpgoaunnicd sco(mVOpoCusn)dtos t(hVeOaCtms)o tsop thheer eatamndosrpehdeurcee apnrdo creesdsuincge epnroercgeysscinogst se.nAerngye xceomstps.l aAryn
seexqeumepnltaiaryld sueqalu-ecunrtiianlg dpuraolc-ceussriinsgs cphreomceastsi zies dscihneSmchateimzeed1 i.n Scheme 1.

Scheme 1. A sequential dual-curing process. The monomers depicted as green and light blue
Scheme1.Asequentialdual-curingprocess.Themonomersdepictedasgreenandlightblueparticipate
participate in the first curing reaction (RXN 1) leaving the dark blue monomers unreacted. At a later
in the first curing reaction (RXN 1) leaving the dark blue monomers unreacted. At a later stage
stage (RXN 2), the dark blue monomers polymerize with the available light blue functional groups
(RXN2),thedarkbluemonomerspolymerizewiththeavailablelightbluefunctionalgroupsandalso
and also homopolymerize. A grafted IPN is obtained.
homopolymerize.AgraftedIPNisobtained.
Successful sequential dual-curing processing requires that (a) both polymerization reactions are
Successfulsequentialdual-curingprocessingrequiresthat(a)bothpolymerizationreactionsare
selective and compatible so that no undesired inhibition or reactivity effects take place; and (b) they
selectiveandcompatiblesothatnoundesiredinhibitionorreactivityeffectstakeplace;and(b)they
can be triggered using different stimulus such as UV light or temperature, or else they have
canbetriggeredusingdifferentstimulussuchasUVlightortemperature,orelsetheyhavesufficiently
sufficiently different reaction rates so that they can be controlled from a kinetics point of view in
differentreactionratessothattheycanbecontrolledfromakineticspointofviewintermsoftime
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.
advancedapplications.
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
enhancementofthepropertiesofagivenbasematerial. Forinstance,post-polymerizationofexcess
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
propertiesofthermosets[7,8]. Inothercases,thecrosslinkingreactionistriggeredbymeansofUV
light leading to fast cure in the outermost layers, but extended through-cure can take place more
lightleadingtofastcureintheoutermostlayers,butextendedthrough-curecantakeplacemoreslowly
slowly and reach completion by means of a thermal polymerization process in the deepest layers [9].
andreachcompletionbymeansofathermalpolymerizationprocessinthedeepestlayers[9]. Likewise,
Likewise, for certain applications such as coatings that are cured by UV light, the use of dual-curing
forcertainapplicationssuchascoatingsthatarecuredbyUVlight,theuseofdual-curingformulations
formulations together with the use of photoabsorbers can make it possible to achieve complete, or at
togetherwiththeuseofphotoabsorberscanmakeitpossibletoachievecomplete,oratleastsufficient,
least sufficient, cure in shadowed areas [10]. In the synthesis of IPNs [11–15], the control of the dual-
cureinshadowedareas[10]. InthesynthesisofIPNs[11–15],thecontrolofthedual-curingprocess
curing process plays a role in the morphology. Materials with particular morphological features can
playsaroleinthemorphology. Materialswithparticularmorphologicalfeaturescanbedeveloped
be developed depending on whether the polymerization processes take place in a simultaneous or
dependingonwhetherthepolymerizationprocessestakeplaceinasimultaneousorsequentialway,
sequential way, and whether or not there is crosslinking between the two networks.
andwhetherornotthereiscrosslinkingbetweenthetwonetworks.
Dual curable systems are also available in which the two curing reactions proceed
Dualcurablesystemsarealsoavailableinwhichthetwocuringreactionsproceedsimultaneously.
simultaneously. In such systems, the ultimate goal of employing a dual-cure formulation is
Insuchsystems,theultimategoalofemployingadual-cureformulationisenhancingend-product
enhancing end-product properties rather than an intermediate processing possibility. For example,
propertiesratherthananintermediateprocessingpossibility. Forexample,mixturesofmultifunctional
mixtures of multifunctional acrylate and epoxy monomers can be crosslinked simultaneously under
acrylateandepoxymonomerscanbecrosslinkedsimultaneouslyunderUVlightandwiththeuseof
UV light and with the use of radical and cationic photoinitiators. Decker and coworkers [16] showed
radicalandcationicphotoinitiators. Deckerandcoworkers[16]showedthesynthesisofIPNsfrom
the synthesis of IPNs from a 50:50 mixture (by weight) of 1,6-hexanediol diacrylate (HDDA) and
a50:50mixture(byweight)of1,6-hexanedioldiacrylate(HDDA)andepoxidizedsoybeanoil(ESO)
epoxidized soybean oil (ESO) under intense UV irradiation. The plasticizing effect of ESO facilitated
Polymers2018,10,178 3of24
underintenseUVirradiation. TheplasticizingeffectofESOfacilitatedhigherconversionofHDDA
comparedtoaneatformulation. Suchacrylate/epoxideIPNsaresuitableasfast-dryingprotective
coatings,compositesandopticalcomponents.
The use of photolatent and thermally-latent catalysts, for base- or acid-catalysed reactions or
radicalpolymerizationreactions[17–19]cancontributetothedesignofefficientone-potdual-curing
formulations. These formulations have good storage stability of the uncured system and stable
intermediateproperties. Theuseofblockedspeciescanalsobeexploitedforthedesignofdual-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
polymerizationprocessisthermallyactivated[22].
Theintermediateandfinalpropertiesofdual-curingmaterialsstronglydependonthenetwork
structureattheendofeachpolymerizationprocess. Somecriteriafordual-curingformulationdesign
orpreliminarynetworkstructureanalysisbasedonidealnetworkbuild-upmodelshavebeenoutlined
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
relationshipsindual-cureprocessing.
Alargenumberofreactionscanbeusedindual-curingthermosettingformulations[1]but“click”
typereactionsareofspecialinterest. TheclickconceptwasfirstcoinedbySharplessetal. torefertoa
varietyofchemicalreactionsthatareorthogonal,selective,andhighlyefficient[26,27]. Clickreactions
havethefollowingcharacteristics: (a)highyields;(b)regioandstereospecificity;(c)insensitivityto
oxygenorwater;(d)mild,solvent-less,oraqueousreactionconditions;(e)orthogonalitywithother
commonorganicreactions;and(f)applicabilitytoawiderangeofavailablestartingcompounds[28,29].
Theclickchemistryconcepthasfoundaparticularinterestinpolymerchemistryleadingtoa“paradigm
change”: A modular construction approach is used [27,30,31] rather than conventional synthetic
approaches. Productpurificationismostlyunnecessarythankstotheselectivityandquantitativeyield
ofclickreactions. Inanextensivereview,Tuncareportedthecombinationoftwoormoreorthogonal
clickprocessestosynthesizecomplexarchitectures[32].
Inthefieldofthermosets,clickreactionsallowthepreparationofwell-definednetworkstructures.
Theycanbecarriedoutundermildandsolvent-freeconditionsandthereforecanbedesignedinan
environmentallyfriendlymanner. Asvariouschemicalfunctionalitiesshowclickbehaviourandeach
haschemicallyandmechanisticallydistinctreactionpathways,theycanbecombinedindualcuring
methodologies. Somecommonclickreactionsreportedforthepreparationofdual-curablematerials
areoutlinedinScheme2.
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vsealsetcsteiolenc toiof ncoomfcmomermciaerllcyia allvyaailvaabillea,b laef,foarffdoarbdlaeb alenadn edaesailsyil yhahnadnldelde dacarcyrylaltaetse.s .BByy ccaarreeffuullllyy sseelleeccttiinngg
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ainpdpulisctartiiaoln aspspulcihcaatisoinnsd usustcrhia lacso aintidnugsst[r1ia3l, 33c,o3a4t]i,nagdsh [e1s3iv,3e3s,3[345],, 3a6]d,hineskisve[3s 7][,35a,n3d6]b, ioinmkas te[r3i7a]l,s [a3n8d].
Abicormylaatteergiarolsu [p3s8r]e. aAdcilryylraetaec tgwroiuthpas vreaarideitlyy orfesaucbt swtriatthe sao vwairnigettyo othf esucabrsbtroant–ecsa orbwoinngd otuob tlheeb ocnardbaonnd–
ictasrnbeoing hdboouubrline gboelnedct raonndw itist hnderigawhbinogurcianrgb oenleycltgroronu wp,itwhhdircahweinnhga cnacrebsothneylr egarcotuivpit, ywohfitchhe ednohuabnlceebso tnhde.
Trehaecrteifvoirtey, othf etyhea rdeowuibdlee lbyounsded. Tinhearnefuomreb, etrhoeyf raeraec twioindse[l3y9 u–4se4d]t ihna tac naunmfitbtehre ocfl ircekacctoinocnesp [t3.9–44] that
can fiAt tphreo mcliicnke ncotnclcicekptr.e a ctionofacrylatesistheMichaeladdition. Michaeladdition[39]isdefinedas
the1,A4- padrodmitiionnen(ot rclciockn jruegaactteioand odfi taiocrny)loafterse siso nthaen cMe-iscthaabeilli azdeddictaiornba. nMioicnhsateol aacdtdivitaitoend [d3o9u] ibsl edbefoinndeds.
Mas icthhea e1l,-4ty-apdedaitdiodnit i(oonr rceoancjtuiognastea raedudsiteidonw) iodfe lryesionndaunacel-csutaribnilgizperdo cceasrsbeasn,iboencsa utos eaocftitvhaetevda rdieotuybolef
bonds. Michael-type addition reactions are used widely in dual curing processes, because of the
commerciallyavailablenucleophiles(Michaeldonors)andactivateddoublebondcompounds(Michael
variety of commercially available nucleophiles (Michael donors) and activated double bond
acceptors)thatcanbeusedinsuchprocesses.
compounds (Michael acceptors) that can be used in such processes.
TypicalMichaeldonorsareamines, thiolsandacetoacetates. ThegroupofMichaelacceptors
Typical Michael donors are amines, thiols and acetoacetates. The group of Michael acceptors is
is more numerous and includes acrylate esters, acrylamides, maleimides, alkyl methacrylates,
more numerous and includes acrylate esters, acrylamides, maleimides, alkyl methacrylates,
cyanoacrylates and vinyl sulfones. Of all these different acceptors, acrylates have a suitable
cyanoacrylates and vinyl sulfones. Of all these different acceptors, acrylates have a suitable
combination of reactivity, versatility, availability and cost. Although there are Michael acceptors
combination of reactivity, versatility, availability and cost. Although there are Michael acceptors with
withahigherreactivity,suchasvinylsulfones[24],thereisavastselectionofcommerciallyavailable
a higher reactivity, such as vinyl sulfones [24], there is a vast selection of commercially available and
andaffordableacrylates. Incontrast,widelyavailablemethacrylateshavemuchlowerreactivitydue
affordable acrylates. In contrast, widely available methacrylates have much lower reactivity due to
tothestericeffectofthemethylgroup[45]andarethereforelessusedasMichaelacceptors.
the steric effect of the methyl group [45] and are therefore less used as Michael acceptors.
As co-reactant in Michael additions, the most common nucleophylic monomers are thiols
As co-reactant in Michael additions, the most common nucleophylic monomers are thiols (thiol–
(thiol–Michaeladdition),amines(aza-Michaeladdition),andacetoacetates. Asthesepolymerization
Michael addition), amines (aza-Michael addition), and acetoacetates. As these polymerization routes
routes of acrylates usually fit click criteria, they are used widely in dual-curable formulations,
of acrylates usually fit click criteria, they are used widely in dual-curable formulations, in
incombinationwithotherclickreactions,orwithothernon-clickreactions.
combination with other click reactions, or with other non-click reactions.
Avarietyofbasicand,especially,nucleophiliccatalystscanbeusedtotriggerandcontrolMichael
A variety of basic and, especially, nucleophilic catalysts can be used to trigger and control
additions [39,46]. Michael-type (mostly thiol–Michael) reactions can be combined in dual-curing
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
Polymers2018,10,178 5of24
systemswithavarietyofotherreactionssuchasradical-mediatedpolymerizationsinacontrolled
andsequentialway. However,thecomplexdependenceofthiol–Michaeladditionsonthesolvent,
natureandconcentrationofcatalyst,polarityandbasic/nucleophiliccharacterofthecatalystmakes
Polymers 2018, 10, x FOR PEER REVIEW 5 of 25
itdifficulttooptimizethekineticsandextensionofthereactionandtominimizesidereactions[47].
Thepcroendtrioctllieodn aonfdt hseeqoupentitmialu wmayr.e Hacotwioenvecro, nthdei tcioomnpslbexa sdeedpeonndtehnecep oKf ath(inole–gMaticivheaello agdadriittihonms oonf tthhee acid
dissoscoialvtieonnt, cnoantustraen atn)do cfotnhcioenlstraantidonb aosf ecsatiaslynsott, ppoolsasriibtyl eanddu ebatsoict/hneuccolenocpuhrirliecn cchearoafcbtears oefa tnhde cnautacllyesot phile
medimataekdest hiti doilf–fMicuilcth toa eolprtiemacizteio tnhes k[4in6e]t.icTs haendu esxeteonfssiotrno onfg thbea rseeasctoiornn auncdl teoo mphinilimesizlee asiddse troeaactvioenrsy fast
reacti[o47n],. bTuhet pmreadkiecstiopnr eopf athrae toiopntimduifmfi cruealtct[i2o4n, 4c6o]n.dWitioenaks bnausecdle oonp thhiele psKaal (lnoewgatthiveec loongatrroitlhmof otfh tehein itial
acid dissociation constant) of thiols and bases is not possible due to the concurrence of base and
reactionrate,showinganinductionperiodthatcanbefurthertunedbyadditionofsmallquantitiesof
nucleophile mediated thiol–Michael reactions [46]. The use of strong bases or nucleophiles leads to a
anacid(protonicspecies)[48]. Theanionicpolymerizationofacrylates,whichreliesuponMichael
very fast reaction, but makes preparation difficult [24,46]. Weak nucleophiles allow the control of the
additionchemistry,isanothersidereactionthatcannotbedisregardedtakingplaceduringthecuring
initial reaction rate, showing an induction period that can be further tuned by addition of small
of thiol/acrylate systems [39]. Photobase generators can be used in Michael addition reactions to
quantities of an acid (protonic species) [48]. The anionic polymerization of acrylates, which relies
designlatentdual-curingsystems[45,49,50]butaradicalinhibitorisneededtosuppresstheradical
upon Michael addition chemistry, is another side reaction that cannot be disregarded taking place
medidatuerdinsgi tdhee rceuarcintigo onfs th[4io9l–/a5c1r]y.late systems [39]. Photobase generators can be used in Michael addition
Rreaadctiicoanls- mtoe ddeisaitgend larteeanct tdiounals-csuurcinhga ssysatcermysl a[4te5,h49o,5m0]o bpuotl ya mraedricizaal itniohniboitrort hisi onle–eednede troe asuctpipornescsa nbe
foundthein radduicaall- cmuerdiniagtedsy ssidteem resac[1ti]o.nsT [h4e9s–e51r]e. a ctions, although not fitting the click concept, are also
highlyverRsaadtiilceala-mndedciaantebde rneeaficttiofrnosm suscehl eacst iavceryalcattiev haotimonopboylymmeearnizsatoiofna onru tmhiboel–reonfet hreearcmtioanl icnainti abteo rsor
found in dual-curing systems [1]. These reactions, although not fitting the click concept, are also
photoinitiators[52–57].
highly versatile and can benefit from selective activation by means of a number of thermal initiators
2.1. Tohri oplh–oAtcorinyiltaitaetoRresa [c5t2io–n57].
T2.h1e. Tmhiools–tAccormylamte oRneaacntidonc lickableco-reactantsforacrylatesarethiols. Acrylatesreactwiththiols
viatwodistinctmechanisms: (a)Radicalmediatedthiol–acrylatereaction;(b)Thiol–acrylateMichael
The most common and clickable co-reactants for acrylates are thiols. Acrylates react with thiols
addition. The radical mediated route is shown in Scheme 3. The thiyl radical is formed either by
via two distinct mechanisms: (a) Radical mediated thiol–acrylate reaction; (b) Thiol–acrylate Michael
UVirardadditiiaotnio. nThien rtahdeicpalr emseednicaeteodf ropuhtoe tios isnhiotiwatno irns ,Sochrebmye t3h. eTrhme athliaycl triavdaitciaol nis[ f5o2r]m.eOdn ecitehethr ebyr aUdVic alis
generirartaeddi,atiitoins aind dtheed ptoretsheencdeo oufb plehbotooninditoiaftothrse,a ocrr yblya tethmeromnaol macetrivtaotiyoine l[d52a]n. Oacnrcyel itcher ardaidciacla.l Inis turn,
thisrgaedniecraaltsepd,e icti ies eaidthdeedr ptor othpea dgoautebslet hboronud gohf trheeaacctriyolnatew mitohnaonmoetrh teor yaicerldy laant ea,coryrliacb rsatdriaccatls. Ian htuyrdnr, ogen
fromtthhies rtahdioiclatlo spyeiceiled etihtheerth piroolp–aagcarytelsa ttehrroeuagchti orenacptrioond wucitth. Iannoptohleyrm acerryilcatsey, sotre mabss,traacctosm a phyedtirtoiognene xists
from the thiol to yield the thiol–acrylate reaction product. In polymeric systems, a competition exists
betweenthechain-growthhomopolymerizationoftheacrylateandthestep-growthmechanismofa
between the chain-growth homopolymerization of the acrylate and the step-growth mechanism of a
thiol–enepolymerization. IthasbeenshownpreviouslybyCramerandBowman[58]thattherate
thiol–ene polymerization. It has been shown previously by Cramer and Bowman [58] that the rate
constantforacrylatepropagationis1.5timeshigherthanthatofhydrogenabstractionfromthethiol.
constant for acrylate propagation is 1.5 times higher than that of hydrogen abstraction from the thiol.
Because of this, a radical mediated thiol–acrylate reaction cannot be considered click since thiols
Because of this, a radical mediated thiol–acrylate reaction cannot be considered click since thiols
remainpartiallyunreacted.
remain partially unreacted.

SchemScehe3m.Re a3d. Ricaadli-cmale-mdieadteiadtetdh itohlio–la–carcyrlyaltaetep poollyymmeerriizzaattiioonn. .TThhe eprporpoapgaagtiaotnio kninkeitnice ctoicncstoannstt (akn1)t w(kas) was
1
reported to be 1.5 higher than the hydrogen abstraction kinetic constant (k2) [58].
reportedtobe1.5higherthanthehydrogenabstractionkineticconstant(k )[58].
2
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aoft ha itohlaiotelaaten iaonni,own,h wichhiccho uclodutldak teakpela pcleacveia vtiwa otwdoif dfeirffeenrtemnte mcheacnhiasnmissm,ass, sahs oswhonwinn Sinch Secmheem4e[ 549 []5.9].

SScchheemmee 44.. TThhiiooll––MMiicchhaaeell rreeaaccttiioonn mmeecchhaanniissmm.. TThhiioollaattee aanniioonn mmaayy ffoorrmm vviiaa ttwwoo ddiissttiinncctt mmeecchhaanniissmmss::
CCaattaallyyssiiss bbyy bbaassee oorr nnuucclleeoopphhiillee.. ((EEWWGG rreepprreesseennttss aann eelleeccttrroonn wwiitthhddrraawwiinngg ggrroouupp..))
DDeeppeennddiinngg oonn tthhee ppKKa anandd nunculceloepohpihliicl isctrsetrnegntght ohf othfeth ceatcaalytaslty, stht,et hfoermfoartmioanti oofn thoef tthhieotlhatieo lcaatne
a
follow either mechanism. When a strong base such as 1,5,7-triazabicyclo[4.4.0]dec-5-ene (TBD) is
canfolloweithermechanism. Whenastrongbasesuchas1,5,7-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,thebasecatalysedmechanismisfavouredforthethiol–Michaelreaction. Ontheotherhand,
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]ortertiaryphosphines[46]. Thiol–Michaelreactions
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. Asaresult,thisreactionhasbeencombinedwithmanyotherclickreactionstodevelop
dual-curing systems over the years.
dual-curingsystemsovertheyears.
2.2. Aza-Michael Addition
2.2. Aza-MichaelAddition
Acrylates can also undergo Michael addition in the presence of amines. The so-called aza-
AcrylatescanalsoundergoMichaeladditioninthepresenceofamines. Theso-calledaza-Michael
Michael additions have the same mechanism as thiol–Michael reactions (Scheme 5). Amines as
additionshavethesamemechanismasthiol–Michaelreactions(Scheme5). AminesasMichaeldonors
Michael donors have the following advantages in comparison to thiols or acetoacetates: (a) many
havethefollowingadvantagesincomparisontothiolsoracetoacetates: (a)manysuitableaminesare
suitable amines are commercially available, including amine-terminated hyperbranched polymers;
commerciallyavailable,includingamine-terminatedhyperbranchedpolymers;(b)catalystsarenot
(b) catalysts are not required since amines can act as both nucleophiles and bases; (c) radicals or other
requiredsinceaminescanactasbothnucleophilesandbases;(c)radicalsorotheractivespeciesarenot
active species are not formed during the first stage of curing; (d) the presence of tertiary amines
formedduringthefirststageofcuring;(d)thepresenceoftertiaryaminesformedduringaza-Michael
formed during aza-Michael addition overcomes the intrinsic oxygen inhibition of acrylate free-
additionovercomestheintrinsicoxygeninhibitionofacrylatefree-radicalpolymerizations[62]and
radical polymerizations [62] and allows the curing to be performed in non-inert environments and
allows the curing to be performed in non-inert environments and (e) these tertiary amines can
(e) these tertiary amines can act as co-initiator when type II photoinitiators (i.e., photoinitiators that
act as co-initiator when type II photoinitiators (i.e., photoinitiators that require a co-initiator or
require a co-initiator or synergist to produce initiating radicals) are used. Due to the low reactivity of
synergist to produce initiating radicals) are used. Due to the low reactivity of formed secondary
formed secondary amines, aza-Michael reaction may not reach completion but further crosslinking
amines,aza-Michaelreactionmaynotreachcompletionbutfurthercrosslinkingduringsubsequent
during subsequent photoirradiation leads to complete acrylate conversion [42].
photoirradiationleadstocompleteacrylateconversion[42].
Polymers2018,10,178 7of24
PPoloylmymeresr s2 2001188, ,1 100, ,x x F FOORR P PEEEERR R REEVVIIEEWW 77 o of f2 52 5

Scheme 5. Aza-Michael addition between a primary amine and an acrylate. The resulting secondary
SScchheemmee 55.. AAzzaa--MMiicchhaaeell aaddddiittiioonn bbeettwweeeenn aa pprriimmaarryy aammiinnee aanndd aann aaccrryyllaattee.. TThhee rreessuullttiinngg sseeccoonnddaarryy
amine may further undergo aza-Michael addition at a slower reaction rate.
aammiinnee mmaayy ffuurrtthheerr uunnddeerrggoo aazzaa--MMiicchhaaeell aaddddiittiioonn aatt aa sslloowweerr rreeaaccttiioonn rraattee..
2.3. Acetoacetate–Acrylate Reaction
22..33.. AAcceettooaacceettaattee––AAccrryyllaattee RReeaaccttiioonn
Acetoacetate–acrylate reaction [39] is a good example of Michael addition to acrylates.
AAcceettooaacceettaattee––aaccrryyllaattee rreeaaccttiioonn [[3399]] iiss aa ggoooodd eexxaammppllee ooff MMiicchhaaeell aaddddiittiioonn ttoo aaccrryyllaatteess..
Mechanistically, the reaction proceeds through an enolate anion, which is formed after the
MMeecchhaanniissttiiccaallllyy,, tthhee rreeaaccttiioonn pprroocceeeeddss tthhrroouugghh aann eennoollaattee aanniioonn,, wwhhiicchh iiss ffoorrmmeedd aafftteerr tthhee
deprotonation of the active methylene. The deprotonation is facilitated by the base catalyst (Scheme
ddeepprroottoonnaattiioonn ooff tthheea accttiviveem meeththyylelennee..T Thheed depeprortootnoantaitoinonis isfa fcaicliitlaittaetdedb ybtyh tehbea bseascea tcaaltyaslty(sSt c(hSecmheem6e).
6). The enolate then adds to the acrylic double bond. The adduct is formed and the base is regenerated
T6)h. eThene oelnaotelatthee tnheand addsdtos ttoh ethaec arycrliyclidco duobulbelbe obnodn.dT. Thehea daddduuctcti sisf oforrmmeedda annddt thhee bbaassee iiss rreeggeenneerraatteedd
once proton transfer occurs. The rate determining step of the reaction is the attack of the enolate to
oonnccee pprroottoonn ttrraannssffeerro occccuurrss..T Thheer arateted deteetremrminiinnigngst setpepo fotfh tehree arcetaicotnioins tihs ethaett aacttkaockf tohfe tehneo elantoelatotet htoe
the acrylate. The reaction kinetics is highly dependent on base strength. The formed adduct may
athcery alactrey.laTthee. Trehaec trioenacktiionnet ikcisniestihcisg hisl yhdigehpleyn ddeenptenondebnats eonst rbeansget hs.trTehnegtfhor. mTehde afodrdmuectdm aadyduunctd mergayo
undergo a second Michael addition, but at a slower rate.
aunsedceorngdo aM siecchoanedla Mddicihtiaoenl, abdudtiatitoans, lbouwt eart raa stelo.wer rate.

Scheme 6. Michael reaction mechanism. The Michael donor is deprotonated by the base catalyst. The
Sfcohremmede 6a.n Mioinc haatetal crkesa ctthioe na cmryeclahtaen disomub. lTeh beo Mndic haanedl dreogneonre irsa tdeesp trhoet obnaastee. dT bhye tahded buacste f ocarmtaleyds tm. Tahye
Scheme6. Michaelreactionmechanism. TheMichaeldonorisdeprotonatedbythebasecatalyst.
fofurmrtheder aunnidoenr gaott aac skesc othned abcurty sllaotwe edro Mubicleh abeol nadd daintido nr.e generates the base. The adduct formed may
Theformedanionattackstheacrylatedoublebondandregeneratesthebase.Theadductformedmay
further undergo a second but slower Michael addition.
furtherundergoasecondbutslowerMichaeladdition.
2.4. Radical Homopolymerization of Acrylates
2.4. Radical Homopolymerization of Acrylates
2.4. RaFdoirc athlHe soamkoep oofly cmomeripzlaettieonneosfsA, wcrey plarteessent the well-known radical mediated photopolymerization
reacFtioorn t hoef saackryel aotfe cso imn pSlcehteemnees s7,. w Teh ep rreesaecntito tnh es twaretlsl -wknitohw thne r apdhiocatol lmyseids ioaft etdh ep ihnoittioaptoorly tmhaetr iyziaetldiosn
Forthesakeofcompleteness,wepresentthewell-knownradicalmediatedphotopolymerization
retwacot ieoqnu oafl layc rreyalacttievse i nra Sdcichaelms. eT h7.e Tnheex tr estaecpti oisn t hstea rintsi twiatiitohn t hoef tphheo cthoalyinsi sb yo ft hthe er eiancittiiaotno ro ft haa rta ydiiecladl s
reactionofacrylatesinScheme7. Thereactionstartswiththephotolysisoftheinitiatorthatyields
twwoit he qauna lalycr yrelaatcet ivmeo rnaodmicearl.s .T Thhe ec hnaeixnt sptreopp iasg tahtees i nbiyti arteiaocnti oonf twheit chh oatihne br ym thone ormeaecrtsio. nT eorfm ai nraadtiiocna l
twoequallyreactiveradicals. Thenextstepistheinitiationofthechainbythereactionofaradical
woictchu arsn wahcreynl athtee mproonpoamgaetri.n gT hpeo lcyhmaeinr rperaocptsa geiattheesr bwyi trhe aanctoiothne rw girthow oitnhge rp omlyomnoemrice rrsa.d Ticearlm oirn watiitohn
with an acrylate monomer. The chain propagates by reaction with other monomers. Termination
oac cpurrism warhye rna dthicea pl.r o pagating polymer reacts either with another growing polymeric radical or with
occurswhenthepropagatingpolymerreactseitherwithanothergrowingpolymericradicalorwitha
a primary radical.
primaryradical.
Polymers2018,10,178 8of24
Polymers 2018, 10, x FOR PEER REVIEW 8 of 25

SScchheemmee 77.. RRaaddiciacal lmmeeddiaiatetedd pphhoototoppoolylymmeerirzizaatitoionn oof faaccrryylalatetess. .I: I:PPhhotootioninitiitaitaotro,r ,RR: :RRaaddicicaal,l ,MM: :
MMoonnoommeerr,, PP:: PPoollyymmeerr cchhaaiinn..
33. .AAccrryyllaattee--BBaasseedd DDuuaall--CCuurriinngg SSyysstteemmss
33.1.1. .TThhioiol–l–AAccrryylalattee aanndd TThhiiooll––EEppooxxyy RReeaaccttiioonnss
AAlltthhoouugghh tthhee rraaddiiccaall--mmeeddiaiatetedd tthhioiol–l–aaccrryylalatete rreeaacctitoionn ddooeess nnoott ggeenneerraalllyly eexxhhibibitit cclliicckk
cchhaarraacctteerriisstticicss, ,itit hhaass bbeeeenn ccoommbbiinneedd wwiitthh ootthheerr cclliicckk rreeaaccttiioonnss ttoo oobbttaaiinn mmaatteerriiaalsls wwiitthh ssiiggnnififiiccaannttlyly
llooww ssoolluubbllee ccoonntteenntt.. JJiiaann eett aall.. [[5500]] oobbttaaiinneedd hhyybbrriidd oorrggaanniicc nneettwwoorrkkss bbyy tthhiiooll––eeppooxxyy//tthhiiooll––aaccrryyllaattee
ppoollyymmeerriizzaattioionn pprroocceesssseess inin wwhhicichh bbootthh rreeaaccttioionnss pprroocceeeeddeedd sseeqquueenntitaialllyly wwitihthoouut tccleleaar rsseeppaararatitoionn. .
TThhee ccuurirningg wwasa stritgriggegreerde dbyb iyrrairdriaadtiioanti ownitwh iUthV UliVghlti gbhyt thbey utshee ouf sae pohfoatobpahsoet ogbeanseeragtoern e1r,5a,t7o-r
t1r,i5a,z7a-btriicayzcalob[i4cy.4c.l0o]d[4e.c4-.50-]edneyc-l 5t-eetnrayplhteetnryalpbhoernatyel b(oTrBaDte·H(TBBPDh4·)H cBaPphab4)lec aopf ainb lseitouf ginenseitruatginegn ear asttrinongga
bsatrsoe nagndb afrseee arnaddicfarelse, wraidthic iaslosp,rwopityhltihsoioplrxoapnythltohnioe l(xITanXt)h aosn peh(oItToXse)nassitpizheort toos eexntseintidze trheto weaxvteelnedngtthhe
awbsaovrepletinognt hraabnsgoer ptoifo ntrhaen gceaotafltyhset. caTtahley stf.reTeh erfardeeicraalsd icfaalcsilfiatactileidta tebdotbho ththtihoilo–la–carcyrlyaltaet erraaddiiccaall
ppoollyymmeerrizizaattioionn aass wweellll aass aaccrryyllaattee hhoommooppoollyymmeerriizzaattioionn, ,lleeaaddiningg ttoo mmaatteerriiaalsls wwiitthh uunnrreeaacctteedd tthhiiooll
ggrroouuppss.. HHoowweevveerr,, aasst htehpe hpohtoobtoabseasgee ngeernateorrataolsro allisboe rlaitbeedraatsetdro an gsbtraosneg( TbBaDse), t(hTiBoDl–)M, itchhioael–lMadidchitaioenl
aadlsdoittiooonk aplsloa cteooankd pclaocme paennds actoemdppeanrtslaytefodr ptharetlsyto fiocrh itohme esttroiicchimiobmaleatnricce i.mTbhaelraensceea.r Tchheer srefsoeuanrdchtehrast
ftohuionld– athcrayt ltahtieorl–eaaccrtyiolantse arneadctaicornysl aatnedh aocmryolpaotel yhmomeroizpaotliyomnewriezraetifoanst werearen dfamsteorr eanedffi mcioenret ethffaicnietnhte
tthhaino lt–heep othxiyolr–eeapcotixoyn sre(a>c9t5io%nsc o(>n9v5e%rs icoonnvinerasimona titne rao mfsaettceorn odfs saencodnmdsin auntdes m,riensupteecst,i vreeslpy)e,catinvdeltyh),e
atnhdio tlhceo nthvieorls cioonnvinercsrieoanse idncwreiathseadn winicthre aanse inincrtehaesee pino xthyec oepntoexnyt .content.
IInn aa rerceecnetn ptappaepre [r61[]6,1 t]h,et haeutahuotrhs oorfs thoifs trheivsierwev pierwesepnrteesde na tnedovael npohvoetlolpahteontto dlauteanl-tcudruea tlh-cioulr–e
atchriyolla–taec–reyplaotxey– seypsotxeyms. yUsntedmer. UUVn dliegrhUt, Vtheli gphhto,ttohbeaspeh goetonberaasteogr elinbeerraattoers ali bsterroantegs baassetr tohnagt cbaatsaelytsheast
tchaet athlyiosle–sMthicehatheilo rle–aMctiicohna aetl rroeoamct itoenmapterraotoumre.t Temo ipneirtiaattuer teh.e Tseocoinnidti actueritnhge rseeaccotinodn bcuetrwinegenre eapcotixoyn
abnedtw reemenaienpionxgy thainodlsr, etmhea sinyisntegmth iiso hlse,attheeds aybsotevme 8is0 h°Cea. tTehdea ubsoev oef8 a0 r◦aCd.icTahl einuhsieboitfoar erandsuicraeldi nthhaibt intoor
reandsiucarle dmethdaiattendo arcardyilcaatel mhoemdioapteodlymacerryilzaatteiohno tmoookp opllyamcee. rTizhaet ifoinnatlo mokateprliaacle p.roTpheertifiens aslumggaetsetreidal
pportoepnetriatile sins uagpgpelsitceadtiopnot esnutciha lains acphpemliciactailolny sruecshistaasncth ienmdiucastlrlyiarle csiosatatinntgisn,d audshtreisailvceosa, tianngds ,ealdechtersoinveics ,
manadtereilaelcst.r o nicmaterials.
IItt iiss aallssoo rreeppoorrtteedd iinn tthhee lliitteerraattuurree tthhee pprreeppaarraattiioonn ooff hhyybbrriidd ppoollyymmeerr nneettwwoorrkkss ((HHPPNNss)) ffrroomm
oonnee--ppoott tthhiiooll––aaccrryylalattee––eeppooxxyy mmixixttuurreess. .JinJi nete tala. l.[6[36]3 ]cucureredd aa ssttooicichhioiommeetrtricic ccoommbbininaatitoionn ooff aa
ddiiaaccrryyllaattee aanndd aa ddiieeppooxxiiddee uussiinngg ddiiffuunnccttiioonnaall oorr mmuullttiiffuunnccttiioonnaall tthhiiooll ccrroosssslliinnkkeerrss.. TThhee nnuucclleeoopphhiilliicc
tthhiiooll––aaccrryyllaattee rreeaaccttiioonna annddt htihoilo–el–peopxoyxrye arcetaiocntiowna swcaasr riceadrroieudt soimutu lstiamneuolutasnlyeoauts8l0y◦ Cat in80th °eCp riens etnhcee
porfe1se,8n-cdei aozfa 1b,i8c-ydcilaoz[a5b,4i,c0y]culnod[5e,4c-,07]-uennede(Dc-B7-Uen).eD (eDpBeUn)d. iDngepoenntdhiengfu onnc ttihoen afluitnyctoiofnthaelittyh ioofl ,trhees uthltiionlg,
rmesautletriinagls mexahtiebriitaelds aewxhidibeitreadn gae owfipdreo prearntigees cohfa rparcotepreirzteidesb ychdairffaecrteenritziaelds cbayn ndinifgfecraelonrtiimal estrcyan(DniSnCg) ,
cdaylonraimmeictrmy e(cDhSaCn)ic, adlyannaamlyisci sm(DecMhaAn)icaanld asncaalnynsiisn g(DeMlecAtr)o annmd isccraonscnoinpgy (eSleEcMtr)omn emthicordoss.copy (SEM)
methods.
Polymers2018,10,178 9of24
3.2. Thiol–AcrylateandThiol–Ene(orThiol–Yne)Reactions
Chatanietal.[64]usedselectiveandsequentialthiol–divinylsulfoneandthiol–Michaeladditions
tocreatematerialswithindependentnetworkstructuresandtwodifferentglasstransitiontemperatures
showingmultipleshape-memoryeffect. Bothreactionswerecatalysedbynucleophilicphosphine
catalysts. Itwaspossibletoseparatetheprocessintotworeactionstepssincethetwothiolmonomers
used(amercaptoacetoacetateandamercaptopropionate)showedsignificantlydifferentreactivities.
Chanetal.[65]showedthepossibilityofcombiningphosphine-catalysedMichaeladditionofthiols
toacrylatesfollowedbyUV-inducedradicalthiol–yneadditioninacontrolledandsequentialway.
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
radicalphotoinitiatorforthethiol–enereactionforthepreparationofholograms. Asimilarapproach
wasusedforsequentialthiol–Michaelandthiol–ynesystems[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
withallylfunctionality. Theseallylmoietieslaterundergoaradicalthiol–enereactiontoyieldfully
crosslinkeddual-networks.
3.3. Thiol–AcrylateandAcetoacetate–AcrylateReactions
UsingacetoacetatestoaccompanythiolsasMichaeldonorsisastrategytoachievesequentialityin
dual-curingprocessessincetheprotonaciditiesofacetoacetateandthiolgroupsaredisparate.Recently,
ourgroup[43]developeddual-curablethiol–acetoacetate–acrylatethermosetsbyusingthiol–Michael
additiontoacrylatesatroomtemperaturefollowedbyMichaeladditionofacetoacetatestoacrylates
atmoderatelyelevatedtemperature. Thedifferentaciditiesoftheprotonsonthiolandacetoacetate
groups, and the favorable pK of the base catalyst ensured two separate curing stages. Using a
a
photobasegenerator,itwasalsopossibletodesignlatentcuringsystemsoutofthesamemixtures.
Similarlytootherdual-curingsystems,thefinalmaterialpropertiescouldbetailoredbychangingthe
structureofthemonomersandtherelativecontributionofbothMichaeladditionreactions,byonly
changingthemonomerproportion.
3.4. Thiol–AcrylateandThiol–IsocyanateReactions
Thiol–isocyanate coupling and thiol–Michael addition reactions were combined by
Matsushimaetal.[69]tosynthesizematerialswithtunablemechanicalproperties. Thiolisocyanate
co-catalysed by phosphine–acrylate was faster than the thiol–Michael addition at low-phosphine
concentration. Therefore,thecuringstepswereeasilyseparable. Althoughphosphineswerenotbasic
enoughtoinitiateathiol–isocyanatereaction,thephosphoniumenolategeneratedvianucleophilic
additionofthephosphinetoacrylatedoublebondactsasastrongbasethatcandeprotonatethethiol
andproducethethiolateanion,whichcanthenreactwithisocyanategroups. Inthepresenceofexcess
acrylategroups,afinalphotopolymerizationstepcouldbeusedtoproduceternarynetworks.
3.5. CombinationofTwoThiol–MichaelAdditionsonDifferentAcceptors
Dualcuringsystemscanbedesignedbasedonasinglereactionmechanism,butusingmonomers
whosereactionkineticshavedifferenttemperaturedependenciessothatthetwostagescanbecarried
out at different curing temperatures. As an example, thiol–Michael addition was employed in a
sequentialcuringprocessusingtwomonomerswithdistinctelectrophilicstrengths: vinylsulfones
andacrylates[64]. Theformerreactedatroomtemperature,whereasthelatterreactedat90◦C.
AcrylatesaremorereactivethanmethacrylatesinMichaeladditionsduetotheinductiveeffect
andsterichindranceofthemethylgroupinthemethacrylate. Thiol–Michaelreactionswithacrylates
and methacrylates have been be employed in dual curing formulations by choosing an adequate
combinationofcatalysts: Aconventionaltertiaryamineforthethiol–Michaeladditiontotheacrylate
Polymers2018,10,178 10of24
indarkconditionsandaphotolatentbaseleadingtothereleaseofastrongerbaseforthethiol–Michael
additiontothemethacrylate[45]. Theseformulationswereusedinphoto-patterningwithspatialand
temporalcontrol.
3.6. MichaelAdditionFollowedbyHomopolymerization
Inordertoobtainstorableandprocessablepolymersatthefinalofthefirststageofcuringandfinal
networkswithoptimumpropertiesfordifferentapplications,thepreparationofdualmaterialsfrom
off-stoichiometricformulationshasrecentlygainedinterest[59].Themonomerinexcessshouldbeable
toundergohomopolymerizationreaction,withoutanyinterference. Thefirststageisaself-limiting
clickreactionbetweentwomultifunctionalmonomerswithanexcessofoneofthetwomonomers.
ThesecondstagereactionisathermallyorUV-inducedpolymerizationoftheexcessofunreacted
groups. Thematerialsobtainedafterthefirstcuringstagecanbegelledorungelledandlooselyor
tightlycrosslinkedattheendofthesecondcuringstage,dependingonthefunctionalityandfeedratio
ofthemonomers.
Thecombinationofstep-growth(usuallyfirststage)andchain-growth(usuallysecondstage)
polymerizationreactionsconfersthesesystemsagreatversatility,andcompletecuringcanbegenerally
achievedattheendofthesecondcuringstep. Crosslinkedpolymernetworksobtainedduringthe
firststageofcuringviastep-growthpolymerizationreactionshavesomeremarkablefeaturessuchas
homogeneity,highcapacityformechanicalenergyabsorptionnearitsglasstransitiontemperature,
highgelpointconversionandlowpolymerization-inducedshrinkagestresscomparedtochain-growth
polymers. In applications that require high T value and hardness at ambient temperature,
g
the relatively low T values and crosslinking densities of the step-growth intermediate polymers
g
canincreasesignificantlyafterthesecondstageofcuringviachain-growthhomopolymerization.
AsMichaeladditionreactionsareself-limiting,allMichaeldonorspecieswillhavereactedby
theendofthefirststageofadual-curingscheme. TheremainingMichaelacceptorspecies,usually
acrylates,canhomopolymerizeinthesecondcuringstage.
Moszner et al. [70,71] synthesized networks from multifunctional acetoacetates (as Michael
donors)withmultiacrylates(asMichaelacceptors)using1,5-diazabicyclo-[4.3.0]non-5-ene(DBN)as
thebasecatalyst. Tailormadematerialscouldbepreparedinatwo-stepprocess: Michaeladdition
followedbyradiationcuringofexcessacrylate. Ananalysisofcrosslinkedpolymersshowedthat
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
onthehomogeneity,conversionandcrosslinkingdensityofthesynthesizednetworks. Duringthe
second stage, the soft Michael network was converted into a hard crosslinked material with high
Shorehardness. Thepreparedmaterialswereinterpenetratingpolymernetworkscovalentlybonded.
Tosynthesizesimilarmaterials,Moyetal.[72]usedMichaeladditionofacetoacetatestomulti-acrylate
followedbyphotopolymerizationofacrylatesthatwereinexcessintheinitialmixture. TheMichael
additionhasbeenusedtosynthesizeliquidoligomersthatfinduseinUV-curableformulations[73].
This alleviates the need for toxic photoinitiators, which would be indispensable in conventional
solvent-basedformulationstoensurethecompletecureofthickfilms[37].
Despite the promising results obtained by Moszner et al., few studies have deepened in the
formationofnetworksbasedonacetoacetatesandacrylates. Probably,therelativelyhighcomplexity
ofsynthesizedacetoacetatesofdifferentfunctionalitiesandstructurealongwiththerelativelylower
reactivityofthesecondacidicprotoninthemethyleneoftheacetoacetatearethereasonsforthescarce
numberofpublicationsonthisspecifictypeofMichaeladdition[74].
Takingadvantageofthebehaviourassociatedwithclickreactions,Bowman’sgroup[41,75,76]has
developedanoveltwo-stagereactivesystemusingoff-stoichiometricthiol–acrylateformulations,ina
similarwaytotheonedevelopedbyMoszneretal.[70,71]withacetoacetate–acrylateformulations.
In the first step, a polymer network is formed via base-catalysed thiol–Michael addition. In the