Thiol‐ene polymers are formed by the stoichiometric reaction of multifunctional enes with multifunctional thiols. While numerous other thiol‐vinyl reactions are possible through a variety of mechanisms as presented throughout this book, the reaction considered in this chapter as the “thiol‐ene” reaction proceeds via a radical‐mediated step growth mechanism that can be initiated by light, peroxides, thermal initiators or any system whereby radicals are generated. The reaction of thiols with enes was first observed in 1905,¹  their use in forming polymers was discovered in 1926,²  and in the 1930s, the thiol‐ene reaction mechanism for radical polymerization was first proposed.³  A systematic study of the reactions of dienes with dithiols was performed in the 1940s with the findings summarized in a patent.⁴  The systematic examination of these reactions demonstrated that the reaction was general in nature, proceeded under mild reaction conditions, reacted rapidly to achieve high yields, gave a specific composition of products (the anti‐Markovnikov addition product of thiols to enes), and exhibited few or no side products – whence this collection of attributes more recently has led to the thiol‐ene reaction meeting all of the basic requirements for being considered a click reaction.⁵  This study and numerous others since have clearly demonstrated that thiol‐ene radical reactions uniquely combine the benefits of click reactions with the spatial and temporal advantages that come from its ability to be photoinitiated.

It wasn’t until the 1970s that thiol‐ene polymerizations were developed for commercial applications, such as relief printing plates, conformal coatings on electronics, and floor coatings by W. R. Grace and Co.^6–8  However, these and other potential applications for thiol‐enes gave way to acrylate and methacrylate systems, which became the commercial photopolymerization systems of choice. The most notable exception was for optical adhesives developed and sold by Norland since the 1970s. In the early 1990s there was again renewed interest in thiol‐ene photopolymerizations, largely in concert with an increased interest in all non‐acrylate photopolymers due to concerns over toxicological issues associated with acrylate monomers. Additionally, the rapid polymerization rate of thiol‐enes, as well as their inherent ability to polymerize in the presence of oxygen, are highly desirable characteristics. These unique properties, along with many others, led to continued interest and development in thiol‐ene polymerizations. Another review was published in 1993,⁹  resulting from Loctite’s interest in thiol‐ene systems as coatings. The fundamental understanding, application potential, and overall interest in thiol‐enes has continued to grow with several reviews since 2000,^10–14  indicating the resurgence of interest in thiol‐ene chemistry, particularly as one of the click‐reaction family.

Though thiol‐ene chemistry and polymerization have been known for many decades, the use of thiol‐ene photopolymerizations is still not widely prevalent commercially, with the photopolymerization of acrylate and methacrylate systems continuing to dominate the commercial radiation curing markets. Acrylate and methacrylate systems are widely utilized commercially for applications including coatings, adhesives, dental materials, contact lenses, and photolithographic processes.^15–17  There exists significant potential to expand application of photopolymerization technology to applications in nanotechnology, biomaterials, high‐resolution lithography, polymer and surface functionalization, electro‐optics, high‐impact energy absorbing devices, optical switching arrays, and many others. Critical to achieving expanded application is an improvement in polymerization and material performance. The classical acrylate and methacrylate photopolymerizations encompass several significant drawbacks including inhibition of the polymerization by oxygen,^18–20  polymerization induced stress development,^21,22  and the formation of highly heterogeneous polymer networks.^23–25  Significant research is aimed at improving the properties and performance of photopolymerizations to achieve expanded application and the resurgence of interest and development of thiol‐ene photopolymerizations has been one of the thrusts of this research. Thiol‐enes exhibit numerous polymerization characteristics that directly address the drawbacks of the classical acrylate/methacrylate photopolymerization systems. Thiol‐ene photopolymerizations exhibit rapid and simplified polymerization kinetics, are relatively uninhibited by oxygen, form low stress homogeneous polymer networks, and are able to achieve a wide range of material properties. Each of these unique thiol‐ene properties will be discussed in detail.

The thiol‐ene reaction occurs as a result of the radical‐mediated addition of thiol functional groups across carbon–carbon double bonds (enes) (Scheme 1.1). The thiyl radical attacks the least substituted side of the double bond giving the anti‐Markovnikov addition product.²⁶  Traditional thiol‐ene systems utilize ene monomers, such as allyl ethers and vinyl ethers, that do not homopolymerize. However, thiol monomers can react with a multitude of different vinyl moieties, including those that do homopolymerize such as acrylates, methacrylates, and acrylamides.

The propagation mechanism of the thiol‐ene photopolymerization is an alternation of addition and chain transfer reactions, as outlined in Scheme 1.2. The thiyl radical adds across carbon–carbon double bonds via an addition reaction, forming a carbon radical. Carbon radicals abstract hydrogen from thiol functional groups, forming thiyl radicals. For systems where the ene monomers are utilized that do not homopolymerize, such as vinyl ethers, allyl ethers, and norbornenes, this addition/chain transfer process continues cyclically as seen in Figure 1.1, forming the basis of the thiol‐ene step growth polymerization mechanism. The step growth mechanism results in a 1∶1 stoichiometric consumption of thiol and ene functional groups resulting in near ideal molecular weight and network structure development as predicted for step growth polymer molecular weight evolution. The molecular weight of the system increases slowly, resultant from the chain transfer step following every addition step, building up dimers, trimers, etc. before higher molecular weight species and gelation are ultimately achieved. As such, the polymerization kinetics are also relatively simple over the majority of the polymerization, in contrast to conventional acrylic free radical chain growth polymerizations where high molecular weight species are rapidly formed resulting in rapid gelation and complex diffusion mediated kinetics. The use of multiple types of ene functional groups and/or enes that also homopolymerize (such as acrylates and methacrylates) leads to an additional homopolymerization propagation step (Scheme 1.2). When homopolymerizable enes are utilized, such as with acrylates and methacrylates, significant homopolymerization of the (meth)acrylate occurs, leading to a combination of both step and chain growth molecular weight evolution which can be characterized as dual cyclical reactions, as seen in Figure 1.2. These step‐chain growth systems are kinetically more complicated, but also provide a unique platform enabling much greater control and greater range of achievable polymerization and polymer properties.

Radicals are extremely efficient at catalyzing the reaction of thiols and enes. The reaction of radicals with oxygen is also an extremely efficient process with reactivities of radicals towards oxygen being around four orders of magnitude greater than the reactivity towards propagation.^27,28  In conventional radical photopolymerizations, the reaction with oxygen serves to inhibit the polymerization, as the ensuing peroxy radical that is formed is unreactive towards additional propagation. This inhibition of the polymerization occurs from oxygen that is dissolved in the monomer formulation as well as oxygen that is able to diffuse into the system during polymerization of non‐laminated films. In both cases, oxygen inhibition has a huge impact on photopolymerizations resulting in tacky (uncured) surface layers and/or requiring the use of high irradiation intensities, high initiator concentrations, multiple initiator types, and inert/nitrogen blankets. Uniquely, the thiol‐ene polymerization, due to its unique addition/chain transfer propagation mechanism, is not strongly impacted by oxygen inhibition. The propagating radicals are reactive with oxygen, forming peroxy radicals. However, the peroxy radicals, though not reactive towards additional propagation, readily abstract hydrogen from thiol groups (Scheme 1.3). The hydrogen abstraction generates an alkylhydroperoxide and a thiyl radical and the polymerization thus continues with only a single, rapid additional reaction step, rather than being inhibited or effectively terminated by a reaction with oxygen. Numerous studies have been performed that have characterized polymerization rates in thiol‐ene systems with and without the presence of ambient oxygen and in comparison to acrylic systems.^10,29–32  Figure 1.3 illustrates conversion versus time results for thiol‐allyl ether and thiol‐acrylate systems exposed to either ambient oxygen or as a laminate. In both cases the samples are spread to a thickness of 6 μm. In the thiol‐allyl ether system the polymerization rate is almost unaffected by the ambient conditions. In the thiol‐acrylate system there is a modest reduction in both polymerization rate and conversion, which is remarkable for an acrylate system with low initiator concentration, low irradiation intensity, and only 6 μm thick!

Initation is the process whereby radicals are generated and subsequently react with either thiol or ene functional groups to generate thiyl or carbon radicals that start the polymerization process. Type II abstraction initiators (typically benzophenone) have long been utilized to initiate thiol‐ene reactions.⁹  In these systems, benzophenone or a similar compound absorbs ultraviolet irradiation and goes into the excited triplet state where it abstracts hydrogen from a thiol functional group generating a pinacol radical and a thiyl radical. The pinacol radical is relatively unreactive except towards termination. The thiyl radical enters the thiol‐ene reaction cycle as outlined in Scheme 1.2. Type I cleavage initiators exhibit significantly better initiation efficiencies than abstraction initiators and are also effective at initiating thiol‐ene polymerizations. Cleavage initiators, either ultraviolet, such as hydroxy cyclohexyl phenyl ketone and 2,2‐dimethoxy‐2‐phenylacetophenone, or visible such as diphenyl (2,4,6‐trimethylbenzoyl)‐ phosphine oxide (TPO) directly cleave into primary radicals that readily initiate thiol‐ene reactions by either hydrogen abstraction from a thiol or addition across an ene. Interestingly, the camphorquinone/amine initiation system that is commonly utilized for many visible light initiated polymerizations is extremely inefficient in thiol‐ene systems. Figure 1.4 illustrates the polymerization rate in a methacrylate/thiol‐ene system with a UV photoinitiator (2,2‐dimethoxy‐2‐phenylacetophenone), a TPO derivative (bis acyl phosphine oxide), and a camphorquinone/amine combination. Both of the cleavage initiators are extremely efficient, whereas the camphorquinone/amine system exhibits minimal polymerization. The improved efficiency of cleavage initiators relative to benzophenone is illustrated in Figure 1.5, where it is seen that a thiol‐ene system proceeds much more rapidly when initiated with 2,2‐dimethoxy‐2‐phenylacetophenone relative to a system initiated with benzophenone. Thermal initiators that degrade into initiating radicals upon heating, such as the commonly utilized azoisobutylnitrile (AIBN), are also efficient at initiating thiol‐ene reactions though they do have some drawbacks as well.^33,34  However, the use of thermal initiators in some cases, causes the thiol‐ene monomer formulation to be unstable even at ambient conditions.

One of the unique aspects of thiol‐ene polymerizations is that for ultraviolet irradiation, no added photoinitiator molecules are required to initiate the polymerizations.^10,30,35,36  In systems where ultraviolet light centered around 365 nm is utilized, polymerization depths of up to 25 inches have been achieved resultant from the low absorptivity of the monomers/polymers with the absence of added photoinitiator.³⁰  In these cases, and as illustrated in Figure 1.5, the initiation rate is dramatically reduced relative to systems containing added photoinitiator systems. When 254 nm light is utilized, initiation rates are extremely rapid, however polymerization depths are limited due to high absorptivity. The mechanism of initiation without added photoinitiators is not well understood. It has been postulated that a charge transfer complex between the thiol and ene functional groups results in a complex that can absorb photons and initiate polymerization.^37–40  The charge‐transfer complex was substantially studied and demonstrated to initiate the polymerization via consumption of oxygen.^37–39  However, no absorption from this complex can be seen been spectroscopically and the charge transfer complex does not appear to be responsible for photoinitiation. The rate of photoinitiation without added photoinitiators and using 365 nm irradiation has been shown to be directly proportional to the concentration of ene functional groups,³⁶  however a specific mechanism has not yet been found that is consistent with the experimental data. The mechanism of initiation with 254 nm irradiation is ascribed to direct cleavage of thiol functional groups,³⁶  a process that is known to occur in gas phase thiols.^41,42 

Temination can occur by any number of reactions, but is often dominated by the various radical–radical recombinations of thiyl and carbon radicals. The termination mechanism(s) has a strong effect on polymerization kinetics. Given the thiol‐ene polymerization mechanism, if one assumes that bimolecular radical–radical recombination is the dominant termination mechanism, the polymerization rate under pseudo‐steady state conditions would be expected to scale by the initiation rate to the ½ power (R_p∝R_i^1/2). Indeed several investigations dating back to the 1970s have demonstrated that this classical polymerization rate scaling behavior^6,43  occurs in at least some of the thiol‐ene reactions. In addition, the type of ene used as well as the presence of any dissolved oxygen or other impurities in the resin are critical to the termination mechanism, which was clearly identified in several systems not to be dominated by bimolecular termination reactions.⁴⁴ 

In most (meth)acrylate chain growth polymerizations, radical propagation results in high molecular weight polymers and oligomers being formed almost immediately and diffusion limits termination reactions very early in the polymerization leading to the phenomenon of autoacceleration and complicated polymerization kinetics. By contrast, chain transfer is an integral component of the step growth thiol‐ene mechanism, resulting in uniform molecular weight build‐up with delayed molecular weight evolution. Thus, the termination reaction rates generally remain high throughout the polymerization, resulting in consistent polymerization kinetics throughout much of the polymerization. The reduced diffusion limitations also lead to relatively high overall functional group conversions.

The thiol functional group is extremely versatile, participating in numerous chemical reactions. For radically mediated reactions, any non‐sterically hindered terminal ene can react in the manner depicted in Scheme 1.1. As such there are an incredibly diverse number of different types of ene functional groups that can be utilized in thiol‐ene reactions. Thiol functional groups have less diversity with the most common thiols including alkyl thiols, thiol propionates, thiol glycolates, and thiol phenols. Typical thiol and ene functional groups are depicted in Figure 1.6. The propagation reaction is exothermic with higher enthalpies for electron poor double bonds and lower enthalpies for electron rich double bonds. Reaction enthalpies range from 10.5 kcal mol^–1 for an electron‐rich vinyl ether to 22.6 kcal mol^–1 for an electron poor maleimide.⁴⁵ 

Reaction rates vary dramatically with different types of enes with terminal enes being the most reactive and electron‐rich (vinyl ethers and allyl ethers) and strained (norbornene) enes reacting more rapidly than electron‐deficient enes; internal 1,2‐substituted enes exhibit much lower overall reaction rates.⁴⁶  Numerous thiol and ene functional groups have been evaluated for their relative reactivities.^6,9,10,35  Figure 1.7 illustrates an example of a thiol monomer polymerized with a divinyl ether, a diallyl ether, and a diyne. It is readily observed that the vinyl ether reacts extremely rapidly followed by the allyl ether and with the yne group being the slowest. There are still questions about the specific order of reactivities, as numerous experiments have been performed over a wide range of experimental reaction conditions. The relative reactivities loosely are as follows with norbornene>vinyl ether>propenyl>alkene∼vinyl ester>n‐vinyl amide>allyl ether∼allyltriazine∼allylisocyanurate>acrylate>unsaturated ester>maleimide>acrylonitrile∼methacrylate>styrene>conjugated dienes.

One of the distinct features of thiol‐enes is that each ene functional group reacts only once with a thiol group whereas in chain growth polymerizations, each ene group reacts twice. An advantage of the ene group only reacting once is that thiol‐enes exhibit less volume shrinkage during polymerization than equivalent (meth)acrylate systems. One of the disadvantages of the single ene reaction is that thiol‐enes also exhibit lower crosslink density, and hence often lower modulus and T_g, than equivalent (meth)acrylate systems. An exception to the single reaction paradigm of ene functional groups is the yne triple bond. As depicted in Scheme 1.4, an yne functional group reacts with thiols to form a vinyl sulfide and the vinyl sulfide can subsequently react with another thiol. Outside of the dual reactivity of the yne functional groups, all of the other characteristics of the reaction follow the ideal click reaction paradigm of thiol‐ene systems, where the reaction is highly efficient, with the ensuing network being near ideal in homogeneity and relatively unaffected by oxygen. The radical mediated thiol‐yne reaction was initially observed and characterized in the 1940s–1960s.^47–51  More recent interest has followed from the overall resurgence in interest in the unique properties of thiol‐ene polymerizations^52–58  with a review being published in 2010.¹³ 

From the standpoint of polymerization kinetics, the reaction of the thiol with the yne is slower than the subsequent reaction of thiol with vinyl sulfide.^52,56  The relative reactivities of a range of different yne moieties with an alkyl thiol has been studied with the relative reaction rates being 1‐octyne>propargyl acetate>methyl propargyl ether>2‐octyne. In the case of cyclooctyne, methyl propargylamine, and ethyl propiolate the ynes react with thiols (rapidly for cyclooctyne and very slowly for methyl propargylamine and ethyl propiolate), but the subsequent addition of thiol with the vinyl sulfide was not observed to occur.⁵⁶  Resultant from the dual reactivity of the yne functional group, the crosslink density of thiol‐yne based polymer networks is substantially increased relative to an equivalent thiol‐ene polymer network. A comparison of properties of equivalent thiol‐ene and thiol‐yne systems in Table 1.1 illustrates this point, showing dramatic increases in crosslink density, modulus, and T_g.

The nature of the step growth polymerization reaction, as depicted in Figure 1.8, is such that if mono‐thiol monomers are reacted with mono‐ene monomers, only dimer products result. Reacting di‐thiol and di‐ene monomers in an ideal thiol‐ene reaction results in the formation of linear polymers. However, it is worth noting that many starting materials are not 100% pure and contain fractions that are less than the specified functionality. These impurities significantly affect the molecular weight evolution, and especially in the case of di‐thiols and di‐enes high molecular weight linear polymers are therefore difficult to achieve. Only when multi‐functional monomers (average functionality>2) are utilized do crosslinked polymer networks results. In these cases, the thiol‐ene polymerization proceeds via an ideal step growth polymerization with molecular weight evolution, gel point, and network structure similar to any other traditional step growth polymerization such as epoxy‐amines, alcohol‐isocyanates, etc. As such, the Flory–Stockmayer equation (eqn (1.1)) accurately predicts the gel point conversion (α) in ideal thiol‐ene systems

where r is the molar ratio of thiol to ene functional groups, f_thiol is the thiol monomer functionality, and f_ene is the ene monomer functionality. The Flory–Stockmayer equation and ideal step growth network evolution are only valid when negligible ene homopolymerization occurs.

To generate a more complete understanding of molecular weight evolution in thiol‐ene systems, models have been developed based on combined kinetic and statistical approaches to describe molecular weight evolution, gel point conversion, and crosslink density of thiol‐ene systems.^59–61  The models include pure step growth and mixed mode step‐chain growth systems. In traditional thiol‐ene step growth systems, with negligible amounts of ene homopolymerization, molecular weight evolution is consistent with any other step growth polymerization process. Cyclization has a significant impact on gel point, particularly with lower functionality monomers, with increased levels of cyclization increasing gel point conversions. In cases where homopolymerization occurs, such as in thiol‐acrylate systems, molecular weights (prior to gelation) are increased and the gel point is shifted towards lower conversions.⁶¹  In these cases, the ratio of thiyl radical propagation to chain transfer (k_p/k_ct) impacts the gel point with increased ratios of propagation to chain transfer resulting in earlier gel points. Both thiol‐acrylate and thiol‐ene‐acrylate systems demonstrate a readily controllable network evolution with the ability to manipulate network properties such as gel point conversion and crosslink density through changes in monomer functionality and the initial stoichiometric thiol to ene ratios. The ability to control network properties ultimately correlates with a concomitant ability to control polymerization and polymer properties in ways not available through either pure step growth or pure chain growth polymerizations.

Based on the initiation, propagation, and termination mechanisms of the thiol‐ene polymerization, equations can be derived that describe the polymerization kinetics of ideal, binary thiol‐ene systems.^62,63  The polymerization rates of thiol and ene functional groups are described by eqn (1.2) and (1.3), respectively. Eqn (1.3) describes the polymerization rate as it relates to thiol monomers (R_psh), where the rate of consumption of thiol functional groups is related to the chain transfer kinetic parameter (k_ct), the thiol functional group concentration, and the carbon radical concentration. Eqn (1.4) describes the polymerization rate as it relates to ene monomers (R_pcc), where the rate of consumption of ene functional groups is related to the thiyl radical propagation kinetic parameter (k_psc), the carbon radical propagation kinetic parameter (k_pcc), the concentration of ene functional groups, and the concentration of thiyl and carbon radicals.

Utilizing these equations, along with equations describing rates of initiation and termination, models have accurately confirmed and predicted thiol‐ene polymerization kinetics over a range of functional group chemistries, stoichiometric ratios, and polymerization conditions.^43,62–64  The modeling can also be expanded to include ternary systems.⁶⁴ 

Given the cyclical nature of the reaction, and for systems where carbon radical propagation is negligible (k_pcc≪k_ct), i.e. traditional binary thiol‐ene step growth systems, thiol and ene functional groups are equivalently consumed at the same rate and eqn (1.3) and (1.4) represent equivalent reactions. Several limiting cases have been identified for these classical thiol‐ene systems.⁶³  For cases where propagation and chain transfer are equivalent, k_CT≈k_p, the polymerization rate is ½ order in both thiol and ene functional group concentration (R_p∝[SH]^1/2[CC]^1/2). For cases where chain transfer is the rate limiting step, k_CT≪k_p, the polymerization rate is first order in thiol functional group concentration (R_p∝[SH]¹ ). For cases where thiyl radical addition is the rate limiting step, k_CT≫k_p, the polymerization rate is first order in ene functional group concentration (R_p∝[CC]¹ ). In all cases, the reaction is overall first order in functional group concentration, with the particular dependencies being dictated by the nature and reactivitiy of the functional groups and radical species. The polymerization characteristics are more strongly dependent on the chemical nature of the ene functional groups. For allyl ether functional groups, chain transfer is the limiting step and the reaction is first order dependent on thiol concentration. For ring strained enes, such as norbornenes, and vinyl ethers, propagation and chain transfer are near equivalent and the reaction is ½ order in both ene and thiol concentration. For vinyl silazanes, chain transfer is the limiting step and the reaction is first order dependent on ene concentration. Figure 1.9 illustrates an example of a thiol‐allyl ether polymerization where chain transfer is the rate limiting step and the polymerization rate is first order in thiol functional group concentration and zero order in allyl ether functional group concentration.

The kinetic equations describing thiol‐ene polymerization kinetics can also be used in combination with a modified rotating sector technique to experimentally quantify the kinetic parameters.^43,64  To date thiol‐allyl ether, thiol‐norbornene and thiol‐vinyl ether systems have all been characterized via this technique and their propagation, chain transfer, and termination kinetic parameter values are given in Table 1.2. Propagation kinetic parameters for thiyl radicals are on the order of 10⁶  L mol^–1 s^–1 and chain transfer kinetic parameters are on the order of 10⁵–10⁶ L mol^–1 s^–1, where chain transfer kinetic parameters on the order of 10⁶ represent the more rapid thiol‐norbornene and thiol‐vinyl ether systems and chain transfer kinetic parameters of 10⁵ represent the less rapid thiol‐allyl ether system. These kinetic parameters are in the same range as propagation kinetic parameters for chain growth systems,²⁸  with acrylate propagation kinetic parameters typically ranging from 10⁴–10⁶. The termination kinetic parameters are much higher than typical termination kinetic parameters in chain growth systems. However, the termination kinetic parameters are similar to those in chain growth systems (on the order of 10⁷–10⁹ L mol^–1 s^–1) during the very early stages of polymerization (<5% conversion) before the onset of diffusion limitations.⁶⁵  While termination kinetics rapidly decrease in chain growth polymerizations after the onset of diffusion limitations, the molecular weight evolution in thiol‐ene systems is such that the propagation and chain transfer kinetic parameters remain relatively constant until much higher conversions are achieved.^59,62–64 

The mechanism of polymerization and knowledge of kinetic parameters enables the ability to accurately model and predict polymerization kinetics in thiol‐ene systems over a range of polymerization conditions and functional group chemistries. This ability also extends to understanding and modeling of more complex ternary systems such as thiol‐ene‐ene and thiol‐ene‐acrylate.⁶⁴  In these systems simple changes in the relative concentrations and reactivity of the ene functional group have significant impact on the gel point, crosslink density, and network homogeneity.^59,61,64,66  In thiol‐ene‐ene systems where neither ene is homopolymerizable, the relative consumption of functional groups is proportional to the propagation kinetic parameters of the ene monomers and independent of the chain transfer kinetic parameter. In thiol‐ene‐acrylate systems where the ene monomer is not homopolymerizable, and in a mixed mode step‐chain growth polymerization, the relative conversion of ene and acrylate functional groups depends on both the propagation and chain transfer kinetic parameters.

Unlike a typical chain growth radical mechanism, each ene double bond in a thiol‐ene polymerization reacts with one thiol containing monomer instead of the two monomers that are generally coupled to each double bond. Thus as a thiol‐ene polymer is formed, there is less contraction of the van der Waals distances separating the monomers as the polymer continues to be formed. The calculated shrinkage associated with a thiol‐ene polymerization is between 12 and 15 cm³ mol^–1 per reacted double bond, in contrast with the shrinkage associated with a (meth)acrylate polymerization at 22–23 cm³ mol^–1. Thiol‐ene photopolymerizations, by virtue of progressing via the step growth mechanism and thereby exhibiting delayed gelation, give rise to materials that have distinct advantages in terms of material properties. One of the significant advantages in terms of material properties is the low stress that is resultant from the delayed gelation during polymerization. Delayed gelation also results in a greater fraction of the volume shrinkage occurring prior to gelation, reducing the amount of post‐gel shrinkage. Thiol‐ene polymerizations also show reduced stress within the polymer network. Typically, the formation of a glassy polymer results in the development of stress within the network, as the post‐gelation shrinkage that occurs within a highly crosslinked network results in permanent strain within the network as the network structure now resists deformation. However, in thiol‐ene polymerizations, the reduced shrinkage and the delayed gel point of the step‐growth mechanism result in reduced stresses within the network.

In a novel approach to further reducing the stress within thiol‐ene networks, addition‐fragmentation capable functional groups such as allyl sulfide moieties, which can react with the thiyl radicals formed at any point of the polymerization, were incorporated into the polymer backbone. By incorporating allyl sulfide moieties into multifunctional thiol and ene monomer systems a covalent adaptable network is created in which the allyl sulfide continuously participates in addition‐fragmentation reactions such that the bond structure of the network is covalent, yet each individual crosslink can be broken and re‐formed.^66–68  The ability to reversibly fragment and reform the backbone and crosslinked structure of the network serves to alleviate the stresses and strains within the network even after gelation. Shrinkage stress reductions of up to 75% have been demonstrated through incorporation of allyl sulfide moieties.⁶⁹  The use of allyl sulfides to reduce shrinkage stress has also been demonstrated in thiol‐yne systems,⁵⁴  as well as high modulus thiol‐ene‐methacrylate systems.⁷⁰ 

A few limited investigations have directly evaluated structure–property relationships in thiol‐ene polymers and evaluated the effect of a range of monomer chemistries and functionalities on polymer properties such as modulus, T_g, hardness, etc.^71–74  Due to the chemical nature of thiol‐ene systems, a broad range of different thiol and ene monomer structures, chemistries, and functionalities can be utilized. Judicious selection of these monomers can therefore be exploited to formulate systems with an incredibly broad range of polymerization kinetics, network structures, and material properties. In many cases the polymerization and material properties of thiol‐ene systems are not achievable by any other classical photopolymerization processes.

There are numerous different classes of thiol‐ene formulations, with the classical binary thiol‐ene systems, utilizing non‐homopolymerizable ene monomers, being the most common. The classical binary systems exhibit the well known advantageous properties of rapid polymerizations, no oxygen inhibition, and formation of homogeneous polymer networks. One of the drawbacks of these systems is that it is difficult to achieve high modulus and T_g. For many thiol‐ene polymer networks, the available monomers and resultant flexible sulfide bonds results in low T_g, low modulus networks at ambient temperatures. For applications that desire high modulus and T_g such as dental restoratives and automotive and aerospace resins, the low T_g of thiol‐ene networks is a detriment. Numerous approaches have been utilized to increase the mechanical properties of thiol‐ene polymers. Multifunctional carbamates synthesized by sequentially reacting aliphatic and aromatic diisocyanates with a tetrafunctional thiol yield thiol monomers that result in systems with excellent hardness and impact properties along with T_g’s of up to 108 °C due to the extensive hydrogen bonding within the network.⁷⁵  In another approach, the use of norbornenes to form the thiol‐ene network results in networks with T_g>80 °C, largely due to the mobility restriction that norbornenes impart to the network.⁷⁴  The use of sequential/hybrid curing methodologies or hybrid systems have also been demonstrated to achieve thiol‐ene materials with improved modulus and T_g.^76–79  Crosslinking linear polymers is a technique that can result in improved mechanical and physical properties of the crosslinked material. Several studies have shown that thiol‐ene crosslinking reactions resulted in improved physical properties and were seen to be more efficient when compared with thermally catalyzed crosslinking reactions traditionally used to crosslink linear polymers.^80–83 

The use of thiol‐acrylate and thiol‐methacrylate systems is a simple way to expand the achievable material properties of thiol‐ene systems to include high modulus and T_g.^10,11  These types of systems exhibit both step growth thiyl addition and chain transfer as well as (meth)acrylate chain growth homopolymerization. This step‐chain growth mechanism is represented schematically in Figure 1.2, where the step growth thiyl radical addition and chain transfer mechanism is represented by the first cycle and the thiyl radical addition and chain growth mechanism is represented by the second cycle. These binary systems enable achievement of a broader range of material properties resultant from the combination of step and chain growth polymerization mechanisms, as well as the large number of commercially available (meth)acrylates and their increased crosslink density. The presence of the thiol in the (meth)acrylate system imparts a number of beneficial characteristics to the polymerization and polymer properties. Incorporation of the thiol monomer, and hence chain transfer step, into the formulation dramatically reduces the sensitivity of the film to oxygen inhibition. The use of low concentrations of thiols has been demonstrated to lead to systems readily curable with both UV and visible irradiation in the presence of oxygen.³¹  Typically, 1–10 wt% thiol monomer is sufficient to reduce oxygen inhibition and form tack free polymers even in films that are only a few micrometres in thickness.²⁹  The reduced oxygen inhibition allows films to be cured with significantly less photoinitiator (and photoinitiator combinations) and reduced irradiation intensities while still maintaining rapid polymerization rates. In many cases, the polymerization rates are dramatically increased by up to 10 fold or even greater depending on the oxygen level. Additionally, the chain transfer step that is resultant from the presence of the thiol results in slower and more uniform molecular weight development that delays the gel point and reduces the amount of polymerization induced shrinkage stress. The delayed gel point also results in achievement of higher overall functional group conversions. For relatively small amounts of thiol, typically 5–20 wt% depending on the thiol and (meth)acrylate molecular structures, the modulus and glass transition temperature are not significantly impacted. Perhaps the one disadvantage of adding thiols to (meth)acrylates is that the amount of thiol that can be added is limited due to the nature of the polymerization.^35,84  The thiol monomer can only participate in the polymerization via chain transfer and subsequent thiyl radical addition to (meth)acrylate groups. If too much thiol is present, the (meth)acrylate groups will be completely consumed leaving unreacted thiol monomer in the final polymer. The specific ratios of thiol and (meth)acrylate that can be accommodated vary widely dependent upon the types of thiols and (meth)acrylates utilized and the desired polymer properties. Additionally, thiol‐methacrylate polymerizations proceed relatively slowly often leading to low functional group conversions.

The limitations of binary thiol‐ene and thiol‐(meth)acrylate systems can be overcome to achieve an even broader range of properties by utilizing ternary (or greater) systems comprising thiols, enes, and (meth)acrylates. The advantages of thiol‐ene systems are largely maintained when using these thiol‐ene‐(meth)acrylate ternary systems. With this approach, the mechanistic propagation pathways become much more complex providing the opportunity to manipulate the polymerization and polymer properties in ways not possible with binary systems.^64,85  Ternary thiol‐ene systems enable greater control over material properties by incorporating multiple monomer types. Additionally, via the step growth mechanism the network architecture is also dictated by the relative reactivities of the different functional groups. One of the distinctive properties of thiol‐ene systems is that a 1∶1 ratio of thiol to ene functional groups is required for achievement of full conversion of both functional groups and maximum mechanical properties such as modulus and T_g. Even slight deviations from a 1∶1 ratio of functional groups can result in a dramatic reduction these properties. The use of ternary systems, and especially thiol‐ene‐(meth)acrylate systems, provides a consistency of material properties over a range of functional group ratios, reducing the need for an exact 1∶1 ratio of functional groups.

An example of ternary systems being used to achieve high modulus is seen in dental restorative materials. The rapid polymerizations to high conversion, lack of oxygen inhibition, and low shrinkage and stress is an ideal combination for application of thiol‐enes as dental restorative materials.^86,87  In combination with methacrylate monomers in thiol‐ene‐meethacrylate systems, high modulus dental restorative materials are also achieved while maintaining the advantages of the thiol‐ene polymerization.^53,87–89  Interestingly, reductions in shrinkage stress for thiol‐ene‐methacrylate systems are greater than for traditional thiol‐ene systems. The increased reduction in shrinkage stress is resultant from a hybrid polymerization mechanism for thiol‐enes and methacrylates. In these systems, the reaction often proceeds in two relatively distinct stages; a first stage that is dominated by methacrylate homopolymerization and a second stage that is dominated by thiol‐ene polymerization.^90,91 

Resultant from the thiol‐ene step growth mechanism, thiol‐enes exhibit uniform network formation with low shrinkage and stress. Additionally, the gel point occurs at relatively high conversions relative to chain growth polymerizations followed by a rapid increase in crosslink density and modulus. Combined with the ability to overcome oxygen inhibition, these properties differentiate thiol‐enes from other photopolymerization techniques and lead to significant polymerization and material property advantages for a wide range of applications.

The incorporation of sulfur into thiol‐ene networks imparts a higher refractive index than comparable organic networks. The refractive index of thiol‐ene polymers formed from typical thiol and ene monomers (such as pentaerythritol tetra(3‐mercaptopropionate) and triallyl triazine trione) is commonly ∼1.54 without the incorporation of any aromatic substituents. In fact, thiol‐yne reactions have yielded polymers with refractive indexes up to 1.65 due to high concentrations of sulfur within the material.⁹² 

Much research has investigated the use of thiol‐enes in the field of electro‐optics. Thiol‐ene systems have been used in applications that vary from flexible display components, photonic crystals, lens components, nanofiber high‐oxygen‐barrier networks, nanopillars and microbridges.¹⁰  The uniform molecular weight development and delayed gel point of thiol‐ene polymerizations result in the formation of PDLCs and HPDLCs with diffraction gratings that have excellent diffraction efficiency, high switching speeds and low switching voltages that make them desirable for use in photonic lasers, spectrometers for chemical and biological sensors and nanostructured devices. There are more than 50 research publications investigating the use of PDLCs and HPDLCs.¹¹  Thiol‐ene polymerizations have been explored extensively as optical adhesives because of the outstanding optical clarity, high refractive index and ability to form nearly stress‐free materials that do not cause optical distortion.⁹³ 

Thiol‐ene polymers have been demonstrated as ideal substrates for imprint lithographic applications.^33,94–98  Polymeric microdevices have been fabricated with improved feature qualities and high aspect ratios without warping or stress induced surface wrinkling.^99,100  Similarly, thiol‐ene polymer derived ceramic devices have demonstrated improved curing and feature qualities.^101,102 

The uniform network formation of thiol‐ene polymers results in networks that are very homogeneous and thereby exhibit glass transition regions that are much narrower than comparable chain growth systems. Uniquely, the narrow glass transition regions lead to extremely efficient energy absorption and dissipation for systems with glass transition temperatures near to ambient temperature.^103,104  The narrow glass transition region also leads to distinct and rapid shape memory polymer actuation and superior shape retention.¹⁰⁵  The uniform networks can also be tailored to serve as excellent barrier membranes for gas transport.¹⁰⁶ 

Thiols are well known to exhibit a distinct odor that is often considered offensive. In some cases, the odor can be dealt with using traditional masking agents. Alternatively, eliminating thioglycolic acid or mercaptopropionic acid reduces the odor of thiols and a number of thiols are available with reported dramatic reductions in odor due to improved synthetic and purification procedures.^107,108  Thiols are also available from Yodo Chemicals with little to no odor. The odor issues associated with thiols are strongly correlated to the molecular weight of the monomers. Low molecular weight monomers such as ethane or butane dithiol exhibit extremely strong and offensive odors while higher molecular weight and commonly utilized trimethylolpropane tris(3‐mercaptopropionate) (MW=399) and pentaerythritol tetra(3‐mercaptopropionate) (MW=489) exhibit much milder odors, particularly when appropriately purified. Once polymerized, crosslinked thiol‐ene polymers that achieve high conversion exhibit minimal if any residual odor.

Thiol‐ene monomer systems polymerize extremely rapidly and also exhibit minimal inhibition by oxygen and other traditional quinone based stabilizers. As such, thiol‐ene systems can be rapidly polymerized with little to no inhibition period to form tack‐free surfaces with low concentrations of photoinitiator and low irradiation intensities. While this feature is ideal in formulating ultra‐fast and energy efficient systems, the drawback is that thiol‐ene mixtures may exhibit poor shelf stability at room temperature. Unstabilized shelf lives of thiol‐ene systems can range from hours to months with the most rapidly polymerizing thiol‐ene systems (vinyl ethers and norbornenes) often exhibiting the poorest shelf stability. Depending on the particular thiol and ene monomers utilized, there are numerous mechanisms resulting in poor stability. The thiol‐ene reactions can be base‐catalyzed for enes substituted with electron withdrawing groups, with the base‐catalyzed Michael addition reaction of thiols to acrylates being a very well known reaction. Decomposition of peroxide impurities results in generation of radicals that can initiate polymerization, while oxygen can initiate the polymerization through the ground‐stage charge transfer complex.^37,38 

Numerous investigations have examined strategies for stabilizing thiol‐ene systems including the use of vinyl radical scavengers,¹⁰⁹  phosphorous acid and hindered phenolic antioxidant stabilizers,¹¹⁰  thermal stabilizers such as pyrogallol, hydroquinone, catechol, and 1,2,3‐trihydroxybenzene,^111,112  sulfur and potassium iodine–iodine mixtures,¹¹³  hindered phenolics,^114–118  phosphines and triarylphosphites,¹⁰⁷  and phosphoric acid.¹¹⁹  Different inhibitors and combinations of inhibitors have varying effectiveness for different thiol‐ene monomer systems and must be evaluated on a case‐by‐case basis. Perhaps the most effective and simplest stabilizer for a broad range of thiol‐ene systems is the aluminium or ammonium salt of N‐nitrosophenylhydroxylamine.^116,120 

Traditional stabilizers such as pyrogallol, hydroquinones, and catechol improve the stability of thiol‐ene systems; however, rather than creating an initial polymerization inhibition period where the inhibitors are consumed before the reaction commences, they tend to act as polymerization retarders.¹¹¹  Additionally, stabilization often comes at the price of reduced polymerization kinetics and functional group conversion. Stabilizers are more effective in combination with acrylates, methacrylates, styrene derivatives, and conjugated 1,3‐dienes.¹⁰ 

Many polymer films degrade oxidatively when subjected to elevated temperature. Uniquely, thiol‐ene polymer films have been found to exhibit excellent thermal stability.¹²¹  Thiol‐ene films exhibited no weight loss after being held in air at 200 °C for 40 min and the onset of thermal decomposition was greater than 250 °C with a temperature ramp of 10 °C min^–1. In thiol‐ene networks, a thioether group is formed every time a thiol group adds across a double bond. Thioethers are known to act as antioxidant stabilizers for many commercial polymers when used in combination with radical scavengers.^122–124  In addition to oxidative stability thiol‐enes have also demonstrated excellent optical stability with minimal discoloration after exposure to 50 °C for three years (Norland Optical Products Literature, www.norlandproducts.com), which correlates to greater than 20 years stability at ambient conditions. Thiol‐ene films using monomers that do not contain aromatic groups have also been shown to exhibit low yellowing in simulated UV weathering experiments.¹⁰