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Introduction

Presently, composite restoratives suffer from several significant drawbacks associated with the chain growth nature of the methacrylate-based free radical polymerization process. These problems include the significant shrinkage that occurs during the polymerization process and the corresponding stress that arises due to early gelation, thermal expansion mismatch , moisture uptake by the sample following polymerization , the presence of extractable, unreacted monomer following cure , and inhibition of the polymerization by oxygen .

Recently, we have considered the implementation of dental restorative materials based on resins comprised of either binary thiol–ene or ternary methacrylate–thiol–ene compositions. Thiol–ene-based resins exhibit a step growth radical polymerization mechanism in contrast to the chain growth polymerization mechanism of (meth)acrylate systems. The thiol–ene polymerization mechanism comprises the addition of a thiyl radical to an ene functional group, followed by chain transfer to a thiol, thus regenerating the thiyl radical . Traditional ene monomers utilized in thiol–ene systems (allyl ether, vinyl ether, norbornene, etc.) are not homopolymerizable, resulting in a true step growth radical polymerization mechanism . As a result of the step growth polymerization mechanism, thiol–ene photopolymerizations have numerous kinetic advantages relative to the more common dimethacrylate-based materials. Thiol–ene polymerizations uniquely result in reduced volume shrinkage per double bond and a significantly delayed gel point conversion. The combination of reduced shrinkage and delayed gelation promotes significant reductions in shrinkage stress . Additionally, the unique polymerization mechanism also delays the vitrification process, resulting in delayed autodeceleration and ultimately leading to higher functional group conversions as compared to analogous chain growth systems. When oxygen interacts with either carbon or thiyl radicals to form a peroxy radical, chain transfer to thiol functional groups minimizes the effects of oxygen inhibition . Since thiol–ene systems exhibit reduced inhibition, initiation in these systems is more effective. This phenomenon is often evidenced by a lack of an inhibition period before the polymerization begins and can also be observed by achieving increased cure depths with minimal tacky layer formation on any air-exposed surface.

Concomitant with the reduction in volume shrinkage in thiol–ene systems is a reduction in crosslink density and therefore one of the drawbacks of thiol–ene systems is that they generally exhibit reduced mechanical properties relative to dimethacrylate-based systems . However, the use of thiol–enes as reactive diluents in ternary formulations (e.g. as a replacement for triethylene glycol dimethacrylate) uniquely results in a synergistic combination of both thiol–ene polymerization kinetics and shrinkage dynamics with dimethacrylate mechanical properties . Additionally, the methacrylate–thiol–ene systems often exhibit a pseudohybrid polymerization whereby the first stage is dominated by methacrylate homopolymerization and chain transfer to thiol and the second stage is dominated by thiol–ene polymerization . The hybrid nature of the methacrylate–thiol–ene polymerization results in even greater reductions in shrinkage stress without compromising mechanical properties. When compared to BisGMA/TEGDMA formulations, methacrylate–thiol–ene resins were demonstrated to exhibit increased methacrylate conversion, equivalent cure speed and flexural modulus, and decreased polymerization shrinkage stress .

In methacrylate–thiol–ene systems, the thiol functional groups are consumed by chain transfer from both the ene-centered radicals (allyl ether or norbornene) and the methacrylate-centered radicals. However, the ene functional groups are consumed only by thiyl radical propagation as they do not react with either of the vinyl-centered radicals in the polymerization. Therefore, the traditional 1:1 stoichiometry that is optimal for thiol–ene polymerizations results in low ene conversion and is suboptimal in regards to conversion and mechanism for ternary methacrylate–thiol–ene formulations. By increasing the thiol to ene ratio, the ene conversion and ultimately crosslinking are increased. Additionally, since volume shrinkage is proportional only to the amount of double bonds that react and is not dependent at all on the thiol reaction extent, as the thiol concentration is increased, there are fewer double bonds available to cause volume shrinkage . Thus, with increased thiol content, further reductions in shrinkage stress were obtained while maintaining mechanical properties and increasing overall functional group conversion .

For resin systems to achieve the desired property requirements and function as a dental restoration, they must be utilized as composites . Most dental restorative materials are comprised of 60–87 wt.% glass filler to improve mechanical properties and reduce volume shrinkage . Typically, the larger filler particles provide primary reinforcement as well as radioopacity to the composite while the smaller fumed silica nanoparticles enable increased overall filler loading along with an improved surface finish.

In this study, methacrylate–thiol–ene systems are evaluated as composite systems with glass fillers relative to composite dimethacrylate control resins where each of these resin formulations was filled to the same consistency with inorganic glass fillers. The composites were then initiated with visible light and evaluated for polymerization kinetics, volume shrinkage, depth of cure, water absorption and solubility, flexural modulus, flexural strength, and polymerization shrinkage stress. We hypothesize that the ternary methacrylate–thiol–ene composite systems will exhibit equivalent or improved performance relative to the dimethacrylate controls and that off-stoichiometric thiol to ene ratios would further enhance properties.