We used real-time Fourier transform infrared to monitor the conversion of both thiol and ene (vinyl) functional groups independently during photoinduced thiolene photopolymerizations. From these results, the stoichiometry of various thiol-ene and thiol-acrylate polymerizations was determined. For thiol-ene polymerizations, the conversion of ene functional groups was up to 15% greater than the conversion of thiol functional groups. For stoichiometric thiol-acrylate polymerizations, the conversion of the acrylate functional groups was roughly twice that of the thiol functional groups. With kinetic expressions for thiol-acrylate polymerizations, the acrylate propagation kinetic constant was found to be 1.5 times greater than the rate constant for hydrogen abstraction from the thiol. Conversions of thiol-acrylate systems of various initial stoichiometries were successfully predicted with this ratio of propagation and chaintransfer kinetic constants. Thiol-acrylate systems with different initial stoichiometries exhibited diverse network properties. Thiol-ene systems were initiated with benzophenone and 2,2-dimethoxy-2-phenylacetophenone as initiators and were also polymerized without a photoinitiator.

Composite dental restorations represent a unique class of biomaterials with severe restrictions on biocompatibility, curing behavior, esthetics, and ultimate material properties. These materials are presently limited by shrinkage and polymerization-induced shrinkage stress, limited toughness, the presence of unreacted monomer that remains following the polymerization, and several other factors. Fortunately, these materials have been the focus of a great deal of research in recent years with the goal of improving restoration performance by changing the initiation system, monomers, and fillers and their coupling agents, and by developing novel polymerization strategies. Here, we review the general characteristics of the polymerization reaction and recent approaches that have been taken to improve composite restorative performance.

The mechanism and kinetics of thiol-ene photopolymerizations utilizing a tetrafunctional thiol monomer copolymerized with acrylate, norbornene, vinyl ether, and vinyl silazane functionalized ene monomers are successfully modeled and experimentally characterized. Modeling predictions demonstrate that the reaction orders in thiol-ene systems are controlled by the ratio of thiyl radical propagation to chain transfer kinetic parameters (k p/kCT). Ratios of kinetic parameters (kp/kCT) were found to vary significantly with the ene functional group chemistry and to have a dramatic impact on polymerization kinetics. For high ratios of kp/kCT, polymerization rates are first order in thiol functional group concentration and nearly independent of ene functional group concentration. For kp/kCT values near unity, polymerization rates are approximately 1 /2 order in both thiol and ene functional group concentrations. When kCT is much greater than kp, polymerization rates are first order in ene functional group concentration and nearly independent of the thiol functional group concentration. In thiol-allyl ether and thiol-acrylate systems, the step growth polymerization rates are first order in thiol functional group concentration (Rp ∝ [SH]). For thiol-norbornene and thiol-vinyl ether systems, polymerizations are nearly 1 /2 order in both thiol and ene functional group concentrations (Rp ∝ [SH] 1/2 [CdC] 1/2 ). In thiolvinyl silazane systems, polymerization rates are approximately first order in ene functional group concentration (Rp ∝ [CdC]) and independent of thiol functional group concentration. A theory is proposed which states that the effect of functional group chemistry on kp/kCT is controlled primarily by ene functional group electron density (kp) and carbon radical stability (kCT).

A thiol monomer is shown to copolymerize with vinyl ether, allyl, acrylate, methacrylate, and vinylbenzene monomers. These thiol−ene polymerizations are photoinitiated without the use of photoinitiator molecules. It is seen that the polymerization proceeds more readily when initiatorless samples are irradiated with light centered around 254 nm as compared to 365 nm light. To demonstrate resistance to oxygen inhibition, thin polymer films of 3−15 μm are polymerized while exposed to ambient air. Without photoinitiator molecules present, light is attenuated only by the monomer and polymer. This feature leads to greater penetration of ultraviolet light and allows for the polymerization of extremely thick polymers. Thick cures of up to 25 in. are obtained using a thiol−vinyl ether system.