| Thiol-ene polymerizations react via a step growth mechanism between thiol and vinyl functional groups. Investigation of thiol-olefin reactions have been conducted by several groups, leading to the application of thiol-ene resins and polymers as adhesives, electronic coatings, flooring, and optical fiber coatings. More recently, attempts to overcome material property limitations and significant polymerization stress associated with acrylate polymerizations have sparked a renewed interest in thiol-ene polymerizations. This resurgence of thiol-ene polymerizations has lead to an improved understanding of thiol-ene polymerization kinetics and network structure, demonstrating their rapid kinetics, advantageous mechanical properties, lack of oxygen inhibition, and delayed gelation. Furthermore, current investigations have demonstrated the possibilities of photoinitatorless thiol-ene polymerizations, as well as demonstrated reduced polymerization shrinkage and stress when compared with methacrylate polymerizations. Despite the many studies of thiol-vinyl polymerizations, the relationship between monomer chemistry or reaction methodology and property development, particularly polymerization stress, in thiol-ene polymerizations is not thoroughly understood.; To improve the fundamental understanding of monomer chemistry on property development, novel vinyl and thiol monomers were synthesized and evaluated. The results indicated that norbornene functionalized vinyl monomers enhance polymer Tg as compared to vinyl monomers of similar backbone chemistry, as well as produce lower polymerization shrinkage and stress. In addition, thiol monomers containing carbamate functional groups were demonstrated to have improved glass transition temperatures due to hydrogen bonding and exhibit reduced vinyl homopolymerization. Secondly, oligomerization was investigated as a means to reduce polymerization shrinkage and stress by reducing the concentration of reactive vinyl functional groups in the polymerizing resin. The results showed that oligomerization is a useful reaction scheme for reducing resin polymerization stress, with specific application to the preparation of thiol-ene dental restorative materials. In addition, experimental and modeling results indicated that the stress development in thiol-ene resins is highly dependent on the extent of conversion, and thereby the modulus development. Furthermore, the extent of stress relief by oligomerization was demonstrated to be a function of monomer functionality and resin modulus.; Finally, hybrid thiol/vinyl ether and thiol-ene/thiol-epoxy polymerizations were explored as a method in which to develop thiol-ene materials with tailorable properties and low polymerization stress. Synergistic material properties and controlled network development were demonstrated in hybrid thiol/vinyl ether polymers using varied initiation conditions. In addition, the monomer ratio in thiol-ene/thiol-epoxy hybrid polymerizations was demonstrated to affect resin conversion and mechanical properties. As compared to current resins used in dental restorative, thiol-ene/thiol-epoxy polymerizations were shown to have similar Tg and flexural modulus while significantly reducing polymerization stress. |
