Developing Low Shrinkage Stress Thiol-Ene Dental Resins & Two-color Photopolymerization Chemistries

Abstract

Photopolymerizations are an important class of chemical reactions that find ubiquitous use in our daily lives. The intimate interaction of light and matter in these systems allows unprecedented spatial and temporal control of polymerization lending these systems to innumerable applications in manufacturing of electronics, medical devices, and consumer products. This dissertation focuses on two of the most well-known applications of photopolymerizations: dental restorative composites and additive manufacturing.

To address the deleterious effects associated with polymerization-induced shrinkage stress in methacrylate dental resins, silane-based norbornyl-bearing thiol–ene oligomers were investigated for dental restorative resins. The glassy polymeric materials prepared here demonstrated glass transition temperatures (Tg) exceeding 100 °C, being on par with those of widely-used methacrylate polymers. Simultaneously, this approach yielded over 60% reduction in polymerization shrinkage stress relative to a thiol–ene standard. Subsequently, allyl sulfide addition-fragmentation chain-transfer chemistry was demonstrated to further lower polymerization-induced shrinkage stress of these high Tg polymers to over 80% reduction. The impact of this chemistry on polymerization shrinkage stress exceeded and eclipsed that of the pre-oligomerization, polymers without significant Tg reduction. To examine the potential for thiol–ene compatible adhesive monomers for adhesion of thiol–ene restorative materials to dental tissue and eliminate the need for methacrylate-based adhesives, acid-containing allyl-bearing monomers were prepared and evaluated for their impact on photopolymerization kinetics and thermomechanical properties.

Following the improvements to dental restorative composites, the development of two-color irradiation chemistries for application in additive manufacturing is discussed. Hexaarylbiimidazoles (HABIs) were discovered to act as efficient photoinhibitors in acrylate, methacrylate, and vinyl ether/maleimide chain-growth photopolymerizations. Photoinhibition was then demonstrated as an effective adhesion-prevention strategy in continuous stereolithographic additive manufacturing – which allowed for vertical print rates up to 2 m/hr, the highest reported print rates to date. Additionally, to counteract the deleterious effects of the long-lived transient inhibitor species, diffusion-limited HABIs were synthesized and tested. Sharper contrast in polymerization inhibition was achieved at the expense of inhibition efficacy. To broaden the scope of two-color irradiation chemistries in volumetric additive manufacturing, butyl nitrite was found to be a suitable photoinhibitor of free radical chain-growth photopolymerizations. Through this means, the efficacy of butyl nitrite to inhibit photopolymerizations under near-UV irradiation enabled volumetric confinement of photopolymerization through perpendicular irradiation. This unique chemistry was demonstrated to selectively photoinhibit free radical polymerizations in a dual-cure cationic and radical monomer system, paving the way for support-free stereolithographic additive manufacturing.