The performance and longevity of polymer composite dental restorative materials is still limited, primarily due to complications from shrinkage induced polymerization stresses. Other drawbacks involve a lack of toughness, thermal expansion mismatch, moisture uptake following polymerization, extractable, unreacted monomer following cure, and oxygen inhibition. Shrinkage induced stresses are primarily associated with the chain growth nature of the methacrylate-based free radical polymerization process that leads to high volume shrinkage and early gelation. Shrinkage stress is alleviated either by reducing the concentration of reactive methacrylate functional groups or delaying gelation during polymerization. We propose a radical shift in the polymerization mechanism from a chain growth polymerization to a mixed mode step-chain growth polymerization. This shift is achieved by the incorporation of a thiol-ene component as the reactive diluent, which has two distinct advantages. The thiol-ene polymerization mechanism results in lower volume shrinkage and delayed gelation. Additionally, we propose the incorporation of molecular fillers of controlled architecture and size. Appropriately fabricating, designing and incorporating molecular fillers will reduce the concentration of reactive groups and increase the filler loading beyond the present levels with corresponding mechanical property and shrinkage benefits. Since the reactive functionality is incorporated throughout the molecular filler rather than only at the surface, the molecular filler will be better integrated before polymerization, which prevents aggregation, and following polymerization, which improves toughness and other mechanical properties. Synergistically combining these two developments into a composite system will result in composite systems that exhibit both dramatically reduced shrinkage and stress and improved mechanical properties.
These aims are predicated on the hypothesis that appropriate materials synthesis and design coupled with optimization of the polymerization mechanism will lead to enhanced polymeric dental composites with respect to shrinkage stress, composite mechanical properties, polymerization speed (or reduction in the initiator content), moisture uptake, reduced extractables and improved biocompatibility. Results to date have already demonstrated a higher double bond conversion, lower volume shrinkage induced stress, improved mechanical properties, and near elimination of oxygen inhibition.
Because of the widespread use of esthetic composite dental restorative materials, significant improvements in their reliability and performance will have a very broad and positive effect on the oral health of the general public. Improvements in these materials to date have been driven by modifications in the filler phase where this application addresses the continuing deficiencies of the polymer phase through a novel chemistry approach.
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