Covalent adaptable networks as dental restorative resins: Stress relaxation by additionfragmentation chain transfer in allyl sulfide-containing resins

Objectives. The aim is to demonstrate significant polymerization shrinkage stress reduction in model resins through incorporation of additionfragmentation chain transfer moieties that promote network stress accommodation by molecular rearrangement. Monomers containing allyl sulfide linkages are incorporated to affect the shrinkage stress that arises during photopolymerization of model resins that contain an initiator and dimethacrylates. Radical-mediated allyl sulfide additionfragmentation is enabled during polymerization. We hypothesize that allyl sulfide incorporation into methacrylate polymerizations promotes stress relaxation by enabling network adaptation. Methods. A 1:2 mixture of tetrathiol and allyl sulfide-containing divinyl ethers is formulated with glass-forming dimethacrylates and compared to controls where the allyl sulfide is replaced with a propyl sulfide that is incapable of undergoing additionfragmentation. Simultaneous shrinkage stress and functional group conversion measurements are performed. The Tg is determined by DMA. Results. Increasing allyl sulfide concentration reduces the relative stress by up to 75% in the resins containing the maximum amount of allyl sulfide. In glassy systems, at much lower allyl sulfide concentrations, the stress is reduced by up to 20% as compared to propyl sulfide-containing systems incapable of undergoing additionfragmentation chain transfer. Significance. Shrinkage stress reduction, typically accompanying free-radical polymerization, is a primary focus in dental materials research and new product development. Allyl sulfide additionfragmentation chain transfer is utilized as a novel approach to reduce stress in ternary thiol-ene-methacrylate polymerizations. The stress reduction effect depends directly on the allyl sulfide concentration in the given ternary systems, with stress reduction observed even in systems possessing super-ambient Tgs and low allyl sulfide concentrations. Published by Elsevier Ltd on behalf of Academy of Dental Materials.