Thiol-Enes as Low-Shrinkage, Low-Stress, High-Tg Dental Restorative Materials

Objective: The objective of this work was to investigate the feasibility of formulating novel dental restorative materials that utilize a step-growth thiol-ene photopolymerization. Particularly, we are aiming to significantly reduce the polymerization shrinkage and shrinkage stress while retaining or improving physical properties as compared to current dimethacrylate-based systems. Methods: Fourier Transform Infrared spectroscopy was utilized to monitor polymerization kinetics. To measure shrinkage stress, a novel experimental setup capable of simultaneous shrinkage stress and double bond conversion measurement on exactly the same sample was utilized. Results: Due to their step growth mechanism, thiol-ene polymers result in polymer materials with significant reductions in shrinkage and dramatic reductions in shrinkage stress as compared to current methacrylate composites. The tetrathiol/triallyl thiol-ene system exhibits 6.8% volume shrinkage as compared to 7.3% for the BisGMA/TEGDMA system. The maximum shrinkage stress developed for the tetrathiol/triallyl system was approximately 14% of the maximum shrinkage stress of the Bis-GMA/TEGDMA system. Additionally, the maximum polymerization rate of the tetrathiol/triallyl system is six times faster and reaches 85% overall conversion as compared to only 60% conversion for Bis-GMA/TEGDMA cured under the identical irradiation/initiation conditions. While the initial kinetic, shrinkage and stress, and overall conversion results are encouraging, the performance of this commercially available thiol-ene system is not completely equivalent to the Bis-GMA/TEGDMA system in regards to mechanical properties. The glass transition of the thiol-ene system is 62 °C as compared to 75 °C for the Bis-GMA/TEGDMA system. To address this, we have synthesized and examined novel thiol and ene monomers that lead to improved cure behavior and mechanical properties. Conclusions: The dramatically reduced shrinkage stress, increased polymerization rate, and significantly increased functional group conversion leading to decreased leachable species indicate the potential benefits for the use of thiol-ene systems as dental restorative materials. Supported by NIDCR F32 DE15906-01A2.