Ternary Thiol-Ene Systems as Low-Stress, High-Tg Dental Restorative Materials

Objectives: The objective of this work was to investigate ternary thiol-ene-methacrylate systems as dental restorative materials. Ternary thiol-ene-methacrylate systems combine advantages of both step-growth thiol-ene and chain growth methacrylate systems in that they exhibit rapid polymerization kinetics, low oxygen inhibition, high functional group conversion, low shrinkage and shrinkage stress, and excellent mechanical properties. Specifically, 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: Dynamic mechanical analysis was utilized to measure Tg. A materials testing system was utilized to measure flexural modulus. Shrinkage stress was measured with a modified tensometer capable of simultaneous shrinkage stress and double bond conversion measurements. Results: Commercially available thiol-ene systems are not equivalent to current dimethacrylate resin systems such as BisGMA/TEGDMA in regards to mechanical properties. To address this, we have synthesized and evaluated norbornene monomers. Thiol-norbornene systems were shown to exhibit glass transition temperatures in excess of 80 °C, however these systems do not exhibit Tgs that are in excess of Tcure and also exhibit low flexural modulus. Ternary thiol-norbornene-mechacrylate systems comprised of 30 wt% of a stoichiometric mixture of pentaerythritol tetra(3-mercaptopropionate) (PETMP) and trimethylolpropane trinorbornene (TMPTN) or bisphenol-A ethoxylate dinorbornene (BPAEDN), and 70 wt% EBPADMA were evaluated. PETMP/TMPTN/EBPADMA and PETMP/BPAEDN/EBPADMA systems exhibit Tgs that are in excess of 40 °C above Tcure and significantly improved flexural modulus. PETMP/TMPTN/EBPADMA system exhibits a final stress of 0.35±0.12 MPa, an 80% decrease in polymerization stress relative to BisGMA/TEGDMA. Conclusions: The dramatically reduced shrinkage stress, increased polymerization rate in the presence of oxygen, and significantly increased functional group conversion leading to decreased leachable species indicate potential benefits for the use of ternary thiol-ene-methacrylate systems as dental restorative materials. Supported by NIDCR F32 DE15906-01A2 and a corporate grant from Confi-Dental.