Hybrid Methacrylate/Thiol-ene System for Novel Dental Resins

Objectives: Step-growth thiol-ene photopolymeirzation has demonstrated increased polymerization rate and final conversion while providing reduced shrinkage stress over chain-growth methacrylate photopolymerization. However, insufficient mechanical properties and poor shelf-life stability considerably restrict thiol-ene application as dental filling materials. To overcome these disadvantages, hybrid systems that contain both methacrylate and thiol-ene monomers have been investigated. Methods: Ethoxylated bisphenol A dimethacrylate (BisEMA) and urethane dimethacrylate (UDMA) were combined with thiol-ene components that contain a tetrathiol (PETMP) with a triallyl triazine (TATATO). Hydroquinone monomethyl ether (MEHQ) and an aluminum salt of N-nitrosophenylhydroxylamine (Q-1301) were studied as stabilizers. Resin viscosity was monitored in accelerated shelf-life stability testing at 50 degC. Real-time, simultaneous shrinkage stress and double bond conversion were measured. Flexural strength and dynamic mechanical analysis were also characterized. Results: When PETMP/TATATO was incorporated with UDMA or BisEMA matrix resin, significantly increased stability against spontaneous polymerization was observed. PETMP/TATATO with 0.03 wt% Q-1301 gelled within two months, whereas for BisEMA/PETMP/TATATO system with 0.03 wt% Q-1301, viscosity only increased by 15% within six months. The photopolymerization kinetics study showed that the final double bond conversion was increased from 67% for BisEMA/TEGDMA to over 88% for BisEMA/PETMP/TATATO. Although the final conversion of BisEMA/PETMP/TATATO is much higher than that of BisEMA/TEGDMA, the maximum shrinkage stress developed was only about 1.5 MPa, as compared to 2.7 MPa for BisEMA/TEGDMA control system. Due to the step growth nature of thiol-ene polymerization, delayed gelation and vitrification of BisEMA/PETMP/TATATO was observed. Furthermore, our optimum BisEMA/PETMP/TATATO system has demonstrated much higher flexural strength (93.7±7.6 MPa) and flexural modulus (2.4±0.2 GPa) as compared to the PETMP/TATATO system (71.4±2.7 MPa and 1.6±0.3 GPa, respectively). Conclusions: Hybrid methacrylate/thiol-ene materials offer a new route to create formulations with not only enhanced shelf-life stability and mechanical properties, but also reduced shrinkage stress. Support: Confi-Dental Products Co.