Hydrogen Bonding Interactions in Dental Resins

Common monomers in dental restorative resins, such as urethane dimethacrylate (UDMA) and Bis-GMA, exhibit extended hydrogen bonding interactions that account for their moderate to high viscosities. Objective: This investigation evaluates hydrogen bonding in dimethacrylate monomers and the effect these interactions may have on photopolymerization kinetics. Methods: Dilute carbon tetrachloride solutions of UDMA and Bis-GMA as well as the neat monomers were analyzed by mid-IR spectroscopy focusing on the respective N-H and O-H absorption regions. In near-IR monitored photocuring studies, three base monomers [Bis-GMA, UDMA and ethoxylated bisphenol A dimethacrylate (EBPADMA)] were copolymerized with either glycerol dimethacrylate (GDMA) or neopentyl glycol dimethacrylate (NPGDMA) as diluent comonomer. This gives comonomer pairs in which both, one or neither of the monomers can donate hydrogen bonds. Homopolymerizations were conducted to compare potential synergistic effects obtained in terms of polymerization rate and final conversion. Results: Lengthening of O-H bond in Bis-GMA due to hydrogen bonding with carbonyl and O-H groups is significantly greater than the analogous effect on the N-H in UDMA. This, along with structural constraints limiting Bis-GMA to intermolecular hydrogen bonding, likely accounts for the significantly higher viscosity of Bis-GMA compared with UDMA. Homopolymerizations of the diluent monomers showed GDMA had a higher polymerization rate, which can be explained by its greater hydrogen bond related viscosity, while the non-hydrogen bonding NPGDMA reached higher conversion due to less mobility restriction. In copolymerizations, all resins containing NPGDMA provided significantly greater rate increases over NPGDMA homopolymerization result than did GDMA resins relative to its homopolymerization. In addition, all the NPGDMA resins reached significantly higher final conversions than the GDMA resins. Conclusions: Synergistic improvements rather than additive effects were observed when the non-hydrogen bonding NPGDMA diluent monomer was combined with either Bis-GMA or UDMA as hydrogen bonding comonomers in photopolymerizations. Supported by NIDCR/NIST IA Y1-DE-1021-02