Synthesis and Characterization of Methacrylate-Functionalized Betulin Derivatives as Antibacterial Comonomer for Dental Restorative Resins

Secondary caries is the primary cause of composite restoration failures, resulting from marginal leakage and bacterial accumulation in the oral environment. Antibacterial dental composites, especially antibacterial monomers, have emerged as a promising strategy to inhibit secondary caries, which is pivotal to prolonging the lifespan of dental restorations. In this work, monomethacrylate- and dimethacrylate-functionalized betulin derivatives (M1Bet and M2Bet) were synthesized via an esterification reaction and served as antibacterial comonomers to develop novel dental resin formulations, in which M1Bet and M2Bet were incorporated to partially or completely replace bisphenol A glycerolate dimethacrylate (Bis-GMA). The control resin was a mixture based on Bis-GMA and tri(ethyleneglycol) dimethacrylate (TEGDMA) with a weight ratio of 50:50 (5B5T). The effect of the resin compositions and the chemical structures of M1Bet and M2Bet on the rheology behavior, optical property, polymerization kinetics, mechanical performance, cell viability, and antibacterial activity of dental resins were systematically investigated. Among all materials, the 1M2Bet4B5T resin with 10 wt % substitution of Bis-GMA by M2Bet exhibited comparable viscosity, higher light transmittance, improved degree of conversion, and mechanical properties compared with 5B5T. After incubation for 24 h, this optimal resin also possessed the best antibacterial activity against Streptococcus mutans, which had a significantly lower bacterial concentration (1.53 × 109 CFU/mL) than 5B5T (9.03 × 109 CFU/mL). Introducing betulin-based comonomers into dental resins is a potential strategy to develop antibacterial dental materials without sacrificing physical–mechanical properties.