Effect of side-group methylation on the performance of methacrylamides and methacrylates for dentin hybridization

The stability of the bond between polymeric adhesives to mineralized substrates is crucial in many biomedical applications. The objective of this study was to determine the effect of methyl substitution at the α- and β-carbons on the kinetics of polymerization, monomer hydrolytic stability, and long-term bond strength to dentin for methacrylamide- and methacrylate-based crosslinked networks for dental adhesive applications. Methods: Secondary methacrylamides (α-CH₃ substituted = 1-methyl HEMAM, β-CH₃ substituted = 2-methyl HEMAM, and unsubstituted = HEMAM) and OH-terminated methacrylates (α- and β-CH₃ mixture = 1-methyl HEMA and 2-methyl HEMA, and unsubstituted = HEMA) were copolymerized with urethane dimethacrylate. The kinetics of photopolymerization were followed in real-time using near-IR spectroscopy. Monomer hydrolysis kinetics were followed by NMR spectroscopy in water at pH 1 over 30 days. Solvated adhesives (40 vol% ethanol) were used to bond composite to dentin and microtensile bond strength (μTBS) measured after 24 h and 6 months storage in water at 37 °C. Results: The rate of polymerization increased in the following order: OH-terminated methacrylates ≥ methacrylamides > NH₂-terminated methacrylates, with minimal effect of the substitution. Final conversion ranged between 79% for 1-methyl AEMA and 94% for HEMA. 1-methyl-HEMAM showed the highest and most stable μTBS, while HEMA showed a 37% reduction after six months All groups showed measurable degradation after up to 4 days in pH 1, with the methacrylamides showing less degradation than the methacrylates. Additionally, transesterification products were observed in the methacrylamide groups. Significance: Amide monomers were significantly more stable to hydrolysis than the analogous methacrylates. The addition of a α- or β-CH₃ groups increased the rate of hydrolysis, with the magnitude of the effect tracking with the expected base-catalyzed hydrolysis of esters or amides, but opposite in influence. The α-CH₃ substituted secondary methacrylamide, 1-methyl HEMAM, showed the most stable adhesive interface. A side reaction was observed with transesterification of the monomers studied under ambient conditions, which was not expected under the relatively mild conditions used here, which warrants further investigation.