Photopolymerization experiments and properties of some urethane/urea methacrylates tested in dental composites

Hydrophobic silyl-urea/urethane methacrylates (UHM-1, UHM-2) and a modified BisGMA derivative (BisGMA-UMA) were synthesized and used as photopolymerizable co-monomers in the preparation of dental composites, besides other urethane dimethacrylates containing oligoethylene oxide segments and carboxylic groups. The homo- and copolymerization behaviour of the synthesized monomers were investigated by FTIR spectroscopy using camphorquinone (CQ) and 4-(dimethylamino)-phenylacetic acid (DMPheAA) as a photoinitiating system, as well as by photo-DSC. The first hybrid monomethacrylate, N-(methacryloyloxyethyl-N’-diethoxymethylsilylpropyl)-urea showed the highest value for the maximum polymerization rate (0.117 s-1) followed by BisGMA analogue (0.085 s-1) and N-methacryloyloxyethyl-N’-bis(triethoxysilylpropylcarbamoyloxyethyl)-urea (0.069 s-1), whereas the degree of conversion decreased from 75.1% (UHM-2) to 41.6% (BisGMA-UMA). Polymerization shrinkage (PS) measured by picnometry for the mixture BisGMA-UMA/TEGDMA (4.78 vol.%) was lower than that exhibited of the control organic matrix BisGMA/TEGDMA (6.85 vol.%) at the same dilution. The addition of one of the obtained monomers into a mixture based on urethane oligomers led to a polymerization shrinkage varying between 8.33 and 6.97 vol.%. A number of properties of the composites formulated with our urethane methacrylates, a urethane dimethacrylate, BisGMA, and glass filler (Zr/Sr glass, quartz, and diatomite) were also determined including the surface damage caused by the Vickers indentation to establish their utility in dentistry.