Towards a controlled photopolymerization of dental dimethacrylate monomers: EPR studies on effects of dilution, filler loading, storage and aging.

Electron paramagnetic resonance (EPR) was used to study the kinetics of methacrylate radical formation in the monomer mixture 2,2-bis [4-(2-hydroxy-3-methacryloxyprop-1-oxy) phenyl] propane (Bis-GMA)/triethylene glycol dimethacrylate (TEGDMA), in the presence of a photo-initiator system (camphorquinone, CQ/N,N-dimethyl-p-toluidine, DET). Curing-time dependences on the filler (0-40 wt%) and TEGDMA content (15-90 wt%) were evaluated; the influence of irradiation protocol, uncured sample storage time and aging of cured systems were also studied. EPR enabled observing at least two different kinetic regimes during polymerization. The final radical concentration decreased both with Bis-GMA and filler content. However, a reverse trend was obtained when the relative photo-initiator concentrations were considered. Filler also showed a significant effect on the radical life-time reduction. Irradiation protocol and storage time of uncured matrices showed to affect the free radical concentration. The observed changes on the EPR signal lineshape with post-curing time suggests that the distribution of CH(2) conformations also changes with time.