The polymerization behavior and photoinitiation process in the visible light‐cured dental composite resins were studied using ESR and IR spectroscopies. The concentration of methacryl propagating radical corresponded to the transmittance of irradiated light. While the radical was stable and long‐lived in the cured resin, its concentration quickly decreased in the oral cavity due to the ventilation of atmospheric oxygen accompanied by respiration. Camphorquinone radical in the presence of tertiary amine easily initiated the radical polymerization. A decrease in monomer consumption was explained by the inhibition effect of oxygen in the photoinitiation process. Post‐curing by the residual radical proceeded effectively in the deeper part of the resin.
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