AbstractA new photoredox catalyst is described wherein a phenothiazine dye in its excited state oxidizes a catalyst to a free radical. This free radical efficiently initiates the polymerization of metal acrylates giving rise to polymers which are insoluble in the monomer‐containing medium. The polymers precipitate as insoluble colloidal particles. Optical transmission measurements are used to follow the polymerization and to calculate the number and size of the polymer particles. The effect of various factors such as the nature of the catalyst and the photooxidant, the pH and the temperature of the medium on photopolymerization are described. It is indicated that this new system may be useful for rapid photographic imaging and display application