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A 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 applications.Dye-sensitized photopolymerization of acrylic and vinyl monomers has been studied by many investigators (1-22). It has been suggested that the use of visible radiation in photochemical sequences leading to polymerization required the presence of an electron donor and oxygen (2, 15, 23-26). Most investigators have not recognized the importance of direct photochemical generation of the free-radical initiator. Usually, studies have centered around initiator systems which required multiple reactions or whose free radicals were very inefficient for initiating polymerization.Dyes of certain classes, in particular, the phenothiazine dyes, are strong oxidants in their photoexcited state (probably the triplet state). Redox catalyst initiation of polymerization (27) has been shown to be a very efficient means of polymerizing vinyl and acrylic monomers. Photochemically induced redox catalysis, referred to here as "photoredox catalysis," has proved useful to initiate polymerization directly, and with efficiencies which are greater than any heretofore reported for dye-sensitized photopolymerization. The present paper deals with a photoredox catalyst system consisting of a phenothiazine dye photooxidant, such as methylene blue and thionine, and an organic sulfinic compound, such as p-toluenesulfinic acid. The system does not require oxygen to function efficiently, but on the other hand, it is not particularly adversely affected by the presence of oxygen. Organic sulfinic acids, particularly when activated by aromatic amines, have been proposed and studied for initiating the polymerization of vinyl and acrylic monomers (28, 29). The system described in this paper, along with results presented in other papers of this series ( l ) , has led to dry-processed photographic techniques and information storage and retrieval systems. EXPERIMENTAL MATERIALS AND METHODSBarium diacrylate was prepared by neutralizing distilled acrylic acid with a distilled water solution of barium hydroxide octahydrate. The pH was usually adjusted to 5.0 to 7.0 with excess acrylic acid and the solution filtered. In some preparations the solution was concentrated under vacuum, and in other preparations undistilled acrylic acid was used, followed by treatment with activated carbon and filtration. Generally, the concentratio...