Photochemical modifications of high polymers by visible light

Methods have been developed in our laboratory to induce oxidation—reduction reactions with visible light using dyes as sensitizers. Applying this general technique to high polymers one can photochemically crosslink or decrosslink a polymer by changing the oxidation state of certain groups. Thus, gelatin containing dichromate ion, a dye, and a compound which serves as an electron donor for the light‐excited dye only becomes crosslinked by visible light. This is due to the photoreduction of chromate ion to chromic ion, the latter crosslinking the polymer. Similarly polyacrylamide which is crosslinked by complexation with mercuric ion and which is sensitized will decrosslink rapidly with visible light due to photoreduction of mercuric ion to mercurous ion. Mercaptans are photooxidized in the presence of sensitizing dyes. Hence polymers containing thiol groups can be crosslinked by dye‐sensitized photooxidation. This process can be reversed by far ultraviolet light.