This invention involves a derivative of an organic photocatalyst called 10-phenylphenothiazine (PTH). This derivative is conjugated to a thermally responsive gel to create Gel-PTH in a simple three-step synthesis. When Gel-PTH is swollen in a monomer-containing solution, it provides a transparent framework through which initiators and monomers can diffuse into and begin polymerizing via photo-CRP and heat-responsive reactions. When the Gel-PTH is removed from the solution, when the temperature is increased, or when the light is turned off, the polymerization reaction is effectively turned off. In this way, polymerization is controlled by three separate stimuli—light, heat, and presence of the photocatalyst. The Inventors tested Gel-PTH catalysis using different types of RAFT and ATRP initiators and seven different types of monomers—NIPAAM (N-isopropylacrylamide), DMA (dimethylacrylamide), nBA (n-butyl acrylate), MA (methyl acrylate), EGMEA (ethylene glycol methyl ether acrylate), HEA (2-hydroxyethyl acrylate), and IBA (isobutyl acrylate.) They were able to create at a large variety of polymers with molar masses, ranging from 14.8 kDa to 46.6 kDa, by adjusting the molar ratios of the monomer, initiator, and PTH, by using different solvents, and by increasing or decreasing the time of the reaction. Furthermore, the inventors demonstrated that the Gel-PTH catalyst could be easily recycled and re-used in multiple polymerization reactions with no loss of efficiency.