Abstract <p>A promising direction in the field of controlled polymer synthesis is photopolymerization via the organocatalyzed atom transfer radical polymerization (O-ATRP), which relies on the use of metal-free organic catalysts. The application of organic compounds as catalysts not only reduces the environmental impact but also enables the use of the obtained polymeric materials in industrial sectors sensitive to trace amounts of metals. Traditionally, heteroaromatic polycyclic compounds with an extended conjugated π-bond system, particularly aryl-substituted phenothiazines, are used as O-ATRP catalysts. This work investigated the possibility of polymerizing a series of methacrylic monomers via the O-ATRP mechanism in the presence of 10-(1-naphthyl)phenothiazine as a catalyst and various bromine-containing initiators under irradiation with 365 nm light. It was shown that the polymerization process is accompanied by an increase in the polymer molecular weight with conversion. The number-average molecular weights are consistent with theoretically calculated values. The use of a bifunctional initiator, ethylene bis(2-bromoisobutyrate), allows for the controlled synthesis of polymers with various molecular weights. The possibility of using the obtained polymer samples as macroinitiators for post-polymerization and block copolymerization, catalyzed by 10-(1-naphthyl)phenothiazine under UV irradiation, was demonstrated.</p>