Irradiating a 2.8 mM solution of 1 in MA with light of 365 nm (1.5 W) led to full polymerization within only 15-19 min (Figure ). This polymerization process initiated by 1 is highly exothermic. A corresponding UV/vis spectral analysis demonstrated photoisomerization and consumption of the photoproducts of 1 in MA leading to polymerization until only remaining open 1 was left in the polymer matrix (Figure ). The UV/vis experiment was performed at a lower 5.9 10 -2 mM concentration of 1 in MA, which led to the necessity of prolonged light irradiation but allowed direct monitoring. The polymerization takes place between 1 h and 2 h 30 min at this concentration of 1 and it was visible that it starts along the area of the light beam entering the quartz cuvette. Localized polymerization is therefore possible up to a certain point if the concentration of 1 was drastically reduced (Figure ). Addition of an equimolar amount of diphenyliodonium hexafluorophosphate (DPI) to the mixture of 1 and MA did not enhance the polymerization speed when irradiating with light of 365 nm (1.5 W, Figure ). DPI is a cationic reductant capable of forming phenyl radicals. However, if the sample containing 2.8 mM 1 and 2.8 mM DPI in MA was simultaneously irradiated with 365 nm (1.5 W) and 450 nm (400 mW) light, full polymerization occurred within 10 min (Figure ). In this case, compound 1 acts as a photosensitizer and DPI as a radical source starting the polymerization. To prove the radical nature of the polymerization mechanism, we prepared samples containing 1 + 2,2,6,6-Tetramethyl(piperidin-1-yl)oxyl (TEMPO) in MA, and 1 + DPI + TEMPO in MA, and irradiated them with 365 nm light (1.5 W), as well as with a combination of 365 nm (1.5 W) and 450 nm (400 mW) light (Supporting Table , Supporting Figure ). The addition of TEMPO, a well-known quencher of radical polymerization of acrylic acids, successfully prevented the polymerization to PMA. The latter finding confirmed the radical nature of the polymerization mechanism.