A new photocatalyzed atom transfer radical polymerization (ATRP) procedure starting directly from a copper(II) bromine/phenanthroline (phen) mixture in the presence of triethylamine as a reducing agent is described. Under the irradiation of a compact blue LED lamp, the polymerization of methyl methacrylate (MMA) conducted to PMMAs with narrow molecular weights distributions (M-w/M-n similar to 1.10). The good chain end fidelity of the products was validated in subsequent chain-extension experiments, using them as macroinitiators, either by conventional thermal ATRP or by photocatalyzed ATRP. The efficient reinitiation under light irradiation was also evidenced by a "light ON/OFF" experiment. The respective effects of several parameters on the polymerization kinetics were studied, including light intensity, the nature of the solvent, the molar ratio of the ligand, and the nature of the counterion. Besides the essential generation of the excited species [Cu(phen)(2)](+)*, which will undergo an oxidative quenching as the key step of this photocatalytic cycle, supplementary investigations by UV-vis spectroscopy revealed an additional role of light, which also favored the regeneration of the activator. This complementary contribution may consist in a light-triggered exchange of ligands involving minor Cu(II) species, which absorb light in the blue wavelengths domain and are in equilibrium with [Cu(phen)(2)Br](+) as the predominant Cu(Il) complex. Interestingly, this photocatalyzed ATRP mechanism exhibited a good tolerance to oxygen and inhibitors, as demonstrated by the efficient synthesis of PMMAs with relatively narrow molecular weights distributions (M-w/M-n < 1.30) in the presence of air and/or 4-methoxyphenol (MEHQ).