Abstract The photochemically initiated polymerization offers distinct advantages in controlling the molecular weights and the polydispersity of PMMA-prepolymers. Such prepolymers possessing a polymer content of up to 20% per weight are of a steadily increasing importance in industrial “casting-processes”. Experiments were performed in a batch reactor equipped with a medium pressure Hg arc, and the influence of initiator concentration, reaction temperature, and reaction time on rate of conversion and polydispersity was investigated. Benzoin (BN), benzoin-methylether (BME), and Irgacure 651 ® (IGC) were employed as photo-initiators. Quite similar results were observed for all three initiators, although their quantum yields of initiation are distinctly different. The starting concentration of the initiators was the most decisive factor effecting the prepolymer molecular weights ( M ̄ n and M ̄ w ) and, consequently, also the polydispersity. The experimentally observed behavior may be explained by the non-uniform distribution of the photochemically generated radicals, causing a significant broadening of the molecular weight distribution (MWD). The reaction temperature was investigated at 25 and 40 °C and was found to be of lesser effect on the molecular weight distribution.