Focusing on the primary experimental variables that affect the polydispersity of poly(methyl methacrylate) during its photochemically initiated production, the effects of initiator (benzoin) concentration and flow rate of the reaction mixture were evaluated. The results led to the design and construction of a new reactor concept providing the enhanced control of the production rate of initiator radicals. The immersion-type photochemical reactor was equipped with a pulsed XeCl excimer radiation source emitting at 308 nm, which could be operated within a wide range of frequencies (840 Hz to 46 kHz). The effects of pulse frequency, initial initiator concentration, and flow rate on the molecular weight distribution of the pre-polymer (polydispersity) were evaluated by performing a series of experiments based on a three-dimensional experimental design (Doehlert matrix) and corresponding polynomial models. The results of the response surface analyses showed that the chosen variables are indeed controlling the production rate of initial radicals, which in turn is the principal factor effecting the pre-polymer polydispersity.