We studied the living nature on the formation of hyperbranched poly(methacrylic acid ester) by free radical photopolymerization of 2-(N, N-diethyldithiocarbamyl)ethyl methacrylate (DTEM) in benzene. We performed the first-order time-conversion plots in this polymerization system, and the straight line in the semilogarithmic coordinates indicated that the polymerization is first order in monomer. The number-average molecular weight (M-n) of hyperbranched polymers increased with increasing conversion. The free radical polymerization proceeded with controlled radical mechanism. Subsequently, we treated the kinetics of initiation and propagation steps of the active A* and B* (side group) sites using model compounds. The degradation rates of two types of dithiocarbamate (DC) groups at A and B sites agreed well with theoretical trends in C-S bond dissociation energies predicted by the density functional theory for model compounds. The reactivity of the initiating B* was less than that of propagating A* groups. The degree of branching of hyperbranched polymers was discussed using both reaction rates.