We studied, with a kinetic approach, the living nature and degree of branching (DB) of the formation of hyperbranched polystyrene by the free-radical photopolymerization of 2-(N,N-diethyldithiocarbamyl)methylstyrene in benzene. The free-radical polymerization proceeded with a living radical mechanism. Subsequently, we treated the kinetics of the initiation and propagation steps of the active A* and B* (side-group) sites with model compounds. The degradation rates of two types of dithiocarbamate groups at the A and B sites agreed well with theoretical trends for C-S bond dissociation energies predicted by the density functional theory for model compounds. The reactivity of the initiating B* groups was greater than that of the propagating A* groups. DB of the hyperbranched polymers was estimated to be 0.31 with both reaction rates in terms of Muller's equation. This result supported the DB value (0.39) estimated from NMR data. The hyperbranched molecules obtained in this work were not perfectly dendritic but had somewhat defective structures. (c) 2005 Wiley Periodicals, Inc.