The branching copolymerization of styrene with triethylene glycol dimethacrylate (tri-EGDMA) as the branching agent was carried out using atom transfer radical polymerization (ATRP) in anisole at 90 degrees C. The resulting copolymers were analyzed using H-1 NMR and triple detection size exclusion chromatography (TD-SEC). The NMR analysis shows that the pendent vinyl groups react even in the early stages of the polymerization. Analysis of the changes in the molecular weight and polydispersity of the copolymers suggests that the reaction system contains three components: the primary chains, the slightly branched chains comprising of two primary chains, and the highly branched chains consisting of more than three primary chains. The coupling reaction mainly takes place between the primary chains, resulting in the slightly branched chains in the early stages of the reaction, the weight fraction of the branched chains and the degree of branching increase gradually with monomer conversion, highly branched chains mainly form at relatively high monomer conversions.