To obtain highly branched graft copolymers, styrene (St) was grafted onto poly(vinyl p-nitrobenzoate) (PVNB) as a trunk polymer through the chain transfer reaction of growing polystyrene (PSt) radicals to the pendent aromatic nitro groups on the trunk polymer. The number of PSt branches increased with St concentration at constant concentrations of PVNB and azobisisobutyronitrile (AIBN) as an initiator, and decreased with AIBN concentration at constant PVNB and St concentrations. The maximum number of branches attained was 43 (Pn of PVNB was 970), which corresponds to 23 monomer units of PVNB per PSt branch. It is confirmed from the results of infrared spectroscopy that the addition of the growing polystyrene radicals occurs not at the benzene rings but at the nitro groups on the benzene rings. Polymerization of St was also carried out in the presence of isopropyl p-nitrobenzoate (IPNB) as a model compound of PVNB. IPNB was found to retard the polymerization of styrene more strongly than PVNB. The chain transfer constant of the polystyrene radicals to IPNB was more than twice as large as that to PVNB.