Hyperbranched polymers with numerous pendent norbornene functionalities have been synthesized via the radical polymerization of a novel asymmetrical divinyl monomer hearing a higher reactivity methacrylate group and it lower reactivity norbornene group. Mediated by a rapid reversible addition-fragmentation chain transfer (RAFT) equilibrium, the concentration of polymeric chain radicals is decreased, and thus the gelation did not occur until higher monomer conversions (ca. 90%). An increase in reaction temperature call also significantly promote the formation of the hyperbranched structure owing to the decreased stability of the intermediate radicals derived from the norbornene group, which was confirmed by a model copolymerization system of two single vinyl monomers with similar structures to the vinyl groups in the asymmetrical divinyl monomer. Furthermore, Tri-SEC and conventional Sin-SEC as well as H-1 NMR. analyses show that hyperbranched structures and higher molecular weights were formed at moderate-to-high monomer conversion stage, whereas the early polymerization stage only afforded linear polymers with a relatively narrower molecular weight distribution.