Amphiphilic double-brush copolymers (DBCs) with each graft site quantitatively carrying both a hydrophilic poly(ethylene oxide) (PEO) graft and a hydrophobic polystyrene (PSt) graft are synthesized by sequential reversible addition-fragmentation chain transfer (RAFT) polymerization and ring-opening metathesis polymerization (ROMP). These DBCs are used as both surfactants and polyfunctional RAFT agents in the radical polymerization of St in aqueous dispersed media. Miniemulsions with narrowly dispersed St-based nanodroplets are readily obtained after ultrasonication of the reaction mixtures. Without the presence of crosslinker, chain-extension polymerization of St from the DBCs yields well-defined polymeric latexes with narrow size distributions. However, with the presence of divinylbenzene (DVB) as the crosslinker, vesicular polymeric nanoparticles are formed as the major product. Such crosslinking-induced change in morphology of the resulting latex nanomaterials may be ascribed to the increase of interfacial curvature in the heterophase systems during crosslinking polymerization. (c) 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2014, 52, 3250-3259