Core crosslinked star (CCS) polymers, which have crosslinked poly (divinyl benzene-co-styrene) [P(DVB-St)] core and multiple arms of polystyrene-b-poly(ethylene oxide) diblock copolymer (PEO-b-PS) [denoted as PEO-b-PS/P(DVB-St) CCS], were synthesized via atom transfer radical polymerization(ATRP). PEO-b-PS/P(DVB-St) CCS polymer was spherical with average diameters of scores of nanometers from transmission electron microscopy (TEM) and dynamic light scattering (DLS), and blended with diglycidyl ether of bisphenol (DGEBA) and 4,4'-diamino diphenyl methane (DDM) in tetrahydrofuran (THF). With 5 or 10 wt% PEO-b-PS/P(DVB-St) CCS polymer, spherical core-shell nanodomains with average diameters of 29 or 32 nm were observed from atomic force microscopy (AFM), which were randomly distributed in the resultant thermosets. Considering the difference in miscibility of the epoxy with P(DVB-St) and PEO-b-PS after and before curing reaction, a reaction-induced microphase separation (RIMPS) mechanism was proposed to account for the formation of the core-shell nanodomains in the thermosets. During curing, the RIMPS of PS subchain occurred but was confined by P(DVB-St) core, resulting in formation of thermoplastic PS shell around the crosslinked core. Such core-shell nanodomain could be easily etched away by THF, whereas the control thermosets containing PEO/P(DVB-St) CCS polymer could not be etched by THF. The glass transition temperatures (T(g)s) of the epoxy thermosets containing PEO-b-PS/P(DVB-St) CCS polymer were significantly improved compared with pure epoxy thermosets. (C) 2010 Elsevier Ltd. All rights reserved.