Well-defined asymmetric amphiphilic ABA' block copolymers composed of poly(ethylene oxide) monomethylene ether (MPEO) with different molecular weights as A or A' block and poly(styrene) (PS) as B block were synthesized by the combination of atom transfer radical polymerization (ATRP) and click reactions. First, bromine-terminated diblock copolymer poly(ethylene oxide) monomethylene ether-block-poly(styrene) (MPEO-PS-Br) was prepared by ATRP of styrene initiated with macroinitiator MPEO-Br, which was prepared from the esterification of MPEO and 2-bromoisobutyryl bromide. Then, the azido-terminated diblock copolymers MPEO-PS-N-3 were prepared through the bromine substitution reaction with sodium azide. Propargyl-terminated MPEO with a different molecular weight was prepared under the basic condition from propargyl alcohol and p-toluenesulfonyl-terminated MPEO, which was prepared through the esterification of MPEO and p-toluenesulfochloride using pyridine as solvent. Asymmetric amphiphilic ABA' block copolymers, with a wide range of number-average molecular weights from 1.92 x 10(4) to 2.47 x 10(4) and a narrow polydispersity from 1.03 to 1.05, were synthesized via a click reaction of the azido-terminated diblock copolymers and the propargyl-terminated MPEO in the presence of CuBr and 1,1,4,7,7-pentamethyldiethylenetriamine (PMDETA) catalyst system. The structures of these ABA' block copolymers and corresponding precursors were characterized by NMR, IR, and GPC. (C) 2008 Wiley Periodicals, Inc. J Appl Polym Sci 111: 560-565,2009