A macro-ate initiator (MAI), prepared by polycondensation of azobiscyanopentanoyl chloride (ACPC) and polyethyleneglycol having average molecular weight of 1000 (PEG1000), was found to show enough surface activity to be usable as an initiator/surfactant (inisurf). Emulsion polymerization of styrene (St) using this MAI as inisurf was carried out and (AB)(n)-type poly(PEO-b-PSt) block copolymer (where n represents the block-multiplicity) was obtained. In comparison with solution copolymerization, the emulsion block copolymerization gave PEO-b-PSt with higher molecular weight due to increased block multiplicity.