The synthesis of an amphiphilic triarm star copolymer based on polystyrene, poly(ethylene oxide) and poly(epsilon-caprolactone) blocks has been achieved by a novel strategy which consists in the preparation of a diblock copolymer, polystyrene-block-poly(ethylene oxide), having a protected anionic initiator group at the junction of the two blocks. After deprotection, this function is activated by a coloured and weakly basic carbanion. The generated alcoholate initiates the epsilon-caprolactone anionic polymerization.