AbstractInteresting phenomena were observed in the polymerization of styrene and ethylene oxide into PEO‐PSt‐PEO triblock copolymers using potassium naphthalene as initiator and tolusene as a solvent. The relationship between the phenomena and molecular weight of the blocks was studied. The purified and characterized triblock copolymer showed mesomorphic behavior in toluene solution which depolarized polarized light increasingly as the concentration was increased. The structure of the triblock copolymer was also studied by small‐angle laser light scattering and by electron micro