Hard soft hard triblock copolymers based on poly(ethylene oxide) (PEO), poly(2-naphthyl glycidyl ether)block-poly[2-(2-(2-methoxyethoxy)ethoxy) ethyl glycidyl ether]-block-poly(2-naphthyl glycidyl ether)s (PNG-PTG-PNGs), were synthesized by sequential ring-opening polymerization of 2-(2-(2-methoxyethoxy)ethoxy)ethyl glycidyl ether and 2-naphthyl glycidyl ether using a bidirectional initiator catalyzed by a phosphazene base. Four PNG-PTG-PNGs had different block compositions (f(wt,PNG) = 9.2-28.6 wt %), controlled molecular weights (M-n = 23.9-30.9 kDa), and narrow dispersities (D = 1.11-1.14). Most of the PNG-PTG-PNG electrolytes had much higher Li+ conductivities than that of a PEO electrolyte (6.54 x 10(-7) S cm(-1)) at room temperature. Eespecially, the Li+ conductivity of PNG(18)-PTG(107)-PNG(18) electrolyte (9.5 X 10(-5) S cm(-1) for f(wt,PNG) = 28.6 wt %) was comparable to one of a PTG electrolyte (1.11 X 10(-4) S cm(-1)). The Li+ conductivities of PNG-PTG-PNG electrolytes were closely correlated to efficient Li+ transport channels formed by the microphase separation into soft PTG and hard PNG domains.