We report the anionic synthesis of poly(ethylene oxide-b-isoprene-b-ethylene oxide) (OIO′) triblock copolymers and poly(ethylene oxide-b-styrene-b-butadiene-b-ethylene oxide) (OSBO′) tetrablock terpolymers with asymmetrically sized poly(ethylene oxide) (PEO) end blocks using triisopropylsilyloxy-1-propyllithium (TIPSOPrLi) as a protected initiator and diphenylmethyl potassium (DPMK) as the base activator for EO polymerization. All final products and precursors were characterized by size exclusion chromatography (SEC) and nuclear magnetic resonance (NMR) techniques confirming monomodal and narrow molecular weight distributions and controlled molar masses. The growth of independently controlled PEO blocks was additionally demonstrated by the characterization of residual PEO blocks obtained from the ozonolysis of OIO′.