A new method to synthesize functionalizable branched poly(ethylene oxide) (PEO) is presented. Soluble linear poly(1,3-diisopropenylbenzene) (1,3-DIB) samples exhibiting one unsaturation per DIB unit in the chain were obtained through anionic polymerization. These unsaturations served, after reaction with a stoichiometric amount of cumyl-potassium, as efficient initiators for the anionic polymerization of oxirane. Accurate characterization of these samples demonstrated that a far better control of the functionality is reached as compared to the case of poly(DVB) cores.