A bifunctional coupler bearing an azetidinium and an aminochlorohydrin group was synthesized in water as solvent starting with piperazine and epichlorohydrin. Conversion of the bifunctional coupler in aqueous solution with primary and secondary amines (hexylamine and diethylamine) were studied as a model reaction. Using the established reaction conditions, azetidinium-functionalized polytetrahydrofurans were prepared in a controlled way by reacting the coupler in water as solvent with amine-functionalized polytetrahydrofurans.