One-pot synthesis of poly(oxytetramethylene) ionene (POI) composed of one dimethylammonium group in each repeating unit is described, POI was prepared by using the cationic polymerization of tetrahydrofuran, followed by the chain extension reaction of living poly(oxytetramethylene) (POTM) chain with N,N-dimethylaminotrimethylsilane. The weight average molecular weight of the ionene with bromide counter-anion (POI-Br) was 48,000 g/mol and the molecular weight of POTM between the ionic sites was ca. 2100. POI-Br showed polyelectrolyte behavior in polar solvent. The elastomeric film of POI-Br was prepared: its tensile strength at break and elongation at break were ca. 11 MPa and ca. 1900% at 19 degreesC, respectively. Small-angle X-ray scattering, differential scanning calorimetry and dynamic mechanical analysis suggested the formation of microphase-separated structure for POI-Br film.