SYNOPSISElastomeric polymer consisting of poly ( oxytetramethylene ) segments and viologen units was prepared by living cationic polymerization of tetrahydrofuran ( T H F ) , followed by the end capping of the living polymer by 4,4'-bipyridine. The obtained ionene-type polymer ( P T V ) exhibited low tensile modulus and high tensile strength with large extensibility, i.e., it is a n excellent elastomer from its mechanical properties. The water swelling behavior and effects of water and the sort of counter anions on the tensile properties of PTV were investigated. PTV showed high hydrophilicity; e.g., the water content of PTV with bromide counter anions was ca. 40% after swelling in water a t 25°C for 24 h. However, even for the samples with such a high water content, PTVs displayed good elastomeric properties, although the tensile modulus and tensile strength were decreased with increase of water content. The hydrophilicity of PTV was found to depend on the kind of counter anions, which in turn determined the tensile properties of PTVs in a swollen state.