Water-soluble 12,12-ammonium ionenes were prepared via the Menschutkin reaction from 1,12-dibromododecane and 1,12-bis(N,N-dimethylamino)dodecane. A stoichiometric imbalance of monomers controlled the final molecular weights of the polymers. The absolute molecular weights were determined for the first time using an online multiangle laser light scattering (MALLS) detector in aqueous size exclusion chromatography (SEC). Weight-average molecular weights ranged from 4300 to 20 900 g/mol. Relationships between weight-average molecular weight and mechanical properties were established for a series of 12,12-ammonium ionenes using both tensile testing and dynamic mechanical analysis (DMA). Tensile analysis of the higher molecular weight ionenes revealed an average tensile strength of 20 MPa and elongations ranging from 230 to 440%. Dissociation of ionic aggregates was observed at 85-88 degrees C in DMA experiments, and the glass transition temperatures increased with increasing molecular weight (61-88 degrees C). X-ray scattering revealed an amorphous polyethylene peak at approximate to 14 nm(-1) and a sharp ionic group correlation peak at 4.38 nm(-1). These correlations agreed well with the proposed macromolecular structure.