A model polyurethane ionomer is synthesized with regularly spaced ionic groups along the polymer backbone. This model system allows study of the effect of ionic interactions on polymer solutions and to understand the behavior of salt-free polyelectrolyte solutions which suffer from the disadvantages of weak scattering and difficulty in filtration and purification. The ionomer has been studied in a variety of solvents by dynamic light scattering. The underivatized polyurethane exists as single chains in solution. The ionomer solution structure depends on the polarity of the solvent and solvent quality with respect to the ionomer backbone. In low-polarity solvents ionic aggregation takes place, and characteristic polyelectrolyte behavior is observed in polar solvents that are good solvents for the ionomer backbone. It is found that in toluene, a low-polarity solvent, the aggregates consist of physically cross-linked chains whereas in dimethylacetamide, a polar solvent, the aggregates consist of polyions and counterions held together due to electrostatic interactions. In very high-polarity solvents, which are usually poor solvents for the ionomer backbone, hydrophobic aggregation results in the formation of micelles consisting of polyol chains at the core with an outer ionic shell. The results provide useful information on the structure of salt-free polyelectrolyte solutions, some aspects of which have been considered controversial, and also explain the disappearance of polyelectrolyte behavior in very high-polarity solvents.