Application of ionomers is often disturbed by their brittleness originating from limited stretchability of the network strands physically cross-linked by the ionic sites therein. Thus, an effective method of improving the ductility is to increase the length of network strands (and/or entanglements). Considering this point, this study examined linear viscoelasticity (LVE) and nonlinear elongational rheology of unentangled copolymers of hexyl methacrylate (HMA) and the ionic monomer sodium 4-vinylbenzenesulfonate hydrate (SSNa). The ionized SSNa mono- mer, being randomly distributed along the chain backbone at a concentration ranging from <1 to similar to 4 monomers per chain, served as the physical cross-link (or physical branching point). The LVE data showed a sol-to-gel transition, and the ductility of the sample turns out to be strongly related to the degree of gelation. Analysis of those data gave an average length of the network strands, and the ductility of the ionomer samples detected in the nonlinear elongational test was well correlated to this strand length in most cases. An exception was found for the sample slightly above the gel point: the ductility of this sample was much more significant than expected from the strand length, possibly due to the "pseudo-yielding" behavior that reflected exchange of the ionic, physical cross-links, and the resulting motion/displacement of the ionomer chains.