Density-functional calculations on a single, isolated, infinite, periodic chain of poly(cyanoacetylene) have been performed. Thereby more different structures have been considered, partly differing in bond-length-alternation patterns and partly differing in whether the system has a polyacetylene-like or a polyacene-like structure. It is found that all structures with a bond-length alternation are stable, that the systems are semiconductors, and that the polyacene-like structures are the stablest ones. Finally, the consequences of these findings for excitation and transport processes are discussed, and an interesting charge separation for the polyacene-like structures is found.