The first example of phantom ring-closing condensation polymerization for the synthesis of oligoguanidines is presented. A new oligoguanidine with a ring structure was achieved in one step by the condensation reaction of a triamine, like diethylenetriamine, with guanidine hydrochloride. The condensation reaction proceeded by selective ring-closure towards the formation of five-membered rings in the oligomer backbone. The resulting polymer repeat unit structure was different from the starting monomers (phantom polymer) and was formed by elimination of three molecules of ammonia per repeat unit. The inter-, intra-, and intermolecular reaction sequences led to the new structure as proved by different spectroscopic techniques (atmospheric pressure chemical-ionization mass spectrometry, and one-dimensional and two-dimensional homo- and heteronuclear correlation NMR experiments) as well as supported by quantum chemical investigations. Preliminary results regarding antibacterial use of the resulting oligoguanidine were also promising and showed its effect within 15-30 min as an antibacterial material.