Advanced macromolecular engineering requires excellent control over the polymerization reaction. Living polymerization methods are notoriously sensitive to impurities, which makes a practical realization of such control very challenging. Reversible-deactivation radical polymerization methods are typically more robust, but have other limitations. Here, we demonstrate by repeated (ge;10 times) chain extension the extraordinary robustness of the living nucleophilic ring-opening polymerization of N-substituted glycine N-carboxyanhydrides, which yields polypeptoids. We observe essentially quantitative end-group fidelity under experimental conditions that are comparatively easily managed. This is employed to synthesize a pentablock quinquiespolymer with high definition. © 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.