AbstractSummary: Sarcosine‐N‐carboxyanhydride (Sar‐NCA), L‐alanine‐NCA and D,L‐alanine‐NCA were polymerized with benzylamine as initiator in three different solvents: dichloromethane (CH2Cl2), 1,4‐dioxane and dimethylformamide (DMF). The isolated polyaminoacids were characterized by 1H NMR spectroscopy and MALDI‐TOF mass spectrometry. High conversions and degrees of polymerization $\overline {DP}$s close to the monomer‐initiator (M/I) ratios were found for all polypeptides. For polysarcosine which was soluble under the given reaction conditions a narrow monomodal frequency distribution was found. In contrast, a broad frequency distribution was observed, when L‐alanine NCA was polymerized in dioxane and DMF. These results were attributed to a partial precipitation of oligopeptides in the β‐sheet structure, which reduces the reactivity of endgroups for steric reasons. The polymerizations of D,L‐alanine‐NCA showed features in between the extremes of Sar‐NCA and L‐Ala‐NCA.Schematic illustration of the secondary structures formed in a primary amine initiated polymerization of L‐Ala‐NCA.imageSchematic illustration of the secondary structures formed in a primary amine initiated polymerization of L‐Ala‐NCA.