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Molecular weight distribution (MWD) is a key characterization parameter of polymers and has crucial impact on the properties of polymers. The MWD regulation of polypeptoids, an attractive class of biomaterials, has not been reported so far. This study designed and synthesized the photolabile dormant initiator, 2-nitro-4,5-dimethoxybenzylcyclohexylaminecarbamate, for the ring-opening polymerization of N-butyl-N-carboxyanhydride (BuN-NCA). Upon the ultraviolet (UV) light irradiation (360 nm), the photolabile dormant initiator could release the active cyclohexylamine. The release amount was well controlled by manipulating the duration of the applied radiation. The release amount of cyclohexylamine was increased as the time of UV irradiation increased from 1 h to 4 h, resulting in the Mn decrease of the polypeptoid (from 9.6×10³ to 4.6×10³). The release rate of cyclohexylamine was controlled by the intervals of the applied UV irradiation. The early released cyclohexylamine initiated the monomer to form longer chains while the lately released cyclohexylamine initiated the monomer to form shorter chains. Thus, the MWDs of polypeptoids were well regulated in situ. This study provides the possibility to tune the Mn and MWD of polypeptoids in a more precise manner, and also lays the foundation for the investigation of the relationship between the MWD and the properties of polypeptoids. [ABSTRACT FROM AUTHOR]