AbstractIn order to examine the effect of the steric hindrance by the polypeptide side chains on the growth mechanism of polypeptide crystals in the course of polymerization, we studied kinetics of the heterogeneous polymerization ofL‐valineN‐carboxy anhydride (NCA),L‐isoleucine NCA, andL‐leucine NCA initiated by butylamine in acetonitrile and the structures and morphologies of the resultant polymers. In the early stages of polymerization, oligomers formed crystallized into thin ribbon‐like crystals with the antiparallel β‐conformation. The molecular arrangements in the crystals were studied by electron diffractions. In the later stages, the grown chains of poly(L‐valine) and poly(L‐isoleucine) continued to take the β‐structure and their active ends were occluded in the crystals formed, which resulted in the levelling off of the conversion at a low value. On the other hand, poly(L‐leucine) seemed to give the extended chain crystal, which made the conversion very high. This can not be accounted for without assuming the formation of β‐helical chains onto the β‐skeleton crystals formed in the early stages of polymerization as in the case of poly(L