AbstractHeterogeneous polymerizations of γ‐methyl‐L‐glutamateN‐carboxy anhydride (NCA), γ‐benzyl‐L‐glutmate NCA, and β‐benzyl‐L‐aspartate NCA were carried out using butylamine as initiator in acetonitrile at 30°C. The oligomer chains formed in the beginning of the polymerization crystallized into the antiparallel β‐form and thereafter the α‐helical chains grew from the active sites of the β‐chains. The polymerization of γ‐methyl‐L‐gultamate NCA proceeded to 100% conversion and accordingly gave rise to high molecular weight poly(γ‐methyl‐L‐glutamate). The polymerizations of γ‐benzyl‐L‐glutamate NCA and β‐benzyl‐L‐aspartate NCA stopped at 54% and 16%, resp. The levelling‐off of the conversion at such low values, in spite of the α‐helical chain growth, may be due to the occlusion of the active chain ends of the resultant poly(γ‐benzyl‐L‐glutamate) and poly(β‐benzyl‐L‐aspartate) into the crystals. It was concluded that the interlamellar crystallization was induced by the intermolecular interaction between the benzyl groups of the polymer side chains, giving rise to the occlusion of the active growing