The ring-opening polymerization rate of 1-β-cyanoethylazetidine (CA) with 3-hydroxy-1-propanesulfonic acid sultone (HPSS) as an initiator has been investigated in acetonitrile. The polymerization of CA with HPSS proceeded by way of a zwitterion formed by the addition reaction of CA and HPSS. The course of polymerization involved, as an early stage, slow initiation due to the low reactivity of the zwitterion. The propagation constants were obtained as were the enthalpy of activation for propagation (ΔHp* = 12 kcal/mol) and the entropy of activation for propagation (ΔSp* = −31 cal/K·mol).