AbstractAzetidine (1) polymerizes under the influence of various cationic initiators. In methanol and with perchloric acid as initiator, the monomer1is first converted into a dimer (N‐(3‐aminopropyl)azetidine (3)) which then polymerizes further to polymer. When all monomer has disappeared, the reaction mixture consists for 70% of the dimer3. The rate constant of dimer formation at 70°C is 1,50·10−3dm3mol−1s−1. The formation of3is attributed to the higher basicity of1compared to3, which causes a transfer of a proton from the protonated3to1. The structure of the polymer was determined by 300 MHz1H‐NMR spectroscopy. It contains 20% of tertiary, 60% of secondary and 20% of primary am