AbstractN‐Ethylaziridine (1) was polymerized in water with HCl as initiator and copolymerized withN‐(2‐hydroxyethyl)aziridine (2). Rapid propagation and termination reactions were observed. The maximum yield increases with the initiator concentration, whereas the molecular weight decreases. This is due to the difference in basicity between the tertiaryN‐atoms of the two monomers and the tertiary and the terminal secondaryN‐atoms of the polymers, which causes tranfer and termination reactions. Macrocycles with a side chain on a quaternaryN‐atom are formed. During the copolymerization, the more basic1is preferentially incorporated into the copolymer chain. The copolymerization parameters can be estimated roughly on the basis of basicity considerations: r1>1; r2<1. The ratio of the rate constants (propagation to termination) is lower for1than for2. This is due to the fact that the basicity difference between the monomer and the tertiaryN‐atoms of the polymer is higher for1than for2. By reaction of thionyl chloride with the copolymers the desired hydrochlorides of theN‐(2‐chloroethyl) derivatives are obtained. The neutralisation with alkali did not result the free bases, since the products formed are rather due