The polymerization of 1-benzyl-2,2-dimethylaziridine (BDMA) with triethyioxonium tetrafluoroborate (TEFB) under different reaction conditions was found to be impossible, although the initiation reaction (formation of 1-benzyl-1-ethyl-2,2-dimethylaziridinium tetrafluoroborate) was fast and quantitative. BDMA does copolymerize with 1-benzylaziridine (BA) but the amount of BDMA in the copolymer is always inferior to 50%. The reactivity parameters of the copolymerization (in dichloromethane at 20°C) are rBA= 3.1 and rBDMA= 0.0. These results demonstrate the importance of the presence of substituents on the polymerization behaviour of N-alkylaziridines