Anionic polymerization of N-methacryloyl-2-methylaziridine (1) was carried out with 1,1-diphenyl-3-methylpentyllithium (DMPLi) in the presence of LiCl or Et2Zn in THF. Poly(1)s possessing predicted molecular weights and narrow molecular weight distributions (MWD, M-w/M-n < 1.1) were quantitatively obtained at -78 to -40 degrees C within 72 h. The stability of the propagating chain end of poly(l) was confirmed by the quantitative initiation efficiency in the postpolymerization. In the polymerizations initiated with DMPLi/LiCl at the various temperature ranging from -40 to -60 degrees C, the relationship of time-conversion was obtained from the GLC analysis of the residual monomer. The apparent rate constant and the activation energy of the anionic polymerization for 1 were determined as follows: In k(p)(ap)= -5.85 x 10(3)/T + 23.3 L mol(-1) s(-1), and 49 4 U mol-1, P respectively. N-Acryloyl-2-methylaziridine (2) also underwent the anionic polymerization initiated with DMPLi/LiCl at -78 degrees C to give the well-defined polymer. The radical polymerizations of I and 2 with AIBN proceeded in 77 and 100% conversions, respectively. Poly(l) and poly(2) showed the glass transition temperatures at 98 and 54 degrees C but gave the insoluble products at higher temperatures around 150 degrees C through the thermal crosslinking of highly strained N-acylaziri dines. The ring-opening and/or the substitution reactions of aziridine moieties of poly(1) and poly(2) also occurred with nucleophiles such as lithium metboxide and sodium salts of di(ethylene glycol).