A number of poly(alpha-n-alkyl-L-aspartate)s, which are nylon-3 derivatives with an alkoxycarbonyl side group attached to every repeat unit, have been prepared by non-assisted anionic polymerization of the corresponding (S)-4-alkoxycarbonyl-2-azetidinones. Side reactions leading to imidation were found to take place to an extent which is highly dependent on both the polarity of the polymerization solvent and the nature of the alkyl side chain. Highly stereoregular polymers without detectable imidation and molecular weights in the range 10(5) to 5 x 10(5) were obtained in dimethyl sulfoxide for n = 4, 6, 8 and 12 and in CH2Cl2 for n = 1, 2 and 18 (n = number of carbon atoms in the alkyl side group). A study of the thermal stability of these new polyamides showed that they melt with decomposition to largely imidated polymers when heated at temperatures above 250-degrees-C. X-Ray analysis of oriented samples revealed that members with n = 1, 2, 4 and 8 are able to form crystal structures with chains arranged in a helical conformation similar to the alpha-helix typical of polypeptides. This type of conformation seems also to be adopted by all other members of the series. For n = 6 and 12, however, the polymer chains did not crystallize but they became aligned side by side with alkyl side groups remaining in a disordered state.