Poly-beta-amides (nylons 3) were synthesized via the anionic polymerization of a series of 4-alkyl-4-methyl-2-azetidinones where the alkyl group is a methyl, ethyl, propyl, butyl, or pentyl. The "non-assisted" polymerization was conducted under vacuum, in the bulk, at 160 degrees C, using potassium 2-pyrrolidonate as catalyst, whereas the "assisted" polymerization was carried in dimethylsulfoxide, at room temperature, using N-acetylpyrrolidinone-2 as activator but it gave no polymer with a propyl or bulkier side group. Side reactions occur in all cases. X-ray spectra showed that poly(4-alkyl-4-methyl-2-azetidinone)s are amorphous with propyl, butyl, and pentyl groups, and semi-crystalline with methyl or ethyl substituents. Both semi-crystalline polyamides exhibit an extended planar zigzag conformation, with a fiber identity period along the c axis of 4.9 Angstrom. Glass transition temperatures, melting temperatures, and/or decomposition temperatures are also reported.