Catalysts CH3COCo(CO)(3)PPh3 (1) and HCo(CO)(3)PPh3 (2) catalyze the copolymerization of aziridine and carbon monoxide. Catalyst 2 can be conveniently generated in situ via reaction of Na+CO(CO)(4), NN-dimethylamlinium. chloride, and PPh3. The copolymerization alternates at high catalyst loadings to produce poly(beta-alanine). The end groups of the poly(beta-alanine) product are characterized by matrix-assisted laser desorption/ionization time-of-flight mass spectrometry and by comparison of the H-1 NMR spectra of the polymer and a stepwise synthesized model compound. At low catalyst loadings, the polymer product contains both the beta-alanine units and amine units because of nonalternating enchainment of the comonomers. The characterization of the amine units is again supported by comparison of the H-1 NMR spectra of the polymers and the stepwise synthesized model compounds. The molecular weights of the polymers are determined by gel permeation chromatography. The thermal stability of the polymers is probed by differential scanning calorimetry and thermogravimetric analysis.