The selective reductive homo-coupling polymerization of aromatic diisocyanates via one-electron transfer promoted by samarium iodide in the presence of hexamethylphosphoramide (HMPA) produced the corresponding poly(oxamide)s (1) nearly quantitatively. The obtained polymers were insoluble in common organic solvents. The alkylation of 1 with methyl iodide or allyl bromide in the presence of potassium tert-butoxide provided the highly soluble alkylated polymer in good yields. In either case, the alkylation was almost complete, and both N- and O-alkylation proceeded. The ratio of N- and O-methylation was found to be 64 : 36 by H-1-NMR spectrum, and that of N- and O-allylation was 3 : 1 from C-13-NMR analysis. The present polymerization system could be applied to a variety of diisocyanates, including diphenylmethanediisocyanate (MDI), tolylene 2,6-diisocyanate (TDI), 2,6-naphthyl diisocyanate (NDI) and o-tolidine diisocyanate (TODI). The molecular weights of the polymers were estimated by GPC and found to be 2000-9000. The TGA measurement of the corresponding polymers showed T-d10 at 248-320 degrees C.