The nickel(0)-catalyzed cycloaddition copolymerization of 1,4-diethynylbenzene (1) with 4-methylphenyl (2a) or n-octyl (2b) isocyanate to poly(a-pyridone) 3a or 3b, respectively, was studied. Equimolar copolymerization of 1 with 2a or 2b in THF at 60 degrees C produced an insoluble or poorly soluble copolymer. Use of an excess amount of 2a or 2b and the short reaction time of 5-10 min afforded soluble poly(a-pyridone) 3a or 3b, although its molecular weight was not high. Prolonging the copolymerization to 20 h gave insoluble 3a, while it produced soluble branched poly(2-pyridone) 3b with a broad molecular weight distribution, i.e., a high M(w)/M(n) value. The M(w)/M(n) value was found to be a useful criterion for the poly(a-pyridone) branch. Poly(2-pyridone)s 3a,b were identified by IR, H-1 NMR, and C-13 NMR spectroscopies and also by comparing their spectral data with those of a model 2-pyridone compound prepared from phenylacetylene and n-octyl isocyanate.