A nickel (0) catalyst generated from bis(1,5-cyclooctadiene)nickel and a tricyclohexylphosphine ligand effected the efficient 1:1 cycloaddition copolymerization of 1,4-bis(phenylethynyl)benzene (1) with various isocyanates 2 such as 4-, 3-, and 2-methylphenyl isocyanates and n-octyl isocyanate to afford rigid poly(2-pyridone)s with a unique structure having a repeating unit of a phenyl and phenylene-substituted 2-pyridone ring. The alkyl isocyanate had a lower copolymerizability with 1 than the aryl isocyanate and its use in excess of 1 was required for the efficient 1:1 copolymerization to suppress the formation of the diyne trimerization unit in the copolymer. The poly(2-pyridone)s obtained were identified by IR, H-1 NMR, and C-13 NMR spectroscopies and also by comparing their spectral data with those of the cooligomer consisting of two molecules of 1 and one n-octyl isocyanate molecule. The C-13 NMR absorption at delta 141 was found to be due to the phenyl and phenylene ipso carbon atoms of the benzenoid unit, i.e., the diyne trimerization unit, in the copolymer and to be an excellent criterion for the 1:1 copolymerizability of 1 with 2.