We investigated the molecular packing structure and the solid-state polymerization behavior of diacetylene compounds containing a naphthylmethylammonium carboxylate group in the side chain, which forms robust and two-dimensional hydrogen bond networks. The resulting polydiacetylenes (PDAs) showed thermochromic behaviors dependent on the structure of the substituents in the side chain. The maximum wavelength of the absorption bands in the UV-vis spectra of the poly(m,0-DA-Naph)s, which were obtained by the polymerization of the 1-naphthylmethylammonium 2,4-alkadiynoates (m = 4-13, 16), shifted to a longer wavelength region with an increase in the alkyl chain length of the side chain. The poly(m,0-DA-Naph)s showed a reversible color change upon heating in a temperature range wider than that for poly(1-naphthylmethylammonium 10,12-pentacosadiynoate) [poly(12,8-DA-Naph)] due to the different position of the two-dimensional hydrogen bond network supporting the chain structures in the solid state. We further investigated the structure and chromatic properties of PDAs containing the phenylene substituent groups that are directly connected to the conjugating main chain. Eventually, we found the occurrence of the solid-state polymerization of the 4-carboxyphenyl-substituted diacetylenes as not only naphthylmethylammonium salts (m,Ph-DA-Naphs, where m = 8, 10, 12, and 16), but also precursor carboxylic acid derivatives (m,Ph-DA-CO(2)Hs). The poly(16,Ph-DA-Naph)s showed a reversible color change upon heating, but the mechanism was different from that of the other PDAs. The thermochromism of poly(16,Ph-DA-Naph) included the conformational changes in the aromatic substituent and the conjugating backbone structure.