In this study, we report a unique two-step approach to prepare pure and well -organized 10,12-pentacosadiynoic acid (DA)/melamine (MA) cocrystals, from which the topochemically polymerized product, poly(10,12-pentacosadiynoic acid) (PDA)/MA cocrystals, demonstrated reversible thermochromism (note that PDA in its pure crystalline form is thermochromically irreversible). In the first step, co-assembling DA and MA was achieve in a selective solvent for MA (i.e., water) using the non-covalently connected micelle method. In the second step, annealing the DA/MA co-assembly at a temperature higher than the melting point of pure DA crystals (62 degrees C) but below that of the cocrystals (87 degrees C) led to the formation of pure DA/MA cocrystals with a molar ratio of 2:1. The resultant lath-like DA/MA cocrystals were nano-sized and had an alternating DA/MA lamellar structure. Due to the strong intermolecular hydrogen-bonding and pi-pi stacking of MA molecules, the topochemically polymerized PDA/MA cocrystals were completely reversible in thermochromism. This is the first example that reversible thermochromism of PDA is realized through hydrogen-bonding assisted cocrystals with a small molecule. (C) 2016 Elsevier Ltd. All rights reserved.