The topochemical polymerization of diacetylenes (DAs) is closely related to the length of their alkyl chain. DA monomers have two types of alkyl chain side groups: the inner alkyl chain and the outer alkyl chain, that is, the peripheral alkyl chain. Herein, we designed and synthesized a series of DA monomers that possess bis-amide linkages with different peripheral alkyl chains ( n = 6-9; DA1-DA4). The peripheral alkyl chain length of these DA monomers exhibits an odd/even effect on topochemical polymerization. The polymerization of DAs was achieved only when n is an odd number, whereas no polymerization occurred when n is an even number. The odd/even effect on the topochemical polymerization was also investigated using ab initio density functional theory calculations. The thermochromic reversibility of polydiacetylenes (PDAs) was investigated using UV-vis absorption spectroscopy at temperatures ranging from 20 to 60 °C. Monomer DA2 was used as a single-component ink solution in a fountain pen that can be transformed into thermochromic letters on conventional paper. Furthermore, a PDA-embedded polyethylene oxide film was included to monitor the thermochromic reversibility and was found to exhibit excellent thermochromic reversibility between 20 and 100 °C and stability, enabling storage for a few months without deformation. Finally, a green-colored patterned polymer film is readily obtained by a subtractive color (blue and yellow) mixing method and exhibits high thermochromic reversibility at temperatures between 20 and 100 °C.
Keywords: fountain pen; odd/even effect; polydiacetylene; single-component ink; subtractive color mixing; thermochromic.