Helical-structure transition and color changes in aromatic polyacetylenes under mechanochemical conditions: effect of the additive-alcohol chain length

Abstract

This study investigates the control of higher-order polymer structures through mechanochemical (MC) synthesis, focusing on poly(2-ethynylnaphthalene) (P2EN) as a model helical poly(arylacetylene). The products of MC synthesis using linear alcohols (C1–C22) as additives were compared with those of conventional solution synthesis using these alcohols as solvents. Interestingly, the polymer color, which depends on helical conformation, is directly influenced by the carbon-chain length of the alcohol additive in MC synthesis. Short-chain alcohols produce yellow P2EN with extended helices (the cis-transoid structure), whereas long-chain alcohols yield red P2EN with contracted helices (the cis-cisoid structure). This structure-dependent color variation is exclusive to MC synthesis; solution polymerization consistently produces yellow P2EN with extended helices, regardless of the alcohol used. The results of this study suggest that under MC conditions, localized heat and pressure facilitate transitions from metastable cis-transoid to stable cis-cisoid conformations through specific high-affinity polymer/additive interactions. Thus, MC synthesis with appropriate additives can direct the synthesis of polymers with thermodynamically stable higher-order structures. The proposed approach offers an environmentally friendly method for controlling the conformation (and material properties) of polymers, potentially enabling the green industrial production of functional polymer materials.