Phenolic Resins with Semi‐Rigid Polymer Backbones Exhibiting LCST‐Type Phase Separations in the Mixtures of Alcohols and Non‐Polar Solvents

Abstract Research on lower critical solution temperature (LCST)‐type thermo‐responsive polymers has predominantly focused on those with flexible backbones, such as vinyl polymers, while rigid polymers have rarely been investigated due to their low solubility. Although some molecular designs for aqueous systems have been reported, molecular designs in organic media remain unclear due to the difficulty in achieving soluble states at low temperatures. The previous study demonstrates the molecular design for LCST‐type phase separation in organic media based on the control of the solvation through the construction/deconstruction of hydrogen bonds, but only flexible vinyl polymers such as poly(4‐hydroxystyrene) are investigated. In this study, it is aimed to extend the molecular design to rigid phenolic resins, TD‐2131 and KA‐1160, which contain phenolic groups in the main chain. TD‐2131 and KA‐1160 exhibited LCST‐type phase separation in the mixture of 1‐alcohol such as 1‐hexanol and 1‐octanol as hydrogen‐bonding solvents and toluene or cyclohexane as non‐polar solvents in appropriate mixing molar ratios. In mixtures of 1‐alcohols and non‐polar solvents, shorter‐chain 1‐alcohols necessitate higher molar ratios, while longer‐chain 1‐alcohols require lower ratios to induce LCST‐type phase separations. These solubility trends mirror that observed in poly(4‐hydroxystyrene), suggesting that the molecular design applies to both flexible and rigid polymer backbones.