We focus our attention here on semisquaric acid, which is known to show high acidity, as a new proton dissociating group for proton exchange membranes (PEMs). The introduction of a squaric acid group into aromatic polymers was conducted by the reaction of lithiated aromatic polymers and diisopropoxy squarate, followed by treatment with hydrochloric acid. A resulting polyphenylsulfone membrane with the squaric acid group introduced (PPSf‐SQ, IEC = 4.1 meq·g−1) showed proton conductivity of 1.0 × 10−1 S·cm−1 at 80 °C under 95% relative humidity, which indicates that the semisquaric acid has the potential to become an alternative proton‐conducting group for PEMs.
