Fully Aromatic Ionomers with Precisely Sequenced Sulfonated Moieties for Enhanced Proton Conductivity

A series of six fully aromatic ionomers with precisely sequenced sulfonated sites along the polymer chains have been designed, prepared, and characterized as proton-exchange membranes. Two straightforward and efficient synthetic strategies based on Ullmann ether reactions and a Baeyer–Villiger rearrangement were devised to obtain bisphenol monomers with four or six phenylene units linked exclusively by ether bridges to avoid transetherification reactions. Polycondensations of these bisphenol monomers with mono- or disulfonated dihalide monomers gave high molecular weight poly­(arylene ether), poly­(arylene ether sulfone), and poly(arylene ether ketone) homopolymers having microblock-like structures with sulfonated moieties separated by monodisperse nonsulfonated oligo­(ether) spacers. The nanoscale morphology and properties of solvent cast membranes were closely related to the nature of the oligo­(ether) spacers. Small angle X-ray scattering (SAXS) measurements showed intense scattering and very narrow ionomer peaks with second-order features for the polymers with the six-ring spacers. This clearly indicated that the controlled ionic sequencing enabled self-assembly of ionic aggregates with a much higher degree of organization in relation to a corresponding aromatic ionomer with a statistical distribution of the sulfonate groups. At an identical ionic content, the ionomers containing <i>meta</i> ether linkages had lower glass transition temperatures than the all-<i>para</i> materials, leading to a higher water uptake and proton conductivity of the former ionomers. A membrane with an ion exchange capacity (IEC) of 2.05 mequiv g^–1 and containing exclusively <i>para</i> linkages reached the same level of proton conductivity as Nafion at 100% relative humidity (RH), and also had an excellent dimensional stability in boiling water. Under reduced RH, the conductivity of this membrane greatly exceeded that of a membrane based on a statistical copolymer analogue with a similar ionic content