Synthesis of superacid-modified poly(arylene ether sulfone)s via post-bromination

A versatile synthetic method of superacid-modified poly(arylene ether sulfone)s via post-bromination has been developed. Three kinds of high molecular weight poly(arylene ether sulfone)s, in which differences lie in the main chain structures, were synthesized and brominated. Careful control of the reaction conditions enabled selective and quantitative bromination of the polymers. The bromo groups were converted to superacid groups via Ullmann coupling reaction to obtain the title ionomers (FSPE-1a, 1b, and 1c). The chemical structure and the ion exchange capacity (IEC) of the FSPE-1s were characterized by ¹H and ¹⁹F NMR spectra. Tough, flexible, and transparent membranes with IEC ranging from 0.87 to 1.09 meq g^-1 were obtained by solution casting. The FSPE-1 membranes showed comparable properties (chemical stability, phase-separated morphology, water absorbability, and proton conductivity,) to those of our previous version of the superacid-modified poly(arylene ether sulfone) (FSPE) synthesized from brominated monomers (pre-bromination method). The advantages of the post-bromination method have been proven by significant improvement in the mechanical strength of the FSPE-1 membranes since it could provide superacid-modified aromatic ionomers with much higher molecular weight.