Synthesis and properties of grafting sulfonated polymer containing isatin by super acid-catalyzed polyhydroxyalkylation reaction for PEMFC

• We synthesized sulfonate-grafted polymer containing isatin moiety.
• Sulfonate-grafted polymer shows good proton conductivity.
• Grafting-sulfonated polymer formed a bicontinuous type morphology.

Polymer containing isatin was synthesized by super acid-catalyzed carbon–carbon coupling reaction. Propylsulfonic acid was grafted on isatin unit by substitution reaction with potassium salt of 3-bromo-1-propanesulfonic acid. The sulfonic acid composition was regulated at 25–80 mol % of propylsulfonic acid in order to achieve expected ion exchange capacity of maximum 2.0 meq/g. The copolymers were of high molecular weight (inherent viscosity, ηinh = 1.2 dL/g) to afford a tough membrane by solution casting. All these membranes were casted from dimethylsulfoxide (DMSO). The structural properties of the synthesized polymers were investigated by 1H NMR spectroscopy. The membranes were studied by ion exchange capacity (IEC), water uptake, dimensional stability and proton conductivity assessment by comparing with Nafion®. As increasing the IEC values, the small hydrophobic components induced high proton conductivities and proton diffusion coefficients. These kinds of membranes without ether linkages showed low water swelling as well.