Functionalization of Poly(2,6-dimethyl-1,4-phenylene oxide)s with Hindered Fluorene Side Chains for Anion Exchange Membranes

Functionalization of poly(2,6-dimethyl-1,4-phenylene oxide)s (PPO) was achieved by grafting hindered fluorene-based side chains to PPO via the Suzuki–Miyaura coupling reaction using an appropriate catalyst/ligand system. Using the resulting functionalized PPO, anion-conductive copolymers, PPO with fluorene side chains with pendant alkyltrimethylammonium groups, were synthesized via the Menshutkin reaction. The quaternized copolymers yielded ductile, transparent membranes which were soluble in dimethylformamide, dimethyl sulfoxide, and methanol at room temperature. The fluorene side chain containing membranes showed considerably higher hydroxide ion conductivities, up to 176 mS/cm at 80 °C, compared to that of typical anion exchange membranes based on the benzyltrimethylammonium motif. The results of titration and hydroxide ion conductivity measurements demonstrated excellent chemical stability of the fluorene side chain containing anion exchange membranes (AEMs), even after 1000 h immersion in 1 M NaOH at 80 °C. The results of this study suggest a new and scalable route for the functionalization of PPO and preparation of AEMs for practical alkaline fuel cell applications.