A new post-functionalization method of polystyrene (PS) block of polystyrene-b-poly(ethylene-co-butylene)-b-polystyrene (SEBS) is presented. Due to good chemical stability of backbone in alkaline condition, elastic mechanical property, nanoscale phase separation morphology, and commercial availability, SEBS is a promising candidate for preparation of anion exchange membranes (AEMs). Chloromethylation, the most frequently used reaction for AEM synthesis, of the PS block in SEBS generally results in low levels of functionalization or gelation.^{1, 2} To overcome these limitations our group has reported functionalization of SEBS for AEM applications using transition metal-catalyzed C-H borylation and Suzuki coupling reactions.³

Herein, we report another method “bromoalkylation” of aromatic rings at SEBS via the acid-catalyzed Friedel-Crafts alkylation of a tertiary alcohol. Compared to our previous report, this new synthetic route is much simple and does not require noble transition metals, affording the synthetic process more economical and scalable. Bromoalkylated tertiary alcohol is converted to a tertiary carbocation, which readily reacts with p electrons of the aromatic rings at PS block under acidic condition. Quaternary ammonium (QA) functionality and tether length between polymer backbone and cationic head groups were conveniently tunable by changing the structure of brominated tertiaryalcohols. A maximum of 80 mol. % of PS block could be functionalized.

A series of SEBS-based AEMs were prepared with different QA concentrations and tether lengths. The properties of membrane samples were investigated by comparing ion-exchange capacity (IEC), water uptake, OH^- conductivity and morphology. Among them, SEBS-C₃-TMA-0.8 (tether length, n=3; 80% of PS functionalization) has the highest IEC (1.62 mequiv g^-1), and OH^- conductivity of 93 mS cm^-1 at 80 °C in water. Moreover, the IEC and OH^-conductivity of all membranes were preserved after immersing them in 1 M NaOH at 80°C for 500 h.

1. Q. H. Zeng, Q. L. Liu, I. Broadwell, A. M. Zhu, Y. Xiong and X. P. Tu, J. Membr. Sci. 2010, 349, 237-243.
2. L. Sun, J. Guo, J. Zhou, Q. Xu, D. Chu and R. Chen, J. Power Sources 2012, 202, 70-77.
3. A. D. Mohanty, C. Y. Ryu, Y. S. Kim and C. Bae, Macromolecules 2015, 48, 7085-7095.