Synthesis and properties of quaternized polyolefins with bulky poly(4-phenyl-1-butene) moieties as anion exchange membranes

Polyolefin-based anion exchange membranes (AEMs) with well-defined ionic domains for anion transport were synthesized by introducing bulky poly(4-phenyl-1-butene) (P4PB) moieties into quaternized polyolefinic copolymers via heterogeneous catalytic copolymerization of 4-phenyl-1-butene with 11-bromo-1-undecene over a Ziegler–Natta catalyst and subsequent quaternization. Well-defined hydrophobic-hydrophilic separation was observed for the as-obtained membranes, as confirmed by atomic force microscopy (AFM). The membranes showed high hydroxide conductivity (10.4 mS cm⁻¹) despite their low ion exchange capacity (IEC, 0.98 meq g⁻¹). When compared with membranes containing poly(4-methyl-1-pentene) (PMP) or polypropylene (PP) moieties, the membranes obtained herein showed lower conductivities under similar conditions at IEC values higher than 1.2 meq g⁻¹. Excessive swelling of the P4PB membranes resulting from a high water uptake (WU) was assumed to decrease the ion concentration in the membrane. Thus, lower volumetric IEC values were observed for AEMs with IEC higher than 1.2 meq g⁻¹. A maximum hydroxide conductivity of 18.9 mS cm⁻¹ at room temperature was obtained for the P4PB-TMA-43 membrane (IEC = 2.17 meq g⁻¹, WU = 262.4 wt%). Similar to our previous report on polyolefin-based AEMs with PMP or PP moieties, side chains with nine methylene units between the polymer backbone and the quaternary ammonium (QA) cations were very stable and showed no apparent degradation upon storage in a 10 M NaOH aqueous solution at 80 °C for 20 d. We demonstrated that a polyolefin-based molecular architecture combined with poly-QA side chains attached via flexible spacer units with bulky side chain moieties achieved AEMs with efficient phase separation and high alkaline stability characteristics.