Differences in the synthetic method affected copolymer sequence and membrane properties of sulfonated polymers

For energy-device applications, e.g., fuel cells, sulfonated hydrocarbon ionomer membranes seem promising candidates to replace state-of-the-art perfluorosulfonic acid-based ionomers due to easy synthesis, potentially low cost, wide variety of molecular structure, and environmental compatibility. In connection to this, the versatility of the synthesis of sulfonated aromatic polymers using NiBr₂ was studied. The copolymerization of sulfo-dichlorobenzene protected with a neopentyl group was conducted with chlorine-terminated oligomer (1) or three different comonomers (2, 3, 4). The copolymerization proceeded well to obtain high molecular weight copolymers (Mn = 4.8360.1 kDa, Mw = 24.9133 kDa) and yielded flexible membranes, comparable to those obtained with Ni(0) as a polymerization promoter. ¹HNMR spectra suggested differences in the comonomer sequence between the copolymers synthesized with Ni(II) and Ni(0). The sequence was quantified as a “randomness of sulfophenylene (SP) unit” and correlated with the membrane properties. The randomness of SP unit in the sulfonated polyphenylene copolymer (SPP-QP) did not influence the phase-separated morphology under the dry-state as demonstrated by TEM images. SAXS analysis under humidified conditions showed that the SPP-QP with lower randomness of SP unit formed higher periodicity in the self-assembling structures (i.e., the uniform-sized ionic clusters). The increase of the randomness of SP unit from 19% to 47% in SPP-QP led to decrease of the proton conductivity (at 80°C and 20% relative humidity [RH]) by a factor of 1/1.7 and decrease of the elongation (at 80°C and 60% RH) by a factor of 1/27. Similar pattern was noted for other series of the sulfonated copolymer membranes.