Differences in the Synthetic Method Affected Copolymer Sequence and Membrane Properties of Sulfonated Polymers

The versatility of the synthesis of sulfonated aromatic polymers using NiBr2 was investigated. The copolymerization of sulfo-dichlorobenzene protected with a neopentyl group was carried out with chlorine-terminated oligomer (1) or three different comonomers (2, 3, 4). The copolymerization proceeded well to obtain high molecular weight copolymers (Mn = 4.83–60.1 kDa, Mw = 24.9–133 kDa) and provided flexible membranes, comparable to those obtained with Ni(0) as a polymerization promoter. 1H NMR 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 affect the phase-separated morphology under the dry-state as suggested by TEM images. SAXS analysis under humidified conditions revealed 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 caused decrease of the proton conductivity (at 80 °C and 20% 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 tendency was observed for other series of the sulfonated copolymer membranes.