Synthesis and proton conductivity of highly sulfonated poly(thiophenylene)

The synthesis and characterization of poly(thiophenylenesulfonic acid), a novel class of polyaromatic electrolyte possessing up to 2.0 sulfonic acid groups per phenylene unit (m = 2.0), are described. 4-(Methylsulfinyl)diphenyl sulfide (1) was polymerized in sulfuric acid upon heating (<140 °C) or in the presence of SO₃ to yield a sulfonated poly(sulfonium cation) (4), which can be converted to the corresponding sulfonated poly(thiophenylene). The precursor method using the soluble poly(sulfonium cation) makes it easy to control the sulfonation reaction in a homogeneous system. The resulting poly(thiophenylenesulfonic acid), unlike the nonsubstituted polymer, is soluble in water and methanol and can form a transparent film. The sulfonated polymer exhibits a good water affinity and an excellent proton conductivity due to the high carrier concentration. The highest conductivity (σ = 4.5 × 10^-2 S cm^-1) was achieved for the polymer with m = 2.0 at 80 °C.