Intrinsically proton-conducting comb-like copolymers with benzimidazole tethered to the side chains

Intrinsically proton-conducting comb-like copolymers having side chains tethered with benzimidazole units have been prepared and characterized with respect to their thermal properties and proton conductivity. In the first step of the preparation, poly(4-hydroxystyrene) was synthesized by polymerization of tert-butoxystyrene followed by hydrolysis of the butoxy groups. In the next step, poly(allyl glycidyl ether) side chains were grafted from the polyhydroxystyrene (PHSt) backbone. Finally, benzimidazole units were tethered via free radical thiol-ene coupling between 2-(2-benzimidazolyl)ethanethiol and the pendant allyl groups of the side chains. The efficiency of the thiol-ene coupling reaction was found to decrease with increasing length of the side chains, which was attributed to sterical effects. The materials were thermally crosslinkable, most probably via the residual allyl groups. Calorimetry showed that the benzimidazole-tethered copolymers had glass transition temperatures of 50– 60 °C, partly a consequence of the strong hydrogen bonds between the benzimidazole units. The proton conductivity of the fully polymeric materials exhibited a non-Arrhenius behavior and a maximum conductivity of 6.6 μS/cm was reached at 160 °C under anhydrous conditions.