Intrinsically Proton-Conducting Benzimidazole Units Tethered to Polysiloxanes

Polysiloxanes with pendant benzimidazole units have been prepared by free radical thiol−ene coupling reactions of 2-(2-benzimidazolyl)ethanethiol and vinyl-functional polysiloxanes. The latter polymers were prepared by anionic ring opening copolymerization of 1,3,5,7-tetramethyl-1,3,5,7-tetravinylcyclosiloxane and 1,3,5-hexamethylcyclosiloxane. Copolymers with different degrees of benzimidazole functionality were conveniently obtained by varying the monomer ratios. The coupling reaction was found to be very efficient, and the vinyl groups were completely consumed, as confirmed by NMR and FTIR spectroscopy. The glass transition temperature ( T g ) of the benzimidazole functional copolymers increased dramatically with the benzimidazole content at low contents to reach a plateau value just above 50 °C at a content of approximately 33 mol % benzimidazole functional siloxane residues in the copolymer. Conductivity measurements carried out at 60 and 140 °C indicated that the level of polymer segmental mobility, and thus the T g, was the most decisive parameter for the proton conductivity at low temperatures (60 °C), while the benzimidazole concentration was more important at elevated temperatures (140 °C). A conductivity of 7 × 10 -6 S/cm was reached at 140 °C by a polysiloxane carrying 57 mol % benzimidazole-grafted siloxane residues under completely anhydrous conditions.