Polysulfones tethered with benzimidazole

Benzimidazole units have been grafted onto a polysulfone (PSU) backbone via long alkyl thio–ether chains using a two-step procedure. In the first step, lithiated PSU was reacted with 10-undecenoyl chloride to graft PSU with undecenoyl side chains. The second step involved a free-radical thiol–ene coupling reaction between the CC bonds of the pendant undecenoyl chains and 2-(2-benzimidazolyl)ethanethiol. In this reaction, all the CC bonds were converted into thio–ether linkages without any detectable structural degradation, as confirmed by 1H NMR spectroscopy and size-exclusion chromatography. The procedure constitutes a convenient and general pathway to attach functional or mesogenic groups to PSU via long flexible spacers. Thermogravimetry showed that the benzimidazole-functionalized polymers were stable up to 250 °C under nitrogen atmosphere, and that the first degradation step was attributed to the cleavage of the thio–ether bond. While the grafting of the undecenoyl side chains was found to significantly decrease the glass transition temperature (Tg), the subsequent tethering of the benzimidazole only slightly increased the Tg of the grafted PSU backbone. The concentration of benzimidazole was probably too low for the formation of a percolating benzimidazole domain. This explains the quite modest proton conductivity measured under completely dry conditions, e.g. 34 nS/cm at 180 °C for a polymer functionalized with 1.7 benzimidazole units per repeating unit of PSU.