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 C=C bonds of the pendant undecenoyl chains and 2-(2-benzimidazolyl)ethanethiol. In this reaction, all the C=C bonds were converted into thio-ether linkages without any detectable structural degradation, as confirmed by H-1 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 degrees 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 (T), the subsequent tethering of the benzimidazole only slightly increased the T, 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 degrees C for a polymer functionalized with 1.7 benzimidazole units per repeating unit of PSU. (c) 2005 Elsevier Ltd. All rights reserved.