Direct sulfonation and styrene-mediated sulfonation were carried out onto metallocene-based poly(ethylene-co-octene) (POE) and poly(ethylene-co-butene) (PBE) elastomers to impart polarity on the completely nonpolar rubbery matrices and to prepare a new class of elastomer. The influence of styrene-grafting and pendant chain length on the degree of sulfonation was also studied. The effects of sulfonation, styrene grafting and styrene-mediated sulfonation at their optimized levels on various physicomechanical properties were thoroughly investigated, and the resultant properties were correlated with structures of the modified elastomers. Higher extent of sulfonic acid groups were introduced through direct sulfonation in comparison with the styrene-mediated sulfonation, whereas better thermal and mechanical properties were obtained through styrene-mediated sulfonation in comparison with the direct sulfonation process. PBE had shown higher degree of sulfonation and percentage grafting than POE. (C) 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46:8023-8040,2008