Novel semi-crystalline sulfonated copolymers (sPS(B-SA)) have been synthesized by sulfonation of multiblock copolymers of syndiotactic polystyrene with cis-1,4-polybutadiene (sPSB), using a two-stage solution process comprising the addition of thiolacetic acid to the butadiene units, followed by the in situ oxidation of the thioacetyl moieties with performic acid. The sulfonation process is quantitative, chemoselective, cheap and more environmentally benign than similar methods previously reported; in addition it allows achieving high ion exchange capacity values (up to 4.48 equiv. per kg). The sPS(B-SA)s are crystalline, preserving the native polymorphic behavior of the crystalline syndiotactic polystyrene segments. Thin films of sPS(B-SA) samples show a phase separated morphology at the nanometer scale evidenced by tapping mode and tunneling current atomic force microscopy; proton conductive regions embedded in a hard-hydrophobic matrix of syndiotactic polystyrene were actually observed.