Poly(styryl)lithiums (M-n = 1500-2200 g/mol) in benzene were reacted with propylene sulfide (1.3-3 equiv) to prepare the corresponding thiol chain-end-functionalized polymers. The resulting polymers were characterized by thin layer chromatography, elemental analysis, H-1, C-13, and DEPT NMR spectroscopy, and MALDI-TOF mass spectrometry. Analysis of the resulting polymers by MALDI-TOF mass spectrometry indicates that reaction occurs by a ring-opening mechanism, in contrast to precedents showing that the reaction of organolithium compounds with propylene sulfide occurs by a sulfur extrusion mechanism. Poly(styryl)lithium was also reacted with 1.3 equiv of ethylene sulfide. The results from MALDI-TOF mass spectrometry show that the structure of the polymer corresponds to the product resulting from a ring-opening mechanism analogous to the reaction of propylene sulfide with poly(styryl)lithium. More oligomerization was observed for functionalizations with 1.3 equiv of ethylene sulfide (34.6%) compared to propylene sulfide (3.8%). The experimental results are consistent with quantitative thiol functionalization for reactions of poly(styryl)lithium with thiiranes.