Abstract Radical ring-opening polymerizations of a five-membered cyclic vinyl sulfone monomer, 2-vinylthiolane-1, 1-dioxide (VTDO), was carried out by using p-toluenesulfonyl iodide (TosI) and bromide (TosBr) as radical initiators, and the corresponding ring-opened polymer (PVTDO) was obtained. Both TosI and TosBr were found to work as the radical initiators for the polymerization of VTDO in bulk. The use of TosI gave PVTDOs with a ...