A new versatile mediator for conducting “living” radical polymerization was developed based on diselenocarbonyl compounds. After the optimization of substitution groups, a “universal” diselenocarbonyl compound (O4) was developed and successfully applied to the controlled polymerization of both conjugated and unconjugated vinyl monomers, and the desired polymers with predetermined molecular weights and narrow molecular weight distributions were obtained in excellent yields. The characteristic feature of this method is exemplified by the synthesis of block copolymers composed of conjugated and unconjugated monomers, which would be of great importance as functional smart organic nanomaterials. Furthermore, the existence of organoselenium groups in the final polymers would be especially valuable for bio-related applications.