The radical polymerization of styrene was investigated in the presence of five diaryl diselenide compounds with different substitution groups on the benzene ring under visible light irradiation. It showed that bis(2,6-dimethylphenyl) diselenide (DmXDS) with two methyl groups on every benzene ring was the most efficient mediator for preparing polymers with a predetermined molecular weight and narrow molecular weight distribution. The reasons were analyzed through a quantum calculation method. The polymerization behavior of styrene in the presence of DmXDS was further investigated in depth. The results showed a typical living radical polymerization process. Polymers retaining the selenide structures at both the α- and ω-ends were verified by NMR and MALDI-TOF. Such end capped selenide groups could be transformed into terminal vinyl groups with a high efficiency under oxidative conditions, which offers a route to prepare “macromonomers” with a narrow molecular weight distribution.