A polymerization initiator composition includes isomers of the polymerization initiator, thereby preventing instability and inertness of the polymerization initiator and physical property degradation of the SSBR, minimizing by-products and unreacted materials, and remarkably improving a conversion ratio. In addition, when the polymerization initiator composition according to the present invention is used, there is no need to add a polar additive separately upon polymer polymerization, since the polar additive is already added at the time of preparing the polymerization initiator. Further, the polymer initiator composition has high polarity so that the hydrophobic SSBR and the hydrophilic silica can be effectively dispersed.