The reactivity of n-butyllithium-CH3OCH2CH2OLi in toluene (System A) in styrene-butadiene copolymerization reaction was found to be similar to that of the catalyst system n-butyllithium-poly(propylene oxide), which formed copolymers containing more styrene units than the feed monomer ratio at early stage of polymerization. Metalation reaction of toluene to produce benzyllithium was observed to take place in System A. Relative reactivity of each of chemical species present in System A was measured in the reactiom with butadiene, styrene, and 1,1-diphenylethylene, in connection with initiation and propagation reactions of copolymerization. From this study a living end anion possessing larger conjugated system was concluded to be more reactive toward monomers possessing larger conjugated system.