Abstract: As cheme of complex equilibrium and cross propagations between the living species in butadiene-styrene anionic copolymerization by n-BuLi-THFinitiation has been proposed. Accordingly, a copolymer composition equation has been derived and the implications of the apparent monomer reactivity ratio have been discussed. Moreever, the relationship between apparent rate constants (k_B, kS) and the concentration of THF has been established. Finally, the probability of the vinyl group formation in poly-butadiene chain in copolymer from mono-THF coordinated reactive spe-eies has been estimated.