The effect of various experimental conditions on the formation of carboxyl-ter-minated polybutadiene liquid-rubbers prepared by free-radical initiation were investigated with 4, 4''-Azobis (4-cyano-valeric acid) as initiator. TLC separation of polymers by the difference in end-group was applied to the determination of functionality distributions in liquid rubbers. It was found that the liquid rubbers formed in the present free-radical polymerizations have functionalities greater than 2.0. The results of TLC separation indicated that the products include neither no functional nor mono-functional component, while the mono-functionality has been generally found. These lead to the conclusion that termination mechanism in the presently investigated butadiene polymerization in predominately by coupling reaction.