In order to prepare well-defined polymers with halogen end groups, anionic living polymers of isoprene and styrene were allowed to react with excess amounts of alpha,omega-dihaloalkanes in THF at -78 degrees C. The monosubstitution reactions of 1,3-dichloropropane with these anionic living polymers proceeded cleanly to afford quantitatively the polymers with chlorine end groups that possessed controllable molecular weights and narrow molecular weight distributions. On the other hand, the reactions of the anionic living polymers with either alpha,omega-dibromoalkanes or alpha,omega-driodoalkanes proceeded competitively with side reactions leading to dimer formation of the starting living polymers. Thus, the bromine- and iodine-terminated polymers were obtained in 12-93% yields along with undesirable dimers. As side reaction candidates, we speculated on metal-halogen interchange and/or single electron transfer pathways followed by coupling reactions between the reaction intermediates generated by both pathways. By end-capping the polystyryl anion with 1,1-diphenylethylene, one could almost suppress the dimer formation, and the end-functionalized polystyrenes with bromine and iodine were successfully obtained in more than 95% yield.