The anionic polymerization of m-(tert-butyldimethylsilyl)oxymethylstyrene (1) was carried out with s-BuLi in THF at -78 degrees C. Under these conditions, the polymerization proceeds in a living manner to afford polymers with predictable molecular weights and narrow molecular weight distributions. Well-defined block copolymers of 1 with styrene were successfully synthesized. Poly(1)s were transformed quantitatively into poly(m-halomethylstyrene)s (halogen: Cl, Br, I) by treating with BCl3, (CH3)(3)SiCl-LiBr, and (CH3)(3)SiCl-NaI, respectively. The resulting polymers with highly reactive benzyl halides in all monomer units retained the well-controlled structures of their parent polymers. The well-defined poly(m-halomethylstyrene)s were then coupled with living anionic polymers of styrene in THF. Well-defined comblike branched polystyrenes having almost one branch per monomer unit were prepared by coupling poly(m-chloromethylstyrene) both with polystyryllithium in THF at -78 degrees C for 72 h and with polystyryllithium end-capped with 1,1-diphenylethylene (DPE) in THF at -40 degrees C for 168 h. They were also prepared by the coupling reactions of poly(m-bromomethylstyrene)s with the DPE end-capped polystyryl anions in THF at -78 degrees C for 72 h and at -40 degrees C for 168 h. Their backbone and branch chains had well-controlled molecular weights and narrow molecular weight distributions.