The anionic polymerization of benzofulvene (BF) quantitatively proceeded with various initiators, such as sec-BuLi, diphenylmethyllithium, diphenylmethylpotassium, triphenylmethyllithium, and triphenylmethylpotassium, in THF at -78 degrees C to give polymers having predicted molecular weights based on the molar ratios between BF and the initiators and narrow molecular weight distributions (M-w/M-n < 1.1). Although the initiation efficiencies were not quantitative, BF could also be polymerized with benzylmagnesium chloride, potassium tert-butoxide, and the anionic living poly(tert-butyl methacrylate), indicating the high anionic polymerizability. The planar conformation and the polarized electron density of BF obtained by the density functional theory (DFT) calculation supported the observed anionic polymerizability higher than that of 2-phenyl-1,3-butadiene, the acyclic analogue of BF. A series of new block copolymers, polystyrene-b-poly(BF), poly(2-vinylpyriciine)-b-poly(BF), and poly(tert-butyl methacrylate)-b-poly(BF), were synthesized by the sequential anionic copolymerization of BF and the comonomers. On the other hand, no polymerization of styrene and 2-vinylpyridine took place with the living poly(BF), indicating the low nudeophilicity of the propagating indenyl anion formed from BF. BF readily underwent the free-radical polymerization with alpha,alpha'-azobis(isobutyronitrile), while the observed cationic polymerizability of BF was quite low. H-1 and C-13 NMR analyses revealed that the repeating units of poly(BF) consisted of a 1,2-addition unit and a 1,4-addition unit without a 3,4-addition unit regardless of the polymerization conditions. The exo-methylene moiety (CH2=) of BF always participated in the addition polymerization. The addition modes were dependent on the polymerization temperature and not on the solvent or the countercation of the anionic initiator. For instance, polymer obtained with sec-BuLi in THF at 40 degrees C contained 72% of the 1,4-addition unit and 28% of the 1,2-addition unit. Therefore, BF acted as a polymerizable 1,3-diene possessing a fixed transoid framework.