N-Phenylitaconimide (N-PII) can be readily polymerized with basic initiators that involve carbide, nitride, and oxide anions such as sec-butyllithium, lithium diethylamide, and alkali metal tert-butoxide. Poly(N-PII) was obtained in 84, 73, and 78% yields with sec-butyllithium, lithium diethylamide, and lithium tert-butoxide, respectively, in THF at 0 degrees C for 3 h polymerization. The effect of countercation size on the anionic polymerization rate of N-PII (1,1-disubstituted ethylene type monomer with cyclic imide structure) with alkali metal tert-butoxides is inverse to that of N-phenylmaleimide (N-PMI) (1,2-disubstituted ethylene type one); i.e., it was observed that the polymerization rate increased as the ion radius of a countercation decreased. NMR spectroscopic data and molecular orbital calculations suggest that the anionic polymerization of N-PII took place only at the vinylidene group and no appreciable side reaction would occur. In the case of using the initiators with Li+ as a countercation, the molecular weight distribution of poly(N-PII) is apparently unimodal and polydispersity is relatively small. The mechanism of the anionic polymerization of N-PII was discussed in detail. The thermal properties of poly(N-PII) obtained were also presented : it possessed a T-g under its decomposition temperature and could form a film by the cast method while those could not be observed for poly(N-PMI).