A bicyclic pseudourea, 2,3,5,6-tetrahydroimidazo[2,1-b][1,3]oxazole (1), was isolated for the first time, and its cationic and spontaneous polymerizations were examined. Both polymerizations gave poly(1,3-imidazolidin-2-one-1,3-diylethylene) (2) in high yields. The polymer is a crystalline solid with glass transition and melting temperature of 51 degrees C and 286 degrees C, respectively. Thermogravimetry showed that it is stable up to 450 degrees C under nitrogen.