The cationic ring-opening isomerization polymerization of 2,3-dihydro[4,5]benzimidazo[2,1-b]oxazole (DBO) was examined. The polymerization of DBO with methyl trifluoromethanesulfonate proceeded smoothly at 0 degrees C and gave poly(2,3-dihydro-1H-benzo[d]imidazol-2-one-1,3-diylethylene) (polyDBO) in high yield. The polymer was a crystalline solid whose T-g and T-m determined by DSC were 152 degrees C and 343 degrees C, respectively. TGA analysis showed that the polymer was stable up to 485 degrees C under air. Once it was isolated, it was soluble only in strongly acidic media. However, the solution polymerization of DBO could be carried out homogeneously under a high dilution at 0 degrees C in chloroform. A postpolymerization experiment revealed that the polymerization proceeded in a living fashion.