Tautomerism of nucleic acid bases was applied to a new type of polycondensation which was achieved by dehydration of hydroxyethyl purines and pyrimidines or dehydrochlorination of chloroethyl purines and pyrimidines to give polyethers and polythioethers containing purine and pyrimidine bases in the main chain. The polymer was found to be decomposed to the monomer under acidic and basic condition.