AbstractThe synthesis and characterization of uracil‐ and thy mine‐containing polynucleotide analogs with a linear polyethylenimine backbone is described. Hypochromicity is observed for these analogs in DMSO and in DMSO/H2O solutions and found lo increase as the percent water increases. The influence of water on the 240‐ to 280‐nm absorptions of the analogs and their monomer models is correlated with the mobiliiy of the nucleic‐acid‐base‐con tain ing pendant to the polymeric backbone. An impure 5‐fluorouracil analog has been prepared, and its increased solubility in water relative to the corresponding uracil analog has been noted. A thy mine‐containing analog with a PEI/PEOXZ backbone has been synthesized. The analog consists of water‐soluble and insoluble forms c