Citrate esters selectively react with primary (1°) amines on branched polyethylenimine (PEI). As the functionalization occurred, the water soluble PEI transformed into a thermoset. Adjusting stoichiometry and type of citrate ester allowed fine-tuning of hydrophobicity, modulus, and swelling behavior. Based on control experiments of citrate esters with model amines, FTIR analysis of thermosets, swelling experiments, and TGA data, molecular models were constructed to represent the thermosets. Quantifying the hydrophobicity of these molecular models with computational octanol-water (LogPoct) partition coefficients provided a method to assess how hydrophobicity changes during a post polymer modification (PPM) reaction.