Synthesis and Protonation Behavior of Carboxylate-Functionalized Poly(propyleneimine) Dendrimers

Five generations of carboxylate-functionalized poly(propyleneimine) dendrimers have been synthesized starting from a double Michael addition of amine-functionalized poly(propyleneimine) dendrimers to methyl acrylate followed by basic hydrolysis using LiOH in a water/methanol mixture. The dendritic compounds have been characterized using 1 H NMR, 13 C NMR, IR, and ESI−MS spectroscopy. Subsequently, the protonation behavior of these potential complexing agents is studied with potentiometric titrations in 0.1 and 1.0 M KCl solutions. The carboxylate-functionalized dendrimers show a characteristic onionlike shell protonation behavior. The titration curves for different ionic strengths cross at pH 8.5 which is exactly after protonation of the outermost shell of amines ( θ ∼ 0.25). This crossing point is found not to be identical to the point of zero charge θ ∼ 0.5, which is observed at pH 4.5. The Ising model was used to rationalize the complete set of titration data simultaneously for five generations with only one microscopic p K value for each additional shell and a set of three nearest neighbor pair interaction parameters.