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 H-1 NMR, C-13 NMR, LR, 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 (theta similar to 0.25). This crossing point is found not to be identical to the point of zero charge theta similar to 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 pK value for each additional shell and a set of three nearest neighbor pair interaction parameters.