Synthesis and protonation behavior of carboxylate-functionalized poly(propyleneimine) dendrimers

Five generations of carboxylate-functionalized poly(propyleneimine) dendrim ers 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 c ompounds have been characterized using H-1 NMR, C-13 NMR, LR, and ESI-MS sp ectroscopy. Subsequently, the protonation behavior of these potential compl exing 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 i onic strengths cross at pH 8.5 which is exactly after protonation of the ou termost shell of amines (theta similar to 0.25). This crossing point is fou nd not to be identical to the point of zero charge theta similar to 0.5, wh ich is observed at pH 4.5. The Ising model was used to rationalize the comp lete set of titration data simultaneously for five generations with only on e microscopic pK value for each additional shell and a set of three nearest neighbor pair interaction parameters.