Protonation Mechanism of Poly(propylene imine) Dendrimers and Some Associated Oligo Amines

Abstract

The protonation behavior of poly(propylene imine) dendrimers and some related oligo amines was measured using natural abundance 15N-NMR. The chem. shifts of the different shells of nitrogen nuclei as a function of pH were directly interpreted as the degree of protonation for that shell. The thus obtained protonation behavior is described systematically using the Ising model which only requires values for the intrinsic protonation consts. and for the pair interaction free energies between the different nitrogen nuclei. This subsequently leads to a quant. description of the titrn. curves of the individual shells and to a rationalization of the microscopic and macroscopic equil. consts. for these mols. The shell protonation curves were, in general, very unlike Langmuir adsorption isotherms, and hence a single microscopic pK value is insufficient to describe them