A class of amphiphilic macromolecules has been synthesized by combining well-defined polystyrene (PS) with poly(propylene imine) dendrimers. Five different generations, from PS- dendr -NH 2 up to PS- dendr -(NH 2 ) 32 , were prepared in yields of 70 to 90 percent. Dynamic light scattering, conductivity measurements, and transmission electron microscopy show that in aqueous phases, PS- dendr -(NH 2 ) 32 forms spherical micelles, PS- dendr -(NH 2 ) 16 forms micellar rods, and PS- dendr -(NH 2 ) 8 forms vesicular structures. The lower generations of this class of macromolecules show inverted micellar behavior. The observed effect of amphiphile geometry on aggregation behavior is in qualitative agreement with the theory of Israelachvili et al . The amphiphiles presented here are similar in shape but different in size as compared with traditional surfactants, whereas they are similar in size but different in shape as compared with traditional block copolymers.

The self-assembly of amphiphilic dendrimers based on poly(propylene imine) dendrimers of five different generations with up to 64 end groups modified with long hydrophobic chains has been studied. At the air−water interface stable monolayers form in which the dendritic surfactants presumably adopt a cylindrical shape; all the chains are aligned perpendicular to the water surface, and the dendritic poly(propylene imine) core faces the aqueous phase. Electron microscopy and dynamic light-scattering measurements performed on aqueous solutions of the amphiphiles at pH = 1 showed the formation of small spherical aggregates with diameters varying between 20 and 200 nm. X-ray diffraction of cast films of these aggregates revealed bilayer thicknesses of 48−54 Å. A phase transition was detected by measuring fluorescence anisotropy. The theoretical volumes of the dendritic amphiphiles were in good agreement with those calculated from the monolayer experiments and X-ray diffraction data. Hence, the amphiphilic dendrimers within the aggregates in solution have the same highly asymmetric conformation as that proposed at the air−water interface. Calculations showed that the shape of the dendritic poly(propylene imine) core in the aggregates is distorted and that the axial ratio (r b:r a) ranges from 1:2.5 for the first generation to approximately 1:8 for the three highest generation of dendrimer. This behavior illustrates the high flexibility of the poly(propylene imine) dendrimers.

Abstract:The protonation behavior of poly(propylene imine) dendrimers and some related oligo amines has been measured using natural abundance 15 N-NMR. The chemical shifts of the different shells of nitrogen nuclei as a function of pH have been 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 constants and for the pair interaction free energies between the different nitrogen nuclei. This subsequently leads to a quantitative description of the titration curves of the individual shells and to a rationalization of the microscopic and macroscopic equilibrium constants for these molecules.