Polystyrene-poly(propylene imine) dendrimers: Synthesis, characterization, and association behavior of a new class of amphiphiles

A new class of amphiphilic macromolecules has successfully been synthesized by creating well-defined diblock copolymers of polystyrene (PS, M(n) = 3.2 x 10(3), M(w)/M(n) = 1.04) with poly(propylene imine) dendrimers. A polystyrene core molecule with a primary amine end-group was prepared by a quantitative three-step modification procedure of acid-functionalized polystyrene. On this core molecule, five different generations from PS-dendr-NH2 up to PS-dendr-(NH2)(32) were constructed in high yields. The molecular structure of the block copolymers was analyzed in detail with NMR and IR spectroscopy and electrospray mass spectrometry. With conductivity measurements and monolayer pressure-area isotherm determinations, we observed generation-dependent amphiphilic behavior. Dynamic light scattering and transmission electron microscopy showed that amphiphile geometry had a distinct effect on aggregation behavior; this is in qualitative agreement with Israelachvili's theory. Critical association concentrations determined with the pyrene probe luminescence technique were as low as 5 x 10(-7) M. The amphiphiles presented here, therefore, show a resemblance to traditional block copolymers with regard to size and stability, and are similar to surfactants with respect to tuning of the shape.