Solution size variation of linear and dendritic bis-MPA analogs using DOSY-H NMR

Dendrimers are globular, multi-functional, monodisperse macromolecules with perfect structure fidelity. Their architecture is composed of a series of branched polymeric arms, composed within "wedges", that emanate from a central core. Their structure contains a high density of functional groups located at their periphery, referred to as the "outer shell". Due to their globular structure, it is assumed that the relative "size" of a dendrimer does not fluctuate greatly between solvents. This may be due to the inability of the branched arms, or wedges, to significantly expand or collapse (comparative to analogous linear polymers) owing to steric barriers from branching, especially at higher generations. In contrast, it is expected that a linear polymer, of similar molecular weight to a dendrimer analog, would have a greater degree of size variation dependent on solvent-polymer interactions. This stems from its innate flexibility and greater conformational flexibility. For this investigation, analogous dendritic and linear bis-MPA polyesters as well as poly(caprolactone) (PCL) were analyzed using size-measuring techniques including gel permeation chromatography (GPC) and diffusion ordered spectroscopy-nuclear magnetic resonance (DOSY- 1 H NMR). The size and size variability of bis-MPA dendrimers is shown to be smaller by DOSY- 1 H NMR than their linear analog, PBBM. This was accomplished using five different solvents and for the first time confirms, experimentally, what has been theorized.