Dramatic effect of solvent on the NMR spectral resolution of hyperbranched polyethers

Abstract

NMR spectroscopy has served as a powerful tool to discriminate between the various types of subunits present in hyperbranched polymers. This discrimination is critical for the determination of degree of branching (DB), which is an important structural parameter that distinguishes hyperbranched structures from dendrimers. However, often times spectral signatures due to the various subunits, namely linear (L), dendritic (D) and terminal (T), are not well-resolved. We demonstrate here that a dramatic enhancement in the peak resolution could be achieved in a series of hyperbranched polyethers, by recording the proton NMR spectra in an aromatic solvent, such as benzene, instead of the usual CDCl₃. The assignment of the various peaks was confirmed by comparison of the longitudinal relaxation times (T₁) of different protons belonging to the individual subunits. Interestingly, the chemical shift differences between the different types of subunits persist even when fairly long spacer segments separate the branch-points in these hyperbranched polyethers. This permitted the determination of the DB of an entire series of hyperbranched polymers of varying branching densities without the need for comparison with model compounds.