Sequential assignments of polymers by 2D NMR techniques: application to stereochemical configuration assignment of poly(acrylic acid)

Two‐dimensional heteronuclear single quantum coherence — total correlation spectroscopy (2D HSQC‐TOCSY) was used to make end‐to‐end sequential assignments and determine stereochemical configuration at the triad, tetrad and pentad levels of poly(acrylic acid). The pseudo three‐dimensional experiment allows heteronuclear ( 13 C‐ 1 H) as well as homonuclear ( 1 H‐ 1 H) corelations in a single 2D NMR experiment. The stereochemical configuration assignment is based on sequential identification of neighboring species. For example, a mmr tetrad resonance (methylene‐ 13 C) is determined by its correlations to the mm and mr triad resonances (methine) that constitute it. Hence, it is independent of chemical shift and peak intensity estimates that are requisites of the conventional method. This self‐consistent technique enables absolute assignments of all magnetically unique tetrads and some of the pentad resonances. In addition, some resonances in the methylene and methine regions that have been difficult to characterize by conventional techniques were unequivocally assigned to endgroup resonances and other polymerization by‐products of the low‐molecular‐weight poly(acrylic acid).