3D NMR Studies of C-Centered n-ad Structures in Poly(ethylene-c o-n-butylacrylate-c o-13C-carbon monoxide) (PolyEBC)

Three-dimensional HCACX and HCACX−HH−TOCSY NMR experiments (J. Magn. Reson. 2004, 168, 352) at 750 MHz have been used to investigate the resonance assignments and monomer sequence distributions of C-centered triads and pentads in poly(ethylene-co-n-butyl acrylate-co-13C-carbon monoxide), poly(EBC*) terpolymer prepared from 13C-labeled carbon monoxide. These experiments provide spectra with correlations among mutually coupled atoms with δCO plotted along the f 1 dimension and δCA and δH of adjoining groups plotted along the f 2 and f 3 dimensions, respectively. The correlation among signals in three planes in a 3D NMR spectrum facilitates selective detection of the resonances from the structure fragments near the 13C labeled ketone carbonyl groups. Moreover, these experiments provide enormous spectral resolution, permitting the identification of numerous unresolved signals that are overlapped in 2D 1H−13C HMBC NMR spectrum. The correlation of resonances involving three different nuclei (δH, δCA, and δCO) provides unambiguous atomic connectivity information in the polymer backbone that facilitates analysis of comonomer composition, stereo-sequence distribution and branching structures. Occasionally, these experiments provide additional proof of the methods adopted to perform chemical shift assignments from 2D HMBC NMR experiments.