| Poly(ethylene-co-n-butylacrylate- co-carbon monoxide) (polyEB*C) samples, selectively 13C-labeled in different position of the n-butyl acrylate monomer, were characterized by using two-dimensional (2D) and three-dimensional (3D) pulsed field gradient (PFG) 750 MHz NMR spectroscopy. In the past decades, nuclear magnetic resonance (NMR) has become a powerful tool for studying the microstructures of polymers. Early works involved one-dimensional (1D) NMR, which, although useful in solving various problems, had many limitations that lead to ambiguity and errors. Two-dimensional (2D) NMR surmounts many of these limitations, making the process of obtaining unambiguous NMR resonance assignments a routine one, in the process of identifying the monomer and stereosequence distribution, chain-end and branching structures and tacticity of copolymers. 2D-1H/13C-heteronuclear single quantum coherence (HSQC) and heteronuclear multiple bond correlation (HMBC) experiments were conducted by selectively exciting the enhanced resonances in the spectra of two polymer samples, one polymer resulting from synthesis with 1-13 C-n-butylacrylate monomer and a second polymer obtained from a synthesis with 2-13C-n-butylacrylate monomer, to elucidate the complex structure of the terpolymer. High-resolution 2D-NMR combined with 13C-labeling of the polymer greatly simplifies the 2D-NMR spectra, selectively enhances the weak peaks from low occurrence B-centered triad structures and aids in their resonance assignments. However, the 2D-NMR spectra of the terpolymer are still quite complex because of severe overlap of the resonances arising from the large number of structures from permutations of monomer sequence, chain-end and branching structures and requires further spectral simplification for complete and unambiguous resonance assignments. Triple resonance three-dimensional (3D) NMR was found to be a solution to this problem if an abundant third NMR-active nucleus like 19F or 31P is present in the polymer. Unfortunately, most of the commercial synthetic polymers, like polyolefins, polyacrylates and polyesters are hydrocarbon-based or lack a suitable third NMR active nucleus. Selective 13C-labeling of all or one carbon in a monomer and use of shaped pulses (which treat the resonances from the labeled 13C as if they were from a third NMR active nucleus) along with 3D NMR is a solution to this complex problem as it greatly simplifies the spectra by dispersing the resonances into a third dimension and provides atomic connectivity information among three different nuclei. Therefore, a suite of new 3D NMR techniques were developed for the characterization of hydrocarbon-based terpolymers and are used to disperse and assign the 1H and 13C resonances of 13 C-labeled poly(ethylene-co-1,2,3-13C3- n-butylacrylate-co-carbon monoxide). The 3D HC ACX and HCACX-HH-TOCSY experiments provide enormous spectral dispersion, permitting the resolution of many signals that overlap in the 2D-HSQC and HMBC spectra and gives atomic connectivity information in three different nuclei. In addition, to aid in the unambiguous monomer and stereosequence assignment of the terpolymer, three poly(ethylene- co-n-butylacrylate) (polyEB) copolymer samples were also characterized with the help of 2D-HSQC and HMBC 750 MHz NMR spectroscopy. |
