Multidimensional Nmr Studies Of The Polymers

Author : Deepa Savant
Publisher :
ISBN 13 :
Total Pages : 134 pages
Book Rating : 4.:/5 (115 download)

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Download or read book Multidimensional NMR Studies of Terpolymers Poly(ethylene-co-vinyl Acetate-co-carbon Monoxide) and Poly(ethylene-co-1-hexene-co-carbon Monoxide) written by Deepa Savant and published by . This book was released on 2006 with total page 134 pages. Available in PDF, EPUB and Kindle. Book excerpt: A series of poly(ethylene-co-vinyl acetate-co-carbon monoxide) (polyEVC) terpolymer samples with varying monomer compositions were studied using a variety of one- and two-dimensional nuclear magnetic resonance (NMR) techniques on a 750 MHz NMR instrument. NMR is one of the powerful techniques to study the microstructures of polymers. For a long time, one-dimensional (1D) NMR was used to identify polymer structures. Although useful for solving problems at initial stages, it can only help to approximate the assignment of resonances in complex polymers. In such cases, two-dimensional (2D) NMR can be used to disperse the resonances in a second dimension, thus solving overlap of resonances. The resonances can also be confirmed by correlation experiments. In the study of unlabeled polyEVC polymers 1D 1H and 13C NMR experiments were used for the primary assignment of resonances. The spectra were found to contain extreme overlap due to various combinations of resonances from the three monomers used in the polymer. To overcome this problem, 2D NMR experiments were used. 2D 1H/13C heteronuclear single quantum correlation (HSQC), heteronuclear multiple-bond correlation (HMBC) and HSQC followed by total correlation (HSQC-TOCSY) experiments were used. These experiments not only helped to confirm the assigned resonances but were also useful to assign new resonances from low probability units. Thus, due to improved dispersion, the study was not just restricted to triad levels but in some cases assignment could also be done at the tetrad or pentad levels. It was also possible to identify and assign distinct resonances from chain-ends and short chain branches. In spite of the dispersion in 2D NMR, some regions of the spectra could not be assigned unambiguously. This is due to extreme overlap of resonances from large number of n-ads produced from the monomers and stereosequences. To circumvent this problem the study was continued using three-dimensional (3D) NMR experiments. The 3D experiments used so far needed a third NMR active X-nucleus; but most of the commercially used polymers lacked one. To study the polymers which are primarily hydrocarbon based and thus are deficient in a third NMR active X-nucleus, a new suite of 3D NMR experiments was introduced recently. These experiments require that at least the structures of interest had 13C enrichment. This allows treatment of the two carbon nuclei as two different NMR active centers which can be selectively excited at two different times and correlated in a 3D spectrum. This approach was recently tested and applied in the structural study of an ethylene based polymer which was labeled selectively at n-butyl acrylate positions. Here, the two 3D NMR experiments, gHCACX and gHCACX-HH-TOCSY, were used for the structure elucidation of a similar polymer of ethylene, vinyl acetate and carbon monoxide which was selectively labeled at the two adjacent olefinic carbons of vinyl acetate. The 3D experiments provided enormous spectral dispersion, permitting the resolution of vinyl acetate signals which showed a tremendous overlap in 2D spectra. The two complimentary experiments facilitated the identification and assignment of resonances up to the pentad level which could not even be identified from 2D NMR experiments. These experiments were further applied in the study of another terpolymer poly(ethylene-co-1-hexene-carbon monoxide) (polyEHC*) which was selectively 13C enriched at ketone carbonyl position. The polymer was prepared with high concentrations of carbon monoxide (C) to achieve alternating polymer where C alternated with either ethylene (E) or 1-hexene (H). 1D proton/carbon and 2D HSQC/HMBC experiments were used initially to assign the resonances. HMBC experiment was not very useful here as it failed to separate correlations from different environments near carbonyl carbons. 3D NMR experiments were extremely useful here to obtain connectivity information in the polymer. Two different environments were identified around the ketone carbonyl carbon. With the aid of 3D NMR, it was possible to confirm the alternating nature of this polymer.