Light-induced yellowing of selectively 13C-enriched dehydrogenation polymers (DHPs). Part 2, NMR assignments and photoyellowing of aromatic ring 1-, 3-, 4-, and 5-13C DHPs

Abstract

Light-induced yellowing of lignocellulosicmaterials has been studied using 13C-enriched DHP (dehydrogenation polymer), selectively 13C-enriched at positions 1, 3, 4, and 5 in the aromatic ring, and quantitative solution state 13C NMR spectroscopy. The NMR study confirmed the results of previous studies using side-chain labeled DHP, mainly that coniferyl alcohol end groups are degraded with the subsequent formation of a- carbonylic structures, e.g. vanillin end groups. The developed technique renders the potential to follow chemical changes of aromatic carbons 1, 3, 4, and 5, independently from each other. Although a drastic photoyellowing of the sheets was observed, definitive chemical shifts corresponding to quinoid carbons could not be discerned in the spectra. The reasons for this may be that the amount of quinoid carbons formed was too low to be detected, with the experimental setup used, or that the quinoid photoproducts, if formed, were not extractable with the solvent used or had reacted further forming non-quinoid structures.

Citation

Parkas, Jim; Paulsson, Magnus; Terashima Noritsugu; Westermark, Ulla; Ralph, Sally. 2004. Light-induced yellowing of selectively 13C-enriched dehydrogenation polymers (DHPs). Part 2, NMR assignments and photoyellowing of aromatic ring 1-, 3-, 4-, and 5-13C DHPs. Nordic pulp and paper research journal. Vol. 19, no. 1 (2004): Pages 44-52