A comparison of lignin polymer models (DHPs) and lignins by P-31 NMR spectroscopy

Fractionated guaiacyl (G) and guaiacyl/syringyl (GS) DHPs prepared by continuous (Zutropf, ZT) and discontinuous (Zulauf, ZL) dehydrogenation schemes were subjected to quantitative P-31 NMR spectroscopy and their various hydroxyl groups determined. Two milled wood lignin samples from softwood and hardwood species were also examined. It was found that GS-DHPs resemble more GS milled wood lignins than G-DHPs resemble G milled wood lignins. The total phenolic-OH contents of ZT-DHPs are always lower than those of ZL-DHPs, in good agreement with the theory which predicts more beta-O-4 linkages in the former. Furthermore, in agreement with the dehydrogenation theory, the bulk character of G-DHPs results from the formation of condensed units. In GS-DHPs, the S units are etherified early in the polymerization process, while the bulk character of such samples is mainly due to G-units. Regarding the secondary hydroxyl groups the results obtained for the G-DHPs are extremely low. This underlines again the principal differences of G-DHPs compared with GS-DHPs and MWLs. The erythro to three ratio of the G-type samples was found to vary between 1 and 1.5, indicating only minor dependence on molar mass or mode of preparation. The erythro to three ratio in GS-type samples was found to vary from 1.6 to 4.3, showing the highest value for low molar mass ZL-DHPs and lowest values for the cherry tree MWL. Copyright (C) 1996 Elsevier Science Ltd