Despite the vast amount of information available, it is impossible to draw the exact chemical structure of native and technical lignins. There is incomplete information on the exact chemical structures of all lignin structural units, their linkage modes and more importantly the sequence of lignin subunits in the macromolecule. However, visualization of current information in a plausible lignin structural model is certainly useful for understanding structural features of lignin. Herein, we combine available analytical data, both from own work and literature, with recent findings on the absolute molecular weights of MWL to suggest an up-to-date structural model of softwood and hardwood MWLs as well as main technical lignins. Spruce MWL consists of 25 C9-units on average being highly branched and crosslinked with short chains. In addition to etherified 5-5’ moieties, as widely accepted branching/crosslinking points, our analysis indicates that about half of lignin branching/crosslinking points are located in the side chain. Quantitative 13C NMR indicates that the branches are apparently of aliphatic ether (alkyl-O-alkyl) type at the a- or/and g-positions of the side chain (with intact b-O-4 linkages). The exact structure of these moieties is not fully defined yet: spectral data indicates slight structural diversity, offering challenges for precise 2D NMR elucidation. The degrees of branching of hardwood (HW) MWLs are lower than that of softwood, but still significant. Structural models of technical (e.g., kraft softwood) lignins are much more challenging due to extremely high diversity of their structural elements. Therefore, we suggest a correct way to vizualize their structure that reflects available structural information but avoids claiming unjustified features.

On the vizualizaton of lignin structure

Macromolecular Chemistry: The Second Century

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ACS Spring 2020 National Meeting & Expo

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.