On the structure of softwood kraft lignin†
Two constitutional structural schemes are proposed attempting to unify and rationalize a series of focused NMR and chromatographic determinations aimed at providing an integrated picture for the structure of softwood kraft lignin. The complexity of native softwood lignin when coupled with the complexity of the kraft pulping process is known to lead to a rather heterogeneous material that has eluted us to date. The present work embarks at applying state-of-the-art quantitative 1D and 2D NMR methods on carefully isolated softwood kraft lignin samples and fractions. The accumulated data, when coupled with size exclusion chromatography, mass spectrometric analyses and literature accounts that pertain to the chemistry of kraft pulping, provide the following picture for softwood kraft lignin. Softwood kraft lignin is composed of two distinct fractions that can be separated by using anhydrous acetone. The acetone insoluble fraction is a somewhat branched polymeric material that still contains a variety of native wood lignin bonding patterns, albeit in significantly reduced abundance, as well as new structures induced during the process. The acetone soluble fraction is a significantly more branched and less polymeric material with an abundance of chemical structures that may be created when oligomeric phenols react under kraft pulping conditions. To account for the presence of the various moieties in these two fractions, kraft pulping fragmentation and repolymerization chemistries are extensively invoked, including radical processes initiated by sulfur.