Improved understanding of technical lignin functionalization through comprehensive structural characterization of fractionated pine kraft lignins modified by the Mannich reaction

Amino alkylation of technical lignins via the Mannich reaction can be used as an analytical derivatization technique, but it is also used to improve kraft lignin hydrophilicity or metal ion adsorptive capacity. Recent studies have indicated the potential for increased amination under acidic conditions; however, the effects of pH on the amination of kraft lignins remain poorly understood because indirect quantification methods are often used. A quantitative technique combining 2D HSQC NMR and 13C NMR spectroscopy was used in this study to directly probe the structure of high and low molecular weight fractions of an industrial pine kraft lignin aminated in acidic or basic media with dimethylamine or diethylamine. The degree of amine substitution was two times higher for low molecular weight lignin compared to the high molecular weight fraction. Neither pH nor amine species had any significant effect. New evidence confirms that amine substitution of pine kraft lignin takes place selectively ortho to non-etherified phenolic hydroxyl groups and displaces carboxylic acid groups of benzoic- and cinnamic-type subunits. Other pathways were proposed and tested, but substitution did not occur at sterically hindered sites. Elemental analysis has been the most widely used indicator of Mannich condensation, but unfortunately overestimates amine substitution because nitrogen-containing side products and impurities can be included in the total nitrogen content. Multiple methods should always be employed when characterizing complex, heterogeneous biopolymers like lignins to avoid misinterpretation of results.