Methylation of softwood kraft lignin with dimethyl carbonate

The reactivity and functionality of technical lignin requires reliable modulation in order to be used as a precursor for a variety of applications. A green alternative for lignin methylation using dimethyl carbonate (DMC) is reported and this paper discusses our efforts toward optimization and structural elucidation for such reactions. It is demonstrated that softwood kraft lignin can be progressively and reproducibly methylated to different extents using DMC in the presence of sodium hydroxide or cesium carbonate as bases with the latter requiring milder reaction conditions. C-13 NMR, FT-IR, and quantitative P-31 NMR spectroscopic analyses were used to document and understand the structural changes occurring within the methylated lignin derivatives. While the phenolic hydroxyl groups of lignin are methylated, the reduction in aliphatic -OHs is also observed in control and methylation reactions, most likely via a solvent mediated intramolecular rearrangement reaction. As anticipated, the methylation induced thermal stability, elimination of thermally induced crosslinking and lowering of the glass transition temperature. Overall, the developed chemistry offers a green alternative to a much sought derivatization reaction that adds value to an otherwise intractable and underutilized biopolymer.