Lignin is the largest renewable feedstock for aromatic compounds, making its valorization an important challenge to overcome during the green transition. One of the major hurdles in valorizing lignin is its heterogeneous nature and complex structure, making the raw material characterization particularly challenging. 
One of the key parameters for valorization is knowing the molecular weight of lignin, which is usually determined by Size-Exclusion Chromatography (SEC). In this work, we have investigated an alternative method, determining the molecular weight of lignins by Nuclear Magnetic Resonance (NMR) spectroscopy. Diffusion Ordered SpectroscopY (DOSY) is a common method for confirming proper grafting in polymer synthesis and determining the molecular weight of a polymer, which works by measuring the diffusion of a polymer in a solvent and comparing it to the diffusion of a polymer standard. Some publications have also shown successful determination of the molecular weight of lignin with DOSY, reporting values comparable to those from SEC methods.1–3 
With this work, we present an in-depth investigation into utilizing DOSY to determine the molecular weight of lignin. Various factors (such as lignin fractionation, sample concentration, agglomeration, and choice of standards) affected the calculated values for the molecular weight. Sample concentration significantly affected the diffusion coefficient of every lignin tested.  Agglomeration was observed to increase the variability of a lignin’s diffusion coefficient but could be mitigated, for example by filtration. Two commonly used linear polymers, poly(methyl methacrylate) and poly(styrene sulfonate), were used as calibration standards for the molecular weight determination of lignin. Tannins with known weight-average molecular weight were also analyzed to validate the results from the calibration curves.
Keywords: Lignin, diffusion NMR, molecular weight determination