Fractionation and DOSY NMR as Analytical Tools: From Model Polymers to a Technical Lignin

Abstract

One key challenge hindering the valorization of lignin is its structural complexity. Artificial lignin-like materials provide a stepping stone between the simplicity of model compounds and the complexity of lignin. Here, we report an optimized synthesis of an all-G β-O-4 polymer 1 designed to model softwood lignin. After acetylation, the polymer Ac-1(V) was fractionated using a protocol that involved only volatile organic solvents, which left no insoluble residue. Using diffusion ordered spectroscopy NMR in combination with gel permeation chromatography, it was revealed that this fractionated material behaved like a flexible linear polymer in solution (average α > 0.5). Acetylated kraft lignin was subsequently processed using the same fractionation protocol. By comparison with the model polymer, we propose that the acetylated kraft lignin is composed of two classes of materials that exhibit contrasting physical properties. One is comparable to the acetylated all-G β-O-4 polymer Ac-1 , and the second has a significantly different macromolecular structure.

Using Fractionation and Diffusion Ordered Spectroscopy to Study Lignin Molecular Weight.

“…The data in Table S16 imply that a generic set of MHS scaling parameters are applicable to a wide range of lignins. Overall MHS analysis based on 151 data points that were obtained in this and previous studies [21] allows us to propose the generic scaling parameters for lignin of α = À 0.63 � 0.02 and log K = À 8.05 m 2 s À 1 � 0.06 when a solution of lignin at 86 mg mL À 1 in d 6 -DMSO is analysed at 295 K (Figure 3). These parameters could be used to provide an estimate of M w from the diffusivity of lignins analysed solely by a DOSY-NMR experiment.…”

“…Considering the DOSY NMR component of the analysis, we have previously reported that the heavy fractions, with log D values below À 10.6 m 2 · s À 1 , exhibited a significantly higher α scaling factor. [21] This observation was attributed either to the hardware limits of the gradient coil in the standard NMR equipment or to dramatic changes in lignin structure. The latter was further investigated using EPR which showed, in accordance with our hypothesis, that these fractions contained considerably more radicals than the lighter fractions.…”

“…The plot of measured GPC-determined M w against M w derived from DOSY for all Chart also includes data for G1, G2, KL-1 and KL-2 fractionations from our previous report on acetylated model polymer and acetylated Kraft lignin fractionation. [21] Fractions that generated log D values below À 10.6 m 2 s À 1 were not included. 151 data points clearly shows that the largest residuals from the identity line occur at high M w ( Figure S7).…”

“…[20] We have previously shown that diffusion ordered spectroscopy (DOSY-NMR) has potential for use in monitoring lignin MW. [21] This technique initially measures the diffusivity of molecules in solution and this parameter can then be related to the MW of both small molecules [22,23] and polymers including lignin. [15,[24][25][26] In our previous report, diffusion coefficients of lignin fractions of variable MW were linearly correlated with MW data generated via GPC using the Mark-Houwink-Sakurada (MHS) relationship.…”

“…[15,[24][25][26] In our previous report, diffusion coefficients of lignin fractions of variable MW were linearly correlated with MW data generated via GPC using the Mark-Houwink-Sakurada (MHS) relationship. [21] These fractions were generated by a selective dissolution solvent fractionation that used acetylation to enhance the solubility of Kraft lignin in acetone. It is important to note that the DOSY data was generated using a standard NMR probe, making the method accessible to any lab with a modern NMR spectrometer.…”