Is NMR Combined with Multivariate Regression Applicable for the Molecular Weight Determination of Randomly Cross-Linked Polymers Such as Lignin?

Abstract

The molecular weight properties of lignins are one of the key elements that need to be analyzed for a successful industrial application of these promising biopolymers. In this study, the use of 1H NMR as well as diffusion-ordered spectroscopy (DOSY NMR), combined with multivariate regression methods, was investigated for the determination of the molecular weight (M w and M n) and the polydispersity of organosolv lignins (n = 53, Miscanthus x giganteus, Paulownia tomentosa, and Silphium perfoliatum). The suitability of the models was demonstrated by cross validation (CV) as well as by an independent validation set of samples from different biomass origins (beech wood and wheat straw). CV errors of ca. 7–9 and 14–16% were achieved for all parameters with the models from the 1H NMR spectra and the DOSY NMR data, respectively. The prediction errors for the validation samples were in a similar range for the partial least squares model from the 1H NMR data and for a multiple linear regression using the DOSY NMR data. The results indicate the usefulness of NMR measurements combined with multivariate regression methods as a potential alternative to more time-consuming methods such as gel permeation chromatography.

Is the Calibration Transfer of Multivariate Calibration Models between High- and Low-Field NMR Instruments Possible? A Case Study of Lignin Molecular Weight

“…Gel permeation chromatography (GPC) measurements were used to determine the M w of the lignin samples. The GPC measurement conditions are described by Burger et al…”

“…These calibrations are based on the recently published method for the molecular weight determination of this complex heteropolymer using 1 H NMR and PLS regression. 14,15 As lignin is a promising raw material for use in a sustainable chemical industry, methods for its fast, accurate, and reliable characterization are required.…”

“…Gel permeation chromatography (GPC) measurements were used to determine the M w of the lignin samples. The GPC measurement conditions are described by Burger et al 14 1 H NMR Measurements. For the NMR measurements, 50 mg of lignin was dissolved in 700 μL of DMSO-d 6 + 0.03% (v/ v) tetramethylsilane.…”

“…Weight average molecular weight ( M w ) calibrations of lignin samples were developed on a high-field NMR instrument at 600 MHz and two benchtop instruments at 43 and 60 MHz, respectively. These calibrations are based on the recently published method for the molecular weight determination of this complex heteropolymer using 1 H NMR and PLS regression. , As lignin is a promising raw material for use in a sustainable chemical industry, methods for its fast, accurate, and reliable characterization are required.…”

Diffusion-Ordered Spectroscopy for Rapid and Facile Determination of Consumer Plastic Molecular Weight

“…DOSY has mainly been applied thus far to a limited number of polymer types (e.g., polymers with corresponding standards for GPC such as polystyrene, or those with well-defined end groups to compare to NMR measurements), but its application is theoretically universal for different polymers. For example, DOSY has recently been applied to determine the MW of natural polymers such as lignin and heparin, relating 1 H and DOSY spectra to GPC-determined MWs using multivariate correlations. , DOSY should therefore be a suitable alternative method to GPC for the analysis of plastic consumer products, which span a wide range of polymer types and presumably MWs. Furthermore, the potential for DOSY as a method to determine polymer MWs for experimental work (e.g., degradation of polymers in engineered or environmental systems) has only recently been introduced and not yet fully validated.…”