Determination of molar-mass parameters of TEMPO-oxidized pulps by SEC/MALLS/RI

Molar-mass (MM) and MM distributions of TEMPO-oxidized pulps (TEMPO: 2,2,6,6-tetramethylpiperidine-1-oxyl radical) measured under the same conditions as those of the original pulps are important for understanding the changes of MM parameters of cellulose molecules in pulps during TEMPO-catalyzed oxidation. The carboxy groups in TEMPO-oxidized pulps prepared from softwood and hardwood bleached kraft pulps were esterified in high yield by trimethylsilyl diazomethane using a newly established protocol, which enabled complete dissolution in 8% (w/w) lithium chloride/N,N-dimethylacetamide (LiCl/DMAc). A linear relationship was obtained between the carboxy contents (0.05–2.68 mmol/g) of TEMPO-oxidized pulps and the signal area ratio of methyl/C1 carbon atoms in solid-state ¹³C NMR spectra of the esterified products. Next, 1% (w/v) LiCl/DMAc solutions of the original pulps and TEMPO-oxidized and then esterified products were subjected to size-exclusion chromatography with refractive index and multi-angle static laser light scattering detection. Chromatographic elution patterns, MM plots, conformation plots, and related MM parameters of the pulps and esterified products were obtained with good reproducibility. The results showed that the changes of the degree of polymerization and its distribution for TEMPO-oxidized pulps can be investigated in terms of carboxy content, mass recovery ratio, and TEMPO-catalyzed oxidation conditions.

Graphical abstract