Chemical structures and molar masses of water-soluble TEMPO-oxidized products prepared from 20 % NaOH-treated...

TEMPO-catalyzed oxidation is a unique method for converting primary C6-hydroxymethyl groups in water-insoluble regenerated cellulose materials to sodium C6-carboxylate groups in water at room temperature to provide water-soluble polyglucuronates. In this study, 20 % NaOH-treated bacterial cellulose (BC), cotton lint (CL), and ramie cellulose (RC) were oxidized to prepare water-soluble polyglucuronates with high degrees of polymerization and high mass recovery ratios. Solid-state CP/MASS ¹³C NMR spectra of the water-soluble products indicated that they contained considerable amounts of C2/C3-ketone hydrate structures (50-60 % of glucuronosyl units). These ketone hydrate structures were reduced to hydroxy groups using NaBH₄ to form almost pure poly β-(1 → 4)-glucuronic acids. When the 20 % NaOH-treated and never-dried wet CL and RC samples were oxidized, the mass recovery ratios of the water-soluble fractions reached 87 % and 98 %, respectively. Although the mass-average degree of polymerization (DP_w) values of the 20 % NaOH-treated BC, CL, and RC samples were 622-1617, the water-soluble oxidized products exhibited DP_w values as low as 79-136. Therefore, the formation of ketone hydrate structures in water-soluble oxidized products prior to reduction was clarified, and the depolymerization mechanisms of the oxidized products with NaOCl/NaOBr during oxidation was proposed.