Abstract
Lignin-based materials, ligno-p-cresol and ligno-2,4-dimethylphenol were synthesized directly from native lignins through the phase-separation system composed of phenols (p-cresol or 2,4-dimethylphenol) and 72% sulfuric acid. Under alkaline conditions, C1-p-cresol and 2,4- dimethylphenol units nucleophilically attacked adjacent C2, followed by the cleavage of aryl ether linkages to give similar fragments, indicating that both lignophenols had the same switching function. The ligno-p-cresol was highly hydroxymethylated (HM) on the cresolic nucleus to givenetwork-type growing pre-polymers. On the other hand, the ligno-2,4-dimethylphenol hadreactive sites in only terminal units to give linear-type growing pre-polymers. The degree of network-or linear-type polymerization could be controlled by themixing of these pre-polymers. The resulting polymer chains were cleaved at the switching points (1,1-bis(aryl)propane2-O-aryl ether units) under alkaline conditions, leading low molecular weight subunits (switching function). Recyclable lignocellulosic composites were prepared by the combination of cellulose and HM lignophenol. The water resistance of composites got greatly higher with increasing ratio of HM-ligno-p-cresol to HM-ligno-2,4-dimethylphenol. Under alkaline conditions, the composites were re-separated into low molecula rweight lignophenol and cellulosemoieties. The controlled mixing ratio of HM-ligno-p-cresol and HM-ligno-2,4-dimethylphenol leads to the accurate control of mechanical and recycling propertie sof the composites.
