Polymerization of lignin model compounds with formaldehyde in acidic aqueous medium

Abstract

The reaction of formaldehyde with lignin model compounds in acidic medium was shown to give fast crosslinking of alkyl‐substituted phenolic and etherified phenolic lignin model compounds at positions meta to the aromatic hydroxy groups. This reaction differs from the reaction of formaldehyde with phenolic lignin model compounds in alkaline conditions, where the reaction with formaldehyde always occurs at positions ortho/para to the aromatic hydroxy group., The reaction of formaldehyde with lignin in acidic medium have considerable potential for the crosslinking of lignin, particularly heavily condensed alkali lignin, for use in polymeric products.

The Modification of Lignin at Positions 2 and 6 of the Phenylpropanoid Nucleii - Part II1: Hydroxymethylation of Lignin Model Compounds

“…This compound (1) was preferred to a softwood (guaiacylic) model compound owing to its symmetry and the absence of any vacant positions ortho and para to the aromatic hydroxy group. acetoxy groups of the proton NMR spectra, were taken as the yields of hydroxymethylation.…”

“…The hardwood model compound 4-hydroxy-3,5-dimethoxyphenyl ethane (1) was used for the initial hydroxymethylation reactions. This compound (1) was preferred to a softwood (guaiacylic) model compound owing to its symmetry and the absence of any vacant positions ortho and para to the aromatic hydroxy group.…”

Modification of Lignin at the 2- and 6-Positions of the Phenylpropanoid Nuclei

“…4C). With NMR-SPI (selec tive population inversion) techniques it was unevocably proved that the methylene linkage was situated at the 6-positions (13).…”

“…The hydroxymethylation of the hardwood model (13) was first attempted by using a 20 molar excess formaldehyde in 1,1N HC1 in 50% aqueous dioxane (14). Since IS has two reactive sites (2 and 6), this means a ten-fold excess of formaldehyde per reactive site.…”

“…The controlled polymerization of several lignin model compounds at the "meta" positions was subsequently attempted (13):…”