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

Abstract

Lignin model compounds were reacted with formaldehyde in acid medium at positions meta to the aromatic hydroxy groups. Both phenolic and etherified phenolic lignin model compounds were shown to give fast polymerization or hydroxymethylation of the meta positions depending on the conditions used. This reaction differs from the reaction of formaldehyde with phenolic lignin model compounds under alkaline conditions, where the reaction with formaldehyde always occurs at positions ortho/para to the aromatic hydroxy group. The conditions developed for the polymerization and hydroxymethylation were also evaluated on industrial lignin. The reaction of formaldehyde with the meta positions of lignin clearly have considerable potential for the use of lignin, particularly heavily condensed alkali lignin, in polymeric applications.Lignin, present as a waste material in the spent liquors of the pulping industry, constitutes a potentially useful raw material for the production of various polymeric products. This has repeatedly been demonstrated in the past by its application in products ranging from wood adhesives to plastics (1). In these applications the lignin is crosslinked to increase its molecular mass or to form a rigid three-dimension ally crosslinked structure.The features on lignin that were utilized for these polymerization or modification reactions are as follows (compare Fig. 1):1. phenolic hydroxy group; 2. aliphatic hydroxy group; 3. unsubstituted 3-or 5-positions on C9 units; and 4. structures that can form quinonemethide intermediates.

Polymers From Renewable Resources

“…That this is indeed the branching mode was suggested by the fact that model (79), which simulates a polymer chain before hydride-ion abstraction and which cannot lead to conjugated patterns for lack of sufficient precursor length, does not react with 5-methylfuryl alcohol (43b), which simulates a monomer or polymer end-group. Contrary to what had recently been suggested, 196 it is therefore not an acid-catalyzed condensation between CH 2 OH and CH 2 groups that engenders the branches, but instead the equivalent reactions involving CH 2 OH and -CH= groups (except the H-3 and H-4 of furanic moieties).…”

“…It has been shown by very detailed studies involving model compounds 79 that both guaiacyl and syringyl units are susceptible to electrophilic substitution reactions at the meta, or 3 and 5, positions with respect to the phenolic OH groups, i.e. the permanently free sites in structures (21)- (23).…”

“…Again, part of the answer came from the use of model compounds. The 2,5'-dimethylated 'dinners' (79), (80) and (81) of the three furanic alcohols mentioned above in the context of polycondensation experiments were prepared together with the corresponding 'trimer' (82).…”

“…Treatment of these molecules with a hydride-ion abstractor, such as a dioxolenium salt, reveals that (79) and (80) give rise to the same UV-visible spectrum, characterized by a strong peak at 495 nm, whereas (81) does not react. It seems reasonable to postulate that the loss of H~ creates the corresponding secondary and tertiary carbenium ions from (79) and (80), which are very similar in structure and which are responsible for the visible absorption associated with the high delocalization of the positive charge over the entire difuranic structure.…”

“…It seems reasonable to postulate that the loss of H~ creates the corresponding secondary and tertiary carbenium ions from (79) and (80), which are very similar in structure and which are responsible for the visible absorption associated with the high delocalization of the positive charge over the entire difuranic structure. With (81), the lack of mobile hydrogen atoms makes the system unreactive towards the hydride ion abstractor.…”