Lignin is formed by removal of water from sugars to create aromatic structures. These reactions are not reversible. There are many possible monomers of lignin, and the types and proportions depend on the source in nature. Some typical monomers are shown in the sketch :

The OH groups (either the alcoholic OH's on the chains or the phenolic OH's on the aromatic rings) can react with each other or with the aldehyde or ketone groups. When an OH reacts with another, an ether linkage is formed. As we have seen, an OH reacts with an aldehyde to form a hemiacetal. The reactions of OH groups with ketones forms ketals. An early stage in the condensation of various monomers to form lignin is shown in the next sketch :

There are several groups shown in red that can react further. Some will simply extend the polymer while others would establish cross linking. The monomer that is shaded in orange has three of its functional groups linked to other monomers, so it is starting a branch or cross link. The large lignin molecules fill three dimensions and are heavily cross linked. Sometimes lignin is isolated as a brown powder, but more often it is a gummy mixture of lignins with a wide range of molecular weights.

Lignin resists attack by most microorganisms, and anaerobic processes tend not to attack the aromatic rings at all. Aerobic breakdown of lignin is slow and may take many days. Lignin is nature's cement along with hemicellulose to exploit the strength of cellulose while conferring flexibility.

while on sabbatical leave, ESB, Porto, Portugal July 1996