Lignin is a complex, irregular biopolymer that binds cells together providing strength and durability to all plants. After cellulose, lignin is the most abundant source of renewable carbon. Currently, the majority of lignin is burned to provide energy in the pulping of cellulose. However, there are many new applications of lignin that offer promising research avenues.
Although all lignins vary in their chemical composition, they are composed of similar basic units. The Freudenberg model1 describes this general chemical composition as a dendritic network polymer of phenyl propene basic units. Isolated lignins are categorized as sulfur-containing lignins or sulfur-free lignins. Of the two categories, the sulfur-containing lignins are more commercialized. However, both types hold great promise for future innovations and breakthroughs.
1. Freudenberg, Karl. (1965). "Lignin: its constitution and formation from p-hydroxycinnamyl alcohols." Science 148.3670:595-600.