Abstract
The chemical recalcitrance of lignin limits the industrial processing of biomass, which could be addressed by so-called designer lignins. Dehydrogenation polymers (DHPs) formed by artificial lignification of monolignols, enable studies on structure-property relationships independently of genetic information. Thin films of phenolic acid esters of cellulose were prepared and used for quartz crystal microbalance with dissipation monitoring (QCM-D) experiments to investigate surface polymerization in real-time. The phenolic anchor groups significantly influenced lignification speed, deposited mass, and rigidity of resulting DHP layers. Linkage types in the lignin structure were quantified by HSQC NMR spectroscopy. Polymerization efficiency was increased in the order ferulate < p-coumarate < caffeate. Among the tested anchors, protocatechuate groups were excellently performing the reaction, while vanillate and p-hydroxybenzoate led to minimal deposition of DHPs. Lignification behavior could be correlated with radical stability of phenolic anchor groups and the formation of benzodioxane structures of caffeate moieties. The presence of caffeate units that undergo trapping reaction, prevents cross-linking of cell wall components and enhances digestibility. Moreover, the benzodioxane motif increased rigidity and linearity of lignin, which is advantageous for material science applications, e.g. for bio-based carbon fibers.