**2. Lignin Polymers and Antioxidant Properties**

The review of lignin polymers as potential antioxidants for biodegradation of polymers and antimicrobial treatment [29], tumor-targeted drug delivery, food fortification [28], and diabetes treatment [15] is supported by the chemical composition of lignin structures, abundance in the natural environment, and ease of production; conservative estimates suggest that at least 70 million tons of lignin are generated from pulp [35]—of which 2% is converted to value-added products including lignin for biological, pharmaceutical and chemical industry applications [35]. The widespread availability of lignin in plant species predicts cost. The availability of lignin is predicted by the rate of lignin biosynthesis, which is responsible for biotic and abiotic stress management, organ/tissue development, and growth [36]; these processes are mediated by a broad class of enzymes and genes. Higher expression of CCR, CCoAOMT B, C3H, 4CL, and F5H was associated with pronounced tissue growth [37]. Since the genes predict the presence of G, S, and H units [16], the genetic composition of the different plant species strongly predicts the utility of the plant lignin in tumor-targeted drug delivery, food fortification [28], biodegradation of polymers, and antimicrobial treatment [29]. The observations made by Liu et al. [16] are valid, considering that the G, S, and H units which comprise sinapyl alcohol, coniferyl alcohol, and p-coumaryl alcohol, respectively, predict the rate of lignin monomer copolymerization. The function of different genes on plant species is heightened in Table 4.



The focus on selected plant species as precursors for lignin-based polyurethanes, dopamine polymerization, polydopamine and copolymers, polymerization of inulin, enzyme-catalyzed polyphenol polymerization, antioxidant quercetin polymers, polyquercetin and quercetin copolymers, and antioxidant properties, antioxidant terpene polymers, polylimonine synthesis and properties, antioxidant random copolymers, and gallic acid grafted polymers is beyond the scope of this research inquiry. This means that the desired chemical properties of lignin-based compounds are considered in general without a specific emphasis on a particular plant species.

Lignin molecules have vast antioxidant properties due to the presence of certain functional groups such as p-hydroxy acetophenone extracted from oil palm fronds [38]. The antioxidant properties of lignin-derived polymers are predicted by the precursors. Different precursors have different antioxidant properties, which are predicted by the presence of carbohydrates, G + S phenols, Pi-conjugated carbons, ArC (Py-products with primary, secondary and tertiary carbons on the side chains), carboxylic groups, OH groups, and phenols [14]. The number of functional groups that predict chemical behavior considering the chemical structure of lignin is poorly understood—the aliphatic OH and phenol groups increase the probability for functionalization of the compound [10] and biological and pharmacological function. The antioxidant properties and higher binding affinity of the chemical functional groups in lignin enable the material to bind to bile acids in the intestines—a process that facilitates serum control.

The antioxidant properties are also integral to tumor suppression—animal models suggest that lignin reduces the adverse effects associated with different carcinogens, including 3,2-dimethyl-4-ami-biphenyl [15]. The phenolic compounds isolated from the different lignin structures have been proven to inhibit microbial growth through the inhibition of oxygen-mediated reactions, ATP depletion, and interference with the intracellular pH [35].

In other cases, organic functional groups within the lignin structure (carvacrol, thymol, and cinnamaldehyde) trigger bacterial lysis and damage the cell membrane [35]. The lignin compounds extracted from corn Stover have exhibited appropriate antioxidant properties in eliminating free radical initiators in the red blood cells such as AAPH (2,2-azobis (2-amidinopropane)) [35]. The trends and applications of lignin as a natural antioxidant supersede the antimicrobial effects; this means that natural lignin best functions as a free radical scavenger/borrower [35]. The utility of the antioxidant properties transcends pharmaceutical applications—the chemical antioxidant properties offer protective benefits to the skin and eyes; this means that topical formulations can be prepared from lignin extracts for cosmetic applications. Other potential commercial applications can be explored due to the heterogeneity of lignin structures drawn from diverse sources. Moreover, research has demonstrated that it is possible to customize the behavior of lignin in different applications to stimulate the desired biological function [35].

Even though diverse plants are a suitable source of lignin, only selected sources are used for commercial applications due to wide variations in the lignin composition by weight 10–40 wt%, the need to balance resource use and promote green economy [39]. In other cases, the herbaceous weight composition of biomass is lower 15–25% (*w*/*w*) [15]; this explains why most lignin is derived from byproducts during pulp production [15,39]. The production-related requirements have practical implications on the choice of lignin precursors for biological, polymer processing industries, agricultural, cosmetics, textiles, and agricultural applications. This observation is supported by the link between the precursor and chemical properties and production methods [35]. Certain sources are associated with a higher cost of production, low yield extraction, higher energy use, poor solubility, and presence of chemical impurities; this means that the choice of fractionation methods of technical lignin predicts commercial utility [35].

Even though precursor-specific chemical functional groups might be ideal in the development of lignin-based antioxidants for pharmaceutical, medical and packaging applications, other factors have to be taken into account, including the availability of the plant species and lignin content. The *Quercus* plant species have an extremely low lignin content (3.8%); this is in contrast to 54% in *Acacia auricuriformis* [36]. The high concentration of lignin in the latter justifies its use as a preferred source of antioxidants for a broad array of applications. However, there is significant information asymmetry about the performance of different plants with variable concentrations of lignin—most studies focus on selected plant species [14], with proven benefits. The bias towards selected plant species could have practical implications in commercial applications.
