*1.2. Natural Phenolic Polymers (Lignin and Proanthocyanidins (PAs))*

Lignin is a naturally occurring polymer/heterogeneous biomacromolecule that supports the connective tissues in plants [13–15]. The rigidity, tensile strength, resistance to chemical degradation, and pressure-resistance of lignin are linked to the phenyl propane created using coniferyl alcohol, p-coumaryl alcohol, and sinapyl alcohol units [16]—precursorspecific structural modifications of lignin translate to unique, different structural modifications. The microscopic-level changes improve the utility of the material in the production of plastics, paints, films, PU-based forms, nutritional supplements, resins and coatings, beverage additives, food additives, adhesive binders, carbon fiber composites, and enhancement of the structural properties of existing materials [17]. The application of lignin-based compounds in dopamine polymerization, synthesis of polydopamine and copolymers, polymerization of inulin, enzyme-catalyzed polyphenol, polymerization, antioxidant quercetin polymers, polyquercetin and quercetin copolymers, and antioxidant properties, and antioxidant terpene polymers is explored under Sections 2 and 3.

Proanthocyanidins are a class of natural phenolic compounds derived from natural phenols (flavan-3-ols) and are fundamental polyphenolic components of the human diet, including red wine, cocoa, dark chocolate, orange juices, grapefruit, parsley, black and green tea, onions, olives, broccoli, and apple skin [18,19]. The PAs are critical to the investigation of sustainable and natural monomers, synthesis, and applications of antioxidant polymers considering that they are the second most naturally occurring and abundant plant polyphenols after lignin [18]. In addition, PA compounds possess ideal chemical properties for industrial and biological applications. For example, cinnamon-derived polyphenols have been proven to have superior anti-inflammatory characteristics–a factor that is instrumental in the treatment of inflammation-related conditions, including inflammatory bowel disease, rheumatoid arthritis, asthma, cancer, diabetes, cardiovascular diseases, degenerative musculoskeletal diseases, and neurodegenerative conditions. The need for plant-based alternatives is supported by the limitations of traditional non-steroidal anti-inflammatory drugs (NSAIDs) [20], the manifestation of appropriate antioxidant properties [21,22], and a variety of naturally occurring precursors, including agro-wastes.
