*3.1. Polydopamine and Copolymers*

The chemistry of polydopamine (PDA) copolymers predicts the synthesis of derivative materials and promising applications in biomedicine, environmental, and energy applications. The application of PDA is predicted by the presence of the different functional groups (azido, alkyl-thiol, amino, carboxyl derivative groups, and carboxyl and alkyl groups) and the synthesis process [62]. There are three feasible mechanisms for polydopamine synthesis from derivatives, namely oxidative copolymerization of the mixtures (comprising of copolymers and monomers), and oxidative polymerization of DA analogs with additional functional groups (see Figure 4). *Polymers* **2021**, *13*, x FOR PEER REVIEW 15 of 33

**Figure 4.** Polydopamine polymerization from different analogs [62]. **Figure 4.** Polydopamine polymerization from different analogs [62].

#### Melanoidins Melanoidins

Melanoidin compounds are byproducts of the Maillard reaction and the reduction of amino acids, sugars and proteins during food preservation and processing [61]. In most cases, the compounds are drawn from coffee processing—the roasting of coffee bean components results in the generation of brown-colored nitrogenous compounds with high molecular weight [62], referred as melanoidins. Melanoidin compounds are byproducts of the Maillard reaction and the reduction of amino acids, sugars and proteins during food preservation and processing [61]. In most cases, the compounds are drawn from coffee processing—the roasting of coffee bean components results in the generation of brown-colored nitrogenous compounds with high molecular weight [62], referred as melanoidins.
