Vanillin-Based Polymers—part II: Synthesis of Schiff Base Polymers of Divanillin and Their Chelation with Metal Ions

Condensation of renewable resources-based monomer divanillin with alkyl diamines gives Schiff base polymers with degree of polymerization (DP) ~25–32 in 88–95% yield. These yellow polymers are insoluble in water and common organic solvents, slightly soluble in warm DMSO, DMF and dissolves in aqueous NaOH. The polymers were characterized using FT-IR, 1H, and 13C NMR spectroscopy and by comparison with the model compound N,N′-bis(vanillidene)-1,3-propanediamine. Polymer prepared by condensation of divanillin and 1,6-diaminohexane is shown to chelate with Cu(II), Fe(II), and Co(II) metal ions in basic aqueous methanol. 1. Introduction Synthesis of polymeric and composite materials from renewable resources-based feedstock has attracted an increasing attention over the last two decades due to concerns over dwindling petroleum resources [1, 2]. Some of the renewable resources-based monomers currently under study are triglycerides [3, 4], 1,3-propanediol [5], furfural [6, 7], 2,5-furandicarboxylic acid or its derivatives [8–10], lactic acid [11], and levulinic acid [12, 13]. Vanillin (1, Figure 1) is another biomass-derived compound that can be considered as a potential renewable feedstock chemical. This phenolic-aldehyde was first isolated from vanilla extract used as a common food flavoring, which is derived from orchids of the genus Vanilla, and primarily from the Mexican species, Vanilla planifolia. Recent advancements in the production of vanillin by metal catalyzed air oxidation of abundant lignin have promoted the status of this old world flavor chemical to another promising renewable feedstock material for the chemical and polymer industries [14–16]. Alternative biotechnology-based approaches applying fungi, bacteria, plant cells, and genetically engineered microorganisms are also currently under study [17] for the large scale production of this highly functionalized versatile building block. Even though the lignin derived vanillin is becoming relatively easily accessible, still there are only a handful of reports on attempts to utilize vanillin or vanillin derivatives as monomers for the renewable resources-based polymer synthesis. In one example, Gosling et al. have recently reported [18] the preparation of 3-carboxy muconate as a monomer by an enzymatic transformation of vanillin. Furthermore, as a proof of the principle, they demonstrated that trimethyl ester derivative of 3-carboxy muconate can be copolymerized with styrene to give novel polyesters. In another recent example, vanillin-derived dihydroferulic acid has been used in the preparation of