Enzymatic Synthesis of a Partially Bio-Based Schiff Base Polymer: Evaluation of Structural, Morphological, and Thermal Properties

In this study, a novel vanillin-derived aromatic monomer was synthesized and enzymatically polymerized to obtain a functional polymer with enhanced structural and thermal characteristics. The monomer containing an azomethine linkage (V-DEPDA) was first prepared by condensing vanillin with N,N-diethyl-p-phenylenediamine, isolated from its oxalate salt. Subsequently, the monomer was subjected to oxidative polymerization using the horseradish peroxidase (HRP)/H2O2 catalytic system, yielding a conjugated aromatic polymer, P(V-DEPDA). The structural features of the synthesized monomer and polymer were elucidated by Fourier Transform Infrared (FT-IR) and Proton Nuclear Magnetic Resonance (1H NMR) spectroscopy. FT-IR and 1H NMR confirmed the successful formation of azomethine bonds and structural shifts post-polymerization, resulting from multi-fractional chain growth. Gel Permeation Chromatography (GPC) analysis verified macromolecular formation, yielding a number-average molecular weight (Mn) of 3300 g/mol and a remarkably narrow polydispersity index (PDI) of 1.36. Scanning electron microscopy (SEM) examined the surface morphology. Thermal properties, weight loss percentages, and decomposition behavior were evaluated by thermogravimetric analysis (TGA) at a heating rate of 10 °C/min. P(V-DEPDA) exhibited exceptional improvements in thermal stability, with the onset decomposition temperature (Ton) shifting from 208 °C (monomer) to 370 °C (polymer) and the char yield at 1000 °C increasing dramatically from 6.3% to 41.4%. These results demonstrate that this eco-friendly enzymatic route successfully yields highly robust, carbon-dense Schiff base polymers that are promising for high-temperature and sustainable applications. Overall, this study expands the scope of HRP-catalyzed polymerization by successfully converting a custom bio-based Schiff base monomer into a highly conjugated functional polymer framework.