Sustainable Lignin–Acrylonitrile Copolymers via Aqueous Free Radical Polymerization for Carbon Materials Development

Abstract This study investigates a bio-based precursor for carbon materials by directly copolymerizing unmodified LignoBoost lignin (LB) with non-fossil derived acrylonitrile (AN) via aqueous free radical polymerization (FRP), forming lignin–polyacrylonitrile (PAN) graft-type copolymers. In contrast to conventional approaches, the process avoids organic solvents and lignin functionalization. Key parameters such as lignin activation, the order of reactant addition, and the reactivity of functional groups were analyzed. Activation was achieved through in situ radical generation without introducing permanent chemical modifications. Copolymers with increasing LB initial contents (3.4, 17.5, and 33 wt%) were synthesized and characterized in comparison to polyacrylonitrile (PAN) homopolymer based on molecular weight (GPC), molecular structure (FTIR, 1 H NMR, and EA), and thermal stability (TGA and DSC). Higher LB initial content (33 vs. 3.4 wt%) resulted in lower Mw (24206 vs. 56075 g/mol) and higher lignin incorporation in the copolymer (33.0 vs. 18.0 wt%). The new copolymers exhibited enhanced thermal stability compared to PAN and LB, as reflected by higher char yield at 900 °C (39–42 wt% vs. 27 wt% for PAN and 32 wt% for LB). DSC results indicated a more controlled cyclization process, as evidenced by lower heat release per temperature interval (|ΔH| = 2.78–4.42 J/g/°C vs. 4.85 J/g/°C for PAN) and lower onset temperatures of the exotherms (177.9–208.4 °C vs. 234.7 °C for PAN). The lignin-PAN copolymers, obtained from renewable and non-fossil derived raw materials and through a sustainable and potentially scalable process, showed improved thermal behavior compared to PAN, highlighting their potential as precursors for carbonizable polymer systems and carbon-based materials.