Fractionated and Methacrylated Kraft Lignin as a Reactive Macromonomer for Multifunctional, Partially Bio-Based PMMA Copolymers

Developing sustainable polymeric materials requires effective use of renewable aromatic feedstocks. Lignin is a promising biobased candidate, but its structural heterogeneity and broad molecular-weight distribution limit its direct use in polymer synthesis. Herein, solvent fractionation was used to isolate a low-molecular-weight softwood kraft lignin fraction that was then methacrylated and copolymerized with methyl methacrylate (MMA) by microwave-assisted free-radical polymerization. A Box–Behnken design was used to optimize copolymer molecular weight by varying azobis(isobutyronitrile) (AIBN) loading, temperature, and reaction time. Under optimum conditions (8 mg of AIBN, 70 °C, 60 min), the copolymer with Mw = 2.09 × 105 g/mol was obtained. Methacrylated lignin (ML) feeds of 0–30 wt % were evaluated, with maximum tensile strength obtained at 20 wt % ML feed (68.0 MPa). At 30 wt % ML feed, radical-scavenging activity (RSA) reached approximately 67% at 33.33 mg/mL, compared with approximately 17% for neat poly(methyl methacrylate) (PMMA). After two dissolution–recasting cycles, the lignin-containing copolymer films retained 75–80% of their initial tensile strength. This method demonstrates that lignin fractionation and methacrylation offer a potential approach to creating multifunctional, partially biobased PMMA copolymers with tunable mechanical properties, enhanced antioxidant activity, and good solution processability.