Mass Spectrometric Characterization, Antioxidant Activity, and Computational Investigation of Styrene–Carboxyphenylmaleimide Copolymers

The effect of positional isomerism on the physicochemical and functional properties of styrene–carboxyphenylmaleimide (St–CPMI) copolymers remains insufficiently explored despite the increasing importance of maleimide-based functional polymers for advanced material applications. In the present study, styrene copolymers containing ortho-, meta-, and para-carboxyphenylmaleimide (CPMI) units were systematically investigated to elucidate the influence of substituent position on their structural characteristics, electronic properties, and antioxidant activity. The copolymers were comprehensively characterized by high-resolution liquid chromatography–electrospray ionization quadrupole time-of-flight mass spectrometry (LC–ESI–QTOF–MS), which confirmed the successful incorporation of CPMI moieties into the polymer backbone and revealed characteristic fragmentation pathways involving sequential decarboxylation, imide ring stabilization, and polymer chain cleavage. The antioxidant properties were evaluated using the DPPH free radical scavenging assay, demonstrating a pronounced dependence on the positional isomerism of the carboxyl substituent. Among the investigated copolymers, the ortho-substituted derivative exhibited the highest antioxidant activity with an IC₅₀ value of 194.16 ± 4.31 μg mL⁻¹, followed by the meta- (217.17 ± 5.62 μg mL⁻¹) and para-substituted (264.29 ± 6.48 μg mL⁻¹) analogues. To establish structure–property relationships, density functional theory (DFT) calculations and Hansen solubility parameter (HSP) analysis were employed to evaluate electronic structure, charge distribution, intermolecular interactions, and solvent compatibility. The combined experimental and computational results demonstrate that positional isomerism markedly affects the molecular architecture and antioxidant performance of St–CPMI copolymers. In particular, the ortho isomer provides the most favorable balance of electronic stabilization and radical scavenging efficiency, highlighting its potential as a promising building block for the design of multifunctional, thermally stable, and antioxidant polymeric materials for advanced engineering and biomedical applications.