SYNTHESIS AND CHARACTERIZATION OF POLY(LIMONENE) OBTAINED BY O-ATRP: THERMAL STABILITY AND ANTIOXIDANT ACTIVITY

Keywords: Terpene; Bioactive Compounds; O-ATRP; Green Chemistry.

Abstract: The incorporation of bioactive compounds into food packaging is a promising strategy for improving food preservation. Among these compounds, terpenes exhibit functional potential; however, their limited thermal stability and volatility may restrict their direct application in polymer matrices. Polymerization may improve these properties, favoring their use as bioactive agents. This study aimed to synthesize poly(limonene) (PLM) via Organocatalyzed Atom Transfer Radical Polymerization (O-ATRP), characterize its chemical structure and thermal stability, and evaluate its antioxidant activity. Synthesis was performed in bulk using N,N,N',N'',N''-pentamethyldiethylenetriamine (PMDETA) as the electron donor, benzophenone as the photoinitiator, and 2,2,2-tribromoethanol (TBE) as the initiator. After nitrogen purging for 10 min, the reaction proceeded under UV irradiation at 365 nm and 40 °C for 6 h. The product was then oven-dried at 60 °C for 24 h to remove residual monomer. While conventional terpene oligomer synthesis methods rarely exceed 10% yield after 6 h, this route achieved approximately 8% under mild conditions, without solvents or metal catalysts, aligning with green chemistry principles. Fourier-transform infrared spectroscopy (FTIR) revealed spectral changes after the reaction, including broadening of the band at 3000–2850 cm⁻¹, attributed to C–H stretching, and a decrease in the band near 887 cm⁻¹, associated with limonene's exocyclic unsaturation. These changes suggest consumption of the exocyclic double bond during polymerization. Thermogravimetric analysis showed that the main mass-loss stage occurred between approximately 180 and 400 °C, with a maximum degradation rate near 320 °C and 4–5% residual mass at 600 °C, suggesting thermal stability under typical polymer processing conditions. PLM (0.5% m/v ethanol) exhibited mean antioxidant activity values of 98.75% using ABTS and 78.38% using DPPH. In comparison, limonene monomer (LIM; 97%) showed values of 10.97% and 3.38%, respectively. These results indicate enhanced free-radical scavenging capacity following limonene polymerization, supporting PLM's application as an antioxidant additive in polymer matrices for bioactive food packaging. Its renewable origin, photoinduced synthesis at low temperature, solvent-free conditions, and potential for greater thermal stability and lower volatility than the monomer further support its relevance for food-packaging applications.

Acknowledgments: This study was financed, in part, by the São Paulo Research Foundation (FAPESP) – grant #2022/02728-0, #2024/07555-2, #2025/03776-7, #2025/16617-4, and #2026/00774-6. This study received partial financial support from the National Council for Scientific and Technological Development (CNPq), grant #301184/2025-9 and #444408/2024-0; and Coordination for the Improvement of Higher Education Personnel (CAPES): Finance code 001.