Synthesis of sustainable eugenol/hydroxyethylmethacrylate-based polymers with antioxidant and antimicrobial properties

Abstract

Eugenol is a phenolic monoterpenoid, obtained mainly from clove oil and lignin, with a peculiar chemical structure containing an allyl group and a phenol group, which can be easily subjected to chemical modification. The phenol group also endows eugenol with antimicrobial activity and the ability to scavenge reactive oxygen species. Here, we present eugenol as a building block for the obtainment of antimicrobial and antioxidant bio-based polymers. So far, the approaches followed to obtain EU-based polymers involved phenol group functionalisation with polymerisable moieties, which inevitably implied the loss of the antioxidant activity of eugenol. In contrast, herein, an efficacious and low environmental impact synthetic strategy was set up to obtain an eugenol-methacrylate (EUMA) monomer bearing a free phenol group. EUMA was copolymerised with 2-hydroxyethyl methacrylate (HEMA) at different volume percentages (10%, 30% and 50%). The EUMA homopolymer was also synthesised. The obtained amorphous polymers were as thermally stable as pHEMA but more flexible (lower T_g) and hydrophobic (less swellable in water) than pHEMA. The rheological tests evidenced that all of the EU-containing polymers have good inherent elastic properties, which were retained also at high deformation frequencies (up to 80 Hz). Thanks to the presence of phenol groups in the side chain of the polymers, the pHEMA-EU copolymers showed significant radical scavenging activity and also antimicrobial activity towards a strain of Staphylococcus epidermidis. Overall, the synthesised eugenol-methacrylate monomer has the potential to be copolymerised with a wide number of different acrylate or vinyl monomers, thus allowing one to prepare a library of polymers with the desired physical and biological properties. The cutting-edge antioxidant and antimicrobial properties shown by the prepared copolymers open an interesting perspective towards the use of these materials in different application fields, including the food sector and biomedical field.