**7. Antioxidant Terpene Polymers**

Similar to polyphenols, tocotrienols, tocopherols, quercetin, α-lipoic acid, N-acetyl cysteine, melatonin, polyquercetin, gallic acid, captopril, taurine, vitamins, and catechin terpenes are antioxidant polymers synthesized via natural and artificial polymerization processes [86–90]. However, the synthesis and commercial application of terpenes as antioxidants in food packaging and biomedical industries has attracted less research interest compared to polyphenols, vitamin C and D, and quercetin polymers—a phenomenon that is linked to the unique function and features of different terpenes compounds (D-limonene, βionone, geraniol, eugenol, myrcene) [91]. For example, each of the listed terpenes functions

as an ideal insecticide (in citral mixtures) due to the antibacterial and antifungal properties (see Table 8); there are no notable side effects on human health given the diminished neurotoxicity risks.

**Table 8.** Classifications of terpenes and their potential application as pesticides [91].


Despite the potential for application in various manufacturing applications, there has been minimal industry interest in terpene and terpene derivatives. Winnacker et al. [88] attributed this phenomenon to inadequate understanding of how terpenes could be combined with other natural and synthetic polymers to augment material characteristics (such as tensile strength, photo-degradation, elongation at break, antimicrobial properties, and biocompatibility) [88]. Moreover, even though the feedstock is cheap and widely available, there are gaps in knowledge concerning effective chemical processes to reduce material costs, and determining potential new applications for terpene derivatives and monomers remain underexplored [88].

The most widely applied procedures include radical initiator-free addition and solvation of thiols to terpenes such as R +/− and S−/− limonene and -β-pinene; chain and step-growth polymerization techniques involving acrylic monoterpenes myrcene (7-methyl-3-methylene-1,6-octadiene) and ocimenes and Bayer–Villiger oxidation of menthone into methide [89]. The application potential of terpenes includes the synthesis of terpene/fatty acid-based polyesters for industrial applications [89]. The reaction parameters, including conversion efficiency for polyester synthesis via copolymerization of propylene oxide (PO), are depicted in Table 9 [88].

**Table 9.** Mediated copolymerization of PO, time conversion, MW and Tg [88].


In other cases, terpenoid-based polymeric resins have been employed in stereolithographic 3D printing [87] and as starting materials in the production of sustainable polymers and advanced materials [88]. The renewed industry attention on terpenes and their derivatives is validated by the progress made in controlled polymerization, interesting structures, and the broad scope of industrial application, renewability, and abundant feedstock.
