*Polylimonine Synthesis and Properties*

The synthesis of polylimonine has practical implications on the production of terpenes, given that the former classes of chemical compounds are optically active terpenes, which exhibit antioxidant behavior [92]. Commercial applications of limonene and polylimonene include food preservation, enhancing the flavor and aroma of foods [93]. A fundamental constraint in the development of polymeric products globally is the recyclability of the precursor and the ecological effects; this remains a priority concern in polylimonene synthesis, considering that most precursors are sourced from fossil fuels [94]. In light of this challenge, the synthesis of non-toxic and renewable plastics is a priority for manufacturers. Such plastics would be ideal substitutes for existing non-renewable polymers. Traditionally, ideal precursors for renewable polymers are starch, essential oils, and wood. However, there have been growing research interests in the production of renewable and non-toxic plastics from terpenes.

Terpenes are naturally found in citrus plants and essential oils. Other sources of terpenes include tea, thyme, cannabis, Spanish sage, and citrus fruits (lemon, orange, mandarin) [91]. Beyond the ubiquity of the precursors, terpenes (D-limonene in particular) are ideal based on their chemical structure and the presence of isoprene units; these units are key enablers for addition-based polymerization processes. The choice of D-limonene as a precursor in the current case is supported by the worldwide production capacity [94] and compatibility with microbial mediated synthesis processes, including PMD, Codon optimized MVK, and PMK (see Table 10).


**Table 10.** Microbial-based synthetic routes for polylimonene through microbial mediated synthesis processes [93].

The biotechnology-based production of limonene from microorganisms does not address production-related constraints, including the molar mass loss/lack of sufficient molar mass, selection of an acceptable monomer for conversion. Most of the precursors for terpenes have practical drawbacks and limitations—a factor influencing the choice of orange peel-based precursors—a precursor proven useful in the synthesis of polylimonene through recycling polymerization [94]. Other sources of terpenes include tea, thyme, cannabis, Spanish sage, and citrus fruits (lemon, orange, mandarin) [91]. Beyond the ubiquity of the precursors, terpenes (D-limonene in particular) are ideal.

Under standard conditions, Coelho and Vieira synthesized polylimonene by placing D-limonene in a solution containing Ziegler-Natta catalysts and Lewis acids in the first stage. The molar mass of the mixed species was below 1000 g/mol [94]. The second stage entailed the initiation of free-radical polymerization using benzoyl peroxide. The concentration of the free radical initiator is adjusted accordingly to attain the desired conversion efficiency. In the current case, the desired conversion efficiency was 12% [94]. The reduced copolymerization rates demonstrate there is an inverse link between the D-limonene ratio, molar masses, and concentration of styrene, methyl methacrylate, and n-butyl acrylate).
