## 1 Introduction
PS has a resin identification code of 6, therefore, it is recyclable, although its biodegradation can take hundreds of years and it is also relatively resistant to photo-oxidation [16]. As mentioned, PS is widely used, and now there is an abundance of it in outdoor environments. This includes coastal areas, especially in its foam form, and increasingly in the oceans as styrene monomers (SM), styrene dimers (SD), and styrene trimers (ST). PS poses a significant hazard to marine life in its high molar mass form through entanglement and smothering, and in its low molar mass form, potentially through bioaccumulation [16]. Consequently, PS appears in the red non-biodegradable section of Figure 1 with other fossil fuel based plastics, meaning PS production leaves a large carbon footprint. Moreover, due to the short lifespan ofpackaging, a large proportion of PS ends up in landfills, and approximately 13-20% of the solid waste produced is composed of plastic [17]. Although some European Union (EU) members have banned using landfills, approximately 50% of plastic waste still ends up in one [18]. In developing countries, plastic consumption and pollution (often marine) is more serious, and it is also less regulated; plastic usage there is believed to be greater than the world average due to more rapid rates of economic growth and urbanization [18; 19; 20]. Life Cycle Assessment (LCA) studies have shown that the mechanical recycling of plastic materials is more favorable than other management procedures in relation to overall energy use and emissions contributing to global warming [21]. The potential pathway of this study's overall process is summarized in Figure 2. The quality of waste plastic materials for use in processes such as this one relies on the degree of degradation, the degree of mixing (whether there are impurities or not), and the presence of low molecular weight compounds (additives, contaminants, and degradation products) [22; 23; 24]. Figure 1: Plastics and bio-based polymers’ diversity and comparison diagram. Abbreviations include: PA—Polyamide, PBAT—Polybutyrate Adipate Terephthalhalate, PCL—Polycaprolactone, PE—Polyethylene, PE—Polyethylene Furanoate, PES—Polyether Sulphone, Polyethylene Terephthalhalate—PET, PP—Polypropylene, PC—Polycarbonate, PHA—Polyhydroxyalkanoate, \(\gamma\)-PGA—Poly(Gamma Glutamic Acid), PLA—Poly(Lactic Acid), PS—Polystyrene, PBS—Poly(Butylene Succinate), PBSA—Poly(Butylene Succinate Adipate), PVC—Polyvinyl chloride, PVOH—Poly(vinyl alcohol), adapted from [13]. The * indicates polymers that have bio-based origins plus fossil fuel synthesis routes. The mechanical recycling of styrene polymers, such as high impact polystyrene (HIPS), has increased over the last decade due to the European Directives on Waste of Electric and Electronic Equipment (WEEE) and End-of-Life Vehicles (ELV) as sytrenic plastics are used in their manufacture [23]. HIPS are a multiphase system with a PS rigid matrix and dispersed rubber particles (polybutadiene) that enhances the blend's impact properties [23]. For environmental, social, and economic reasons, innovative and diverse strategies need to be engaged with to encourage the use of alternative biobased plastics that can equal, or even surpass, conventional mass plastics. Waste general purpose PS (GPPS) is a carbon source that could be used in this context to provide an avenue for value-added biopolymer synthesis through fermentation. The utilization of waste PS in this manner would also be in line with the recent global effort to transform Europe's (and eventually the world's) economy into a more sustainable one [25]. In fact, to implement the Circular Economy Action Plan, in January 2018, the European Commission endorsed a new set of measures that includes plans to modify the way plastics are designed, manufactured, applied, and recycled. They also intend that by 2030, all plastic packaging should be recyclable [25].