## 1 Introduction  
Polyethylene (PE) is the most highly manufactured and utilized polymer in the world, with continuously projected growth as consumer demand in automobile, construction, packaging and other applications continues to rise [1, 2]. In 2019, the global demand for PE was estimated at 47 million metric tons, due in part to broadened ethylene feedstock sources such as shale gas, coal, and bio-based materials [3]. PE is a highly recyclable plastic material since it has high thermal stability [4]. However, the rate of PE recycling was only 29.3% in 2018 in the United States (US) [5]. Plastic recycling rates vary around the globe, but the overall effective reuse of the material is minimal. The broader pool of polymer waste is mostly landfilled (~79%) or incinerated (12%) [6]. Another growing practice for waste PE and other olefin polymers is pyrolysis, which requires high temperatures (400-600 \({}^{\circ}\)C) to convert these polymers into smaller molecules for fuel or further processing [7, 8, 9, 10]. A problematic outcome of the over nine billion metric tons of plastic generated since 1950 is a global environment containing a growing amount of persistent waste plastic [11]. Much of this waste contaminates surface waters, where it ultimately fragments into even more problematic microplastics [12]. A large pool of data has been published on the presence of microplastics in the oceans [13; 14], freshwater reservoirs [15; 16], wetlands [17], sediments [18] and other environments [19]. PE is a commonly identified microplastic in water and other environmental media [20; 21]. The distribution and fate of PE microplastics are dictated by a number of material factors, including size, shape, density and additives, and environmental conditions, such as temperature, pH, sunlight exposure, biofilms, wind and waves [22]. PE microplastics are expected to mostly subsist on water surfaces since their density is less than water--ranging from 0.86 g mL-1 for amorphous PE, to near 1.0 g mL-1 for crystalline PE [23]. Chemical transformations (weathering) of microplastics in the environment are initiated by factors such as light, heat, and/or reactions with reactive oxygen species and radicals--e.g., superoxide (O2\({}^{\bullet-}\)), hydroxyl radicals (\({}^{\bullet}\)OH), singlet oxygen (O\({}^{\bullet}\)), and/or ozone (O\({}_{3}\))--present in the environment. Highly implicated in the chemistry of materials in aqueous environments are the strongly oxidizing \({}^{\bullet}\)OH generated by the interaction of solar ultra-violet (UV) light and/or secondary chemical reactions, and typically present in a steady-state concentration range of 10\({}^{-18}\) to 10\({}^{-16}\) M in surface waters [24; 25]. Additional radicals can also be created by UV light absorption by various water solutes, such as nitrate, nitrite, and dissolved organic matter, leading to additional reactive species capable of inducing chemical change in microplastics [26]. While many studies of microplastics' transformations have been conducted in the presence of consistent light and oxygen (O\({}_{2}\)), much less is known about the weathering under reduced dissolved O\({}_{2}\) concentrations, especially their chemical reactivity, products released, and the lifetime of these particles. It is therefore important to explore the fate of microplastics under low O\({}_{2}\) to anaerobic conditions, where any radical-promoted chemical weathering would be followed by cross-linking and/or other reactions [27]. Radiation chemistry has been widely used to explore the stability of plastics and to manipulate polymeric materials [28; 29; 30; 31]. Additionally, water radiolysis can be used to generate reactive oxygen species whose reactions simulate the natural weathering of microplastics [32; 33].