Trojan horse repeat sequences for chemically triggered recycling of polyethylene terephthalate
Date
2024-12
Authors
Dileep, Dhananjay
Major Professor
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Cochran, Eric
Wang, Qun
Kraus, George
Winter, Arthur
Narasimhan, Balaji
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Abstract
The Earth faces a plastic deluge, and current industrial recycling strategies are unable to control plastic accumulation, resulting in pollution and energy loss. Polyethylene terephthalate (PET), a semicrystalline polyester, comprises a significant portion of this waste. It is widely prevalent in the packaging, beverage, and textile industries due to its excellent thermomechanical properties and chemical resistance. This widespread use and environmental persistence manifest as a massive volume of post-consumer waste. Further, most post-consumer plastic waste is improperly sorted, complicating recycling efforts. Moreover, the pursuit of high-performance materials often leads to blending and chemically compatibilizing different plastics at the molecular level, creating materials with unprecedented properties but unintended resistance to easy recycling. Additionally, most plastics are derived from fossil fuels. The scarcity of fossil fuels, coupled with increased demand and plastic production, results in higher greenhouse gas (GHG) emissions, contributing to global warming and climate change. These challenges necessitate swift and practical solutions.
This research recognizes the implications of plastic pollution, its impact on biodiversity and ecosystems, and the limitations of current recycling methods. It aims to conduct in-depth research into the depolymerization mechanisms of PET and propose strategies to incorporate depolymerizability into the polymer design. Advanced materials research is crucial for developing innovations that make plastics more recyclable.
PET comprises a dense network of ester linkages that can be broken down (solvolysis) in highly nucleophilic environments at elevated temperatures and pressures. However, this process demands significant energy. The proposed approach involves modifying the polymer backbone by introducing molecular "decoys" that respond to specific triggers under mild conditions. These reactive molecules are expected to act as catalysts for depolymerization, enhancing the rate without being significantly consumed. These molecules, dubbed "Trojan horse moieties," would be incorporated into the polymer chain to serve as initiation sites for depolymerization. As the polymer breaks down into smaller oligomers, the reaction zone can progress into the bulk of the material, further accelerating depolymerization. This process could lead to significant cost savings in chemical recycling through a relatively minor modification—incorporating these Trojan horses. The economic and environmental impact of efficient chemical depolymerization could revolutionize the plastics industry, enabling the production of high-quality recycled plastic while reducing reliance on fossil fuels.
Additionally, this project aims to synthesize PET monomers from non-food-based, bio-derived sources like limonene (a terpenoid) to enable PET production from renewable feedstocks. Alternative feedstocks for terephthalic acid are also being explored to meet the high demand for PET. Given the significant disparity in market volumes between terephthalic acid and limonene, sugars are also being investigated as a potential source of aromatic compounds that can be oxidized to terephthalic acid. The ultimate goal is to create a bio-based, chemically recyclable PET that encourages commercial adoption, providing the benefits of plastic while promoting its responsible use.
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dissertation