Chemical Recycling of Polymers Synthesized by Controlled and Free Radical Polymerization
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Date
2024
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Doctoral Thesis
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yes
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Abstract
Plastics, or synthetic polymers, are widely used in various aspects of modern life, from everyday items like cups and plastic bags to specialized applications such as photoresists and antireflective coatings. While plastics offer industry-disrupting benefits like affordability, lightness, and durability, their excessive use within a linear economy framework has caused many environmental issues. These issues include plastic accumulation in landfills and oceans, microplastic contamination, and greenhouse gas emissions from incinerated waste. Moreover, the transition to a more sustainable circular economy faces hurdles due to the lack of incentives for chemical companies.
To address these issues and move towards a circular economy, extensive research in recycling is imperative. Recycling methodologies generally fall into two categories: mechanical and chemical. While mechanical recycling is a straightforward melt-and-remold process, it results in products of lower quality and thus the process is called “downcycling.” Chemical recycling offers a solution by converting polymers into value-added small molecules or reverting them back to their original building blocks, the monomer. The latter process, depolymerization, creates the most ideal smallest closed-loop cycle as the regenerated monomer can be repolymerized into virgin-grade polymeric materials. It also opens up the option to create different materials from the same monomer. However, the depolymerization of vinyl polymers containing all-carbon backbones is challenging because of the lack of heteroatom-associated weak links (e.g. polyesters). Conventional methods to depolymerize polymers rely on extreme heating, leading to side reactions that diminish product quality. Therefore, there is a pressing need for milder depolymerization strategies for vinyl polymers.
In this thesis, we demonstrate methodologies that facilitate the depolymerization of vinyl polymers, specifically polymethacrylates, under significantly milder conditions. We place emphasis on lowering the kinetic barrier to depolymerization by either installing labile endgroups at the chain terminus or by utilizing photocatalysis to trigger a midchain-initiated depolymerization. The thesis comprises three main topics: uncontrolled radical depolymerization, controlled radical depolymerization, and depolymerization of commercial products.
In the first part of the thesis, we demonstrate near-quantitative depolymerization of various polymethacrylates synthesized by reversible addition-fragmentation chain-transfer (RAFT) polymerization. These polymethacrylates contain a labile thiocarbonylthio end-group that can be cleaved at 120 °C to produce a chain-end radical which initiates the depolymerization process. Importantly, the end-group can also be recovered after depolymerization and reinstalled at the chain-end of the repolymerized, second-generation polymethacrylate.
In the second part of the thesis, we demonstrate the possibility of a controlled radical depolymerization by employing significant chain deactivation during depolymerization. This overcomes the limit of uncontrolled depolymerizations in which the average molecular weight does not meaningfully change regardless of conversion. In contrast, in a controlled depolymerization, a gradual decrease in the molecular weight is observed as a result of concurrent chain deactivation. As an added benefit, such uniform unzipping of chains enables, for the first time, the analysis of monomer sequence in a copolymer via ¹H NMR.
In the final part of the thesis, we discuss a method to photocatalytically depolymerize commercial polymethacrylatessuch as Plexiglas. Commercially available polymethacrylates do not contain any labile end-groups and hence a completely different approach is required to depolymerize these materials. We utilized a midchain-initiated depolymerization approach wherein a carbon-centered radical is created on the polymer backbone via C-H hydrogen atom transfer. This end-group-independent depolymerization strategy unlocks many benefits, including near-quantitative depolymerization of high-molecular-weight (10⁶ g/mol) polymers, high tolerance to non-depolymerizable comonomers, and multigram-scale depolymerization of commercial products.
Overall, this thesis provides a deeper understanding of the factors that govern the depolymerization of polymethacrylates through well-defined end-groups and further develops these findings to depolymerize commercial products. This sets the foundation to develop more environmentally conscious and economically viable strategies to chemically recycle plastics.
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Examiner : Anastasaki, Athina
Examiner : Carriera, Erick M.
Examiner : Choi, Tae-Lim
Examiner : Klok, Harm-Anton
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ETH Zürich
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09644 - Anastasaki, Athina / Anastasaki, Athina