Advances in Chemical Recycling of Plastic Waste for High-value Monomers and Chemicals

The global accumulation of plastic waste necessitates a shift from mechanical recycling—which currently recovers less than 10% of material and often results in downcycling—toward robust chemical recycling strategies. Polyurethanes (PUs) are prime candidates for such “circular” approaches due to their reactive C-O and C-N bonds. Among chemical recycling methods, acidolysis offers a promising pathway for closed-loop recovery; however, the fundamental chemical principles and kinetic drivers of the process remain a “black box.”
Our group has undertaken a comprehensive mechanistic and kinetic investigation into the acidolysis of polyurethane foams. By systematically varying the carboxylic acid reagents, we evaluated how acid transport phases, molecular structure, and electronic properties dictate depolymerization rates and product distributions. Our findings identify distinct rate-limiting regimes and establish a set of design rules to predict how reagent selection influences depolymerization efficiency.
Leveraging these rules, we identified optimal acids and conditions that enable the facile recovery of polyols, the primary feedstock in PU synthesis. We also addressed the remaining scalability barriers, providing a roadmap for transitioning polyurethane waste management from a linear “take-make-waste” model to a potentially sustainable, chemically driven circular economy.

Polymer length, i.e. molecular weight, is a critical polymer parameter that dictates their properties. The preparation of polymers with controlled molecular weights has relied on controlled or living polymerization such as radical, ionic or ring-opening polymerization. Obtaining controlled molecular weights in step growth polymerization is generally challenging due to the step-wise reaction mechanism with the requirement of controlling reaction equilibrium by removing condensates. Here, we report precision depolymerization of step-growth polymers and their valorization. We first developed organocatalysts for efficient depolymerization of step-growth polymers such as poly(ethylene terephthalate) (PET), polyamide (PA), and polycarbonate (PC), via glycolysis, and correlated their design rules using Hammett analysis. To precisely control molecular weights of the deconstructed products, we tailored the type of catalyst, catalyst loading, ethylene glycol concentration, and reaction temperature. In addition to each experiment conducted by a researcher, we successfully established autonomous chemistry workflow coupled with artificial intelligence, that enables the synthesis of depolymerized polymers and oligomers with precision length and functionality. We further strategized to synthesize new upcycled polymers from those well-defined deconstructed building blocks of precision molecular weights. For example, precisely deconstructed PA or PC oligomer building blocks were used for synthesizing new upcycled polymers, that exhibit exceptional adhesion or elastic properties. These upcycled polymers can be deconstructed again to well-defined blocks in closing-loop. This presentation will update our progress on precision depolymerization and valorization of step-growth polymers.

Waste plastics can be pyrolyzed at scale (i.e., 5–330 kilotons per year) to produce an olefin rich pyrolysis oil. Olefins are the central building blocks of the petroleum industry [1]. These olefins can then undergo hydroformylation to form aldehydes, serving as intermediates for the synthesis of various petroleum chemicals [2]. In this work, we demonstrated a process for producing high-purity (>95%) carboxylate surfactants from post-consumer recycled high-density polyethylene (PCR-HDPE) [3]. The approach involves the thermal depolymerization of PCR-HDPE via pyrolysis, followed by fractional distillation to isolate C9–C14 olefins. These olefins undergo hydroformylation using cobalt carbonyl catalysts to generate aldehydes, which are subsequently oxidized to carboxylic acids using Pinnick oxidation under mild aqueous-phase conditions. Neutralization of the resulting carboxylic acids with sodium hydroxide produces plastic-derived carboxylate surfactants (PDCs) in the form of sodium carboxylates. Subsequent purification steps ensure surfactant-grade purity and enable accurate assessment of physicochemical properties. The resulting PDCs are evaluated for critical micelle concentration, foamability, surface tension reduction, and calcium ion tolerance, demonstrating competitive behavior with conventional anionic carboxylate surfactants. This route provides a sustainable alternative for surfactant production, reducing reliance on fossil-derived feedstocks and valorizing plastic waste streams through chemical upcycling.
[1] Li, H., et al., and Huber, G. W. Green Chemistry, 24, 8899-9002 (2022).
[2] Li, H., et al., and Huber, G. W. Science, 381 (6658), 660-666 (2023).
[3] Li, H., et al., and Huber, G. W. Angewandte Chemie, 137 (50), e202517471 (2025).

Global plastic production reached 390.7 MMT in 2021, a 44% increase over 10 years. The United States generated the largest amount of plastic waste, accounting for 18% of global production, after China. The Environmental Protection Agency reported that 4.5 MMT of the 32.4 MMT of plastic generated in the US in 2018 were recycled, with the remaining waste disposed of in landfills. Discarded plastic waste pollutes land and ocean environments and migrates into human and animal bodies as microplastics. Finding environmentally friendly, cost-effective approaches to recycling end-of-life plastics is a grand challenge. Plastic pyrolysis has the highest TRL among various chemical upcycling technologies. However, an incomplete understanding of the underlying mechanisms due to the heterogeneity of real-world waste plastics creates challenges in controlling product quality and optimizing processes. In addition to further improving the commercial prospects of pyrolytic technology, developing novel technologies to reduce process energy demands and increasing product selectivity are necessary to improve the feasibility of a circular economy. This talk will describe current strategies for managing post-consumer plastics and present our research activities on upcycling waste plastics via pyrolysis or non-thermal plasma. The talk will discuss the pyrolysis of post-consumer polyolefins to hydrocarbons and how common additives, pigments, and fillers affect thermal and catalytic pyrolysis. It will also discuss our approach to using non-thermal plasma as a highly tunable platform for producing versatile chemicals, such as oleochemicals, aromatic hydrocarbons, and butene. Finally, we will also discuss the challenges to overcome in obtaining, handling, and converting post-consumer waste plastics on a commercial scale.

We are investigating catalytic materials and methods that regulate the cleavage of C–H bonds or C–C bonds in polyolefins, to introduce functional groups at selected positions or to create narrow distributions of shorter, partially deconstructed chains. This approach involves the design and synthesis of 3D porous inorganic metal oxide architectures which contain catalytic sites in well-defined positions in the material, along with spectroscopic investigations and theoretical models of polymer adsorption and translocation in the pores. In parallel, we are developing catalytic sites and reactions that break C–C and C–H bonds in aliphatic hydrocarbon polymers. As these catalytic sites are incorporated into 3D architectures and studied in polyolefin deconstruction reactions, our team is developing theoretical, kinetic models and in situ spectroscopic methods for studying the ‘macromolecular’ mechanisms that influence the average chain lengths of products and the dispersity of product distributions. Such approaches using micro or mesoporous materials can lead to processive catalysis, whereby a polymer chain is adsorbed into the pores of the inorganic oxide and is successively cleaved into smaller fragments without release of the ever-shortening polymer chain. Nanoparticles, responsible for C-C bond cleavage, localized in the pores at uniform distances from the pore mouth, then cleave polyolefin chains into semi-regular smaller chain lengths. We will present our studies of these architectures and catalytic reactions in the selective deconstruction of polyolefins.

Recycling is critical to reducing plastic pollution and developing a circular plastics economy. However, the implementation of recycling processes at a large scale is challenging due to variations in plastic waste feedstock composition, low recycled plastic yields, and high energy or chemical demands. This talk outlines how the Bio-Optimized Technologies to keep Thermoplastics out of Landfills and the Environment (BOTTLE) consortium utilizes quantitative analysis to help guide the design of plastic recycling processes. Using the chemical recycling of PET as an example, this talk will explore how stringent process modelling, techno-economic analysis, and life cycle assessment are used to evaluate the technical feasibility, cost, and environmental impact of a chemical recycling technology. By highlighting the key cost and environmental impact drivers of the recycling process, these analysis tools enable targeted process optimization and help inform future research priorities to ensure competitive and sustainable plastic recycling.

Efficient recycling of plastics requires a thorough understanding of complex deconstruction mechanisms intertwined with the structure-property relationships of these feedstocks for optimizing the conversion. Continuum models are limited to tracking the overall decomposition of lumped polymeric species and the temporal evolution of products and provide little information on the influence of the structure and morphological properties, such as crystallinity, on the depolymerization mechanisms (Fig. 1a). Tracking specific chain sequences and changes in the morphology is important when studying glycolytic deconstruction at temperatures ranging between the glass transition temperature and the melting point of semicrystalline polymers, like poly(ethylene terephthalate) (PET). This study aims to develop a detailed Kinetic Monte Carlo (kMC) framework to gain insights into the structure-property relationships of PET and the impact on glycolytic deconstruction.

In this work, the initial chain length distribution of PET is determined by assuming a Schulz-Flory distribution function. The spherulitic morphology is then defined by the distributions of tails, ties, loops, and crystalline stems present in the PET chains, which are generated using random-walk simulations based on their probabilities of formation. KMC simulations of catalytic PET glycolysis at discrete time steps for reaction temperatures between 150-190 ^oC suggest a competition between the random, chain scission reactions of PET and polymerization reactions of BHET at different reaction timescales. Mapping the reactions and distributions (Fig. 1b) suggests that interlamellar amorphous domains like tie chains and loops undergo sequential random scission forming tails and disrupt the spherulitic morphology. While the end chain scission of free amorphous domains and tails resulted in BHET monomer, it has been observed that the reactivity of crystalline stems governs the monomer yield at longer reaction times. Such frameworks are promising in developing predictive models for the solvent-based depolymerization of condensation polymers and gaining insights into the structure-function relationships.