A Review on Sustainable Upcycling of Plastic Waste Through Depolymerization into High-Value Monomer

Abstract

Plastic waste accumulation is one of the most pressing environmental challenges of the 21st century, owing to the widespread use of synthetic polymers and the limitations of conventional recycling methods. Among available strategies, chemical upcycling via depolymerization has emerged as a promising circular approach that converts plastic waste back into valuable monomers and chemical feedstocks. This article provides an in-depth narrative review of recent progress in the upcycling of major plastic types such as PET, PU, PS, and engineering plastics through thermal, chemical, catalytic, biological, and mechanochemical depolymerization methods. Each method is critically assessed in terms of efficiency, scalability, energy input, and environmental impact. Special attention is given to innovative catalyst systems, such as microsized MgO/SiO₂ and Co/CaO composites, and emerging enzymatic systems like engineered PETases and whole-cell biocatalysts that enable low-temperature, selective depolymerization. Furthermore, the conversion pathways of depolymerized products into high-purity monomers such as BHET, TPA, vanillin, and bisphenols are discussed with supporting case studies. The review also examines life cycle assessment (LCA) data, techno-economic analyses, and policy frameworks supporting the adoption of depolymerization-based recycling systems. Collectively, this work outlines the technical viability and sustainability benefits of depolymerization as a core pillar of plastic circularity and monomer recovery, offering a path forward for high-value material recirculation and waste minimization.

1. Introduction

Plastics have become an indispensable part of modern life due to their versatility, lightweight nature, durability, and cost-effectiveness [1,2,3]. From packaging materials and textiles to medical devices and automotive parts, plastics have permeated virtually every sector of human activity [4,5,6]. Since their large-scale industrial production began in the mid-20th century, the use of plastics has grown exponentially. According to data from the United Nations and the OECD, global plastic production exceeded 390 million tonnes in 2021, and this figure is expected to double by 2040 if current trends persist. However, this rapid growth has come at a significant environmental cost [7,8,9]. One of the most pressing concerns surrounding plastic usage is its persistence in the environment [10]. Most conventional plastics, such as polyethylene (PE), polypropylene (PP), polystyrene (PS), and polyethylene terephthalate (PET), are derived from non-renewable petroleum resources and are not readily biodegradable. Once discarded, they can persist in landfills or natural environments for hundreds of years, leading to widespread environmental contamination. Plastic waste has been detected in marine ecosystems, terrestrial habitats, and even remote polar regions, posing severe threats to wildlife and ecosystems. The rise of microplastic particles less than 5 mm in size has further complicated the issue. These particles, resulting from the breakdown of larger plastic debris or directly released in product formulations, have infiltrated food chains, drinking water supplies, and even the human body, raising health concerns that are still not fully understood [2,5]. Given the linear “take-make-dispose” model of plastic production and consumption, the accumulation of plastic waste is reaching crisis levels. The 3Rs—Reduce, Reuse, and Recycle—are no longer sufficient as standalone strategies. While source reduction and reuse are ideal, they are not always feasible at scale [7]. Recycling, although widely advocated, faces multiple technological and economic limitations, prompting the need for innovative, sustainable solutions to manage plastic waste [11,12,13].

Conventional plastic waste management methods—primarily mechanical recycling, incineration, and landfilling—have significant drawbacks. Each approach, while capable of reducing immediate waste buildup, introduces new environmental or economic concerns [14,15,16]. Mechanical recycling involves the physical reprocessing of plastic waste into new products, usually by sorting, cleaning, shredding, melting, and remolding. While this method is widely practiced, especially for thermoplastics like PET and HDPE, it suffers from several limitations. Material degradation is the tendency of plastics to degrade in quality with each cycle due to thermal and mechanical stress, limiting the number of times they can be recycled. Contamination implies that mixed plastic waste streams, coloring agents, and additives reduce recyclability. Incompatible polymers are difficult to separate, and contamination by food or chemicals often renders waste non-recyclable [17,18,19]. In downcycling, the most mechanically recycled plastics are downcycled into lower-value products (e.g., plastic lumber), rather than being converted back into materials of equal or higher value. Only about 9% of global plastic waste is effectively recycled, with the rest ending up in landfills, incinerators, or the environment [20,21]. Landfilling remains the most common disposal method in many countries due to its simplicity and low initial cost. However, it comes with serious drawbacks. Plastics in landfills can take centuries to decompose, leaching harmful additives into the soil and groundwater. Plastics contain significant embedded energy from fossil fuels. Landfilling discards valuable carbon-based resources that could otherwise be recovered. Urbanization and population growth are reducing the availability of land for waste disposal, especially in developing countries. Incineration with energy recovery, or “waste-to-energy” (WTE), involves burning plastic waste to generate electricity or heat. While this reduces waste volume and recovers some energy, it poses several issues. Air pollution: Incineration can release toxic gases and particulates, including dioxins and furans, unless highly controlled. Carbon footprint: Plastics are hydrocarbon-based; their combustion contributes to greenhouse gas emissions. Public opposition: WTE plants are often met with community resistance due to pollution and health concerns. In light of these challenges, the current waste management paradigm is clearly unsustainable. A shift toward circular economy principles where materials are kept in use for as long as possible and waste is minimized requires new technologies and processes that go beyond traditional recycling. This is where upcycling and monomer recovery come into play.

Upcycling, unlike traditional recycling, refers to processes that convert waste materials into new products of higher quality or value, often through chemical transformations (Figure 1) [22,23,24]. In the context of plastic waste, upcycling seeks to break down polymers into their original monomers or valuable intermediates, which can then be used to synthesize new polymers or other chemical products [25,26,27]. This chemical recycling or depolymerization approach offers several key advantages over mechanical recycling: Closed-loop recycling: By recovering monomers, plastics can be recreated with properties equal to virgin materials, enabling truly circular use. Versatility: Chemical recycling can handle mixed, contaminated, or composite plastic waste streams that are unsuitable for mechanical recycling. Value addition: Monomer recovery allows the creation of high-purity building blocks, specialty chemicals, or even fuels, increasing the economic value of waste. Integration with green chemistry: Emerging methods focus on using benign solvents, renewable catalysts, and energy-efficient processes, making the approach more environmentally friendly. One of the most widely studied examples of plastic upcycling is the depolymerization of PET into bis(2-hydroxyethyl) terephthalate (BHET) or terephthalic acid (TPA), both of which can be used to synthesize new PET [28,29,30]. Similar strategies are being explored for polyurethane (PU), polylactic acid (PLA), and other thermoplastics [11,15]. With increasing emphasis on sustainable development goals (SDGs), especially those related to responsible consumption and production (SDG 12), climate action (SDG 13), and life below water (SDG 14), upcycling offers a viable pathway for reducing environmental impact, conserving resources, and fostering innovation in materials science [20,26]. However, the technological readiness, economic feasibility, scalability, and environmental performance of different upcycling methods vary widely. Thus, there is a pressing need for comprehensive reviews that evaluate and compare these methods in the context of sustainability [31,32,33].

The primary aim of this review is to provide a comprehensive and critical overview of current advancements in the upcycling of plastic waste through depolymerization into useful monomers, focusing on the sustainability of the processes. Specifically, this review seeks to do the following: Examine the types of plastics amenable to chemical depolymerization and identify the monomers or valuable intermediates that can be recovered. Analyze and compare technological approaches to depolymerization, including thermal, catalytic, chemical, and enzymatic methods. Highlight the role of green chemistry in making these processes more environmentally benign and resource-efficient. Assess the life cycle implications, energy requirements, and carbon footprint of upcycling techniques. Explore the economic and policy frameworks needed to support the industrial adoption of plastic upcycling technologies. Identify current research gaps, emerging trends, and future directions for scalable and sustainable plastic circularity. By synthesizing recent findings from the peer-reviewed literature, case studies, and industrial reports, this article aims to serve as a valuable reference for researchers, industry stakeholders, and policymakers seeking to implement or scale sustainable plastic upcycling solutions. Emphasis will be placed not only on technical feasibility but also on real-world applicability and environmental impact, offering a holistic understanding of how plastic waste can be transformed from a global problem into a valuable resource.

2. Overview of Plastic Waste and Its Environmental Impact

2.1. Types of Plastics and Their Applications (PET, PE, PP, PS, PVC, etc.)

Plastics are synthetic polymers derived predominantly from fossil fuels, and they exhibit a wide range of properties that make them suitable for various commercial and industrial applications. Their durability, flexibility, light weight, and resistance to moisture and chemicals have led to an exponential increase in their production and utilization. Plastics are broadly categorized into two groups: thermoplastics and thermosets [21]. Thermoplastics can be softened by heating and reshaped multiple times, making them more suitable for both mechanical and chemical recycling processes [25]. In contrast, thermosets undergo irreversible chemical changes during curing, resulting in a rigid, crosslinked structure that cannot be remolded or recycled by conventional means. Among thermoplastics, polyethylene terephthalate (PET) is one of the most widely used types, particularly in beverage bottles, food containers, and textile fibers [18,27]. Its excellent barrier properties, strength, and transparency make it especially well-suited for packaging applications, particularly for storing liquids [33]. High-density polyethylene (HDPE) and low-density polyethylene (LDPE) are other forms of polyethylene used extensively in household and industrial packaging. HDPE is found in containers for milk, detergent, and automotive products, while LDPE is used in films, plastic bags, and squeeze bottles. Both types of PE are highly versatile and form a significant portion of municipal plastic waste. Polypropylene (PP) is another prevalent plastic, found in products like yogurt containers, automotive components, and medical devices. Its high melting point and chemical resistance make it suitable for both domestic and industrial applications. Polystyrene (PS), commonly used in disposable utensils, cups, and packaging foams, is notorious for its brittleness and environmental persistence. It exists in both rigid and expanded forms (EPS, or Styrofoam), the latter being particularly difficult to recycle due to its low density. Polyvinyl chloride (PVC) is extensively used in construction (pipes, window frames), healthcare (blood bags, tubing), and electrical applications. It contains chlorine and various additives that make it resistant to weathering and fire but also complicate its recyclability and environmental safety. Other specialty plastics like polycarbonate, polyurethane, and acrylics are used in high-performance applications such as electronics, automotive parts, insulation, and coatings. These “other” plastics often contain complex additives and layered structures, rendering them even more difficult to recycle by conventional means. Despite their functional diversity, most plastics are derived from non-renewable feedstocks and exhibit poor degradability, leading to persistent environmental challenges. Each plastic type comes with its own set of challenges in waste collection, sorting, and end-of-life processing, especially when mixed or contaminated in post-consumer waste streams [34,35,36].

Recycling Codes of Common Plastics

Plastics are categorized by recycling codes, also known as resin identification codes (RIC), which classify plastic polymers based on their chemical composition and recycling potential [6,9]. These codes are represented by a number (1 through 7) inside a triangular recycling symbol typically printed on plastic products. Code 1 (PET or PETE) refers to polyethylene terephthalate, commonly used in beverage bottles and food containers; it is widely recycled and often processed via chemical or enzymatic depolymerization into BHET or TPA [12,14]. Code 2 (HDPE) stands for high-density polyethylene, found in detergent bottles and milk jugs; it is mechanically recyclable [11,12]. Code 3 (PVC) is polyvinyl chloride, used in pipes, window frames, and blister packaging, but it is difficult to recycle due to chlorine content and additives [12,16]. Code 4 (LDPE) refers to low-density polyethylene used in plastic bags and squeeze bottles; its recycling is limited due to film contamination [12,17]. Code 5 (PP) is polypropylene, found in yogurt cups and automotive parts; it has growing recycling potential, although still limited by infrastructure [11,15]. Code 6 (PS) represents polystyrene and expanded polystyrene (EPS), used in disposable cutlery and packaging foam; its recycling is minimal due to economic and technical barriers [12,19]. Finally, Code 7 (Other) includes all other plastics such as polycarbonate, PLA, and multilayered composites; these are the most challenging to recycle and are often targeted by advanced chemical or catalytic upcycling methods [12,29]. Including these codes provides clarity on polymer identity and recyclability, and is crucial for waste sorting, material recovery, and policy regulation. Understanding and leveraging these recycling codes is essential for designing targeted upcycling technologies and improving circularity in plastic waste management systems [3,7].

2.2. Global Statistics on Plastic Waste Generation

The global production and consumption of plastics have reached staggering levels. According to the United Nations Environment Programme (UNEP), the world produced approximately 391 million tonnes of plastic in 2021, a figure that is expected to nearly double by 2040 if current consumption patterns continue unabated [37,38,39]. This massive production has been accompanied by an equally alarming rate of plastic waste generation, with over 10 billion tonnes of plastic estimated to have been produced since the 1950s [5,31]. Only a fraction of this plastic waste has been effectively managed. Globally, just about 9% of all plastic waste ever generated has been recycled. Around 12% has been incinerated, and the remaining 79% has either been landfilled or dumped into the natural environment [14,19]. These figures reveal a systemic failure in plastic waste management infrastructure and policies, particularly in low- and middle-income countries where open dumping and informal recycling are common [31,32]. Sectoral contributions to plastic waste are heavily skewed towards packaging, which accounts for nearly 40% of global plastic demand. Packaging plastics are typically designed for single use and have short life cycles, making them one of the biggest contributors to plastic pollution [40]. Other major sectors include construction, textiles, automotive, electronics, and consumer goods [17,26]. Textiles, largely composed of synthetic fibers such as polyester (a form of PET), are emerging as a major source of microplastic pollution [41,42,43]. The COVID-19 pandemic significantly exacerbated the plastic waste crisis. The surge in the use of personal protective equipment (PPE), takeout food packaging, and disposable medical supplies added an estimated 8 million tonnes of pandemic-related plastic waste to the global load. Much of this waste was inadequately managed, often ending up in landfills, waterways, and oceans. Current recycling systems are inadequate to handle this growing influx. In the United States, only around 5–6% of plastic waste is currently recycled, with most municipal systems relying on downcycling or exporting waste to other countries. In the European Union, the recycling rate is better, at around 30%, but still insufficient to address the scale of the problem. Moreover, inconsistencies in regulations, collection systems, and market demand for recycled materials further limit the effectiveness of global recycling efforts [44,45,46].

2.3. Environmental Consequences of Plastic Accumulation

Plastic waste accumulation has emerged as one of the most pressing environmental challenges of our time [47,48,49]. Its persistence in both terrestrial and marine ecosystems leads to a cascade of ecological disruptions, affecting wildlife, food security, human health, and climate change mitigation efforts. In marine environments, an estimated 11–12 million tonnes of plastic waste enter the oceans every year. This waste forms floating garbage patches, such as the Great Pacific Garbage Patch, and degrades into microplastics that sink into the seabed or remain suspended in the water column. Marine animals often mistake plastic for food or become entangled in debris, leading to injury, starvation, or death. It is estimated that over 700 marine species, including seabirds, turtles, and whales, are affected by plastic ingestion or entanglement [50,51,52]. On land, plastic pollution disrupts soil health and terrestrial food chains. Agricultural films and packaging can break down into microplastics, altering soil structure and microbial diversity [41,45]. Microplastics have been detected in soil, plants, and even in agricultural runoff, suggesting a potential route into the human food chain [29,33]. Microplastics and nanoplastics are now found in virtually every environmental compartment—air, water, soil, and ice—and have also been detected in human blood, lungs, and placenta [42,48]. These particles are not only physical contaminants but also act as vectors for toxic chemicals such as persistent organic pollutants (POPs), heavy metals, and endocrine-disrupting compounds. Although the full health implications are still under study, early evidence links microplastic exposure to inflammation, oxidative stress, and potential hormonal imbalances. The production and disposal of plastics also contribute significantly to greenhouse gas emissions. Incineration of plastic waste releases carbon dioxide and toxic compounds like dioxins and furans. Even the slow photodegradation of plastics in sunlight can produce methane and ethylene, both potent greenhouse gases. Life cycle analyses indicate that by 2050, the cumulative emissions from plastic production and incineration could exceed 56 billion tonnes of carbon dioxide equivalent, undermining global climate goals. Moreover, plastic waste disproportionately affects marginalized and low-income communities. In many countries, informal waste pickers and recyclers work in unsafe and unhygienic conditions, often exposed to toxic fumes and contaminated waste. This environmental injustice adds a social dimension to the already complex issue of plastic pollution [53,54,55].

2.4. Need for Circular Economy and Sustainable Waste Management

The traditional linear economic model based on the principles of take, make, use, and dispose is fundamentally unsustainable [56,57,58]. This model not only leads to the depletion of finite resources but also results in waste accumulation and environmental degradation. A shift toward a circular economy offers a more viable and sustainable alternative. In a circular economy, the value of products and materials is maintained for as long as possible, waste generation is minimized, and resources are reintroduced into the production cycle through reuse, repair, recycling, or upcycling [8,19]. For plastics, this means designing products that are easier to disassemble and recycle, creating standardized material flows, and developing technologies for chemical recycling that can regenerate plastics back into their monomeric or feedstock forms [11,27]. Central to this vision is the concept of design for recyclability. Many current plastic products are composed of multilayered or composite materials that are difficult to separate or recycle. By designing plastics with fewer additives and using mono-materials, the efficiency of recycling systems can be significantly improved. Likewise, product stewardship where manufacturers take responsibility for the post-consumer stage of their products can incentivize sustainable design and end-of-life management [26,29]. Upcycling, or the conversion of plastic waste into products of equal or higher value, plays a crucial role in achieving a circular plastic economy. Unlike mechanical recycling, which often leads to material degradation (downcycling), upcycling through depolymerization can regenerate plastics into their original monomers or create new chemicals and fuels [31,33]. These recovered monomers can then be used to manufacture virgin-quality plastics, closing the loop and reducing the demand for fossil fuel-based raw materials. From a policy perspective, extended producer responsibility (EPR), plastic taxes, and bans on single-use plastics are regulatory tools that can facilitate the transition to a circular model. Investment in infrastructure, public–private partnerships, and consumer awareness campaigns are also critical components of a holistic waste management strategy [50,51]. Furthermore, the integration of green chemistry principles such as the use of renewable feedstocks, energy efficiency, and benign solvents can ensure that recycling and upcycling processes themselves do not contribute to pollution. Life cycle assessments (LCAs) must be employed to evaluate the environmental impact of new technologies, ensuring that solutions are not only technically feasible but also ecologically sound [53]. In conclusion, addressing the plastic waste crisis demands more than incremental improvements to existing systems. It requires a paradigm shift toward circularity, where innovation in materials science, chemical engineering, and policy converge to transform plastic waste from a global liability into a valuable resource [59,60,61].

3. Concept of Plastic Upcycling

3.1. Definition of and Distinction Between Recycling, Downcycling, and Upcycling

The management of plastic waste has evolved significantly over the past few decades as environmental concerns, policy regulations, and technological advancements have reshaped global strategies [62,63,64]. Among the emerging concepts gaining traction in recent years is plastic upcycling, a process distinct from the conventional notions of recycling and downcycling. To appreciate the value of upcycling, it is essential first to understand how it differs from other forms of waste management. Recycling is traditionally defined as the process of converting waste materials into new materials and objects. In the context of plastics, recycling typically refers to mechanical recycling, where plastic waste is collected, sorted, washed, melted, and reprocessed into pellets that can be used to manufacture new products. While this approach diverts plastic from landfills, it often results in a degradation of material properties due to thermal and mechanical stress during processing. Downcycling, a term often associated with mechanical recycling, refers to the conversion of materials into new products of lower quality or functionality than the original. For instance, recycled PET from bottles may be transformed into carpet fibers or insulation materials. While downcycling retains some material utility, it fails to preserve the full economic and functional value of the plastic. Each recycling cycle reduces the polymer’s molecular weight, mechanical strength, and optical clarity, limiting its reuse in high-performance applications. In contrast, upcycling is a process that transforms plastic waste into products or materials of equal or higher value than the original. Unlike mechanical recycling, which is limited by contamination, polymer degradation, and material mixing, upcycling typically employs chemical, catalytic, or enzymatic processes that break down plastics into their fundamental building blocks (monomers or valuable intermediates). These recovered compounds can then be purified and reused to synthesize new high-quality plastics or entirely different chemical products such as fuels, lubricants, or specialty chemicals [26,45]. Upcycling offers the potential to reverse the thermodynamic degradation seen in mechanical recycling. By converting polymers into monomers or platform chemicals, upcycling maintains the molecular integrity of the material and allows for the production of virgin-quality plastics, thereby extending their life cycle indefinitely. Moreover, the inclusion of green chemistry and advanced catalyst systems in upcycling processes enhances efficiency and reduces environmental impact, making it a vital innovation in sustainable materials science.

3.2. Advantages of Upcycling over Traditional Recycling Methods

Plastic upcycling offers a wide array of advantages over traditional recycling methods, particularly in terms of product quality, feedstock flexibility, environmental sustainability, and economic viability [65,66,67]. As the limitations of mechanical recycling become more apparent, upcycling is emerging as a more robust and promising alternative for managing the growing volume of plastic waste. One of the most significant advantages of upcycling is its ability to produce high-purity monomers or specialty chemicals. While mechanical recycling is constrained by polymer contamination and degradation, upcycling can selectively cleave polymer chains and recover the original chemical constituents. This closed-loop regeneration ensures that plastics do not lose their functionality over time. Another major benefit is the ability to handle mixed and contaminated waste streams. Conventional mechanical recycling requires careful sorting of plastic types (e.g., separating PET from polyethylene), and even minor contamination can render entire batches unusable. In contrast, many upcycling processes, particularly those involving chemical or catalytic depolymerization, can tolerate higher levels of heterogeneity, additives, dyes, and even food residues. This capability significantly reduces preprocessing costs and expands the range of plastics that can be treated effectively. From an environmental perspective, upcycling aligns closely with the principles of green chemistry and sustainability. Many new upcycling technologies employ non-toxic solvents, bio-based catalysts, and operate at lower temperatures, thereby reducing greenhouse gas emissions, energy consumption, and the generation of hazardous byproducts. For instance, enzymatic depolymerization of PET using PETase enzymes can occur under mild aqueous conditions with minimal environmental burden. These eco-friendly approaches are far superior to incineration or landfilling, both of which contribute to pollution and climate change. Upcycling also presents an opportunity for economic value creation. Unlike downcycled products, which have limited commercial applications, upcycled materials often serve as precursors to high-value products in sectors such as packaging, textiles, automotive, and pharmaceuticals. This potential to generate revenue from waste transforms plastic from a liability into a valuable asset, providing economic incentives for both industry and policymakers to invest in upcycling technologies. Furthermore, upcycling can enhance resource efficiency by reducing the demand for virgin fossil-based feedstocks. The extraction, refining, and polymerization of petrochemicals to produce plastics are energy-intensive processes that contribute significantly to global carbon emissions. By recovering monomers from plastic waste, upcycling reduces the need for primary resource extraction and fosters material circularity, contributing to climate change mitigation efforts. Despite these advantages, it is important to recognize that upcycling technologies are still in various stages of development and deployment. Scalability, cost-effectiveness, and compatibility with existing waste management infrastructure remain challenges that must be addressed through continued research, innovation, and policy support [68,69,70].

3.3. Role of Upcycling in Achieving a Circular Economy

The transition from a linear to a circular economy represents a systemic transformation in how resources are consumed and managed [71,72,73]. In this model, waste is not merely a byproduct but a resource to be reintegrated into the production cycle. Plastic upcycling plays a pivotal role in this transformation by enabling the regeneration of material value at the molecular level, thereby closing the loop in the plastic value chain. At its core, the circular economy aims to design out waste and pollution, keep products and materials in use, and regenerate natural systems. Plastic upcycling directly supports these goals by converting post-consumer waste into high-value monomers or feedstock chemicals that can be reintroduced into industrial supply chains. This continuous reuse of plastic-derived chemicals reduces the need for virgin material production, minimizes environmental leakage, and promotes long-term resource sustainability. Moreover, upcycling aligns with circular design principles, encouraging manufacturers to consider end-of-life management during the product development phase. If plastics are designed with depolymerization in mind by reducing cross-linking, additives, and multilayered components they can be more efficiently upcycled at the end of their useful life. This proactive design strategy reduces the complexity of waste streams and improves the feasibility of closed-loop recycling systems. Incorporating upcycling into the circular economy also enhances resilience in supply chains. By localizing waste-to-resource conversion, communities and industries can reduce their dependence on international supply chains for virgin plastic and fossil resources. This depolymerization not only reduces transportation emissions but also creates local employment opportunities in the waste management and recycling sectors. Additionally, plastic upcycling supports the achievement of several United Nations Sustainable Development Goals (SDGs). Specifically, it contributes to SDG 12 (Responsible Consumption and Production) by promoting sustainable material use, SDG 13 (Climate Action) through emissions reduction, and SDG 14 (Life Below Water) by mitigating marine plastic pollution. The integration of upcycling technologies into national and regional waste management strategies is thus essential for global sustainable development. Governments and international agencies are increasingly recognizing the value of upcycling in policy frameworks and investment programs. Extended producer responsibility (EPR), eco-design regulations, and plastic taxes can incentivize the adoption of upcycling technologies by shifting the cost burden from consumers to manufacturers. Simultaneously, public–private partnerships and funding mechanisms are vital to support the commercialization of promising upcycling methods and infrastructure. Despite its promise, successful implementation of upcycling in the circular economy requires overcoming several barriers. These include the technological maturity of upcycling processes, economic competitiveness compared to virgin plastic production, and the lack of standardized regulations or certifications for upcycled materials. Furthermore, public awareness and consumer behavior play a crucial role; the success of circular initiatives hinges on community engagement and responsible consumption. To fully realize the potential of plastic upcycling in the circular economy, a multi-stakeholder approach is essential. Scientists must develop efficient, scalable technologies; policymakers must create enabling regulations; industries must innovate in product design and supply chain management; and consumers must adopt sustainable behavior. When all sectors work in concert, plastic upcycling can evolve from a niche concept to a cornerstone of circular sustainability in the plastics industry [74,75,76].

4. Depolymerization Techniques for Plastic Waste

4.1. Overview of Depolymerization Processes

Depolymerization refers to the process of breaking down long polymer chains into smaller units, ideally reverting them to their original monomers or other valuable chemical intermediates. This approach is at the heart of chemical recycling and upcycling, as it offers the potential for closed-loop material recovery that preserves the quality of the polymer and eliminates the need for fossil-derived virgin materials. Unlike mechanical recycling, which often results in the degradation of plastic properties, depolymerization can restore plastic to its fundamental building blocks, enabling the creation of virgin-quality materials. Several types of depolymerization processes exist, each with its own set of mechanisms, catalysts, reaction conditions, and target materials. These include thermal, chemical, catalytic, and biological methods. While the ultimate goal of all these techniques is the same—to recover useful compounds from plastic waste—their approach to bond cleavage, specificity to polymer types, and environmental impacts differ significantly. Recent advances have also led to the development of hybrid approaches that combine different depolymerization strategies to enhance performance, selectivity, and sustainability. Understanding the principles and variations of these depolymerization techniques is essential for assessing their viability in large-scale plastic waste management systems [77,78,79].

4.1.1. Thermal Depolymerization

Thermal depolymerization involves the application of high temperatures to break down polymers into smaller molecules without the use of specific catalysts or reagents [80,81,82]. The process is typically conducted in the absence of oxygen (pyrolysis) or in the presence of limited oxygen (thermal oxidation), leading to the formation of monomers, oligomers, oils, gases, and char [21,24]. This method is highly versatile and can be applied to a wide range of plastic types, including polyethylene (PE), polypropylene (PP), polystyrene (PS), and mixed plastic waste streams [16,17,22]. Pyrolysis operates at temperatures ranging from 300 to 800 °C and decomposes polymers into hydrocarbon mixtures that can be further refined into fuels or chemical feedstocks. For example, PE and PP can be thermally cracked into diesel-like oils and waxes [23,49]. Polystyrene is especially amenable to thermal depolymerization, often yielding styrene monomer with relatively high selectivity [2,11]. One of the main advantages of thermal depolymerization is its ability to handle contaminated and heterogeneous waste, including multilayer packaging and non-recyclable plastics [16,28]. However, the process is energy-intensive, often requiring external heat sources and robust reactor materials to withstand extreme conditions. Moreover, the non-selective nature of pyrolysis can lead to complex product mixtures that require further separation and purification, which adds to the operational costs and environmental burden [22,35]. Thermal depolymerization also generates gaseous emissions, including CO₂, CO, and volatile organic compounds (VOCs), which must be carefully managed to prevent environmental pollution [79,80]. Despite these challenges, the scalability and simplicity of thermal depolymerization continue to make it a prominent option, particularly for applications where mechanical recycling is not feasible [83,84,85].

A novel thermally triggerable polymer blend system was developed to enable rapid and efficient depolymerization at relatively low temperatures, making it ideal for applications in transient electronics and smart degradable materials (Figure 2a–c) [81]. The study focused on blending cyclic polyphthalaldehyde (cPPA) a depolymerizable polymer with a low ceiling temperature (−42 °C) with a polymeric thermoacid generator, poly(vinyl tert-butyl carbonate sulfone) (PVtBCS). Upon heating, PVtBCS undergoes thermal decomposition at around 85 °C, releasing acidic species that catalyze the depolymerization of cPPA into volatile byproducts, primarily o-phthalaldehyde monomer as confirmed by Raman spectroscopy. This approach successfully reduced the thermal depolymerization onset temperature of cPPA by 22 °C with just 2 wt% PVtBCS, as shown in thermogravimetric analysis (TGA). The incorporation of increasing concentrations of PVtBCS (0.5–2.0 wt%) significantly accelerated the depolymerization rate, with complete depolymerization achieved within 15 min at 85 °C for the 2 wt% blend. Gel permeation chromatography (GPC) and dynamic mechanical analysis (DMA) further confirmed that both molecular weight and mechanical integrity dropped sharply under these mild thermal conditions, indicating efficient material disintegration. This synergistic polymer blend strategy demonstrates a controlled, thermally activated degradation pathway where a self-degrading additive (PVtBCS) triggers the breakdown of an otherwise stable but depolymerizable matrix (cPPA). The resulting films degrade cleanly, leaving less than 2 wt% solid residue after 80 min at 85 °C, making them highly suitable for single-use devices, recyclable packaging, and eco-friendly electronics where rapid disappearance of polymeric components is desirable.

Thermal RAFT (Reversible Addition–Fragmentation chain Transfer) depolymerization has recently emerged as a powerful method for the chemical recycling of polymethacrylates, offering a pathway to regenerate monomers under controlled conditions (Figure 3a–d) [82]. However, the fate of the RAFT end-group, a key feature in reversible addition–fragmentation chain-transfer polymerization, remained poorly understood. This study investigates the byproducts formed from RAFT end-groups during thermal depolymerization of polymethacrylates and provides new insights into the depolymerization mechanism. The main small molecule identified was a RAFT methacrylate unimer (DP = 1), a structure that is difficult to synthesize through conventional single-unit monomer insertion methods. Remarkably, this unimer exhibited superior performance as a RAFT agent in the polymerization of methyl methacrylate (MMA), leading to faster monomer conversion and narrower molecular weight distributions compared to standard commercial RAFT agents like 2-cyano-2-propyl dithiobenzoate. In addition to the unimer, solvent-derived byproducts were detected, especially during the early stages of the depolymerization reaction. Their consistent presence across different monomers, RAFT agents, and solvents implies a significant role of the solvent in initiating depolymerization likely through a solvent-mediated radical generation mechanism. Despite variations in the chemical environment, the formation of the unimer and solvent-derived fragments remained reproducible, indicating a robust, generalizable decomposition pathway. These findings not only clarify the decomposition fate of RAFT-functionalized polymers but also reveal a novel route to synthesize RAFT-active unimers, offering new opportunities for the design of precision polymerization tools. Overall, this work enhances the mechanistic understanding of thermal RAFT depolymerization and demonstrates that the end-group chemistry can be harnessed for functional material synthesis, rather than merely being treated as a degradation artifact.

4.1.2. Chemical Depolymerization

Chemical depolymerization employs chemical agents such as acids, bases, alcohols, or water to cleave the chemical bonds in polymers under controlled conditions [25,26]. This method is especially useful for condensation polymers, such as polyethylene terephthalate (PET), polyamides (nylons), and polyurethanes, which contain hydrolyzable ester, amide, or urethane linkages [86,87,88]. In PET recycling, several chemical depolymerization routes are well-established, including hydrolysis, methanolysis, glycolysis, and aminolysis [48,49]. For example, glycolysis of PET involves the reaction with ethylene glycol to produce bis(2-hydroxyethyl) terephthalate (BHET), which can be purified and repolymerized into new PET. Methanolysis yields dimethyl terephthalate (DMT) and ethylene glycol, both of which are valuable industrial monomers [16,19]. Hydrolysis, performed under acidic, basic, or neutral conditions, breaks PET down into terephthalic acid (TPA) and ethylene glycol [47,56]. The specificity and selectivity of chemical depolymerization make it suitable for high-purity monomer recovery. Moreover, chemical depolymerization typically operates under milder temperatures compared to thermal methods, particularly when using catalysts or optimized solvents. However, this technique often requires strict separation of plastic types, as the reagents are usually tailored for specific polymer chemistries [62,69]. A key drawback is the use of corrosive and potentially hazardous chemicals, which raises concerns about reactor materials, worker safety, and wastewater treatment. Additionally, chemical consumption and the need for reagent recovery or neutralization contribute to the environmental footprint [71,77]. Nonetheless, continued innovations in solvent systems and process integration are improving the feasibility of chemical depolymerization on a commercial scale [89,90,91].

In pursuit of scalable and sustainable approaches for PET recycling, this study introduces a novel glycolytic depolymerization method utilizing a microsized MgO-incorporated SiO₂ catalyst (Figure 4) [91]. Unlike conventional nanoparticle-based catalysts, which pose challenges in recovery and may contribute to residual contamination in the product stream, the microsized MgO/SiO₂ catalyst offers enhanced operational ease and cleaner product separation. The catalyst was synthesized via wet impregnation and characterized for its catalytic activity across varying doses and reaction durations. Results revealed a notable BHET yield of 95.1%, achieved with exceptional selectivity and purity exceeding 99.85%, as confirmed by HPLC and NMR. Importantly, no detectable metal residues were found in the final product by ICP-MS, highlighting the system’s environmental compatibility and suitability for high-purity applications. Despite concerns that increased particle size might compromise catalytic performance, the microsized catalyst demonstrated efficiency on par with or superior to traditional nanoscale catalysts. Its larger size facilitated straightforward post-reaction separation, thereby reducing downstream processing complexity and minimizing catalyst loss. This characteristic directly addresses a longstanding limitation in chemical PET recycling, where fine catalysts are difficult to recover without affecting product quality. By merging effective catalytic activity with operational simplicity, this approach supports a closed-loop recycling system for PET that aligns with circular economy principles. The method provides a promising foundation for industrial-scale implementation, where yield, purity, and ease of catalyst recovery are critical parameters. This advancement underlines the potential of engineered heterogeneous catalysts not only to improve process efficiency but also to uphold sustainability standards across the PET life cycle. The results contribute meaningfully to the field of chemical recycling and represent a significant step toward practical, clean, and economically viable PET upcycling.

A novel and scalable approach to the chemical upcycling of polyethylene terephthalate (PET) waste is demonstrated through mechanochemical depolymerization, providing a solvent-minimized, high-yield pathway to terephthalic acid (TPA) (Figure 5) [92]. This study represents the first kilogram-scale depolymerization of PET using planetary and oscillating ball mills, enabling the conversion of post-consumer PET waste, including used textiles and plastic bottles, into high-purity monomers. Extensive process optimization was conducted by varying critical parameters such as milling speed, liquid-to-solid ratio, jar material, and milling duration. The use of methanol as a liquid-assisted grinding agent, in conjunction with sodium hydroxide as a catalyst, proved essential for effective depolymerization. A TPA yield exceeding 97% was consistently achieved in planetary ball mill conditions (400 rpm, 80 min, 1000 g PET), while small-scale trials (0.2–50 g PET) in an oscillating ball mill delivered up to 100% yield of TPA in as little as 40 min. This methodology presents a significant departure from conventional solution-based depolymerization strategies (e.g., hydrolysis and glycolysis), as it eliminates the need for large volumes of solvents and operates under solvent-free or solvent-assisted grinding conditions, thus offering environmental and economic advantages. The process also benefits from energy-efficient mechanical activation, which enhances reaction kinetics and allows for effective breakdown of the PET backbone. Moreover, this approach is feedstock-flexible, accommodating diverse PET waste streams, including colored, multilayered, and textile-derived PET, without compromising product quality or yield. The results firmly establish mechanochemical depolymerization as a viable route for industrial-scale PET recycling, aligning with circular economy goals and offering a practical solution to mitigate both plastic pollution and resource inefficiency. By enabling high monomer recovery rates without complex downstream purification or catalyst separation steps, this technique offers a compelling pathway for the closed-loop recycling of PET into value-added chemical feedstocks.

4.1.3. Catalytic Depolymerization

Catalytic depolymerization enhances chemical or thermal processes by introducing catalysts that lower activation energies, improve selectivity, and enable reactions at lower temperatures and pressures [92,93,94]. The catalysts can be heterogeneous (solid) or homogeneous (liquid) and may include metal-based catalysts, zeolites, ionic liquids, or organocatalysts [19,52]. Catalytic approaches are particularly advantageous for olefinic polymers such as PE, PP, and PS, as well as for more complex plastics [12,46]. One notable example is the hydrogenolysis of polyolefins using ruthenium- or platinum-based catalysts, which selectively cleave C–C bonds to produce hydrocarbon fuels or waxes [29,31]. These reactions can be tuned to control product chain length, making them useful for fuel production or specialty chemical synthesis. Similarly, polystyrene can be catalytically depolymerized into styrene monomer or oligomers using solid acid catalysts under relatively mild conditions [44,47]. Catalytic depolymerization of PET and polycarbonates has also shown promising results. For instance, metal–organic frameworks (MOFs) and ionic liquids have been investigated as catalysts in glycolysis reactions, offering higher yields and faster reaction times compared to traditional methods. While catalytic processes can significantly reduce energy consumption and reaction time, they often face challenges related to catalyst deactivation, cost, and recovery [51,66]. Catalysts may be poisoned by impurities or degraded over multiple cycles, necessitating robust regeneration protocols. Additionally, some metal-based catalysts rely on rare or expensive elements, raising concerns about economic viability and material sustainability [71,78]. Nevertheless, catalytic depolymerization remains one of the most flexible and promising routes for plastic upcycling, with ongoing research focused on developing low-cost, earth-abundant catalysts and process intensification techniques [95,96,97].

In an effort to valorize lignin waste and enhance the quality of bio-oil derived from its depolymerization, a series of cobalt-supported calcium oxide (Co/CaO) catalysts were developed and evaluated for their catalytic efficiency under a range of reaction conditions [95] (Figure 6). The catalysts, containing 2, 5, 10, and 15 wt% Co, were systematically tested across varying solvents (water and methanol), temperatures, reaction times, and catalyst loadings. Among the tested formulations, the 10 wt% Co/CaO catalyst emerged as the most effective, achieving a maximum bio-oil yield of 60.2 wt% when methanol was used as the solvent at 160 °C for 60 min. In contrast, the yield using water as a solvent under similar conditions was significantly lower, at 26.6 wt%, demonstrating the superior role of alcohols in promoting lignin depolymerization. Detailed product analysis using GC-MS, ¹H NMR, and FT-IR revealed a marked improvement in the chemical complexity and value of the bio-oil obtained through catalytic routes. Compared to non-catalytic processes, the catalytic depolymerization not only increased the total phenolic content but also favored the production of methoxy- and alkyl-substituted phenols, particularly vanillin-like compounds, which accounted for up to 58.7% selectivity in the product mixture. This shift in product distribution reflects the synergistic interaction between cobalt and calcium oxide, which promotes selective bond cleavage in lignin’s complex aromatic structure. Another significant finding was the substantial improvement in the higher heating value (HHV) of the catalytic bio-oil, measured at 35.5 MJ/kg, compared to the original lignin feedstock (24.5 MJ/kg), underscoring the potential of this process for energy recovery applications. Additionally, the Co/CaO catalyst exhibited excellent recyclability and thermal stability, maintaining high bio-oil yields over multiple cycles without significant deactivation or structural degradation. The findings underscore the critical role of bimetallic and multifunctional catalyst design in tailoring lignin valorization processes and advancing the development of renewable fuel and chemical production pathways.

The chemical recycling of super engineering plastics—materials known for their exceptional thermal stability, chemical resistance, and insolubility—has long remained a challenge due to the strength and rigidity of their main chain structures, particularly aromatic ether linkages. In this study, a novel catalytic depolymerization-like strategy is introduced for the decomposition of such materials, including polyetheretherketone (PEEK), polysulfone (PSU), PEES, PPSU, PESU, and PEI, through the selective cleavage of carbon-oxygen bonds using thiols (Figure 7a,b) [96]. The process is enabled by a synergistic catalyst system composed of a bulky organic phosphazene base (P4-tBu) and inorganic bases such as tripotassium phosphate (K₃PO₄) or cesium carbonate (Cs₂CO₃), which together facilitate efficient bond scission under moderate temperatures (Figure 7a,b). This method also benefits from polar aprotic solvents such as DMAc, which aid in solubilizing otherwise insoluble polymers and enhancing reaction efficiency [96]. The reaction yielded electron-deficient arenes with thiofunctional groups and bisphenols, which are valuable not only as depolymerization products but also as reactive intermediates for further functionalization. The study demonstrated that the thioarenes produced could be readily converted into amino, sulfonium, or fluorinated arenes, the latter of which are key building blocks for synthesizing new super engineering plastics. Notably, the method was successfully applied to composite materials, including carbon- and glass-fiber-reinforced PEEK, and even to commercial products, such as a baby bottle made from PPSU, highlighting its practical applicability to real-world waste streams. This catalytic system offers a rare and significant advancement in the upcycling of high-performance polymers that are otherwise considered non-recyclable through conventional chemical means. By transforming thermally and chemically robust plastics into monomer-like, reactive small molecules, the approach supports both closed-loop recycling and molecular upcycling, contributing to material circularity. Furthermore, the platform demonstrates the versatility of thiol-based nucleophilic strategies in overcoming the decomposition barriers of advanced polymeric architectures. These findings open new opportunities for the recycling and refunctionalization of high-value plastic waste, thereby supporting broader goals in sustainable materials chemistry and circular engineering.

4.1.4. Biological Depolymerization

Biological depolymerization, also known as enzymatic depolymerization, uses microorganisms or their enzymes to break down polymers into monomers or low-molecular-weight compounds under environmentally benign conditions [98,99,100]. This method has gained significant attention due to its low energy requirements, high specificity, and potential for integration with green technologies. One of the most well-known examples is the use of PETase, an enzyme discovered in the bacterium Ideonella sakaiensis, which hydrolyzes PET into terephthalic acid and ethylene glycol. The enzymatic process operates under mild temperatures (~30–70 °C) and neutral pH, producing high-purity monomers with minimal environmental impact. Subsequent studies have engineered variants of PETase and MHETase (mono-hydroxyethyl terephthalate hydrolase) with enhanced activity, thermal stability, and broader substrate scope. Other polymers being explored for biological degradation include polyurethanes, polycaprolactone (PCL), polylactic acid (PLA), and even certain blends of polyethylene with oxidizable additives. Fungi and bacteria capable of producing depolymerizing enzymes such as esterases, lipases, and cutinases are being identified through metagenomics and synthetic biology. Despite its environmental advantages, biological depolymerization is currently limited by slow reaction rates, low polymer accessibility, and enzyme cost. Crystalline or highly hydrophobic plastics, like PE and PP, are particularly resistant to enzymatic attack. Moreover, scaling up enzymatic processes for industrial use poses logistical challenges, including maintaining bioreactor conditions and downstream purification of products. Nonetheless, the biotechnological potential is enormous. Coupling enzyme engineering with directed evolution and protein design can yield more efficient biocatalysts, making this method increasingly attractive for future sustainable plastic recycling efforts [101,102,103].

In the quest for sustainable and biologically driven lignin degradation methods, this study explores the potential of Schizophyllum commune, a hyper-performing laccase-producing white-rot fungus, for selectively degrading lignin in various crop residues under in vitro conditions (Figure 8A–H). Laccase enzymes derived from immobilized fungal cells showed significantly greater lignin-depolymerizing efficiency compared to purified and recombinant forms. Lignin degradation reached 30–40% in finger millet straw and sorghum stover, 27–32% in paddy straw, 21% in wheat straw, and 26% in maize straw using laccase from the cell immobilization method, whereas treatment with purified or recombinant laccase yielded only around 20% degradation across the tested residues. These findings underscore the advantage of enzyme production via immobilized fermentation systems in improving lignin breakdown efficiency. Crucially, in vitro dry matter digestibility (IVDMD) analysis demonstrated improved feed utilization potential of treated residues. Finger millet straw exhibited the highest digestibility (54–59%), followed by wheat and paddy straw (33–36%), and maize straw (16%) when treated with immobilized laccase. The control group using recombinant and purified laccase showed substantially lower digestibility values (13–14%), suggesting a strong correlation between lignin removal and digestibility enhancement. These results position immobilized cell-derived laccase as a cost-effective and scalable biocatalyst for pre-treatment of lignocellulosic biomass, particularly in the ruminant feed industry, where selective lignin removal is key to improving nutrient bioavailability. Additionally, the use of submerged fermentation for laccase production is noted to be more controllable and consistent than solid-state fermentation, which, although traditionally used, leads to excessive organic matter losses and is unsuitable for feed preparation. The cell immobilization technique, therefore, provides a promising and environmentally friendly approach to enzyme production, enabling better yields, reusability, and long-term operational stability. The outcomes of this study support the adoption of enzymatic lignin depolymerization as a green, economically viable method with applications not only in livestock nutrition but also in industrial bioprocessing, bioenergy, and biorefinery sectors. To build on these findings, pilot-scale enzyme production and in vivo feeding trials are recommended to validate digestibility improvements and enzyme performance under practical conditions. The promising results from this research underline the multifunctional potential of laccase, especially when produced via advanced biotechnological methods like cell immobilization, reinforcing its role in sustainable agriculture and bio-based industries.

In the ongoing pursuit of sustainable plastic waste management, this study presents a highly efficient whole-cell biocatalyst system for the complete depolymerization of polyethylene terephthalate (PET) into its monomeric constituents terephthalic acid (TPA) and ethylene glycol (EG) at ambient temperature (Figure 9). Traditional enzymatic strategies for PET degradation have been hindered by incomplete conversion and accumulation of intermediates like MHET due to the isolated or inefficient co-expression of PET-hydrolyzing enzymes (PHEs). Addressing this bottleneck, the authors engineered a Saccharomyces cerevisiae-based whole-cell system that displays both FAST-PETase and MHETase using a cellulosome-inspired trifunctional scaffoldin on the yeast surface (Figure 9) [102]. This configuration promotes enzyme co-localization and substrate channeling, enabling full degradation of amorphous PET films without pretreatment. The system yielded 4.95 mM TPA in 7 days at 30 °C, demonstrating no accumulation of intermediates and consistent activity over six reusability cycles. Notably, dual-site attachment of FAST-PETase on the scaffoldin further enhanced yield by up to 1.8-fold, while the presence of MHETase increased monomer production nearly fivefold compared to PETase alone. The system’s simplicity of recovery via centrifugation or filtration and its operation under mild, environmentally friendly conditions offer distinct advantages for scale-up and integration into industrial workflows. Beyond complete recycling, this biocatalyst platform holds significant potential for PET upcycling. The high-purity TPA and EG products obtained can be repolymerized into virgin-quality PET, or biologically converted into polyhydroxyalkanoates (PHA), protocatechuic acid (PCA), gallic acid (GA), and lycopene, all of which serve as precursors for value-added industrial products. The use of divergent cohesin–dockerin pairs in the scaffoldin design enables modular enzyme assembly, allowing expansion of this system into multi-enzyme cascades for one-pot biotransformations. Additionally, its applicability extends to microplastic degradation in wastewater streams, broadening its environmental relevance. This innovative whole-cell biocatalyst marks a significant step toward realizing a circular plastic economy, offering a robust, scalable, and flexible strategy for both recycling and upcycling PET waste using synthetic biology, enzyme engineering, and microbial platform design.

4.2. Comparative Analysis of Methods: Efficiency, Scalability, Environmental Impact

Each depolymerization method offers distinct benefits and faces unique limitations. When comparing thermal, chemical, catalytic, and biological depolymerization, several critical performance metrics must be considered: efficiency (yield and selectivity), energy consumption, scalability, environmental footprint, and feedstock flexibility. Thermal depolymerization is highly scalable and can process a wide range of mixed and contaminated plastics [104,105,106]. However, it is energy-intensive and often yields complex product mixtures requiring additional refinement. While suitable for fuel production, its selectivity toward monomer recovery is limited. Chemical depolymerization offers high selectivity and monomer purity, particularly for condensation polymers like PET and polyurethane. It requires relatively milder conditions than thermal methods but is limited by chemical costs, waste generation, and incompatibility with mixed waste streams. The need for feedstock pre-sorting also complicates scalability. Catalytic depolymerization provides a middle ground, combining the versatility of chemical methods with improved energy efficiency and reaction rates. The use of tailored catalysts allows for high product selectivity and moderate operational conditions. However, catalyst development, stability, and cost are significant barriers to widespread implementation. Biological depolymerization stands out for its eco-friendliness and specificity. It produces minimal emissions and waste, and the reaction conditions are benign. However, its applicability is currently limited to a narrow range of plastics and is constrained by slow kinetics and enzyme costs. Ongoing advancements in enzyme engineering and microbial biotechnology may eventually improve its commercial viability. In summary, no single depolymerization method is universally superior. The optimal approach depends on factors such as polymer type, contamination level, product goals, and local infrastructure. Hybrid solutions and integrated systems may offer the best path forward [107,108,109].

4.3. Emerging Hybrid and Green Technologies

As the demand for efficient and sustainable plastic recycling intensifies, researchers are increasingly turning to hybrid and green depolymerization technologies that combine the strengths of existing methods while minimizing their weaknesses [110,111,112]. These approaches seek to enhance reaction performance, reduce environmental impact, and improve economic feasibility. One promising avenue is the integration of catalytic and enzymatic depolymerization, where enzymes are used to pre-treat plastic waste, increasing surface area and introducing functional groups that make the material more susceptible to catalytic cleavage. This synergy can reduce energy requirements and improve overall conversion rates. Similarly, combining thermal pretreatment with enzymatic hydrolysis can enhance depolymerization of highly crystalline or hydrophobic plastics like PET. Green solvents, such as ionic liquids, deep eutectic solvents (DES), and supercritical fluids, are also being explored to replace traditional hazardous solvents. These materials often offer higher solubility for polymers, tunable polarity, and low volatility, making them ideal media for depolymerization reactions. Some ionic liquids even act as both solvent and catalyst, streamlining the process. Microwave- and ultrasound-assisted depolymerization represent other innovative methods that improve heat and mass transfer during the reaction, reducing energy consumption and accelerating reaction kinetics. These technologies can be particularly beneficial for heterogeneous systems or reactions that typically suffer from low rates under conventional heating. Advancements in reactor design such as continuous flow systems, membrane reactors, and modular processing units are also enhancing the scalability and efficiency of depolymerization methods. These designs offer better control over reaction conditions and facilitate integration into existing waste management infrastructure. Lastly, the development of machine learning and AI models for reaction optimization is accelerating the discovery of new catalysts and process parameters. These tools can significantly reduce experimental workload and guide data-driven innovations in depolymerization technology.

5. Conversion of Depolymerized Polymers to Useful Monomers

5.1. Target Monomers and Their Industrial Significance

The primary goal of plastic depolymerization is the recovery of valuable monomers or chemical intermediates that can be reintroduced into industrial production cycles [113,114,115]. These monomers, when extracted with high purity and efficiency, serve as crucial raw materials in the manufacturing of new polymers, effectively closing the loop in a circular plastic economy. Unlike downcycled products, which often suffer from inferior quality and limited applicability, monomer recovery enables the creation of virgin-equivalent polymers, ensuring both material performance and market competitiveness. Among the most sought-after monomers are terephthalic acid (TPA) and bis(2-hydroxyethyl) terephthalate (BHET) from polyethylene terephthalate (PET), styrene from polystyrene (PS), propylene glycol and polyols from polyurethanes (PU), lactic acid from polylactic acid (PLA), and adipic acid and hexamethylenediamine from nylon-based polyamides. These compounds are essential in sectors such as packaging, automotive, textiles, electronics, and construction. For instance, TPA and BHET are central to the global polyester industry, forming the backbone of PET used in beverage bottles and synthetic fibers. Polyols recovered from PU are vital in the production of rigid and flexible foams, coatings, and adhesives. Lactic acid serves not only as a feedstock for biodegradable plastics like PLA but also finds application in the food, pharmaceutical, and cosmetics industries. Similarly, adipic acid and diamines are used extensively in the production of engineering plastics, particularly nylon-6 and nylon-6,6, which are in high demand for high-strength applications. A comparative overview of various plastic depolymerization and upcycling technologies is presented in

Table 1. Comparative summary of major plastic upcycling technologies.

Plastic Type	Upcycling Method	Process Conditions	Products Obtained	TRL Level
PET	Chemical Glycolysis	180–220 °C, EG, catalyst (e.g., ZnAc2, MgO/SiO2)	BHET, TPA	7–8
PET	Enzymatic Depolymerization	30–70 °C, pH 7–9, engineered PETase/MHETase	TPA, EG	5–6
PU	Chemical Hydrolysis/Glycolysis	150–220 °C, acid/base/glycol	Polyols, amines	6–7
PS	Thermal Pyrolysis	450–550 °C, inert atmosphere	Styrene monomer, gases, oils	7–8
PE/PP	Catalytic Hydrogenolysis	200–300 °C, H2, Ru/CeO2 catalyst	Alkanes, fuels, waxes	5–6
Mixed Plastics	Supercritical Solvolysis	>350 °C, 250 bar, water/ethanol	Monomers, oils	4–5
PLA	Alcoholysis	130–160 °C, ethanol or methanol	Lactate esters	6–7
Nylon-6	Acid Hydrolysis	200–250 °C, HCl, water	Caprolactam	6–8

, summarizing key parameters such as plastic type, method, process conditions, products obtained, technology readiness level (TRL), and environmental indicators. The economic value of these monomers, coupled with growing environmental concerns, underscores their importance in plastic waste valorization strategies. Efficiently converting plastic back into these useful building blocks not only reduces the demand for virgin fossil resources but also opens up new avenues for sustainable materials innovation and industrial symbiosis.

5.2. Pathways from Depolymerized Products to Monomer Recovery

The process of converting depolymerized plastic waste into high-value monomers involves multiple chemical pathways and separation steps, tailored to the specific polymer type and depolymerization method used [116,117]. These pathways may vary significantly depending on whether thermal, chemical, catalytic, or biological depolymerization is employed. However, the general principle involves breaking down the polymer chains into intermediate compounds, followed by purification and conversion to desired monomers. In chemical depolymerization, particularly of condensation polymers like PET and polyurethanes, the depolymerization step itself often directly yields monomeric units. For PET, glycolysis produces BHET, methanolysis yields DMT and ethylene glycol, and hydrolysis results in TPA and EG. These compounds can typically be isolated by crystallization, filtration, or distillation, depending on their chemical nature and solubility. For thermoplastics like PE and PP, which are composed of C–C backbone chains, the depolymerization process typically yields a complex mixture of oligomers, alkanes, alkenes, and waxes, which must be further cracked or catalytically refined to isolate target monomers such as ethylene or propylene. In these cases, additional catalytic cracking or steam reforming steps may be required, making the process more complex and energy-intensive. Biological depolymerization usually involves enzyme-mediated hydrolysis under mild conditions. In the case of PET, enzymes such as PETase and MHETase sequentially degrade the polymer to produce mono-hydroxyethyl terephthalate (MHET) and eventually TPA and EG. After enzymatic hydrolysis, these products can be filtered and purified for reuse. The challenge in such processes lies in enhancing enzyme efficiency and ensuring the purity of the recovered monomers without extensive downstream processing. In some cases, hybrid systems are employed to improve monomer recovery. For example, a combination of mechanical pretreatment (e.g., shredding and heating) followed by chemical or enzymatic depolymerization can improve yield and selectivity. Once the depolymerized mixture is obtained, it undergoes product separation, a critical step that affects the quality, cost, and downstream usability of the monomers. Overall, the successful recovery of monomers hinges on the integration of effective reaction chemistry with efficient separation technologies, such as chromatography, solvent extraction, membrane filtration, or distillation. Innovations in process intensification, such as continuous-flow systems and reactive separations, are helping to streamline these pathways and make monomer recovery more viable at commercial scale.

5.3. Case Studies: PET to BHET/TPA, PU to Polyols, etc.

To illustrate the real-world application and success of monomer recovery from plastic waste, several case studies stand out. These examples not only demonstrate technical feasibility but also highlight how various industries are incorporating circular strategies to reduce environmental impact and enhance material efficiency [118,119]. A well-documented case is the depolymerization of PET to BHET and TPA. Companies such as Loop Industries and Carbios have developed proprietary technologies for enzymatic and chemical depolymerization of PET, respectively. Loop Industries employs a patented chemical process that converts PET from a variety of waste streams, including mixed and colored plastics, into purified TPA and EG. Carbios, on the other hand, utilizes engineered enzymes to hydrolyze PET into TPA and EG with high efficiency. The recovered monomers are then reused to produce new PET bottles and fibers, offering a truly closed-loop solution. Both companies have demonstrated that their methods can operate at industrial pilot scales, signaling the viability of commercial deployment. Another important example is the chemical recycling of polyurethanes (PU) to recover polyols. Polyurethane foams, widely used in furniture, insulation, and automotive components, are traditionally considered non-recyclable due to their cross-linked structure. However, chemical depolymerization via glycolysis or acidolysis can break down PU into its original polyol and isocyanate components. Companies like Repsol and Covestro have developed processes that recover polyols from end-of-life mattresses and insulation foams. These polyols can be reused in new PU formulations, reducing reliance on petroleum-derived raw materials. Polystyrene (PS) is another target for monomer recovery due to its depolymerization into styrene monomer via thermal or catalytic methods. Agilyx, an American company, has commercialized a pyrolysis-based process that converts post-consumer PS waste into styrene oil, which is further purified and polymerized into new PS products. This technology has been successfully deployed at industrial scales and is gaining traction in both the U.S. and Europe. In the case of nylon-6, chemical recycling through hydrolytic or acidic depolymerization can regenerate caprolactam, the monomer used in its original production. Companies such as Aquafil have built full-scale recycling plants that transform fishing nets, carpets, and industrial waste into high-purity caprolactam, which is then repolymerized into virgin-grade nylon fibers under the brand name Econyl^®. These case studies underscore the practicality, economic potential, and scalability of monomer recovery from plastic waste. They also highlight the importance of tailored depolymerization methods that match the chemistry of the target polymer and the end-use requirements of the recovered monomer.

5.4. Quality and Purity Concerns in Monomer Production

While the technological potential of monomer recovery is immense, one of the critical challenges facing the widespread adoption of depolymerization-based recycling is ensuring the quality and purity of the recovered monomers. These parameters are essential not only for successful repolymerization but also for regulatory compliance and performance assurance in end-use applications. Monomers recovered from plastic waste often contain trace impurities, such as additives, dyes, fillers, stabilizers, plasticizers, and degradation products. These contaminants can adversely affect the polymerization process, leading to materials with poor mechanical, thermal, or optical properties [120]. For example, in PET production, impurities in BHET or TPA can cause discoloration, reduced intrinsic viscosity, and poor crystallinity in the final product, rendering it unsuitable for applications like food-grade packaging. The presence of metal residues from catalysts, residual solvents, or reaction byproducts also raises concerns for health and safety. For monomers intended for medical or food contact applications, stringent regulatory standards apply, necessitating the use of high-purity reagents and rigorous purification steps. Analytical techniques such as high-performance liquid chromatography (HPLC), gas chromatography-mass spectrometry (GC-MS), and nuclear magnetic resonance (NMR) are routinely used to monitor monomer purity and structural integrity. Achieving high purity often requires additional separation and refining steps, which can increase process complexity and cost. Crystallization, vacuum distillation, solvent extraction, and membrane filtration are among the techniques used to purify monomers, each with its own advantages and limitations. For example, TPA can be recrystallized from aqueous solutions, while BHET may require multiple crystallization and filtration stages to remove oligomers and colorants. Another concern is the variability of feedstock, especially in post-consumer plastic waste streams, which can result in inconsistent product quality. This variability necessitates robust preprocessing systems for sorting and cleaning the input materials, as well as adaptive process control mechanisms during depolymerization and purification. Moreover, economic trade-offs exist between purity, yield, and cost. Excessive purification may reduce overall yield and energy efficiency, while inadequate purification can compromise polymer performance and customer confidence. Therefore, optimizing the balance between process efficiency and product quality is a key area of research in monomer recovery technologies. To build market trust in recycled monomers, standardized certification and quality benchmarks are needed. These could include industry standards for monomer purity, traceability protocols, and labeling systems that verify compliance with environmental and health regulations. Governments, in collaboration with industry and academia, have a critical role to play in establishing these standards and supporting the development of high-purity recycling infrastructure.

6. Life Cycle Assessment and Sustainability Considerations

6.1. Energy Consumption and Carbon Footprint

The assessment of energy consumption and carbon emissions is fundamental in evaluating the overall sustainability of plastic depolymerization and monomer recovery technologies. Life Cycle Assessment (LCA) is the standard methodology used to quantify these impacts by evaluating the inputs and outputs of a system across its life stages from raw material extraction to processing, use, and end-of-life treatment [121,122]. In the context of plastic upcycling, LCAs help determine whether the energy required to break down and reconstruct polymers is justified when compared to producing virgin polymers from fossil resources. Traditional plastics, especially those derived from petrochemicals, are highly energy-intensive. For example, the production of 1 kg of virgin PET consumes around 80–90 MJ of energy and emits approximately 2.8–3.4 kg CO₂-equivalent (CO₂-e). These values are even higher for engineering plastics like polycarbonates or polyamides, which require more complex synthesis and purification [123,124]. By contrast, depolymerization-based upcycling methods have shown promising reductions in net energy use and carbon emissions, especially when powered by renewable energy and optimized for efficiency. Among depolymerization methods, chemical and biological processes tend to have lower energy requirements than thermal depolymerization. For instance, the glycolysis of PET to BHET consumes significantly less energy than pyrolysis or gasification, especially when integrated with heat recovery systems. Biological depolymerization processes, although slower, operate under ambient or near-ambient conditions, using enzymes that catalyze reactions without external heating. This drastically reduces operational energy input and, consequently, greenhouse gas emissions. However, not all upcycling routes offer net environmental gains. If fossil-based energy sources power the depolymerization process, the carbon savings may be marginal or even negative compared to virgin plastic production [125,126]. Therefore, the energy source mix, whether from fossil fuels, grid electricity, or renewables, is a decisive factor in determining the carbon footprint of a given process [127,128]. Hybrid systems that incorporate solar, wind, or waste heat as auxiliary energy inputs are increasingly being investigated to mitigate this concern. Another important consideration is the energy and emissions associated with purification and product separation. The removal of impurities, solvents, or byproducts in monomer recovery can be energy-intensive, especially when high purity standards are required [129]. Advanced techniques like membrane filtration, crystallization, and low-temperature distillation are more energy-efficient than traditional methods, but the choice of technology and process optimization remains crucial in minimizing carbon impacts. Overall, LCAs conducted on chemical recycling pathways, particularly those focusing on PET and PU, have demonstrated carbon footprint reductions ranging from 30% to 80% compared to virgin polymer production, depending on process design and energy sourcing. These results highlight the importance of system boundaries and scenario modeling in producing reliable sustainability metrics [130].

6.2. Environmental Benefits of Monomer Recovery

Monomer recovery from plastic waste presents several compelling environmental advantages beyond just energy and carbon savings [25,66]. Unlike mechanical recycling, which often leads to polymer downcycling and lower-quality products, chemical and enzymatic depolymerization preserve the material’s integrity and functionality. This leads to a higher rate of reuse and less demand for raw resource extraction, contributing to resource conservation, pollution prevention, and ecological restoration [131,132]. One of the most important environmental benefits of monomer recovery is its contribution to reduced plastic leakage into the environment. Conventional recycling systems are often unable to process contaminated, colored, or multilayered plastics, which are consequently discarded into landfills or incinerated. In contrast, upcycling technologies that recover monomers can handle a broader range of plastic types and qualities, enabling the treatment of plastic that would otherwise contribute to ocean or terrestrial pollution. Additionally, monomer recovery supports reduction in land-use requirements and toxic emissions [133,134]. For example, landfilling plastics not only occupies valuable space but also poses long-term threats through leachate contamination and microplastic formation. Incineration, while reducing volume, results in air pollution through the release of dioxins, heavy metals, and CO₂. Upcycling offers a cleaner end-of-life option, especially when the recovered monomers are used to manufacture products with long life spans or in high-value applications [135,136]. Monomer recovery also aids in closing material loops and reducing the need for virgin petrochemical extraction. This directly limits environmental degradation associated with oil drilling, gas flaring, and chemical refining. Tapping into post-consumer plastic waste as a secondary raw material aligns with circular economy principles and has a lower ecological footprint compared to sourcing new hydrocarbons [136,137]. Moreover, some monomer recovery technologies are integrated with green chemistry practices, such as using bio-based solvents, non-toxic catalysts, and aqueous systems. These approaches minimize hazardous byproducts, reduce process toxicity, and improve worker and environmental safety. The development of benign depolymerization agents and enzyme-based systems furthers these sustainability goals and provides alternatives to more aggressive or polluting methods. Another critical benefit is the reduction in water and soil contamination. Many plastic products contain additives like phthalates, flame retardants, and pigments that can leach into ecosystems. Efficient depolymerization and monomer recovery systems are capable of separating and treating these substances, thereby reducing the risk of environmental exposure. Finally, the integration of monomer recovery into urban and industrial ecosystems can promote industrial symbiosis, where the waste output of one sector becomes the input of another. This not only increases resource efficiency but also fosters collaboration across industries, leading to more sustainable production networks [138].

6.3. Economic Feasibility and Scalability of Processes

Beyond environmental metrics, the economic viability of depolymerization and monomer recovery technologies plays a critical role in determining their commercial adoption [139]. While the concept of circularity is appealing, it must be cost-competitive, scalable, and compatible with existing supply chains to be practical on a global scale. Economic assessments consider factors such as capital investment, operational costs, market value of recovered monomers, and regulatory incentives or penalties [110,116]. One of the primary economic advantages of monomer recovery is the potential to generate high-value outputs from low-cost or even negative-cost inputs (e.g., plastic waste). Monomers such as TPA, BHET, styrene, polyols, and caprolactam command significant market prices, making their recovery attractive for industries aiming to reduce raw material costs [139,140]. Additionally, producing these monomers from waste materials provides price stability against fluctuations in petroleum markets. However, the initial capital expenditure (CAPEX) for building depolymerization plants can be substantial. Specialized reactors, separation units, and purification equipment are required, along with skilled labor and safety infrastructure. Operational expenditures (OPEX) are also influenced by energy use, reagent costs, catalyst life cycles, and maintenance requirements [141,142]. For chemical and thermal processes, energy consumption remains a major cost driver, while enzymatic systems face high enzyme production and stability costs.

Economies of scale can mitigate some of these challenges. Large-scale facilities capable of processing 10,000–50,000 tonnes of plastic per year can reduce per-unit costs through optimized logistics, continuous processing, and better waste aggregation [143]. Moreover, co-locating recycling units near plastic manufacturing or waste collection hubs can reduce transportation costs and improve integration with upstream and downstream operations. Public policy plays a pivotal role in improving economic feasibility. Extended Producer Responsibility (EPR) schemes, carbon credits, subsidies, and plastic taxes can create economic incentives for adopting sustainable recycling methods [144,145]. For instance, regulations mandating minimum recycled content in plastic packaging boost demand for high-quality recovered monomers, thus strengthening the business case for depolymerization technologies. Business models based on product-as-a-service, chemical leasing, and waste buyback can also enhance profitability and stakeholder engagement. Startups and consortia investing in advanced recycling infrastructure are increasingly receiving funding from venture capital, corporate sustainability funds, and green bonds, suggesting a growing market appetite for circular economy ventures. Nonetheless, challenges persist in achieving universal scalability. These include regulatory uncertainty, feedstock inconsistency, lack of infrastructure in developing regions, and market resistance to recycled materials due to perceived quality concerns. Addressing these issues will require coordinated action among policymakers, industry leaders, and researchers to de-risk innovation and scale up proven technologies [55,67].

6.4. Comparison with Conventional Plastic Management Strategies

When compared with traditional plastic waste management approaches such as landfilling, incineration, and mechanical recycling, monomer recovery through depolymerization offers superior environmental, economic, and material performance in many contexts [146,147]. However, its effectiveness is conditional on the type of plastic, technology maturity, and infrastructure support. Landfilling, the most common end-of-life route for plastics globally, is the least desirable from a sustainability perspective (

Table 2. Comparative analysis of plastic depolymerization techniques.

Criteria	Thermal Depolymerization	Chemical Depolymerization	Catalytic Depolymerization	Biological Depolymerization
Target Polymers	PE, PP, PS, Mixed plastics	PET, PU, Nylon, PC	PE, PP, PET, PS	PET, PLA, PCL
Temperature Range	300–800 °C	150–250 °C	180–350 °C	30–70 °C
Reaction Time	Minutes–hours	Hours	Minutes–hours	Hours–days
Catalyst Requirement	Optional	Often not required (acid/base catalysts used)	Required (e.g., metal, zeolite, MOF, ILs)	Enzymes (e.g., PETase, cutinase)
Selectivity	Low (broad product range)	High (monomer-targeted)	High to moderate	Very high (bond cleavage)
Feedstock Flexibility	High (mixed, contaminated waste)	Moderate–low (requires clean feedstock)	Moderate	Low (pure polymers preferred)
Energy Consumption	Very high	Moderate	Moderate to high	Low
Environmental Impact	High (CO2, VOCs emissions)	Moderate (chemical use, waste generation)	Moderate (depends on catalyst and conditions)	Low (greenest method)
Scalability	High (already commercialized)	Medium (semi-commercial)	Medium (emerging tech)	Low (lab/pilot scale)
Main Products	Hydrocarbons, oils, gases	Monomers (e.g., BHET, TPA, EG)	Monomers, fuels, lubricants	Monomers (e.g., TPA, EG)

). It offers no material recovery, occupies valuable land, and poses long-term risks of leachate contamination and greenhouse gas emissions. Moreover, landfilled plastics persist for hundreds of years, exacerbating microplastic pollution and resource wastage. Incineration with energy recovery can reduce waste volume and generate electricity or heat, but it comes with high carbon emissions and toxic air pollutants such as dioxins, furans, and heavy metals [148]. While incineration provides some energy recovery, it results in the permanent loss of material value, contradicting the principles of circularity and material conservation. Mechanical recycling, while more environmentally friendly than landfilling or incineration, is limited in scope and effectiveness. It typically requires clean, sorted, and single-type plastic streams. The process often degrades polymer quality, resulting in lower-performance products with limited reuse cycles. Moreover, mechanical recycling is ineffective for mixed, contaminated, or multilayered plastics, which constitute a significant portion of municipal plastic waste [149,150]. In contrast, monomer recovery through depolymerization offers a pathway to high-quality, closed-loop recycling. It enables the reuse of materials without sacrificing quality, extends product life cycles, and reduces dependence on virgin petrochemicals. It also accommodates a broader range of plastics and contamination levels, making it more inclusive in its treatment capacity. LCAs comparing these options have shown that monomer recovery typically has lower environmental impacts per functional unit than incineration and mechanical recycling when high-purity recovery is achieved. For example, studies have demonstrated that chemically recycled PET results in up to 65% fewer carbon emissions compared to virgin PET, and 30–40% fewer than mechanically recycled PET, depending on energy sourcing and process integration [151]. That said, monomer recovery is not a universal replacement for all traditional methods. It must be integrated as part of an hierarchical plastic waste management system, where prevention, reuse, mechanical recycling, and chemical upcycling coexist and complement each other. For example, mechanical recycling may remain preferable for simple, uncontaminated waste streams due to its lower cost and energy use, while monomer recovery is better suited for complex or hard-to-recycle plastics. In conclusion, monomer recovery presents a transformative opportunity for advancing sustainability in plastic waste management. With the right technological innovations, policy frameworks, and economic models, it can shift the global plastics economy toward a circular and regenerative system that minimizes waste, conserves resources, and reduces environmental harm [21,69].

6.5. Limitations and Comparative Assessment of Plastic Waste Valorization Approaches

Despite the growing attention and optimism surrounding plastic upcycling, several limitations, particularly technological and financial, hinder its large-scale implementation [152,153]. Moreover, understanding the relative merits and constraints of upcycling, recycling, and downcycling is crucial to determining the optimal strategy for plastic waste valorization in specific contexts. Most chemical or catalytic upcycling technologies, especially those aimed at producing monomeric compounds with high purity, are still in developmental or pilot stages [154]. Processes such as enzymatic depolymerization or organocatalytic cleavage demand highly specialized conditions—controlled temperatures, specific catalysts or enzymes, and careful feedstock pretreatment. Moreover, existing technologies often target a limited range of plastics, such as PET or PU, while polymers like PE and PP remain challenging due to their chemically inert backbones [155]. Multilayered, colored, or contaminated plastics further complicate the depolymerization process. The current waste management infrastructure is largely optimized for mechanical recycling and landfilling. Upcycling often requires entirely new processing facilities with advanced reactors, separation units, and material handling capabilities. Integration into municipal waste streams demands compatibility with existing collection and sorting systems, which is currently limited [156,157]. High-performance upcycling processes typically require relatively pure or well-characterized input materials to function effectively. However, real-world plastic waste is heterogeneous and contaminated with dyes, additives, and residues. This necessitates labor- and cost-intensive preprocessing, including sorting, washing, and in some cases, chemical pretreatment. While some upcycling methods operate under mild conditions (e.g., enzymatic systems), many chemical and catalytic processes still involve substantial energy inputs, particularly for heating and separation stages [158,159]. Additionally, certain routes use hazardous reagents or generate secondary waste streams that must be treated before disposal, impacting their overall environmental and economic sustainability. Capital costs (CAPEX) for constructing depolymerization facilities are significantly higher than for mechanical recycling plants. Operational expenditures (OPEX), including energy, catalyst, enzyme, and solvent costs, add further burden. Although the recovery of high-value monomers can offer long-term returns, the initial investment deters widespread industrial adoption. Moreover, market competition with inexpensive virgin polymers, derived from fossil fuels, undermines the economic viability of upcycled materials in the absence of regulatory incentives or subsidies. There are currently limited global standards for certifying upcycled monomers or materials, particularly for high-end applications such as food contact or medical products. The lack of harmonized regulations, coupled with consumer hesitancy, impairs market confidence in upcycled products [160].

6.6. Comparative Analysis: Upcycling vs. Downcycling vs. Recycling

Plastic upcycling through depolymerization has emerged as a promising solution to address the mounting plastic waste crisis, but its large-scale adoption remains constrained by significant technological and financial barriers [161]. At the technological level, upcycling methods—particularly chemical, catalytic, and enzymatic depolymerization—are still in varying stages of maturity, with many confined to laboratory-scale demonstrations or pilot-scale trials. These methods often require precise reaction conditions, specialized catalysts or enzymes, and tight control of feedstock purity [162,163]. While PET and certain polyurethanes have shown good compatibility with existing depolymerization techniques, widely used polyolefins like polyethylene (PE) and polypropylene (PP) are far more resistant to chemical cleavage due to their saturated hydrocarbon backbones. Consequently, the current suite of upcycling technologies is not universally applicable across all plastic types [164]. Moreover, the heterogeneous nature of post-consumer plastic waste, including mixtures of polymers, contamination by dyes, food residues, and additives, poses a challenge to selective depolymerization [165]. Advanced preprocessing steps such as sorting, washing, or even chemical pretreatment are typically required to ensure process compatibility and efficient conversion. These preparatory requirements not only increase the complexity of upcycling operations but also contribute significantly to overall processing costs. Infrastructure is another limitation; existing waste management systems are mostly optimized for mechanical recycling or landfill disposal, and integrating upcycling facilities into municipal waste systems demands new investments in reactor systems, separation units, and logistics tailored to chemical recycling workflows. On the financial front, the economic viability of upcycling technologies is hindered by both high capital expenditure (CAPEX) and operational expenditure (OPEX). The construction of specialized depolymerization facilities involves high upfront costs for reactors, purification units, safety systems, and skilled personnel. Additionally, many depolymerization reactions require high energy input, expensive or rare catalysts, and post-reaction purification steps to isolate monomers at acceptable levels of purity [166]. This renders the process more capital- and resource-intensive compared to conventional mechanical recycling. Even enzymatic depolymerization, though energy-efficient and environmentally benign, suffers from high enzyme production costs and slow reaction kinetics, further limiting its economic competitiveness. Furthermore, the market for upcycled monomers must compete with virgin petrochemical-derived products, which are often cheaper due to economies of scale and fossil fuel subsidies. Without regulatory incentives such as carbon pricing, extended producer responsibility (EPR), or mandates for recycled content, upcycled monomers struggle to establish a foothold in the commodity polymer market [167,168]. A lack of global standards or certifications for upcycled materials, especially those intended for food-grade or medical applications, also undermines consumer confidence and commercial uptake. Taken together, these technological and financial barriers form a significant hurdle to the widespread deployment of upcycling technologies. In light of these limitations, it is important to compare upcycling with other plastic waste management strategies—specifically recycling and downcycling—to contextualize its potential within a broader waste valorization framework. Mechanical recycling, the most widely practiced form of recycling, involves physically processing plastics through sorting, cleaning, melting, and re-extruding into new materials [169]. While this method is cost-effective and well-established, it is limited by material degradation after each cycle, contamination sensitivity, and incompatibility with mixed or multilayered plastics. Recycled materials often suffer from diminished mechanical and aesthetic properties, making them suitable only for low-end applications—a phenomenon termed downcycling. Downcycling, therefore, is a subset of recycling where plastic waste is transformed into products of lesser value or function than the original, such as turning PET bottles into fibers for carpets or insulation. While downcycling extends the life of materials and delays disposal, it fails to preserve the inherent value of polymers and does not support a true circular economy. By contrast, upcycling aims to regenerate waste plastics into monomers or high-value intermediates that can be used to create virgin-equivalent or entirely new materials. This approach offers numerous advantages: it enables closed-loop recycling by restoring monomeric building blocks; accommodates a wider range of plastic feedstocks including mixed or contaminated streams; and delivers higher-value outputs such as specialty chemicals, fuels, or pharmaceuticals [170]. Moreover, the incorporation of green chemistry principles, like using bio-based catalysts, low-toxicity solvents, and low-temperature processes, can further enhance the sustainability profile of upcycling. However, these benefits come at the cost of greater technological complexity and economic input. When comparing the three approaches in terms of value retention, product quality, environmental impact, and scalability, mechanical recycling and downcycling rank lower in terms of circularity and long-term resource efficiency. Mechanical recycling is less capital-intensive and more scalable but offers limited quality retention and poor compatibility with mixed waste. Downcycling is the least desirable in a circular economy framework due to permanent value loss and eventual disposal. Upcycling, while still emerging, represents the highest-value pathway, especially when integrated with renewable energy and supported by regulatory frameworks that promote recycled content and penalize virgin polymer usage [171,172]. In terms of environmental impact, upcycling can substantially reduce carbon emissions, conserve raw materials, and mitigate plastic pollution provided that clean energy and efficient separation systems are employed. Nonetheless, challenges remain in terms of process scalability, standardization, and cost-efficiency. Innovations in catalyst development, enzyme engineering, and process intensification are urgently needed to address these limitations. Additionally, a robust policy framework including subsidies, tax incentives, public–private partnerships, and investment in R&D can accelerate the commercialization of upcycling technologies. In conclusion, while mechanical recycling and downcycling will continue to play roles in plastic waste management, they are inherently limited in achieving long-term circularity. Upcycling offers a technically superior and environmentally sustainable alternative, though it currently faces significant technical and economic hurdles. A balanced and integrated approach that utilizes mechanical recycling for simpler streams and reserves upcycling for complex or contaminated plastics backed by strong policy support represents the most promising pathway toward a sustainable, circular plastics economy [173,174].

7. Recent Advances and Future Perspectives

7.1. Recent Breakthroughs in Upcycling and Depolymerization

In recent years, rapid advancements in materials science, catalysis, and biochemistry have significantly enhanced the feasibility and efficiency of plastic upcycling and depolymerization. These breakthroughs have moved beyond theoretical or lab-scale innovations and have begun to demonstrate tangible impacts through pilot projects, start-ups, and industrial collaborations [175]. Among the most notable developments are new catalytic systems, engineered enzymes, and process intensification technologies that allow for the selective conversion of plastic waste into high-value monomers and specialty chemicals. One of the most celebrated discoveries has been the identification and engineering of PET-degrading enzymes, particularly PETase from Ideonella sakaiensis. Originally identified in 2016, PETase can hydrolyze PET into mono(2-hydroxyethyl) terephthalate (MHET), and when paired with MHETase, complete depolymerization to terephthalic acid and ethylene glycol becomes feasible [26,55]. Through protein engineering and directed evolution, researchers have significantly enhanced the thermostability and catalytic efficiency of PETase. Companies like Carbios have scaled up this innovation, demonstrating enzymatic recycling of post-consumer PET at industrial pilot levels, including colored bottles and multilayer packaging. Simultaneously, significant progress has been made in catalytic depolymerization, particularly for traditionally hard-to-recycle plastics like polyethylene (PE) and polypropylene (PP) [5,16]. The catalysts selectively convert long polymer chains into liquid alkanes, lubricants, or waxes, potentially replacing petroleum-based products. Another innovation comes from solvolysis approaches for polycarbonates, polystyrene, and polyurethane [23,66]. New solvent systems, including ionic liquids and deep eutectic solvents, enable efficient depolymerization of these plastics at low temperatures, with minimal use of toxic chemicals. These breakthroughs have reduced process times, improved product selectivity, and minimized environmental impact. For instance, methanolysis of polycarbonate into bisphenol A and dimethyl carbonate has achieved nearly 100% conversion efficiency under mild catalytic conditions. Thermal depolymerization has also seen advancements, particularly in microwave-assisted pyrolysis and plasma cracking, which offer faster reaction times, lower energy inputs, and better control over product composition. These technologies can be applied to mixed plastic streams, improving versatility and throughput. Furthermore, machine learning and artificial intelligence (AI) are increasingly being integrated into catalyst design and process optimization. AI models help predict reaction pathways, identify optimal catalyst structures, and reduce experimental trial-and-error, accelerating the development of next-generation upcycling technologies. Collectively, these breakthroughs represent a shift from generalized recycling to precision upcycling, where specific polymers are targeted and converted into value-added materials using tailored pathways. This trend is paving the way for high-efficiency, low-footprint recycling systems that can be customized to local waste streams and industrial needs [36,99].

7.2. Integration with Renewable Energy and Green Chemistry

Sustainability in plastic upcycling is not solely determined by the recycling process itself but also by how it integrates with renewable energy sources and adheres to green chemistry principles [176]. As climate change concerns and resource constraints intensify, the future of plastic management depends on systems that are both technologically effective and environmentally benign [11,29]. A significant emerging trend is the use of renewable energy such as solar, wind, and geothermal to power depolymerization processes, particularly those that require heat or electricity. For instance, electrically heated pyrolysis reactors or microwave systems can be directly powered by solar panels or grid-supplied renewable electricity, significantly reducing the carbon footprint. Companies like Plastic Energy and Brightmark are already exploring integration with green energy systems to improve the net sustainability of their operations. Another innovative concept involves coupling solar-driven photoreforming with plastic depolymerization. In this process, solar energy is used not only to degrade polymers but also to generate useful byproducts such as hydrogen gas. This approach has the dual benefit of reducing plastic waste and producing clean energy, making it particularly attractive in regions with abundant sunlight. While still in early research phases, photoreforming has shown potential for application to PET and PLA under UV and visible light in the presence of semiconducting catalysts like TiO₂. Green chemistry principles are also shaping the design of depolymerization reactions. Efforts are being made to use non-toxic, biodegradable solvents, minimize reagent use, and operate under ambient or near-ambient conditions [56,91]. Deep eutectic solvents, for example, are being developed as greener alternatives to conventional organic solvents for polymer dissolution and depolymerization. Likewise, organocatalysts and biocatalysts are being favored over heavy metal-based systems, reducing toxicity and facilitating easier catalyst recovery. Another area of integration is the use of bio-based feedstocks and hybrid polymer systems, where conventional plastics are blended with biodegradable or enzymatically cleavable components. This design approach makes the materials more amenable to future upcycling, encouraging design-for-recycling and compatibility with biological and chemical degradation pathways. Water and energy recovery systems are also being incorporated into modern depolymerization plants, helping to reduce the overall environmental burden. Waste heat from reactors can be used to preheat incoming materials, and water used in enzymatic hydrolysis can be filtered and reused, creating closed-loop systems that improve sustainability metrics. In summary, aligning plastic upcycling technologies with renewable energy and green chemistry is not only necessary for reducing their environmental impact but also vital for long-term economic and regulatory viability. This systems-level integration is what distinguishes next-generation recycling infrastructure from traditional models and will be key to achieving circularity in the plastics industry [66,81].

7.3. Policy Support, Regulations, and Public–Private Partnerships

Technological innovation in plastic upcycling cannot succeed in isolation; it must be supported by robust policy frameworks, strategic regulations, and strong collaboration between public and private sectors [177]. Policy-driven innovation is a crucial driver for transforming research into scalable industrial solutions, especially in waste management and resource recovery sectors. Globally, several governments are introducing Extended Producer Responsibility (EPR) policies, mandating manufacturers to take responsibility for the post-consumer phase of their products. These regulations create a powerful economic incentive to invest in sustainable design and end-of-life management, including chemical recycling and upcycling solutions. For instance, the European Union’s Circular Economy Action Plan and Plastics Strategy include clear targets for recycled content and single-use plastic reduction, encouraging industries to explore monomer recovery as a route to compliance [178]. In the United States, state-level mandates and incentives, such as California’s minimum recycled content laws for beverage containers, are beginning to push companies toward higher-quality recycling solutions. Similarly, countries like Japan, South Korea, and Canada are adopting zero-waste roadmaps that emphasize resource recovery and high-value recycling, often with financial and policy support for pilot projects and research. On a multilateral level, organizations like the United Nations Environment Programme (UNEP) and the OECD are coordinating global action on plastic pollution, including frameworks that promote international cooperation on recycling technologies, standards, and monitoring. These initiatives are essential for harmonizing practices, sharing knowledge, and building capacity in developing countries where plastic waste challenges are acute [96,98].

Public–private partnerships (PPPs) are emerging as a powerful tool to bridge the gap between research, industry, and policy. Collaborations between academic institutions, municipal authorities, and corporations have led to the establishment of pilot plants, joint ventures, and consortia focused on upcycling innovation. For instance, Project STOP and the Alliance to End Plastic Waste (AEPW) have initiated large-scale investments in infrastructure and innovation in Southeast Asia, with upcycling as a strategic priority. Private companies are also forming industry alliances, such as the Circular Plastics Alliance (CPA) in Europe and the Closed Loop Partners in the U.S., which aim to invest in and scale up chemical recycling technologies [170]. These initiatives are helping create a market ecosystem where recycled monomers are competitively priced and readily available, closing the economic loop and supporting a more circular supply chain. In addition, standardization and certification systems are being developed to verify the quality and sustainability of upcycled materials. Eco-labels, recycled content certification, and lifecycle auditing tools are becoming essential for product differentiation and consumer confidence. Despite this progress, significant regulatory challenges remain. Inconsistent definitions of chemical recycling, unclear classification of upcycled materials, and fragmented waste collection systems hinder scale-up. A coordinated policy approach that includes technology-neutral standards, R&D incentives, infrastructure funding, and international cooperation is essential to unlock the full potential of plastic upcycling [29,34].

7.4. Research Gaps and Future Research Directions

While considerable progress has been made, numerous research gaps and technical challenges continue to limit the widespread deployment of plastic upcycling technologies [179]. Addressing these gaps through targeted research and innovation will be essential to transform pilot-scale successes into global circular solutions. One of the most pressing challenges is the lack of efficient technologies for mixed and multilayer plastics. Most current methods are optimized for single-polymer streams, such as PET or PS, and perform poorly with complex waste mixtures containing additives, dyes, or barrier layers. Developing universal or adaptive depolymerization processes, possibly using modular catalysts or enzyme cocktails, remains a critical research priority [21]. Another major gap is the low efficiency of enzymatic depolymerization for non-polyester plastics, such as PE, PP, and PVC. While enzyme systems for PET have advanced rapidly, similar tools for polyolefins are still in early research stages. High-throughput screening, metagenomics, and synthetic biology could accelerate the discovery and development of new enzymes capable of degrading these recalcitrant polymers under ambient conditions [71,85]. The design of advanced catalysts is also a key focus area. Catalysts that are selective, reusable, and stable under varying conditions are needed for large-scale deployment. In particular, heterogeneous catalysts with tunable activity, resistance to fouling, and compatibility with industrial systems are critical for reducing process costs and increasing reliability. Another underexplored area is real-time process monitoring and control. Inconsistent waste feedstocks can lead to variable product quality and process instability. Integrating machine learning, real-time sensors, and automation into depolymerization systems can help optimize yield, energy efficiency, and product purity. Additionally, techno-economic and environmental impact assessments are still limited in scope and region-specific. Comprehensive, multi-region LCAs and cost-benefit analyses are needed to guide investments and policy decisions [180]. These should account for local energy mixes, waste availability, labor costs, and transportation infrastructure. Societal and behavioral research is equally important. Understanding consumer preferences, perception of upcycled materials, and behavioral barriers to recycling can inform design, communication, and policy strategies that promote adoption. Education and awareness campaigns, combined with behavioral economics tools, can help close the loop from the demand side. Finally, the future of plastic upcycling may lie in systems-level innovations—integrated processes that combine material design, recycling, and reuse within closed industrial ecosystems. Research into circular product design, where materials are engineered for compatibility with upcycling from the outset, will be essential. Concepts like upcyclable-by-design polymers, smart packaging, and embedded material tracers are likely to become key components of next-generation recycling systems [181]. In conclusion, the next decade will be critical for transforming the vision of plastic upcycling into reality. By closing research gaps, fostering interdisciplinary collaboration, and aligning science with policy and market needs, we can lay the groundwork for a sustainable, circular future for plastics [54,91].

8. Conclusions

The transition toward a sustainable and circular plastic economy requires innovative technologies capable of transforming plastic waste into high-value resources. This review has highlighted the potential of depolymerization-based upcycling to enable closed-loop recycling of various plastic streams. Techniques such as thermal and catalytic depolymerization, enzymatic hydrolysis, and mechanochemical processing demonstrate considerable promise in converting post-consumer plastics into reusable monomers and chemical intermediates. Case studies including the use of MgO/SiO₂ catalysts for PET glycolysis, Co/CaO systems for lignin-derived plastic analogs, and whole-cell engineered biocatalysts for PET depolymerization illustrate the breadth and scalability of these approaches. Importantly, the integration of green chemistry principles, renewable feedstocks, and multi-functional catalysts has begun to overcome long-standing challenges related to product purity, reaction efficiency, and environmental impact. Furthermore, upcycling strategies that produce functional monomers such as vanillin, BHET, bisphenols, or fluorinated arenes, open doors not only for recycling but also for the synthesis of new, high-performance materials. Despite encouraging progress, further research is needed to optimize reaction conditions, enzyme engineering, and hybrid catalytic systems for mixed plastic streams. Regulatory alignment, public–private investment, and international collaboration will be key in scaling up these technologies for real-world applications. The depolymerization-driven plastic upcycling offers a viable, scalable, and sustainable pathway to reduce environmental plastic burden while reclaiming material value, thereby laying a robust foundation for a circular economy in polymers.