1. Challenges to the sustainable development of polymeric materials
Thanks to their advantageous properties, which include low density, processability, and chemical resistance, organic polymers have become indispensable materials in modern society, with widespread applications across the packaging, consumer-product, textile, electrical, transportation, building-and-construction, industrial machinery, agriculture, healthcare, and other industries. Global polymer production currently exceeds 500 million tonnes annually. However, this massive scale of manufacturing and consumption has created critical sustainability challenges in terms of resource consumption and environmental impact.
1.1. A resource-dependency crisis
Contemporary polymer production remains predominantly reliant on finite fossil resources (i.e., petroleum, natural gas, and coal) formed through geological processes over millions of years. Over 90% of plastics are still manufactured from virgin fossil feedstocks. Alarmingly, plastics production is projected to account for 20% of global oil consumption by 2050 under current growth trajectories. With accelerating extraction rates depleting these non-renewable reserves, the polymer industry faces imminent resource constraints that threaten its long-term viability unless alternative feedstocks are developed.
1.2. Environmental impacts of unrecycled polymer waste
Current recycling infrastructures remain inadequate for most commercial polymers. Approximately 79% of discarded polymer products ultimately enter landfills, incinerators, or the natural environment, causing cascading ecological consequences: ① The annual leakage of 23 600 t of dissolved organic carbon from marine plastic debris fundamentally alters oceanic ecosystems [
1]; ② wildlife mortality from plastic ingestion and entanglement disrupts food chains; ③ the leaching of polymer additives such as plasticizers and flame retardants demonstrates confirmed endocrine-disrupting effects in humans [
2]; and ④ microplastic contamination has been detected in placental tissues, indicating transgenerational exposure risks [
2]. These compounding crises have elevated polymer waste management to a top-priority global challenge. Developing sustainable solutions that balance society’s material demands with ecological preservation is now one of the most pressing imperatives for materials science and circular economy strategies.
2. Ways to address the sustainable development of polymeric materials
The complete life cycle of polymeric materials constitutes a complex system that begins with resources and ends with the materials’ environmental impact (
Fig. 1). Currently, the production of such materials primarily relies on fossil resources such as petroleum, natural gas, and coal. Alternative pathways utilizing renewable resources such as biomass are also gaining attention. These raw materials are transformed into polymerizable monomers, which are then synthesized into high-molecular-weight polymers and further processed into various polymeric products. Throughout their service life, these products undergo aging and performance degradation before ultimately being discarded. For non-disposable products, material design and modification such as increasing aging resistance and environmental durability can extend their service life, effectively reducing waste generation and resource consumption over an equivalent period of use. Once products reach the end of their life and become waste, their subsequent disposal pathways determine their final environmental impact. At present, collected polymer waste is treated through various methods, including physical or chemical recycling, incineration, landfilling, and composting or biodegradation. The proportion of each treatment method continuously evolves with technological advancements. Both recycling—including energy recovery through incineration—and compostable degradation help mitigate environmental pollution from solid waste. Even landfilling, which does not reduce the volume of solid waste, can confine waste to designated sites. However, uncontrolled leakage of waste into the environment remains inevitable. If the leaked polymer waste can fully degrade into environmentally harmless substances such as carbon dioxide and water, it will cause minimal pollution; otherwise, polymer waste becomes a major source of solid waste pollution. Thus, reducing the amount of nondegradable polymeric waste is a key priority in mitigating post-disposal environmental contamination.
2.1. Developing new raw material sources
From a resource perspective, the whole world is rich in bio-based resources. Lignin and cellulose make up the largest proportion of these, but they are usually discarded without full and effective utilization. Therefore, using renewable bio-based resources as raw materials and synthesizing bio-based polymeric materials with excellent performance through chemical or biological processes can effectively reduce the polymer industry’s consumption, dependence on fossil resources, and carbon emissions, thereby promoting the sustainable development of polymeric materials. Certain natural macromolecules, such as cellulose and starch, can be directly used in the production of polymeric materials through simple processing. However, it is necessary to convert most other biomass resources into chemicals, especially the monomers required for the preparation of polymeric materials. These biomass-derived monomers can be polymerized to obtain polymers—just like monomers derived from fossil resources—and then processed and modified through the addition of various additives to finally make products with specific properties. Thus far, many research results regarding polyesters, polyamides, polyurethanes, epoxy resins, and other materials based on biomass raw materials have been published. Nevertheless, the current application of bio-based polymers in life and industry is still far less than that of petroleum-based polymers because the latter have a complete and mature industrial chain and are low in cost. Therefore, in order to promote more extensive and popularized use of bio-based polymeric materials, such materials must either be used as a substitute for existing materials in some function or have performance advantages over existing materials.
Carbon dioxide (CO2) is a safe and non-toxic renewable resource. Thus, the use of CO2 and its transformation products as raw materials for the preparation of polymeric materials—such as polycarbonates, polyurethanes, polyureas, polyesters, and polyamides—not only is an effective way to utilize CO2 to realize the goals of a carbon peak and carbon neutrality but can also slow the consumption of fossil resources and thereby promote the sustainable development of polymeric materials. However, due to the thermodynamic stability and kinetic inertia of CO2, its conversion often requires harsh reaction conditions and cumbersome treatment processes, which in turn lead to resource consumption and CO2 emissions. Thus, there is an urgent need for the development of new CO2-conversion methods and technologies.
2.2. Recovery of existing waste polymeric materials
In the case of plastics, the largest class of polymeric materials, 95% of plastic packaging is discarded after first use. According to the current growth rate of plastic production and the proportion of plastic wastes, the cumulative amount of global plastic wastes will exceed 1.2 × 10
10 t by 2050 [
3]. In fact, this is a waste of valuable and unconventional carbon resources. In addition, the complete life cycle of plastics produces a large amount of carbon emissions, including emissions from the extraction and refining of crude oil to obtain the monomer, the polymerization of the monomer to obtain high-molecular-weight polymers, the molding processing of these polymers to produce different plastic products, and the incineration (rather than recovery) of the plastic products after their end of use. The data show that 1 t of plastic will produce as much as 5-10 t of CO
2 in its entire life cycle [
4]. It is estimated that the carbon emissions generated by global plastic wastes will reach nearly 1.4 × 10
9 t in 2030. From now until 2050, the global production and incineration of plastics are expected to cumulatively emit 5.6 × 10
10 t of CO
2-equivalent, which will account for 15% of the global carbon budget [
5,
6]. Therefore, effectively recycling the waste polymeric materials that can be collected will not only avoid environmental problems but also reduce dependence on fossil resources and decrease carbon emissions. For polymeric materials that cannot be collected or that are leaked into the environment, it is necessary to consider new polymers that degrade naturally in the environment.
There has been concern over the recovery of polymeric materials since the 1990 s, but progress has been limited. Problems with existing recovery methods include high energy consumption, low utilization rates, and low added value of the recycled products. Moreover, in some cases there is the enduring issue of the "secondary waste generation from waste treatment processes." These operational and systemic problems result in a recovery rate of less than 15% for plastic wastes [
7]. Based on different objectives, the recovery of polymeric materials is divided into energy recovery and matter recovery. The former is mostly carried out by incineration. Matter recovery is further divided into "physcycling" or "mechcycling" (physical or mechanical recovery), "chemcycling" (chemical recovery), "physchemcycling" or "mechchemcycling" (physical and chemical recovery or mechanical and chemical recovery), and "biocycling" (biological recovery) (
Fig. 2) [
8,
9]. In recent years, increasing attention has been paid to the quality, performance, and value of the recycled products, which are used to define their downcycling or upcycling, as well as being important factors in determining the feasibility of recovery methods [
10,
11].
When the recycled products are the starting polymers or the monomers/building blocks of the waste polymeric materials, the recovery process is called closed-loop recovery, that is, recycling, with the former being called "physical recycling" and the latter being called "chemical recycling." Of these, chemical recycling is always desirable if the cost is effective. Upcycling is a recovery process in which waste polymeric materials are converted into products with higher performance quality and/or higher economic/environmental value than their current state or their simple physical recovery products. This recovery process not only includes the conversion route or method but also emphasizes the ability to improve the ratio of performance to price for the converted products; that is, the upcycling process focuses on the recovery of high-value materials. Downcycling is a process of converting waste polymeric materials into products with lower performance and value than the materials had in their pristine state.
Upcycling and recycling are possible with different recovery methods.
Physical recovery is simple in operation, but it often leads to low added value and poor performance of the recycled products due to thermal and mechanical effects, making it more of a downcycling process.
Biological recovery involves using the action of microorganisms, fungi, or their secreted enzymes to degrade waste polymeric materials into reusable substances; while this process is characterized by environmental friendliness, it has low recovery efficiency in most cases.
Chemical recovery can break polymer chains into monomers or high-value small molecule/macromolecule products through controlled degradation and holds promising potential for closed-loop recovery or upcycling. For example, the chemical recovery of polyolefin is mainly based on thermal cracking with a high reaction temperature generally above 400 °C; the products are a mixture of light hydrocarbon gas, pyrolysis oil, and solid paraffin [
12]. Similarly, thermosets with a stable crosslinked structure can be degraded into oligomers with the assistance of a strong acid, base, or oxidant [
13,
14,
15]. However, these methods present several practical barriers, including high energy consumption, poor reaction selectivity, complex product compositions that complicate seperation and purification, and challenges in managing used solvents.
In recent years, a new generation of electrocatalytic and photocatalytic strategies has emerged, offering more energy-efficient, selective, and sustainable routes to convert plastic waste into high-value chemicals or materials. At the same time, precision catalysis is expected to dominate future advances, with catalysts specifically designed to recognize and depolymerize complex or mixed plastic streams under mild conditions. Innovations in single-site heterogeneous catalysis, enzymatic degradation, and dual-function catalytic systems capable of both decomposition and value-added conversion in one-pot processes are actively being explored. Nevertheless, a critical issue that must be considered is the severe mismatch between the large quantities of degradation products generated and their limited reutilization potential, which not only undermines economic viability but also leads to resource waste and reduced marketability of the outputs. In addition, many of the polymeric materials widely used in the market are mixed polymeric materials containing plasticizers, flame retardants, fibers, or inorganic fillers, and relatively few reports are available on the chemical recovery of mixed polymeric materials. Scholars have recently emphasized the urgent need to carry out systematic and in-depth research on the recovery of mixed polymeric materials [
16,
17].
The essential characteristics of the chemical bonds of existing waste polymeric materials complicate the recycling of these materials. Moreover, the structures of these polymers (including their aggregation state), their material compositions, and other factors affect the conversion process and its efficiency, as well as the properties of the recycled products. Therefore, it is extremely challenging to convert waste polymeric materials into high-value-added products. To solve these problems, it is necessary to explore new strategies for the efficient catalytic degradation and recovery of polymeric materials based on their physical and/or chemical characteristics. The ideal recovery approach should meet the following criteria as comprehensively as possible:
(1) Waste-to-product volume alignment. The output volume and market demand of recycled products must match the volume of generated waste, while maintaining a performance and value equivalent to or higher than the original materials (polymers/auxiliary components), enabling either closed-loop recycling (same-grade reuse) or upcycling (higher-value applications).
(2) Efficiency and sustainability. High yield and the easy separation of recycled products with full utilization should be achieved, greatly facilitated by optimized solvent management (e.g., the use of green solvents or solvent-free routes), alongside a green and low-carbon footprint throughout the recycling process.
(3) Economic viability. The process must remain competitive in the market without relying on subsidies or policy incentives, ensuring intrinsic profitability.
In this way, the huge waste of resources represented by discarded polymeric materials can be avoided, the recovery value of these materials can be increased, their serious harm to the ecological environment can be eliminated, and the carbon emissions generated by their production, use, and end-of-life cycle can be reduced.
2.3. Designing new polymeric materials for the future
Most existing polymeric materials were originally designed and manufactured with a primary focus on service performance, with little consideration being given to recyclability. This has led to significant challenges in waste recovery, including low recycling efficiency and complex processing requirements. To address this issue, it is necessary to design and synthesize new recyclable polymers that are easily recycled and reutilized after their service life and can be recycled either to their starting polymers (i.e., through physical recycling or closed-loop physical recycling) or to their starting monomers (i.e., through chemical recycling or closed-loop chemical recycling). Polymers recycled via physical recycling almost inevitably exhibit worse performance than the original polymers, resulting in the downcycling of polymeric materials. In contrast, chemical recycling offers an opportunity to revert waste polymeric materials back to their monomers for repolymerization into virgin materials, without altering the properties of the materials or the economic value of the polymers. In particular, repeatably chemically-recyclable polymers that can be chemically recycled completely or almost completely to their starting monomers, which can then be polymerized into the starting polymers, should be designed at the molecular level to inherently balance service performance with closed-loop recyclability. This will make it possible to dramatically increase the recovery rates of post-consumer materials.
We propose two complementary strategies. The first involves the design and synthesis of novel polymers that integrate essential performance characteristics with complete depolymerizability. The second strategy focuses on modifying existing polymers by incorporating a small amount of tailored co-monomeric structures into their backbones. These structures would be designed to preserve—and potentially enhance—the original properties or add new functionalities while enabling the complete depolymerization of the polymer into its constituent monomers, thereby facilitating closed-loop chemical recycling. As an example of the first strategy, poly(
p-dioxanone) (PPDO) exhibits sufficient service stability at ambient temperature, provided that residues of catalysts, monomers, and solvents are well controlled. When subjected to catalytic depolymerization coupled with vacuum distillation at 160 °C, PPDO can be almost entirely depolymerized into its monomer within just 8 min [
18]. Similarly, ring-opening polymerization-derived gem-disubstituted aliphatic polycarbonate (poly(5,5-dimethyl-1,3-dioxan-2-one), PDTC) demonstrates good thermal stability, mechanical strength, and service stability. Under catalytic depolymerization and vacuum distillation at 170 °C, both the monomer-recovery rate and the purity of the recovered monomer exceed 99% [
19]. As for the strategy of incorporating co-monomers, the introduction of a small fraction of such units into conventional polymers—such as poly(ethylene terephthalate) and polycarbonate—has been shown to significantly enhance their recyclability and impart additional functionalities, while allowing them to largely retain their original performance [
20,
21].
Single- use polymeric products are not suitable for collection in some applications. In such cases, biodegradation is an important supplement to the treatment of these waste polymeric materials. The development of repeatably chemically recyclable biodegradable polymers was first proposed by Wang and coworkers in 2011 [
22,
23], and has attracted more and more attention only until recent ten years. The ideal polymers for producing disposable polymeric products should satisfy the following criteria:
(1) They can be chemically recycled into their polymerizable monomers without separation and purification under mild conditions with a high yield, which can be completely reused for polymer synthesis, establishing a closed-loop chemical cycle.
(2) They are completely biodegradable in natural environments such as soil, fresh water, ocean water, and so forth, forming CO2 and water or substances harmless to human health and the environment.
(3) They exhibit a cost and comprehensive properties (e.g., facile processing/formability and mechanical properties) comparable to those of conventional polymeric materials used in the same application fields.
(4) Their degradation rate can be controlled for special applications, matching their service lifetime with post-consumer degradability through enzymatic, hydrolytic, photolytic, or other pathways.
(5) Whenever possible, their monomers can be derived from renewable bio-based feedstocks with sustainable sourcing, such as biomass, which is more conducive to low carbon emissions and demonstrates environmental superiority in life-cycle assessment (LCA) metrics compared with conventional petrochemical analogs.
According to these criteria, repeatably chemically recyclable biodegradable polymers—which are actually types of sustainable polymers—represent the most viable candidates and next-generation polymers for ideal single-use polymeric materials. Such polymers would enable the synergistic integration of closed-loop monomer recovery through industrial depolymerization with environmental degradation pathways under unmanaged disposal scenarios. Thus, they should be prioritized as the strategic focus for future research and development in sustainable polymeric materials industries.
In addition, as non-covalent interactions and dynamic covalent bonds are characterized by dynamic reversibility, polymers constructed by crosslinking polymer chains with these chemical bonds have re-processable and chemically recyclable properties. Thus, the construction of reversible crosslinked polymers opens up a new and important way to solve the problem of environmental pollution from plastics and avoid wasting end-of-life polymeric materials [
24,
25,
26]. Although important advances have been made with the new generation of recyclable polymeric materials based on monomer recovery and reversible bonds, the synthesis costs and physical properties of these new materials are hardly comparable with those of conventional polymeric materials. Therefore, these materials cannot yet satisfy the demands of large-scale production and practical applications.
3. Conclusions
The sustainable development of polymeric materials not only concerns the future of the polymeric materials industry but is also closely linked to the sustainable development of the global ecological environment, making it a global problem that urgently requires a solution. To address this issue, several key pathways are essential. First, vigorously developing renewable resources such as bio-based materials is a key measure to reduce the consumption of fossil resources. Second, it is necessary to realize the recycling and upcycling of the large amount of existing waste polymeric materials through efficient catalytic degradation and green transformation. Third, the sustainable development of polymeric materials can also be promoted by developing new recyclable polymeric materials based on covalent bonds or dynamic bonds in order to realize polymerization-depolymerization-polymerization and crosslinking-decrosslinking-crosslinking recycling, as well as by improving material properties such as aging resistance and incorporating self-healing capabilities to extend materials’ service life. Finally, for single-use/disposable polymeric products, the repeatably chemically-recyclable biodegradable polymers are the ideal polymers, which will enable the synergistic integration of closed-loop monomer recovery through industrial depolymerization with environmental degradation pathways under unmanaged disposal scenarios.
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