Introduction to plastics in a circular economy

We are delighted to edit this themed collection for Polymer Chemistry devoted to recent advances in plastics in a circular economy. Plastics have become the material of choice for the 21st century. This is in large part because of the undeniable fact that these materials combine unrivalled functionality with low cost, low density, durability, safety and processability. However, durability, one of the greatest properties of plastics, together with deficient plastic waste management platforms, has created severe worldwide environmental consequences, significant materials value losses for the global economy, and depletion of finite natural resources. These issues have been highlighted in the recent Royal Society of Chemistry report on ‘Science to enable sustainable plastics’ (https://rsc.li/progressive-plastics). Concurrently there is a need to change from our current linear production model based on production, use and disposal to a plastic circular economy model. This themed collection illustrates the latest developments in the area of circular plastic economy. At the time of publication of this editorial, the collection is composed of 10 research articles and 3 review articles dealing not only with the chemical recycling of commodity plastics such as poly(ethylene terephthalate) (PET), polypropylene (PP) or poly(bisphenol A carbonate) (BPA-PC), but also with the design of intrinsically recyclable plastics.

Nowadays only a small portion of commodity plastics are recycled. For instance, on average only 15% of PET, the largest recycled commodity plastic due to robust technology for PET sorting, is recycled. With mechanical recycling arguably the most successful industrial example, residual contaminants present in polymer waste lead to significant deterioration of properties during reprocessing. In order to maintain the value of PET, Andrew Dove and coworkers report an inexpensive dual-catalytic approach based on Lewis acids and organic bases to mediate the chemical recycling of PET (DOI: 10.1039/C9PY01920K). They found that tuning the strength of the acid–base interaction could greatly improve the catalytic activity and stability of this catalytic system, thus enhancing the selectivity and activity of the chemical recycling of PET. The authors suggested that research in this direction could lead to a closed-loop PET cycle. Besides chemical recycling, plastic waste has recently been explored as an alternative feedstock to prepare new monomers, polymers or materials. In this context, Kazuki Fukushima and coworkers investigated the use of PET as a source to prepare bis-benzimidazole and bis-benzoxazole using an organocatalytic depolymerization approach (DOI: 10.1039/D0PY00436G). They demonstrated that 1,5,7-triazabicyclo[4.4.0]dec-5-ene (TBD) facilitated the one-pot heterocycle formation from methyl benzoate, o-phenylenediamine (OPD) and 2-aminophenol (2AP). Thus, the TBD-catalyzed depolymerization of PET in the presence of OPD and 2AP was demonstrated to be a new method for the production of bis-benzimidazoles and bis-benzoxazoles. Similarly, Jeremy Demarteau and coworkers investigated the use of PET to create high added-value terephthalamide diols (DOI: 10.1039/D0PY00067A). In the presence of a thermally stable acid–base mixture, they obtained a series of terephthalamide diols that were further copolymerized with dimethylesters for the synthesis of poly(ester-amide)s.

Alongside PET, 60–70% of worldwide production is based on polymers that are composed of chemically inert C–C bonds. These polymers are thermodynamically non-favored for chemical recycling and therefore are mostly landfilled or incinerated. In order to maintain the value of the material, Frank A. Leibfarth and coworkers present a metal-free C–H fluoroalkylation strategy to functionalize commodity polymers by taking the advantage of the innate reactivity of aromatic polymers. They investigated a series of organic photoredox catalysts to incorporate trifluoromethyl groups onto aromatic polymers without altering their molar masses and dispersities to enhance the properties of commercially available polymers like polystyrene. They argued that this method performed well for post-consumer waste, representing a conceptual approach towards chemical upcycling of aromatic-containing polymers (DOI: 10.1039/C9PY01884K). Indeed, this method was also valid for the post-functionalization of commodity polymers such as BPA-PC. Apart from post-functionalization, plastic waste based on BPA-PC has been largely explored for chemical recycling, as reviewed by Jeung Gon Kim (DOI: 10.1039/C9PY01927H). He summarizes research studies on the chemical recycling of BPA-PC with a special focus on the chemical upcycling of BPA-PC into valuable chemical feedstocks to highlight the present and the future of BPA-PC recycling.

While chemical recycling is desired to retain a material's value after the end of its life, it is also important to design plastics that are derived from renewable sources and could potentially biodegrade, moving towards an environmentally closed circular system. Along this line, Keiji Numata and coworkers synthesized polypeptides containing nylon units by chemoenzymatic polymerization (DOI: 10.1039/D0PY00137F). They found that copolymers containing nylon 4 type structures greatly improved the performance of these polypeptides without affecting their biodegradability. According to the authors, these findings will open the door to the use of polypeptides in processes that require prior thermal treatment. Similarly, Karen Wooley and coworkers designed sugar-based magnetic hybrid nanoparticles by means of ring opening polymerization (ROC) of cyclic carbonates (DOI: 10.1039/D0PY00029A). They managed to use these biodegradable nanoparticles effectively to tackle environmental pollution posed by marine oil spills. Moreover, these nanoparticles suffer hydrolytic degradation, alleviating concern regarding potential microplastic contamination. Apart from polycarbonates and polyamides, bioderived polyesters have gained attention to replace non-biodegradable materials in different applications. Hence, Sergio Torres-Giner and coworkers reviewed the most recent advances in poly(hydroxy acids), a specific type of polyester, derived from hydroxy acids (DOI: 10.1039/D0PY00088D). Their review focused on the self-condensation of these hydroxy acids and evaluated their end-of-life options, considering not only their biodegradability but also their potential to be chemically recycled.

Despite the undeniable benefits of chemical recycling approaches to solve the problems arising from plastic waste accumulation, new plastic materials will be required that are not only designed for performance and durability, but also for recyclability. Thus, Eugene Chen and coworkers designed vinyl lactone acrylic bioplastics with high chemical recyclability (DOI: 10.1039/D0PY00786B). They found that the presence of cyclic vs. linear esters unexpectedly greatly enhanced the chemical recyclability of vinyl monomers, reaching notable values (up to 76% of the pure monomer) with negligible char formation. Similarly, Karin Odelius and coworkers explore the polymerization of natural δ-lactones to thermodynamically stable polymers and their recyclability (DOI: 10.1039/D0PY00270D). They found that copolymerization could be used to circumvent the poor equilibrium behavior of δ-lactones. Hence, using two distinct monomers could lead to unique macromolecular structures to create high molecular weight polymers without significantly affecting their ability to be chemically recycled.

In order to expand the library of fully recyclable polyesters, Michael Shaver and coworkers designed polyesters with alicyclic repeat units in the backbone (DOI: 10.1039/D0PY00448K). Their rational design was based on the superior properties of polymers containing cyclic repeat units within the backbone, together with recent reports suggesting that the presence of a flexible methylene linker between cyclohexyl rings favors chemical recycling. In spite of the low molecular weights attained due to competing reactions during polymerization, commercially available catalysts to some extent afforded the desired polyester, which has a T_g of 69 °C and could lead to interesting materials. Besides conventional ROP, entropically-driven ROP of macrolactones has also shown great potential to prepare intrinsically recyclable aromatic polyesters. This method exploits the occurrence of ring-chain equilibria between polyester chains and macrocyclic oligoesters in the presence of a suitable catalyst to tune the monomer–polymer equilibrium. Antxon Martínez de Ilarduya and Sebastián Muñoz Guerra reviewed the procedures that have been recently developed to obtain these polyesters, as well as their recovery by cyclodepolymerization (DOI: 10.1039/D0PY00258E).

While thermoplastics account for 80–85% of total plastic production, thermosets are of great interest for high-performance applications, but are particularly impractical for recycling because they cannot be reprocessed. Recently, the incorporation of dynamic interactions in polymer networks has enabled the design of reprocessable thermosets. To shed some light on this area, Frederik Wurm and coworkers synthesized intrinsic flame-retardant recyclable thermosets containing covalently installed phosphonates as flame-retardant units (DOI: 10.1039/D0PY00275E). The covalently installed phosphonate groups in the polymer backbone allowed the preparation of an additive-free thermoset with flame retardant character. The authors suggest that this concept could be further exploited to design other additive polymer networks with full recyclability.

We thank all the authors for their contributions to this themed collection on this exciting field.

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