Polymer Recycling and Upcycling: Recent Developments toward a Circular Economy

Abstract

Introduction The increasingly unsustainable generation and accumulation of plastic waste has become one of the great global challenges we face as a society. Indeed, the urgency of the need to address this issue is reflected in Target 12.5 in Goal 12 of the United Nations Sustainable Development Goals, which aims to “substantially reduce waste generation through prevention, reduction, recycling and reuse by 2030”. 1 Eight years have now passed since the 2030 Agenda for Sustainable Development was adopted by all United Nations Member States in 2015, which emphasizes just how critical and urgent this need has become. This, in turn, has driven substantial research efforts in plastic recycling and upcycling to close the loop on production, reduce waste, and facilitate the transition to a circular plastic economy. In this joint virtual issue, the editors of ten ACS journals who regularly publish research in this area have each selected recently published works that they consider to have advanced the field and bring us closer to realizing the important global goal of sustainability in the production and use of plastics. This editorial introduces the selected Articles, Letters, Perspectives, and Reviews that have contributed to tackling this key issue. Readers will observe a variety of approaches taken by the works featured in this Virtual Issue, ranging from developing technologies to directly recycle single-use plastics, to chemical means of converting plastics back into useable chemicals, and finally to the production of renewable monomers and polymers. Direct Recycling of Plastics The take-home message in a thought-provoking perspective by Epps, Korley, and co-workers (DOI: 10.1021/jacsau.1c00191) is that there is no single solution to achieving polymer circularity. The diversity of molecular chemistries and architectures of polymers, which enable their widespread use, means that the approach to life-cycle management will differ for each polymer class. Building on this, Uekert and co-workers (DOI: 10.1021/acssuschemeng.2c05497) noted that identifying the correct approach to life-cycle management will require full consideration of technical, economic, and environmental impacts. They compared various closed-loop recycling technologies for the most common commercial polymers, and concluded that mechanical recycling outperformed all other technologies. Indeed, a significant impediment to the mechanical recycling of mixed plastic waste is the efficiency of sorting plastic into streams containing polymers designated for similar treatment and reprocessing. Larder and Hatton (DOI: 10.1021/acspolymersau.2c00040) discussed the importance of sorting plastic waste to enable a circular economy. In particular, they discussed how photoluminescent-based labeling or doping could aid in the detection and, hence, sorting of plastic waste. While not a new approach, as discussed in the review, in recent years, there have been increased efforts of photoluminescent labeling to improve plastic-waste sorting, including the development of a pilot plant. As noted above, the mechanical mixing of waste streams into a new polymer blend is one route to achieve circularity. However, phase separation of dissimilar polymer components leads to heterogeneous materials with unfavorable properties. To overcome this challenge, additives that promote interfacial compatibilization may be used to reduce or suppress phase separation. In a perspective, Ellison and co-workers (DOI: 10.1021/jacsau.1c00500) discussed the challenges of mechanical blending and the potential of multiblock copolymers with diverse architectures to effectively compatibilize polymer blends. In one example, Ellison, Hillmyer, and co-workers (DOI: 10.1021/acsmacrolett.2c00601) demonstrated the in situ formation of polyethylene-terephthalate-polyethylene (PET–PE-PET) triblock copolymers to enhance compatibilization of PET and PE mixed plastic waste. Another well-known approach to compatibilize polymer blends is via reactive compatibilization. Segalman and co-workers (DOI: 10.1021/acs.macromol.3c00060) demonstrated the use of noncovalent ionic bonding to suppress macroscopic phase separation. It was shown that well-ordered blends were obtained using polystyrene (PS) and polydimethylsiloxane end-functionalized with acid and base moieties, respectively. In contrast, polymers containing weak acid and base end-groups were less ordered. The experimental results, combined with self-consistent field theory, provide a blueprint for blending plastic waste via ionic bonding. Reprocessing thermosets that do not naturally contain dynamic linkages represents a long-standing challenge for mechanical recycling. To make progress toward this challenge, Ellison, Dichtel, and co-workers (DOI: 10.1021/acscentsci.0c00083) demonstrated that the postsynthetic introduction of dibutyltin dilaurate into model and commercially available polyurethane foams could enable reprocessing at elevated temperature via twin-screw mixing. The dibutyltin dilaurate facilitated dynamic carbamate exchange reactions at high temperatures, thus preserving similar properties pre- and post-reprocessing. Beyond mechanical recycling, chemical-based approaches to recycle plastics are also being pursued. Yet, the ability to directly break down (or depolymerize) and subsequently rebuild (or repolymerize) a polymer is still challenging. In their investigation, Xu, Wang, and co-workers (DOI: 10.1021/acs.macromol.1c02085) reported on the development of a “DE-RE Polymerization” strategy to recycle polylactide via a chemical-based approach. The key to their innovation was the use of zinc bis[bis(trimethylsilyl)amide], which enabled both the breakdown and synthesis of lactide under mild conditions. The study demonstrated the controllable cleavage of polylactide and subsequent repolymerization with different commercial samples, thus representing a promising approach toward utilization. Chemical Upcycling of Plastics The conversion of waste plastic into valuable and functional chemicals, i.e., chemical upcycling, is a research topic of immense interest within the chemical sciences community, as highlighted by the collection of articles from the ACS portfolio journals. The upcycling of single-use PE is a global challenge that many researchers are investigating. In one example, Delferro and co-workers (DOI: 10.1021/acscentsci.9b00722) used catalytic depolymerization, enabled by Pt nanoparticles supported on SrTiO3 perovskite nanocubiods, to break down PE into low molecular weight lubricants and waxes in 170 psi H2 and 300 °C. The favorable PE adsorption to Pt and the electronic and geometric properties of the Pt-supported catalyst are key to selective PE hydrogenolysis. Yet, in another example, a Ruthenium-based catalyst was shown to break down PE into liquid alkanes in 20 bar H2 at 200 °C (DOI: 10.1021/jacsau.0c00041). Ru nanoparticles supported atop carbon converted PE into liquid alkanes with a 45% yield (by mass) after 16 h of reaction time. Pan, Zhang, and co-workers (DOI: 10.1021/acs.iecr.2c01287) investigated the pyrolysis of low-density PE using a metal-modified HZSM-5 zeolite catalyst under CO2 and at 550 °C. Their investigation revealed the evolution of aromatic hydrocarbons of benzene, toluene, ethylbenzenes, and xylene (BTEX). The examples highlighted above were shown to be effective in commercial PE, thus demonstrating a promising route for PE upcycling. Relatedly, Vlachos et al. (DOI: 10.1021/acscatal.1c00874) demonstrated the use of an Ru-supported titania catalyst to deconstruct polypropylene in 30 bar H2 at 250 °C. The reaction conditions resulted in high oil yields (∼66–80%) and low gasification, which was attributed to the dynamic adsorption/desorption of the catalyst and the polymer. Expanding beyond polyolefins, Skrydstrup and co-workers (DOI: 10.1021/jacsau.1c00050) demonstrated the use of iridium-MACHO under 30 bar H2 and 150–180 °C for the hydrogenation of polyurethanes (PU) to low molecular weight chemicals. Using commercially available PU of either foam or solid structure, Ir-MACHO catalytic hydrogenation resulted in the formation of anilines and polyols with high yields. Finally, McInnes, Qi, Xiao, and co-workers (DOI: 10.1021/jacs.2c01410) demonstrated the upcycling of PS into benzoic acid, formic acid, and acetophenone in 1 bar O2 using an acid catalyst. A better understanding of factors influencing polymer degradation is crucial to advancing approaches to deconstruct polymers in a controllable manner, as highlighted in a series of articles. Epps and Christoff-Tempesta (DOI: 10.1021/acsmacrolett.3c00276) discussed the current state of affairs and the potential of ionic liquids as a medium to assist in polymer deconstruction. Van Geem et al. (DOI: 10.1021/acs.iecr.0c05414) demonstrated the use of tree-based kinetic Monte Carlo and artificial neural networks to elucidate the three-step degradation of poly(styrene peroxide), including the evolution of value-added products. Meanwhile, Jahnke and co-workers (DOI: 10.1021/acscatal.1c03963) developed a new approach to accurately measure the degradation rate. Using impendence spectroscopy is a unique way to measure, in real-time, the enzymatic degradation of PET. The capacitance was sensitive to changes in film thickness that occurred due to enzymatic degradation. Conversion of polymer to monomer via depolymerization, including an understanding of the thermodynamics of polymerization and repolymerization, remains a challenge (DOI: 10.1021/jacs.1c11197); yet some chemistries present a more favorable situation. Sumerlin and co-workers (DOI: 10.1021/acsmacrolett.2c00603) demonstrated that the reactive end-groups of polymers made by controlled radical polymerization may be targeted to facilitate depolymerization. In particular, they investigated the influence of light on the depolymerization of poly(methyl methacrylate) containing thiocarbonythio end-groups prepared by RAFT polymerization. Under optimal light conditions, 87% depolymerization occurred within 1 h. Johnson and workers (DOI: 10.1021/jacs.2c05374) demonstrated the conversion of PS into repolymerizable fragments by incorporating thionolactones, as a cleavable comonomer. In the case of PU, Jehanno, Sardon, and co-workers (DOI: 10.1021/acssuschemeng.2c05647) demonstrated selective C–O cleavage of the urethane group in the presence of secondary amines under N2. In contrast, completing the reaction in the presence of primary amines unselectively broke both C–O and C–N bonds, thus providing less control over the final products. In another approach to making PU more sustainable, Bakkali-Hassani, Caillol, and co-workers (DOI: 10.1021/acs.macromol.2c01184) suggested, in a call-to-action perspective, that more attention should be focused on transcarbamoylation as a reaction that could enable a more sustainable future for PU. Finally, Wu and co-workers (DOI: 10.1021/acs.iecr.2c03393) recently prepared urethane-based epoxy vitrimers with the capability of either being reprocessed or fully degraded, thus leading to a material with multiple end-of-life sustainable options. Lignin will enable the development of new bioderived polymers, yet it also offers opportunities for upcycling. In one study, Barta, Hirsch, and co-workers (DOI: 10.1021/acscentsci.9b00781) described a three-step approach to transform lignocellulose into biologically active compounds. In particular, they employed the “LignoFlex” process and deep eutectic solvents to create benzazephines, i.e., pharmaceutical-relevant molecules, without the production of waste byproducts. In another example, photoassisted partial depolymerization of lignin was carried out under mild conditions using tetrabutylammonium decatungstate that facilitated bond cleavage via hydrogen atom transfer reactions (DOI: 10.1021/acscentsci.2c01257). The partially depolymerized lignin was used as a feedstock to create recyclable dynamic polymer networks. Development of Sustainable Monomers and Polymers There has been tremendous progress in creating monomers and polymers from renewable and sustainable resources. Here, we highlight several recent investigations. Hong et al. (DOI: 10.1021/acspolymersau.2c00001) demonstrated the production of methylene butyrolactone monomer via a zinc-mediated allylation-lactonization reaction with several biorenewable aldehydes. The polymerization of the monomer and subsequent depolymerization of the polymer presented an acrylic material with closed-loop cyclability. In another example of creating polymers starting with sustainable feedstock, Jannasch, Vares, and co-workers (DOI: 10.1021/acs.biomac.2c00452) used citric acid as the starting feedstock to synthesize rigid spirodiols that were further polymerized into poly(β-thioether ester)s via thiol–ene polymerization. To create mechanically robust and recyclable biobased thermosets, Du, Zhu, and co-workers (DOI: 10.1021/acssuschemeng.1c07523) reacted vanillin-based dialdehydes and trialdehydes with renewable diamines. The high aromatic content led to an improvement in the mechanical properties of the thermosets. Yet, they could be degraded under mild acidic conditions. Covalent adaptable networks employ dynamic bonds that enable their reprocessing. Rusayyis and Torkelson (DOI: 10.1021/acsmacrolett.2c00045) synthesized and used a hindered-urea-based cross-linker to produce polymethacrylate networks. The use of the dynamic cross-linker enabled the formation of addition-type polymer networks that are fully reprocessable without a reduction in mechanical properties. Relatedly, Xu, Liu, and co-workers (DOI: 10.1021/acs.biomac.2c01230) developed a self-healing and reprocessable poly(thiourethane-urethane using terpene-derived polysiloxanes functionalized with isobornyl and thiol groups. CO2-based polymers from biosourced monomers with enhanced thermal stability were prepared by Liu, Lu, and co-workers (DOI: 10.1021/acscatal.1c01376). In particular, they synthesized completely recyclable polycarbonates from CO2 and biosourced epoxides. The thermal properties of the polycarbonates could be tuned by changing the substituent group at the carbamate linkage. Recyclability back to monomer was achieved by thermolysis in the presence of a catalyst at 200 °C. PETs are the most common class of thermoplastic polyesters used in various applications, and production proceeds via terephthalic acid (TA) and ethylene glycol (EG). Improving the sustainability of PET is a continuing challenge. A GE patent from 1959 reported the synthesis of polyesters using methoxyterephthalic acid (MA) and EG with properties similar to the TA derivative. To enable the production of biobased MA, Skydstrup and co-workers (DOI: 10.1021/jacsau.3c00092) used lignin derived from softwood as the feedback. Subsequent MA polymerization into methoxy poly(ethylene terephthalate) resulted in a material with good thermal properties. Enhancing the performance of sustainable and biocompatible polymers remains an enduring challenge, and has thus far limited their replacement of traditional polymers from nonrenewable sources. In one example, the low flexibility and poor toughness of PLA have limited its use in elastomeric applications. In a simple but effective approach, Gallos, Allais, and co-workers (DOI: 10.1021/acs.biomac.1c00002) melt blended PLA with biobased ferulic acid to create a transparent and flexible material with improved mechanical properties. π–π stacking between PLA and ferulic acid enabled the elastomeric properties, as suggested by the molecular modeling. Pitet and co-workers (DOI: 10.1021/acspolymersau.2c00019) demonstrated the reuse of postconsumer PET as a feedstock to produce thermoplastic copolymers with fatty-acid-derived soft blocks. To produce poly(butylene terephthalate) (PBT), PET was combined with 1,4-butane diol and a renewably derived fatty acid dimer. The thermal and mechanical properties of PBT-based copolymers make them attractive for injection molding. Future Outlook As illustrated by the above highlighted articles, ACS journals are a home to showcase the most significant advances in the science and technology of polymer recycling and upcycling, and the development of plastics from sustainable resources. These articles, combined with others from the ACS portfolio, demonstrate the shared commitment from the global chemical science community to address one of the most enduring challenges to create a more sustainable future. Progress toward a circular plastics economy is notable, yet, as revealed in the collection of articles, much work remains to be done, including creating new sustainable monomers and new technologies to deconstruct polymers into meaningful products. In addition, recycling thermosets, particularly those that do not contain inherent dynamic bonds, deserves continued attention. Finally, a circular and interconnected economy requires that scientists and engineers consider how other resources, i.e., water, energy, and agriculture, are conserved to create a sustainable plastic future.