Emerging horizons in polymer applications

Abstract

Polymers are ubiquitous in life; they are used across many length scales, from bulk engineering to precision nanomedicine. The broad applicability of polymers stems from the myriad of different architectures and chemistries that can be produced. In essence, a polymer is a large macromolecule consisting of many repeat units, a concept first proposed by Hermann Staudinger in 1920. However, the use of polymers predates this; natural polymers have been used throughout human history and synthetic polymers such as polyvinyl chloride and Bakelite were invented in the late 19th century. Throughout the 20th century, a plethora of synthetic polymers have been developed. This editorial will highlight the cutting-edge research recently reported across Materials Horizons and Nanoscale Horizons, covering four critical research areas: catalytic polymer materials, polymers in additive manufacturing, self-healing polymeric materials, and recyclable/sustainable polymers.

Calum T. J. Ferguson

Calum T. J. Ferguson is an assistant professor in the School of Chemistry at the University of Birmingham. Previously, he was a group leader in the O’Reilly group in Birmingham in the Landfester Department at the Max Planck Institute for Polymer Research, Germany. He has published over 35 peer-reviewed research papers in scientific journals, mainly on photocatalytic polymers. He is a photocatalytic materials chemist researching new conjugated and vinyl-based polymers as sustainable photoredox active materials for producing commodity chemicals.

Kostas Parkatzidis

Kostas Parkatzidis is a Swiss National Science Foundation Postdoc Fellow in the group of Professor Zhenan Bao at Stanford University (United States), working on the molecular design of polymer-based skin-inspired materials for various applications. Kostas obtained his PhD from ETH Zurich (Switzerland) under the supervision of Professor Athina Anastasaki where he focused on the development of advanced polymer synthesis and chemical recycling methodologies. He also holds an MSc in organic chemistry and a BSc in materials science and engineering obtained from the University of Crete (Greece). Since 2023, Kostas has served as a Materials Horizons Community Board Member.

Polymers can vary in structure with differences in the monomer and repeat units, microstructure and morphology. The microstructure of polymers is integral to their physical properties and, therefore, their applicability. Polymers can be found as either linear unbranched, branched, crosslinked or network polymers, with each geometry suiting different applications. Moreover, monomer functional groups encompass almost all imaginable chemistries, creating a near-limitless range of polymers that can be produced. This versatility has allowed polymers to be used for specialist applications, such as precision medicine, energy storage, and electronics.

Polymers dominate modern life, leading to what has been termed the ‘plastic age’. However, concerns have arisen due to the release of plastic waste into the environment. Striking images of plastic waste in marine environments has led to negative public opinion and an outcry for more sustainable solutions, resulting in an ever-increasing focus on bio-based and recyclable polymers to reduce environmental impact. Herein, we summarize some of the topics in the field of polymer science published in Materials Horizons and Nanoscale Horizons journals.

Catalytic polymer materials

Polymeric materials have been used to enhance catalytic properties, with implementation in organo-, electro-, thermal and photocatalysis. Catalytic polymers typically come in two forms: first, the polymer can act as a support material, incorporating a catalytic species (e.g., small molecule or nanoparticle); second, conjugated polymers can act as catalysts, typically owing to their electronic properties.

In this section, we will discuss advances in both supported catalysts and conjugated catalysts.

The term supported catalyst is generally not favoured as it does not fully explain the importance of the polymer, which can increase the catalyst's recyclability and stability. Moreover, polymers can enable photocatalysts to be used in aqueous environments and with biomaterials, enabling biocompatibility. For example, Crocker et al. modified a linear polymer with a small molecule flavin-based catalyst to create enzyme-inspired biocompatible photocatalysts (https://doi.org/10.1039/C9NH00199A). Here, a linear polydopamine polymer was selected due to its high biocompatibility and excellent charge carrier properties. Two strategies are available for creating a photocatalytic modified polymer: either the post-polymerization modification of the polymer with a catalyst or the creation of a catalytic monomer that can be readily polymerized. The latter approach is generally favoured in the literature due to problems associated with modifying polymers; however, the catalyst incorporated into the polymer must be stable under the polymerization conditions. Crocker et al. produced a flavin-based monomer that was statistically polymerized into a linear polydopamine chain, which precipitated during polymerization to create nanoparticles around 200 nm in diameter. Incorporating the flavin into the polymer nanoparticle dramatically increased its photooxidation by a factor of ∼20 and its photoreduction potential ∼10-fold. This highlighted a potential synergistic relationship between polydopamine and flavin within the hybrid material, which may result in stabilizing reactive intermediates and/or enhancing charge transfer.

Similarly, using polymers, Chen et al. increased the biocompatibility of inorganic catalytic nanoparticles. They demonstrated that the cerium oxide (CeO₂) nanoparticles could be functionalized on the surface with an oligochitosan biopolymer through electrostatic interactions (https://doi.org/10.1039/D2NH00572G). This modification allowed the catalytic nanoparticles to be loaded onto the surface of Spirulina platensis, a cyanobacterium, to create a biohybrid catalyst. This catalyst could then work in tandem with naturally occurring enzymes to mitigate superoxide anion radicals produced in the body, leading to an anti-inflammatory effect.

It is not only the ability of polymers to allow biocompatible reaction conditions to be used; they can also broaden the scope of reaction solvents that can be used for synthetic chemistry. Lee et al. developed an ingenious way to increase the dispersibility of photocatalytic metal organic framework (MOF) crystals in organic solvents without sacrificing reactivity (https://doi.org/10.1039/C8MH01342J). A bio-based substrate (pollen) was modified with pH-responsive hairs by surface-initiated atom transfer radical polymerization using 2-(dimethylamino)ethyl methacrylate as a monomer. A range of MOFs could then be incorporated into this polymer hair corona. Notably, Cu₂(bdc)₂(dabco) was immobilized within the hairs, enabling catalysis in a broad range of solvents with increased performance compared to the native MOF. Moreover, the system also displays pH responsiveness as a function of the polymer hairs' protonation and deprotonation, leading to swelling of the corona and access of substrates and light to the photocatalytic MOF.

Further to using polymers as a support to enhance catalyst performance, conjugated polymers have been used for a range of different catalytic applications. Materials Horizons and Nanoscale Horizons have recently published excellent reviews in this field, specifically on processing polymer photocatalysts (https://doi.org/10.1039/D4MH00482E), porous graphitic carbon nitride (https://doi.org/10.1039/D0NH00046A), organic-electro-photo-catalytic covalent organic frameworks (https://doi.org/10.1039/C9MH00856J), and conjugated porous polymers for chemical synthesis (https://doi.org/10.1039/C9MH01071H). Further to these reviews, cutting-edge research in conjugated photocatalytic polymers has recently been reported. For example, Chen et al. surface-modified an aerogel with a conjugated polymer network for seawater desalination and uranium extraction (https://doi.org/10.1039/D4MH01055H). The surface modification of a SiO₂ aerogel creates a more versatile, high-surface-area photocatalytic material with enhanced mass transfer properties compared to bulk materials. These materials have significant potential applications in the remediation of water contaminants, including radioactive uranium. The photocatalytic polymer grafted aerogel efficiently reduced uranium(VI) to uranium(IV), demonstrating its potential for radioactive remediation.

In addition to remediating uranium, You et al. reported the formation of photocatalytic covalent organic frameworks (COFs) with a core–shell structure for phenol degradation and hydrogen evolution (https://doi.org/10.1039/D4MH01596G). A type II heterojunction is formed from a combination of a urea-linked perylene diimide polymer and a β-ketoenamine-linked covalent organic framework. Polymer synthesis allows these two semi-conducting components to be covalently linked and grafted together to allow excellent charge transfer, creating a highly efficient catalyst. This clearly demonstrates that new catalysts can be designed and optimized using classical techniques in polymer science.

Polymers in additive manufacturing

Another important field where polymers have found great application, both in industry and academia, is additive manufacturing (3D printing). 3D printing has revolutionized the production of polymer-based materials by continuously advancing printing techniques that enhance speed, resolution, and accessibility. Alongside these technological developments, significant efforts in material design have introduced innovative properties to printed objects, driving further progress in the field (https://doi.org/10.1039/D4MH01160K).

Several works have been published across Materials Horizons and Nanoscale Horizons, highlighting the progress in this field. One particularly transformative innovation is light-based 3D printing, which utilizes photopolymerization reactions to solidify inks with precise spatiotemporal control. Recent advancements by Ehrmann's group highlights the potential of dual-wavelength printing, where orthogonal chemistries have expanded printing methodologies (https://doi.org/10.1039/D4MH01261E). By leveraging distinct polymerization mechanisms activated by different wavelengths, the group has enabled the simultaneous creation of multi-material 3D objects with complex mechanical properties. The concept of multi-material components for 3D printing has opened the door to creating flexible and deformable objects, marking a departure from the traditional rigidity of 3D-printed structures. By employing water-in-oil emulsion inks, where water acts as a pore-forming agent, and the continuous phase consists of a stretchable polymer, Magdassi, Beccai and co-workers have developed porous 3D structures with tunable mechanical properties (https://doi.org/10.1039/D3MH00773A). This capability to adjust porosity enables the creation of highly stretchable materials, offering unique advantages for applications in soft robotics and actuators. Such designs pave the way for innovations in materials with mechanical properties engineered across multiple scales.

Tuning these mechanical properties of the 3D printed materials has allowed the development of soft and conformable objects, advancing the development of bioelectronics, soft robotics, and health-related devices. Mecerreyes, Malliaras and co-workers have developed printable conductive polymer mixtures using deep eutectic solvents combined with poly(3,4-ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS), a widely recognized conductive polymer system (https://doi.org/10.1039/D3MH00310H). This innovative approach enhances ionic-electronic conductivity, which is critical for bioelectronics. By fine-tuning the viscosity of the polymer-solvent mixture, they successfully utilized direct ink writing to produce adhesive electrodes for in vivo electromyography measurements, showcasing excellent conformability and potential for wearable applications.

Despite these advancements, a major limitation of current 3D printing technologies lies in the inability to chemically recycle most polymer-based objects due to the inertness of the materials and the irreversible polymerization processes. Addressing this challenge, Nguyen and Du Prez developed novel chemistries that enable the creation of functional and chemically recyclable materials (https://doi.org/10.1039/D4MH00823E). This recent study introduced a closed-loop 3D printing process utilizing dynamic chemistry. The authors synthesized an acrylate photocurable polymer material incorporating dynamic β-amino ester cross-linkers, which serves as an ink for 3D printing. This material can undergo decross-linking via a transesterification reaction, allowing the depolymerization of the polymer network. Such a reversible process, operating under mild and environmentally friendly conditions, holds immense potential for sustainable 3D printing practices.

Self-healing polymeric materials

Self-healing polymeric materials have been reported ubiquitously in research and can be used in a variety of applications. Different polymer chemistry approaches can be employed to achieve self-healing, using covalent or supramolecular interactions, which, under appropriate conditions, can reform the damaged polymer to its initial state. Recently, supramolecular interactions have attracted significant attention because they require milder conditions to restore the damaged polymer compared to covalent bonds. Among the various supramolecular interactions, hydrogen bonding (H-bonding) has emerged as a potent tool in developing self-healing polymers. Due to their reversibility, H-bonding interactions enable polymeric materials to regain their original functionality after sustaining damage. In addition to H-bonding, other supramolecular dynamic interactions—including hydrophobic forces, metal ligand coordination, electrostatic interactions, π–π stacking, guest–host interactions, and van der Waals forces—have been explored to create robust and dynamic polymer networks that heal efficiently while maintaining mechanical and chemical integrity (https://doi.org/10.1039/D3MH00236E). Self-healing materials have the potential to significantly advance emerging fields such as wearable organic electronics. This field requires devices that combine comfort for the applied tissues with electrical properties that enable signal recording, cargo delivery, and monitoring of the healing process. In a recent study, Cicoira's group utilized the well-known conductive polymer blend PEDOT:PSS and combined it with polyurethane and polyethylene glycol, achieving excellent self-healing properties through hydrogen bonding interactions (https://doi.org/10.1039/D4MH00203B). The hydrogen bonds between the customized polymers facilitate both mechanical and electrical recovery after damage, ensuring optimal performance. Notably, due to the reversibility of hydrogen bonds, the material exhibits exceptional recyclability—an important yet rarely demonstrated feature—maintaining its functionality over multiple reprocessing cycles. When devices are used to monitor physiological processes in the body, such as wound healing, additional properties—such as adhesion to the skin or inner tissues—are required. At the same time, as the wound heals, the device should be able to adapt to the changing environment while also providing a protective barrier against external factors. In this context, Wang, Li, Huang and co-workers have demonstrated a novel skin-adhesive wound dressing designed to integrate multiple functionalities for diagnostic applications, including strong adhesion to various surfaces, self-healing capability, and intrinsic antimicrobial properties (https://doi.org/10.1039/D3MH02064A). This material's success lies in combining several key strategies—dynamic multiple hydrogen bonds, ionic interactions, and cationic chain segments—which synergistically impart all the desired properties. Incorporating moieties capable of forming dynamic hydrogen bonds alongside ionic groups that establish ionic cross-links enhances self-healing and adhesion properties. Meanwhile, the cationic moieties serve as intrinsic antimicrobial agents, protecting the wound from infection.

To target the final application in highly dynamic human tissues, the developed devices should be able to adjust to deformation by possessing stretchability while maintaining toughness. These two mechanical characteristics are often conflicting, making the development of materials that are both robust and highly stretchable particularly desirable. In a recent study, Zha's group developed a PDMS-based elastomer incorporating two types of hydrogen bonds—strong decuple and weaker quadruple interactions (https://doi.org/10.1039/D3MH01265D). Introducing these hydrogen-bonding moieties into the PDMS creates a dynamic cross-linked network with significantly enhanced mechanical properties. The novel elastomer exhibits remarkable stretchability and toughness while demonstrating excellent self-healing capabilities. Beyond hydrogen bond-based self-healing polymeric materials, other chemical strategies can also be employed to achieve desirable properties. For example, Li and co-workers have developed a novel self-healing material based on adaptable network and disulfide bonds, which was utilized to create a pneumatic robotic arm (https://doi.org/10.1039/D2MH01056A). This polymeric material is designed around two key principles: a coordination-adaptable network and dynamic disulfide bonds. The synergy between these two principles enables forming a material with tunable mechanical properties. At room temperature, the material exhibits high load-bearing capacity, while at elevated temperatures, it can be actuated pneumatically, demonstrating soft-rigid switching behavior. This unique property arises from the adaptable network, which maintains strong bonding interactions at lower temperatures but dissociates at higher temperatures. By integrating this adaptable network with a disulfide-based self-healing elastomer, the final material combines self-healing capabilities with rigidity and mechanical property switching. This approach exemplifies the potential of hybrid material design, merging covalent and supramolecular interactions to achieve unique functionalities. Aside from chemistry-specific interactions used in self-healing polymeric materials, a new concept based on bottlebrush polymers has been developed by Wu, Liu and co-workers to achieve self-healing in a chemistry-neutral manner (https://doi.org/10.1039/D3MH00274H). This approach relies solely on the unique topology of bottlebrush polymers, which consist of a linear backbone with densely grafted side chains. The interactions between these side chains have proven highly beneficial for designing self-healing materials, as demonstrated in a recent study. The principle behind this polymeric material's remarkable—up to 100%—self-healing efficiency is rooted in physical entanglements introduced by the long side chains alongside van der Waals interactions. These factors create a highly dynamic system capable of autonomously repairing damage. A key advantage of this chemistry-neutral concept is its ability to enable self-healing under diverse environments, including water, acidic, and alkaline solutions—conditions that pose significant challenges for most self-healing materials. This work highlights the critical role of polymer architecture in material design and demonstrates how structural topology can be leveraged to achieve advanced functionalities.

Recyclable/sustainable polymers

While polymers have greatly improved our lives, they come at a cost—the alarming plastic crisis. The rate of plastic production far exceeds that of recycling, posing a serious challenge to society. The topic of chemical recycling is of paramount importance across all fields of polymer science. In the last few years, significant efforts have been dedicated to advancing knowledge of sustainable polymers, including recycling of produced polymers as well as potential ways to replace the fossil-fuel based monomers with renewable bio-based alternatives (https://doi.org/10.1039/D2MH01549H).

Addressing the plastic crisis is a multi-component problem and there is no single solution that will overcome all the problems associated with the polymers. However, significant efforts have been made to recycle specific polymers, such as polystyrene, that are readily used in our everyday lives. Catalytic approaches based on external stimuli have been demonstrated to be a promising potential root to chemically recycle this class of polymers via depolymerization, and, thus, monomer generation, or chemical upcycling, where other value-added small molecules can be produced (https://doi.org/10.1039/D2MH01215D). Biomass can reduce our dependence on fossil fuels by providing new starting materials for polymer synthesis. This not only helps decrease reliance on fossil fuels but also enables the production of polymers with unique properties. Recent studies have identified several highly promising bio-based molecules as building blocks for sustainable polymers that can undergo depolymerization on demand, regenerating the virgin monomer. One notable example, by Kim, Lee and co-workers is lipoic acid as a monomer (https://doi.org/10.1039/D4MH00868E). This versatile monomer polymerizes upon light activation, while under basic conditions, its disulfide bonds break, leading to depolymerization and monomer recovery. Beyond sustainability, lipoic acid-based polymers offer opportunities for designing materials with unique properties. By introducing side chains into these polymers, researchers have optimized the synthesis of dynamic networks that efficiently dissipate energy, yielding advanced sustainable materials with enhanced mechanical properties.

Conclusions

The last century was dedicated to developing highly efficient polymerization approaches and understanding the mechanisms of polymerization reactions. Over time, polymers have found a plethora of applications, driving the evolution and foundation of numerous fields. However, throughout this development, minimal attention has been given to the end-of-life of these materials. Looking ahead, we believe taking a step back is necessary to re-evaluate material production principles. This reassessment will develop much-needed new chemistries, enabling the creation of polymer-based materials with intrinsic sustainability. This shift will foster the development of novel materials with unique properties and lay the foundation for the emergence and growth of new fields. Materials Horizons and Nanoscale Horizons look forward to supporting this endeavour by publishing high-quality and conceptually novel polymer research, spanning topics discussed in this Editorial and more.

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