Teaching an Old Dog New Tricks: Sustainable Polymers

The world's first-ever human-made synthetic polymer, “Parkesine”, as it was called, was introduced in 1862 by Alexander Parkes at the London International Exhibition. “Parkesine” was a moldable plastic material (as an alternative to ivory and horn) that Parkes discovered while trying to develop a synthetic substitute for shellac for waterproofing. Whilst the product was not a commercial success, Parkesine represented an important first step in the development of human-made plastics. Since then, synthetic polymers have been an integral part of modern life, as they have been providing many societal benefits including those strongly and positively connected to sustainability (for instance food packaging to prevent spoilage or membranes for efficient water purification). Nevertheless, a linear economy approach, also called a “take-make-use-dispose” value chain, in which raw materials are transformed into products that are then used until being disposed of as waste, does not take into consideration any end-of-life burden. Hence, a new plastic paradigm, which aims to achieve a long-term sustainable future for plastics and requires integration along the entire value chain, from design to reuse, has been developed within the last decades. The so-called circular (polymer) economy has emerged as a holy grail along the concept of green chemistry as two ambitious approaches to promote sustainability. On the one hand, green chemistry, as a multidisciplinary field, covers areas such as synthesis, solvents, catalysis, raw materials, products, and efficient processes, and reflects the efforts of academia and industry to address challenges related to sustainable development of the chemical industry. On the other hand, the circular economy emphasizes the reuse of resources instead of being continuously discarded and represents a value chain approach in which the end-of-life of a product is taken into consideration from the moment it is developed. Inevitably, the COVID-19 pandemic and lockdown measures along with the ongoing war in Ukraine are putting the ambitions and timeline for reaching social, economic, and environmental sustainability at risk, despite the enormous efforts to reach the UN Sustainable Development Goals by 2030 and developments within the fields of green chemistry, sustainable chemistry, along with the circular economy and chemistry, which we are encountering in the practice of on a daily basis. Accordingly, within this special issue on sustainability in Macromolecular Chemistry and Physics—based on 24 contributions—leading experts from all over the world present new trends and greener pathways that fit into a circular (polymer) economy in order to enhance the sustainability of synthetic macromolecules ranging from material design (Talita M. Lacerda and co-workers, article 2100501) to waste management (Bernhard von Vacano and Hannah Mangold, article 2100488), with particular focus on tools to reduce the negative impacts of plastics on the environment throughout their life cycle (Venkateshwaran Venkatachalam and co-workers, article 2200046), the use of renewable resources (Philip B. V. Scholten and Monique B. Figueirêdo, article 2200017; Audrey Llevot and Thomas Vidil, article 2100494; Hatice Mutlu and co-workers, article 2100497), and the design of (bio)degradable and/or recyclable materials (Hendrik Frisch et al., 2100472), amongst others. In other words, we show that the concept of sustainability, with particular focus on macromolecules, can be improved at many levels of the process, from the source of monomers employed, to reduced energy consumption in synthesis and processing. In fact, it is challenging to provide a complete, comprehensive overview for all subjects covered. Nevertheless, we—as the guest editors—are delighted to present a very broad set of manuscripts addressing the various fields of action under the main umbrella of sustainability. Regarding sustainability, research in the last decades, either in academia or industry, has focused mainly on replacing the depleting fossil raw materials with renewable alternatives. Particularly, plant oils, polysaccharides (mainly cellulose and starch), sugars and wood amongst others are used increasingly in the production of renewable polymers. Indeed, the design of synthesis of polymers from renewable resources is one of the main concepts which is well represented in this collection. For instance, Michael A. R. Meier and colleagues (2200010) report the sustainable synthesis of isocyanate free poly(ester urethane)s using renewable diols as starting materials – particularly, 2,3-butanediol (a promising bulk chemical favouring decarbonization). Henri Cramail and José M. Asua (2100437) have emphasized how the synergistic combination of nonisocyanate polyurethanes (NIPU) with other polymers can be a promising pathway toward the formation of novel sustainable materials. An aromatic polyether network of lignin inspired Frederik Diness and colleagues (2100484) to design and synthesize high-temperature polymers similar to poly(bisphenol A sulfone) or polyether sulfone from lignin-derived small molecule, i.e. 4-hydroxybenzyl alcohol (4-HBA), via SNAr reaction. In a similar manner, Katrien V. Bernaerts and coworkers (2100461) presented a proof-of-concept for using depolymerized lignin monomers and oligomer mixture as such without fractionation for anticorrosive resin synthesis, and benchmarks its usability with lignin oligomeric fractions separated via either extraction or membrane separation. Exploitation of agricultural waste as a renewable starting material to produce various valuable products is attracting the attention of academic, industrial, and other practitioners. In line with this, Prakash P. Wadgaonkar and Henri Cramail (2100449) have shown that cashew nutshell liquid (CNSL) derivative, a byproduct of the cashew industry, stands out as a unique renewable starting material amongst other, leading to low glass transition (co)polycarbonates that could be effectively applied in soft touch applications, data storage and optical devices. Alternatively, naturally occurring flavan-3-ol, catechin, was explored as a polyphenolic feedstock for the synthesis of benzoxazine monomer, which in turn was crosslinked to deliver a thermally stable resin for various high-temperature sustainable applications (Bimlesh Lochab, 2100458). As the use of plant-based renewable resources alone will not suffice to tackle the issue of sustainability, the utilization of carbon dioxide as chemical feedstock has been recognized as a solution to meet the demand of virgin polymers in the future. Likewise, Shunjie Liu, Xianhong Wang and colleagues (2100403) report the terpolymerization of carbon dioxide with cyclohexene oxide and terminal epoxides using simple aluminum porphyrin complexes to facilitate a chemical modification of the highly brittle poly(cyclohexene carbonate). As aforementioned, alternative feedstocks are required to ensure the sustainable production of polymers and functional materials. Excess waste sulfur generated by the petroleum industry is in fact a widely available and underused building block. Nevertheless, there has been a resurgence in using it as a starting material for polymers and materials. In an exemplary manner, Justin M. Chalker et al. have employed elemental sulfur as a main component for the synthesis of polysulfide terpolymer in the presence of canola oil and dicyclopentadiene via inverse vulcanization (2100333). Due to the unique S-S crosslinks formed in this process, the polysulfide polymers, as bulk structural materials, can react at their interfaces and covalently bond together, and appear as promising materials for the construction industry. While most of the abovementioned renewable-based polymers often feature oxygenated linkages and repeat units (e.g. ester, carbonate, urethane etc.), renewable polyacrylates which are built around carbon–carbon bonds were also reviewed by Fiona L. Hatton, Gerard Lligadas et al. (2200005). Environmentally benign and green synthetic reaction protocols in addition to technologies are the foremost objectives in the present scenario to ensure sustainability. Accordingly, multicomponent reactions (MCRs) are aligning substantially with green chemistry principles due to their high atom economy, new bond-forming efficacy, inexpensive, easy separation and purification of products, alongside minimal waste generation. Inspired by this, the team of C. Remzi Becer (2100408) applied the Ugi 4CR polymerizations of the commercially available biobased diacid, diamine, and aldehydes in the presence of isocyanides to give a series of N-substituted polyamides with high biomass content and tunable physical properties. In similar manner, light induced polymerization reactions have been recognized as a tool to progress green and sustainable chemistry. Accordingly, Pinar Cakir Hatir and coworkers (2200002) utilized photopolymerization as a green and economical fabrication process to deliver environmentally friendly hydrogels which are based on oleyl alcohol (OA) derivatives. The OA-based hydrogels have been postulated to be suitable biomaterials for tissue engineering and biomedical engineering applications. Harnessing the advantages of photopolymerization in a sustainable manner, Kei Saito and his colleagues (2100493), have prepared highly cross-linked and well-defined polymer microparticles with good thermal stability. The strategy involved the use of a 4-arm coumarin based monomer, expected to yield highly cross-linked microspheres via [2+2] cycloadditions. The results demonstrated the successful synthesis of cross-linked microspheres, and it was found that the particle size can be readily tuned by varying the reaction parameters. Photoinitiators (PIs) are crucial components of the photopolymerization process, determining the photopolymerization efficiency and various properties of the final product. Moreover, an important criterion for a photopolymerization process to qualify as environmentally friendly is visible-light activation. Accordingly, Duygu Avci and her team (2100450) have designed a new water soluble, visible-light activated, low migration, polymerizable, possibly nontoxic photoinitiator based on a poly(β-amino ester) derivative, which in turn was prepared from commercially available poly(ethylene glycol) diacrylate. Importantly, via differential scanning photo-calorimetry (photo-DSC), it was shown that the synthesized PIs reveal faster photopolymerization kinetics compared to the commercial photoinitiator thioxanthone, thus having the potential to be attractive photoinitiators for use in various UV curing applications. Historically, electrospinning was based on the use of hazardous solvents that harm the environment, and hence make the broad use of the process unfeasible. Nevertheless, Mustafa M. Demir and his team (2100438) detail a “fine-tuned” electrospinning method that mitigates the environmental and hazardous material disposal risks of traditional electrospinning while satisfying health and safety protocols. Last but not least, the transformation of the obtained graft copolymers to electrospun nanofibers facilitated their use as support materials for antibacterial surfaces. While a substantial body of research has been devoted to establishing structure–property relationships to develop new sustainable polymers, an emerging area of investigation is to jointly establish structure–toxicity relationships of microplastics, e.g., for polystyrene-based microplastics (George Vamvounis, 2100485, Chor Y. Tay, 2100454). Finally, Hatice Mutlu and Leonie Barner discussed the importance of and difference between green chemistry, sustainable chemistry, and circular chemistry as indispensable concepts for a sustainable future of polymer chemistry (2200111). In summary, we can clearly see that a key aspect to foster sustainability in the long term requires the collaborative effort of a diverse group of scientists on the fundamental questions addressing not only green chemistry but in general the concept of circular (polymer) economy/chemistry. Hence, we cordially thank the authors for their important contributions to this themed collection, and hope that this special issue will be an inspiration for researchers to further innovate within this stimulating and impactful area of polymer science. Thus, we, as guest editors, are grateful to all authors for sharing their research and insights in the special issue of Sustainability. The authors declare no conflict of interest. Hatice Mutlu studied chemistry at Marmara University and Bogaziçi University (Turkey). Subsequently, she obtained her Ph.D. from the Karlsruhe Institute of Technology (KIT) in the group of M. A. R. Meier. She is currently working as a senior researcher and acting as the Deputy Director of the Institute of Biological Interfaces 3 (IBG3) at KIT. She was selected as one of the 2020 Emerging Investigators in Polymer Chemistry by Polymer Chemistry, RSC. She is also a recent member of the International Advisory Board of Macromolecular Chemistry and Physics. Her research interests focus on the development of new polymer-forming reactions and novel conjugation chemistries with a particular focus on the design of novel sustainable sulfur-based polymeric materials. Leonie Barner is the inaugural director of the Centre for a Waste-Free World at the Queensland University of Technology (QUT, Brisbane, Australia). She received her Ph.D. in physical chemistry in 1998 (Georg-August-Universität, Göttingen, Germany). During her career, she has worked in industry, German research institutes (Fraunhofer, Helmholtz), and Australian universities (University of New South Wales, QUT). Besides her research interest in macromolecular synthesis and characterization, she leads a transdisciplinary research center that follows a holistic approach to develop technologies and processes to reduce waste, while using social science knowledge to catalyze change and reduce barriers to participation and adoption.