Polymers for a Sustainable Future.

Abstract

Over the last century, polymers are everywhere we look, have transformed our daily lives, and gained global acceptance. From the clothes we wear, to the utensils used by scientists around the world in labs, the current rate of activities and standard of living couldn't be maintained without polymers and plastics in all shapes or forms. The majority of the polymers we consume everyday are still obtained from fossil fuels and the rate, but also efficacy, with which these polymers are produced in terms of low waste production and low energy consumption is truly astonishing and highlights the power of continuous refinement and improvement of these extraction and fractionation processes. In addition to other factors, this efficiency, which translates into low costs, is still a high hurdle for bio-based, (bio)degradable, or more sustainable polymer alternatives to conquer the market. Nevertheless, a multitude of start-ups and several pilot and full-sized plants have been built solely dedicated to the production of bio-based and bio-degradable polymers on an industrial scale. These steps toward more sustainable polymers have been underpinned by governments or governing bodies emphasising the need for a sustainable future. Whether these are the sustainable development goals of the UN,[1] the pledge for carbon neutrality within the next 20 years, as stated by China or New Zealand, amongst others, or the action plan toward a circular economy as put forward by the European Union,[2] all have provided guidelines along with tight restrictions as to what future economies and communities will look like. In addition to these top-down regulations, the changing perception of plastics within the public provides a customer-based incentive for companies to provide sustainable solutions to the general public. All of these have heavily impacted the direction of polymer science over the last ten years with a five-fold increase in papers with the concepts of “polymer” and “sustainability”. However, care must be taken as to what really constitutes a sustainable polymer. “Sustainability” is not an absolute or hard description but rather exists on a continuum describing the source of starting materials, the process, and end-of-life issues, etc. Importantly, it is always described as a comparison to other processes, whether established or on a lab scale, and must in general be viewed through the lens of life cycle analysis. Recent publications give insights into how this can be achieved for polymers.[3-5] While the initial focus of the field was on obtaining renewable and sustainable building blocks in an efficient manner using catalyzed synthetic routes, the field has broadened to constructing more complex materials systems from these renewable/sustainable building blocks, controlling their architecture more closely, as well as looking at the whole life cycle of the polymeric material. This special issue in Macromolecular Rapid Communications tries to give a glimpse of what “polymers for a sustainable future” could look like and shows that this systemic change is being addressed by leading experts all over the world. The majority of publications within this special issue look at the production of polymeric materials using natural resources such as lignin, cellulose, and chitin. These poorly soluble, abundant building blocks provided by nature itself, are still difficult to process and the transformation into monomers and polymers is still a real challenge. Based on cellulose, Yang, Heinze, Wang, and co-workers created paper-based wearable electronics with high adhesion, reflectivity, and conductivity which are comparable to fossil-fuel based alternatives (2000499). The communication by Zhu, Shi, and co-workers investigates the use of chitosan and agarose in hydrogels to trigger actuation and shape deformation by water and/or heat which could provide sustainable actuators for soft robotics applications (2000342). Further advanced applications are explored by Lu, Hallinan Jr., Chung, and co-workers who use up to 35 wt% lignin in their solid polymer electrolyte materials (2000428) while Baroncini, Stanzione III, and co-workers compare soft- and hardwood lignin for lithium-ion battery applications (2000477). Cellulose, the most abundant biopolymer in the world, features as both a filler for nanocomposites in the work by Pakdel, Dubé, and co-workers (2000448), and as a polymer which can be modified with other polymers using a grafting-from approach, as in the work of Kelley, Gramlich, and co-workers (2000531), and by using small fluorescent molecules for biosensing applications, as demonstrated in the work of Wang, Zhou, and co-workers (2000497). A fundamental insight into the oxidative TEMPO-functionalisation is given by Jiang, Fan, and co-workers (2000501). Cheng, Zhang, Zhang, and co-workers use imidazolium-based ionic liquids to modify cellulose acetate to make membranes. These membranes show a drastically increased CO2 permeability and are promising for gas separation applications (2000494). Another naturally abundant resource, namely chitin, is transformed into microspheres by a freeze-drying method and the method's effect on the microsphere morphology is investigated by Ying, Zhang, and co-workers (2000502). Silk fibroin is another natural material which features in this special issue. In the contribution by Xiao, Pei, Ling, and co-workers, the fundamental understanding of the self-assembly process of these silk nanofibrils is investigated and provides guidance for the future development of silk-based materials (2000435). The advances in fractionation and catalytic valorisation of lignin from biomass is featured in the review by Al-Naji, Antonietti, and co-workers (2000485) while Ma, Chen, Nang and co-workers look at this bioresource and how it can be transformed into poly(urethanes) (2000492). At the same time, Laprise, Kerton, Kozak, and co-workers describe a three-step synthesis method for preparing non-isocyanate polyurethanes (NIPUs) from fish waste, which is a resource largely untapped to date in terms of chemical potential (2000339). Overall NIPUs feature strongly in this issue with an alternative approach toward fully renewable NIPUs being presented by Filippi and Meier by attaching lignin and plant oil-based derivates through thiol-ene click chemistry (2000440). NIPU-based hydrogels also appear in the communication by Bourguignon, Detrembleur, and co-workers (2000482) while the promise of NIPUs for adhesive applications is highlighted by Gomez-Lopez, Sardon, and co-workers (2000538). Akin to the highly controlled orientation of amino acids in proteins or other organic molecules, the control of the syndiotacticity of a polymer can oftentimes lead to a material with improved mechanical, or thermal properties compared to their atactic analogue. In this special issue, a highly isoselective lactide polymerisation is reported by Wan, He, Zhang and co-workers (2000491) while Magliozzi, Grau, Cramail, and co-workers highlight the effect of tacticity on properties of poly(hydroxyurethanes) (2000533). Bexis, Coulombier, Dove, and co-workers report on the ring-opening polymerisation of a propargyl-functionalised cyclic carbonate to yield a versatile system for telechelic and block copolymer synthesis (2000378). A thermally stable poly(ethylene terephthalate-co-lactic acid) copolymer is presented by Zhou, Xiang, Zhu, and co-workers for melt-spinning into fibres (2000498). In another communication, a pathway towards poly(thiocarbonates) is presented by Yang, Zhang, and co-workers using Lewis acid/base pairs starting from carbon disulfide and ethylene oxide, two low-cost bulk chemicals (2000472). A water degradable carbon fiber reinforced epoxy vitrimer with high tensile strengths is described by Liu, Zhang, and co-workers (2000458). Further reviews in the special issue include the recent advances in biobased acrylates and discuss the production of these with respect to commercial implementation by Fouilloux and Thomas (2000530). Another overview by Wang and Tao looks at the progress in catalyst developments for O-carboxyanhydrides polymerisations with a special emphasis on organocatalysts (2000535). Within the above-mentioned Circular Economy, the end-of-life of polymers is gaining more importance and constitutes a highly interdisciplinary and complex field. Indeed, end-of-life issues have global implications as plastic debris continues to accumulate in freshwater and marine environments. Schyns and Shaver outline the main ways of recycling common polymers found in real life waste streams and highlight avenues to upcycle waste polymers through mechanical recycling which normally lead to lower quality, low mechanical property materials (2000415). Recyclability of photoresins is addressed in the review by Voet, Loos, and co-workers with the focus on next generation 3D-printing photo resins which are biodegradable, bio-based, or recyclable (2000475). In summary, the trajectory of sustainable polymers has moved from using renewable materials to contemplating “sustainability” in a broader sense as described by the notion of a Circular Economy. From our perspective, the 12 Principles of Green Chemistry[6] serve as an initial guide to achieve sustainable polymers. While this qualitative approach was very beneficial, insight provided by life cycle analysis, the E-factor,[7] and other quantitative metrics, indicates the concept of “sustainability” exists on a continuum and requires attention to a number of facets. Only through a holistic approach, from cradle to grave, can concrete and real sustainable solutions be obtained, pushing polymers into a new era. Philip B. V. Scholten studied chemistry at the University of Warwick, UK, and obtained his master's degree in 2015 under the supervision of Prof. Andrew P. Dove. He then undertook a Ph.D. under the supervision of Dr. Christophe Detrembleur (Université de Liege, Belgium) and Prof. Michael A. R. Meier (Karlsruhe Institute of Technology, Germany) where he investigated the synthesis of novel copolymers based on renewable resources and ethylene using reversible deactivation radical polymerization. He then joined the Adolphe Merkle Institute (Fribourg, Switzerland) in 2019 as a post-doctoral researcher and since June 2020 he has been working on his Marie Curie Individual Fellowship “DECOMPOSE.” His research interests include renewable and sustainable polymer chemistries, stimuli-responsive, and bio-inspired material systems. Jie Cai is a full professor at Wuhan University. He received his B.S degree (2001) and Ph.D. degree (2006) from Wuhan University (Prof. Lina Zhang's group). He then worked as a JSPS postdoctoral fellow in Prof. Shigenori Kuga's group at the University of Tokyo. He was appointed as an associate professor (2009) and professor (2012) in the College of Chemistry and Molecular Sciences (CCMS) at Wuhan University. His research focus is on biomacromolecules to understand structure-function relationships, with particular emphasis on studies related to “green” solvents for cellulose, chitin and chitosan, and self-assembly, biomaterials and tissue engineering. Robert T. Mathers investigates sustainability issues related to degradation of plastics in the oceans, synthesis with renewable components, and predicting polymer hydrophobicity with partition coefficients (LogP) to guide synthetic efforts, optimize physical properties, and reduce waste. Rob obtained a Ph.D. in polymer science at The University of Akron (2002) in the field of anionic polymerization with Prof. Roderic Quirk. After two years of postdoctoral research at Cornell University with Prof. Geoffrey Coates, he joined Pennsylvania State University. Presently, Rob is professor of chemistry at the New Kensington campus. To expand his background in polymer science and sustainability issues, Rob has taken a sabbatical at Carnegie Mellon University (2011-2012) with Prof. Krzysztof Matyjaszewski as well as a more recent European sabbatical tour in 2019 that included visits with Prof. Andrew Dove at University of Birmingham, Prof. Christophe Thomas at Chimie ParisTech and Prof. Michael Meier at Karlsruhe Institute of Technology (KIT).