Introduction to the themed collection on sustainable polymers

Following the Second World War, the world witnessed the advent of the synthetic polymer industry and polymers have since shaped the modern world in many ways. Today, the annual global production of plastics and polymeric materials comes to nearly 370 million tonnes. Under the form of plastic materials, polymers insulate our homes and bring clean water to our cities, they keep our food fresh, make our vehicles lighter and safer, and are essential components of our mobile phones and computers. Plastics have also revolutionised medicine and the health sector, from blood bags, catheters and implants, to facemasks and contact lenses. In addition, polymers are found in millions of consumer and industrial products in liquid polymer formulations, from the paints on our walls to the shampoo and detergents in our cupboards.

As advantageous as polymers are in everyday life, we have recognized over the last decades that their traditional use is unsustainable. Plastic materials are typically made from fossil feedstocks, and – as a flipside of their durability – are hardly degradable. Hence, we tap into a resource that, for many reasons, will not be available anymore in the future, and we create increasing amounts of waste. This waste can often in the best case only be burned, in the worst it will pollute the environment and threaten the living spaces of animals or humans. Through littering or landfill disposal, discarded plastics pollute the natural world, with plastics of all sizes, down to the nanoscale, being detected in many ecosystems. While recycling has been an ongoing endeavour for a long time already, still no breakthrough has been reached that could fundamentally change the way we treat plastic materials.

It is also safe to say that in the future polymers will continue to be essential. Avoidance of their use can only be a limited measure. However, we can also use them in order to create a more sustainable society. Many emerging technologies aiming at reducing our reliance on fossil fuels and our CO₂ emissions indeed require plastic materials. For example, using light-weight plastics in electric vehicles is greatly beneficial to their autonomy, while batteries themselves rely on housings made of plastics and may apply polymers as electrolytes and other active components. At the renewable power generation stage, wind turbine blades also require plastic composites and adhesives which are resistant to the elements and are long-lasting.

Despite these benefits, using plastics and polymers on such a large global scale is posing environmental challenges and, as described above, the solution can only be to reduce the reliance on fossil feedstocks, and by providing better recycling and degradation pathways. Over the past few decades, several initiatives have been launched across the globe to make polymers and the entire polymer industry sustainable and fit for the future. Changing the status quo necessitates a major collaboration between sciences and engineering, materials and product design, manufacturing and supply chains, humanities and policy, as well as economics and businesses.

Sustainability across the entire life cycle of polymers, including its manufacturing, must be a core design feature of the polymers of the future. Chemistry can help in many ways, by developing efficient ways to recycle the polymers we use today, as well as by creating replacements which are made from sustainable starting materials in an energy-efficient way, which have multiple end-of-life options, and which have reduced environmental impact.

Despite tremendous efforts by the academic and industrial community, more research is still urgently needed. This themed collection brings together contributions from leading researchers in this field, offering a snapshot of the current scientific landscape in introducing sustainable solutions into polymer chemistry. Sustainability can be improved in many different aspects of the whole process, from the feedstocks of monomers used, to process development over biodegradability to reduced energy consumption in synthesis and processing. It is difficult to give a complete, comprehensive overview for all areas, yet we are delighted to present a good mix of articles addressing the various fields of action.

Research around sustainable polymers is a multi-faceted topic, which is demonstrated by the breadth of original investigations and reviews featured in this collection. Bio-based acrylates are reviewed (DOI: 10.1039/D0PY01222J), as well as degradable polyesters (DOI: 10.1039/D1PY00014D and DOI: 10.1039/D0PY01226B), renewable BPA alternatives (DOI: 10.1039/D1PY00909E) and lignin feedstocks for polymer synthesis (DOI: 10.1039/D1PY00694K).

Renewable polymers

One aspect which is heavily represented in this collection is the development of polymers from renewable resources, whether they are based on lignin (DOI: 10.1039/D1PY00694K, DOI: 10.1039/D1PY01017D, DOI: 10.1039/D1PY00818H, DOI: 10.1039/D1PY00851J), renewable acids (DOI: 10.1039/D1PY00754H, DOI: 10.1039/D1PY00561H, DOI: 10.1039/D1PY00415H) and lactones (DOI: 10.1039/D1PY00811K), terpenes (DOI: 10.1039/D1PY00570G, DOI: 10.1039/D1PY00931A, DOI: 10.1039/D1PY00035G), or carbohydrates (DOI: 10.1039/D1PY00784J, DOI: 10.1039/D1PY00753J). Some polymers have been also made using monomers directly derived from carbon dioxide (DOI: 10.1039/D0PY01757D). While these renewable polymers often feature oxygenated linkages and repeat units (e.g. polyester, carbonates, urethanes etc.), the collection also showcases renewable polyolefins and polyacrylates which are built around carbon–carbon bonds.

Greener synthetic methods

Another prominent aspect of research in sustainable polymers involves the development of greener methods for polymer synthesis. This themed collection includes reports of energy-efficient photochemical reactions (DOI: 10.1039/D1PY00734C, DOI: 10.1039/D1PY01060C) and aqueous emulsion-type polymerisation (as well as the use of supercritical CO₂ as a clean reaction medium for polymer synthesis (DOI: 10.1039/D1PY00415H)). Catalysis remains one of the main tools that chemists have in their toolbox to make polymerisation reactions more energy efficient (DOI: 10.1039/D1PY01340H), tolerant to impurities which are difficult to remove (DOI: 10.1039/D1PY00931A), and more selective, including to avoid the use of protecting group strategies (DOI: 10.1039/D1PY00014D), or to control the tacticity of polymers so that desired material properties can be achieved (DOI: 10.1039/D1PY00669J). Advances in chemo-enzymatic cascade catalysis are key to synthesise novel monomers from renewable feedstocks (DOI: 10.1039/D0PY01471K).

Improving properties

Obtaining materials with adequate thermomechanical properties is at the core of sustainable polymers research. Several strategies are being investigated to control and improve the properties of polymers (including those from renewable feedstocks). In particular, covalent adaptable networks can afford reprocessable crosslinked materials (DOI: 10.1039/D1PY00754H, DOI: 10.1039/D1PY00811K). Post-polymerisation modification is also another strategy (DOI: 10.1039/D1PY00879J), including to improve the thermal properties of polymers while retaining other intrinsic features, like degradability (DOI: 10.1039/D1PY00656H).

Degradability, chemical recycling and toxicology impact and design rules

The end of life of polymers is finally at the centre of many studies from this themed collection. Current research actively seeks to maximise the number of closed loop life cycle of polymers, for example by developing materials with controlled degradability and biodegradability, which, when recovery or mechanical recycling/reprocessing is not possible, can be turned back into their original monomers, broken down into smaller building blocks or upcycled into other useful products (DOI: 10.1039/D0PY01519A). Using these approaches, molecules are effectively kept in play as long as possible, and this can propose a new end of life for materials such as thermosets, which traditionally could not be recycled or degraded (DOI: 10.1039/D0PY01226B).

Finally, while a lot of research has been focusing on establishing structure–property relationships to develop new performant sustainable polymers, an emerging area of investigation is to jointly establish structure–toxicity relationships (DOI: 10.1039/D1PY00909E). Only then will safe, functional and renewable alternatives to less sustainable polymers be within our reach.

The present themed collection was conceived to draw attention towards new trends and encourage future discoveries to make polymers more sustainable. We thank the authors for their important contributions and hope that this collection will be an inspiration for researchers to further innovate within this stimulating and impactful area of polymer science.

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