The Looming Problem of Lithium-Ion Battery Waste

By Eloi Grignon, Ph.D. student, Member-at-Large for the GCI

Since their commercialization in 1991, lithium-ion batteries (LIBs) have gradually come to pervade our daily lives. Their ubiquity is achieved through our phones and laptops (you are likely reading these words via energy supplied by a LIB), where they are used to power not only our communication with one another, but also the myriad other tasks that we have come to delegate to our devices. Increasingly, LIBs are powering how we move, too, as is evidenced by the several million battery electric vehicles already on the road.1 With the production of electric vehicles set to skyrocket – the British and French governments have already pledged to ban sales of fossil-powered vehicles by 2040 – and the possibility of using LIBs for storage of grid electricity, it is clear that LIBs are not going anywhere, either.2 Indeed, spent batteries are expected to be generated at a rate of 2 million metric tons per year by 2030.3

And yet, there is no clear idea of what is to happen to these batteries once they’ve served their purpose. Currently, fewer than 5% of LIBs in the US and Europe are recycled while the rest end up in landfills.3

Since a LIB is densely comprised of several costly metals (Figure 1), it is fair to liken used batteries to enriched ore.3 It follows, then, that complete disposal of millions of metric tons of such a material represents a tremendous waste.

Figure 1.

Figure 1. Breakdown of LIB constituents. From Ref [3].

Recycling could curb the waste by salvaging this ‘ore’ and supplying it to LIB manufacturers at a cheaper price than that of virgin materials, thereby reducing LIB cost. Moreover, less of the material would have to be mined and treated in the first place. This is especially important when considering the impacts of both processes: mining has obvious environmental consequences while ore treatment is typically energy-intensive and can release harmful gases such as SOx.4 Furthermore, 10-25% of global cobalt production is mined by ‘artisanal’ workers in the DRC, many of whom lack proper wages and equipment. The UNICEF estimated in 2012 that 40,000 children were employed in such mines.5 From this perspective, LIB landfilling has a heavy economic, environmental, and moral opportunity cost attached.

In addition to wastefulness, landfilling LIBs also has direct negative consequences. Over time, the toxic constituents of the LIBs tend to flow into the soil, eventually leaching into the groundwater and accumulating in various organisms. These toxins can make their way up to humans, thus extending the health hazard to people. The harm imposed by discarded LIBs on the environment is not without some degree of irony, as LIBs have long been celebrated as a key cog in the establishment of a greener future. Evidently, this detrimental end-of-life scenario presents an incongruity.

Given the benefits of LIB recycling, it is clear that technical and economic barriers, rather than lack of purpose, are responsible for the poor recycling rates.

One such barrier results from the complicated composition within LIBs, which renders separation and recovery of all components difficult. For instance, smelting can effectively recover the heavy metals nickel, cobalt, and copper but fails to salvage lithium and the electrolyte. While hydrometallurgical (chemical leaching) methods can recover more components, they necessitate acids, hydrogen peroxide, and 7 m3 of water per ton of LIB.6 Needless to say, this is not ideal.

Another technical issue is the great variability between LIBs – different manufacturers tend to use different components and so there is no one universal composition (this pertains mainly to vehicles). As such, a recycling firm is at the mercy of its feedstock – for an input collected from many sources, there is no guarantee that 1 ton of batteries will yield a given amount of, say, cobalt.

These issues appear blatant when considering as a counterexample the success story of lead-acid battery recycling, whose simple and standardized composition – about 60% lead – enables an easy recycling process that claims nearly all (99%!) of used batteries.4

The low recycling rates are also due to economic factors. The end-to-end recycling process is energy-intensive and requires many steps, thus increasing costs. A firm operating such a process must carefully assess whether their repurposing protocol is cheap enough to supply materials that are price-competitive with mined materials. Due to the lack of LIB standardization and high volatility of constituent prices, this assessment is far from trivial and the business represents a clear risk. This risk is further exacerbated by the uncertainty of what the future of energy storage may resemble. In the arms race for higher energy density, new technologies arise frequently, thus threatening to render state-of-the-art materials (and so, recycling processes) obsolete.

While the above paragraphs appear rather pessimistic, it should be noted that we are only at the onset of the LIB boom. Indeed, the field of LIB recycling is still gaining traction and it is expected that serious investments will aid in the development of more efficient recycling techniques. To this end, the US DoE (through the $15 million ReCell Center) and the UK-based ReLib project have pledged to fund and support R&D in LIB recycling.3

There is also a clear interest from the private sector as is evidenced by the numerous startup firms currently designing their own protocols, including the Toronto-based company Li-Cycle.7

Another approach to sustainable energy storage is to circumvent the need for recycling breakthroughs altogether by designing the LIB differently from the start. For instance, the use of organic materials that are easily recyclable is increasingly explored for use in devices.1 Not only are these materials favourable in the end-of-life stage, but their production is also cheap, accessible, and environmentally benign.

In any case, scientists, engineers, and policymakers must come together to address the issues caused by LIB landfilling. And quickly, too, because the storm is coming (Figure 2).

Figure 2. A possible scenario for the growth of electric vehicle sales in the next decade. PLDVs = passenger light duty vehicles; LCVs = light commercial vehicles; BEVs = battery electric vehicles; PHEV = plug-in hybrid electric vehicles. From Ref [1].

References

  1. Poizot, P.; Gaubicher, J.;  Renault, S.;  Dubois, L.;  Liang, Y.; Yao, Y. J. C. R., Opportunities and Challenges for Organic Electrodes in Electrochemical Energy Storage. 2020.
  2. Gardiner, J. J. T. G., The rise of electric cars could leave us with a big battery waste problem. 2017, 10.
  3. It’s time to recycle lithium-ion batteries. C&EN Global Enterprise 2019, 97 (28), 29-32.
  4. Gaines, L. J. S. M.; Technologies, The future of automotive lithium-ion battery recycling: Charting a sustainable course. 2014, 1, 2-7.
  5. Frankel, T. C.; Chavez, M. R.; Ribas, J. J. T. W. P., The cobalt pipeline. Tracing the path from deadly hand-dug mines in Congo to consumers’ phones and laptops. 2016, 30.
  6. Larcher, D.; Tarascon, J.-M. J. N. c., Towards greener and more sustainable batteries for electrical energy storage. 2015, 7 (1), 19-29.
  7. https://li-cycle.com/about-us/.

Electrosynthesis: A Green Methodology for Organic Chemistry

By Dr. Matthew Leech, post-doctoral fellow, University of Greenwich

According to the International Energy Agency (IEA), global energy-related carbon dioxide emissions were estimated to be approximately 33 gigatonnes (Gt).1 It is perhaps unsurprising that the two largest sources of carbon dioxide emissions arise from transportation and the generation of electricity (Figure 1).  However, what is surprising is that the aggregate global emissions of the pharmaceutical sector far outweigh those of the automotive industry – 52 million tonnes vs 46.4 million tonnes in the year 2018.1

Figure 1

Figure 1: World carbon dioxide emissions by sector between 1990 and 2010. Source – https://ourworldindata.org/co2-and-other-greenhouse-gas-emissions

 

This is because a large proportion of chemical syntheses in an industrial setting are conducted under thermochemical conditions, meaning that they must be heated to high temperatures for prolonged periods of time.  These conditions are very energy intensive, and the reagents used are often hazardous and difficult to remove from the final product.  The latter is particularly problematic for redox reactions (reactions involving the addition or removal of electrons), where toxic metal complexes are often used.

In contrast, the electrochemical synthesis of organic molecules, also known as Organic Electrosynthesis, uses the cheapest and most versatile redox agent available on the market, electricity, to perform chemical transformations at room temperature.  Organic electrosynthesis has experienced a resurgence in interest in recent years, owing to the increasing availability of affordable and standardised equipment,2 which has improved experiment reproducibility, and the growing desire by the chemistry community to find milder and greener reaction conditions. A typical modern academic electrosynthesis setup can be seen in Figure 2.

 

Figure 2

Figure 2: Modern organic electrosynthesis equipment, comprising of a combined stirrer/potentiostat, cell lid, 10 mL glass cell, and two carbon graphite electrodes.

 

Aside from saving energy by switching from thermochemical to electrochemical synthesis, electrosynthesis is often safer than traditional methods, as it reduces or eliminates the need for toxic solvents and catalysts.  In 2019 it was reported that substituted lactones, which are known for their fungicidal, antibiotic, and anticancer properties, can be synthesised using photochemistry (Scheme 1).3 However, the reported procedure necessitates the use of a potentially toxic nickel catalyst, in addition to a solvent mixture comprising of benzene and 1,4-dioxane, both of which are suspected carcinogens.  Fortunately, a complimentary electrochemical method was reported in the same year which not only removed the need for the nickel catalyst, but also uses methanol as a solvent, which is less toxic and is considered to be greener by comparison (Scheme 1).4

Scheme 1

Scheme 1: Synthesis of substituted γ-butyrolactones via photochemical (top) and electrochemical (bottom) methods.

 

Photochemistry also necessitates the use of a photocatalyst, a compound which can convert absorbed light into useable electrons.  These are normally based upon third-row transition metals such as iridium, which makes them very expensive (a typical photocatalyst costs between £40 – 60 for 0.1 g), which usually limits the uptake of photochemical methods within industry.  In contrast, electricity is cheap, with 1 mole of electrons (6.02×1023 electrons) costing around £0.83.4

 

Furthermore, through electrosynthesis, it is possible to avoid the use of reactive and often highly toxic reagents by generating them in-situ.  For example, methoxymethyl (MOM) ethers, which are used as protecting groups in organic chemistry, are normally synthesised using chloromethyl methyl ether (CMME) which, aside from being flammable and toxic, is known to cause cancer.  However, it is now possible to synthesise these ethers using electrosynthesis at room temperature, using a very low current, inexpensive graphite electrodes,  and greener solvents (Scheme 2).5,6

Scheme 2

Scheme 2: Electrochemical synthesis of methoxymethyl ethers from carboxylic acid derivatives in methanol at room temperature using graphite electrodes.

 

But organic electrosynthesis is not limited to university laboratories, as electrosynthetic methodologies can often be scaled-up to be used in industrial laboratories with relative ease.  Perhaps the most well-known example of industrial scale electrosynthesis is the electrohydrodimerisation of acrylonitrile into adiponitrile by Monsanto, which is a key intermediate in the manufacturing of nylon (Scheme 3).7–9  Under aqueous conditions, it has been possible to synthesise approximately 300 000 tons of adiponitrile worldwide, with the sole by-product being oxygen, arising from the oxidation of water.

Scheme 3

Scheme 3: Industrial electrochemical synthesis of adiponitrile from two acrylonitrile molecules using cadmium and stainless-steel electrodes under aqueous conditions.

 

In summary, organic electrosynthesis represents a modern, greener, more economical, and safer alternative to traditional chemical syntheses.  With the advent of new and more user-friendly equipment, the field has experienced a resurgence in interest, with many new methodologies being developed in recent years.  Through the use of electrosynthesis, the challenge of making the pharmaceutical sector greener is starting to look much more achievable.

References

 

1            L. Belkhir and A. Elmeligi, Carbon footprint of the global pharmaceutical industry and relative impact of its major players, J. Clean. Prod., 2019, 214, 185–194.

2            M. Yan, Y. Kawamata and P. S. Baran, Synthetic Organic Electrochemistry: Calling All Engineers, Angew. Chem. Int. Ed., 2018, 57, 4149–4155.

3            L. E. Overman, N. A. Weires and Y. Slutskyy, Facile Preparation of Spirolactones by an Alkoxycarbonyl Radical Cyclization Cross-Coupling Cascade, Angew. Chem. Int. Ed., 2019, 58, 8561–8565.

4            A. Petti, M. C. Leech, A. D. Garcia, I. C. A. Goodall, A. P. Dobbs and K. Lam, Economical, Green and Safe Route Towards Substituted Lactones via the Anodic Generation of Oxycarbonyl Radicals, Angew. Chem. Int. Ed., 2019, 58, 16115–16118.

5            X. Luo, X. Ma, F. Lebreux, I. E. Markó and K. Lam, Electrochemical methoxymethylation of alcohols – A new, green and safe approach for the preparation of MOM ethers and other acetals, Chem. Commun., 2018, 54, 9969–9972.

6            C. G. W. van Melis, M. R. Penny, A. D. Garcia, A. Petti, A. P. Dobbs, S. T. Hilton and K. Lam, Supporting-Electrolyte-Free Electrochemical Methoxymethylation of Alcohols Using a 3D-Printed Electrosynthesis Continuous Flow Cell System, ChemElectroChem, 2019, 6, 4144–4148.

7            D. E. Danly, Development and Commercialization of the Monsanto Electrochemical Adiponitrile Process, J. Electrochem. Soc., 1984, 131, 435C.

8            M. M. Baizer, Electrolytic Reductive Coupling, J. Electrochem. Soc., 1964, 111, 215.

9            M. C. Leech, A. D. Garcia, A. Petti, A. P. Dobbs and K. Lam, Organic electrosynthesis: from academia to industry, React. Chem. Eng., 2020, Advanced Article, DOI: 10.1039/D0RE00064G

 

 

A Quirky Chemistry Break: Solvent-less Polymerization with Ball Milling

By Hyungjun Cho, member-at-large for the GCI

In April of 2019, the International Union of Pure and Applied Chemistry (IUPAC) named reactive extrusion as part of the 10 chemical innovations that could have high impact in society [1]. One of the reactive extrusion technique that is of interest is the use of ball mills instead of solvents to mix reagents. An exciting example of such a reaction is solvent-less polymerization. This method of making polymers in a ball mill has been blossoming in recent years but has not attracted much mainstream attention in the chemical community. This article will present the pivotal publication that re-invigorated interest the field of solvent-less polymerization.

There is the typical list of ingredients required for making polymers: 1) monomer, 2) an initiator and/or catalyst, 3) a constant input of energy, and 4) a solvent. Like in organic chemistry, the solvent is almost always critical to the success of the process. Although some solvent-less polymerization techniques are have known for (such as bulk [2] and solid-state polymerization [3]), these methods are not widely applicable and often requires elevated temperatures, which needs a lot of energy. In contrast, ball milling polymerization is a solvent-free technique that is at its early stages of development, but may avoid these issues.

 

Figure 1

Figure 1. A cartoon diagram of the vibrational ball milling process.

 

Ball milling is a mechanochemical process, where solid chemical samples are added to a grinding jar with solid metal balls (Figure 1). The grinding jar is shaken or spun which results in collisions between the balls and the chemical material. This generates high instant pressure and heat through impact and friction [4].

Although ball mills are conventionally used to break down polymers [5], a handful of reports demonstrated the synthesis of polymers using ball mills [6-10]. In 2014, Ravnsbaek et al. synthesized the polymer poly(p-phenylene vinylene) using a ball mill [5]. This work seems to have reinvigorated the field of ball mill polymerization, and is briefly presented here.

Figure 2

Figure 2. Solvent-less synthesis of poly(2-methoxy- 5-2′-ethylhexyloxy phenylene vinylene) in a ball mill. ACS Macro Lett. 2014, 3 (4), 305–309 [5]. Copyright 2014 American Chemical Society.

Briefly, the solid chemicals bis(chloromethyl)-methoxy-ethylhexyloxy-benzene and potassium tert­-butoxide were added to a zirconium oxide grinding jar, along with a zirconium oxide ball (diameter = 10 mm) (Figure 2). The reaction mixture was shaken horizontally at a frequency of 30 Hz at room temperature to synthesize poly(2-methoxy- 5-2′-ethylhexyloxy phenylene vinylene) (MEH-PPV). The authors found that after ca. 10 minutes of shaking, the polymers grew to its maximum average molecular weight (Mn = 35 kDa) (Figure 3). The size dispersity of the polymer was broad (Đ ~ 4) which is typical of stepwise growth polymerizations. The yield was ca. 60% after 10 minutes of shaking and did not improve with longer shaking time. This was especially remarkable because similar polymerization in solution typically requires several hours and elevated temperature [11-13].

Figure 3

Figure 3. The synthesis and degradation of MEH-PPV with respect to ball milling time Reprinted with permission from Ravnsbæk J. B. and Swager, T. M. ACS Macro Lett. 2014, 3 (4), 305–309 [5]. Copyright 2014 American Chemical Society.

In another set of experiments, Ravnsbaek et al. obtained a pre-made sample of MEH-PPV with a larger Mn (Mn =150 kDa) and exposed it to the same grinding conditions used for their synthesis of MEH-PPV. The authors found that the Mn of the larger polymer sample decreased over time to 35 kDa (Figure 3), which suggested that there exists a maximum size of polymer that can be synthesized using a ball mill.

Since this report by Ravnsbaek et al., other research groups have been using ball mills for solvent-less polymerizations. The Borchardt group [14-16], the Kim group [17-18], and the Song group [19] published their findings within the last 4 years. These reports collectively demonstrate many novel ball mill polymerizations, including co-polymerization (more than one monomer) [15-16,19], metal catalyzed polymerization (palladium-catalyzed polymerization) [15-16], and the synthesis of non-conjugated polymers (i.e. poly(lactic acid) and polyurethanes) [17-19].

In 2019, Christensen et al. published an article in Nature Chemistry showing their work regarding a new type of polymer called the vitrimer [20], which was synthesized as the product of the reaction of ketones and amines in a ball mill. While the focus of this article was on the chemistry of the polymer, it is noteworthy that the ball mill polymerization has been highlighted in a high impact journal. This will likely encourage more polymer research groups to look into this unconventional technique as it has a lot of room for growth and improvement, and even more potential to reduce the use of solvent in polymer production.

References

  1. Gomollón-Bel, F. Ten Chemical Innovations That Will Change Our World: IUPAC Identifies Emerging Technologies in Chemistry with Potential to Make Our Planet More Sustainable. Chemistry International 2019, 41 (2), 12–17.
  2. Han, X.; Fan, J.; He, J.; Xu, J.; Fan, D.; Yang, Y. Direct Observation of the RAFT Polymerization Process by Chromatography. Macromolecules 2007, 40 (15), 5618–5624.
  3. Duh, B. Effects of Crystallinity on Solid-State Polymerization of Poly(Ethylene Terephthalate). Journal of Applied Polymer Science 2006, 102 (1), 623–632.
  4. Janot, R.; Guérard, D. Ball-Milling in Liquid Media: Applications to the Preparation of Anodic Materials for Lithium-Ion Batteries. Progress in Materials Science 2005, 50 (1), 1–92.
  5. Ravnsbæk, J. B.; Swager, T. M. Mechanochemical Synthesis of Poly(Phenylene Vinylenes). ACS Macro Lett. 2014, 3 (4), 305–309.
  6. Huang, J.; Moore, J. A.; Acquaye, J. H.; Kaner, R. B. Mechanochemical Route to the Conducting Polymer Polyaniline. Macromolecules 2005, 38 (2), 317–321.
  7. Murakami, S.; Tabata, M.; Sohma, J.; Hatano, M. Mechanochemical Polymerization of Acetylene. Journal of Applied Polymer Science 1984, 29 (11), 3445–3455.
  8. Posudievsky, O. Yu.; Goncharuk, O. A.; Barillé, R.; Pokhodenko, V. D. Structure–Property Relationship in Mechanochemically Prepared Polyaniline. Synthetic Metals 2010, 160 (5), 462–467.
  9. Posudievsky, O. Yu.; Goncharuk, O. A.; Pokhodenko, V. D. Mechanochemical Preparation of Conducting Polymers and Oligomers. Synthetic Metals 2010, 160 (1), 47–51.
  10. Thorwirth, R.; Stolle, A.; Ondruschka, B.; Wild, A.; Schubert, U. S. Fast, Ligand- and Solvent-Free Copper-Catalyzed Click Reactions in a Ball Mill. Commun. 2011, 47 (15), 4370–4372.
  11. Jin, S.-H.; Kang, S.-Y.; Yeom, I.-S.; Kim, J. Y.; Park, S. H.; Lee, K.; Gal, Y.-S.; Cho, H.-N. Color-Tunable Electroluminescent Polymers by Substitutents on the Poly(p-Phenylenevinylene) Derivatives for Light-Emitting Diodes. Mater. 2002, 14 (12), 5090–5097.
  12. Liu, B.; Lü, X.; Wang, C.; Tong, C.; He, Y.; Lü, C. White Light Emission Transparent Polymer Nanocomposites with Novel Poly(p-Phenylene Vinylene) Derivatives and Surface Functionalized CdSe/ZnS NCs. Dyes and Pigments 2013, 99 (1), 192–200.
  13. Schwalm, T.; Wiesecke, J.; Immel, S.; Rehahn, M. The Gilch Synthesis of Poly(p-Phenylene Vinylenes): Mechanistic Knowledge in the Service of Advanced Materials. Macromolecular Rapid Communications 2009, 30 (15), 1295–1322.
  14. Grätz, S.; Borchardt, L. Mechanochemical Polymerization – Controlling a Polycondensation Reaction between a Diamine and a Dialdehyde in a Ball Mill. RSC Adv. 2016, 6 (69), 64799–64802.
  15. Grätz, S.; Wolfrum, B.; Borchardt, L. Mechanochemical Suzuki Polycondensation – from Linear to Hyperbranched Polyphenylenes. Green Chem. 2017, 19 (13), 2973–2979.
  16. Vogt, C. G.; Grätz, S.; Lukin, S.; Halasz, I.; Etter, M.; Evans, J. D.; Borchardt, L. Direct Mechanocatalysis: Palladium as Milling Media and Catalyst in the Mechanochemical Suzuki Polymerization. Angewandte Chemie International Edition 2019, 58 (52), 18942–18947.
  17. Lee, G. S.; Moon, B. R.; Jeong, H.; Shin, J.; Kim, J. G. Mechanochemical Synthesis of Poly(Lactic Acid) Block Copolymers: Overcoming the Miscibility of the Macroinitiator, Monomer and Catalyst under Solvent-Free Conditions. Chem. 2019, 10 (4), 539–545.
  18. Ohn, N.; Shin, J.; Kim, S. S.; Kim, J. G. Mechanochemical Ring-Opening Polymerization of Lactide: Liquid-Assisted Grinding for the Green Synthesis of Poly(Lactic Acid) with High Molecular Weight. ChemSusChem 2017, 10 (18), 3529–3533.
  19. Oh, C.; Choi, E. H.; Choi, E. J.; Premkumar, T.; Song, C. Facile Solid-State Mechanochemical Synthesis of Eco-Friendly Thermoplastic Polyurethanes and Copolymers Using a Biomass-Derived Furan Diol. ACS Sustainable Chem. Eng. 2020, 8 (11), 4400–4406.
  20. Christensen, P. R.; Scheuermann, A. M.; Loeffler, K. E.; Helms, B. A. Closed-Loop Recycling of Plastics Enabled by Dynamic Covalent Diketoenamine Bonds. Chem. 2019, 11 (5), 442–448.

A Greener Future for Chemicals

By Jose Jimenez Santiago, member-at-large for the GCI

Over the last four decades, the development of the chemical industry has transformed the global economy and provided a better quality of life for societies worldwide. However, the mass production and disposal of persistent chemicals has been a threat for the environment and the human health. Understanding the effect of chemicals in the environment is necessary in order to replace current harmful molecules with more benign ones1. Fortunately, chemist have learned from negative past experiences and the next generation of chemicals are expected to cause less toxicity and be less environmentally persistent.

Some chemicals have had a negative impact on wildlife populations, some of those effects appear after several years of exposure and bioaccumulation of the toxic molecules in the environment (Figure 1). The first step to prevent further tragedies in wildlife populations is to know the environmental impact of every chemical that is released into the market. However, in places where this data is available, like the United States and Europe, the percentage of chemicals with known environmental information is relatively small. There are around 75, 000 to140, 000 chemicals on the market, out of those, empirical data on persistence is available for only 0.2%, bioconcentration data for only 1% and aquatic toxicity for 11%2,3. Without this information, how can we make an accurate risk assessment of those chemicals? Moreover, molecules with high persistence in the environment may show a negative impact after many years of accumulation. It is important to keep in mind that that for most molecules on the market, we do not know the extent of their negative effects on the environment in the short term and long term2,4.

Jose blog picture

Figure 1. Examples of wildlife scenarios where chemicals have had or are having population effects 1,6,7,8.

 

 

It is hard to shift to a greener chemical industry that prioritizes the environment due to the cost involved in testing the toxicity of molecules. However, there have been advances from the government and chemistry community towards that goal:

  1. More regulation of chemicals in the environment.

Previously, chemical regulations were targeting the emission of a limited number of pollutants into the environment. New regulations, such as Registration, Evaluation, Authorization and Restriction of Chemicals (REACH2006) in the European Union, are looking to ensure that new chemicals entering the market will conform to minimum human safety and environmental standards5. Thus, manufacturers are responsible for evaluating the impact of new compounds being introduced into the market.

  1. Analytical development and computer models

Many analytical methods have achieved low limits of detection (LOD). It is now possible to identify all the molecules present in a sample. For example, these methods have been used to analyze urban runoff water and identify unusual pollutants4. With the help of these new, sensitive methods approach it is possible to investigate pollution incidents and identify the industrial location responsible for those incidents.

One major ethical concern when running toxicity tests is the large number of animals needed2. In addition, the many thousands of chemicals yet to be tested make this approach unreasonable. Recently, computer models were used to predict which chemicals will be of greatest concern; namely chemicals which are persistent, bioaccumulative and toxic (PBT). In this survey, out of 95,000 chemicals, only 3-5% were likely to be PBT4. These tolls can make feasible the lab test of the most hazardous chemicals and will reduce the ethical concerns.

  1. Better wastewater treatment

The incorporation of a secondary biological treatment into wastewater has considerable benefits for the water quality and chemicals reduction. The Activated Sludge Process is the most common method and has been applied in cities all around the world. For example, in China, 81% of the water distributed to the urban population undergoes the Activated Sludge Process9. This shows how we can find cost effective ways to remove chemicals that have accumulated in the environment for many years.

The amount of chemicals used worldwide, their production, diversity, and incorporation has never been greater1. As chemists, it is important to understand the environmental challenges that we face. In addition, chemists should be aware that chemical problems require chemical solutions. We can be pessimistic about the current status of pollutants, but there are tangible reasons to be optimistic about solutions and methods to reduce the negative impact of our current chemical production. Learning from the past is the starting point to ensure that the next generation of chemicals will have less negative impact on the environment.

 

  • Johnson, A.C., Jin, X., Nakada, N., Sumpter, J. P. Science, 2020, 367, 384-387.
  • Egeghy, P.P., Judson, R., Ganwal, S., Mosher, S., Smith, D., Vail, J., Cohen Hubal, E. A. Total Environ. 2012, 414, 159-166.
  • Judson et al., Environ. Health Perspect. 2009, 117, 685–695.
  • Strempel, M. Scheringer, C. A. Ng, K. Hungerbühler, Environ.Sci. Technol. 2012, 46, 5680–5687.
  • European Commission, Introduction to REACH regulation, 2019; https://ec.europa.eu/environment/chemicals/reach/reach_en.htm
  • P. Desforges et al., Science, 2018, 361, 1373–1376.
  • L. Oaks et al., Nature, 2004, 427, 630–633.
  • D. Sayer et al., Environ. Sci. Technol. 2006, 40, 5269–5275.
  • H. Zhang et al., Environ. Int. 2016, 92–93, 11–22.

 

The Great Step Backwards: Polymer to Monomer

By Hyungjun Cho, member-at-large for the GCI

There is a movement to develop a new type of product life system called ‘the circular economy’ [3]. Part of this movement aims to manufacture products from recycled or raw materials, and after its useful lifetime, re-introduce the product (now considered waste) as recycled material. The motivation for the introduction of the circular economy is to minimize the need for virgin raw material, especially when it originates from non-renewable resources. This effort is being spearheaded by the Ellen MacArthur Foundation with major industry partners like Google, Unilever, Solvay, and Philips, among others [3]. A critical component to the function of the circular economy is developing the capability to turn waste into a desirable product.

There are several methods of recycling all the different types of materials we use in every day life. This blog will discuss a niche in the ‘plastic to monomer’ field. Evidently, in April of 2019, IUPAC named ‘plastic to monomer’ as part of the 10 chemical innovations that could have high impact in society [6]. Before discussing ‘plastic to monomer’, I must clarify the term ‘plastic’. Generally, plastic is made up of many polymer chains that are physically entangled with one another. A macroscopic analogy is when many electrical wires (think of Christmas tree lights) become entangled: the wires are stuck to each other and the rigidity of the ball of wire is greater than the rigidity of a single wire.

Much like the type of wire influences the tangled ball it forms, the chemical structure of the polymer influences the material properties of the plastic. Examples of properties of plastics include rigidity, elasticity, malleability, gas permeability, friction to skin, transparency, and many others. The polymers that are used for commercial plastic products have been studied and developed for decades to be able perform a specific function. For example, polyvinyl chloride and polystyrene were initially discovered in the 1800s [2,8]. Thus, it would be ideal if the currently used polymers can be de-polymerized back into monomers for recycling purposes. This would be a major move by the plastics industry to become environmentally friendly.

The conventional method to turn polymers into monomer is thermal decomposition. Samples of polymer can be heated to high temperature (typically 220-500 °C) to break some of the bonds that hold the monomers together [10]. When this occurs, radicals can form at the site of the broken bond, which can lead to de-polymerization [10]. The required temperature and how much monomer is formed is dependent on the chemical structure of the monomers that are formed. Thermal decomposition to recover monomer is suitable only for a few types of polymers, such as poly(α-methylstyrene), which has ceiling temperature of 66 °C to propagate depolymerization; the monomer recovery after thermal decomposition of poly(α-methylstyrene) is excellent at 95% [11]. However, for polymers like polyethylene (PE, the most produced polymer) and polypropylene (PP, 2nd most produced polymer), the monomer recovery yield is poor (0.025-2%) [11]. In some cases such as polyvinylchloride (PVC, 3rd most produced polymer), thermal decomposition is even more problematic because PVC will release harmful hydrochloric acid and vinylenes upon heating [11]. Thus, the monomer recovery is poor (1 %) and the process is highly corrosive.

Therefore, one of the key challenges to address for ‘polymer to monomer’ is to perform de-polymerization at a low temperature. There are 4 recent publications that explore this challenge [5,7,9,12]. In general, the authors synthesized polymers using reversible-deactivation radical polymerization (RDRP) techniques and explored the de-polymerization reactions they encountered. Below is a brief highlight from the publications from the Haddleton group [9] and the Gramlich group [5].

Picture1

Scheme 1: De-polymerization of RAFT polymers with trithioester end-group [5]. Reproduced from ref. [5] with permission from The Royal Society of Chemistry.

Flanders et al. polymerized methacrylate monomers, including methylmethacrylate (MMA), using reversible addition-fragmentation chain-transfer (RAFT) polymerization with a trithioester chain-transfer agent (CTA) [5]. This type of polymerization places trithioester end-group at end of the polymer chain (Scheme 1). Typically, this end-group is used to re-start the polymerization at the trithioester end of the polymer. However, as we will see, it may have another function. The authors isolated the polymer, then re-dissolved the polymer in 1,4-dioxane at 70 °C (Scheme 1). This caused monomers to be released from the polymer chains at a temperature much less than the ceiling temperature of MMA, which is 227 °C [13]. Analysis of the polymer after partial de-polymerization demonstrated that the trithioester end-group was still attached to the polymer and the size dispersity (range of polymer ‘molecular weight’) was low, which suggested that the de-polymerization was moderated by the trithioester end-group. The authors observed 10-35% de-polymerization after heating at 70 °C for 12-60 hrs.

Picture2

Scheme 2: ATRP of NIPAM in carbonated water, followed by de-polymerization [9]. Reproduced from ref. [9] with permission from The Royal Society of Chemistry.

Lloyd et al. used an alkylbromide initiator, Cu-based catalyst system to polymerize N-isopropylacrylamide (NIPAM) in Highland Spring carbonated water at 0 °C (Scheme 2) [9]. This type of polymerization places a halide at the end of the polymer chain. The authors monitored the monomer conversion into polymer using 1H-NMR spectroscopy. They measured that ca. 99% of the monomer was converted into polymer chains within 10 min. Unexpectedly, in the next 50 min. the authors observed 50% de-polymerization. The authors attempted to optimize de-polymerization conditions by changing the pH, using dry ice in HPLC grade water instead of Highland Spring carbonated water, etc. which led to 34-71% de-polymerization after 0.5-24 hrs. Years later, the same group used a very similar polymerization condition to polymerize NIPAM [1]. This time, non-carbonated water was used as the solvent and they did not report any de-polymerization.

The reports on RDRP followed by de-polymerization highlighted here are not yet ready to make an impact to ‘plastic to monomer’. The authors admit that the mechanism of de-polymerization is unknown. However, these seem to be the first set of reports on de-polymerization occurring at low temperatures. Perhaps these publications could be the birth of the reversible-deactivation radical de-polymerization (RDRDe-P) field. This is especially intriguing because RDRP have already been studied for decades in academia and are being adopted by the polymer industry [4]. Companies like BASF, Solvay, DuPont, L’Oréal, Unilever, 3 M, Arkema, PPG Industries, etc. already claimed patents for technology and products based on RDRP [4]. Somewhat ironically, RDRP was also part of the IUPAC’s 10 chemical innovations for impact on society but not for its potential to recycle polymer [6].

The polymers of the future may not be made from monomers abundantly used today, but the polymers of the future may be degradable through a low energy process.

References

  1.  Alsubaie, F.; Liarou, E.; Nikolaou, V.; Wilson, P.; Haddleton, D. M. Thermoresponsive Viscosity of Polyacrylamide Block Copolymers Synthesised via Aqueous Cu-RDRP. European Polymer Journal 2019, 114, 326–331.
  2. Baumann, E. Ueber Einige Vinylverbindungen. Justus Liebigs Annalen der Chemie 1872, 163 (3), 308–322.
  3. Circular Economy – UK, USA, Europe, Asia & South America – The Ellen MacArthur Foundation https://www.ellenmacarthurfoundation.org/ (accessed Jan 5, 2020).
  4. Destarac, M. Industrial Development of Reversible-Deactivation Radical Polymerization: Is the Induction Period Over? Chem. 2018, 9 (40), 4947–4967.
  5. Flanders, M. J.; Gramlich, W. M. Reversible-Addition Fragmentation Chain Transfer (RAFT) Mediated Depolymerization of Brush Polymers. Chem. 2018, 9 (17), 2328–2335.
  6. Gomollón-Bel, F. Ten Chemical Innovations That Will Change Our World: IUPAC Identifies Emerging Technologies in Chemistry with Potential to Make Our Planet More Sustainable. Chemistry International 2019, 41 (2), 12–17.
  7. Li, L.; Shu, X.; Zhu, J. Low Temperature Depolymerization from a Copper-Based Aqueous Vinyl Polymerization System. Polymer 2012, 53 (22), 5010–5015.
  8. Liebig, J. Justus Liebig’s Annalen Der Chemie. Annalen der Chemie 1832, 1874-1978.
  9. Lloyd, D. J.; Nikolaou, V.; Collins, J.; Waldron, C.; Anastasaki, A.; Bassett, S. P.; Howdle, S. M.; Blanazs, A.; Wilson, P.; Kempe, K.; et al. Controlled Aqueous Polymerization of Acrylamides and Acrylates and “in Situ” Depolymerization in the Presence of Dissolved CO2. Commun. 2016, 52 (39), 6533–6536.
  10. Microwave-Assisted Polymer Synthesis. Springer eBooks 2016
  11. Moldoveanu, Șerban. Analytical Pyrolysis of Synthetic Organic Polymers; Techniques and instrumentation in analytical chemistry; Elsevier: Amsterdam ; Oxford, 2005.
  12. Sano, Y.; Konishi, T.; Sawamoto, M.; Ouchi, M. Controlled Radical Depolymerization of Chlorine-Capped PMMA via Reversible Activation of the Terminal Group by Ruthenium Catalyst. European Polymer Journal 2019, 120, 109181.
  13. SFPE Handbook of Fire Protection Engineering, 5th ed.; Hurley, M. J., Gottuk, D. T., Jr, J. R. H., Harada, K., Kuligowski, E. D., Puchovsky, M., Torero, J. L., Jr, J. M. W., Wieczorek, C. J., Eds.; Springer-Verlag: New York, 2016.