February 28, 2023 by GreenChemUofT

The Polystyrene Problem

By Victor Lotocki, Ph.D. Student in the Seferos Research Group at the University of Toronto

It isn’t a surprise that the world has a plastic problem that impacts human and environmental health, yet we continue to over-rely on the material. In Canada, for example, plastic is found in 95% of manufactured goods, most of which are single-use.¹ Recognizing the issue, in October 2020, Canada’s Minister of Environment and Climate Change announced the government’s plan to achieve zero plastic waste by 2030.² Since only 9% of plastics are recycled in the country, Canada’s Zero Plastic Waste Agenda emphasizes ramping up recycling regulation and technology, and critically, many common single-use plastics including plastic checkout bags, straws, stir sticks, six-pack rings, and cutlery were also promised to be banned. Unfortunately, a recent report from Environmental Defense revealed that the Government of Canada will need to institute new substantial measures to increase the prevention, reuse, and recycling of plastics, as a million tonnes of waste will continue to be generated even in the best-case scenario.³ Due to current difficulties in managing its recycling as well as the dangers it poses to human and environmental health, polystyrene is a particularly relevant target for new recycling strategies and regulatory action.

Polystyrene is the most common waste item found in the South Atlantic, Indian, as well as Pacific oceans, where it accumulates in the Great Pacific Garbage Patch.⁴ More concerning is that of the major plastic pollutants, which also include polyethylene and polyethylene terephthalate, polystyrene is the fastest to degrade into microplastics,⁵ which have been linked to oxidative stress, inflammation, and metabolic disorders in humans.⁶ Styrene monomers and oligomers, which are anticipated carcinogens, in particular, have been found to leech out of polystyrene microplastics.⁷ This issue is also found closer to home. For example, in Lake Ontario, researchers have found an alarming 760 particles per kilogram of sediment sampled, with each of these particles being larger than 2 mm.⁸

Most polystyrene comes in the form of expanded polystyrene foam, which includes Styrofoam, and is notoriously difficult to recycle, with about 10% of it being recycled overall in Canada. What’s more, only 35% of communities within Canada even collect polystyrene, to begin with.⁹ Currently, the only province or territory in Canada in which every locality collects Styrofoam products is British Columbia, while most others vary by region.³ Manitoba and Prince Edward Island don’t collect polystyrene, while Nunavut has no plastic collection program in place at all. One of the challenges in collecting Styrofoam relates to its low density. Since most of its volume is air, it would take up a lot of space that could more efficiently be filled using denser plastics, and it could break apart in transit, contaminating other recyclables. As a result, many regions cannot justify the cost required for collection. Here in Toronto, we even have to pay companies such as to collect our polystyrene waste.⁹

Another issue with polystyrene, along with other common plastics, is in its starting material sourcing. The production of polystyrene is strongly tied to the petroleum industry, as the two key starting reagents, benzene, and ethylene, are generated from it through methods including steam cracking and catalytic reformation.¹⁰ Benzene is alkylated with ethylene in an acid-catalyzed reaction to form ethylbenzene, typically in the liquid phase with AlCl3 or zeolitic catalysts. Then, styrene is produced from ethylbenzene through catalytic dehydrogenation, before finally being polymerized into polystyrene. As over 99% of global ethylbenzene is used in this process, the production of the two is strongly linked.¹¹ Overall, the manufacture of polystyrene is unsustainable from the very beginning of the process as it involves the emission of greenhouse gases and other pollutants (Figure 1). However, polystyrene production will be difficult to ramp down as it’s very cheap and will continue to be economical as long as crude oil and natural gas are in demand.

Figure 1. Synthesis of polystyrene from petroleum industry-sourced benzene and ethylene.

The other major issue associated with expanded polystyrene foam recycling stems from the fact that its lightweight cellular structure cannot be easily reproduced following the more common mechanical recycling methods; therefore, we need to turn to other methods for recycling. Pyrolysis, or thermal degradation, is the most widely studied form of degradation for plastics, and its advantages include being able to handle more contamination than mechanical recycling.¹² Polystyrene pyrolysis is typically conducted at 350–700 °C, and more than 90% of the material by weight is consistently retained.¹⁰ Most of the material yielded by pyrolysis consists of styrene monomers and oligomers, allowing for reuse after repolymerization (Figure 2). The rate-determining step for the degradation of polystyrene through pyrolysis is β-scission on the polymer chain. However, this is mostly true for polystyrene containing many branching points, typically produced through radical polymerization. For higher-grade linear polystyrene synthesized using methods such as anionic polymerization or controlled radical polymerization, decomposition begins with scission at the chain end, generating the necessary radicals for further decomposition. Unfortunately, polymers with fewer branching points are also more thermally resistant, so they require higher temperatures, and therefore more energy-intensive conditions, for chains to begin breaking down.

Figure 2. Thermal degradation of polystyrene. Reproduced from reference [10].

Although public facilities may be lagging behind, a few Canadian start-ups have fortunately stepped forward to try and tackle the polystyrene recycling problem. GreenMantra is a Brantford, Ontario-based start-up which has patented a method for the catalytic depolymerization of polystyrene using pyrolysis. Their recovered styrene monomers and other styrenic by-products have found commercial applications after repolymerization.¹³ In 2019, GreenMantra reached a joint development agreement with INEOS Styrolution and has begun providing their degraded polystyrene to replace a portion of INEOS Styrolution’s feedstock in manufacturing plastic goods, thereby creating a closed polystyrene cycle.¹⁴ More recently, INEOS Styrolution has also collaborated with Agilyx, based in Oregon, to open up a polystyrene recycling plant in Illinois that will be capable of recycling polystyrene contaminated with food back into food-grade plastics.¹⁵ In Montreal, Pyrowave is a company that uses a similar catalytic decomposition process but heated with microwave reactors that effectively decontaminate polystyrene from food and other organics, in forming styrene monomers for further use.¹⁶

Polystyvert is another Montreal-based company that has developed a rather different, yet arguably more effective way of recycling polystyrene. Instead of degrading it, Polystyvert uses cymene, an essential oil, which dissolves polystyrene, but not other contaminants that can then be filtered out.¹⁷ After separating the polystyrene from cymene using a patent-pending technology, the cymene can itself be recycled for later dissolution. Since dissolution only affects the state and not the chemical structure of polystyrene, the recycled material has virtually identical properties to the original product, and it can be re-foamed with a blowing agent. Crucially, this technology could address one of the largest current issues with expanded polystyrene foam recycling – the size and low density of the material. Immersing the material at the point of collection, or more realistically at the sorting facility, could give the economical impetus for polystyrene waste management needs.

All in all, green chemists are making decent progress in tackling the issues of polystyrene recycling. What’s left is for Canadian regulation to catch up. Right now, only British Columbia, Ontario, and Quebec have plans for systems that can reliably measure the amount of plastic waste that is collected, sorted, and sent to processing facilities, yet only Quebec has set targets for the amount of recycled material that should be used in new products.³ More importantly, the industrial sector, which is responsible for most plastic packaging waste, is not currently subject to legislation making it responsible for its own plastic products.

References

(1) The Role of Chemistry in a Circular Economy; Chemistry Industry Association of Canada, 2020. https://canadianchemistry.ca/wp-content/uploads/2020/07/The-Role-of-Chemistry_ENG_Web-FINAL.pdf.

(2) Canada One-Step Closer to Zero Plastic Waste by 2030; Environment and Climate Change Canada, 2020. https://www.canada.ca/en/environment-climate-change/news/2020/10/canada-one-step-closer-to-zero-plastic-waste-by-2030.html.

(3) Canada’s Zero Plastics Packaging Waste Report Card; Environmental Defence, 2022. https://environmentaldefence.ca/wp-content/uploads/2022/10/Environmental-Defence-Zero-Plastics-Waste-Report-Card_September-9-2022-October-7-2022.pdf.

(4) Eriksen, M.; Lebreton, L. C. M.; Carson, H. S.; Thiel, M.; Moore, C. J.; Borerro, J. C.; Galgani, F.; Ryan, P. G.; Reisser, J. Plastic Pollution in the World’s Oceans: More than 5 Trillion Plastic Pieces Weighing over 250,000 Tons Afloat at Sea. PLOS ONE 2014, 9 (12), e111913. https://doi.org/10.1371/journal.pone.0111913.

(5) Biber, N. F. A.; Foggo, A.; Thompson, R. C. Characterising the Deterioration of Different Plastics in Air and Seawater. Mar. Pollut. Bull. 2019, 141, 595–602. https://doi.org/10.1016/j.marpolbul.2019.02.068.

(6) Yee, M. S.-L.; Hii, L.-W.; Looi, C. K.; Lim, W.-M.; Wong, S.-F.; Kok, Y.-Y.; Tan, B.-K.; Wong, C.-Y.; Leong, C.-O. Impact of Microplastics and Nanoplastics on Human Health. Nanomaterials 2021, 11 (2), 496.

(7) Kwon, B. G.; Koizumi, K.; Chung, S.-Y.; Kodera, Y.; Kim, J.-O.; Saido, K. Global Styrene Oligomers Monitoring as New Chemical Contamination from Polystyrene Plastic Marine Pollution. J. Hazard. Mater. 2015, 300, 359–367. https://doi.org/10.1016/j.jhazmat.2015.07.039.

(8) Ballent, A.; Corcoran, P. L.; Madden, O.; Helm, P. A.; Longstaffe, F. J. Sources and Sinks of Microplastics in Canadian Lake Ontario Nearshore, Tributary and Beach Sediments. Mar. Pollut. Bull. 2016, 110 (1), 383–395. https://doi.org/10.1016/j.marpolbul.2016.06.037.

(9) Chung, Emily. Most Styrofoam Isn’t Recycled. Here’s How 3 Startups Aim to Fix That. CBC News. 2019. https://www.cbc.ca/news/science/styrofoam-chemical-recycling-polystyrene-1.5067879.

(10) Li, H.; Aguirre-Villegas, H. A.; Allen, R. D.; Bai, X.; Benson, C. H.; Beckham, G. T.; Bradshaw, S. L.; Brown, J. L.; Brown, R. C.; Cecon, V. S.; Curley, J. B.; Curtzwiler, G. W.; Dong, S.; Gaddameedi, S.; García, J. E.; Hermans, I.; Kim, M. S.; Ma, J.; Mark, L. O.; Mavrikakis, M.; Olafasakin, O. O.; Osswald, T. A.; Papanikolaou, K. G.; Radhakrishnan, H.; Sanchez Castillo, M. A.; Sánchez-Rivera, K. L.; Tumu, K. N.; Van Lehn, R. C.; Vorst, K. L.; Wright, M. M.; Wu, J.; Zavala, V. M.; Zhou, P.; Huber, G. W. Expanding Plastics Recycling Technologies: Chemical Aspects, Technology Status and Challenges. Green Chem. 2022, 24 (23), 8899–9002. https://doi.org/10.1039/D2GC02588D.

(11) Ullmann ́s Encyclopedia of Industrial Chemistry, 6th ed.; Wiley-VCH: 1999, 1999.

(12) Davidson, M. G.; Furlong, R. A.; McManus, M. C. Developments in the Life Cycle Assessment of Chemical Recycling of Plastic Waste–A Review. J. Clean. Prod. 2021, 293, 126163.

(13) Di Mondo, D.; Scott, B. Reactor for Treating Polystyrene Material. U.S. Patent 11,072,676.

(14) INEOS Styrolution and GreenMantra Sign JDA to Advance Polystyrene Chemical Recycling. INEOS Styrolution. Brantford, Ontario 2019. https://www.ineos-styrolution.com/news/ineos-styrolution-and-greenmantra-sign-jda-to-advance-polystyrene-chemical-recycling.

(15) INEOS Styrolution and Agilyx Advance Polystyrene Chemical Recycling Plant in Channahon, Illinois. INEOS Styrolution. Aurora, Illinois 2019. https://www.ineos-styrolution.com/news/ineos-styrolution-and-agilyx-advance-polystyrene-chemical-recycling-plant-in-channahon-illinois.

(16) Doucet, J.; Laviolette, J.-P. Catalytic Microwave Depolymerisation of Plastic for Production of Monomer and Waxes. U.S. Patent 11,518,864.

(17) Roland, C. Ô. T. É. Processes for Recycling Polystyrene Waste. U.S. Patent 11,407,878.

October 4, 2020 by GreenChemUofT

Monomer Spotlight: Multifunctional and Renewable Itaconic Acid

By Nina-Francesca Farac, Ph.D. student, Social Media Coordinator and Blog Coordinator for the GCI

As chemists and material scientists strive to create a sustainable chemical industry, chemical building blocks derived from renewable resources have become a research necessity¹. In 2004, the U.S. Department of Energy reported 12 building blocks attainable from biomass which have high potential for high-value chemicals or materials.^2–4 One of the listed biorenewable building blocks is itaconic acid (IA). IA is an inexpensive, non-toxic, and readily available compound produced by the fermentation of glucose or other biomass sources such as corn, rice, or lignocellulosic feedstock.^1,2 Importantly, IA is produced through an industrially scaled glucose fermentation process with an estimated global production of 80,000 tons/year and a price of around 2 USD/kg.⁵ Given this compound’s potential to become economically competitive with petroleum-based sources, its manufacturing capacity is expected to grow at a rate of 5.5% each year between 2016 and 2023.^5,6

IA or methylene succinic acid has a trifunctional structure with two carboxylic acid groups and an α,β-unsaturated double bond in the backbone (1, Figure 1). These functional groups make IA a promising monomer for a variety of polymeric reactions.^5,7,8 The polymerization of IA and its derivatives has been extensively studied to access a wide range of new and renewable advanced materials.

**Figure 1**. Chemical structure of itaconic acid (1) and its ester derivatives (2, 3) along with acrylic (4) and methacrylic acid (5).

Most attention has been focused on radical polymerization of IA (Figure 2a) and its various alkyl esters such as dimethyl itaconate (2) and dibutyl itaconate (3) due to their structural similarities to the traditional monomers for poly(meth)acrylates, acrylic acid (4) and methacrylic acid (5). More recently, the step-growth polymerizations of itaconate derivatives have been investigated for the synthesis of numerous polyesters (Figure 2b).^5,9

**Figure 2**. Select polymerization pathways for itaconate.

Several other chemical transformations have been explored to generate interesting materials from IA-derived polymers, such as thermoplastics and thermosets. Nonetheless, researchers argue that a significant amount of chemical space remains unexplored for the synthesis of high-value materials from IA.¹ Moreover, several IA-derived polymers tend to have high glass-transition temperatures (T_gs) that limit their application.

In effect, Trotta et al.¹ set out to design new polymers from IA and to expand the library of IA-derived renewable materials. Published in ACS Sustainable Chemistry & Engineering in December 2018, Trotta & colleagues were able to synthesize and characterize bio-sourced thermosets and thermoplastics that are almost completely derived from IA and whose mechanical properties are tunable. The authors developed scalable and efficient syntheses of three step-growth monomers from an IA derivative (Figure 3a). These monomers were then used to access functional polyesters that can be used for making thermosets and thermoplastics that are both mechanically and thermally stable (Figure 3b).

*Figure 3¹. (a) Bio-sourced monomers from IA. (b) Bio-sourced thermoplastics and thermosets derived from IA.*

1. Synthesis & Characterization of Step-Growth Monomers from IA

First, a saturated diester (MS), a saturated diol (MB), and an unsaturated diester (CS) step-growth monomers were efficiently synthesized from commercially available dimethyl itaconate (DMI), an IA derivative (Figure 3a). The DMI-to-CS transformation was motivated by the instability of DMI itself in step-growth polymerization with diols. A scalable Diels-Alder reaction was carried out to circumvent this issue. The diene of choice was isoprene since it was recently demonstrated that isoprene can be derived from IA¹⁰, allowing for the synthesis of a stable monomer (CS) that is made completely from IA.

2. Step-Growth Polycondensation Polymerizations – Synthesis & Characterization

Next, binary (i.e. two-monomer) and ternary (i.e. three-monomer) step-growth polycondensations were carried out to make various amorphous polymers with low T_gs (e.g. -31 to -9 °C) and relatively high molar masses (>10 kg/mol). The low T_gs suggest easier processability and a wider range of applications for these polyesters. Thermogravimetric analysis confirms the relative stability of these polymers, particularly PMBCS (Figure 4) whereby no retro-Diels Alder reaction was observed along the polymer backbone at temperatures above 200 °C.

**Figure 4.**¹ Synthesis of (a) fully unsaturated poly(MB-alt-CS) (PMBCS), (b) fully saturated poly(MB-alt-MS) (PMBMS), and (c) statistical ternary PMBCS_x-stat-PMBMS_1-x.

3. IA-Derived Thermosets

By controlling the feed ratio of the CS monomer, the authors produced ternary polyesters with tunable amounts of unsaturation along the polymer backbone. This gave rise to polyesters that can be efficiently cross-linked using thiol-ene click chemistry to generate thermosets.

To continue with the theme of bio-sourced materials, the authors used a potentially renewable tetrathiol cross-linker (highlighted in purple, Figure 5b), to cross-link the ternary polymer, PMBCS_x–stat-PMBMS_1-x (Figure 5c).

**Figure 5**. (a) A model reaction for thiol-ene “click” chemistry on PMBCS. (b) A potentially renewable retro-synthetic route to the tetrathiol cross-linker. (c) Cross-linking reaction of crude PMBCS_x-stat-PMBMS_1-xwith the tetrathiol cross-linker and DMPA initiator to give IA-derived thermosets.

The authors characterized both the thermal and mechanical properties of the synthesized thermosets. The cross-linking reaction afforded thermosets with slightly higher T_gs than their respective prepolymers, suggesting decreased flexibility of the polymer backbone due to cross-linking. The Young’s modulus (E), which is a measure of a material’s elasticity, was the largest for thermosets with the highest cross-linking densities. It was also observed that these mechanical properties are tunable by varying the molar mass between cross-links.

4. IA-Derived Thermoplastics

Finally, the authors prepared well-defined triblock polymer thermoplastics that can be prepared from IA-derived α-methylene-γ-butyrolactone (MBL)¹¹ (Figure 6), giving thermoplastics that are almost completely derived from IA.

**Figure 6.**¹ Chain extension of HO-PMBMS-OH to give PMBL-PMBMS-PMBL.

Tensile testing data indicate that the MBL chain-extended triblock polymer is indeed thermoplastic. The triblock polymer exhibits ductile tensile properties with a Young’s Modulus and tensile strength far exceeding those of the IA-derived thermosets.

5. Green Metrics

A sustainable chemical industry not only requires the use of bio-sourced/biorenewable substrates but also relies on safe and energy-efficient synthetic processes that do not generate any additional waste streams. In effect, the authors evaluated the small molecule transformations and polymerizations disclosed above using green metrics¹² – measures to quantify the efficiency and/or environmental performance of a chemical process as it relates to the principles of green chemistry.^13,14 Isolated yields, atom economies (AEs)¹⁵, and process mass intensities (PMIs)¹⁶ were reported for select reactions as listed in Table 1.

**Table 1**. Green metric evaluations of various reactions presented by Trotta et al. ^aDefined as the percent of the molecular weight of the desired product compared to the molecular weight of all starting reactants. ^bDefined as the ratio of the mass (in kg) of all the raw material used in the synthesis of the desired product (including all reagents, mass of solvent(s) and other materials such as silica used in purification) to the mass (in kg) of the isolated product.

Most of the small molecule transformations and polycondensations are high yielding with a few exceptions. The low yield of 60% for PMBCS in reaction 4 is attributed to low polymer recovery during the purification by precipitation step.¹ For reactions 2 and 3, yields >90% are achievable but on smaller scales; however, the authors desired scalable syntheses, making yields of 82% and 67% acceptable for these purposes.

The AE for addition reactions like reactions 1 and 3 is 100% whereas reactions with undesired byproducts have lower AE. Trotta et al. propose alternative synthetic routes that would result in increased AE for reactions 2, 4, 5, & 6 but these were not explored. Besides, a higher AE does not guarantee a high yielding and low PMI process and such suggestions require further investigation. On the other hand, this emphasizes the challenge scientists are faced when attempting to develop a sustainable process; it is generally difficult to implement all criteria outlined by the principles of green chemistry without having a trade-off somewhere within the process.

The PMI metric nicely showcases the impact of solvent on process efficiency. PMI values closer to 1 indicate a smaller mass of material required to synthesize 1 kg of product.^1,16 PMI is low for most reactions listed in Table 1, reflecting little to no use of solvents (i.e. neat conditions) or other reagents during the reaction, workup, and/or purification steps. Comparing reactions 1 through 3, the synthesis of CS has an increased PMI of 12 due to the use of solvent during workup. Similarly, there is a sharp rise in PMI for polymers that are purified by dissolving and precipitating them from solution (reactions 4 and 5). In contrast, polymers that don’t require purification have a significantly lower PMI (reaction 6).

6. Summary

Overall, this paper is one of many that demonstrate the growing potential of itaconic acid as a multifunctional and renewable monomer for a wide range of polymerizations. The authors were largely successful in producing new bio-sourced and mechanically (as well as thermally) stable thermosets and thermoplastics from IA-derived materials.

Although the green metric evaluations identify several sustainable features of the reactions developed by Trotta et al., one aspect that puts into question how ‘green’ the reactions are is the type of catalyst used. Each small molecule transformation (reactions 1 – 3 in Table 1) uses either a platinum group metal or a rare earth metal – both of which are considered critical/endangered elements that are essential for use but subject to supply risk.¹⁴ It is noted that the catalyst loading for these reactions is low with amounts below 5 mol% or 5 wt%; however, it is not enough to reduce catalyst use. Complete catalyst recyclability is another factor that should be implemented. On this basis, the authors did show that the Sc(OTf)₃ catalyst used in the synthesis of CS can be recovered at a 97% yield prior to distillation of the crude polymer product. The ability to recycle the other catalysts were not mentioned.

Furthermore, it would have been beneficial if the authors quantified the biorenewable content of their synthesized thermosets and thermoplastics instead of using general statements such as “almost completely derived from IA”. Other studies investigating the synthesis of polymeric materials from renewable sugar-derived precursors typically state the percentage of biobased content within their resulting materials (e.g. wt% in biorenewable content). For instance, a 2015 study¹⁷ described the biobased content of thermosets derived from IA-based polyesterss in the ranges of 78 and 88 wt%. The quantification of biorenewable content would unambiguously validate the work of Trotta et al. and better gauge their impact within the literature.

REFERENCES

(1) Trotta, J. T.; Watts, A.; Wong, A. R.; Lapointe, A. M.; Hillmyer, M. A.; Fors, B. P. Renewable Thermosets and Thermoplastics from Itaconic Acid. ACS Sustain. Chem. Eng. 2019, 7 (2), 2691–2701.

(2) Noordzij, G. J.; Van Den Boomen, Y. J. G.; Gilbert, C.; Van Elk, D. J. P.; Roy, M.; Wilsens, C. H. R. M.; Rastogi, S. The Aza-Michael Reaction: Towards Semi-Crystalline Polymers from Renewable Itaconic Acid and Diamines. Polym. Chem. 2019, 10 (29), 4049–4058.

(3) Werpy, T.; Petersen, G. Top Value Added Chemicals from Biomass Volume I – Results of Screening for Potential Candidates from Sugars and Synthesis Gas; 2004.

(4) Bozell, J. J.; Petersen, G. R. Technology Development for the Production of Biobased Products from Biorefinery Carbohydrates—the US Department of Energy’s “Top 10” Revisited. Green Chem. 2010, 12 (4), 539–555.

(5) Robert, T.; Friebel, S. Itaconic Acid – a Versatile Building Block for Renewable Polyesters with Enhanced Functionality. Green Chem. 2016, 18 (10), 2922–2934.

(6) Transparency Market Research, Market Report Itaconic Acid, 2015; 2015.

(7) Geilen, F. M. A.; Engendahl, B.; Harwardt, A.; Marquardt, W.; Klankermayer, J.; Leitner, W. Selective and Flexible Transformation of Biomass-Derived Platform Chemicals by a Multifunctional Catalytic System. Angew. Chemie – Int. Ed. 2010, 49 (32), 5510–5514.

(8) Medway, A. M.; Sperry, J. Heterocycle Construction Using the Biomass-Derived Building Block Itaconic Acid. Green Chem. 2014, 16 (4), 2084–2101.

(9) Kumar, S.; Krishnan, S.; Samal, S. K.; Mohanty, S.; Nayak, S. K. Itaconic Acid Used as a Versatile Building Block for the Synthesis of Renewable Resource-Based Resins and Polyesters for Future Prospective: A Review. Polym. Int. 2017, 66 (10), 1349–1363.

(10) Abdelrahman, O. A.; Park, D. S.; Vinter, K. P.; Spanjers, C. S.; Ren, L.; Cho, H. J.; Zhang, K.; Fan, W.; Tsapatsis, M.; Dauenhauer, P. J. Renewable Isoprene by Sequential Hydrogenation of Itaconic Acid and Dehydra-Decyclization of 3-Methyl-Tetrahydrofuran. ACS Catal. 2017, 7 (2), 1428–1431.

(11) Trotta, J. T.; Jin, M.; Stawiasz, K. J.; Michaudel, Q.; Chen, W. L.; Fors, B. P. Synthesis of Methylene Butyrolactone Polymers from Itaconic Acid. J. Polym. Sci. Part A Polym. Chem. 2017, 55 (17), 2730–2737.

(12) Tobiszewski, M.; Marć, M.; Gałuszka, A.; Namies̈nik, J. Green Chemistry Metrics with Special Reference to Green Analytical Chemistry. Molecules 2015, 20 (6), 10928–10946.

(13) Tang, S. L. Y.; Smith, R. L.; Poliakoff, M. Principles of Green Chemistry: Productively. Green Chem. 2005, 7 (11), 761–762.

(14) Dubé, M. A.; Salehpour, S. Applying the Principles of Green Chemistry to Polymer Production Technology. Macromol. React. Eng. 2014, 8 (1), 7–28.

(15) Jiménez-González, C.; Constable, D. J. C.; Ponder, C. S. Evaluating the “Greenness” of Chemical Processes and Products in the Pharmaceutical Industry—a Green Metrics Primer. Chem. Soc. Rev. 2012, 41 (4), 1485–1498.

(16) Jiménez-González, C.; Ponder, C. S.; Broxterman, Q. B.; Manley, J. B. Using the Right Green Yardstick: Why Process Mass Intensity Is Used in the Pharmaceutical Industry to Drive More Sustainable Processes. Org. Process Res. Dev. 2011, 15 (4), 912–917.

(17) Dai, J.; Ma, S.; Wu, Y.; Han, L.; Zhang, L.; Zhu, J.; Liu, X. Polyesters Derived from Itaconic Acid for the Properties and Bio-Based Content Enhancement of Soybean Oil-Based Thermosets. Green Chem. 2015, 17 (4), 2383–2392.

January 29, 2020 by GreenChemUofT

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, 2^nd most produced polymer), the monomer recovery yield is poor (0.025-2%) [11]. In some cases such as polyvinylchloride (PVC, 3^rd 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].

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.

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.