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 (Tgs) 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 Tgs (e.g. -31 to -9 °C) and relatively high molar masses (>10 kg/mol). The low Tgs 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 PMBCSx-stat­-PMBMS1-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, PMBCSxstat­-PMBMS1-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 PMBCSx-stat­-PMBMS1-x with 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 Tgs 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.

Green Marketing in the Plastic Era: Honesty or Hype?

By Nina-Francesca Farac, Member-at-Large for the GCI

The impact of human activity on climate and the environment has moved beyond a mainstream headline. It has come to the point where we are considered the dominant influence on our ecosystems and geology, so much so that there is a buzzword for it: ‘Anthropocene’. Within the Anthropocene, our greatest challenge is lessening the effects of our immense footprint on Earth, mainly caused by consumption of fossil fuels and our obsession with plastics. Consequently, there has been a considerable spike in eco-friendly or ‘green’ marketing of numerous products labeled as ‘organic’, ‘biodegradable’, or ‘sustainable’ ranging from fuels, cars, skincare, all the way to clothing1. One common advertising theme for several everyday products is post-consumer recycled materials and their incorporation into the design and production of such commodities.  But to what extent are the advertised claims legitimate and whether they allow for a circular economy (e.g. make, use, recover)? Here, we will cover the chemistry of popular sustainable alternatives to plastics and compare them to their non-sustainable counterparts to assess whether the ‘green’ hype is valid.

Recycling Plastics: Single-Use vs. Biodegradable vs. Compostable

Let’s start with why commonly used plastics, including single-use plastics, pose such an environmental liability. The reality of plastic recycling is that it is far less efficient in practice than one would hope. The types of plastics that can be recycled, the number of times they can be recycled and reused starts with the ubiquitous recycling symbols found on the bottom of plastic products2. A common misconception is to equate the presence of this symbol to the ability of recycling a given type of plastic; however, this is not the case. The truth is just because there is a recycling sign doesn’t necessarily mean it gets recycled3. According to Resin Identification Codes (RICs), plastics are organized into 7 categories according to the temperature at which the material has been heated, and this numerical categorization is only indicative of the kind of plastic it is, and not necessarily its recyclability (Fig. 1).

Image 1

Figure 1. Resin Identification Codes (RICs) designating the seven categories of plastics, the corresponding chemical structure of each polymer, and graphical illustrations of common plastic products of each type.

In other words, just because we place it in a blue bin doesn’t mean it gets recycled. In fact, an astonishing 91% of plastics are not recycled2. You may be wondering, “how are recycling rates that low?” As with any commodity, recycling is ultimately determined by the market.  If there is a demand, recyclers and companies will pay for post-consumer recyclables; but, without market demand, recycling bares no profit and placing them in a blue bin doesn’t make a difference3. For example, out of the seven categories of plastics depicted in Figure 1, only PET has a high recycling value (i.e. the price of PET scrap is high) while other plastics are projected to see a drop in recycling rates (at least in North America)4–6. In addition, certain types of everyday plastics are simply not recyclable, such as plastic bags, straws, and coffee cups (the latter is not possible unless the paper exterior is separated from the plastic interior)3; in effect, these items are tossed together in the “everything else” category #7 (Other) as non-recyclables and mainly contribute to plastic waste generation. Other limiting factors include the inability to recycle dirty plastic and how the quality of plastic is downgraded each time plastic is recycled7.

Since many everyday items are plastic-based and plastics are a staple of modern life, the most common sustainable alternative to single-use plastics are bioplastics, a.ka biodegradable plastics (Figure 2).

Image 2

Figure 2. Types of biodegradable plastics in use today, their chemical structures, and their applications.

Consumer confusion often arises when the terms “biodegradable” and “compostable” are used interchangeably, although they do not convey the same concept. Biodegradable plastics are a class of polymers that can break down by the action of living organisms into natural byproducts such as water, biomass, gases (e.g. N2, CO2, H2, CH4), and inorganic salts within a reasonable amount of time8. The issue with this definition is that many plastic products eventually degrade; for instance, low density polyethylene (LDPE, category #4 – Fig. 1) has been shown to biodegrade slowly to carbon dioxide (0.35% in 2.5 years) and thus can be considered a biodegradable polymer according to the above description9. Because certain definitions of biodegradability do not state a time limit or timeframe within which degradation should occur, consumers can be easily misled, and companies can hide behind this ambiguity. It is assumed, however, that a biodegradable product has a degradation rate that is comparable to that of its application rate, i.e. the break down process is fast such that product accumulation in the environment does not occur.

To understand why certain polymers biodegrade and others do not, one has to consider the chemical structure of biodegradable polymers along with the mechanisms through which polymeric material are biodegraded. Structurally, many biodegradable polymers, both natural and synthetic, often contain amide, ester, or ether bonds10. Those deriving from biomass (i.e. agro-polymers) include polysaccharides (glycosidic bonds via condensation of a saccharide hemiacetal bond and an alcohol) and proteins (chains of amino acids linked via amide groups). The other major category is biopolyesters, which are typically derived from microorganisms or are synthetically made (Figure 3).

Image 3

Figure 3. Categories of biodegradable polymers.

Mechanistically, biodegradation is defined as a process caused by biological activity, especially driven by enzymes, but it can occur simultaneously with – and sometimes even initiated by – abiotic process such as photodegradation and hydrolysis9. From the chemical perspective, biodegradation can occur in the presence of oxygen (aerobic, Equation 1.1) or in the absence of oxygen (anaeriobic, Equation 1.2), where Cpolymer represents either a polymer or a fragment from an earlier degradation process9.

Cpolymer + O2→CO2 + H2O + Cresidue + Cbiomass (Aerobic biodegradation,1.1)

Cpolymer→ CO2 + CH4 + H2O + Cresidue + Cbiomass (Anaeriobic biodegradation,1.2)

Complete biodegradation is said to occur when no Cresidue remains, and no oligomers or polymers are left to be further broken down9. As polymers represent major constituents in living cells that have a high turnover rate, i.e. they are constantly degraded in response to environmental changes and metabolic requirements, numerous microorganisms are capable of breaking down naturally occurring polymers as a result of millions of years of adaption. However, for many new synthetic polymers invented in the last 100 years (categories 1-6, Fig. 1) which find their way into the environment, such biodegradation mechanisms have yet to be developed. Other key factors affecting polymer biodegradation include copolymer composition and environmental factors such as pH, temperature, and water content9. This suggests that even if a product is made from bioplastics, it doesn’t necessarily mean it will fully decompose. If such products end up in landfills, for instance, the low oxygen content of such an environment impedes complete degradation. Furthermore, although bioplastics fall within category 7 (Fig.1), these plastics aren’t suitable for recycling and can even degrade the quality of plastic if added to a recycled mixture.

In contrast, compostable materials can break down into water, carbon dioxide, inorganic salts and biomass at the same rate as cellulose, or roughly 90 days11,12. In addition, compostable plastics must disintegrate fully and be indistinguishable in the compost while leaving no toxic material behind. Although compostable plastics appear to have more environmental benefits, this material is equally limited by an inability to biodegrade in a landfill and being incompatible with mixed recycled plastics11. When it comes to biodegradable and compostable plastics, these products make for sustainable alternatives only if they are destined for the appropriate composting facilities whereby the specific conditions for their complete biodegradation are met.

In short, today’s environmental pressures have urged the mainstream production of sustainable alternatives to an ever-growing plastic problem. Although socially responsible plastic products exist with the intention of lessening their environmental footprints, their legitimacy as sustainable alternatives lies in their proper disposal and complete integration into a given environment without any adverse or toxic effects. For a more concrete circular economy, products made of glass and metal can be recycled infinitely without losing product quality, have no need for additional virgin material in the recycling process, and do not generate waste during the process3. Ultimately, a future with a reduced plastic impact depends not only on closed-loop recycling habits, but also on consumer education and awareness about how these products are made and disposed of.

References

(1)        Schmuck, D.; Matthes, J.; Naderer, B. Misleading Consumers with Green Advertising? An Affect–Reason–Involvement Account of Greenwashing Effects in Environmental Advertising. J. Advert. 2018, 47 (2), 127–145.

(2)        Lu, C. The Truth about Recycling Plastics. Mitte. 2018. https://mitte.co/2018/07/18/truth-recycling-plastic/

(3)        Sedaghat, L. 7 Things You Didn’t Know About Plastic (and Recycling). https://blog.nationalgeographic.org/2018/04/04/7-things-you-didnt-know-about-plastic-and-recycling/

(4)        Toto, D. The Value of Plastics. https://www.recyclingtoday.com/article/recyclingtoday-1011-plastics-value/

(5)        Dell, J. U.S. Plastic Recycling Rate Projected to Drop to 4.4% in 2018 https://www.waste360.com/plastics/us-plastic-recycling-rate-projected-drop-44-2018.

(6)        A Circular Economy For Plastics In Canada: A bold vision for less waste and more value. http://circulareconomyleaders.ca/downloads/A_Circular_Economy_for_Plastics_in_Canada.pdf

(7)        Hopewell, J.; Dvorak, R.; Kosior, E. Plastics Recycling: Challenges and Opportunities. Philos. Trans. R. Soc. B Biol. Sci. 2009, 364 (1526), 2115–2126.

(8)        Gross, R. A.; Kalra, B. Biodegradable Polymers for the Environment. Science (80-. ). 2002, 297 (5582), 803–807.

(9)        Bastiolo, C. Handbook of Biodegradable Polymers; 2005. Rapra Technology Limited.

(10)      Vroman, I.; Tighzert, L. Biodegradable Polymers. Materials (Basel). 2009, 2 (2), 307–344.

(11)      https://pelacase.com/blogs/news/5-things-you-probably-didnt-know-about-compostable-plastics

(12)     https://www.food.ee/blog/compostable-vs-recyclable-vs-biodegradable/