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.

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.
Green Polymer Chemistry: Approaches, Challenges, Opportunity

Green Polymer Chemistry: Approaches, Challenges, Opportunity

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

I was recently inspired by an episode of podcast by NPR’s Planet Money called Oil #4: How Oil Got Into Everything. It told the story of Leo Baekeland’s invention of Bakelite, which is the plastic that made many commodities affordable for the masses.

Plastic is made of polymers, and many of the common items we use are made from one or more of these polymers. Examples of these polymers are polystyrene, polymethylmethacrylate, and polyethylene and some examples of common items that contain these polymers are Styrofoam™, Plexiglas®, and plastic bags, respectively. Polymers are synthesized by forming bonds between many molecules of same structure, called monomers.

Conventionally, these monomers are produced from chemicals derived from oil, which is a non-renewable feedstock. Environmentally conscious scientists have been trying to make polymers in a more eco-friendly way. The biggest challenge lies in how we obtain monomers from renewable sources.

There are two main approaches to this challenge. The first approach is to produce currently used monomers, such as styrene, from a renewable source. A literature review by Hernandez et al.1 called this approach bioreplacement. The biggest progress in this

oct-1

Figure 1. Engineered metabolic pathway to produce styrene from glucose. (1)

approach has been made by engineering the metabolic pathways of bacteria cultures. McKenna et al. 3 have been able to feed glucose to engineered E. coli to produce styrene and release it in the culture medium they are incubating in. The E. coli flask cultures were able to produce styrene to reach concentrations of up to 260 mg/L1,3. Figure 1 shows the metabolic pathway from glucose to styrene.

While this method of producing monomers is promising, there are road blocks that are hindering progress. The biggest issue is toxicity of styrene to the E. coli, which limits the maximum concentration of styrene in the bacterial culture (E. coli can only tolerate up to 300 mg/L styrene1,3). Other challenges that exist with using bacteria include long incubation times, obtaining poor yield of desired product relative to amount of glucose added, and scale up. Looking down the road, these kinds of limitations may prevent this method from being economically and practically viable.

The second approach is called bioadvantage. Polymer chemists take chemicals that are already being produced from renewable feedstock, synthesize polymers, and use said polymers to produce polymer products in hopes of replacing already existing polymer materials. There are many molecules that are being studied for this purpose such as cellulose, starch, anethole, methylene-butyrolactone, and myrcene.

oct-2

Figure 2. Conventional monomers (styrene, methylmethacrylate, ethylene) and their potentially renewable counterparts. Renewable counterpart monomers tend to be structural analogues of conventional monomers.

During the podcast by Planet Money, research by the Hillmyer group from University of Minnesota was featured. They aim to synthesize eco-friendly polymer using monomers from renewable feedstock (the bioadvantage method). After many failures to produce viable polymer from corn, coconut, orange peels, etc., they were able to develop a polymer synthesized from a menthol derivative obtained from peppermint2.

A critical challenge to bioadvantage polymers is the need for years of study and passing a battery of regulatory tests before they are adopted. The petroleum based polymers that are being used today already have been researched for decades, which allows them to be used easily by industry. By extension, bioadvantage polymers will need to match or exceed their performance in terms of strength, durability, flexibility, and other properties we require from our plastic. Even when industry is willing to allocate resources to adopt eco-friendly polymers, sometimes it’s the consumers that prove to be even less accommodating. We observed this with the biodegradable bag fiasco by Sun Chips.

It should be mentioned that both bioreplacement and bioadvantage polymers are not necessarily biodegradable. Therefore, we should not call them green polymers.

I will conclude with this: I see the impact that plastic has on our daily lives and I see demand for polymers. Being able to make eco-friendly polymers economically will change the world around you, literally. As Planet Money teaches, the world works in a supply-demand swing. When the kinks in the supply side of eco-friendly polymers are fixed, demand for them will present itself. How soon eco-friendly plastics will develop will depend on us. As green chemists, we should see that the biggest impact we might have in the future, will be making eco-friendly polymers.

References

(1)   Hernández, N.; Williams, R. C.; Cochran, E. W. Org. Biomol. Chem., 2014,12, 2834-2849

(2)   Hillmyer, M. A.; Tolman, W. B. Acc. Chem. Res., 201447 (8), pp 2390–2396

(3)   Mckenna, R.; Nielsen, D. R. Metab. Eng. 2011, 13 (5), 544–554.