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.
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
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).
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.
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, PMBCSx–stat-PMBMS1-x (Figure 5c).
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.
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.
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
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(6) Transparency Market Research, Market Report Itaconic Acid, 2015; 2015.
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(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.
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