29 Aug Cyclic carbonates: a sustainable and greener alternative to toxic compounds used in polymer synthesis

Posted at 10:42h in Articles SP, News by Clementine

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September 2023

Introduction

Cyclic carbonates are a class of compounds that is attracting growing interest both at academic and industrial levels, as their preparation and applications present several attractive features in the context of green chemistry and sustainability. The most common and efficient route for preparing cyclic carbonates, also at industrial level, is the cycloaddition of CO₂ to epoxides. This reaction offers several advantages in the context of green and sustainable chemistry, as it uses a renewable, non-toxic and widely available reactant as carbon dioxide. It displays 100% atom efficiency, since all the reactants are incorporated into the product, and can be carried out efficiently without the need for solvents (Figure 1).¹

Synthesis of cyclic carbonates from CO2 and epoxides and selected applications

Figure 1. Synthesis of cyclic carbonates from CO₂ and epoxides and selected applications ¹

Among cyclic carbonates, propylene carbonate is the one most commonly used as green solvent. Propylene carbonate displays suitable properties as a polar aprotic solvent, having a large temperature range in which it is liquid (i.e. between -49 and 242°C), relatively low viscosity, and being colorless and odorless. From the health and safety point of view, propylene carbonate is an excellent solvent, as it displays low vapor pressure, low toxicity, low flammability and is noncorrosive. The properties of propylene carbonate and its eco-friendly characteristics make it an attractive, sustainable alternative to conventional, more harmful, polar aprotic solvents such as N,N-dimethylformamide (DMF) and N-methyl-2-pyrrolidone (NMP).^{1, 2}

High-purity ethylene carbonate and, to a lesser extent, propylene carbonate find an important application as solvent components in the formulation of electrolytes for lithium batteries. Cyclic carbonate provides the high dielectric constant component needed to dissolve lithium salts, thus generating a conductive electrolyte.^{1, 2}

The uses of cyclic carbonates in lithium batteries will be studied in the following article.

Cyclic carbonates can also undergo ring-opening reactions with alcohols and amines, opening up possibilities as reactants for chemical syntheses. The reaction of cyclic carbonates with primary and secondary amines generates urethane groups. When carried out with a diamine, this reaction leads to the formation of polyhydroxyurethanes (PHUs) and offers a greener alternative to conventional polyurethanes, which are based on the reaction of toxic isocyanates with polyols.^{1, 3}

Cyclic carbonates in Polyhydroxyurethanes (PHUs)

Polyurethanes constitute an important class of polymer products, which are typified by the presence of urethane (carbamate) linkages. Having an estimated market size of approximately 20 million tons per year, the main application of polyurethanes is in the production of rigid and flexible foams, coatings and adhesives. The synthesis of polyurethanes typically involves an addition reaction between a di- or polyisocyanate and a compound with at least two hydroxyl groups (Figure 2). However, the use of di- or polyisocyanates is not preferred due to the health issues associated with these compounds. Furthermore, isocyanates are commercially produced using the toxic phosgene as precursor, which is another reason to search for greener alternatives.²

Figure 2. Synthesis of polyurethanes vs. synthesis of polyhydroxyurethanes

One of the most promising strategies to bypass the use of toxic isocyanates is the synthesis of polyhydroxyurethanes (PHUs) by reacting a bis-cyclic carbonate with a bis-amine (Figure 2). Indeed, this reaction not only has 100% atom economy but also benefits from the availability of a large portfolio of poly(cyclic carbonate)s, which can be synthesized by a facile chemical [3+2] CO₂ insertion into the corresponding epoxy precursor.⁴

Bio-based PHUs derived from vanillin

As an alternative to the use of hazardous phosgene-based isocyanates for polyurethane preparation, non-isocyanate PHUs based on 5-membered cyclic carbonates have been developed. While there is no doubt about the reduced carbon footprint in comparison with isocyanate chemistry, petroleum is still the main source for the production of cyclic carbonates, especially those based on aromatic precursors.

Recently, Sardon and co-workers reported the preparation of bio-based PHUs from vanillin.⁴ Vanillin, with its aldehyde and phenol functionality, represents a very versatile platform for the synthesis of a variety of monomers for polymer synthesis. It can be isolated from the oxidative depolymerization of Kraft lignin using peroxodicarbonate as “green” oxidizer for the degradation (Figure 3).⁵ The aromaticity of vanillin can be highly valuable for non-isocyanate polyurethane production, as it can potentially increase the cohesion of polymer materials due to strong pi-interactions, which limit chain mobility. This can lead to materials with properties analogous to those of petroleum-based PUs, but with a ‘‘green’’ feedstock that has reduced environmental impact.⁴

Figure 3. Oxidative degradation of Kraft lignin to produce vanillin ⁵

In the paper of Sardon et al., three different vanillin-derived bis-cyclic carbonates were synthesized: vanillin alcohol-based bis-carbonate, hydroquinone-based bis-carbonate and vanillin acid based bis-carbonate (Figure 4). Subsequently, each monomer was reacted with two different bis-amines, i.e. 1,4-butanediamine (BDA) and 1,6-hexamethylenediamine (HDMA), to yield six different PHUs. PHUs derived from vanillin showed glass transition temperatures (T_g) ranking from 41°C to 66°C.

Figure 4. Functional bis-carbonates derived from vanillin

In the case of the bisphenol A material, the observed Tg was 56°C. It is thus envisioned that vanillin-based PHUs could potentially be a safer alternative to harmful bisphenol A-based PHUs and provide a useful strategy for CO₂revalorization, especially considering that vanillin is an abundant by-product of Kraft lignin production.⁴

Reprocessable and recyclable PHU networks

Isocyanate-free chemistry and the introduction of dynamic bonds are a promising combination toward the development of more sustainable polyurethane (PU) networks. Recently, Odelius and co-workers reported the synthesis of reprocessable isocyanate-free PU networks by introducing dynamic disulfide covalent bonds in PHU thermosets.⁶ This kind of reversible covalent material belongs to the range of so-called covalent adaptable networks (CANs). CANs have the ability to reversibly rearrange their network structure through exchange reactions of dynamic covalent bonds under external stimuli such as heat, pH, and UV light. Owing to this topological rearrangement, CANs can be easily recycled, repaired and reprocessed (Figure 5).^7-9

Figure 5. Covalent Adaptable Networks (CANs) as an intermediate bridge between thermosets and thermoplastics ⁹

Odelius et al. successfully synthesized a series of disulfide-based PHU CANs through the step-growth polymerization between five-membered cyclic carbonates and cystamine, evaluating the effect of the cyclic carbonate structure, cross-link density and moisture absorption on their properties. The structural diversity of the monomers realized PHU networks with a finely tuned profile of thermal (T_g from -9 to 44 °C), mechanical (E from 0.2 to 1700 MPa), and viscoelastic properties (E’ from 0.03 to 5.5 MPa at 1 Hz). Facile recycling (100°C, 20 min) of the networks was enabled thanks to the rapidly exchanging disulfide bonds and the modulated cross-link density. All of the CANs showed excellently retainable properties after one reprocessing cycle, while multiple reprocessing cycles were also feasible as shown for the network synthesized only with trimethylolpropane tricyclocarbonate (TMPTC) and cystamine (Figure 6).⁶

Figure 6. Structure of the monomers employed for PHU CANs, representation of network formation, and mechanical properties of TMPTC-Cystamine material before and after reprocessing cycles ⁶

This methodology, avoiding toxic reagents and preventing waste generation, make this approach an attractive and greener pathway to PU networks taking a step toward a circular plastic economy.⁶

Related R&D Products

Vanillyl alcohol bis(cyclocarbonate)

SP-68-015

Cystamine

SP-2-4-001

TMP tricyclocarbonate

SP-3-00-003

In the next article, discover the advantages of using cyclic carbonates in lithium batteries.

References

Pescarmona, P. P., Cyclic carbonates synthesised from CO2: Applications, challenges and recent research trends. Current Opinion in Green and Sustainable Chemistry 2021, 29, 100457 >
Kamphuis, A. J. et al., CO2-fixation into cyclic and polymeric carbonates: principles and applications. Green Chemistry 2019, 21 (3), 406-448 >
Bobbink, F. D. et al., En route to CO2-containing renewable materials: catalytic synthesis of polycarbonates and non-isocyanate polyhydroxyurethanes derived from cyclic carbonates. Chemical Communications 2019, 55 (10), 1360-1373. >
Fanjul-Mosteirín, N. et al., Bio-based non-isocyanate poly(hydroxy urethane)s (PHU) derived from vanillin and CO2. Materials Advances 2023, 4 (11), 2437-2448. >
Zirbes, M. et al., Peroxodicarbonate as a Green Oxidizer for the Selective Degradation of Kraft Lignin into Vanillin. Angew Chem Int Edit 2023, 62 (14), e202219217. >
Pronoitis, C. et al., Structurally Diverse and Recyclable Isocyanate-Free Polyurethane Networks from CO2-Derived Cyclic Carbonates. ACS Sustainable Chemistry & Engineering 2022, 10 (7), 2522-2531. >
Bowman, C. N.; Kloxin, C. J., Covalent Adaptable Networks: Reversible Bond Structures Incorporated in Polymer Networks. Angew Chem Int Edit 2012, 51 (18), 4272-4274. >
Denissen, W. et al., Vitrimers: permanent organic networks with glass-like fluidity. Chemical Science 2016, 7 (1), 30-38. >
Alabiso, W.; Schlögl, S., The Impact of Vitrimers on the Industry of the Future: Chemistry, Properties and Sustainable Forward-Looking Applications. Polymers 2020, 12 (8), 1660. >