Valuing CO2 in the development of polymer materials

Reducing the concentration of carbon dioxide in the atmosphere is a major challenge for humanity as well as for living species on Earth. Not least because of the adverse effects of climate warming caused by the anthropological emissions of the CO2. We are interested in a complementary approach to capturing and storing CO2, which is to use CO2 as a raw material for chemical reactions. Specifically, it is a question of using the CO2 for the synthesis of certain types of biocompatible polymers. For the time being, these remain restricted to polycarbonates for economic reasons. The use of CO2 is also motivated by its abundance as a potentially unlimited source of carbon. The challenge of this chemical process is to use appropriate catalysts to synthesize polycarbonates in sufficient quantities and with interesting physical properties. There is also talk of producing biodegradable and biocompatible polycarbonates to ensure a relatively green footprint compared to conventional polycarbonate production processes.


Introduction
The increase in emissions of carbon dioxide (CO 2 ) into the atmosphere, due to the consumption of fossil fuels (coal, gas and oil), is the main cause of climate change. Because CO 2 is a greenhouse gas, it blocks heat from the planet. Its high concentrations in the atmosphere, reaching a level above 415 parts per million in 2019 [1] (historical record) have led to an increase in global temperature with devastating effects on the environment and human well-being ( Fig. 1).
Although the CO 2 can be eliminated by natural processes, the current rate of CO 2 emissions exceeds that of natural elimination. This has led to growing public dissatisfaction, putting pressure on governments around the world to take action to reduce emissions from CO 2 and exploit its abundance in the atmosphere.
The top priority is therefore to avoid CO 2 emissions, mainly by developing renewable energies, also reducing energy consumption and improving energy efficiency. At the same time, energy companies are also working on Carbon Capture and Storage (CCS) which is a major strategy that can be used to reduce CO 2 emission. There are three methods for CCS: pre-combustion capture, oxyfuel process, and post-combustion capture. Among them, post-combustion capture is the most important one because it offers flexibility and it can be easily added to the operational units [2][3][4]. Beside the geological storage, the CO 2 mineralization or the algae feeding are two complementary technologies to "sequester" CO 2 [5].
Beyond these interesting approaches that tackle with the stakes with the stakes of reduction CO 2 emissions, there is also another approach, which interestingly sees CO 2 as a new chemical building block. Hence, even if chemistry accounts only for 10% of oil and gas consumption, thus same percentage of CO 2 emission, there is still interest to reduce environmental impacts of chemistry through the use of renewable resources, to follow the principles of Green Chemistry. Hence, considering CO 2 as a raw material, known as Carbon Capture and Use (CCU), notably, as a basic brick in the production of polymers, may have also interest to follow the principles of Green Chemistry. Hence, CO 2 could be a renewable and inexhaustible platform chemical, produced locally from multiple sources and find utility for the synthesis of commodities (methanol, urea, (in)organic carbonates, formic acid), fuel (methane, alcanes) and particularly polymers. The polymers annual production is around 400 million tonnes compared to the global CO 2 emission (nearly 40 Gt/y), the utilization of CO 2 a raw material to fabricate polymers will only show marginal effect on the anthropologic CO 2 emissions and climate change but it could draw the reuse of CO 2 and improves the circularity of polymer economy [6,7].
Until now, the production of polymers has been mainly based on petrochemical raw materials, i.e. mainly oil. However, we dispose of finite fossil resources while CO 2 is inexhaustible. In addition, converting oil into precursor chemicals also consumes energy, resulting in further CO 2 emissions, even if industries has developed improved energy efficacity for last decades. Moreover, the chemical industry has already made a lot of progress in implementing CO 2 as a new raw material [8][9][10][11][12][13][14][15][16][17].
The use of CO 2 for the manufacture of plastic is beneficial to the environment in two ways: First, the CO 2 is directly incorporated into the polymers and partially replaces oil as a raw material. In order to compare efficacy of the whole process from cradle to gate (raw material), a Life Cycle Assessment would be required. Recently, some companies have studied such analyses to improve their processes either conventional or new [18].
There are two chemical approaches to incorporating CO 2 into the development of polymer materials (Fig. 2): either directly using it as a polymer construction block, or indirectly through the involvement of CO 2 -based monomers to form other polymers. Both approaches are proven possible and feasible to a certain extent [19][20][21][22][23][24][25][26][27].

The direct approach
Chemical fixation of CO 2 is very interesting from a carbon use perspective, as CO 2 is a low-cost, non-toxic, carbon-rich resource that can be used to produce value-added chemicals and materials. The most promising area of use of CO 2 is the co-polymerization of CO 2 and epoxide monomers to obtain Polycarbonates (PCs), which has been pioneered by Inoue et al. in 1969 [28]. These polymers not only have excellent barrier properties to oxygen and water, but they also have excellent biocompatibility and biodegradability. Given their exceptional properties, PCs can be used as non-polluting materials for the production of disposable medical products, in addition to food packaging, adhesives and composite materials, etc. (Fig. 3) However, it is necessary to mention that the thermodynamic stability of the CO 2 , limits its use as a reagent. Hence the value of using energy-rich reagents such as epoxides in the presence of suitable catalysts (Fig. 4) [29][30][31][32][33][34][35][36][37][38].
Indeed, since the pioneering work of Inoue et al. who prepared Polypropylene Carbonate (PPC) from the CO 2 and Propylene Oxide (PO), using a heterogeneous catalytic water/diethylzinc system, the copolymerization of epoxides and CO 2 has been widely developed. In general, the catalysts used for copolymerization of CO 2 and epoxides are components based on transition metals or metals from the main group of elements that are abundant on earth, such as Zn, Co, Cr, Mg, Al, which are either insoluble or soluble in the reaction system during copolymerization.
The most accomplished CO 2 /epoxide copolymerization systems are based on the Transition Metal Complexes Cr (III), Co (III) or Zn (II) with Schiff base ligands. In the case of CO 2 copolymerization and Propylene Oxide (PO), Polypropylene Carbonate (PPC) with a molar mass of up to 300,000 g mol À1 can be prepared using a recyclable catalyst ((salen)Co(III)) [39].

Catalysts
Depending on the type of catalyst, the reaction can be directed to produce a specific type of polymer. A wide range of metal complexes (Zn, Cr, Co, Al, Fe) with the general formula "LnMX" (Ln is a suitable, often multidentate ligand; X is typically a halide or other suitable initiating group; M is typically a di-or trivalent metal cation) have been designed to promote the ROCOP of CO 2 with epoxides ( Fig. 5) [40,41].
These catalysts can be roughly divided into two categories: -The homogeneous catalysts, represented by cobalt and chromium with salen ligands. Homogeneous catalysts are more active and selective compared to heterogeneous ones, and are more suitable for research because their chemical structures are well defined. However, these first ones require a multi-step synthesis due to the complexity of the ligands (Fig. 6). However, regardless of the catalysts used in copolymerization, the polycarbonates produced are contaminated with metals, making them toxic and coloring during degradation. A disposal step after polymerization is therefore necessary to ensure a wide applicability, especially when it comes to products dedicated to electronic processing or in packaging.
Recently, great importance has been placed on processes that are environmentally friendly. This has resulted in the use of catalysts based on metal complexes of the main group.
Indeed, with catalysts such as Co(III) and Cr(III) that are effective, traces of metal residues are obtained inside the resin that can lead to problems of toxic and coloured degradation, which limits their fields of applicability. These affect their performance and therefore limit their applications. However, this is not the case for aluminum, one of the first metals studied as a catalyst since the discovery of CO 2 copolymerization and epoxides. Aluminum is becoming abundant on land, cheap and biocompatible. Aluminum complexes are also known to catalyze a wide range of other polymerization reactions, which would allow the development of CO 2 -based sequential copolymers other than epoxides. However, due to the competitive homopolymerization of aluminum-catalyzed epoxides, more work will be needed to improve its catalytic effects [42].

The indirect approach
The CO 2 can also be used to produce chemical components for polymer synthesis, particularly urea. Urea is widely used in the fertilizer industries as a rich source of nitrogen. Urea is non-toxic and is a raw material for the production of many important chemical compounds such as various plastics, resins and adhesives. Nowadays, approximately 200 Mt/y of urea is produced in the world. To produce this amount of urea, 150 Mt/y of CO 2 is required. The most common method for production of urea is reforming natural gas, producing both carbon dioxide and ammonia. Carbon dioxide can also be produced from combustion. Hence, considering, CO 2 , nitrogen, and water as the main products of the combustion process, capturing of CO 2 and removing water from the combustion gas can lead to a high purity N 2 . This N 2 could react with hydrogen to yield ammonia. Therefore, the production of urea by reaction of ammonia with CO 2 , captured from combustion flue gases, would increase the economic justification of CO 2 capture unit [43].
This opens up the possibility of obtaining a range of thermosetting polymers, such as Urea-Formaldehyde (UF) resins and Melamine-Formaldehyde (MF), as well as industrial plastics, such as Polyoxymethylene (POM) or Polymethylmethacrylate (PMMA). In the first case, UF resin can be obtained from urea, which is directly produced by the CO 2 's reaction with Ammoniac, and Formaldehyde, which is obtained from methanol derived from the CO 2 ; similarly, the ingredients of MF resins are Melamine, obtained from urea, and Formaldehyde. For the examples provided last, the POM can be produced from the CO 2 via formic acid, and PMMA from methyl Acrylate obtained from methanol derived from CO 2 [44].

Properties of polycarbonates synthesized from CO 2
Despite interesting properties, polycarbonates from catalyzed CO 2 reactions and epoxides exhibit a non-variable structure, linked to alternation of CO 2 and epoxide units, which limits their properties. Moreover, the absence of aromatic ring, compare to conventional polycarbonates, reduces their thermo-mechanical properties. Hence, the limited properties of these PCs can be extended by combining them with biopolymers, such as starch, or synthetic polymers, such as PMMA, where CO 2 -based PCs confer biodegradability or act as an important additive. In addition, these polycarbonates can also be treated using traditional processing techniques, such as injection molding or molten extrusion. This makes it possible to produce on an industrial scale. PCs from Bisphenol A-based CO 2 can also be synthesized to produce superior traditional polycarbonates, via greener pathways without toxic phosgene. Additionally, high Tg CO 2 -based polycarbonates have been reported by Kindermann et al. who succeeded in the copolymerization of CO 2 and limonene oxide to obtain polycarbonates with unprecedented high glass transition of up to 180°C [45]. However, currently, PCs produced by conversion of CO 2 are mainly end functionalized with hydroxyl groups and used as poly (carbonate-ether) polyols, for the synthesis of Polyurethane (PU) with diisocyanates [46].

Conclusion
In conclusion, it is important to note that while this area of research is hardly new, the use of CO 2 as a raw material remains one of the most interesting and visionary technologies. With limited fossil resources, the use of CO 2 as a chemical raw material is a promising approach to global carbon management. In this sense, the use of CO 2 to produce polymers has an obvious ecological advantage over conventional polymers. It is even possible to improve this situation by following the different approaches of direct and indirect use. The present review was focused on CO 2based materials that could be manufactured from precursors already produced in large scale (such as urea) or that are reaching the industrial scale (PC). Pioneering examples already show that the chemical use of CO 2 for the production of polymers on an industrial scale is possible. Hence, the use of CO 2 as an alternative raw material in the chemical industry is still in its infancy. Future efforts will aim to demonstrate the potential of this gas and initiate a possible change of image, from a greenhouse gas harmful to the environment to a new useful and sustainable raw material.