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CO₂-derived Polymers

Polymers derived from CO₂ are an expanding field in materials chemistry, offering sustainable alternatives to conventional synthetic polymers. Resource scarcity and a growing interest in developing low-carbon “circular” economies have fuelled research into CO₂ based polymers, as the chemical industry increasingly prioritizes the mitigation of CO₂ emissions and the development of renewable chemical processes. Currently, the vast majority of commercial polymers are synthesized from fossil fuels (petrochemicals). The depletion of such resources has fuelled research into polymer development from renewable resources like CO₂. CO₂ is an attractive source of carbon, due to its natural abundance, low cost, nontoxicity, and nonflammability. Moreover, CO₂ produced as a waste product in other industrial processes can be recycled for reuse in polymer synthesis. While the utilization of carbon dioxide feedstock is still limited, due to the relative inertness of CO₂ and the large amounts of energy required for such reactions, recent breakthroughs in catalytic pathways have made it a viable replacement for environmentally burdensome industrial processes. Carbon dioxide utilization technologies are becoming ever more economically feasible, with new processes being developed on an industrial scale. The large global demand for plastic (over 407 million tonnes of plastic were produced in 2017) has also provided incentives for more sustainable plastics. Particularly successful reactions include the copolymerization of carbon dioxide with highly reactive epoxides to yield polycarbonates, which can then be converted into polyurethanes, and the catalytic chemical fixation of CO₂ by carbonation of oxiranes, oxetanes, and polyols to produce environmentally favourable di- and polyfunctional cyclic carbonates, which can then be used to form non-isocyanate polyurethane (NIPU). Such technologies are being incorporated into long-term action plans as economies transition to completely renewable energy sources.

Properties and Applications of CO2-Based Polymers

Polycarbonates from CO2 and epoxides have useful and attractive material properties. They are thermoplastic, highly transparent, UV stable, and have a high Young’s modulus. They also break down cleanly, giving them an environmentally friendly afterlife. Polycarbonates developed by Gregory et al. exhibited enhanced thermal properties, with high glass-transition temperatures. They also displayed minimal degradation in water and CdCl₂. These properties give them the potential for biomedical applications, particularly given their biodegradability and biocompatibility (devices made from such polymers do not need to be removed in invasive secondary surgeries). Heterogenous CO₂ polymerization produces highly interconnected macroporous polymeric networks, while fluorinated polymers can be used in medical devices and as hydrophobic coating. Aliphatic polycarbonates have excellent thermal and mechanical properties, along with enhanced scratch-resistance and favourably low costs. Their temporal stability and biodegradability make them favourable for use in medical devices, and they can also be modified by altering side chains and polymeric branching. Polyurethanes and carbamates have widespread applications in the agri-chemical, preservative, cosmetic, and medical industries. PEC and other polycarbonates have also been favourably assessed for drug delivery, controlled drug release, model protein release, biocompatibility, drug-eluting stent coating, and as tissue scaffold materials. NIPU-epoxy coatings also exhibit improved adhesion on wet surfaces, thermal stability, and wear resistance, making them attractive for biomedical applications.

History

1950s-2000

Polymers derived from CO₂ have enjoyed a sustained interest, even as they only recently have become commercially viable. Patent literature from the 1950s described processes which could generate polyureas from CO₂ and diamines. In 1969, Inoue et al. reported the copolymerization of CO₂ with an oxirane over a zinc catalyst. Research from Inoue (1976), Rokicki and Kuran (1981), and Super and Beckman (1997), demonstrated the copolymerization of CO₂ with various cyclic ethers, although their catalytic yields were not commercially practicable. Patents for CO₂-based polymers filed in the 1980s similarly only demonstrated low catalytic activity (generally 50 grams polymer per gram of metal; for comparison, polyolefin activity can be over 100,000 grams polymer per gram of metal). Processes were generally not conducive to commercial applications due to high production costs, low rates of reaction, or disadvantageous physical properties.

Interest in polymers synthesized from CO2 increased in the 1990s, with new reactions of CO₂ with diyne co-monomers. Increased catalytic activity was also shown in the copolymerization of CO₂ with cyclohexene oxide. Darensbourg and Holcamp developed various phenoxy zinc catalysts, with improved catalytic activity (over 400 grams of polymer per gram of zinc). Super et al. developed a high-activity fluorinated zinc catalyst to generate copolymers of CO₂ and cyclohexene oxide. Coates et al. demonstrated the use of a high activity zinc catalyst in the copolymerization of CO2 and cyclohexene oxide at low CO₂ pressure. Such a process was more industrially feasible, as CO₂ reactions often require high temperatures and pressures, making them energetically inefficient. CO and olefin copolymers were also developed by Shell in the 1990s, using palladium-based catalysts.

During the late 1990s, the EPA, interested in methods for pollution prevention, funded investigations into the potential of biocatalytic polymer syntheses from carbon dioxide. The project, led by Alan Russell (University of Pittsburgh), used supercritical CO₂ as a solvent to investigate the solubility of various monomers. From these CO₂ soluble monomers, fluorinated polyesters were synthesized. Increased solubility of fluorinated diols produced polymers with greater molecular weights. One such fluorinated polyester, synthesized from divinyl adipate and 3,3,4,4,5,5,6,6-octafluorooctan-1,8-diol, resulted in a polymer with a weight average molecular weight of 8232 Da. Using supercritical CO₂, they were able to increase the efficiency of polymerization almost 10,000 fold.

Supercritical carbon dioxide (scCO₂) also became widely used in CO₂-derived polymers during the 1990s. scCO₂ is a non-flammable, non-toxic, relatively inert, and recyclable solvent. Polymerization reactions can be carried out in scCO₂ via step-growth and chain-growth mechanisms, with the most common techniques including radical polymerization, including free radical, atom transfer radical polymerization (ATRP), reversible addition–fragmentation chain transfer polymerization (RAFT), and nitroxide-mediated radical polymerization (NMP) (Boyère). ScCO2 is also used in the homogeneous synthesis of fluoropolymers via free radical polymerization and cationic polymerization.

2000-2019

Reactions have become ever more efficient and productive. Banerjee et al. activated organic substrates with caesium carbonate, prior to reaction with CO₂. They found that CO₂ reacts with 2-furan carboxylate (FC), which can then form furan-2,5-dicarboxylic acid (FDCA), in turn generating polyethylene furandicarboxylate (PEF). FC can can be derived from biowastes, like sawdust, and PEF is a sustainable alternative to the petrochemical plastic polyethylene terephthalate (PET). (Over 45 million tonnes of PET are produced annually, making it an attractive target for a more sustainable replacement.) Kyoko Nozaki et al. demonstrated the copolymerization of CO2 with butadiene, using a metastable lactone intermediate for a favourably exothermic polymerization. They were able to successfully incorporate up to 33 mol% (29 wt%) of CO₂ into their derived polymer. Ming Luo et al. used carbon dioxide and its sulfur analogues (CO₂, CS₂ (carbon disulfide), and COS (carbonyl sulfide)), in their polymer synthesis. They found that a copolymerization of alternating one-carbon blocks and epoxides could yield new polycarbonate and polythiocarbonate copolymers. Using a zinc-cobalt(III) double metal cyanide complex [Zn-Co(III) DMCC] catalyst, they were able to form new C-O and C-S bonds. This process lends itself to the synthesis of polymers with customized structures and properties.

Olefins and polyolefins can also be produced from CO₂ using renewably sourced hydrogen and modified Fischer-Tropsh catalysts. Aliphatic CO₂-derived carbonates are also a developing area of research, along with the production of aromatic polycarbonates from CO₂, eliminating the need for phosgene, a toxic reagent. Carbamates can also be formed by reacting N-nucleophiles with CO₂, creating useful precursors for the synthesis of isocyanates, which are used in the formation of poly(urethane)s. Dimethyl carbonate (DMC), another polymer precursor traditionally produced from phosgene and methanol, can also be produced more safely and sustainably from CO₂-derived methanol and CO₂, or urea and CO₂.

While copolymerization catalysts comparable in productivity to the Ziegler-Natta catalyst in polyolefin production have not yet been developed, PPC and other CO2-based copolymers have been developed industrially. PPC is becoming more competitive with polyolefins and polyethylene, and yields materials with higher tensile, flexural, and tear strength, as well as improved adhesion and load bearing capacities. Catalytic chemical fixation of carbon dioxide through the carbonation of oxiranes, oxetanes, and polyols also present routes to the the formation of non-isocyanate polyurethane (NIPU), via either the polycondensation of diacarbamates or acyclic dicarbonates with diols or diamines, or, alternatively, the polyaddition by ring-opening polymerization of di- and polyfunctional cyclic carbonates with di- and polyamines.

Industrialization and Future Developments

As a cornerstone of the European Vision 2030 plan, CO₂ recycling has enjoyed widespread development over the past ten years. The European Union’s strategic action plan to achieve resource-efficiency, the Smart CO₂ Transformation (SCOT) project, relies upon the successful implementation of synthetic CO₂ reactions, through the use of 'waste' CO₂ recycled from industry. Through such initiatives, SCOT anticipates that “Carbon Dioxide Utilization (CDU) can contribute to make European industry resource-efficient, greener, sustainable and competitive.”

In May 2009, Germany initiated the most ambitious research program into CO₂-based chemical processes, called “Technologies for Sustainability and Climate Protection: Chemical Processes and Use of CO₂.” This program generated over 150 research and development projects between 2010 and 2016, with state and industrial funding of over 150 million euros. In June 2015, a further initiative was announced, entitled “CO₂Plus—Broadening the Raw Material Base by CO₂ Utilization.”

However, while progress has been rapid (there was a 100-fold increase in CO₂ utilised in polycarbonate production between 2013 and 2016, as well as a commensurate increase in polyurethane production), CO₂ processes are still in the fledgling stage. The main impediments to industrial implementation remain technical, as new reactions still lack commercial competitiveness with traditional processes. Moreover, energy input in CO₂ reactions must be from renewable energy supplies, which is challenging for traditional chemical plants.

Nevertheless, a number of commercial polyurethane plants were due for completion in 2016, and the BMBG funding initiative in Germany resulted in a Bayer Material Science plant for polyol production from CO₂, a precursor for polyurethane, with production capacity of several 1000 tonnes per year. Their process incorporates up to 22 wt% CO₂ polyether carbonate polyols, with optimal flexibility in the polymer chains. The plant envisages expansion into a 100ktpa facility by 2023. BMBF funding has enabled potential technologies to be scaled up to the industrial scale, in order to achieve the 'Energiewende.' In addition, Evonik Industries AG and Siemens AG are working on a two-year joint research project launched 18 Jan 2018, called Rheticus, to convert CO via electrolysis and fermentation processes into butanol and hexanol, plastic feedstocks. The first plant based on this research is scheduled to enter production by 2021. Another BMBF funded project involving Siemens and BASF developed a blended plastic with similar properties to ABS, using PPC from CO₂ obtained from power plant emissions, producing a material made from 70% green polymers. This plastic has been used as in vacuum cleaner covers by Bosch-Siemens-Hausgeräte (BSH).

In the UK, Econic Technologies is producing catalysts for polycarbonate production on a small commercial scale. BASF is investigating sodium acryelate, DME, methanol, and polymer production from CO₂, although as of 2016 such processes were still in the research phase. TNO in the Netherlands and SINTEF in Norway are both investigating industrial syntheses of cyclic carbonates, although both (as of 2016) are still in the research phase.

Outside of Europe, Novomer has piloted the production of a range fo CO2 derived Polyols under the Converge tradename, which contain up to 50% weight CO2 and have superior material properties. These polypropylene carbonate (PPC) polyoles are targeted for uses as adhesives, coatings, sealants, elastomers, and rigid and flexible foams. These polyols, when used in conjunction with existing formulations, produces foams with higher tensile and tear strength and increased load bearing capacity, as well as up to 40-50% lower flammability than conventional polyether, polyester, and polycarbonate polyols. Ford Motor Co has expressed an interest in developing such foams and plastics for vehicle use. Novomer also manufactures aliphatic polycarboates from epoxides reacted over a catalyst with CO₂. Asahi Kasei Corp. also produces aromatic polycarbonates from CO₂ and bisphenol-A. Newlight Technologies has developed a plastic called Air Carbon, made from CO₂ and methane, which has been used by the furniture manufacturer KI and by Sprint for mobile phone cases.

In March 2018, Nova-Institute published the first worldwide technology report on CO₂ based polymers, entitled “Carbon dioxide (CO2) as a chemical feedstock for polymers – technologies, polymers, developers and producers.” The study provides an overview of polymer production and pathways, including aromatic phosgene-free polycarbonates (PC), aliphatic polycarbonates (APC), such as polypropylene carbonate (PPC), polyethylene carbonate (PEC), polylimonene carbonate (PLimC), and polyurethanes (PUR) that are synthesised with CO2-based polyols. It also includes the fermentation of CO2 or CO2-rich syngas, producing lactic acid or succinic acid, which serve as building blocks in the synthesis of polymers like polylactic acid (PLA) and polybutylene succinate (PBS). Polyhydroxyalkanoates (PHAs) can also be directly derived through CO₂ fermentation. Electrochemical pathways, including the production of polyethylene terephthalate (PET) from monoethylene glycol (MEG),are also detailed. In all, the report lists about 30 companies from Asia, Europe, and North America involved in commercial CO₂-based polymer production.

See also

Polymer

Copolymer

Polycarbonate

Carbon dioxide