In general, several different types of resin are labeled "plastic". Among them, polycarbonates are polymers that contain a carbonate group in the molecular structure. Polycarbonate was developed in 1959 by Bayer in Germany and GE in the United States at about the same time, and in the 1970s it became widely used due to its excellent properties. The high impact resistance of polycarbonates (approximately 200 times greater than that of glass; polycarbonates show no breakage, even when struck by a hammer) makes them particularly suitable for the fabrication of construction-site helmets, protective goggles, and industrial parts. Moreover, their transparency and resistance to deformation enables their utilization as CD and DVD substrates (which can read and write data using laser beams). With an exponential increase in the demand for optical storage media in the 1990s, the production volume of polycarbonates increased significantly.

Polycarbonates are mainly (nearly 80%) produced by the phosgenation process that uses toxic phosgene (COCl₂) and bisphenol A as starting materials. Phosgene, with extremely high reactivity, undergoes facile reaction with bisphenol A to form a carbonate group (carbonate bond).

Although the synthesis of polycarbonates involves a simple one-step reaction of phosgene and bisphenol A, the actual synthesis process is extremely complex and problematic (from the viewpoint of GSC). First of all, the process uses toxic phosgene as a starting material.

Furthermore, methylene chloride, which is suspected to be carcinogenic, is used as a solvent. Methylene chloride is highly volatile (boiling point of 40°C) which makes its recovery difficult, and dissolves easily in water which makes its recovery from wastewater difficult. Moreover, a large amount of sodium chloride is produced as a byproduct. Impurities containing chlorine remain inside the synthesized polycarbonate resin, requiring the resin to be thoroughly washed if it is to be used as an optical material, which increases the amount of wastewater. The treatment for this wastewater results in tremendous environmental impact and costs.

It is extremely challenging to replace a mainstream process technology with a new alternative. Nevertheless, the expected increase in the demand for polycarbonates prompted several companies (including Asahi Kasei) to start developing alternative polycarbonate production processes without phosgene in the 1980s.

Each company strived to develop an environmentally-benign and low-cost production process. Asahi Kasei focused on "the reason why phosgene had to be used" while developing the new production process. Then, for the first time in the world, a polycarbonate production process in which carbon dioxide and ethylene oxide replaced phosgene and were reacted with bisphenol A was developed.

In this process, ethylene carbonate is formed from carbon dioxide and ethylene oxide, which is then converted to diphenyl carbonate and polymerized with bisphenol A to produce polycarbonate. The byproduct ethylene glycol is the end product. This process does not use toxic phosgene or methylene chloride; instead, it utilizes carbon dioxide (discharged during the production of ethylene oxide) as the starting material. Furthermore, waste liquids and waste products are reduced so that the burden for their treatment is small, making the process an exceptional one from the viewpoint of GSC.

A reaction in which diphenyl carbonate is used in place of phosgene and polymerized with bisphenol A to produce polycarbonate has been known. This reaction is called transesterification, which produces polycarbonate and the by-product phenol. However, this process is an equilibrium reaction. Therefore, to manufacture polycarbonates with a high degree of polymerization (suitable for industrial use), the mixture of starting materials is heated to a high temperature and phenol is continuously removed under reduced pressure during the process.

Conventionally, the starting material (diphenyl carbonate) is synthesized from phenol and phosgene. The use of phosgene would mean that the new process would be no different from the conventional phosgenation process. The developers once again considered the fact that phosgene (COCl₂) is formed of carbon monoxide (CO) and chlorine (Cl₂). Phosgene is used for polycarbonate synthesis owing to its structure and high reactivity. The main structural feature of polycarbonates is the carbonate group (which originates from carbon monoxide), and the high reactivity of phosgene originates from chlorine, which is a halogen. Both, carbon monoxide and halogens are toxic.

Therefore, they thought of using carbon dioxide, which also has the necessary structure, in place of carbon monoxide.

Carbon dioxide is inexpensive and readily available; however, it is stable and typically unreactive. Thus, ethylene oxide (which is highly reactive, but significantly less toxic than phosgene) was used to synthesize ethylene carbonate from carbon dioxide. However, the ethylene carbonate is not highly reactive. Thus they tried to obtain the final object through sequential conversions by transesterification towards a more highly reactive substance. Although the proposed synthetic strategy involved a larger number of steps than the phosgenation process, it aimed to "kill two birds with one stone" by eliminating the use of toxic materials (phosgene, carbon monoxide, and chlorine) and ensuring the utilization of carbon dioxide (the byproduct generated in ethylene oxide plants) as a starting material after purification.

It is difficult to use this new process for the production of diphenyl carbonate monomers. Although the chemical reaction appears simple on paper, the equation represents an equilibrium state; therefore, the reaction might be too slow to be practicable. In industrial manufacturing, a high reaction efficiency (which indicates the amount of starting material that is converted into the target substance) is vital. This is why phosgene has been used until now. Phosgene exhibits high reactivity, enabling the efficient synthesis of polycarbonates. On the other hand, phosgene also reacts easily with biological components, which causes it to be highly toxic.

In the newly developed process, ethylene carbonate (formed from the typically unreactive carbon dioxide) is sequentially converted to dimethyl carbonate and diphenyl carbonate with higher reactivity. The transesterification is an equilibrium reaction in which the starting material is more stable than the product; thus, the reaction does not spontaneously proceed toward the target substance. For example, methyl phenyl carbonate is generated during the synthesis of diphenyl carbonate from dimethyl carbonate and phenol. However, the equilibrium constant of this reaction, which indicates how easily the reaction will proceed, is extremely small (in the order of 10^-3 to 10^-4). The reason why the developers took on the challenge of this seemingly impossible reaction is that they believed that developing a new process will not only contribute to the environment, but also become an outstanding technology that would change society.

Continuously removing a product from the reaction system facilitates the progress of a non-spontaneous equilibrium reaction toward the product side. Therefore, continuously removing the byproduct methanol could facilitate the production of diphenyl carbonate. Consequently, a production process using the "reactive distillation method" was developed. In this method, reaction and distillation are simultaneously carried out in a single reaction vessel. Therefore, methanol can be evaporated and removed during transesterification, enabling the forward reaction to proceed efficiently. However, this method has been rarely applied in industrial manufacturing because it is difficult to control.

Many studies were patiently conducted in the laboratory regarding optimum conditions for carrying out reactive distillation, and the results were reflected in a large-scale "reactive distillation column" having a refined structure fit for industrialization. The newly developed reactive distillation column comprised several chambers (stacked in 10 or more layers) where reactions occurred; these chambers were heated from below (to evaporate methanol). According to the developers, this technology was the most difficult to develop.

Computer simulations, which have progressed rapidly in this era, contributed significantly toward the development of this technology.

Development of a unique polymerization device for improving the degree of polymerization The synthesis of polycarbonates involves the polymerization of diphenyl carbonate and bisphenol A along with the continuous removal of phenol. With the progress of the reaction, the degree of polymerization and molecular weight increase, increasing the viscosity of the reaction mixture. Consequently, phenol gradually becomes less likely to evaporate, and polymerization ceases. In such cases, an agitation apparatus is used to agitate the mixture, which is heated to facilitate the evaporation of phenol. However, the polycarbonate production process requires low-pressure conditions. Using an agitation apparatus would discolor the polycarbonate product (to yellow) owing to the effect of atmospheric oxygen, making it unsuitable for final products.

Therefore, developers formulated a new polymerization technology to produce high- quality polycarbonates with a high degree of polymerization.

Immersing a polycarbonate (pre-polymer) with a low degree of polymerization and low molecular weight into acetone generates a porous crystal (in the solid state). Heating this crystal pre-polymer under reduced pressure promotes polymerization, generating polycarbonates with a high degree of polymerization (through solid-state polymerization).

However, liquid-phase polymers with high fluidity, which are easier to handle than solid- state polymers, are preferred for industrial applications. Thus, the developers considered several methods for realizing a state having large specific surface area in the molten state without mechanical agitation, similar to the porous pre-polymer crystal in the solid state polymerization. In the end, they found that by heating and melting the prepolymer under reduced pressure and allowing it to drop by gravity into a stringlike form, the produced phenol was removed by foaming and evaporation, and at the same time, the foaming action caused the polymer to be agitated so that the reaction would proceed effectively. The developers were certain that high-quality polycarbonate could be formed by using this method. Subsequently, a polymerization device based on these principles (the gravity-based non-agitation melt polymerization method) was designed and industrialized.

Following figure shows the device comprising cylindrical container and wires (stretched to the vertical direction). Under the high-temperature and reduced-pressure conditions, a reaction mixture containing the pre-polymer was allowed to drop (by gravity) along the wires. Then, the phenol foams and evaporates, and polymerization of diphenyl carbonate proceeds. This method did not require the agitation of the mixture, thereby preventing the entry of air into the mixture, enabling the final product to retain its original color. Furthermore, the conditions for the polymerization can be easily controlled, and various types of polycarbonates having low molecular weight to high molecular weight can be produced with high quality.

By developing the "process for synthesizing monomers (diphenyl carbonate) using reactive distillation" and the "process for synthesizing polymers (poly carbonate) with a new polymerization device," Asahi Kasei became the first company in the world to manufacture polycarbonates from carbon dioxide, ethylene oxide, and bisphenol A. More than 20 years had passed before the first plant started operation in 2002. It can be presumed that cooperation bridging the boundaries among technology experts responsible for developing the reactions, processes and devices, and their extraordinary enthusiasm and tenacity to resolve the continuously arising issues one by one provided the background for accomplishing this great achievement.

Upon taking a closer look at this process, it is apparent that the carbon dioxide emissions into the atmosphere which is the byproduct of ethylene oxide synthesis can be suppressed because the carbon dioxide formed during the production of ethylene oxide is used as the starting material. Furthermore, the methanol and phenol which are added during the reaction are circulated within the system and reused. Ethylene glycol, another end product, is used as starting material for polyester fibers and is not disposed. Moreover, ethylene glycol can be produced with less energy as compared to the conventional method.

The non-phosgene polycarbonate production process developed by Asahi Kasei is green and sustainable. The cost for equipment for this process is one-half compared to that of the phosgenation process because the overall process is simple: toxic substances requiring strict control and handling are not used, treatment for by-products, waste products and impurities are reduced, and continuous production is possible. Thus, costs for starting materials and electricity can also be suppressed. In the past, phosgene leakage accidents have occurred in phosgene- producing plants. The adoption of the non- phosgene polycarbonate production process could significantly reduce such accidents. The Asahi Kasei production process was used to manufacture 660,000 tons of polycarbonate in 2012, which is 14% of the total amount of polycarbonates produced globally. As approximately half of the newly established polycarbonate production plants have adopted this process, polycarbonate production using this green and sustainable process is expected to increase significantly in the future.

After commercialization, the developers have continued to improve this new technology. In the non-phosgene process developed by Asahi Kasei, ethylene oxide and the byproduct CO₂ are used as starting materials. Ethylene oxide is difficult to transport. Therefore, in the absence of ethylene oxide production plants in the vicinity, ethylene carbonate or dimethyl carbonate (which can be easily transported) are produced and transported to polycarbonate production plants.

Asahi Kasei has also developed a new process called the "dialkyl carbonate (DRC) process" for manufacturing diphenyl carbonate without ethylene oxide; demonstration tests are currently underway. The challenge for new technology still continues today.