The synthesis of polycarbonate (PC) is a process of building polymer chains through polymerization reactions between monomers. The core of this process lies in the effective connection of bisphenol A (BPA) with a carbonate source to form a polymer with specific structures and properties. Currently, the mature industrial synthesis methods are mainly divided into two categories: the phosgene method and the transesterification method. These two processes have their own characteristics in terms of raw material selection, reaction mechanisms, and environmental impact.
The phosgene method was the earliest route to achieve industrialized production. Its principle is to utilize the interfacial condensation reaction between BPA and phosgene under alkaline conditions. In the reaction, the phenolic hydroxyl groups of BPA form phenolates under the action of alkali, which then undergo nucleophilic substitution with phosgene dissolved in the organic phase, gradually forming carbonate bonds and precipitating the polymer. The advantages of this process are fast reaction speed, narrow molecular weight distribution of the product, and stable optical and mechanical properties, making it suitable for producing high-purity PC resin. However, phosgene is a highly toxic gas, requiring extremely high operational safety standards, and it produces large amounts of hydrogen chloride and sodium chloride as byproducts, resulting in a heavy wastewater treatment burden and posing challenges to the environment and occupational health.
To overcome the environmental and safety risks of the phosgene process, transesterification (also known as melt polycondensation) has been widely adopted. This method uses bisphenol A and diphenyl carbonate (DPC) as raw materials. Under an inert atmosphere and with a catalyst, a transesterification reaction produces phenol and oligomers. The phenol is then removed under high temperature and high vacuum conditions, followed by polycondensation to obtain high molecular weight PC. Transesterification eliminates the need for phosgene, uses relatively mild raw materials, and the byproduct phenol can be recycled, making the overall process more in line with green chemical engineering principles. However, this route requires precise control of reaction conditions; for example, temperature, vacuum level, and catalyst type directly affect molecular weight growth and product color, necessitating accurate regulation to ensure consistent performance.
In recent years, to further enhance sustainability and process economy, non-phosgene melt transesterification technology has gradually incorporated bio-based or recyclable carbonate sources, exploring low-carbon emission pathways. Furthermore, copolymerization modification synthesis has been applied to adjust the toughness, heat resistance, or processing fluidity of PC by introducing small amounts of third monomers or chain segment regulators during polymerization to achieve targeted performance optimization.
Overall, the synthesis methods of PC have evolved from the early highly toxic and risky routes to safer, cleaner, and more controllable ones. Process selection needs to comprehensively consider product quality, environmental protection requirements, and economic efficiency. In the future, green and functional synthesis will remain the key directions for technological development.