Making polycarbonates without employing phosgene: An overview on catalytic chemistry of intermediate and precursor syntheses for polycarbonate

Abstract

Conventional production of aromatic polycarbonates (PCs) involves interfacial polycondensation between phosgene (COCl2) and bisphenol A (BPA). This COCl2 process has several drawbacks such as environmental and safety problems involved in using the highly toxic COCl2 as the reagent, which resulted in the formation of chlorine salts of a stoichiometric amount, and in using copious amounts of methylene chloride as the solvent. For these reasons, environmentally friendly processes for PC production without COCl2 have been developed such as melt transesterification of BPA and diphenyl carbonate (DPC). However, COCl2-free production of DPC is not easy yet because of its severe equilibrium constraint, and therefore obtaining DPC efficiently is the most important step to develop a successful COCl2-free PC process. DPC can be obtained via two methods without using COCl2: two-step synthesis of DPC from dimethyl carbonate (DMC) and phenol and direct oxidative carbonylation of phenol. The DPC reacts with BPA to form PC precursors, which are amenable to the subsequent polycondensation step to obtain high molecular weight PC. Alternatively, BPA can be directly carbonylated with DMC or CO. In this work, we review and discuss different reaction routes and the involved catalytic chemistry of possible COCl2-free PC syntheses based on literature as well as our own experimental results. We compare reaction characteristics and the nature of PC precursors produced from different synthetic routes and provide a perspective on an improved COCl2-free PC process