Technology to copolymerize carbon dioxide and propylene oxide (PO) was first discovered in 1960s, but has not been commercialized due to the formation of cyclic propylene carbonate (CPC) by a “back-biting” reaction, which leads to the formation of unstable, low-molecular-weight copolymers. This limitation has essentially been eliminated, thanks to a new catalyst developed by Professor Kyoko Nozaki at the Department of Chemistry and Biotechnology, Graduate School of Engineering, the University of Tokyo (Japan; edlinks.chemengonline.com/6901-536).
The new catalyst — a bis-(piperidinyl methyl)-salen cobalt(III) complex with two acetate ligands — is synthesized by the reaction of cobalt acetate with the corresponding disalicylidenediamine, followed by oxidation in the presence of an excess amount of acetic acid and air. The catalyst enables the selective formation of copolymers with alternating CO2 and an epoxide, such as PO, 1-butene oxide, and 1-hexene oxide.
For example, the catalyst has been used to make a copolymer of regularly alternating CO2 and PO molecules with a number-average molecular weight of 26,500. The reaction takes place in a DME (1,2-dimethoxyethane) solvent under 14 bar CO2, with a 99% yield and a 97% selectivity. CPC formation is suppressed by capping the copolymer terminus by a proton coming from the piperidinium branch of the catalyst. This copolymer has a high (250°C) decomposition temperature, making it suitable for injection molding. It also has a low birefringence, a high gas permeability and flexibility, and is biodegradable.
The commercial production of such copolymers provides an opportunity to utilize CO2, thus reducing the amount of this greenhouse gas released into the atmosphere. Taking this opportunity, a project has started to study the commercialization of aliphatic polycarbonates made from CO2 and epoxides. Supported by the New Energy & Industrial Technology Development Organization (Kawasaki, Japan), the project involves three universities (including the University of Tokyo) and four companies from Japan.