New Chemistry for Renewable Raw Materials
Kyoko Nozaki, The University of Tokyo
Currently, fossil fuels are dominant carbon resource for chemical production. Because depletion of petroleum is anticipated, finding an alternative resource is an emerging requirement for chemical industry. Carbon dioxide is one of the attractive feedstock in a sense that it is an abundant, inexpensive, and less toxic compound. Aiming at fixation of CO2 into organic materials, we focused our attention to the synthesis of aliphatic polycarbonate by the alternating copolymerization of propylene oxide with CO2.
2. Alternating Copolymerization of Epoxide with CO2
The copolymerization of epoxides with CO2 has been intensively studied in the last decade as one of the most promising processes for CO2 utilization. The concomitant production of cyclic carbonate is an incident problem in the reaction of propylene oxide. Recently, several highly active catalysts based on Co/salen complex have been developed. However, such a high reactivity also leads to the formation of PC at higher conversion (>50%): the polymerization mixture solidified to retard the intermolecular propagation, resulting in relative acceleration of intramolecular degradation, so-colled back-biting. Thus, high catalytic activity has been incompatible with the suppression of cyclic carbonate formation.
In order to achieve the high epoxide conversion without production of cyclic carbonate, we designed cobaltate complex 1 with two acetate ligands and a salcy-type ligand. The key of the catalyst design is the piperidinyl and piperidinium arms. When the two acetate ligands initiate the copolymerization, a piperidinium arm should control nucleophilicity of the propagating species by protonating the anionic propagating species released from the cobaltate center. The protonated propagating species is not nucleophilic enough to form cyclic carbonate through the back-biting, while it can react with carbon dioxide or activated epoxide once it is deprotonated by one of the two piperidinyl groups.
3. Production of molded plastic and plastic film
The polymerization products were dropped into organic solvent and the precipitates were collected and purified. The materials were applied to injection-molding or film process. Further studies on the physical properties are now in progress.
The synthesis of plastic from CO2 seems to be a promising approach for the usage of renewable raw materials in chemical production. Further improvement of the process efficiency is requested in order to realize large-scale production.
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