Copolymerization Of2-methyl Aziridine With CO₂Catalyzed By Bis-salicylaldimine Metal Complexes

Synthesis of polymer materials from carbon dioxide could not only consume the greenhouse gas, but also could alleviate environmental pollution through natural degradation of the produced materials. Polycarbonate copolymerized from carbon dioxide with epoxide compounds has been commercialized in some extent. However, few reports could be found for the copolymerization of carbon dioxide with cyclic amines to synthetize aliphatic polyurethane materials. Due to the low reactivity of carbon dioxide, the design of highly active catalysts is a crucial issue for the carbon dioxide involved copolymerization.In the present thesis, six salicylaldiminato ligands were designed and synthesized by the reaction of salicylaldehyde and its derivatives with aniline compounds through schiff base condensation. Thereafter, bis-salicyladiminato copper(II) and cobalt(II) metal complexes were prepared, and their catalytic behavior for the copolymerization of carbon dioxide with2-methylaziridine was evaluated. The obtained ligands and metal complexes were characterized by using1HNMR, FTIRandEAetc.The three-membered cyclic amine compounds have a strong ring tension. Therefore, in the reaction system of carbon dioxide with2-methylaziridine, two reactions are involved, one is the copolymerization forming carbamate (urethane) linkage, and the other is the ring-opening homopolymerization forming amine linkage. In most cases, the obtained polymer is actually a copolymer containing both the carbamate and the amine linkages. The obtained polymer has enhanced urethane content through the catalysis of bis-salicyladiminato metal complexes, which means a promoted carbon dioxide insertion. A relatively bulky tertbutyl group on the ligand resulted in a lower catalyst activity of the catalyst; while a less bulky methyl group with somewhat electron donating effect on the ligand resulted in an enhanced catalyst activity. The metal complex bearing an electron withdrawing nitro group showed the lowest catalyst activity. It is also found that higher temperature and pressure are beneficial for the copolymerization, especially when carbon dioxide in supercritical state yielding polymers with both higher urethane content and higher molecular weight.Since the polymer obtained from the copolymerization of carbon dioxide with2-methylaziridine contains both hydrophobic carbamate chain segments and hydrophilic amine chain segments, its aqueous solution showes an obvious thermosensitive feature. The alkaline polyamine chain segments provide the polymer a pH sensibility. The concentration of polymer aqueous solution has a less effect on the lowest critical solution temperature (LCST). The LCST just decreased from54℃to50℃when the polymer aqueous solution concentation increased from5mg/mL to50mg/mL. The LCST is greatly influenced by the pH value. In the pH range between8.0and12.0, the polymer aqueous solution showed a gradually decreased LCST with the increase in pH value. At neutral condition (pH7.0), no phase transition was observed from the polymer aqueous solution, while the polymer became insoluble in water when the pH value beyond13.0at room temperature.