Green Synthesis, Properties and Applications of Polyurea

Polyurea is a kind of high performance polymer materials, due to their thermal stability and chemical resistance. However, polyurea is usually prepared from isocyanate, which is toxic and easy to react with water. Therefore, studies on the green synthesis strategy of polyurea and the development of polyurea with various performances are very important for potential utilizations. Herein, the green synthesis methods of polyurea are reviewed, including directly or indirectly using CO₂ as carbonyl block: (1) Polyurea can be prepared by direct polycondensation of CO₂ and diamine. Catalyst can accelerate the reaction process and shift the balance to the right. It is easy to remove water, thereby improve the molecular weight and properties of polyurea by secondary polycondensation. (2) CO₂ can react with methanol, phenol, ethylene oxide and propylene oxide to form dimethyl carbonate, diphenyl carbonate and cyclic carbonate. These carbonates can react with diamine compounds to form carbamates. Then, polyurea can be prepared by self-condensation of carbamate or by transurethane reaction of carbamate with diamine. High molecular weight polyurea can be obtained by the removal of methanol, phenol and so on under mild conditions. (3) CO₂ can react with NH₃ to form urea, and then polyurea is prepared by the condensation reaction of urea and diamine. The produced NH₃ in the reaction process could be easily removed, leading to high molecular weight polyurea. The influence of catalysts, chemical structure and molecular weight of diamines on the properties of polyurea like molecular weight, thermal stabilities and mechanical properties are discussed. In addition, the properties and possible applications of CO₂ based functional polyureas such as thermoplastic polyurea plastics and elastomers are summarized. Finally, the emphasis of future researches and the vast potential for the development of polyurea materials are prospected.