Thermoplastic Polyureas with Excellent Mechanical Properties Synthesized From CO₂-Based Oligourea

Polyureas (PUas) are versatile polymers with a wide range of applications in areas such as coatings, construction, automotive, and aerospace. The isocyanate index (R index) refers to the molar ratio of the equivalent number of isocyanate groups (─NCO) to the total equivalent number of amino groups (─NH₂) or hydroxyl groups (─OH) in the synthesis of PUas or polyurethanes. The relationship between the R index and material properties is less explored in PUa chemistry. Moreover, traditional PUas are thermosets without recyclability or reprocessability, hindering broader industrial adoption. Herein, a series of thermoplastic PUas with excellent mechanical and thermal properties are prepared from a CO₂-based oligourea (OUa) and 4,4'-diphenylmethane diisocyanate (MDI) with different R indices (R index = 1, 1.05, 1.11). The R index has a significant impact on the structure and molecular weight of PUas, thereby significantly affecting their mechanical properties and thermal stability. When the R index is 1.05, the synthesized PUa (PUa-1.05) exhibits the best overall performance, with high tensile strength of ≈75 MPa, Young's modulus of ≈1100 MPa, toughness of ≈104 MJ·m^-3, and great reprocessing capacities. In addition, the PUas possess high thermal stability, with an initial decomposition temperature of 312 °C.