Urea as a Monomer: Synthesis and Characterization of Semi-Aromatic Thermoplastic Non-Isocyanate Polyureas

Abstract

Non-isocyanate polyureas offer a safer alternative to traditional polyureas with the avoidance of health and environmental hazards of isocyanate monomers. This manuscript presents a solvent-free melt polycondensation method using bio-sourceable urea with a combination of aliphatic and aromatic diamines, which bypasses isocyanate precursors. This approach allows direct incorporation of less nucleophilic aromatic diamines, previously challenging in earlier non-isocyanate routes. Adjusting the aromatic diamine content (5-20 wt.%) created polyureas with excellent thermal stability (Td,5% > 318 °C) and widely varied mechanical properties. Most notably, Young's moduli spanned three orders-of-magnitude (3 MPa to 1.1 GPa), from soft elastomers to high modulus thermoplastics. Although atomic force microscopy and X-ray scattering studies revealed amorphous morphologies without distinct nanoscale phase separation, ductile films exhibited excellent mechanical performance due to the presence of the urea functionalities, i.e., associative bidentate hydrogen bonding serving as physical crosslinks. This report demonstrates the efficacy of synthesizing high-performance semi-aromatic polyureas in the presence of an organocatalyst with tunable properties using greener synthetic methods without sacrificing material performance.