Urea as a monomer for isocyanate-free synthesis of segmented poly(dimethyl siloxane) polyureas

Poly(dimethyl siloxane)-containing (PDMS), segmented polyureas represent an important class of high-performance elastomers that leverage the low glass transition temperature (T_g) of PDMS and superior mechanical reinforcement from bidentate hydrogen bonding. Current synthetic methods exclusively employ highly reactive/toxic isocyanate reagents and volatile organic solvents; the latter must be quantitatively removed prior to use. This report details an isocyanate-, solvent-, and catalyst-free synthetic method towards PDMS polyureas using urea and a disiloxane diamine chain extender in the melt phase. Melt polymerization afforded segmented PDMS polyureas, which formed optically clear, mechanically ductile, freestanding films. Observation of distinct thermal transitions with differential scanning calorimetry and dynamic mechanical analysis, corresponding to the respective segments, suggested microphase separation. Tensile and hysteresis measurements corroborated similarities between these PDMS polyureas and their isocyanate-containing analogues with strain at break ranging from 495 to 1180%. This facile, isocyanate-free approach provides a commercially viable alternative to the current industrial process for high performance elastomers.