Synthesis of Low-Dielectric Polyureas by Incorporating Phenyl-Substituted Cage Silsesquioxanes in the Main Chain

Establishing a design concept for low-dielectric constant (ε r ) and low dissipation factor (tan δ) polymeric materials is vital to improve signal transmission rates. Polyurea-based polymers with strong dipole moments have been reported to achieve higher ε r (above 5.0) than most of the traditional polymer dielectrics. However, polyurea-based polymers with low ε r and tan δ values have not been reported yet. In this study, bis(3-aminopropyl)-double-decker-shaped phenyl-substituted silsesquioxane (DDSQ) ( 1 ) was polymerized with various diisocyanates, i.e., methylenediphenyl 4,4′-diisocyanate ( 2a ), 4,4′-diisocyanato-3,3′-dimethylbiphenyl diisocyanate ( 2b ), m -xylene diisocyanate ( 2c ), 1,3-bis(isocyanatomethyl)cyclohexane ( 2d ), 4,4′-methylenebis(cyclohexyl isocyanate) ( 2e ), and hexamethylene diisocyanate ( 2f ), to obtain the corresponding polyureas ( 3a – f ). Optically transparent, colorless, and flexible freestanding films were obtained by casting the polyurea solutions in tetrahydrofuran. Among the tested polymers, polyureas from 2a and 2e exhibited ε r less than 3.0. Incorporation of the DDSQ units produced free volume, resulting in low ε r . Polyureas from 2e and 2f exhibited the lowest tan δ of approx. 7 × 10 –3 . The polyurea from 2e had the lowest urea content and thus had the lowest density and lowest ε r . Additionally, strong hydrogen bonding in the polyurea from 2e inhibited local motion, resulting in a low tan δ even though the urea content was low. Water vapor permeability measurements revealed greater free volume content introduced by the co-oligomerization of polyhedral oligomeric silsesquioxanes (POSS), especially in the polyurea from 2e . Among the reported combinations, the combination of DDSQ and urea groups provides unique low ε r and low tan δ polymeric materials.