This study compares the high strain-rate impact responses of polyurethanes to investigate the molecular dependencies of dynamic stiffening. We use the laser-induced microparticle impact test (LIPIT) platform to perform microscale impacts and measure material response. Dynamic mechanical analysis and differential scanning calorimetry are used to obtain dynamic glass transition temperatures, and solid-state nuclear magnetic resonance spectroscopy is used to probe segmental mobility. Dynamic stiffening in polyurethanes is found to correlate strongly with both glass transition temperatures and segmental mobility and results suggest segmental mobility to be an important determinant of the extent of dynamic stiffening in elastomers. The impact response of polyurethane is compared to that of the more commonly studied polyurea. Dynamic stiffening in both elastomers show a clear dependence on soft segment length, although further differences in impact response may be a result of differences in hydrogen bonding. This study uses polyurethane to systematically understand the molecular dependencies of dynamic stiffening and provides a foundation for further design and testing of dynamic stiffening in synthetic elastomers.