This study elucidates the influence of molecular attributes on the observed dynamic stiffening in select two-component polyurethanes upon high strain-rate impact. Unlike typical segmented elastomers, polyurethanes consisting of poly(tetramethylene oxide), PTMO, and a diisocyanate, but without a chain extender, are investigated. The hexamethylenediisocyanate (HDI)-based polyurethane, HDI - PU, exhibits crystallinity and a much higher ambient storage modulus, as determined by dynamic mechanical analysis at 1 Hz, than that of 4,4'-methylenediphenyldiisocyanate (MDI)-based polyurethane, MDI - PU. In contrast, MDI - PU exhibits a higher glass transition temperature than that of HDI - PU, and a greater dynamic stiffening against silica micro-particle impacts at strain rates between 10(7) and 10(8) s(-1). The variation in dynamic stiffening corroborates well the observed dynamics at the molecular level, as determined via solid-state nuclear magnetic resonance (ssNMR) spectroscopy. The presence of a slower-dynamics component in MDI - PU, as evidenced in the C-13 ssNMR dipolar dephasing time, is used to explain the observed enhanced dynamic stiffening response.