A 4,4＇-diphenylmethane diisocyanate (4,4＇-MDI)-based poly(ester)-poly(urethane) has been synthesized as selectively H-2 labeled at the MDI methylene. The 2H NMR spectrum showed that this elastomer consisted of a rigid and mobile component over a wide temperature range, with the mobile component increasing with temperature. The variation in rigid to mobile transition temperature, T-RM, has been analyzed in terms of hard domain imperfections. Empirically, a linear relationship was found between the transition temperature and an imperfection parameter, i. SAXS was used to convert the NMR parameters to distances on the basis that the hard domain imperfections were related to the domain size. However, physical mechanisms for obtaining such variations in the domain size do not plausibly explain the variation in T-RM Explanations of the hard domain imperfection consistent with this variation are the misregister of the hard segments and misalignment of hard segments within a hard domain.