The hard segments in MDI/diol-based polyurethanes form crystalline domains, and these have been found to exist in different polymorphic structures depending on processing history. Structures of the crystal structures of low molecular weight urethane model compounds possess a number of common features that can be used to predict models for the polymers. Specifically, information is derived concerning the conformation of the diphenylmethane, phenyl-urethane, and chain extender region of the chain. This information has been used to predict the possible conformation of poly (MDI/diol) chains in which the diol region is fully extended (all trans). Less information is available that is relevant to the structures that have a more contracted conformation, but the options to be considered are discussed.