Demonstrated in this paper is the kinetic factor which can control whether the structure in segmented polyurethanes is phase-mixed or phase-separated. According to the solubility parameters, UMDT has greater hard and soft segment incompatibility than UHDI, while HDI/HBPA in UHDI has higher mobility than MDI/HBPA in UMDI due to flexible HDI. There is no observable phase separation in the former, while the latter gave rise to nearly complete phase separation.