A series of segmented polyurethanes (SPUs), based on hexamethylene diisocyanate (HDI) and polytetramethylene glycol (PTMG) as soft segments and then extended with alkylene di( 4-hydroxybenzoate), HE as hard segments, were synthesized in this study. The effect of the number of methylene units in HE (from 2 to 6) on the microphase separation of the SPUs was evaluated by means of a differential scanning calorimeter (DSC) and a polarized optical microscope (POM). The kinetic reaction between HE and HDI was determined by Fourier transform infrared spectroscopy. The rate constants of the second-order kinetics showed that the reactions of HDI and HE with odd numbers of methylene units appeared to be faster than those with even numbers of methylene units. The results from POM showed that the SPUs with even numbers of methylene units in the HE have a higher degree of phase separation between the hard and soft segments than those with odd numbers of methylene units in HE.