The effect of incorporating phthalate, 1,8-naphthalenedicarboxylate, and 1,8-anthracenedicarboxylate structural units on the crystallization rate of PET are evaluated by isothermal and dynamic calorimetry. Although all of the comonomers retard crystallization, the 1,8-naphthalene unit shows no concentration dependence between 2.5 and 10% incorporation, in contrast to the smaller phthalate and larger anthracene units. The greater rate at which the 1,8-naphthalene copolymer crystallizes, relative to that of the other copolymers, is consistent with the notion that the U-turn geometry induces chain folding and nucleates crystallization.