A library of random poly(ethylene terephthalate) (PET), poly(trimethylene terephthalate) (PTT), and seven PET-PTT copolymers has been prepared in a high throughput manner by entropically-driven ring-opening polymerizations of the corresponding macrocyclic oligomers. The products have been investigated by differential scanning calorimetry and wide angle X-ray diffraction. They show that the 50:50 copolymer displays a crystalline phase. The same phase can be formed by in situ transesterification when a 50:50 mixture of PET and PTT is melt blended. Poly(butylene terephthalate) (PBT)-PET and PTT-PBT 50:50 copolymers also show crystal phases. (C) 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 49: 995-1005, 2011