New cyclic oligomers of the copolymer of poly(ethylene terephthalate) (PET) and poly(ethylene isophthalate) (PEI) were isolated and identified. A condensation polymerization was carried out at a high temperature, and the solid-state polymerization that followed yielded the high molecular weight polymer. The oligomers were extracted from the high molecular weight PET-PEI copolymer and separated with preparative high performance liquid chromatography techniques. Their chemical structures and properties were analyzed and determined by 1 H NMR, differential scanning calorimetry, and mass spectroscopy. The oligomers observed at early retention times were a cyclic dimer and cyclic trimers and consisted of [GT] 3 , [GI] 2 , [GI] 3 , [GT] 2 [GI] 1 , and [GT] 1 [GI] 2 .
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