Poly(ethylene terephthalate) (PET) from off-grades of industrial manufacturers was partially and thoroughly depolymerized in order to synthesize PET oligomers and bis(hydroxyethyl) terephthalate (BHET), respectively. Design of experiments and analysis of variance (ANOVA) were applied for optimization of samples. Effects of reaction time, volume of glycol, catalyst concentrations, and particle size of off-grade PET were investigated. The optimal conditions to synthesize PET oligomers (3-8 repeating units) were glycol/PET molar ratio of 1, a weight ratio (catalyst to PET) of 0.5 wt%, using granule-shape. On the other hand, a reaction time of 180 min, a weight ratio (catalyst to PET) of 0.25 wt%, and glycol/PET molar ratio of 5 were obtained as the suitable conditions of BHET production. Then, endcapped PET oligomers, as a compatibilizer for preparing PET nanocomposites, were produced via reaction between maleic anhydride/phthalic anhydride (MA/PhA) composition. The combination of reaction time of 106 min and PhA/MA molar ratio of 0.85 produced the best results based on d-spacing and peak shift of nanocomposite samples. Moreover, the reaction of MA and BHET from glycolyzation of PET was successfully performed at 160 degrees C and 190 degrees C for 8 h. The optimum conditions were compared with a synthesized PET. POLYM. ENG. SCI., 54:417-429, 2014. (c) 2013 Society of Plastics Engineers