Due to the poor solubility of aromatic polyesters in common organic solvents,trifluoroacetic acid is usually used as a co-solvent to increase their solubility for characterizations.However,only few studies have reported the side reactions induced by it.We present here the application of in situ 1H-NMR techniques to explore its effect on the hydroxyl end-groups,which are usually used for the molecular weight determination of polyesters by end-group estimation method.Using bis(2-hydroxyethyl) terephthalate (BHET) as model compound,1H quantitative NMR results show the peak integration of hydroxyethyl end-groups decreased with time via a pseudo-first-order kinetics in d-trifluoroacetic acid/d-chloroform mixture solvent (1∶10,V∶V).This is due to the esterification of hydroxyethyl groups with trifluoroacetic acid,revealed by the 1H-13C gradient-enhanced heteronuclear multiple bond correlation (gHMBC) spectrum.The mixtures of dimethyl terephthalate and BHET with different molar ratios were used to represent poly(ethylene terephthalate) (PET) with different degrees of polymerization,and the effect of trifluoroacetic acid on the estimation of hydroxyethyl groups and subsequent molecular weight determination of polyesters was studied.Our results show that if a relative error of 5％ is allowed,the NMR measurements must be finished within 1.3 h of solution preparation at 25 ℃ in the mixture solvent.The results were confirmed in PET sample,while in poly(ethylene adipate),the obtained esterifaction constant is faster that those in aromatic system.The results can be applied to other polymer systems with alcohol functionalized groups,and used as a guideline for the characterization of polyesters and polyethers by end-group estimation method.