Three end-capped oligomers 35, 41 and 45 related to poly(ethyleneglycol terephthalate) have been prepared starting from t-butylthioterephthalic acid 21 and its ethyleneglycol monoterephthalate derivative 25 terminally protected on the glycol moiety (by the tetrahydropyranyl group) and on the carboxylic acid moiety [by the 2-(2-pyridyl)-group].