Real-time monitoring of the oligomerization reaction of bis(hydroxyethyl terephthalate) by near-infrared spectroscopy and chemometrics

The initial oligomerization of bis(hydroxyethyl terephthalate), BHET, was monitored in situ by near-infrared (NIR) spectroscopy, with a transmission probe. The amount or hydroxyl end groups, which is a function of the degree of oligomerization, and the amount of ethylene glycol (EG) in the reaction solution were successfully predicted by the NIR spectra combined with partial least-squares (PLS) regression using measured values from H-1-NMR. The models for predicting the amounts of the OH end groups and EG were interpreted by plots of regression coefficients. Although the amount of EG was small, it was possible to extract spectral information about EG by use of these loading plots. This study is a sequel to the previous study that used an attenuated total reflection (ATR) infrared (IR) method. In the last part of this paper we compare the present results obtained by NIR spectroscopy with those by ATR-IR spectroscopy.