Determination of end groups as a function of molecular size for aliphatic polyesters by derivatization of end groups and size-exclusion chromatography with infrared detection

Abstract

End groups of poly(ethyleneglycol sebacate) having number average molecular weights less than 2500 were characterized as a function of molecular size by derivatizing end groups separately to form 3,5-dinitrobenzoyl and p-nitrobenzyl esters. A hydroxyl end group was reacted with 3,5-dinitrobenzoyl chloride (DNBC) and a carboxyl end group was reacted with O-( p-nitrobenzyl)- N,N′-diisopropyl isourea (PNBD). After separation of these derivatized polyesters by size-exclusion chromatography, the effluent was monitored by using a highly sensitive infrared detector. Concentrations of polyesters were monitored at 1740 cm −1 for a carbonyl group in the main chain, polyesters derivatized with DNBC at 1560 cm −1 for a hydroxyl end group (characteristic absorption band for the nitro group of DNBC), and polyesters derivatized with PNDB at 1537 cm − for a carboxyl end group (a characteristic absorption band for the nitro group of PNBD). By this technique, three types of polyesters having different end groups were characterized: a diol-type polyester, a mixture of polyesters of a diol type and a mono-ol/monocarboxyl type, and a mixture of polyesters of a dicarboxyl type a mono-ol/monocarboxyl type.