SYNTHESIS AND C-13 NMR CHARACTERIZATION OF ETHYLENE-GLYCOL TEREPHTHALIC ACID HYDROXYBENZOIC ACID COPOLYESTERS

Copolyesters of terephthalic acid, p-hydroxybenzoic acid and ethylene glycol were prepared by high-temperature melt polycondensation, starti ng from different hydroxybenzoic acid derivatives. Methyl p-hydroxyben zoate monomers having the hydroxyl either free, etherified with ethyle ne glycol or esterified with terephthalic acid were used. The chemical structure of the copolyesters obtained was characterized by means of C-13 NMR spectroscopy, and chemical shifts were assigned to the respec tive carbons. The investigation clearly indicates that oxybenzoate uni ts are quantitatively present in the polymer as etherified (on the hyd roxyl group) and esterified (on the carboxylic group) with ethylene gl ycol. Similar analyses were performed on intermediate products formed after the transesterification stage. They point out that at this stage the hydroxyl group is free when starting from methyl p-hydroxybenzoat e, while a noticeable amount of ether is already present when starting from the monomer esterified with terephthalic acid. A reaction scheme encompassing these results and based on the competition between ester ification and etherification process of the phenolic hydroxyl is put f orward.