Polyesters

A polyester is a condensation polymer in which the repeating units are joined by ester links, -COO-. It forms when the -COOH groups of a dicarboxylic acid react with the -OH groups of a diol, eliminating a molecule of water at each link.The best-known example is Terylene (also called PET), made from benzene-1,4-dicarboxylic acid and ethane-1,2-diol:n HOOC-C₆H₄-COOH + n HOCH₂CH₂OH → [-CO-C₆H₄-CO-O-CH₂CH₂-O-]_n + (2n − 1) H₂OEach repeating unit is made from one acid molecule and one diol molecule. Terylene is used as a fibre for clothing (often blended with cotton) and, as PET, to make plastic drinks bottles.A single monomer with an -OH group at one end and a -COOH group at the other can also form a polyester on its own. For example, 2-hydroxypropanoic acid (lactic acid) forms poly(lactic acid).Worked example. Find M_r of the Terylene repeating unit and estimate the number of units in a chain of M_r 19 200. The unit is C₁₀H₈O₄: (10 × 12.0) + (8 × 1.0) + (4 × 16.0) = 120 + 8 + 64 = 192. Number of units = 19 200 ÷ 192 = 100.The ester links can be hydrolysed by hot aqueous acid or alkali. Alkaline hydrolysis gives the diol and the salt of the dicarboxylic acid.