Polyesters

A polyester is a condensation polymer in which the monomers are joined by ester links, COO. In condensation polymerisation, a small molecule (usually water) is lost each time two monomers join.To make a polyester you need monomers with two functional groups each:
• a dicarboxylic acid, HOOC-R-COOH
• a diol, HO-R'-OH
A COOH group of the acid reacts with an OH group of the diol, forming an ester link and eliminating H₂O. Because every monomer has a reactive group at each end, the chain keeps growing. A diacyl chloride can be used in place of the dicarboxylic acid; then HCl is eliminated instead of water.Example: PET (Terylene) is made from benzene-1,4-dicarboxylic acid and ethane-1,2-diol:
n HOOCC₆H₄COOH + n HOCH₂CH₂OH → polymer + 2n H₂OThe repeat unit contains one residue of each monomer joined by ester links: -CO-C₆H₄-COO-CH₂CH₂-O-. PET is used for drinks bottles and clothing fibres.Breaking down polyesters: unlike poly(alkenes), polyesters can be broken down by hydrolysis of their ester links, using acid or alkali, and some are biodegradable. Poly(lactic acid), made from the single monomer 2-hydroxypropanoic acid (which has both an OH and a COOH group), is biodegradable and used for compostable packaging.Worked example: find the monomers of the polyester with repeat unit -CO(CH₂)₄COOCH₂CH₂O-. Break the ester link and add back water: the acid part gives HOOC(CH₂)₄COOH, hexanedioic acid, and the alcohol part gives HOCH₂CH₂OH, ethane-1,2-diol.