Monomers with carboxyl and hydroxyl groups can join together by condensation to form polyesters. A molecule of water is eliminated and an ester bond is formed. Usually a dicarboxylic acid and a diol are reacted.Terylene is a polyester made from ethane-1,2-diol and benzene-1,4-dicarboxylic acid.Lactic acid is a hydroxycarboxylic acid which has both a carboxyl and hydroxyl group and can be polymerised to form poly(lactic acid). Poly(lactic) acid is biodegradable and can be used in food and drink cartons.