The chemical formula of a polyester is (R-OCO-R)n, in which each R is a hydrocarbon. Polyesters are formed by the reaction between dicarboxylic acids and diols. The most common polyester, polyethylene terephthalate, or PET, is formed from terephthalic acid and ethane-1,2-diol. Polytrimethylene terephthalate is formed from terephthalic acid and propane-1,3-diol.
The two reactants of a polyester form an ester linkage with the loss of one water molecule. A full polymer results when a chain of these linkages is formed, with the loss of one water molecule at every ester linkage. The Essential Chemical Industry Online states that polyesters with benzene rings have rigid structures that give them high melting points and greater strength. Polyesters have applications in making bottles, food packaging, water containers and clothing.