21.17 : Types of Step-Growth Polymers: Polyesters

The introduction of polyesters has brought major development to the textile industry. The wrinkle-free behavior of polyester blends has eliminated the need for starching and ironing clothes.

Polyesters are commonly prepared from terephthalic acid and ethylene glycol; the crude product is known as poly(ethylene terephthalate) or PET. However, polyesters are synthesized industrially by transesterification of dimethyl terephthalate with ethylene glycol at 150 °C. The two reactants and the polymer PET are not volatile at this temperature, but the by-product methanol vaporizes from the reaction, thereby driving the reaction to completion.

For PET synthesis, ethylene glycol is obtained by the air-oxidation of ethylene to ethylene oxide, which upon hydrolysis, gives glycol. Terephthalic acid is prepared by the oxidation of p-xylene.

The crude PET can be spun into a Dacron fiber, most commonly used in the textile industry as a clothing material. It can also be fabricated into a film called Mylar. These films are used to prepare magnetic recording tape. Thicker Mylar films are used in compact discs. Poly(ethylene terephthalate) is also blow-molded to make plastic bottles used for soft drinks.

Some polyesters are unstable because they get hydrolyzed in the aqueous medium. Such polyesters find applications where slow degradation is required. For example, the copolymer of glycolic acid and lactic acid is used by surgeons in dissolvable sutures. The copolymer gets hydrolyzed within weeks into starting materials, which get metabolized inside the body.