21.17 : 逐步生长聚合物的类型：聚酯

Polymers in which the monomers are joined together by an ester bond are called polyesters.

Typically, dicarboxylic acids and diols undergo stepwise condensation to yield polyesters.

For instance, terephthalic acid and ethylene glycol undergo Fischer esterification and lose water to form poly(ethylene terephthalate), or PET.

PET can also be prepared from dimethyl terephthalate and ethylene glycol via a transesterification process with the loss of methanol.

Crude PET can be blow-molded to make soft-drink bottles, spun into Dacron fiber, or cast into Mylar film.

Dacron fiber is widely used in the textile industry because of its light weight, high strength, and moisture resistance properties.

Mylar film is used to manufacture magnetic recording tapes, as it is strong, flexible, and resistant to ultraviolet degradation.

Kodel is another polyester prepared by transesterification of dimethyl terephthalate and 1,4-di(hydroxymethyl)cyclohexane.

It is generally used by blending it with wool or cotton to reduce stiffness.