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.

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.

• Polyester Polymerization - Synthesis process where polyesters are made from reactions.
• Poly(ethylene terephthalate) - Commonly known as PET, a type of polyester used in various applications.
• Ethylene Glycol and Terephthalic Acid reaction - Key process for producing PET.
• Dacron - A popular example of a polyester used particularly in textiles.
• Hydrolysis of Polyesters - Decomposition process of some polyesters in aqueous medium, facilitating slow degradation.

• Define Polyester Polymerization – Understand the process and its implications (e.g., polyester polymerization).
• Contrast PET vs Dacron – Know the differences and uses of these two common polyesters (e.g., PET and Dacron).
• Explore Polar and Non-polar Polymers – Learn about how different polymer types, such as polar and non-polar, behave (e.g., Polyester vs Nylon).
• Explain the Step Growth Polymerization Process – Understand how polymers grow incrementally through a step-by-step reaction.
• Apply Understanding of Polyester to Real-life Context – Understand how knowledge about polyesters can be applied to real-world scenarios, like the textile industry.

• What is polyester polymerization and how does it work?
• What are the products of the reaction between ethylene glycol and terephthalic acid?
• What are the uses of Poly(ethylene terephthalate)?

• Students – Understand How polyester polymerization supports scientific learning
• Educators – Provides a clear framework for teaching polymer science
• Researchers – Polyester research is relevant for material science and industrial application
• Textile Enthusiasts – Offers insights into materials used in the textile industry