21.17
Types of Step-Growth Polymers: Polyesters
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Q1: What monomers are used to make polyesters?
Polyesters are typically made from dicarboxylic acids and diols that undergo stepwise condensation. For example, terephthalic acid and ethylene glycol react via Fischer esterification to form poly(ethylene terephthalate), or PET. Alternatively, dimethyl terephthalate and ethylene glycol can be used in a transesterification process. These monomers combine through ester bonds to create the polymer chain.
Q2: How is PET synthesized industrially?
Industrial PET synthesis uses transesterification of dimethyl terephthalate with ethylene glycol at 150 degrees Celsius. At this temperature, the reactants and polymer are non-volatile, but the by-product methanol vaporizes and escapes, driving the reaction to completion. This process efficiently produces crude PET, which can then be processed into various commercial products.
Q3: What are the main applications of PET and its derivatives?
Crude PET has diverse applications: it is blow-molded into soft-drink bottles, spun into Dacron fiber for textiles, and cast into Mylar film for magnetic recording tapes and compact discs. Dacron fiber is valued for its light weight, high strength, and moisture resistance. Mylar film is prized for its strength, flexibility, and ultraviolet resistance, making it ideal for demanding applications.
Q4: What is Kodel polyester and how is it used?
Kodel is a polyester prepared by transesterification of dimethyl terephthalate and 1,4-di(hydroxymethyl)cyclohexane. It is typically blended with wool or cotton to reduce stiffness in textile applications. This copolymer approach improves the handling and comfort properties of fabrics while maintaining the durability benefits of polyester.
Q5: Why are polyesters useful in the textile industry?
Polyesters revolutionized textiles by introducing wrinkle-free behavior, eliminating the need for starching and ironing clothes. Dacron fiber, derived from PET, is widely used because of its light weight, high strength, and moisture resistance. These properties make polyester blends practical for everyday clothing and durable textiles.
Q6: How are biodegradable polyesters used in medical applications?
Some polyesters are intentionally unstable and hydrolyze in aqueous media, making them suitable for applications requiring slow degradation. For example, copolymers of glycolic acid and lactic acid are used by surgeons in dissolvable sutures. These copolymers hydrolyze within weeks into starting materials that are metabolized by the body.
Q7: What are the starting materials for PET synthesis?
Terephthalic acid is prepared by oxidation of p-xylene, while ethylene glycol is obtained by air-oxidation of ethylene to ethylene oxide, followed by hydrolysis. These two starting materials are then combined through transesterification to produce PET. Understanding these precursors is essential for comprehending the complete synthetic pathway of this important polymer.