Polyethylene terephthalate (PET) is used to make synthetic...

Polyethylene terephthalate (PET) is used to make synthetic fibers, such as Dacron; thin films, such as Mylar; and bottles for carbonated beverages. PET is produced from ethylene glycol and either of two monomers, depending on whether the reaction proceeds by dehydration-condensation or by displacement. Write equations for the two syntheses. (Hint: The displacement is reversed by adding methanol to PET at high $T$ and $P .$ )

Polyethylene terephthalate (PET) is used to make synthetic fibers, such as Dacron; thin films, such as Mylar; and bottles for carbonated beverages.
PET is produced from ethylene glycol and either of two monomers, depending on whether the reaction proceeds by dehydration-condensation or by displacement. Write equations for the two syntheses. (Hint: The displacement is reversed by adding methanol to PET at high $T$ and $P .$ )

Key Concepts

Condensation Polymerization

This is a type of chemical reaction in which monomers join together to form polymers while releasing small molecules, typically water, as a byproduct. The process involves functional groups on the monomers reacting to form bonds (such as ester bonds in PET), resulting in a stepwise growth of the polymer chain. In PET production via dehydration-condensation, terephthalic acid and ethylene glycol react to eliminate water and form the polyester linkage.

Transesterification Reaction

Transesterification is a chemical reaction in which an ester is transformed into a different ester through the exchange of the alkoxy group. In the context of PET synthesis, this process involves the reaction of dimethyl terephthalate with ethylene glycol to form the ester linkage while methanol is released as a byproduct. This reaction is reversible, and the conversion of PET back to its monomers under high temperature and pressure with methanol is a key characteristic of the process.

Polymer Synthesis Mechanisms

This concept encompasses the various methods and reaction pathways used to synthesize polymers like PET. Understanding the specific mechanisms—whether via dehydration-condensation or transesterification—is crucial in controlling the molecular weight, structure, and properties of the final polymer product. Each method employs different monomer precursors and reaction conditions that influence the polymerization process and the characteristics of the resulting material.

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