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Nylons and Polyesters

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Klein 27.6 and 27.7. Step-growth polymers students can draw from reactions they know.

  • A polyester from a diacid and a diol, or from a hydroxy acid. PET is the example.
  • A nylon from a diacid and a diamine, or from a lactam. Nylon-6,6 is the example.
  • The linkage is the ester or amide already taught. Show the repeating unit and the small molecule lost.
  • Crystallinity and chain stiffness in a few sentences: why some of these materials are fibers and some are not.

Step-Growth Polymers: Polyesters and Nylons

Moving from radical and ionic chain-growth polymers to step-growth condensation polymers, we examine synthetic macromolecules constructed using functional group reactions students already know: esterification and amide formation. In these step-growth reactions, monomers bearing two reactive functional groups react continuously, eliminating a small molecule (typically water) at every linkage step.

[ Insert Image: Overview of Step-Growth Polyesters and Polyamides ]
Figure 1. Formation of polyester and polyamide backbones via condensation reactions.

1. Polyesters: Polyethylene Terephthalate (PET)

Polyesters are synthesized through step-growth condensation between dicarboxylic acids and diols:

  • The Reagents: Terephthalic acid (benzene-1,4-dicarboxylic acid) reacts with ethylene glycol (ethane-1,2-diol).
  • The Reaction: Each carboxylic acid group condenses with an alcohol hydroxyl group via Fischer esterification, eliminating a molecule of water ($\text{H}_2\text{O}$) per linkage.
  • Example (PET): The resulting polymer is Polyethylene terephthalate (PET), a tough, durable thermoplastic widely used in beverage bottles, food packaging, and polyester textile fibers (dacron).
[ Insert Reaction: Synthesis of Polyethylene Terephthalate (PET) ]
Figure 2. Condensation polymerization of terephthalic acid and ethylene glycol to yield PET.

2. Polyamides: Nylon-6,6

Polyamides—universally known as nylons—are synthesized by reacting dicarboxylic acids with diamines:

  • The Reagents: Adipic acid (a 6-carbon dicarboxylic acid) reacts with hexamethylenediamine (a 6-carbon diamine).
  • The Reaction: The acid groups react with the amine groups to form strong amide linkages ($\text{-CONH-}$), releasing water as a byproduct.
  • Example (Nylon-6,6): Named "6,6" because both starting monomer units contain exactly six carbon atoms. Nylon-6,6 exhibits exceptional tensile strength and elasticity, making it ideal for durable fabrics, ropes, and molded engineering parts.
[ Insert Reaction: Synthesis of Nylon-6,6 Polyamide ]
Figure 3. Condensation of adipic acid and hexamethylenediamine yielding Nylon-6,6.

3. Repeating Units and Small Molecules Lost

In step-growth condensation polymers, the identity of the repeating unit reflects the combined monomer skeletons minus the elements of the small molecule eliminated:

The Small Molecule Lost: Water ($\text{H}_2\text{O}$) is the classic byproduct expelled during polyester and polyamide formation.
  • Repeating Unit Design: For PET, the repeating unit is $-\text{O-CH}_2\text{-CH}_2\text{-O-OC-C}_6\text{H}_4\text{-CO-}$. For Nylon-6,6, the repeating unit is $-\text{NH-(CH}_2)_6\text{-NH-CO-(CH}_2)_4\text{-CO-}$. Brackets enclose these repeating units with a subscript $n$ indicating the degree of polymerization.
[ Insert Structure: Repeating Units of PET and Nylon-6,6 Highlighting Functional Linkages ]
Figure 4. Ester and amide functional groups embedded directly within the polymer backbones.

4. Crystallinity, Chain Stiffness, and Fiber Properties

Why Some Materials Make Excellent Fibers: The macroscopic physical properties of polymers like PET and nylons stem directly from their backbone structure and intermolecular forces. The rigid planar geometry of amide groups in nylons—and the flat aromatic benzene rings in PET—prevent the chains from coiling up randomly. Instead, these rigid chains align parallel to one another and form dense networks of intermolecular hydrogen bonds (in nylons) or strong dipole-dipole and $\pi$-stacking interactions (in PET). This high degree of crystallinity and chain stiffness restricts rotation, resulting in high tensile strength, high melting points, and resistance to stretching—the exact mechanical characteristics that make these polymers premier synthetic fibers.

[ Insert Diagram: Intermolecular Hydrogen Bonding and Crystalline Packing in Polyamides ]
Figure 5. Hydrogen-bonded crystalline sheets imparting high tensile strength to nylon fibers.