Chapter 26 — Case Study 2: Polyamides and Polyesters — Nylon, Kevlar, PET

"Of every kilogram of synthetic polymer made on Earth, about half is held together by amide or ester bonds. Acyl substitution scaled up to industry." — paraphrase from a polymer chemistry text

Polyamides and polyesters are made by repeated nucleophilic acyl substitution between difunctional monomers — a diacid (or diacyl chloride) and a diamine (or diol). Each monomer attacks the next, forming a long chain with periodic amide or ester linkages. This is condensation polymerization (each step releases a small molecule — water, methanol, or HCl — as a byproduct).

This case study traces three of the world's most important polymers, all built by Chapter 26 chemistry.

Nylon-6,6: the original synthetic polyamide

Nylon-6,6 was discovered in 1935 by Wallace Carothers at DuPont (Carothers also made Nylon-6 and several other commercially important polymers; he died at age 41, before nylon was commercialized). It is made from:

  • Adipic acid (HOOC-(CH₂)₄-COOH, a 6-carbon diacid) — produced industrially from cyclohexane via cyclohexanone + cyclohexanol mixture, then oxidation.
  • Hexamethylenediamine (HMDA) (H₂N-(CH₂)₆-NH₂, a 6-carbon diamine) — produced from adipic acid via dinitrile intermediate, then hydrogenation.

The polymerization: 1. Mix adipic acid + HMDA in 1:1 ratio. 2. Heat to ~280 °C for several hours. 3. Each end of the diacid attacks the amine of HMDA; each end of HMDA attacks the COOH of adipic acid. 4. The reaction releases water (since the COOH-amide condensation requires loss of H₂O). 5. After many condensations, a long polymer chain forms.

Mechanism for each acyl substitution step: - Carboxylic acid + amine → amide + water (essentially the reverse of saponification). - This is slow without activation (Section 26.6), but at 280 °C with neutralized salt (the di-amide-of-the-diacid), the reaction proceeds.

The "6,6" naming: 6 carbons in the diacid, 6 carbons in the diamine. (Nylon-6 is from caprolactam, a single 6-membered cyclic amide that opens and polymerizes.)

Properties of nylon-6,6: - Strong, tough, high tensile strength. - Higher melting point than other nylons due to crystallinity. - Used in textiles (clothing, ropes), engineering plastics (gears, bushings), and fishing lines.

The polyamide is held together by: 1. Covalent C-N bonds (the amide linkages) along the chain. 2. Hydrogen bonds between adjacent chains (N-H...O=C between strands).

These hydrogen bonds give the polymer its characteristic strength and crystallinity.

Kevlar: aromatic polyamide

Kevlar (poly-paraphenylene terephthalamide), invented by Stephanie Kwolek at DuPont in 1965, is structurally similar to nylon but with aromatic rings in both monomers:

  • Terephthalic acid (1,4-benzenedicarboxylic acid).
  • p-phenylenediamine (1,4-diaminobenzene).

The polymer chain has aromatic rings every 8 carbons or so, separated by amide linkages. The rings are para-disubstituted, locking the chain in a near-planar zig-zag.

Why Kevlar is so strong: 1. Aromatic rigidity: the para-substituted phenyl rings cannot rotate; the chain is conformationally locked. 2. Hydrogen bonding: amide N-H ...O=C bonds in 3D crystals. 3. Crystallinity: the regularity of the structure (every monomer is identical and ordered) makes Kevlar highly crystalline. 4. High molecular weight: up to 2 million Da, achieved via polymerization in concentrated sulfuric acid.

Tensile strength: ~3.5 GPa (compared to steel's 0.5–1 GPa for the same diameter). Used in body armor, fire-resistant suits, high-performance ropes.

The mechanism is identical to nylon-6,6: acyl substitution between COOH and amine. The advantage of Kevlar is the rigidity from aromatic rings, not a chemistry difference.

PET: polyethylene terephthalate (water bottles, polyester clothing)

PET is the most-produced polyester. Made from: - Terephthalic acid (or its dimethyl ester). - Ethylene glycol (HOCH₂CH₂OH, a diol).

The polymerization: 1. Transesterification with dimethyl terephthalate + ethylene glycol → bis(2-hydroxyethyl) terephthalate + methanol. 2. Polycondensation of bis(2-hydroxyethyl) terephthalate at high T → PET + ethylene glycol (released).

Each step is acyl substitution: an alcohol attacks an ester, displacing the previous alcohol. Many such steps build the chain.

PET is used in: - Water and soda bottles. - Polyester clothing (Dacron, Terylene). - Films (Mylar, Polaroid).

PET is recyclable: the polymer can be hydrolyzed (saponification) to recover terephthalic acid + ethylene glycol, then re-polymerized. Most PET recycling is mechanical (melted and reformed) rather than chemical, but chemical recycling is growing.

Spider silk: the natural polyamide

Spider silk is, chemically, a protein — and a protein is a polyamide. Specifically, spider silk is made from a fibroin polypeptide (mostly glycine and alanine, with some glutamine and proline). It has structural features that make it remarkably strong: - β-sheet domains (extended polymer regions) hydrogen-bonded along chains. - Amorphous flexible regions (random-coil-like) connecting the β-sheets. - Hierarchical assembly: fibers from chains, fibers wound into threads.

The amide bonds are made by ribosomal protein synthesis — the spider's ribosome catalyzes peptide bond formation (acyl transfer from aminoacyl-tRNA to growing chain). This is the same mechanism as DCC coupling, but enzymatically regulated.

Spider silk's tensile strength is remarkable (up to 1.3 GPa for dragline silk), comparable to Kevlar. The polymer is biodegradable (other organisms' proteases hydrolyze it).

Synthetic spider silk has been a research goal for decades. Companies have engineered E. coli or yeast to produce spider silk proteins and then assembled them into fibers; the fibers are used in apparel and biomedical applications. The chemistry of polyamide formation is identical whether the silk is made by a spider or by a yeast: amide bonds via acyl substitution.

Common features of polyamides and polyesters

Common patterns: 1. Difunctional monomers (diacid + diol, or diacid + diamine, or diol + diacyl chloride). 2. Repeated acyl substitution for chain growth. 3. Loss of small molecule (water, methanol, HCl) at each step. 4. High temperature to overcome the kinetic barrier of slow acyl substitution (especially for amides). 5. Crystallinity from H-bonding between strands (in polyamides).

The same chemistry, scaled to industry, makes billions of kilograms of polymer per year. Nucleophilic acyl substitution is the most economically important reaction in synthetic chemistry. Without it, no nylon, no Kevlar, no PET, no plastic bottles.

Forward connections

Chapter 31 (Synthesis Workshop 2) gives practice with multistep synthesis of polyamides and other condensation products. Chapter 33 returns to peptide chemistry — the natural polyamide. Chapter 34 covers fatty acid esters (lipids).

Take-home

  • Polyamides and polyesters are built by repeated nucleophilic acyl substitution between difunctional monomers.
  • Each step is an acyl substitution: a nucleophilic amine or alcohol attacks an electrophilic acid or ester, displacing a leaving group.
  • Industrial polyamides: Nylon-6,6 (textile, plastic), Kevlar (body armor, rigid aromatic).
  • Industrial polyesters: PET (water bottles, clothing).
  • Natural polyamide: spider silk (protein, made by ribosomal acyl transfer).
  • The same chemistry — acyl substitution — connects synthetic polymers to biological proteins. Mechanism is universal; scale and conditions vary.