Condensation Polymers

• How step-growth polymerisation works and how it differs from chain-growth
• Why nylon-6 needs only one monomer but nylon-6,6 needs two
• The structure and synthesis of Dacron/PET from ethylene glycol and terephthalic acid
• Why Bakelite is a thermosetting polymer with a 3D crosslinked network
• How melamine-formaldehyde resin achieves fire resistance
• What PHBV is and why it is genuinely biodegradable
• The key differences between thermoplastic and thermosetting polymers

What is Step-Growth (Condensation) Polymerisation?

In condensation polymerisation, monomers react through their functional groups, and each linkage formation ejects a small molecule (typically H₂O, HCl, or CH₃OH). Unlike chain-growth, ALL molecules in the system react with each other — dimers react with dimers, trimers with trimers, etc. Chain length builds up slowly.

Key test: condensation polymer = loss of small molecule at each bond; addition polymer = no atoms lost.

Functional groups involved:

• −COOH + −NH₂ → amide bond (−CO−NH−) + H₂O → polyamides (nylons)
• −COOH + −OH → ester bond (−CO−O−) + H₂O → polyesters
• −OH + HCHO → methylene bridge (−CH₂−) + H₂O → phenolic resins

Nylon-6

• Monomer: Caprolactam (a single cyclic monomer with 6 carbon atoms)
• Reaction: Ring-opening polymerisation of caprolactam (in hot water/acid, the ring opens and polymerisation proceeds)
• Linkage: Amide (−CO−NH−)
• "6" means: 6 carbon atoms in the repeat unit

n [C₆H₁₁NO]  →  −(−NH−(CH₂)₅−CO−)ₙ−  +  no byproduct (ring-opening)
  caprolactam        Nylon-6

Note: Strictly, nylon-6 from caprolactam is ring-opening, not classic condensation, but it is classified with condensation polymers in NCERT because of the amide bonds formed.

Uses: Ropes, fabrics, bristles, parachute fabric, toothbrush bristles.

Nylon-6,6

• Monomers: Two different monomers
  1. Hexamethylenediamine (HMD): H₂N−(CH₂)₆−NH₂ (6 C atoms)
  2. Adipic acid: HOOC−(CH₂)₄−COOH (6 C atoms)
• The two 6s in the name refer to carbon counts in each monomer
• Linkage: Amide bond; byproduct = H₂O

n H₂N(CH₂)₆NH₂  +  n HOOC(CH₂)₄COOH
→  −[NH(CH₂)₆NH−CO(CH₂)₄CO]ₙ−  +  n H₂O
         Nylon-6,6

Uses: Gear wheels, bearings, bristles, carpets, tyre cords, stockings.

Dacron (PET / Terylene)

• Full name: Poly(ethylene terephthalate) — PET
• Monomers:
  1. Ethylene glycol: HO−CH₂−CH₂−OH
  2. Terephthalic acid: HOOC−C₆H₄−COOH (para)
• Linkage: Ester bond (−CO−O−); byproduct = H₂O
• Type: Polyester

n HO−CH₂CH₂−OH  +  n HOOC−C₆H₄−COOH
→  −[O−CH₂CH₂−O−CO−C₆H₄−CO]ₙ−  +  n H₂O
            PET / Dacron / Terylene

Uses: Clothing fibres (Terylene), plastic bottles (PET bottles), packaging films (Mylar), tyre cords.

PET bottles carry resin code "1". PET is also used in food packaging because it is odourless, transparent, and resistant to gases.

Bakelite — The First Synthetic Plastic

• Monomers: Phenol (C₆H₅OH) + Formaldehyde (HCHO)
• Reaction: Acid or base-catalysed condensation; byproduct = H₂O
• Structure: 3D crosslinked network (thermosetting)
• Invented by Leo Baekeland in 1907

Formation steps:

1. First, phenol + formaldehyde → novolac (linear, thermoplastic intermediate)
2. Novolac + more formaldehyde + heat/pressure → Bakelite (3D crosslinked)

Each phenol can react at ortho and para positions with formaldehyde, creating branches and then crosslinks → rigid 3D network.

Phenol ring − CH₂ − Phenol ring − CH₂ − Phenol ring
                |
             CH₂
                |
Phenol ring − CH₂ − Phenol ring

Properties: Hard, infusible (cannot melt — decomposes before melting), excellent electrical insulator, resistant to heat and chemicals. Uses: Electrical switches, handles, knobs, billiard balls, early telephone casings.

Melamine-Formaldehyde Resin

• Monomers: Melamine (C₃H₆N₆, triazine with 3 −NH₂ groups) + Formaldehyde
• Each melamine can react at 6 NH sites → highly crosslinked 3D network
• Properties: Hard, scratch-resistant, fire-resistant (N atoms release N₂ on burning, smothering flames), can be moulded into shapes
• Uses: Crockery (plates, cups — "Melmac"), laminates (Formica), whiteboards, fire-resistant fabrics

PHBV — A Biodegradable Polymer

• Full name: Poly(hydroxybutyrate-co-hydroxyvalerate)
• Monomers: 3-hydroxybutanoic acid + 3-hydroxypentanoic acid (co-monomers)
• Produced by: Bacteria (Alcaligenes eutrophus / Ralstonia eutropha) as energy-storage granules
• Linkage: Ester bond (polyester)
• Type: Biodegradable — degraded by soil bacteria within weeks to CO₂ and H₂O

Properties: Thermoplastic (unlike Bakelite), biocompatible, piezoelectric. Uses: Packaging films, medical sutures, drug delivery, disposable cutlery — eco-friendly alternatives.

Thermoplastic vs Thermosetting Polymers

Memory trick: "Thermoplastic = can be replasticised (re-shaped)"; "Thermoset = set permanently like a cake — baking irreversible"

Kinetics of Step-Growth: Why Chain Length Builds Slowly

In step-growth, the degree of polymerisation p = fraction of functional groups reacted:

$\bar{X}_n = \frac{1}{1-p}$

To reach n = 100 (100 repeat units), you need p = 0.99 (99% conversion!). This means extremely high conversion is needed for high molecular weight — a reason step-growth polymerisation requires careful stoichiometry and long reaction times.

For chain-growth (addition), high molecular weight is achieved almost immediately (even at low monomer conversion).

Nylon Salt Trick

Nylon-6,6 is industrially made via the nylon salt (hexamethylenediammonium adipate, "66 salt") intermediate:

H₂N(CH₂)₆NH₂  +  HOOC(CH₂)₄COOH  →  ⁺H₃N(CH₂)₆NH₃⁺  ⁻OOC(CH₂)₄COO⁻
                                              "66 salt" (1:1 ratio guaranteed)

The salt ensures exact 1:1 stoichiometry of both monomers (critical for high molecular weight in step-growth). The salt is then heated to give nylon-6,6 + H₂O.

Ester Interchange in PET Production

Industrially, PET is made by ester interchange (transesterification), not direct esterification:

Dimethyl terephthalate + Ethylene glycol → PET + Methanol (byproduct)

Methanol (lower b.p.) is removed easily by distillation, driving the equilibrium forward.

Why Bakelite Cannot be Recycled

Thermosetting polymers like Bakelite have covalent −CH₂− crosslinks between chains in all three dimensions. On heating, there are no free chain ends to flow — the material is one giant macromolecule. Sufficient heat simply breaks covalent bonds, causing charring/decomposition rather than melting. This is fundamentally different from thermoplastics where only weak van der Waals forces hold chains together.

Example 1: Writing the Repeat Unit of Nylon-6,6

Problem: Write the equation for the formation of nylon-6,6 and identify the
type of bond formed.

Monomer 1: H₂N−(CH₂)₆−NH₂   (hexamethylenediamine)
Monomer 2: HOOC−(CH₂)₄−COOH  (adipic acid)

Reaction (between one amine and one acid):
H₂N−(CH₂)₆−NH₂  +  HOOC−(CH₂)₄−COOH
→  H₂N−(CH₂)₆−NH−CO−(CH₂)₄−COOH  +  H₂O    (one amide bond formed)

Polymer repeat unit:
−[NH−(CH₂)₆−NH−CO−(CH₂)₄−CO]ₙ−

Bond type: Amide bond (peptide-like −CO−NH−)
Byproduct: Water (H₂O) — hence it is a condensation polymer

Answer: Nylon-6,6 repeat unit = −[NH(CH₂)₆NHCO(CH₂)₄CO]ₙ−; amide linkage; H₂O lost.

Example 2: Distinguishing Bakelite from PET

Problem: Both Bakelite and PET are condensation polymers. Explain why PET can be
recycled into new bottles but Bakelite cannot be recycled.

Step 1: PET structure
  - Linear polyester chains held together by van der Waals forces and dipole interactions
  - On heating above Tg (~80 °C) and Tm (~260 °C), chains slide past each other
  - Can be melted, moulded, re-solidified → RECYCLABLE (thermoplastic)

Step 2: Bakelite structure
  - 3D covalent crosslinked network (−CH₂− bridges between phenol rings)
  - No individual chains exist — the whole material is one giant molecule
  - Heating breaks covalent bonds → irreversible charring, not melting
  - Cannot be remoulded → NOT RECYCLABLE (thermosetting)

Answer: PET is thermoplastic (linear chains, weak intermolecular forces, meltable).
        Bakelite is thermosetting (3D covalent network, cannot melt, only chars).
        This fundamental structural difference determines recyclability.

1. What small molecule is eliminated when nylon-6,6 is formed from its two monomers?
2. Name the functional group linkage in (a) nylon-6,6 and (b) Dacron.
3. Why is Bakelite considered a thermosetting polymer? What structural feature makes it so?
4. PHBV is produced by bacteria. What type of bond links its monomers and why is it biodegradable?
5. (Stretch) In step-growth polymerisation of nylon-6,6, if 98% of functional groups have reacted (p = 0.98), calculate the average degree of polymerisation. What does this tell you about the challenge of making high-MW nylon?

• NCERT Class 12 Chemistry, Chapter 15 — Polymers
• Sections 15.3.2 (Condensation Polymerisation), 15.5 (Biodegradable Polymers), 15.6 (Polymers of Commercial Importance)
• JEE Main: questions on nylon monomer identification, Bakelite structure (thermosetting), thermoplastic vs thermosetting, ester/amide bond types
• Common MCQ formats: "which polymer contains amide linkage?", "the monomer of nylon-6 is…", "Bakelite is classified as…"

Study strategy: Learn the monomers and linkages for each polymer as a table — monomer(s) → linkage → polymer name. Focus on the 6 vs 6,6 distinction for nylons. Remember: thermosetting = cannot melt = Bakelite and melamine. PHBV is the go-to biodegradable condensation polymer.

Practice MCQs on condensation polymer monomers, linkage types, and thermoplastic vs thermosetting classification in the Condensation Polymers topic bank.

Unsure why nylon-6 from caprolactam is called a condensation polymer if no small molecule is lost? Ask the Drishti AI tutor to explain ring-opening polymerisation and how it bridges the two categories.

Complete the Quick Check and mark in your Drishti progress tracker. Aim for 4/5 before moving to natural and commercial polymers.

• Read: Natural and Commercial Polymers — natural rubber, starch, cellulose, silicones
• Then: Biomolecules — amino acids, proteins, enzymes, nucleic acids
• Practice: Full polymer chapter MCQs (mixed addition + condensation, Medium–Hard)