Condensation Polymerization
Master Step-Growth, Polyamides, Polyesters, and Bakelite.
Unlike Addition Polymerization, where alkene monomers just "snap" together without losing anything, Condensation Polymerization requires the chemical reaction between two different functional groups. When these groups react and link together, they eliminate a small, simple molecule (like water, alcohol, or hydrogen chloride).
1. What is Condensation Polymerization?
Also known as Step-Growth Polymerization, this type of polymerization involves repetitive condensation reactions between two bi-functional or tri-functional monomers.
- Loss of Byproducts: Small molecules such as water ($H_2O$), $HCl$, or $NH_3$ are eliminated during each condensation step.
- Empirical Formula: Because atoms are lost, the empirical formula of the resulting polymer is different from that of the monomers.
- Step-by-Step Growth: The reaction is independent at each step. Two monomers form a dimer, the dimer reacts with another monomer to form a trimer, and so on, until a massive macromolecule is formed.
Figure 1: Condensation reaction eliminating a small molecule (like water) to form a new chemical bond.
2. Important Polyamides (The Nylons)
Polyamides are polymers possessing amide linkages ($-CO-NH-$). These are synthetic fibers known commercially as Nylons. They are characterized by extremely strong intermolecular hydrogen bonding.
A. Nylon 6,6
Formed by the condensation polymerization of Hexamethylenediamine (6 carbons) and Adipic acid (6 carbons) under high pressure and temperature (553 K).
$n\text{H}_2\text{N}-(CH_2)_6-\text{NH}_2$
$\downarrow \text{(553 K, High Pressure)}$
$\textbf{[-OC-(CH}_2\textbf{)}_4\textbf{-CO-NH-(CH}_2\textbf{)}_6\textbf{-NH-]}\mathbf{_n} + 2n\text{H}_2\text{O}$
- Note: The "6,6" refers to the 6 carbon atoms present in each of the two monomers.
- Uses: Making sheets, bristles for brushes, and textiles.
B. Nylon 6
Formed by heating Caprolactam (a 7-membered cyclic amide containing 6 carbons) with water at a high temperature (533-543 K).
$\textbf{[-NH-(CH}_2\textbf{)}_5\textbf{-CO-]}\mathbf{_n}$
- Mechanism: The water helps break the cyclic ring open, converting it into aminocaproic acid, which then polymerizes.
- Uses: Manufacture of tyre cords, fabrics, and ropes.
3. Polyesters (Terylene / Dacron)
Polyesters are polycondensation products of dicarboxylic acids and diols. They contain ester linkages ($-CO-O-$). The most famous example is Terylene (also sold under the brand name Dacron).
It is manufactured by heating a mixture of Ethylene glycol and Terephthalic acid at 420-460 K in the presence of a zinc acetate-antimony trioxide catalyst.
$\textbf{[-O-CH}_2\text{-CH}_2\textbf{-O-CO-C}_6\text{H}_4\textbf{-CO-]}\mathbf{_n} + 2n\text{H}_2\text{O}$
- Properties: Highly crease-resistant and does not shrink easily.
- Uses: Blended with cotton (terycot) or wool (terywool), and used as glass reinforcing materials in safety helmets.
4. Phenol-Formaldehyde Polymers (Bakelite)
These are among the oldest synthetic polymers. They are obtained by the condensation reaction of Phenol and Formaldehyde in the presence of either an acid or a base catalyst. The reaction proceeds in stages:
Phenol reacts with Formaldehyde ($HCHO$) to form ortho and/or para hydroxymethylphenol derivatives.
The ortho derivatives react with each other (eliminating water) to form a linear polymer called Novolac.
Uses: Novolac is widely used in paints.
When Novolac is heated with extra formaldehyde, it undergoes extensive cross-linking at the ortho and para positions, forming an infusible, highly rigid, 3D network solid called Bakelite.
Uses: Making combs, electrical switches, phonograph records, and handles of various utensils.
5. Melamine-Formaldehyde Polymer
This is another highly cross-linked thermosetting polymer formed by the condensation polymerization of Melamine and Formaldehyde.
- Key Property: It forms a very hard, durable plastic.
- Classic Exam Use-Case: It is used in the manufacture of unbreakable crockery (plates, bowls, cups).
Mastery Check: Condensation Polymers
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