Addition polymerization vs condensation polymerization — examples of each

Question

Differentiate between addition polymerization and condensation polymerization. Give two examples of each. Why does condensation polymerization produce a small molecule as a by-product while addition polymerization does not?

(NCERT Class 12 — direct board question, common in NEET)

Solution — Step by Step

In addition polymerization, monomers with double bonds ( $\text{C=C}$ ) join together by opening their double bonds. The entire monomer becomes part of the polymer chain — no atoms are lost.

The repeating unit has the same molecular formula as the monomer.

Examples:

Polyethylene: $n\text{CH}_2\text{=CH}_2 \rightarrow (-\text{CH}_2\text{-CH}_2-)_n$
PVC (polyvinyl chloride): $n\text{CH}_2\text{=CHCl} \rightarrow (-\text{CH}_2\text{-CHCl}-)_n$

In condensation polymerization, monomers with two or more functional groups react with each other, and a small molecule (usually $\text{H}_2\text{O}$ , $\text{HCl}$ , or $\text{NH}_3$ ) is eliminated in each step.

The repeating unit has a different (lower) molecular formula than the sum of monomers.

Examples:

Nylon-6,6: hexamethylenediamine + adipic acid → polyamide + $\text{H}_2\text{O}$
Bakelite: phenol + formaldehyde → phenol-formaldehyde resin + $\text{H}_2\text{O}$

Addition polymerization involves breaking a $\pi$ bond and forming a new $\sigma$ bond — all atoms of the monomer are retained. No atoms need to leave.

Condensation polymerization involves a reaction between two functional groups (like $-\text{OH}$ and $-\text{COOH}$ , or $-\text{NH}_2$ and $-\text{COOH}$ ). When these groups react, parts of each group combine to form a small stable molecule that detaches. For example:

-\text{COOH} + \text{H}_2\text{N}- \rightarrow -\text{CO-NH}- + \text{H}_2\text{O}

The water molecule is made from the $-\text{OH}$ of the acid and the $-\text{H}$ of the amine.

Why This Works

The fundamental distinction comes down to how bonds form. Addition polymers form by chain-growth — a reactive centre (radical, cation, or anion) propagates down the chain, opening double bonds one after another. Since we’re only converting $\pi$ bonds to $\sigma$ bonds, every atom stays.

Condensation polymers form by step-growth — any two monomers (or oligomers) can react at any time, and each reaction creates one new linkage plus one small molecule. The functional groups partially “consume” each other, shedding atoms that form water or another by-product.

Alternative Method — Quick Comparison Table

Feature	Addition	Condensation
Monomer requirement	$\text{C=C}$ double bond	Two functional groups
By-product	None	$\text{H}_2\text{O}$ , $\text{HCl}$ , etc.
Molecular formula of repeat unit	Same as monomer	Different from monomers
Growth type	Chain growth	Step growth
Examples	PE, PP, PVC, Teflon	Nylon, Bakelite, Dacron

For CBSE boards: this table format is exactly what the examiner expects for a 3-mark “differentiate” question. Write at least 4 points of difference with examples. Nylon-6,6 is the most commonly asked condensation polymer — know the monomers (hexamethylenediamine + adipic acid) and the linkage (amide bond, $-\text{CO-NH}-$ ).

Common Mistake

Students often confuse Nylon-6 and Nylon-6,6. Nylon-6 is made from a single monomer (caprolactam) by ring-opening polymerization. Nylon-6,6 is made from two different monomers and is a true condensation polymer. When the question says “condensation polymerization example,” write Nylon-6,6, not Nylon-6. Also, don’t call Nylon-6 an addition polymer — ring-opening is its own category.

Question

Solution — Step by Step

Why This Works

Alternative Method — Quick Comparison Table

Common Mistake

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