Imagine you have a box of identical Lego bricks. Each brick has studs on top and a way to click into the brick below. If you start snapping them together, you get a long chain — nothing is lost, nothing is thrown away. Every single brick you started with ends up in the final chain.
That is the core idea of addition polymerization.
You start with small molecules called monomers (the Lego bricks). Each monomer has a carbon-carbon double bond (C=C). Under the right conditions, these double bonds break open, and the monomers link up end-to-end to form a long chain called a polymer. No water, no ammonia, no small molecule is released as a byproduct. The polymer's molecular formula is an exact multiple of the monomer's formula.
nCH2=CH2initiator(−CH2−CH2−)n
For ethylene (CH2=CH2), the polymer is polyethylene — the plastic in milk jugs and grocery bags. The n can be thousands.
The precise statement
Addition polymerization is a chain-growth polymerization in which unsaturated monomers (containing C=C bonds) add to one another through a reactive intermediate (free radical, carbocation, or carbanion) without the elimination of any small molecule. The reaction proceeds in three stages: initiation, propagation, and termination.
The key points for your exam:
Monomers must have a double bond (or another unsaturated functional group).
No byproduct is formed — the polymer is the only product.
The reaction is chain-growth: once a monomer adds to the active chain end, that end remains active and can add another monomer, and another, very rapidly.
The mechanism can be free-radical, cationic, or anionic, depending on the nature of the reactive intermediate.
How it actually happens (the three stages)
1. Initiation
You need a trigger — an initiator — to break open the first double bond. For free-radical polymerization, a molecule like benzoyl peroxide decomposes to give free radicals. One radical attacks the π-bond of a monomer, creating a new radical at the other end of the monomer.
R⋅+CH2=CH2⟶R−CH2−CH2⋅
Now you have a reactive chain end.
2. Propagation
That radical at the chain end attacks the next monomer's double bond. The chain grows one monomer at a time, with the radical always at the growing tip.
R−CH2−CH2⋅+CH2=CH2⟶R−CH2−CH2−CH2−CH2⋅
This repeats thousands of times in a fraction of a second.
3. Termination
The chain stops growing when two radicals meet and combine (coupling) or when one radical steals a hydrogen from another (disproportionation). The active center is destroyed, and you get a dead polymer chain.
The propagation step is the same in spirit — the active center attacks the next monomer — but the charge and the type of monomer that works best differ. …
The ordinary free-radical route to polymerising alkenes needs high pressure and gives limited control over the product's structure, which is why a specific catalyst system was developed for milder, more controlled polymerisation. …
Ziegler-Natta catalysts enable controlled, stereoregular addition polymerisation of alkenes.
The Ziegler-Natta catalyst is a combination of an organometallic compound, triethylaluminium Al(C2H5)3, and a transition metal halide, titanium tetrachloride TiCl4.
It is used in the industrial polymerisation of ethylene (and other alkenes such as propylene) via a coordination (insertion) mechanism, rather than a free-radical mechanism. This gives:
High density polythene (HDPE) – linear, unbranched, closely-packed chains with high density and rigidity, from ethylene polymerised using Ziegler-Natta catalyst at moderate temperature/pressure. …
Same / Similar Concept — real previous-year questions on the same or a closely similar concept, not this exact question.
CBSE 2025Set D1 markMCQ
Q.The product of addition polymerisation is
(a) PVC
(b) Nylon
(c) Terylene
(d) Polyamide
›Reveal solutionSolution
Addition polymers form from unsaturated monomers with no loss of small molecules; PVC is the addition polymer here.
In addition (chain) polymerisation, monomers containing a double bond add repeatedly with no elimination of any small molecule. Polyvinyl chloride (PVC) is formed by addition polymerisation of vinyl chloride (CH2=CHCl):
High-density polythene forms by addition polymerisation of ethene; Nylon-6, Nylon-6,6 and Dacron are condensation polymers.
Addition polymers form by repeated addition of unsaturated monomers with no loss of any small molecule. HDPE is made by addition polymerisation of ethene (CH2=CH2) -> -(CH2-CH2)-n.
The others are condensation polymers, formed with elimination of a small molecule (like water):