WHY "addition"? Because the monomer is added on — nothing is split off. Contrast with condensation polymers (nylon, polyester) which expel a small molecule like H2O. That is the single most useful distinction.
We will build the rule, not memorise it. Take ethene CH2=CH2.
Step 1 — Initiation. A peroxide initiator breaks into two radicals (species with an unpaired electron, written as a dot):
R−O−O−R⟶2R–O∙
Why this step? A radical is electron-hungry. The π electrons of the C=C are loosely held and easy to grab — perfect prey.
Step 2 — Propagation. The radical attacks one carbon of the double bond. One π electron pairs with the radical's electron to make a new bond; the otherπ electron is left as a new radical on the far carbon:
RO∙+CH2=CH2⟶RO−CH2−CH2∙RO−CH2−CH2∙+CH2=CH2⟶RO−CH2−CH2−CH2−CH2∙
Why this step? The radical just moves to the chain end and keeps eating monomers — a self-perpetuating "chain reaction". This is why it's called chain-growth.
Step 3 — Termination. Two radical chain-ends meet and pair up their electrons (combination), stopping growth.
The net result, ignoring chain ends:
nCH2=CH2peroxide,high P,T−[CH2−CH2]n−
Open the double bond → single bonds to neighbours → repeat unit in −[]n−.
Know the 7 monomers ↔ polymers ↔ uses table by heart.
C–F bond strength explains PTFE's inertness; branching explains LDPE vs HDPE.
Recall Feynman: explain to a 12-year-old
Imagine each monomer is a kid holding both their own hands together (that "holding" is the double bond). When a teacher (the radical) taps a kid, the kid lets go of one of their own hands and instead grabs the hand of the next kid. That kid then frees a hand and grabs the next, and so on — a giant human chain forms, and nobody loses a single finger (no atoms thrown away). If the kids wear different gloves (Cl, F, a flower CH₃, a balloon C₆H₅), the chain behaves differently — that's why we get bags, pipes, non-stick pans, and foam cups all from the same trick.
Dekho, addition polymer ka funda bilkul simple hai: aapke paas ek chhota monomer hota hai jisme ek double bond (C=C) hota hai. Jab polymerisation hoti hai, to ye double bond "khul" jaata hai — ek π bond toot kar uske electrons next monomer ke saath naya single bond bana lete hain. Isliye saare monomers ek lambi chain mein jud jaate hain, aur khaas baat ye ki koi chhota molecule (jaise paani) bahar nahi nikalta. Atoms in = atoms out. Yahi cheez addition ko condensation (nylon, polyester) se alag karti hai, jisme H2O nikalta hai.
Ab maza ye hai ki double bond ke saath jo side group X lagaa hota hai, wahi plastic ki personality decide karta hai. X=H to polyethene (thaili-bottle), X=Cl to PVC (pipe), saari positions par F to PTFE/Teflon (non-stick tawa, kyunki C–F bond bohot strong aur inert hota hai), X=CH3 to polypropylene, X=C6H5 to polystyrene (cup, thermocol), X=CN to acrylic fibre (sweater), aur methyl methacrylate se PMMA (Perspex — kaanch jaisa transparent).
Exam tip: repeat unit likhte waqt double bond mat chhodo — woh khul chuka hai, ab sirf single bonds. Aur hamesha −[]n− ke saath open bonds aur subscript n dikhana, taaki examiner ko pata chale ki ye ek bada macromolecule hai. Reverse mein, agar repeat unit diya ho aur monomer poochha jaaye, to bas do backbone carbons ke beech double bond wapas daal do — monomer mil gaya. Mechanism yaad rakho: Initiation → Propagation → Termination (free radical chain reaction).
