Addition polymers — polyethene, PVC, PTFE (Teflon), polypropylene, polystyrene, PMMA, polyacrylonitrile: Visual walkthrough

We will use ethene first (the simplest monomer) and then swap in a side group $X$ at the very end to get the whole family.

Step 1 — What a double bond actually is (two springs, not one)

WHAT. A single bond between two atoms is one shared pair of electrons — think of it as one spring holding two carbons together. A double bond $C = C$ is two shared pairs — two springs. Chemists give these two springs names:

the σ (sigma) bond: the strong, straight spring lying directly between the two carbons,
the π (pi) bond: a second, weaker spring made of electrons sitting above and below the line joining the carbons.

WHY split them into two names? Because they behave differently. The σ bond is the sturdy skeleton and we will never break it. The π bond is loosely held and far easier to break — it is the spring that snaps open to build the polymer. Naming them lets us say precisely "the π bond opens, the σ bond stays."

PICTURE. Look at the ethene molecule below. The blue line is the σ bond (on the axis). The yellow clouds above and below are the π electrons — notice they are not on the direct line, which is exactly why they are easy prey for an electron-hungry attacker.

Figure — Addition polymers — polyethene, PVC, PTFE (Teflon), polypropylene, polystyrene, PMMA, polyacrylonitrile

Step 2 — Meet the radical: the electron-hungry attacker

WHAT. A radical is any species with a single unpaired electron. Electrons like to live in pairs; a lone unpaired electron is restless and desperate to grab a partner. We draw that lonely electron as a dot $∙$ . A common source is a peroxide $R - O - O - R$ that splits its weak $O - O$ bond into two radicals:

$stays together R - O weak O-O breaks - stays together O - R ⟶ 2 radical: one unpaired e^{-} R - O ∙$

$R - O ∙$ ::: an oxygen carrying one lonely dot electron, itching to pair up.
the arrow ::: "becomes" — the $O - O$ spring snaps, each half keeps one electron.

WHY do we need it? Nothing happens to ethene on its own — the two π electrons are content as a pair. We need a trigger with an unpaired electron to break the peace. This first step is called initiation because it starts everything.

PICTURE. The peroxide breaks in the middle; each fragment walks away holding one dot. The red dot is the reactive electron we will track across the whole page.

Step 3 — The attack: one π electron pairs, one is set free

WHAT. The radical's lonely dot reaches into the π cloud. One π electron leaps out to pair with the radical's dot, forming a brand-new σ bond (a real single bond) to the first carbon. But a bond needs two electrons and the π cloud only donated one — so the other π electron is left stranded on the far carbon as a new dot.

$attacker R O ∙ + one π spring C H_{2} = C H_{2} ⟶ R O - radical moved to the far end C H_{2} - C H_{2} ∙$

$R O - C H_{2}$ ::: the new single bond just formed (old radical dot + one π electron).
$C H_{2} ∙$ ::: the leftover π electron, now the new lonely dot.

WHY does this matter so much? Watch what happened to the count: we destroyed one radical (the $R O ∙$ ) but created a new one at the chain's end. The reactivity did not vanish — it relocated. That is the secret of a chain reaction: the hungry end just keeps moving forward.

PICTURE. Follow the two π electrons (yellow). One becomes the new bond (turns blue); the other slides to the right carbon and becomes the new red dot, ready to strike again.

Step 4 — Propagation: the same move, over and over

WHAT. The new radical at the chain end does exactly what $R O ∙$ did in Step 3 — it attacks the next ethene's π bond, forms a σ bond, and the leftover π electron becomes the next dot, one ethene further along.

$R O - C H_{2} - C H_{2} ∙ + C H_{2} = C H_{2} ⟶ R O - C H_{2} - C H_{2} - C H_{2} - C H_{2} ∙$

each new $- C H_{2} - C H_{2} -$ ::: one monomer swallowed, all atoms kept.
the $∙$ at the end ::: still there, still hungry — the engine of growth.

WHY is this called "chain-growth"? Because the chain grows one monomer at a time from a single active end, like a train adding carriages at the back. Each addition regenerates the reactive dot, so the process is self-perpetuating until something stops it (Step 6).

PICTURE. The chain lengthens rightward; the red dot marches from unit to unit. Notice every backbone bond behind it is now a plain single bond — no double bonds survive inside the chain.

Step 5 — Termination: the chain stops (the degenerate ending)

WHAT. Growth cannot go forever. When two radical chain-ends bump into each other, their two lonely dots pair up into one final σ bond. No dots left ⇒ no more hunger ⇒ growth halts. This is combination termination:

$\sim C H_{2} ∙ + ∙ C H_{2} \sim ⟶ \sim C H_{2} - C H_{2} \sim$

the two $∙$ ::: the last two radicals in the whole batch.
the new $-$ ::: their electrons finally paired; reactivity is gone.

WHY include this edge case? Because a reader must never wonder "does it run forever?" It does not. Termination is why chains have a finite (and slightly random) length — which is exactly why we quote an average molar mass, not one exact number.

PICTURE. Two chains approach; their red dots collide and fuse into a single blue bond. Both dots disappear — the picture is now radical-free.

Step 6 — Collapse the whole story into the repeat unit

WHAT. Zoom out and ignore the tiny $R O$ ends (negligible on a chain of thousands). The whole three-act play (initiate → propagate → terminate) compresses into one tidy equation:

$n monomer, has C = C C H_{2} = C H_{2} peroxide, P, T repeat unit, all single bonds - [C H_{2} - C H_{2}]_{n} -$

$n$ ::: a very large number of monomers (thousands).
$C H_{2} = C H_{2}$ ::: the monomer — the double bond is present.
$- [\dots]_{n} -$ ::: the repeat unit boxed up, with continuation bonds on both sides and subscript $n$ telling you it repeats.
no small molecule on the right ::: this is the fingerprint of addition (contrast condensation, which spits out $H_{2} O$ ).

WHY the brackets and $n$ ? They signal a macromolecule, not a small molecule. The open bonds say "I continue in both directions." Dropping them is the classic exam slip flagged in the parent note.

PICTURE. The left half of the figure shows the messy real chain; an arrow squashes it into the boxed repeat unit on the right.

Step 7 — Swap in $X$ to unlock the whole family

WHAT. Everything above used ethene ( $X = H$ ). But the mechanism never cared what hangs off the carbon. Replace one H with a side group $X$ and the identical dance gives a different plastic:

$n vinyl monomer C H_{2} = C H X ⟶ - [C H_{2} - ∣ C X H]_{n} -$

$X$ ::: the side group that rides along untouched — $Cl \to$ PVC, $F(all) \to$ PTFE, $CH_{3} \to$ PP, $C_{6} H_{5} \to$ PS, $CN \to$ PAN.
the $π$ bond ::: still the one that opens; $X$ is a spectator.

WHY does $X$ change the plastic but not the mechanism? The reaction happens at the $C = C$ ; $X$ just sits on the backbone afterward, altering how chains pack and attract each other (see Intermolecular forces and polymer properties). Same trick, different personality.

PICTURE. Four monomers with different $X$ groups, each opening the same π bond (highlighted) to give a different repeat unit.

The one-picture summary

The whole derivation on one canvas: peroxide splits → radical attacks the π bond → dot relocates and the chain grows monomer by monomer → two dots meet and stop → collapse to the boxed repeat unit → swap $X$ for the family.

Recall Feynman retelling — the whole walkthrough in plain words

Picture each ethene as two kids holding both their own hands together — that self-holding is the double bond, and it's really two grips: a strong one (σ) and a loose one (π). A teacher walks in holding one loose glove in her hand and nothing in the other (that empty hand is the radical dot). She grabs one kid's loose hand — now the kid's other loose hand is free and waving around. That waving hand grabs the next kid's loose hand, whose other hand then waves and grabs the next, and on and on: a human chain grows, and nobody drops a single finger (no atoms lost). It stops only when two waving hands find each other and clasp. Zoom out and you just see a long chain of kids; write it as one kid in brackets with a little $n$ meaning "and thousands more." Finally, give the kids different gloves — a red glove (Cl), a slippery glove (F), a flower (CH₃), a balloon (C₆H₅) — and the same dance builds pipes, non-stick pans, ropes and foam cups. The double bond opened; the gloves just rode along.

Recall Quick self-check

Which spring of the $C = C$ opens during polymerisation? ::: The weaker π bond; the σ backbone stays intact. After propagation, where is the radical? ::: On the far carbon of the newly added monomer — the reactive end just relocated. What stops the chain? ::: Termination — two radical ends pair their unpaired electrons into one bond. What is released as a byproduct? ::: Nothing — that's the fingerprint of addition (vs condensation, which loses $H_{2} O$ ).