Addition polymers — polyethene, PVC, PTFE (Teflon), polypropylene, polystyrene, PMMA, polyacrylonitrile: Question bank

Before we start, one word we lean on everywhere: a repeat unit is the smallest group of atoms that, copied end to end $n$ times, rebuilds the whole chain. Picture a single train carriage that, when coupled to identical carriages, forms the whole train — the carriage is the repeat unit, the train is the polymer.

True or false — justify

True or false: In addition polymerisation the polymer has the same empirical formula as its monomer.

True. No atoms are added or removed — the

C = C

merely opens into single bonds — so the ratio of C:H:X atoms is unchanged; only the molecular mass grows (by a factor of

n

True or false: An addition polymer always loses a small molecule such as water when it forms.

False. That is the signature of condensation polymers like nylon and polyester. Addition polymers expel nothing — atoms in equal atoms out.

True or false: The repeat unit of PVC still contains a

C = C

double bond.

False. The whole point of addition is that the

π

bond opens to form two new single bonds to neighbours; the backbone of the repeat unit

- C H_{2} - C H C l -

contains only single bonds.

True or false: Both HDPE and LDPE are made from the same monomer, ethene.

True. They are chemically identical polyethene; they differ only in chain geometry — LDPE is branched, HDPE is linear — see HDPE vs LDPE — structure and density.

True or false: LDPE is denser than HDPE because its branches add mass.

False. Branches prevent tight packing, so LDPE chains sit further apart — lower density. HDPE's linear chains pack closely and crystallise, giving higher density.

True or false: Because PTFE is fluorine-rich, it must be reactive and easily attacked by acids.

False. The C–F bond is very strong and short, so PTFE is extremely inert and chemically unreactive — that inertness is exactly why it is used for non-stick and corrosion-proof surfaces.

True or false: A saturated molecule with no

C = C

, such as ethane, can undergo addition polymerisation.

False. Addition polymerisation requires an unsaturated

C = C

to open up. With only single bonds there is nothing to break and re-form, so no chain can grow.

True or false: In free-radical propagation the number of unpaired electrons stays constant at one per growing chain.

True. When the radical adds to a

C = C

, one

π

electron pairs off to make a bond and the other becomes a new radical on the chain end — so a radical is consumed and a radical is created, keeping exactly one.

True or false: Termination is when a radical adds yet another monomer.

False. That is propagation. Termination is when two radical chain-ends meet and pair their unpaired electrons, removing radicals from the system and stopping growth.

Spot the error

Spot the error: "The monomer of PTFE is

C H_{2} = C H_{2}

and fluorine is added to the finished chain."

Wrong monomer and wrong order. The monomer is tetrafluoroethene

C F_{2} = C F_{2}

, already fully fluorinated before polymerising — you do not fluorinate a completed polyethene chain.

Spot the error: "

n C H_{2} = C H C l ⟶ - [C H_{2} = C H C l]_{n} -

is the equation for PVC."

The double bond has been left inside the repeat unit. It should read

- [C H_{2} - C H C l]_{n} -

: the

C = C

opens into single bonds, so the backbone has none.

Spot the error: "The repeat unit of polystyrene is

- C H_{2} - C H_{2} -

The phenyl group

C_{6} H_{5}

has been dropped. Styrene is

C H_{2} = C H (C_{6} H_{5})

, so the correct repeat unit is

- C H_{2} - C H (C_{6} H_{5}) -

Spot the error: A student writes the product as a single molecule "

C H_{3} - C H_{2} C l

" with no

n

and no open bonds.

A polymer is a macromolecule. You must show continuation bonds and the subscript

n

- [C H_{2} - C H C l]_{n} -

. Without them you have drawn a tiny molecule, not a chain.

Spot the error: "Nylon-6,6 is an addition polymer because it is a long chain."

Being long does not make it addition. Nylon forms by condensation — it releases water and is built from monomers joined by amide links, not from opening a

C = C

Spot the error: "For a polymer of

M_{r} = 42000

made of ethene (

M_{r} = 28

), the number of monomers is

n = 28/42000

The division is inverted. Since no atoms are lost,

n = M_{polymer} / M_{monomer} = 42000/28 = 1500

Why questions

Why is releasing "no small molecule" the single most useful test for addition vs condensation?

Because it is the one fact that always separates the two: addition keeps every atom (empirical formula preserved), while condensation always expels a small molecule like

H_{2} O

H C l

. Check for a

C = C

monomer and for anything thrown off.

Why does changing only the side group

X

change the plastic's whole personality?

The backbone stays the same, but

X

controls how chains pack and interact — see Intermolecular forces and polymer properties. Big rigid groups (phenyl) stiffen; polar groups (Cl, CN) raise attractions and strength; small F lowers friction.

Why is the process called "chain-growth"?

Because each radical, once formed, keeps adding monomers one after another at the chain end in a self-perpetuating sequence — the chain literally grows step by step until termination stops it.

Why do the

π

electrons of a

C = C

, rather than the

σ

electrons, get attacked by a radical?

The

π

electrons sit above and below the bond axis, are held more loosely and are more exposed, making them easy prey for an electron-hungry radical. The

σ

bond is buried between the nuclei and far stronger.

Why is PAN (acrylonitrile) so useful beyond making sweaters?

On heating, its polymer chains can be converted into rings and then into carbon, so it acts as a precursor to strong carbon fibre — see Carbon fibre from PAN.

Why must the monomer for an addition polymer be a "vinyl" type

C H_{2} = C H X

rather than a fully substituted alkene of any shape?

The general reactive requirement is simply a

C = C

; the

C H_{2} = C H X

form is the standard general case where a single side group

X

hangs off, letting one formula describe the whole family. (Some, like PMMA and PTFE, carry extra substituents, but the

C = C

is always the essential feature.)

Edge cases

Edge case: What is the "

X

" group in polyethene, and does that make it a special case?

X = H

. Ethene is

C H_{2} = C H_{2}

, so it is the symmetrical member of the vinyl family — no distinctive side group, which is why polyethene is the cheapest and most chemically inert of the set.

Edge case: PTFE and PMMA are not simple

C H_{2} = C H X

monomers — how do they still fit "addition"?

They still contain a

C = C

that opens into single bonds with no atom loss. PTFE's monomer is fully fluorinated (

C F_{2} = C F_{2}

) and PMMA's carbon carries two groups (

C H_{3}

and

C O O C H_{3}

), but the addition rule is unchanged.

Edge case: If a peroxide initiator is left out entirely, can the chain still grow?

Not by this route — with no radical to start it, there is nothing to open the first

C = C

. Initiation is the essential trigger; propagation cannot begin without an initial radical.

Edge case: Two different monomers, ethene and chloroethene, are polymerised together. Is the product still an addition polymer?

Yes. As long as every monomer joins via an opening

C = C

with no small molecule lost, it remains addition polymerisation — here it produces a copolymer with both repeat units along the chain.

Edge case: Take PVC and PTFE — same backbone shape, yet PTFE is far more inert. Why does the boundary case of "all H replaced by F" matter so much?

Replacing every H with F surrounds the carbon backbone in an unreactive fluorine sheath held by very strong C–F bonds, shielding the chain far more completely than PVC's single Cl per unit, so inertness and low friction jump.

Edge case: A polymer chain of odd character has

M_{r}

that is not an exact multiple of the monomer mass. What does that tell you?

The small mismatch comes from the chain-end groups (the initiator fragments

R - O

etc.), which are ignored in the

n = M_{polymer} / M_{monomer}

estimate. For large

n

these ends are negligible, so the estimate stays accurate.

Recall One-line self-test before you leave

Cover this and answer: what two things must be true for a reaction to be addition polymerisation? Two things ::: The monomer contains a $C = C$ that opens up, and no small molecule is expelled (atoms in = atoms out).