Small molecules, long chains: how polythene and nylon are made

How to use this simulation

1. It opens on “Polythene” with n = 4. Press play: along a row of carbons each double bond relaxes into a single bond, and green new bonds appear between the monomers.
2. At the end a green bracket wraps the repeat unit, with a small n beside it. That is the template for the whole chain.
3. Choose “Nylon-6,6”. Blue acid molecules and red amine molecules join alternately, and each join lets a little H₂O float away.
4. Slide n. The chain molar mass and the number of water molecules in the readouts change straight away.
5. Compare the unit masses of polythene, polypropene and PVC. Only the side group (H, CH₃, Cl) changes; the chain has the same pattern.

From a shopping bag to a fishing net

A carrier bag in the morning, a water pipe at noon, a nylon shirt at night. They look nothing alike, yet all are polymers: long rows of the same small unit. Change the unit or the way it joins and the bag turns soft, the pipe turns stiff, the thread refuses to snap.

This page shows how the small molecules join and whether anything gets left behind. There are two routes, and the difference between them is one of the favourite exam questions.

Monomer, polymer and repeat unit

A monomer is a small molecule that can join to others. A polymer is the long chain they form. The piece of chain that keeps coming back is the repeat unit, and the number of times it repeats is n, the degree of polymerisation.

Real chains have n in the thousands. Say a polythene chain has molar mass 56,000. One unit weighs 28, so n = 56,000 ÷ 28 = 2,000. The masses use C = 12, H = 1, O = 16, N = 14 and Cl = 35.5.

Two routes: addition and condensation

There are two main kinds of polymerisation. Here is the difference at a glance:

How to read the animation

For addition, the canvas lays out 2n carbon atoms, two per monomer. At first every pair has a double bond (two lines). As the story advances the two lines merge into one and a green new line grows between neighbouring monomers.

If the row is longer than six atoms the chain steps down to the next row like a snake, so you can still follow one unbroken chain. At the end a green bracket marks the first repeat unit.

For condensation, blue and red blocks sit alternately, each block one monomer molecule. When a green line forms between two blocks, a little teal H₂O drifts up and fades. A running count at the top tracks the water released so far.

Addition polymerisation: ethene to polythene

The C=C in ethene is two bonds, one strong and one weak. When the weak one opens, each carbon has a free hand, and each grabs the next molecule's carbon. That is how the chain grows. No atom is lost, so the unit mass equals the monomer mass: 28.

Heat, pressure and a catalyst drive the process. Conditions decide how long the chains are and how branched, which is what makes polythene a soft bag or a stiff bottle.

Condensation polymerisation: nylon-6,6

Two different monomers here: a diacid C₆H₁₀O₄ (mass 146) and a diamine C₆H₁₆N₂ (mass 116). The OH from a –COOH and an H from an –NH₂ leave together as H₂O, and a C–N link (an amide, –CO–NH–) forms in their place.

One repeat unit holds one acid and one amine, with two waters gone. Its formula is C₁₂H₂₂N₂O₂ and its mass 226. The chain's two ends keep an H and an OH, 18 in total. For n units, (2n − 1) waters leave.

Four polymers side by side

The four polymers in the simulation with formulas and unit masses. Every mass is counted from the atoms.

Try it yourself in the simulation

Go in order. Write your guess down first, then check.

• Experiment 1: “Polythene”, n = 2. Watch when the two lines of the double bond merge, and when the green bond appears.
• Experiment 2: set n = 6. The row steps down to a second line. How much did the chain mass grow in the readout?
• Experiment 3: choose “PVC”. Cl hangs under the second carbon. By how much is its unit heavier than polythene?
• Experiment 4: “Nylon-6,6”, n = 3. Six blocks, five joins, five water drops. Match them to the readout.
• Experiment 5: how much water is released in any addition polymer? Then in nylon? Why the difference?

Worked problems

Every number here comes from the same counting the animation does.

Plastic and the waste problem

The toughness that makes polymers useful also makes them a nuisance. Microbes cannot easily break most polymer chains, so a discarded bag can block drains for years, linger in soil and crumble into smaller and smaller pieces that end up in water.

Three habits help: use less (carry a cloth bag), reuse what you have, and collect plastic separately so it can be recycled. Some polymers can be re-melted and some cannot, and newer ones made from plant sources do break down over time. How much of each there is keeps changing, so understand the reasons rather than memorising figures.

Mistakes everyone makes

These cost marks every year.

• Swapping addition and condensation. Addition gives off nothing; condensation releases a small molecule.
• Keeping the double bond in the repeat unit. A polymer has no C=C, only C–C.
• Writing 2n waters for nylon. It is (2n − 1): 2n monomers have 2n − 1 joins.
• Forgetting the n outside the bracket: [–CH₂–CH₂–]ₙ.
• Mixing up the monomer and the repeat unit. For polythene the monomer is ethene and the unit is –CH₂–CH₂–.
• Naming the PVC monomer wrongly. It is chloroethene (CH₂=CHCl).

Exam corner

Try these without looking, then check.

• Give two differences between addition and condensation polymerisation.
• What is the unit mass of polypropene? Answer: 42.
• Name the two monomers of nylon-6,6 with formulas and masses. Answer: C₆H₁₀O₄ (146) and C₆H₁₆N₂ (116).
• How many water molecules leave when a 50-unit nylon chain forms? Answer: 99.
• State one advantage and one problem of plastics.

Revision: the whole page in a minute

Read this just before the exam.

• Monomer = small molecule; polymer = long chain; n = number of units.
• Addition: C=C opens, nothing leaves, mass = n × monomer mass.
• Condensation: two groups react and a small molecule (usually H₂O) leaves.
• Unit masses: polythene 28, polypropene 42, PVC 62.5, nylon-6,6 226.
• Nylon: n units = 2n monomers = (2n − 1) waters.
• Plastics last, so use less, reuse and collect.

Frequently asked questions