Chain Reaction

Chain Reaction: addition polymerisation you can steer

This free game is part of our AQA GCSE Chemistry (8462) course for exams in EXAM YEARS 2027-2029.

What is addition polymerisation?

For AQA GCSE Chemistry (spec point 4.7.3.1, chemistry only), alkenes can be used to make polymers such as poly(ethene) and poly(propene) by addition polymerisation. Many small molecules, called monomers, join together to form very large molecules, called polymers. A monomer for addition polymerisation needs the functional group C=C: one of the two bonds in the double bond opens, which gives each carbon atom a spare bond to join on to the next monomer.

Textbooks show this as a before and after picture. Chain Reaction lets you watch it happen. You are the growing end of a polymer chain in a cutaway reactor at a night-shift polymer works, and the molecules are ball-and-stick models drawn tens of millions of times bigger than real life.

Steer the chain, open the C=C

You steer the glowing open end of the chain with the mouse, a finger or the arrow keys, and the chain follows you like a snake. Touch an ethene molecule, CH₂=CH₂, and you see its double bond (the two orange sticks) open: one half swings back to link to the chain and the other becomes the new open end at the front. The C=C becomes a C–C single bond, so the finished chain is a long zigzag of carbon atoms joined by single bonds. The first join plays in slow motion while Ezra, the reactor supervisor, explains it.

Not everything in the reactor can join. Alkanes such as methane, ethane and propane have only single bonds, so they bounce off, and a card explains why: every carbon already has four bonds and there is no double bond to open. Later you meet chloroethane, CH₃–CH₂Cl, which looks a lot like chloroethene, CH₂=CHCl, but has no C=C.

Same atoms, side groups and orders

An atom check panel counts what goes in and what comes out: every ethene added was C₂H₄, every unit in the chain is still C₂H₄, and there are no other products. That is the key exam idea: in addition polymers the repeating unit has the same atoms as the monomer, because no other molecule is formed in the reaction.

Then the orders get harder. To make poly(propene) you catch only propene, CH₂=CH–CH₃, and you watch the CH₃ hang off the chain as a side group, on the same carbon it was on in the monomer. In level 4 the orders arrive only as a repeating unit, such as (–CH₂–CHCl–)_n or (–CF₂–CF₂–)_n, so you have to work out the monomer before you can catch it. The game ends with a 60-second rush where the orders switch between names and repeating units.

Draw the polymer the exam way

After the chains, you draw the polymer on a pencil board, one step at a time: tap the bond that changes (the C=C becomes C–C), tap where the bonds stick out to join the next units, put the brackets where those bonds pass through them, and put n at the bottom right. The finished board shows the equation, for example n CH₂=CH₂ → (–CH₂–CH₂–)_n. The traps are the mistakes examiners see every year: drawing the CH₃ of propene in the main chain, leaving the C=C in the repeating unit, forgetting the bonds through the brackets and putting n in the wrong place.

You also work backwards: take the two carbons in the chain, remove the bonds that stick out through the brackets and put the C=C back, which turns (–CH₂–CHCl–)_n back into chloroethene, CH₂=CHCl. Quick checks ask you to spot which diagram is an addition polymer and which formula is the repeating unit of poly(propene). A game takes about 8 minutes, works on a computer, tablet or phone, and saves your best score.