In addition polymerisation, many alkene molecules join in a long chain without losing any atoms. The skill is to draw the repeat unit from a monomer, or work back from a section of polymer to its monomer. It extends the addition idea in organic reactions.
How does a monomer become a polymer?
Each monomer has a C=C double bond. In polymerisation, one bond of each double bond opens, and each carbon joins to the next monomer. The result is a long chain of single bonds, with the same atoms as the monomers added together.
The repeat unit is written in brackets with open bonds at each end and a subscript n for the number of units. No double bond remains in the chain.
For ethene, CH₂=CH₂, the repeat unit is [-CH₂-CH₂-]n, poly(ethene).
For propene, CH₃-CH=CH₂, the repeat unit is [-CH₂-CH(CH₃)-]n. The methyl group hangs off the chain as a side group.
For chloroethene, CH₂=CHCl, it is [-CH₂-CHCl-]n, poly(chloroethene).
Worked example
A section of polymer has the repeat unit [-CH₂-CHCl-]. Find the monomer, and then the number of repeat units in a chain of relative molecular mass 31 250.
Step 1, identify the two chain carbons: -CH₂- and -CHCl-.
Step 2, restore the double bond: CH₂=CHCl. This is chloroethene.
Step 3, atom check: repeat unit C₂H₃Cl. Monomer C₂H₃Cl. Same atoms.
Step 4, mass of one repeat unit: 2(12) + 3(1) + 35.5 = 62.5.
Step 5, number of units: 31 250 ÷ 62.5 = 500.
Check: 500 × 62.5 = 31 250.
The mistake to watch for
A common slip is to leave the double bond in the repeat unit.
Mistaken repeat unit: [-CH=CH-]n for poly(ethene).
The double bond is used up in forming the chain, and two hydrogens have been lost.
The correction: the chain between repeat units is all single bonds. Another slip with propene is to put the methyl group into the chain instead of hanging it off as a side group. Count the carbons in the monomer and in the repeat unit, and check they match.
Check yourself
1. Draw the repeat unit for tetrafluoroethene, CF₂=CF₂.
Show answer
[-CF₂-CF₂-]n. The double bond opens, and both carbons keep their two fluorine atoms.
2. Find the monomer of the polymer with repeat unit [-CH₂-CH(CH₃)-].
Show answer
Propene, CH₃-CH=CH₂. Restore the double bond between the two chain carbons.
3. A poly(ethene) molecule has relative molecular mass 14 000. How many repeat units are there?
Show answer
Mr of the repeat unit, C₂H₄, is 28. 14 000 ÷ 28 = 500. Check: 500 × 28 = 14 000.
Where this leads next
Return to ester formation if you want to compare a different kind of joining, and then test the whole module in the organic reactions practice set. The mole and equation-ratio tutor helps with relative mass calculations.
Polymer questions often ask you to combine drawing, naming and a short calculation. Our teachers practise that combination in online one-to-one Chemistry tuition.