Guided lesson: Addition Polymers

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Chemistry

2.Drawing polymers

4CH1-4.45

The second lesson turns to drawing: how a stream of alkene monomers becomes a polymer chain on paper. The trick is the C=C double bond, which opens up to link each monomer to its neighbours. Naming follows a simple pattern too, with poly- fixed to the front of the monomer's name.

1,2,-dichloroethene and the formation of its polymer

Key points

Addition polymerisation only works with monomers that contain a C=C double bond.
One of the two bonds in each C=C breaks, forming the bond to the adjacent monomer.
The resulting polymer chain contains single bonds only.
The polymer's name takes the monomer's name in brackets with poly- added in front.
Propene monomers form polypropene, and ethene monomers form polyethene, as the note's two examples.

First pass

The C=C double bond in each monomer opens, one of its two bonds links to the next monomer, and the chain that results has only single bonds.

Analogy — Picture velcro strips: only strips with the hook side can join, and once joined the hooks are used up, leaving smooth fabric along the chain.

Exam pointer — Practise all four named polymers: poly(ethene), poly(propene), poly(chloroethene) and (poly)tetrafluoroethene.

Going deeper

Only monomers carrying a C=C bond can add to the chain, because breaking one bond of the pair frees each end to bond to neighbours; groups attached to the alkene simply ride along, and naming brackets the monomer name behind poly-.

Analogy — Think of unzipping a jacket halfway: the opened teeth fasten onto the next jacket, and the embroidered patches hang wherever they started.

Exam pointer — In diagrams, make sure one of the two C=C bonds is the one shown breaking, and that the polymer shows single bonds throughout.

Checks

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1.Which structural feature must a monomer have for addition polymerisation?
2.What happens to one of the two bonds in each C=C during polymerisation?
3.What kinds of bonds does the finished polymer chain contain?
4.How is the polymer's name built from the monomer's name?
5.Propene undergoes addition polymerisation. What is the product's name?
6.State the name of the polymer formed when ethene undergoes addition polymerisation.
Written response · recall
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7.State the rule for building an addition polymer's name from its monomer's name.
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8.Explain why the finished polymer chain contains only single bonds even though each monomer carries a C=C double bond.
Written response · explain why
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9.Chloroethene is the monomer used to make PVC. State the systematic name of the polymer it forms, following the note's naming rule.
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3.Deducing monomers and repeat units

4CH1-4.46

The final skill runs in both directions: given a monomer, draw the repeat unit; given a repeat unit, recover the monomer. Three changes do all the work — swap the bond order, adjust the end bonds, and mind the subscript n. Get those right and every named polymer is within reach.

Key points

To draw a repeat unit, change the monomer's double bond to a single bond.
Add a bond to each end, and let them extend outside the brackets.
A subscript n on the bottom right shows that a large number of repeat units are present.
The other groups keep the same order they had around the double bond in the monomer.
To recover the monomer, change the single bond back to a double bond and remove the end bonds and the n.

First pass

Monomer to repeat unit: the double bond becomes single, add extension bonds past the brackets and the subscript n; repeat unit to monomer: reverse the steps.

Analogy — It works like a two-way recipe: fold a page in half, tuck the ends under the clip, and unfold the same page again to recover it.

Exam pointer — The commonest mark loss is forgetting the extension bonds outside the brackets or the subscript n, so check both every time.

Going deeper

The monomer's double bond and the repeat unit's single bond are the same linkage in two states: converting one to the other, adding or removing the end bonds and the subscript n, is the entire change, with every attached group kept in order.

Analogy — Picture a zip on a backpack: zipped is the repeat unit with its extension bonds, unzipped is the monomer with its double bond, and the same teeth change state without rearranging.

Exam pointer — Deduce monomers from repeat units by reopening the double bond exactly between the two bracketed carbons, keeping attached groups in the same positions.

Checks

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1.When drawing a repeat unit from a monomer, what happens to the double bond?
2.What must be drawn at each end of the repeat unit?
3.What does the subscript n represent?
4.When recovering the monomer from a repeat unit, which changes do you make?
5.You are given a repeat unit and asked for its monomer. What is the key step?
6.State what happens to the other groups around the former double bond when a monomer is drawn as a repeat unit.
Written response · recall
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7.State where the bonds at each end of a repeat unit must extend.
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8.Explain why changing the single bond back to a double bond recovers the monomer from a repeat unit.
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9.A student draws a repeat unit with both end bonds inside the brackets and no subscript n. State the two corrections needed to make the drawing correct.
Written response · application
Your answer