Addition Polymer Structure and Naming Revision Notes | A Level AQA Chemistry

1. From Alkene to Polymer: The Core Idea

Picture thousands of ethene molecules lining up and snapping together — each one losing its double bond and linking to its neighbours. That chain of repeating units is an addition polymer, and it is one of the most commercially significant classes of organic compound you will meet at A Level.

Addition polymers form exclusively from alkenes or substituted alkenes. The C=C double bond is the key: one bond in the double bond breaks, and each carbon uses that freed electron to bond to the next monomer unit. No atoms are lost and no small molecule is released — every atom in the monomer ends up in the polymer chain.

Because no atoms are lost, the molecular formula of the repeating unit is identical to the molecular formula of the monomer. This is the first structural relationship you need to be confident with before moving to empirical formulae.

2. Drawing the Repeating Unit from a Monomer

The repeating unit is what you get when you open the C=C double bond of the monomer and replace it with a single bond, then add a bond at each end to show the chain continues. The two dangling bonds are represented by lines extending beyond the brackets, and the brackets enclose the repeating unit with a subscript n.

Worked example: Drawing the Repeating Unit from a Monomer: Chloroethene Example

Draw the repeating unit of the addition polymer formed from chloroethene (CH2=CHCl).

1. Chloroethene: $\text{CH}_2\text{=CHCl}$ — The C=C double bond is the reactive site. One carbon carries two H atoms; the other carries one H and one Cl.
2. Replace C=C with C-C (single bond). Each carbon now has a free valence on each side of the chain. — The pi bond breaks. Each carbon gains one bond to the adjacent monomer unit in the growing chain.
3. Draw: $-\left[\text{CH}_2\text{-CHCl}\right]_n-$ with a bond line extending from each bracket. — The brackets show the unit that repeats. The bond lines outside the brackets indicate the chain continues in both directions. The subscript n represents a large integer.
4. Confirm: two H on one carbon, one H and one Cl on the other — same as the monomer. — No atoms are added or removed. The only change is C=C becomes C-C.

Answer: Repeating unit: $-\left[\text{CH}_2\text{-CHCl}\right]_n-$ enclosed in brackets with bond lines at each end.

The same logic applies to any substituted alkene. Whatever groups are attached to the two carbons of the double bond in the monomer remain attached to those same carbons in the repeating unit — they are spectators to the bond-opening process.

3. Identifying the Monomer from a Polymer Chain Section

Exam questions frequently give you a section of a polymer chain and ask you to deduce the monomer. This is the reverse process, and it requires you to recognise the repeating pattern within the chain and then reintroduce the C=C double bond.

The method is straightforward once you spot the pattern. Look at the backbone carbons in pairs: every two adjacent backbone carbons, together with their substituents, form one repeating unit. Reintroduce a double bond between those two carbons to recover the monomer.

Worked example: Identifying the Monomer from a Polymer Chain Section

A section of a polymer chain shows: -CH2-CH(CN)-CH2-CH(CN)-CH2-CH(CN)-. Identify the monomer.

1. The sequence -CH2-CH(CN)- repeats along the chain. — Every two backbone carbons carry the same set of substituents: two H on one carbon, one H and one CN on the other.
2. Repeating unit: $-\left[\text{CH}_2\text{-CH(CN)}\right]_n-$ — This is the smallest fragment that generates the full chain when repeated.
3. Replace the C-C single bond between the two backbone carbons with C=C: $\text{CH}_2\text{=CH(CN)}$ — Addition polymerisation is the reverse of this step. Reintroducing the double bond gives the monomer.
4. Monomer: $\text{CH}_2\text{=CHCN}$ (propenenitrile, also known as acrylonitrile) — The CN group is a nitrile substituent on carbon-2 of the ethene backbone.

Answer: The monomer is CH2=CHCN (propenenitrile / acrylonitrile).

4. IUPAC Naming of Addition Polymers

The IUPAC system for naming addition polymers is consistent and logical: the polymer name is poly followed by the IUPAC name of the monomer in parentheses. If the monomer name is a single word with no locants or multiplying prefixes, the parentheses are still required by convention at A Level.

To apply this rule, you need to name the monomer correctly first. Monomers are alkenes, so you use standard IUPAC alkene nomenclature: identify the longest carbon chain containing the double bond, number from the end that gives the double bond the lowest locant, and name any substituents as prefixes.

Notice that poly(tetrafluoroethene) is the IUPAC name for the material commonly known as PTFE or Teflon, formed from tetrafluoroethene, CF2=CF2. If the monomer were CH2=CF2, the correct name would be 1,1-difluoroethene and the polymer would be poly(1,1-difluoroethene).

Worked example: Naming an Addition Polymer from its Monomer Structure

Give the IUPAC name of the addition polymer formed from the monomer CH2=CHCH2CH3.

1. Chain: C=C-C-C — four carbons in total, with the double bond starting at carbon 1. — Count all carbons in the longest continuous chain that includes both carbons of the double bond.
2. Numbering from the end nearest the double bond: C1=C2-C3-C4. Double bond at position 1. — The double bond locant is 1, which is lower than it would be (3) if numbered from the other end.
3. Four-carbon chain with double bond at C1: but-1-ene. No substituents. — But = 4 carbons; -1-ene = double bond starting at carbon 1.
4. Polymer name: poly(but-1-ene) — Prefix poly, then the monomer name in parentheses. The locant is retained inside the brackets.

Answer: poly(but-1-ene)

5. Structural Relationships: Comparing Monomers and Polymers

Now that you can move fluently between monomer and polymer in both directions, it is worth consolidating the structural logic. The relationship between monomer and repeating unit is entirely predictable: the carbon skeleton is preserved, all substituents are preserved, and the only change is the conversion of C=C to C-C.

At A Level, AQA does not require you to reproduce the radical mechanism in detail for addition polymerisation, but understanding that the pi bond is the reactive site explains why only alkenes (not alkanes) can act as monomers. An alkane has no pi bond to open, so chain formation cannot occur.

6. Worked Exam-Style Problems

Exam questions on this topic typically ask you to draw structures, identify monomers, or name polymers — often combining two or three of these skills in a single question. The worked examples below model the precision of language and structural detail that earns full marks.

Worked example: Full Exam-Style Question: Monomer to Repeating Unit to Name

The monomer styrene has the structure CH2=CH-C6H5 (phenylethene). (a) Draw the repeating unit of poly(phenylethene). (b) State the IUPAC name of the polymer.

1. Carbon 1: CH2= (two H, no other substituents). Carbon 2: =CH- (one H, one phenyl group C6H5). — These are the only two carbons that change during polymerisation. All other atoms are spectators.
2. Replace C=C with C-C. Draw: $-\left[\text{CH}_2\text{-CH(C}_6\text{H}_5\text{)}\right]_n-$ with bond lines extending from each bracket. — The phenyl group (C6H5) remains on carbon 2. The bracket notation with extending bonds and subscript n is required.
3. Longest chain containing C=C: two carbons (ethene backbone). Substituent on C2: phenyl (C6H5). IUPAC monomer name: phenylethene. — Phenyl = C6H5 substituent; ethene = two-carbon chain with double bond.
4. Polymer name: poly(phenylethene) — The IUPAC name is poly(phenylethene). The common name polystyrene is not acceptable as an IUPAC answer.

Answer: (a) Repeating unit: -[CH2-CH(C6H5)]n- with brackets and extending bond lines. (b) poly(phenylethene)

This topic rewards precision in drawing and naming. The marks are straightforward to earn if you follow a consistent method, but they are equally easy to lose through small structural errors or incomplete notation. The guidance below consolidates the most frequently penalised mistakes seen in AQA mark schemes.

7. Exam Technique and Common Pitfalls

When a question asks you to draw a repeating unit or monomer, the examiner is looking for correct connectivity, correct substituents on the correct carbons, and correct bracket notation. When a question asks you to name a polymer, the examiner expects the full IUPAC format including the parentheses — writing just "polyethene" without parentheses may not score the naming mark.

Together, the skills of drawing repeating units, identifying monomers, and applying IUPAC naming rules cover all six specification requirements for this topic. Practise moving in both directions — monomer to polymer and polymer to monomer — until the structural logic feels automatic, and the naming follows directly from correct monomer identification.