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Addition Polymers

Read repeat units, preserve atoms and substituents, and deduce alkene monomers from their addition polymers.

  • SEC G3 Pure Chemistry 2027

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An alkene monomer becomes part of a long chain without losing its atoms. Follow the two carbons of its double bond: that lets you draw a polymer’s repeat unit and recover the monomer from it.

Read a chain built from monomers

Polymer and repeat unit

A polymer is a very large molecule built from many smaller molecules called monomers. A repeat unit is the structural pattern repeated along the chain. Different monomers and different linkages between them produce different polymers and properties.

The molecule before it joins

A monomer is a small molecule that can join other monomers to form a polymer. Its atoms become part of a repeat unit, but its bonds can change as it joins. Ethene, CH2=CH2, is the monomer of poly(ethene).

Joining many molecules

Polymerisation joins many monomers into polymer molecules. Some polymers are natural, such as starch and proteins. Plastics contain polymers, usually with other substances added to give useful properties; “polymer” and “plastic” are not interchangeable names.

Addition polymerisation

In the addition polymerisation of alkenes, many monomers join and the polymer is the only product. Each C=C becomes a C–C single bond. Each of those two carbons also forms a new bond to a carbon in a neighbouring unit: the original two carbons stay connected.

A different way to join monomers

Condensation Polymers explains nylon and Terylene, whose linkages form with a small molecule such as water as a byproduct.

Track atoms, not just the brackets

For an addition polymer, retain every atom or group attached to the two double-bonded carbons. Replace their double bond with a single bond, add the bonds that join neighbouring units, and use brackets with n to show repetition.

Move between monomer and repeat unit

Ethene forms poly(ethene)

nCH₂ = CH₂ → [-CH₂-CH₂-]ₙ

Here n is the number of monomer units in a chain. It is large and can differ between molecules in a sample. A bond passing through each bracket means that the chain continues; it is not a negative charge.

ethene forming poly(ethene)The monomer ethene: two carbons joined by a double bond. The first carbon carries two H; the second carries two H. An arrow leads to the repeat unit of poly(ethene): the same two carbons joined by a single bond and carrying the same groups, inside brackets, with a bond continuing through each bracket and the subscript n after the closing bracket.nCCHHHHCCHHHHnethenepoly(ethene)
Both carbons and all four hydrogens remain. The C=C becomes single, and each carbon forms a bond to a neighbouring unit; the bonds through the brackets show continuation.

The repeat unit CH2–CH2 has the same atom ratio as ethene: both simplify to empirical formula CH2. This comparison uses the ideal repeating chain, ignoring end groups; do not use it to claim that a real finite chain’s end groups must also match that ratio.

Suitable grades of poly(ethene) make plastic bags and clingfilm. They are light, flexible and resistant to water. Other grades can be more rigid: connect the properties of the particular material to its use.

Keep attached groups in place

For propene, CH2=CHCH3, the two carbons of C=C form the backbone. The CH3 group stays attached to the same backbone carbon; it does not become a third carbon along the repeating backbone.

propene forming poly(propene)The monomer propene: two carbons joined by a double bond. The first carbon carries two H; the second carries H and CH3. An arrow leads to the repeat unit of poly(propene): the same two carbons joined by a single bond and carrying the same groups, inside brackets, with a bond continuing through each bracket and the subscript n after the closing bracket.nCCHHHCH₃CCHHHCH₃npropenepoly(propene)
The methyl group remains a branch on the second backbone carbon. Count four bonds around each carbon before and after polymerisation.

Reverse the change

For an addition polymer derived from one alkene:

  1. Find the two-carbon unit corresponding to one monomer and keep its attached groups.
  2. Remove n, the brackets and the bonds to neighbouring units.
  3. Restore C=C between those two carbons.
  4. Check that each carbon has four bonds.

A chain of identical CH2 groups can be drawn using a one-carbon visual repeat, but recovering its alkene monomer requires the two-carbon monomer-derived unit. Do not automatically select the shortest visible pattern.

Condensation linkages

Continue with Condensation Polymers.

Nylon

Read nylon’s amide linkages.

Terylene

Read Terylene’s ester linkages.

Compare the two routes

The comparison uses monomers, linkages and byproducts.

Plastic disposal

Plastic Waste and Recycling connects durability to disposal problems.

Recycling methods

Physical and chemical recycling act differently on polymer chains.

Recovering monomers

Read polyester depolymerisation.

Making a recycling decision

Recycling trade-offs need evidence about the particular scheme.

Check connections and carbon bonds

A polymer repeat unit is not a free alkene molecule. Keeping C=C as well as bonds to neighbouring units would give five bonds around a carbon in a poly(ethene) repeat unit. Use a single bond between those carbons and leave every attached group in place.

Show what repeats

When asked for a repeat unit, show its bonds through the brackets and n outside. When asked for a monomer, show the double bond and omit the polymer brackets. Use the structure supplied rather than memorising only ethene’s pattern.

Recover a monomer, then build a repeat unit

Modelled example 1

Identify the monomer of poly(ethene)

Core

Problem

A polymer section is ...-CH₂-CH₂-CH₂-CH₂-CH₂-CH₂-.... Draw and name its alkene monomer.
Study the worked solution

  1. Locate one repeating unit

    Method

    Identify the two-carbon unit supplied by one alkene monomer.

    Reason

    Each repeat retains the two-carbon skeleton from one monomer.

    Working

    Repeating unit: -CH₂-CH₂-.

  2. Restore the alkene bond

    Method

    Remove the outward chain bonds and place C = C between the two carbon atoms.

    Reason

    The C=C bond becomes single; each of its carbons forms a new bond to a neighbouring unit.

    Working

    Monomer structure: CH₂ = CH₂.

  3. Name the monomer

    Method

    Name CH₂ = CH₂ as ethene.

    Reason

    It is the two-carbon alkene required for poly(ethene).

    Working

    Monomer: ethene, CH₂ = CH₂.

Guided practice 2

Repeating unit from monomer

About 5 min

Problem

Propene is CH₂ = CHCH₃. Write the repeating unit in poly(propene).

Change the backbone bond and preserve substituents

Backbone bond after polymerisation
Repeating unit

Hints

Hint 1: backbone
Use the two carbon atoms that were joined by C = C as the polymer backbone.
Hint 2: substituent
Keep CH₃ attached to the same carbon; the C=C becomes single and new bonds join neighbouring units.
View solution step by step

  1. Change the bond and join the units

    Method

    Replace C = C with a C-C single bond.

    Reason

    The additional bonding capacity links each monomer to the units on either side.

    Working

    CH₂ = CHCH₃ → -CH₂-CH(CH₃)⁻.

  2. Show continuation

    Method

    Keep an outward bond at each end of the two-carbon unit.

    Reason

    Those bonds show that the same unit continues through the polymer chain.

    Working

    Repeating unit: -CH₂-CH(CH₃)⁻.

Classify a linkage

The classification example is beside the condensation structures.

Identify nylon and Terylene

The partial-structure example distinguishes their linkages.

Recover polyester monomers

The hydrolysis example is beside chemical recycling.

Try without prompts

Mind stretcher 1: An unfamiliar attached atomExtension

A polymer has repeat unit [-CH₂-CHCl-]ₙ. Draw its alkene monomer. Explain why using CH₂ = CH₂ would fail to conserve atoms. You do not need to know the monomer’s name.

Show answer

The monomer is CH₂ = CHCl. Restoring the double bond keeps Cl attached to the second carbon. Ethene would lose that chlorine atom. Each monomer carbon has four bonds: the first has two C–H bonds and C=C; the second has C–H, C–Cl and C=C.

Mind stretcher 2: Spot the wrong monomerExtension

A polymer has repeating unit -CH₂-CH(CH₃)⁻. A student says its monomer is propane, CH₃CH₂CH₃. Explain precisely what is wrong and give the correct monomer.

Show answer

This is an addition polymer, so its monomer must be an alkene with a C = C bond.

CH₃CH₂CH₃ has no C = C, so it cannot form this polymer by addition polymerisation.

The correct monomer is CH₂ = CHCH₃ (propene).

Restore the double bond between the two backbone carbons to obtain propene.

Question a biodegradable claim

Try the disposal claim question.

Practise and check

Use the Organic Chemistry topic check to practise and check your understanding.

Syllabus and review details

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