Synthetic polymers

Chemistry· 4.44–4.50 (section 4(h), 2017 spec)· 20 min read

1. Introduction to Addition Polymerisation★★☆☆☆⏱ 5 min

📘 Definition

Addition polymer

Long chain molecule formed when many small alkene monomers join together by breaking their C=C double bonds, with no small byproducts produced.

All addition polymer monomers contain a carbon-carbon double bond. When polymerisation occurs, the double bond breaks, and each carbon forms a new single bond to a carbon in an adjacent monomer, creating a continuous long chain.

📐 Worked Example

Draw the repeat unit of poly(ethene), given the ethene monomer structure is $C H_{2} = C H_{2}$ .

1
1. Break the C=C double bond in the ethene monomer to form two single bonds on each carbon atom, giving $- C H_{2} - C H_{2} -$
2
1. Place this unit in square brackets, drawing single bonds extending through both sides of the brackets to show links to adjacent repeat units
3
1. Add the subscript $n$ outside the closing bracket to indicate multiple repeats of the unit
4
$[-CH_2-CH_2-]_n$

Exam tip:

You will lose marks if you draw bonds that end at the bracket edge instead of extending through it, as this incorrectly shows no link to adjacent units.

2. Monomer and Repeat Unit Deduction★★★☆☆⏱ 6 min

You will be asked to convert between addition polymer repeat units and monomers in exams. The rule is simple: the number of atoms in the repeat unit exactly matches the number of atoms in the monomer, with only the double bond converted to single linking bonds.

📐 Worked Example

The repeat unit of poly(propene) is $[- C H_{2} - C H (C H_{3}) -]_{n}$ . Deduce the displayed structure of the propene monomer.

1
1. Remove the square brackets, $n$ subscript and the bonds extending out from the repeat unit
2
1. Reform a C=C double bond between the two carbon atoms that formed the backbone of the repeat unit
3
1. The final monomer structure is $C H_{2} = C H C H_{3}$ , with the methyl group attached to one of the double bond carbons

✓ Quick check

What is the correct repeat unit of poly(tetrafluoroethene), given its monomer is $C F_{2} = C F_{2}$ ?
- $[- C F = C F -]_{n}$
- $[- C F_{2} - C F_{2} -]_{n}$
- $[- C F_{3} - C F_{3} -]_{n}$
Reveal answer
1 —
Addition polymerisation breaks the C=C double bond completely, so no double bonds remain in the repeat unit, and no extra fluorine atoms are added.

3. Disposal Problems of Addition Polymers★★☆☆☆⏱ 4 min

Addition polymers are inert (unreactive) because their carbon backbone is made of strong single C-C bonds, which are not broken down by microorganisms in the environment. This means they are non-biodegradable, and remain in landfill sites for hundreds of years.

Burning addition polymers to reduce landfill volume can release toxic gases. For example, poly(chloroethene) (PVC) contains chlorine atoms, so burning it releases hydrogen chloride (HCl) gas, which causes respiratory harm and contributes to acid rain.

📐 Worked Example

State and explain two environmental issues caused by improper disposal of poly(chloroethene) (PVC).

1
1. First issue: PVC is non-biodegradable. Its strong C-C backbone cannot be broken down by microorganisms, so it remains in landfill for hundreds of years, taking up space and releasing pollutants over time.
2
1. Second issue: Burning PVC releases toxic hydrogen chloride (HCl) gas. HCl is corrosive, damages the respiratory system when inhaled, and reacts with water in the atmosphere to form acid rain.

Exam tip:

Always name the specific toxic gas (HCl) when asked about PVC combustion, this will get you full marks instead of partial marks for saying 'toxic gases' generally.

4. Condensation Polymerisation (Higher Only)★★★★☆HL only⏱ 5 min

📘 Definition

Condensation polymerisation (polyester formation)

Reaction between a dicarboxylic acid (two -COOH groups) and a diol (two -OH groups) that forms a long polyester chain and water as a byproduct.

During polyester formation, one -OH group is removed from each carboxylic acid group of the diacid, and one -H atom is removed from each alcohol group of the diol. These combine to form water molecules, while the remaining parts of the monomers join via ester linkages to form the polymer chain.

📐 Worked Example

Draw the repeat unit of the polyester formed from ethanedioic acid ( $H O O C - C O O H$ ) and ethanediol ( $H O C H_{2} C H_{2} O H$ ).

1
1. Remove -OH from both ends of the ethanedioic acid, and remove -H from both ends of the ethanediol: these form 2 molecules of water per repeat unit
2
1. Join the remaining carbonyl carbon of the acid to the remaining oxygen of the alcohol to form ester linkages at both ends of the unit
3
1. Place the combined unit in brackets with bonds extending through both sides, add the subscript $n$
4
$[-OCH_2CH_2OCOCO-]_n$

Some polyesters called biopolyesters are biodegradable, meaning they can be broken down by microorganisms in the environment, reducing plastic waste accumulation.

5. Common Pitfalls

Wrong move:

Leaving a C=C double bond in the addition polymer repeat unit

Why:

Addition polymerisation breaks the double bond of the monomer completely to form single bonds linking adjacent units, so no double bonds remain in the polymer backbone

Correct move:

Replace the double bond in the monomer with single bonds extending out from both carbons for the repeat unit

Wrong move:

Drawing bonds that end at the edge of the repeat unit brackets instead of extending through them

Why:

This is a common marking point loss, as it incorrectly indicates no link to adjacent repeat units in the polymer chain

Correct move:

Draw single bonds through both sides of the square bracket surrounding the repeat unit, before adding the $n$ subscript

Wrong move:

Forgetting that condensation polymerisation produces small byproducts (water)

Why:

The presence of a byproduct is the key defining difference between addition and condensation polymerisation, a frequently tested exam question

Correct move:

State that condensation polymerisation produces both the polymer and small byproducts (e.g. water) when comparing the two processes

Wrong move:

Naming carbon dioxide as the main toxic gas released when burning PVC

Why:

While CO2 is produced, the specific toxic gas expected for chlorinated polymers like PVC is hydrogen chloride, which carries the marks for this question

Correct move:

Name hydrogen chloride (HCl) as the toxic gas released when poly(chloroethene) is burned

Wrong move:

Including extra H and OH atoms in polyester repeat units that are lost as water

Why:

The -OH from the diacid and -H from the diol are removed to form water, so they are not part of the final polymer backbone

Correct move:

Remove the relevant H and OH groups before joining the monomers via ester linkages to form the polyester repeat unit

6. Quick Reference Cheatsheet

Polymer Type	Monomer Type	Repeat Unit Rule	Byproduct	Disposal Notes
Addition Polymer	Alkene with C=C double bond	Break C=C, extend bonds out, bracket + $n$	None	Non-biodegradable, may release toxic gases if burned
Polyester (Condensation, Higher only)	Dicarboxylic acid + diol	Form ester linkages, remove H2O, bracket + $n$	Water	Biopolyesters are biodegradable

7. Frequently Asked

How do I convert an alkene monomer to an addition polymer repeat unit?

Break the C=C double bond in the monomer, extend single bonds out from each of the two former double bond carbons, place the unit in square brackets with bonds extending through both sides of the bracket, and add a subscript $n$ outside the brackets.

What is the key difference between addition and condensation polymerisation?

Addition polymerisation forms only one product (the polymer) from alkene monomers, with no small molecules lost. Condensation polymerisation forms the polymer plus small byproducts (e.g. water) from two different functional group monomers.

What's Next

Now that you have mastered synthetic polymers for Edexcel IGCSE Chemistry, you are ready to consolidate your organic chemistry knowledge and practice exam-style questions. Polymers are frequently tested alongside other functional groups, so make sure you are confident linking alkene, carboxylic acid and alcohol chemistry to polymer formation for high-mark extended response questions. For higher-tier candidates, practice converting between diacid/diol monomers and polyester repeat units to pick up easy marks in Paper 2C. You should also practice past paper questions on polymer disposal, as these 3-4 mark explanation questions are common and easy to score full marks on if you learn the key points.

→Alkenes Revision Guide