Organic chemistry: addition and condensation polymerisation

What you'll learn

This revision guide covers the essential AQA GCSE Chemistry content on polymerisation reactions. You'll learn how small molecules called monomers join together to form large polymer molecules through addition and condensation reactions. Understanding these processes is crucial for answering exam questions on plastics, synthetic materials, and their environmental impact.

Key terms and definitions

Monomer — A small molecule that can bond with other identical molecules to form a polymer chain.

Polymer — A large molecule made from many repeating monomer units joined together through chemical bonds.

Addition polymerisation — A reaction where unsaturated monomers (containing C=C double bonds) join together without any other product being formed.

Condensation polymerisation — A reaction where monomers join together with the elimination of a small molecule, usually water, for each bond formed.

Repeating unit — The basic structural section of a polymer that repeats along the chain, shown in brackets with bonds extending on both sides.

Saturated — A molecule containing only single carbon-carbon bonds (C–C).

Unsaturated — A molecule containing at least one carbon-carbon double bond (C=C) or triple bond.

Polyester — A polymer formed by condensation polymerisation between dicarboxylic acid and diol monomers, containing ester linkages.

Core concepts

Addition polymerisation

Addition polymerisation occurs when alkene monomers join together. The carbon-carbon double bond (C=C) in each monomer breaks, allowing thousands of monomers to link into long chains.

Key features of addition polymerisation:

• Only alkenes (unsaturated hydrocarbons) undergo addition polymerisation
• No other product is formed — atoms are simply rearranged
• The polymer is the only product
• All atoms from the monomers are incorporated into the polymer chain

Common addition polymers you must know:

Drawing addition polymers:

When drawing the repeating unit of an addition polymer:

1. Remove the C=C double bond and replace with a C–C single bond
2. Draw the repeating unit in brackets
3. Add extension bonds on both sides going through the brackets
4. Add an 'n' subscript outside the brackets to show many repeating units

For example, ethene (CH₂=CH₂) becomes poly(ethene) with repeating unit: —(CH₂—CH₂)—ₙ

Condensation polymerisation

Condensation polymerisation involves monomers with two functional groups joining together. Each time two monomers bond, a small molecule (typically water) is eliminated.

Key features of condensation polymerisation:

• Requires monomers with two functional groups
• A small molecule (usually H₂O) is released for each bond formed
• Two different types of monomer are often used
• Creates linkages between monomer units

Types of condensation polymers:

The main condensation polymers at GCSE level are polyesters and polyamides (though polyamides receive less emphasis in AQA GCSE).

Polyesters form when:

• Dicarboxylic acid monomers (with two –COOH groups) react with
• Diol monomers (with two –OH groups)
• Water is eliminated when an ester link forms

Formation of polyesters

The ester link forms through condensation between a carboxylic acid group (–COOH) and an alcohol group (–OH):

–COOH + HO– → –COO– + H₂O

Example monomers for polyester formation:

• Dicarboxylic acid: ethanedioic acid (HOOC–COOH)
• Diol: ethane-1,2-diol (HO–CH₂–CH₂–OH)

These react repeatedly, with each –COOH group reacting with an –OH group, eliminating water molecules and forming ester linkages (–COO–).

Important polyesters:

• PET (poly(ethene terephthalate)) — formed from terephthalic acid and ethane-1,2-diol; used in drink bottles, clothing fibres
• Polyesters are also found in natural materials and synthetic fabrics

Identifying polymer types

You must be able to distinguish between addition and condensation polymers by examining their structure:

Addition polymers contain:

• Only carbon and hydrogen (unless the monomer contained other elements like chlorine in PVC)
• Carbon backbone with no other atoms in the main chain
• No functional groups connecting the repeating units

Condensation polymers (polyesters) contain:

• Ester linkages (–COO–) in the main chain
• Oxygen atoms within the polymer backbone
• Recognisable functional groups joining the units

Drawing repeating units from monomers

For addition polymerisation:

Start with the alkene monomer showing the C=C double bond:

1. Change the C=C to C–C
2. Place in brackets with bonds extending outwards
3. Add subscript n

Example: Propene CH₃–CH=CH₂ becomes —(CH₃–CH–CH₂)—ₙ

For condensation polymerisation (polyesters):

1. Identify the functional groups that will react (–COOH and –OH)
2. Remove H₂O (H from –OH, OH from –COOH)
3. Form the ester link –COO–
4. Draw the repeating unit showing both monomer units joined
5. Place in brackets with extension bonds and subscript n

Identifying monomers from polymer structure

Working backwards from polymer to monomer is a common exam question.

For addition polymers:

1. Identify the repeating unit (shown in brackets)
2. Convert the C–C single bond to C=C double bond
3. Remove the extension bonds
4. This gives you the monomer structure

For condensation polymers:

1. Identify the ester link (–COO–)
2. Break the polymer at these links
3. Add –OH to one side and –COOH to the other (effectively adding back the H₂O)
4. This reveals the two monomers

Properties and uses of polymers

Different polymers have different properties based on their structure, making them suitable for specific applications.

Addition polymers:

• Generally chemically inert (unreactive)
• Do not decompose easily
• Can be flexible or rigid depending on structure
• Most are non-biodegradable

Condensation polymers (polyesters):

• Often stronger and more rigid than addition polymers
• Can be broken down by hydrolysis (reverse of formation)
• Used in fabrics, bottles, and composite materials
• Better biodegradability than most addition polymers

Worked examples

Example 1: Drawing the repeating unit of poly(propene)

Question: Propene has the structure CH₃–CH=CH₂. Draw the repeating unit of poly(propene). [2 marks]

Solution:

Step 1: Identify the C=C double bond in propene (between the two CH carbons)

Step 2: Change C=C to C–C single bond

Step 3: Draw the structure in brackets with extension bonds:

    CH₃
    |
—(—C—CH₂—)—ₙ
    |
    H

Mark scheme:

• Correct structure showing C–C single bond replacing C=C [1 mark]
• Brackets with extension bonds and subscript n [1 mark]

Example 2: Identifying monomers from a polyester

Question: A polyester has the following repeating unit:

—(—O—CH₂—CH₂—O—CO—CH₂—CO—)—ₙ

Identify the two monomers used to make this polyester. [3 marks]

Solution:

Step 1: Locate the ester linkage (–COO–)

Step 2: Break the chain at the ester links: –O–CO–

Step 3: Add H and OH to recreate the original functional groups:

• Add H to the O side → –O–H (alcohol)
• Add OH to the CO side → –CO–OH (carboxylic acid)

Monomers:

• Diol: HO–CH₂–CH₂–OH (ethane-1,2-diol)
• Dicarboxylic acid: HOOC–CH₂–COOH (propanedioic acid)

Mark scheme:

• Correctly identifies ester linkage position [1 mark]
• Correct diol structure [1 mark]
• Correct dicarboxylic acid structure [1 mark]

Example 3: Comparing addition and condensation polymerisation

Question: (a) State two differences between addition and condensation polymerisation. [2 marks]

(b) Poly(ethene) is formed by addition polymerisation. Explain why poly(ethene) is difficult to dispose of. [2 marks]

Solution:

(a) Any two from:

• Addition polymerisation produces only the polymer; condensation polymerisation produces the polymer and a small molecule (usually water)
• Addition polymerisation uses monomers with C=C double bonds; condensation polymerisation uses monomers with two functional groups
• Addition polymers have no functional groups in the main chain; condensation polymers have functional groups (like ester links) joining units

(b)

• Poly(ethene) is unreactive/chemically inert [1 mark]
• Therefore it is non-biodegradable/does not decompose naturally/takes many years to break down [1 mark]

Common mistakes and how to avoid them

• Forgetting extension bonds when drawing repeating units — Always draw bonds extending through both sides of the brackets and add subscript n to show the repeating nature.

• Confusing the monomer with the repeating unit — The monomer contains a C=C double bond; the repeating unit (in addition polymers) shows a C–C single bond with extension bonds.

• Incorrectly identifying what is eliminated in condensation polymerisation — Water (H₂O) is eliminated when an ester link forms, not other molecules. Remember: H comes from –OH and OH comes from –COOH.

• Drawing too many or too few carbons in the repeating unit — Count carefully from the original monomer structure. All atoms from the monomer (except those in eliminated molecules for condensation) must appear in the repeating unit.

• Stating that condensation polymers are always biodegradable — While some condensation polymers can undergo hydrolysis more easily than addition polymers, they are not necessarily biodegradable in the environment.

• Missing out functional groups when working backwards from polyester to monomers — Remember to add back the –OH and –COOH groups (the water that was eliminated) when breaking ester links.

Exam technique for "Organic chemistry: addition and condensation polymerisation"

• "Draw the repeating unit" requires brackets, extension bonds, and subscript n. Without these, you'll lose marks even with the correct structure. Practise drawing these accurately.

• "Name the type of polymerisation" needs the specific term: "addition polymerisation" or "condensation polymerisation". Writing just "addition" or "condensation" may not gain the mark.

• "Identify the monomers" questions often award marks for structure, not names. Draw the full structural formulae showing all bonds and functional groups clearly.

• Extended response questions on polymers and the environment require you to link properties (non-biodegradable, unreactive) to environmental problems (landfill, persistence) and possible solutions (recycling, burning for energy, biodegradable alternatives). Use scientific terminology and write in continuous prose.

Quick revision summary

Polymerisation creates large molecules from small monomers. Addition polymerisation joins alkenes (containing C=C) with no other product; only atoms rearrange. Condensation polymerisation joins monomers with two functional groups, eliminating small molecules like water. Polyesters form when dicarboxylic acids react with diols, creating ester linkages. Draw repeating units in brackets with extension bonds and subscript n. Identify polymer types by examining their backbone structure and functional groups.