Polymers: addition and condensation polymerisation

What you'll learn

Polymerisation forms the foundation for understanding how small molecules combine to create the plastics and synthetic materials used throughout modern life. The CIE IGCSE Chemistry specification requires you to distinguish between addition and condensation polymerisation, draw repeat units, identify monomers from polymer structures, and explain the formation of polyesters and nylon. This topic appears regularly in Paper 2 and Paper 4, often worth 4-6 marks per question.

Key terms and definitions

Monomer — a small molecule that can bond with other identical or similar molecules to form a polymer

Polymer — a large molecule made up of many repeating units (monomers) joined together by covalent bonds

Addition polymerisation — a reaction where many unsaturated monomer molecules (containing C=C bonds) join together to form a polymer with no other products

Condensation polymerisation — a reaction where monomers join together with the elimination of a small molecule (usually water or HCl) at each bond formed

Repeat unit — the smallest section of a polymer chain that, when repeated many times, represents the structure of the whole polymer

Polyester — a polymer formed by condensation polymerisation between dicarboxylic acids and diols, containing ester linkages (-COO-)

Saturated — a molecule containing only single carbon-carbon bonds (C-C), with no C=C double bonds

Thermoplastic — a polymer that softens when heated and can be remoulded repeatedly

Core concepts

Addition polymerisation mechanism

Addition polymerisation occurs exclusively with alkene monomers containing carbon-carbon double bonds (C=C). The mechanism involves three key stages:

Breaking the double bond: The C=C double bond in each monomer molecule opens up. One of the two bonds breaks, leaving a single C-C bond and two unpaired electrons.

Chain formation: Thousands of monomer molecules join end-to-end through these opened bonds, forming long carbon chains with only single bonds.

No by-products: Unlike condensation polymerisation, addition polymerisation produces only the polymer—no small molecules are eliminated.

The general equation can be represented as:

n C=C → [-C-C-]ₙ

where n represents a very large number (typically 1000-10000).

Drawing addition polymers from monomers

To draw the repeat unit of an addition polymer from its monomer structure:

1. Identify the C=C double bond in the monomer
2. Change the C=C to a single C-C bond
3. Add extension bonds (brackets) on either side of the unit
4. Place the subscript 'n' outside the brackets to show repetition

Example with ethene:

• Monomer: CH₂=CH₂
• Repeat unit: [-CH₂-CH₂-]ₙ
• Polymer name: poly(ethene) or polythene

Example with propene:

• Monomer: CH₂=CHCH₃
• Repeat unit: [-CH₂-CH(CH₃)-]ₙ
• Polymer name: poly(propene) or polypropylene

Example with chloroethene:

• Monomer: CH₂=CHCl
• Repeat unit: [-CH₂-CHCl-]ₙ
• Polymer name: poly(chloroethene) or PVC (polyvinyl chloride)

Identifying monomers from addition polymers

Working backwards from polymer to monomer frequently appears in exam questions. The method reverses the polymerisation process:

1. Identify the repeat unit (the section in brackets)
2. Remove the extension bonds from the ends
3. Convert the central C-C single bond to a C=C double bond
4. Ensure each carbon in the double bond has four bonds total

For the polymer [-CH₂-CHCl-]ₙ:

• Remove brackets and extension bonds
• Identify the two central carbons: -CH₂-CHCl-
• Convert C-C to C=C: CH₂=CHCl

Condensation polymerisation principles

Condensation polymerisation differs fundamentally from addition polymerisation in three ways:

Functional groups required: Monomers must contain two reactive functional groups per molecule (bifunctional), such as:

• Dicarboxylic acids: two -COOH groups
• Diols: two -OH groups
• Diamines: two -NH₂ groups

Small molecule elimination: Each time monomers link, a small molecule is eliminated—typically water (H₂O) or hydrogen chloride (HCl).

Linkage formation: The polymer chain contains specific linkages where monomers joined:

• Ester linkages (-COO-) in polyesters
• Amide linkages (-CONH-) in polyamides like nylon

Polyester formation

Polyesters form when dicarboxylic acids react with diols. The most common example tested at IGCSE level involves:

Monomers:

• Ethanedioic acid: HOOC-COOH (or any dicarboxylic acid)
• Ethanediol: HO-CH₂-CH₂-OH (or any diol)

Reaction mechanism:

1. The -COOH group from the acid reacts with the -OH group from the diol
2. Water (H₂O) is eliminated: the -OH from the acid and -H from the diol
3. An ester link (-COO-) forms between the monomers
4. This process repeats at both ends of each molecule, creating a long chain

Simplified equation: n HOOC-COOH + n HO-CH₂-CH₂-OH → [-OC-CO-O-CH₂-CH₂-O-]ₙ + 2n H₂O

The repeat unit shows the ester linkages clearly: [-OC-CO-O-CH₂-CH₂-O-]ₙ

Common polyesters include terylene (PET - polyethylene terephthalate), used in plastic bottles and synthetic fibres.

Nylon formation (polyamides)

Nylon represents the class of polyamides, formed by condensation polymerisation between dicarboxylic acids and diamines.

Monomers for nylon-6,6:

• Hexanedioic acid: HOOC-(CH₂)₄-COOH
• 1,6-diaminohexane: H₂N-(CH₂)₆-NH₂

Reaction mechanism:

1. The -COOH group reacts with the -NH₂ group
2. Water (H₂O) is eliminated: -OH from the acid and -H from the amine
3. An amide link (-CONH-) forms
4. The chain extends as both ends continue reacting

General structure: The repeat unit contains the characteristic amide linkage: [-OC-(CH₂)₄-CO-NH-(CH₂)₆-NH-]ₙ

Nylon applications include clothing fibres, ropes, carpets, and engineering plastics. Its strength derives from hydrogen bonding between polymer chains at the N-H and C=O sites.

Comparing addition and condensation polymerisation

Worked examples

Example 1: Drawing a repeat unit

Question: Styrene has the structure C₆H₅-CH=CH₂. Draw the repeat unit of poly(styrene). [2 marks]

Solution:

1. Identify the C=C double bond (between the two CH carbons, not in the benzene ring)
2. Change C=CH₂ to -C-CH₂-
3. Draw with extension bonds:

    [ CH₂ - CH ]ₙ
            |
           C₆H₅

Mark scheme: 1 mark for correct structure inside brackets, 1 mark for extension bonds and subscript n.

Example 2: Identifying monomers from a polyester

Question: A polyester has the repeat unit [-OC-C₆H₄-CO-O-CH₂-CH₂-O-]ₙ.

(a) Name the type of polymerisation. [1 mark] (b) Draw the structural formula of the diol monomer. [2 marks]

Solution: (a) Condensation polymerisation [1 mark]

(b) The diol section is the part between the two oxygen atoms of the ester links: HO-CH₂-CH₂-OH or HOCH₂CH₂OH [2 marks]

Mark scheme: (a) Must state "condensation" [1]. (b) 1 mark for -CH₂-CH₂- unit, 1 mark for -OH groups at both ends.

Example 3: Comparing polymers

Question: Poly(propene) and nylon are both polymers.

(a) State one difference in how they are formed. [1 mark] (b) Poly(propene) is an addition polymer. Draw the structure of its monomer. [1 mark]

Solution: (a) Poly(propene) forms with no other products OR nylon forms with elimination of water [1 mark]

Alternative acceptable answers:

• Poly(propene) is addition polymerisation, nylon is condensation
• Poly(propene) uses alkene monomers, nylon uses bifunctional monomers

(b) CH₂=CH-CH₃ or CH₃-CH=CH₂ [1 mark]

Mark scheme: Must show C=C double bond with a CH₃ group attached to one carbon.

Common mistakes and how to avoid them

• Mistake: Drawing addition polymer repeat units without extension bonds or the subscript 'n'. Correction: Always include brackets with bonds extending through them on both sides, and place 'n' as a subscript outside the closing bracket. The structure [-CH₂-CH₂-]ₙ is complete; -CH₂-CH₂- alone is just two carbon atoms.

• Mistake: Keeping the C=C double bond in the repeat unit of an addition polymer. Correction: The double bond opens during polymerisation and becomes a single C-C bond in the polymer chain. Addition polymers are saturated—they contain only single bonds in the carbon backbone.

• Mistake: Forgetting to eliminate water when drawing condensation polymerisation equations. Correction: Count the atoms carefully. When -COOH reacts with -OH (in diols) or -NH₂ (in diamines), H₂O is always eliminated. Show this as "+ nH₂O" on the products side of equations.

• Mistake: Confusing which monomers produce which linkages. Correction: Memorise these combinations: dicarboxylic acid + diol → ester link (-COO-); dicarboxylic acid + diamine → amide link (-CONH-). The linkage type determines the polymer class.

• Mistake: Drawing monomers with only one functional group when working backwards from condensation polymers. Correction: Condensation monomers must be bifunctional (two reactive groups). If identifying a diol, both -OH groups must be present; for dicarboxylic acids, show both -COOH groups.

• Mistake: Writing the small molecule eliminated as H₂O in all cases. Correction: While water is most common (from acids reacting with alcohols or amines), other small molecules can be eliminated. Some condensation reactions produce HCl. Always check what groups are reacting.

Exam technique for addition and condensation polymerisation

• "Draw the repeat unit" questions require precise bracketing notation. Marks are frequently lost for missing extension bonds or forgetting the subscript 'n'. Draw the smallest repeating section, ensure bonds clearly extend through the brackets, and position 'n' outside. Typically worth 2 marks: 1 for structure, 1 for notation.

• "Identify the type of polymerisation" commands expect the specific term: "addition" or "condensation". Generic answers like "polymerisation" or "chain reaction" gain no credit. If asked to justify your choice, state whether small molecules are eliminated (condensation) or not (addition), or refer to the monomer type (alkene = addition; bifunctional = condensation).

• Questions comparing polymers often use a table format or ask for differences. Structure answers clearly with one difference per mark. Avoid vague statements—be specific about monomers, products, or bond types. For example: "Addition uses alkenes; condensation uses bifunctional monomers" scores marks; "They use different starting materials" does not.

• Drawing structures from names appears regularly. Know these systematic names: poly(ethene), poly(propene), poly(chloroethene). The monomer name appears in brackets. For condensation polymers, questions may provide the monomer structures rather than names, so focus on recognising functional groups (-COOH, -OH, -NH₂).

Quick revision summary

Polymerisation creates large molecules from small monomers. Addition polymerisation joins alkenes (C=C) without by-products, forming saturated polymers like poly(ethene) and PVC. Condensation polymerisation joins bifunctional monomers (dicarboxylic acids with diols or diamines), eliminating water and forming polyesters or polyamides like nylon. Repeat units show the smallest repeating section with extension bonds and subscript 'n'. Identify polymer type by checking for eliminated molecules (condensation) or alkene monomers (addition). Master drawing structures both forwards (monomer to polymer) and backwards (polymer to monomer) for exam success.