Polymers

This lesson covers polymers — long-chain molecules made from repeating units — as required by the Edexcel GCSE Chemistry (1CH0) specification. You need to understand how addition polymers are formed, how to draw sections of polymer chains, and the difference between thermosoftening and thermosetting polymers.

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What Are Polymers?

A polymer is a very large molecule (a macromolecule) made up of many small, repeating units called monomers joined together.

• Monomer: a small molecule that can join with many other identical molecules to form a polymer.
• Polymer: the long-chain molecule formed when many monomers bond together.
• Polymerisation: the chemical reaction in which monomers join to form a polymer.

The word "polymer" comes from Greek: poly = many, mer = parts.

Exam Tip: Make sure you can define monomer, polymer and polymerisation separately. These are common 1-mark definition questions.

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Addition Polymerisation

Addition polymerisation is the process by which many alkene monomers (molecules with a C=C double bond) join together to form a polymer. No other product is formed — the only product is the polymer.

How It Works

1. The C=C double bond in the alkene monomer "opens up" (one of the two bonds breaks).
2. This frees up a bonding site on each carbon atom.
3. Each monomer then bonds to the next, forming a long chain with C–C single bonds.
4. The process repeats thousands of times.

General Equation

Many monomer molecules → polymer

For example:

n CH₂=CH₂ → –(CH₂–CH₂)–n

Where:

• n is a very large number (thousands or more).
• CH₂=CH₂ is ethene (the monomer).
• –(CH₂–CH₂)– is poly(ethene), commonly known as polythene or polyethylene.

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Examples of Addition Polymers

Poly(ethene) from Ethene

• Monomer: ethene (CH₂=CH₂)
• Polymer: poly(ethene) — –(CH₂–CH₂)–ₙ
• Uses: plastic bags, bottles, packaging

Poly(propene) from Propene

• Monomer: propene (CH₂=CHCH₃)
• Polymer: poly(propene) — –(CH₂–CH(CH₃))–ₙ
• Uses: ropes, carpets, plastic furniture

Poly(chloroethene) from Chloroethene (Vinyl Chloride)

• Monomer: chloroethene (CH₂=CHCl)
• Polymer: poly(chloroethene), commonly called PVC — –(CH₂–CHCl)–ₙ
• Uses: window frames, pipes, insulation for electrical cables

Poly(tetrafluoroethene) from Tetrafluoroethene

• Monomer: tetrafluoroethene (CF₂=CF₂)
• Polymer: poly(tetrafluoroethene), commonly called PTFE or Teflon — –(CF₂–CF₂)–ₙ
• Uses: non-stick coating on pans, waterproof clothing

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Drawing Polymer Sections from Monomers

To draw the repeating unit of a polymer from its monomer:

1. Start with the alkene monomer (which has a C=C double bond).
2. Change the double bond to a single bond (C–C).
3. Add a bond extending out from each end of the repeating unit (to show it continues).
4. Place square brackets around the repeating unit with an n subscript.

Example: Ethene to Poly(ethene)

• Monomer: CH₂=CH₂ (double bond between the two C atoms, each with two H atoms).
• Repeating unit: –CH₂–CH₂– (single bond, with extending bonds at each end).
• Written: –[CH₂–CH₂]–ₙ

Exam Tip: Always show the extending bonds (lines coming out of each side of the brackets) when drawing a polymer. This shows the chain continues in both directions.

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Identifying the Monomer from a Polymer

To identify the monomer from a polymer repeating unit:

1. Look at the repeating unit inside the brackets.
2. Change the single C–C bond back to a double bond (C=C).
3. Remove the extending bonds.
4. The result is the monomer.

Example:

• Polymer repeating unit: –[CH₂–CHCl]–ₙ
• Change C–C to C=C: CH₂=CHCl
• Monomer: chloroethene

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Thermosoftening and Thermosetting Polymers (Higher)

Polymers can be classified by their behaviour when heated:

Thermosoftening Polymers

• The polymer chains are held together by weak intermolecular forces.
• When heated, these weak forces are overcome and the chains can slide past each other.
• The polymer softens and melts.
• When cooled, it solidifies again and can be remoulded.
• This process can be repeated — thermosoftening polymers are recyclable.
• Examples: poly(ethene), PVC, poly(propene).

Thermosetting Polymers

• The polymer chains are held together by strong covalent cross-links between the chains.
• When heated, the cross-links do not break (they are strong covalent bonds).
• The polymer does not soften or melt — it stays rigid.
• If heated too strongly, it will eventually char and decompose (but not melt).
• Thermosetting polymers cannot be remoulded once set.
• Examples: Bakelite (used for electrical sockets), melamine (kitchen worktops), epoxy resins.

Feature	Thermosoftening	Thermosetting
Forces between chains	Weak intermolecular forces	Strong covalent cross-links
Effect of heating	Softens and melts	Does not melt — stays rigid
Can be remoulded?	Yes	No
Recyclable?	Yes (can melt and reshape)	No (cannot melt)
Examples	Poly(ethene), PVC	Bakelite, melamine

Exam Tip: The key difference is the type of force between chains. Thermosoftening = weak intermolecular forces (can be overcome by heat). Thermosetting = strong covalent cross-links (cannot be overcome by heat without destroying the polymer).

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Properties and Uses of Polymers

Polymers have a wide range of properties depending on their structure:

Polymer	Key Properties	Common Uses
Poly(ethene) — low density	Flexible, lightweight, cheap	Plastic bags, packaging film
Poly(ethene) — high density	Stronger, more rigid	Bottles, pipes, containers
PVC (poly(chloroethene))	Rigid or flexible, durable, waterproof	Window frames, pipes, cable insulation
Poly(propene)	Strong, flexible, heat resistant	Ropes, carpets, containers
PTFE (poly(tetrafluoroethene))	Very slippery (low friction), heat resistant, chemical resistant	Non-stick pans, Gore-Tex
Polystyrene	Lightweight, good insulator	Packaging, insulation, disposable cups

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Key Points

• A polymer is a long-chain molecule made from many small monomers.
• Addition polymerisation occurs when alkene monomers (with C=C bonds) join together — the double bond opens up.
• To draw the repeating unit: change C=C to C–C, add extending bonds, put in brackets with ₙ.
• To identify the monomer: change C–C back to C=C in the repeating unit.
• Thermosoftening polymers: weak intermolecular forces between chains — can be melted and remoulded.
• Thermosetting polymers: strong covalent cross-links between chains — cannot melt, rigid, not recyclable.

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Practice Questions

1. What is meant by the terms "monomer" and "polymer"?
2. Draw the repeating unit of poly(ethene) from its monomer, ethene.
3. The repeating unit of a polymer is –[CH₂–CHCl]–. Identify the monomer.
4. Explain the difference between a thermosoftening polymer and a thermosetting polymer.
5. Explain why thermosoftening polymers can be recycled but thermosetting polymers cannot.

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Worked Example: Drawing a Polymer from a Substituted Alkene Monomer

Question: The monomer propene has the structural formula CH₂=CHCH₃. Draw the repeating unit of poly(propene).

Step-by-step answer:

1. Write the monomer with the C=C double bond clearly shown: H₂C=CH–CH₃ (the methyl group –CH₃ is a branch off one of the double-bond carbons).
2. Change the double bond (C=C) to a single bond (C–C).
3. Add extending bonds at both ends of the repeating unit (lines going out to show the chain continues).
4. Enclose the repeating unit in square brackets with n subscript outside.

Repeating unit: –[CH₂–CH(CH₃)]–ₙ

In displayed form: each repeating unit shows one –CH₂– group and one –CH(CH₃)– group, connected by the single bond that was formerly the double bond.