PolymersCambridge IGCSE Chemistry: Revision notes
Section 1
What are polymers and monomers?
Polymers are large molecules made by joining many smaller molecules called monomers together in long chains. Monomers are small organic molecules that can be linked together through chemical bonds to form polymers.
Plastics are materials made from polymers and have become widely used in everyday products. The properties of polymers depend on:
- The type of monomer used
- The length of the polymer chain
- The arrangement of monomer units
- Any cross-linking between chains
Polymer properties directly affect how they can be used and disposed of, which has important environmental implications.
Think of monomers as beads and polymers as a long necklace – individual beads (monomers) are linked together to create the complete chain (polymer).
Section 2
How is poly(ethene) formed through addition polymerisation?
Addition polymerisation is a process where monomer molecules with carbon–carbon double bonds react together to form a polymer without losing any atoms.
Poly(ethene) formation:
- Ethene monomers (C₂H₄) contain a C=C double bond
- Under heat and pressure with a catalyst, the C=C double bonds break
- Single bonds form between adjacent ethene molecules
- This process repeats thousands of times to form a long chain of poly(ethene)
The repeat unit of poly(ethene) is –CH₂–CH₂– (shown in square brackets with an 'n' subscript).
Key features of addition polymerisation:
- Requires alkene monomers with C=C double bonds
- The double bond breaks and becomes single bonds
- No atoms are eliminated during the reaction
- Produces very long chain molecules
When drawing the repeat unit of an addition polymer, always show the bond breaking across the double bond and represent the repeat unit in square brackets with 'n' outside to show it repeats many times.
For poly(ethene): the monomer ethene (CH₂=CH₂) becomes the repeat unit [–CH₂–CH₂–]ₙ. The double bond breaks and each carbon forms a single bond to the next monomer unit.
Section 3
What is the difference between addition and condensation polymerisation?
| Feature | Addition Polymerisation | Condensation Polymerisation |
|---|---|---|
| Monomer type | Alkenes with C=C double bonds | Monomers with two functional groups (e.g. –COOH and –NH₂) |
| What breaks/reacts | C=C double bonds break | Functional groups react |
| Atoms eliminated | No atoms lost | Small molecules eliminated (typically water or methanol) |
| Repeat unit | Contains all atoms from the monomer | Contains all atoms except the small molecules lost |
| Examples | Poly(ethene), poly(propene) | Polyamides (nylon), polyesters (PET) |
Condensation polymerisation occurs when monomers with two functional groups react together. A small molecule (usually H₂O) is eliminated each time two monomer units join. This process creates polymers through the formation of new covalent bonds between the monomers.
Types of condensation polymers:
- Polyamides: formed from a dicarboxylic acid and a diamine
- Polyesters: formed from a dicarboxylic acid and a diol
Students often forget that condensation polymerisation eliminates small molecules. Always identify what is lost (usually water: H₂O) and show this in the equation alongside the polymer formation.
When comparing polymerisation types, examiners expect you to state specifically that addition polymerisation has no atoms eliminated, while condensation polymerisation eliminates small molecules.
Section 4
How are nylon (polyamides) and PET (polyesters) formed?
Polyamides (Nylon): Polyamides are formed by condensation polymerisation between a dicarboxylic acid and a diamine. Each time a bond forms between the acid and amine, a water molecule (H₂O) is eliminated.
The amide linkage formed is: –CO–NH–
This linkage repeats throughout the polymer chain, forming the characteristic structure of nylon.
Polyesters (PET): Polyesters are formed by condensation polymerisation between a dicarboxylic acid and a diol. Water is eliminated each time the acid and alcohol groups react.
The ester linkage formed is: –CO–O–
This linkage repeats throughout the polymer chain. PET (polyethylene terephthalate) is a common polyester used in plastic bottles and clothing fibres.
Key difference in linkages:
- Polyamides contain –CO–NH– (amide linkages)
- Polyesters contain –CO–O– (ester linkages)
Important property: PET can be converted back into its monomers and re-polymerised, making it potentially recyclable and more sustainable than some other plastics.
When identifying or drawing linkages, remember: amide linkages have nitrogen (–CO–NH–) while ester linkages have oxygen (–CO–O–). This distinction is crucial for identifying polymer types in exam questions.
In PET formation: a dicarboxylic acid (benzene-1,4-dicarboxylic acid) reacts with ethane-1,2-diol. For each ester linkage formed, one water molecule is eliminated. The repeat unit contains the ester linkage –CO–O–.
Section 5
What are the environmental challenges caused by plastics?
Disposal issues:
- Landfill accumulation: Polymers are very stable and do not decompose quickly. Plastic waste can persist in landfills for hundreds of years, taking up valuable space and potentially leaching harmful chemicals into soil and groundwater.
- Ocean accumulation: Plastic waste in oceans breaks down into microplastics, which can be ingested by marine organisms, harming wildlife and entering food chains.
- Toxin formation from burning: When plastics are incinerated, incomplete combustion can produce toxic gases such as:
- Carbon monoxide (CO)
- Nitrogen oxides (NOₓ)
- Dioxins and furans (highly toxic compounds)
- Hydrochloric acid (from PVC combustion)
Why the properties of plastics matter: The very properties that make plastics useful (chemical stability, durability, resistance to degradation) are the same properties that make them environmental problems. They do not break down naturally and persist for very long periods.
More sustainable alternatives:
- Use of biodegradable polymers
- Recycling programmes (particularly effective for PET)
- Reduction in overall plastic consumption
- Development of compostable materials
Examiners expect you to explain why plastic properties cause environmental problems: state that polymers are stable/resistant to degradation, then link this to specific disposal challenges (landfill persistence, ocean accumulation, toxic combustion products).
Plastic's environmental problem is a paradox: the strength and durability that makes it useful in products makes it harmful to the environment because it refuses to break down naturally.
Section 6
What are proteins and how do they relate to polyamides?
Proteins as natural polyamides: Proteins are natural polyamides that are formed from many smaller monomer units called amino acids. Amino acids are organic compounds containing both an amino group (–NH₂) and a carboxylic acid group (–COOH).
How proteins form: Amino acids join together through peptide bonds, which are identical to the amide linkages found in synthetic polyamides like nylon. Each time two amino acids join, a water molecule is eliminated through condensation polymerisation.
The peptide bond formed is: –CO–NH– (exactly the same linkage as in polyamides)
Structure of proteins:
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Amino acids contain a central carbon atom bonded to:
- An amino group (–NH₂)
- A carboxylic acid group (–COOH)
- A hydrogen atom
- A variable R group (side chain) unique to each amino acid
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When amino acids condense, the carboxylic acid of one amino acid reacts with the amino group of the next
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The resulting –CO–NH– linkage forms the protein backbone
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The R groups extend from the backbone and determine protein properties
Key similarity to synthetic polymers: Proteins are formed by the same condensation polymerisation mechanism as nylon, with peptide bonds serving the same function as amide linkages in polyamides.
Remember that proteins are condensation polymers just like nylon – examiners expect you to recognise that peptide bonds are amide linkages and that water is eliminated when amino acids join.
When two amino acids condense, the –COOH of one reacts with the –NH₂ of the next, forming –CO–NH– and releasing H₂O. This is the peptide bond that links amino acids in a protein chain.
Must Know
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Polymers are formed from monomers: Small monomer molecules join together in long chains to form polymers, which are used to make plastics.
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Addition polymerisation: Alkene monomers (with C=C double bonds) join directly together without losing atoms. The double bond breaks and becomes single bonds. Example: poly(ethene) from ethene monomers.
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Condensation polymerisation: Monomers with two functional groups react together with the loss of small molecules (usually water). Creates polyamides (–CO–NH– linkage) and polyesters (–CO–O– linkage).
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PET and Nylon: PET is a polyester (from dicarboxylic acid + diol); nylon is a polyamide (from dicarboxylic acid + diamine). PET can be depolymerised and recycled.
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Environmental problems: Plastic's chemical stability means it persists in landfills for centuries, accumulates as microplastics in oceans, and produces toxic gases (CO, NOₓ, dioxins) when burned.
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Proteins are natural polyamides: Amino acids form proteins through condensation polymerisation, creating peptide bonds (–CO–NH–) identical to amide linkages in synthetic polyamides like nylon.
That's the notes covered.
Carry on to the next subtopic.