In simple terms
A friendly intro before the formal notes — no formulas yet.
Linking Up Alkenes
Addition polymerisation is like making a long paper chain from individual rings. Small molecules with a carbon-carbon double bond (monomers) break this bond to link up into a massive chain (polymer) with no other products formed.
Imagine a room full of people, each with their arms crossed (representing the C=C double bond). To form a long conga line, each person must uncross their arms and hold hands with the person in front and behind. The crossed-arm 'monomers' become a single, long hand-holding 'polymer' chain, with the core structure of each person (the atoms) remaining unchanged.
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Addition polymer: C=C opens — repeat unit loses double bond.
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Monomer must contain C=C — no by-product formed.
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Disposal/recycling issues — non-biodegradable polyalkenes.
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Draw one repeat unit from displayed formula.
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Key formulas
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Full topic notes
Formal explanation with the rigour you need for the exam.
The Mechanism of Addition Polymerisation
Addition polymerisation occurs when monomers, which are unsaturated molecules containing a C=C double bond, react together. The reaction is typically initiated under conditions of high temperature, high pressure, and in the presence of a catalyst. During the reaction, the weaker pi (π) bond in each alkene monomer breaks, and the electrons are used to form new, strong sigma (σ) bonds that link the monomers into a long saturated chain.
The general equation for the polymerisation of ethene is:
Where 'n' is a very large integer, representing the number of monomer units that have joined together. The product is poly(ethene).
The monomer must be an alkene or a substituted alkene.
The C=C double bond breaks, and a C-C single bond backbone forms.
No other molecule is produced; the empirical formula of the monomer and the repeat unit are identical.
The process is an 'addition' reaction on a massive scale.
Drawing Polymers and Identifying Monomers
A key skill is to be able to visualise and draw the relationship between a monomer and its polymer. To draw the repeat unit from a monomer, focus only on the two carbons in the double bond. Open the double bond and draw single bonds extending outwards from these two carbons. All other atoms or groups attached to these carbons are then drawn above or below them.
Environmental Impact of Addition Polymers
While incredibly useful, the properties that make poly(alkenes) so versatile also cause significant environmental problems. Their chemical inertness, due to the strong C-C and C-H bonds, means they are not attacked by microorganisms and are therefore non-biodegradable. They persist in the environment for centuries, filling up landfill sites and polluting oceans and landscapes.
Non-biodegradability: Poly(alkenes) are resistant to decomposition by natural processes, leading to long-term plastic pollution.
Landfill: Discarded polymers take up vast amounts of space in landfill sites.
Combustion: Burning plastic waste can release toxic gases. For example, the combustion of poly(chloroethene) (PVC) can produce hydrogen chloride (HCl), a corrosive and acidic gas. Incomplete combustion of any polymer can release toxic carbon monoxide.
Recycling: While a solution, recycling is often difficult and expensive due to the need to sort different types of polymers, which often have different properties and cannot be mixed.
Worked examples
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Propene, , is used to make the polymer poly(propene). Draw one repeat unit of poly(propene) and state its full name.
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Identify the C=C bond: In propene (), the double bond is between the second and third carbons if numbered from the left, but it's easier to see it as being between a group and a group.
A section of a polymer has the following structure: ...-CF-CFCl-CF-CFCl-CF-CFCl-... Identify the monomer that formed this polymer.
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Identify the repeating unit: Look for the shortest section that repeats along the chain. The pattern is [-CF-CFCl-]. This is the repeat unit.
How it all connects
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Glossary
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Revision flashcards
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What is a monomer?
A small molecule, such as an alkene, that can be joined together in large numbers to form a polymer.
Key takeaways
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The monomer must be an alkene or a substituted alkene.
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The C=C double bond breaks, and a C-C single bond backbone forms.
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No other molecule is produced; the empirical formula of the monomer and the repeat unit are identical.
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The process is an 'addition' reaction on a massive scale.
Practice — then mark it
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Addition Polymerisation
Addition Polymerisation
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