Ch 1 · Addition polymerisation · AS & A Level Chemistry 9701
Topic 1 of 1 in Polymerisation — AS & A Level Chemistry 9701 — 57 lessons.
Introduction
Addition polymerisation joins many small alkene monomer molecules into one long polymer chain with no other product formed, giving useful materials whose stability comes at an environmental cost once discarded.
Key ideas — Addition polymerisation
In addition polymerisation, many unsaturated monomer molecules, each containing a mathrmC=C double bond, add to one another with no other product formed, opening the double bond and joining the monomers into a long saturated chain. Ethene monomers join to form poly(ethene), used for plastic bags and packaging: n mathrmCH_2=CH_2 arrow (-CH_2-CH_2-)_n Chloroethene (vinyl chloride) monomers join in the same way to form poly(chloroethene), PVC, used for pipes and window frames.
🧑🏫 Teacher's explanation
Imagine a long chain of people holding hands. In a normal molecule, people are standing separately. In addition polymerisation, everyone is holding hands in a long line. The 'magic' happens when a double bond (like two hands clasped tightly together) breaks and becomes a single bond (one hand holding another). This allows the molecules to link up without losing any atoms.
📖 Definitions
🔍 Worked example — Easy
Calculate the molar mass of a poly(ethene) chain that contains exactly 500 ethene monomers.
🔍 Worked example — Standard
A section of a polymer is shown as -CH_2-CH(Cl)-CH_2-CH(Cl)-. Deduce the structure of the monomer used to create this polymer.
🔍 Worked example — Exam-style
Poly(chloroethene) (PVC) is formed from chloroethene monomers. If 2.2 g of chloroethene is used, what mass of PVC is produced, assuming 100% yield?
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🎯 Exam question
Key ideas — Deducing the repeat unit
The repeat unit of an addition polymer contains the same atoms as the monomer, but with the double bond replaced by a single bond and a bond extending from each of the two carbons that were doubly bonded, shown in brackets with a subscript n. For propene, mathrmCH_2=CHCH_3, the repeat unit is (-CH_2-CH(CH_3)-)_n.
🧑🏫 Teacher's explanation
Think of addition polymerisation like a long chain made of identical paperclips. Each paperclip is a monomer. To make the chain, you don't add any new pieces; you simply 'unclip' the double bond that holds the paperclip's ends together, allowing it to hook onto the next one. The atoms don't change; only how they are connected changes.
📖 Definitions
🔍 Worked example — Easy
Deduce the repeat unit for the polymer formed from ethene, CH_2=CH_2.
🔍 Worked example — Standard
A polymer has the repeat unit -CH_2-CH(Cl)-. Identify the monomer used to create this polymer.
🔍 Worked example — Exam-style
A section of a polymer is shown as -CH(CH_3)-CH_2-CH(CH_3)-CH_2-. Deduce the repeat unit.
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🎯 Exam question
Key ideas — Identifying the monomer from a polymer section
Given a section of polymer chain, the monomer is found by identifying the shortest repeating unit of two carbon atoms along the backbone, restoring the double bond between them and removing the extending bonds. A polymer section -CH_2-CHCl-CH_2-CHCl- has the repeat unit (-CH_2-CHCl-), so the monomer is chloroethene, mathrmCH_2=CHCl.
🧑🏫 Teacher's explanation
Imagine a long chain of paperclips. If you want to find the single 'original' paperclip that was used to make the chain, you look for the smallest repeating pattern. In addition polymerisation, the 'chain' is a long backbone of carbon atoms, and the 'paperclip' is the monomer.
📖 Definitions
🔍 Worked example — Easy
Identify the monomer for the repeat unit: -CH_2-CH(CH_3)-
🔍 Worked example — Standard
A polymer section is given as -CH_2-CH(Cl)-CH_2-CH(Cl)-. Deduce the monomer.
🔍 Worked example — Exam-style
A polymer is formed from the monomer CH_2=CH-CN. Write the structure of the repeat unit.
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🎯 Exam question
Key ideas — Disposal of poly(alkene)s
Poly(alkene)s such as poly(ethene) and poly(chloroethene) are made entirely of strong, non-polar carbon–carbon and carbon–hydrogen (or carbon–chlorine) bonds, so they are chemically inert and not broken down by microorganisms: they are non-biodegradable, persisting in landfill for a very long time. Incinerating them can also be hazardous: poly(chloroethene) in particular releases toxic and corrosive hydrogen chloride gas on combustion, requiring costly gas-scrubbing to prevent pollution.
🧑🏫 Teacher's explanation
Imagine a long, heavy-duty steel chain. If you throw a single link into a pile of leaves, the bacteria in the soil can easily eat the leaves, but they have no way to 'bite' through the steel links. This is exactly why poly(alkene)s are such a problem for the environment. They are made of incredibly strong, non-polar carbon-carbon bonds that act like that steel chain, making them virtually indestructible by natural biological processes.
📖 Definitions
🔍 Worked example — Easy
State the primary reason why poly(ethene) is considered non-biodegradable.
🔍 Worked example — Standard
Poly(chloroethene) is burned to reduce waste. Write a balanced equation for the combustion of poly(chloroethene) showing the production of hydrogen chloride gas. Assume the repeat unit is [CH_2CHCl]_n.
⚠️ Common errors
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🎯 Exam question
Key ideas — Why poly(alkene)s are so unreactive
Unlike their alkene monomers, addition polymers contain no mathrmC=C double bonds once polymerisation is complete: every carbon atom in the backbone is joined only by single bonds. Since electrophilic addition and other alkene reactions all depend on the reactive π-electrons of the double bond, a saturated poly(alkene) chain has no equivalent site of reactivity, which is exactly why these materials resist chemical attack, including by micro-organisms, once formed.
🧑🏫 Teacher's explanation
Imagine a long chain of people holding hands. In an alkene monomer, two people are holding both hands of their neighbor (a double bond), which is a high-energy, unstable position. To make a polymer, they let go of one hand and grab the hand of the person next to them instead. Now, everyone is just holding one hand with one hand (a single bond).\n\nBecause these single bonds are very stable and 'satisfied', there is no 'extra' electron density (the π-bond) looking for a reaction. Most chemical reactions with alkenes require that double bond to act as a target. Without that target, the long chain is chemically 'inert' or unreactive. This is why a plastic bottle doesn't just dissolve or rot away when it sits in the ocean; the molecules are too stable for bacteria to break them down.
📖 Definitions
🔍 Worked example — Easy
Identify the monomer for the following repeat unit: -[CH_2-CH(Cl)]_n-
🔍 Worked example — Standard
Calculate the molar mass (M_r) of a poly(ethene) chain segment consisting of 100 repeat units, -[CH_2-CH_2]_n-. (Use C=12, H=1)
🔍 Worked example — Exam-style
Explain why poly(chloroethene) is resistant to chemical attack compared to chloroethene.
⚠️ Common errors
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🎯 Exam question
Worked example
Question. A section of an addition polymer is -CH_2-CCl_2-CH_2-CCl_2-CH_2-CCl_2-. (a) Deduce the structure of the monomer used to make this polymer. [1] (b) Deduce the repeat unit of the polymer. [1] (c) Suggest one environmental problem from disposing of this polymer by landfill, and one from disposal by combustion. [2]
🔍 Worked example — Easy
Identify the monomer for the following polymer section: -CH_2-CH(CH_3)-CH_2-CH(CH_3)-
🔍 Worked example — Standard
A polymer is made from the monomer CH_2=CH_2. Calculate the molar mass of a polymer chain consisting of exactly 100 monomer units.
🔍 Worked example — Exam-style
A section of an addition polymer is -CH_2-CCl_2-CH_2-CCl_2-CH_2-CCl_2-. (a) Deduce the structure of the monomer. (b) Deduce the repeat unit. (c) State one environmental problem from disposal by combustion.
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