Open the C=C double bond of ethene and the molecules link into long chains of poly(ethene), the plastic in bags and bottles. Learn to draw the repeat unit, work backwards to the monomer, and weigh up what happens to plastics when we throw them away.
Polymers are large molecules built up from many small molecules called monomers. Many plastics are polymers made from chemicals obtained from crude oil. In addition polymerisation, alkene monomers join when their C=C double bonds open; the polymer is the only product. Ethene forms poly(ethene), whose repeat unit is –CH2–CH2–. Extended: draw the polymer from a given alkene, and deduce the monomer from a section of polymer. Plastics are hard to dispose of: they are non-biodegradable and fill landfill sites, they accumulate in oceans, and burning them can release toxic gases. Recycling reduces these problems.
Add reagents across the double bond of an alkene in the simulator, then link many alkene molecules together to make a polymer.
Four quick checks. Each correct answer earns XP and records this skill on your profile.
What is a polymer?
Which monomer is used to make poly(ethene)?
Why can alkenes form addition polymers but alkanes cannot?
Why do plastics cause problems in landfill sites?
Extended. Every addition polymer has a two-carbon backbone repeat unit. Find it, then put the double bond back.
| Monomer | Polymer | Repeat unit | Use |
|---|---|---|---|
| ethene, CH2=CH2 | poly(ethene) | –CH2–CH2– | bags, bottles |
| propene, CH2=CHCH3 | poly(propene) | –CH2–CH(CH3)– | crates, ropes |
| chloroethene, CH2=CHCl | poly(chloroethene), PVC | –CH2–CHCl– | pipes, window frames |
| tetrafluoroethene, CF2=CF2 | PTFE | –CF2–CF2– | non-stick coatings |
Method: take any two adjacent backbone carbons with their side groups, remove the continuation bonds and draw a C=C double bond between them. The CH3 in poly(propene) is a side group, not part of the backbone.
Plastics are useful because they are unreactive and durable. Those same properties make them hard to get rid of.
| Disposal | Problem |
|---|---|
| landfill | non-biodegradable, so they last for hundreds of years and landfill space runs out |
| oceans | plastic waste accumulates; animals swallow it or become trapped; it breaks into microplastics |
| burning | releases carbon dioxide (a greenhouse gas); PVC and others release toxic gases such as hydrogen chloride; incomplete combustion gives carbon monoxide |
| recycling | saves crude oil and reduces waste, but plastics must be collected and sorted by type |
The repeat unit has single bonds only: the double bond belongs to the monomer, not the polymer. Do not include the side group in the backbone (poly(propene) has a two-carbon repeat unit with a CH3 branch, not three carbons in a row). Plastics are made from crude oil, so they are not renewable; "non-biodegradable" means microorganisms cannot break them down.
Unlocks once the four checks above are done. Worth more XP, written to IGCSE Paper 2 and Paper 4 standard.
(Extended) The repeat unit of a polymer is –CH2–CHCl–. What is the monomer?
(Extended) What is the repeat unit of poly(propene)?
Which gas is a toxic product of burning PVC, poly(chloroethene)?
A sample of poly(ethene) has Mr 56 000. How many ethene monomers joined to make one molecule?
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