- describe addition polymerisation as exemplified by poly(ethene) and poly(chloroethene), PVC
- deduce the repeat unit of an addition polymer obtained from a given monomer
- identify the monomer(s) present in a given section of an addition polymer molecule
- recognise the difficulty of the disposal of poly(alkene)s, i.e. non-biodegradability and harmful combustion products
Polymerisation
A-Level Chemistry · Topic 20
20.1
Addition polymerisation
Syllabus
Source: Cambridge International syllabus
In addition polymerisation 加成聚合, many small molecules join into one very long chain, with no other product made.
Each small molecule is a monomer 单体. It must be unsaturated — it has a C=C double bond. The double bond opens up so that the monomers can link together. The long chain that forms is the polymer 聚合物.
Each monomer's double bond opens, and the monomers join end to end into one long chain — with no other product made
Poly(ethene), a common addition polymer, is supplied as tiny pellets a few millimetres across; these are later melted and moulded into bottles, bags and other products
Repeat units
The repeat unit 重复单元 is the small part that is copied again and again along the chain. To find it, take the monomer, change the C=C to a single C–C, and draw bonds going out at each end.
- poly(ethene) 聚乙烯 is made from ethene:
$$n\,\text{CH}_2{=}\text{CH}_2 \rightarrow -(\text{CH}_2{-}\text{CH}_2)_n-$$
- poly(chloroethene) 聚氯乙烯 (PVC) is made from chloroethene:
$$n\,\text{CH}_2{=}\text{CHCl} \rightarrow -(\text{CH}_2{-}\text{CHCl})_n-$$
Finding the repeat unit: change the monomer's C=C to a single bond and draw bonds out at each end; reverse it to find the monomer
Finding the monomer
To go the other way, look at one repeat unit of the polymer, and put the C=C double bond back in. That gives you the monomer.
Worked example. A polymer has the repeat unit $-(\text{CH}_2{-}\text{CH}(\text{CH}_3))_n-$. Identify the monomer and name the polymer. Reverse the rule you used to build it: rub out the bonds sticking out at each end, and turn the single C-C in the backbone back into a C=C. That gives $\text{CH}_2{=}\text{CH}(\text{CH}_3)$, which is propene - so the polymer is poly(propene). Two checks catch most errors: the monomer must have the same molecular formula as the repeat unit (addition polymerisation adds nothing and loses nothing), and the double bond goes back into the backbone, never into the side group.
Explore
Addition polymerisation route
Watch alkene monomers join by opening their double bonds.
Vocabulary
Train
| English | Chinese | Pinyin |
|---|---|---|
| addition polymerisation | 加成聚合 | jiā chéng jù hé |
| monomer | 单体 | dān tǐ |
| polymer | 聚合物 | jù hé wù |
| repeat unit | 重复单元 | chóng fù dān yuán |
| poly(ethene) | 聚乙烯 | jù yǐ xī |
| poly(chloroethene) | 聚氯乙烯 | jù lǜ yǐ xī |
20.1
The problem of disposal
Poly(alkene)s are very hard to get rid of:
- they are non-biodegradable 不可生物降解 — microbes cannot break them down, so they stay in the ground for a very long time.
- their combustion 燃烧 (burning) can release harmful gases. For example, burning PVC gives off toxic hydrogen chloride.
Most addition polymers are non-biodegradable, so they build up as waste in the environment
Poly(alkene) waste is hard to dispose of: it is non-biodegradable, and burning PVC releases toxic hydrogen chloride
Vocabulary
Train
| English | Chinese | Pinyin |
|---|---|---|
| non-biodegradable | 不可生物降解 | bù kě shēng wù jiàng jiě |
| combustion | 燃烧 | rán shāo |
20.1
Exam tips
- Draw the repeat unit in brackets with the two bonds crossing them and $n$ outside; keep the backbone carbons.
- Deduce the monomer from a polymer (and vice versa) — a very common question.
- Addition polymers are inert and non-biodegradable — link to landfill, recycling and incineration issues.
- Do not confuse with condensation (no small molecule is lost in addition polymerisation).