Draw the repeat unit of the addition polymer formed from chloroethene (CH_2=CHCl) and name the polymer. [2 marks]

SingaporeChemistrySyllabus dot point

How do addition and condensation polymers form, and how do their structures relate to their properties and disposal?

Distinguish addition and condensation polymerisation, identify the repeat unit and monomers of a given polymer, describe polyesters and polyamides, and discuss the disposal and environmental impact of plastics

A focused answer to the H2 Chemistry learning outcome on polymers. Distinguishing addition from condensation polymerisation, identifying repeat units and monomers, the structure of polyesters and polyamides, and the disposal and environmental impact of plastics.

Generated by Claude Opus 4.89 min answerUpdated 2026-06-06

Reviewed by: AI editorial process; not yet individually human-reviewed

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What this dot point is asking

SEAB wants you to distinguish addition and condensation polymerisation, identify the repeat unit and the monomers of a given polymer, describe polyesters and polyamides, and discuss the disposal and environmental impact of plastics. Working between monomer and repeat unit, and evaluating disposal methods, are standard exam tasks.

The answer

Addition polymerisation

Many unsaturated monomers (containing C=C) join end to end with no other product. The double bond opens and the monomers link into a long saturated chain. For example, ethene gives poly(ethene):

n\,\text{CH}_2\text{=CH}_2 \rightarrow [\text{-CH}_2\text{-CH}_2\text{-}]_n

The repeat unit has the same atoms as the monomer (just the double bond becomes single bonds joining the chain). Other examples: poly(propene), poly(chloroethene) (PVC), poly(phenylethene) (polystyrene).

Condensation polymerisation

Monomers, each with two reactive functional groups, join with the elimination of a small molecule (usually water or HCl) at each new bond. Two main types:

Polyesters: from a diol and a dicarboxylic acid (or diacyl chloride), forming ester linkages ( $-\text{COO}-$ ) and eliminating water (e.g. Terylene/PET).
Polyamides: from a diamine and a dicarboxylic acid (or diacyl chloride), forming amide linkages ( $-\text{CONH}-$ ) and eliminating water (e.g. nylon).

Identifying monomers and repeat units

For an addition polymer, the repeat unit is drawn with the backbone single-bonded; the monomer is found by putting the C=C back into the repeat unit.
For a condensation polymer, look for the linkage: an ester linkage means a diol and a diacid; an amide linkage means a diamine and a diacid. The monomers are found by adding back the small molecule (water) across each linkage.

Disposal and environmental impact

Addition polymers have a saturated, non-polar carbon backbone with strong C-C and C-H bonds and no hydrolysable groups, so they are chemically inert and non-biodegradable, persisting for a long time. Condensation polymers contain ester or amide linkages that can be hydrolysed, so many are more readily broken down.

Disposal options, each with trade-offs:

Landfill: cheap and simple, but uses space and plastics persist.
Incineration: recovers energy and reduces volume, but can release toxic gases (e.g. HCl from PVC) and $\text{CO}_2$ .
Recycling: conserves resources, but requires sorting by polymer type and quality may degrade.

Worked example

The polymer nylon-6,6 is made from hexanedioic acid and 1,6-diaminohexane. State the type of polymerisation, name the linkage formed, and identify the small molecule eliminated.

The two monomers each have two reactive groups: hexanedioic acid has two $-\text{COOH}$ groups, and 1,6-diaminohexane has two $-\text{NH}_2$ groups.

When a $-\text{COOH}$ reacts with an $-\text{NH}_2$ , an amide linkage ( $-\text{CONH}-$ ) forms, with the elimination of a molecule of water each time. Because a small molecule is eliminated, this is condensation polymerisation, and the product (a polyamide) is nylon-6,6.

Try it: Polymer repeat-unit finder - enter monomers to build the repeat unit, or enter a repeat unit to identify the monomers and polymer type.

Examples in context

Example 1. PET bottles and recycling. Poly(ethylene terephthalate), a polyester made by condensation, can be depolymerised by hydrolysing its ester linkages back to monomers for reuse, which is why PET is among the most recycled plastics. SEAB uses this to contrast the recyclability of hydrolysable condensation polymers with the persistence of addition polymers.

Example 2. Biodegradable polymers. Newer condensation polymers with ester linkages (such as polylactic acid) can be hydrolysed by moisture and microbes, breaking down far faster than poly(ethene). This applied example lets SEAB test the link between the presence of hydrolysable linkages and biodegradability.

Try this

Q1. Draw the repeat unit of the addition polymer formed from chloroethene ( $\text{CH}_2\text{=CHCl}$ ) and name the polymer. [2 marks]

Cue. $[\text{-CH}_2\text{-CHCl-}]_n$ ; poly(chloroethene), PVC.

Q2. State the type of polymerisation and the small molecule eliminated when a diol reacts with a dicarboxylic acid. [2 marks]

Cue. Condensation polymerisation; water is eliminated (forming a polyester).

Q3. Give one advantage and one disadvantage of disposing of waste plastics by incineration. [2 marks]

Cue. Advantage: recovers energy and reduces volume. Disadvantage: can release toxic gases (e.g. HCl from PVC) and $\text{CO}_2$ .

Exam-style practice questions

Practice questions written in the style of SEAB exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.

Specimen (9729)4 marksDistinguish between addition and condensation polymerisation, and for the polyester formed from benzene-1,4-dicarboxylic acid and ethane-1,2-diol, draw the repeat unit and name the small molecule eliminated.

Show worked answer →

Addition polymerisation: many unsaturated monomers (containing C=C) join together with no other product, e.g. ethene to poly(ethene). The repeat unit has the same atoms as the monomer.

Condensation polymerisation: monomers join with the elimination of a small molecule (usually water or HCl) each time a bond forms; the monomers have two reactive functional groups each.

For the polyester from the diacid and the diol, an ester linkage (-COO-) forms each time, eliminating water.

Repeat unit: [-OC-C6H4-CO-O-CH2CH2-O-] with the ester linkages shown; the small molecule eliminated is water.

Markers reward the no-by-product addition definition, the small-molecule elimination for condensation, the ester linkage repeat unit, and water as the eliminated molecule.

2023 (style)4 marksExplain why addition polymers such as poly(ethene) are difficult to dispose of, and describe two methods used to deal with waste plastics, noting an advantage and a disadvantage of each.

Show worked answer →

Addition polymers have a saturated carbon backbone with strong, non-polar C-C and C-H bonds and no groups that can be hydrolysed. They are chemically inert and non-biodegradable, so they persist in the environment.

Disposal methods (any two):

Landfill: simple and cheap, but takes up space and the plastics do not break down for a very long time.
Incineration: recovers energy and reduces volume, but can release toxic gases (e.g. HCl from PVC, CO from incomplete combustion) and CO2.
Recycling: conserves resources and reduces waste, but plastics must be sorted by type and the quality may degrade.

Markers reward the inert, non-biodegradable backbone reason and any two methods with a valid advantage and disadvantage each.