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CIE AS Chemistry Your notes
20.1 Addition Polymerisation
Contents
Addition Polymerisation
Disposal of Polymers
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Addition Polymerisation
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Addition Polymerisation: Polythene & PVC
Addition polymerisation
Addition polymerisation is one of the most important addition reactions of alkenes which form
the basis of the plastic industry
Addition polymerisation is the reaction in which many monomers containing at least one C-C
double bond form long chains of polymers as the only product
Just like in other addition reactions of alkenes, the π-bond in each C-C bond breaks and then
the monomers link together to form new C-C single bonds
A polymer is a long-chain molecule that is made up of many repeating units
The small, reactive molecules that react together to form the polymer are called monomers
A polymerisation reaction can be represented by a general formula or by using displayed
formulae
E.g. poly(ethene) and poly(chloroethene) (also known as PVC) are polymers made up of the
ethene and chloroethene monomers respectively and are commonly used in making plastics
General formula addition polymerisation
The general formulae of the addition polymerisation of ethene (1) and chloroethene (2)
Displayed formula addition polymerisation
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Your notes
The displayed formulae of the addition polymerisation of ethene (1) and chloroethene (2)
Just like any other addition reaction of alkenes, addition polymerisation gives only one product
Deducing repeat units
A repeat unit is the smallest group of atoms that when connected one after the other make up
the polymer chain
It is represented by square brackets in the displayed and general formula
In poly(alkenes) (such as poly(ethene)) and substituted poly(alkenes) (such as PVC) made of
one type of monomer the repeating unit is the same as the monomer except that the C-C
double bond is changed to a C-C single bond
Repeat units in polymers
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Your notes
The repeating units of poly(ethene) and poly(chloroethene) are similar to their monomer except that the
C=C bond has changed into a C-C bond
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Worked example
Your notes
Identify the monomers present in the given sections of addition polymer molecules:
Answer 1:
When ethenol (CH(OH)=CH2 ) is polymerised, the C-C double bond opens to produce a
repeating unit of CH(OH)-CH2 .
This gives the polymer poly(ethenol)
Answer 2:
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To find the monomer, first the repeating unit should be deduced
Repeating units have only 2 carbons in the polymer main chain
Your notes
Since the repeating unit is now found, it can be concluded that the monomer is prop-2-
enoic acid
Answer 3:
Again, the repeating unit only has 2 carbons in the polymer chain which in this case are two
carbon atoms that each contain one OH group
Thus, when ethene-1,2-diol (CH(OH)=CH(OH)) is polymerised, the C-C double bond opens
to produce a repeating unit of CH(OH)-CH(OH) which gives the polymer poly(ethene-1,2-
diol)
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Your notes
Exam Tip
The section of the polymer chain shown inside the square brackets by the structural or displayed
formula is the repeat unit and not the monomer
The monomer is the same as the repeat unit except for that it has C=C bonds instead of C-C
bonds
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Disposal of Polymers
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Disposal of Polymers
Poly(alkenes) are extremely important in everyday life, such as their use as plastics
However, the disposal of these polymers is problematic
Poly(alkenes) are very large alkane molecules which are unreactive and therefore do not undergo
any chemical reactions; they are resistant to chemical attack
Due to their unreactivity, polymers are non-biodegradable and take up to hundreds of years to
decompose when dumped in landfill sites
Throwing away poly(alkenes) therefore causes long-term pollution of the environment
Burning the polymers results in harmful combustion products which again cause the pollution of
the environment
Exam Tip
The disposal of polymers is a challenge due to their unreactivity, their non-biodegradability, and
the formation of harmful combustion products when burnt
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