Addition Polymers (Oxford AQA International A Level Chemistry)
Revision Note
Written by: Alexandra Brennan
Reviewed by: Stewart Hird
Addition Polymerisation
Addition polymerisation is one of the most important addition reactions of alkenes which form the basis of the plastics industry
Addition polymerisation is the reaction in which many monomers containing at least one C=C double bond (alkenes and substituted alkenes) form long chains of polymers as the only product
During polymerisation, the double bond in the monomer opens up and the monomers join together
To name a polymer, the prefix 'poly' is written followed by the name of the monomer in brackets
Ethene forms poly(ethene)
Chloroethene forms poly(chloroethene)
Addition polymerisation gives only one product, just like any other addition reaction
A polymerisation reaction can be represented by a general formula or by using displayed formulae
General formula for addition polymerisation
Displayed formula for addition polymerisation
Deducing repeating units
A repeating 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
For addition polymers, the repeating unit is the same as the monomer except that the C=C double bond is changed to a C-C single bond
Drawing repeating units
Worked Example
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:
To find the monomer, first the repeating unit should be deduced
Repeating units have only 2 carbons in the polymer main chain
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)
Properties of polymers
The polyalkenes formed by addition polymerisation are saturated molecules- they have only single bonds
The C-H and C-C bonds are non-polar which makes them very unreactive and as a result do not biodegrade
Poly(alkenes) consist of polymer chains with van der waals' forces between them
Van der Waals forces are weak but there are thousands and so overall the polymer formed is strong
The properties of a polymer can be changed by altering the conditions under which it is made:
Low density poly(ethene) is formed at high temperature and high pressure
It is flexible and used in plastic bags
High density poly(ethene) is formed at lower temperatures and pressure in the pressure of a catalyst
This more rigid and used in plastic bottles
Examiner Tips and Tricks
Make sure you can do the following:
Draw the repeating unit from a monomer structure
Draw the repeating unit from a section of the polymer chain
Draw the structure of the monomer from a section of the polymer
Uses of Poly(chloroethene)
Poly(chloroethene) is formed from the monomer chloroethene
It is more commonly known as PVC, poly(vinylchloride)
Polymerisation of chloroethene
PVC is a relatively hard and rigid material due to the presence of chlorine
This is because chlorine is more electronegative than carbon so has a partial negative charge whilst the carbon has a partial positive charge
This sets up a permanent dipole which keeps the polymer chains closely packed together
Its rigidity enables it to be used in drainpipes and window frames
Plasticisers
The properties of PVC can be altered by adding a plasticiser
A plasticiser makes a polymer more flexible by preventing the polymer chains from being close to one another
This disrupts the van der Waals forces between the chains, making them weaker
As a result, the chains slide over each other as easily
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