Unit 20
Polymers: the basics
Polymers are large molecules built by linking 50 or more smaller molecules called monomers
Each repeat unit is connected to the adjacent units via covalent bonds
Examples of polymers include PVC and nylon
Many monomers join together to form a polymer
Poly(ethene) is formed by the addition polymerisation of ethene monomers
Addition polymerisation involves the addition of many monomers to make a long chained
polymer
In this case, many ethene monomers join together due to the carbon carbon double bond
breaking
Poly(ethene) is formed by addition polymerisation using ethene monomers
Addition polymers
Extended tier only
Addition polymers are formed by the joining up of many monomers and only occur in monomers
that contain C=C bonds
One of the bonds in each C=C bond breaks and forms a bond with the adjacent monomer
The polymer formed will only contain single bonds
Many polymers can be made by the addition of alkene monomers
Others are made from alkene monomers with different atoms attached to the monomer such as
chlorine or a hydroxyl group
The name of the polymer is deduced by putting the name of the monomer in brackets and
adding poly- as the prefix
For example if propene is the alkene monomer used, then the name is poly(propene)
Poly(ethene) is formed by the addition polymerisation of ethene monomers
Deducing the polymer from the monomer
Polymer molecules are very large compared with most other molecule
Repeat units are used when displaying the formula
To draw a repeat unit:
o Change the double bond in the monomer to a single bond in the repeat unit
o Add a continuation bond to each end of the repeat unit
The bonds on either side of the polymer must extend outside the brackets
(these are called extension or continuation bonds)
o A small subscript n is written on the bottom right hand side to indicate a large number
of repeat units
o Add on the rest of the groups in the same order that they surrounded the double bond
in the monomer
Examples of some addition polymers
The repeat unit for the polymer should have an n in the bottom right hand corner
Deducing the monomer from the polymer
To deduce the monomer from the polymer:
o Identify the repeating unit in the polymer
o Change the single bond in the repeat unit to a double bond in the monomer
o Remove the bond from each end of the repeat unit
Th monomer has been identified, a double bond added and the atoms drawn in
Examiner Tip
You could be asked to draw the repeat unit for any given monomer and vice versa.
You would only need to draw the structure of one monomer if you have been given the polymer.
Condensation polymers
Extended tier only
Condensation polymers are formed when two different monomers are linked together with
the removal of a small molecule, usually water
This is a key difference between condensation polymers and addition polymers:
o Addition polymerisation forms the polymer molecule only
o Condensation polymerisation forms the polymer molecule and one water molecule per
linkage
The monomers have two functional groups present, one on each end
The functional groups at the ends of one monomer react with the functional group on the end of
the other monomer, in so doing creating long chains of alternating monomers, forming the
polymer
Forming nylon
Nylon is a polyamide made from dicarboxylic acid monomers (a carboxylic with a -COOH group
at either end) and diamines (an amine with an -NH2 group at either end)
Each -COOH group reacts with another -NH2 group on another monomer
An amide linkage is formed with the subsequent loss of one water molecule per link
Forming nylon
Nylon is a polyamide formed from a dicarboxylic acid and a diamine
The structure of nylon can be represented by drawing out the polymer using boxes to represent
the carbon chains
Diagram showing a section of nylon
Forming polyesters
PET or polyethylene terephthalate to give its full name, is a polyester made
from dicarboxylic acid monomers (a carboxylic with a -COOH group at either end)
and diols (alcohol with an -OH group at either end)
Each -COOH group reacts with another -OH group on another monomer
An ester linkage is formed with the subsequent loss of one water molecule per link
For every ester linkage formed in condensation polymerisation, one molecule of water is formed
from the combination of a proton (H+) and a hydroxyl ion (OH–)
PET is also used in synthetic fibres as is sold under the trade name of terylene
Forming PET
PET is a polyester formed from a dicarboxylic acid and a diol
The structure of PET can be represented by drawing out the polymer using boxes to represent
the carbon chains
This can be done for all polyesters
Diagram showing a section of PET
Examiner Tip
For coordinated science, you just need to be able to identify the repeat units / linkages in condensation
polymers.
