Polymerisation (Edexcel International A Level Chemistry): Revision Note
Condensation & Addition Polymerisation
Addition polymerisation has been covered in reactions of alkenes
They are made using monomers that have C=C double bonds joined together to form polymers such as polyethene
Condensation polymerisation is another type of reaction whereby a polymer is produced by repeated condensation reactions between monomers
Natural condensation polymers are all formed by elimination of water
Although the process of condensation polymerisation involves the elimination of a small molecule
Condensation polymers can be identified because the monomers are linked by ester or amide bonds
Condensation polymers can be formed by:
dicarboxylic acids and diols
dicarboxylic acids and diamines
amino acids
Polyester
Is formed by the reaction between dicarboxylic acid monomers and diol monomers
Polyester is produced by linking these monomers with ester bonds / links
This polymer structure shows an ester functional group linking monomers together
Formation of polyesters
A diol and a dicarboxylic acid are required to form a polyester
A diol contains 2 -OH groups
A dicarboxylic acid contains 2 -COOH groups
The position of the functional groups on both of these molecules allows condensation polymerisation to take place effectively
When the polyester is formed, one of the -OH groups on the diol and the hydrogen atom of the -COOH are expelled as a water molecule (H2O)
The resulting polymer is a polyester
In this example, the polyester is poly(ethylene terephthalate) or PET, which is sometimes known by its brand names of Terylene or Dacron
Expulsion of a water molecule in this condensation polymerisation forms the polyester called (ethylene terephthalate) (PET)
Formation of polyesters - hydroxycarboxylic acids
So far the examples of making polyesters have focused on using 2 separate monomers for the polymerisation
There is another route to making polyesters
A single monomer containing both of the key functional groups can also be used
These monomers are called hydroxycarboxylic acids
They contain an alcohol group (-OH) at one end of the molecule while the other end is capped by a carboxylic acid group (-COOH)
Both functional groups that are needed to make the polyester come from the same monomer
Polyamides
Polyamides are polymers where repeating units are bonded together by amide links
The formula of an amide group is -CONH
An amide link - also known as a peptide link - is the key functional group in a polyamide
Polyamide monomers
A diamine and a dicarboxylic acid are required to form a polyamide
A diamine contains 2 -NH2 groups
A dicarboxylic acid contains 2 -COOH groups
Dioyl dichlorides can also used to react with the diamine instead of the acid
A dioyl chloride contains 2 -COCl groups
This is a more reactive monomer but more expensive than dicarboxylic acid
The monomers for making polyamides
Formation of polyamides
This shows the expulsion of a small molecule as the amide link forms
Amino acids - formation of proteins
Proteins are vital biological molecules with varying functions within the body
They are essentially polymers made up of amino acid monomers
Amino acids have an aminocarboxylic acid structure
Their properties are governed by a branching side group - the R group
Amino acids contain an amine group, an acid group and a unique R group
Different amino acids are identified by their unique R group
The names of each amino acid is given using 3 letters
For example Glutamine is known as ‘Gln’
Dipeptides can be produced by polymerising 2 amino acids together
The amine group (-NH2) and acid group (-COOH) of each amino acid is used to polymerise with another amino acid
Polypeptides are made through polymerising more than 2 amino acids together
Dipeptides and polypeptides are formed by polymerising amino acid molecules together
Examiner Tips and Tricks
Become familiar with the structures of the different monomers that can be used to make condensation polymers.
Also, remember that exam questions will require you to identify the monomers and also draw the repeating units
Repeating Units
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
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
Worked Example
Draw the repeating unit and identify the monomers used to make the following polymers
Answer:
Worked Example
Identifying monomers
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)
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