Polymerisation (Edexcel International A Level Chemistry)

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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

 

7-7-polymerisation-making-teryle

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

example-dioyl-chloride-monomer

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 Tip

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

Polymerisation Repeating Unit, downloadable AS & A Level Chemistry revision 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

Worked example

Draw the repeating unit and identify the monomers used to make the following polymersMonomers worked example question 1, downloadable AS & A Level Chemistry revision notes

Answer:

Monomers worked example question 2, downloadable AS & A Level Chemistry revision notes

Worked example

Identifying monomers

  • Identify the monomers present in the given sections of addition polymer molecules

polymerisation-worked-example

   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)

Polymerisation Answer 1 Worked example - Identifying monomers, downloadable AS & A Level Chemistry revision notes

Answer 2:

    • To find the monomer, first the repeating unit should be deduced. Repeating units have only 2 carbons in the polymer main chain

Polymerisation Answer 2a Worked example - Identifying monomers, downloadable AS & A Level Chemistry revision notes

    • Since the repeating unit is now found, it can be concluded that the monomer is prop-2-enoic acid

Polymerisation Answer 2b Worked example - Identifying monomers, downloadable AS & A Level Chemistry revision notes

   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)

Polymerisation Answer 3 Worked example - Identifying monomers, downloadable AS & A Level Chemistry revision notes

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Sonny

Author: Sonny

Expertise: Chemistry

Sonny graduated from Imperial College London with a first-class degree in Biomedical Engineering. Turning from engineering to education, he has now been a science tutor working in the UK for several years. Sonny enjoys sharing his passion for science and producing engaging educational materials that help students reach their goals.

Download notes on 5.5.5 Polymerisation