5.5.5 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

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

Answer:

   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)