Addition & Condensation Polymers (Cambridge O Level Chemistry)

• 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 with the polymer being formed containing single bonds only
• 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, change the double bond in the monomer to a single bond in the repeat unit
• Add a 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)
• A small subscript n is written on the bottom right hand side to indicate a large number of repeat units
• Add on the rest of the groups in the same order that they surrounded the double bond in the monomer

Examples of addition polymerisation: polythene and PVC

 

Deducing the monomer from the polymer

• Identify the repeating unit in the polymer
• Change the single bond in the repeat unit to a double bond in the monomer
• Remove the bond from each end of the repeat unit 

Diagram showing the monomer from the repeat unit of an addition polymer (polychloroethene)

• 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:
  • Addition polymerisation forms the polymer molecule only
  • 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
• Hydrolysing (adding water) to the compound in acidic conditions usually reverses the reaction and produces the monomers by rupturing the peptide link

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

The condensation reaction in which the polyamide, nylon is produced

• 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

 

The condensation reaction in which PET is produced

• 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

Table showing differences between addition and condensation polymerisation

Addition Polymerisation	Condensation Polymerisation
Involves one monomer type containing a C=C bond	Involves two monomer types each containing two of the same functional group  OR One monomer type with two different functional groups
One product formed	Two types of product formed- the polymer, and a small molecule (usually water)
Functional group involved is a C=C bond	Involves two functional groups on each monomer