Mer: A mer is a repetitive unit in a polymer.
Addition Polymerisation of Ethylene (Thermoplastics)
- Ethylene consists of 4 hydrogen atoms and 2 carbon atoms joined together by a double bond. (Primary and secondary bond)
- A catalyst is added which introduces a free radical. A free radical is a reactive atom containing an unpaired electron.
- The free radical joins onto one of the monomers causing the weak secondary bond between the carbon atoms to break.
- The mer cannot exist on its own as it now has an unpaired electron.
- It very rapidly links with other mers to form long chain like structures called polymers.
- To stop this reaction an inhibitor is added. This results in a large number of interwoven chains.
- Where these chains touch/overlap, secondary bonds called Van Der Waals forces are formed. These are weak bonds that will allow the polymer to soften whereby its shape can be altered by applying heat or pressure.
Condensation Polymerisation (Thermosetting plastics)
- The polymer molecules react chemically to form new molecules with water/alcohol eliminated as a bi-product.
- It forms strong primary bonds with cross links between chains
- The polymer produced, once moulded, cannot be re-softened/re-moulded as a result
- E.g. Phenol Formaldehyde