Polymerization

Polymers are long chain like molecules which can say macromolecules. Normally they are solids or liquids in the room temperature because of their high molecular mass.

Polymerization is the reaction the monomers convert in to the polymer molecules. Monomer is the starting small molecule. Generally there are three steps in polymerization reaction called initiation, propagation and termination.

Three types of polymerization reactions can be seen.

• Addition polymerization (chain growth polymerization)
• Ionic polymerization
• Condensation polymerization (step growth polymerization)

Addition polymerization

In this type monomers must have unsaturations or double bonds. Free radical initiators or ionic initiators are used to initiation of the polymerization reaction. Otherwise energy can be absorbed from UV or heat.  First initiators react with monomers and make activated monomers which having an active centers. Then propagation happens by active center propagating along the chain. There are two widely available mechanisms of termination called termination by combination and termination by disproportionation. 

 

Example for addition polymerization

Ionic polymerization

Ionic polymerization can be divided in to anionic or cationic according to the charge of the active center. In this type also monomers must have unsaturations or double bonds. 

 

Example for ionic polymerization

 

• Anionic polymerization

Anionic initiators are required.  Negatively charged active centers are propagating along the chain. 

• Cationic polymerization

Cationic initiators are required.  Positively charged active centers are propagating along the chain. 

Condensation polymerization

In this type reactive functional groups of monomers will react with each other and remove small molecules. There is no initiation, termination nor active centers. The reaction will happen as long as they have reactive functional groups like carboxylic acid groups, hydroxyl groups etc. To make chain more than one reactive functional group are needed.

Example for condensation polymerization

This reaction is exothermic one. So to control the chain size we can do things like Cool the system, Use one component excess, use some amount of monomers which are having one reactive functional groups.