The process by which the monomers get linked up is called polymerization. Depending upon the mode of reaction, polymerization is classified as:
This process involves the addition of monomer units to themselves to form a growing chain by a chain reaction mechanism. It is for this reason that the process is also known as chain growth polymerization.
Addition polymerization is achieved by adding a catalyst (known as initiator), which provides some reactive species like free radicals. For example, benzoyl peroxide provides free radical benzyl to initiate the chain polymerization reaction. These free radicals then attack the unsaturated monomer and form a new free radical which goes on successively adding monomers and thus grows the chain.
In this, the monomers combine with the elimination of a small molecule like H2O, ROH or NH3, etc. The reaction is called step growth condensation polymerization and the product formed is called condensation polymer.
The process involves the elimination of by product molecules, therefore, the molecular mass of the polymer is not the integral multiple of the monomer units.
For example, polyester or Terylene is a condensation polymer of ethylene glycol and terphthallic acid.