Preparation of Synthetic Polymers

Classification of polymers based on the mechanism of polymerization

Polymers can also be classified based on the mechanism by which they are polymerized. There are two major types:

i. Addition polymerization

Addition polymerization involves the combination of unsaturated monomers (such as those with double or triple bonds) without the loss of any small molecules.

This process can occur through three mechanisms:

• Free radical polymerization

  1. Mechanism: Initiated by free radicals, which are often generated by organic peroxides (R–O–O–R').

     

  2. Steps:

     1. Chain initiation: A free radical is formed, starting the chain reaction.

        

     2. Chain propagation: The radical reacts with another monomer, adding it to the growing polymer chain.

        

     3. Chain termination: The chain stops growing when two free radicals combine or undergo disproportionation.

        + → R

  Example:

  Monomer

  

  Chain initiation step

  

  Chain propagation step

  

  Chain termination step

  

  Polymer.

• Cationic polymerization

  1. Mechanism: Initiated by an electrophile, which reacts with the monomer to form a cation.
  2. Steps:
     1. Chain initiation: The electrophile (e.g., H+, NO2-) reacts with the monomer, creating a cation.
     2. Chain propagation: The cation adds other monomers to the growing chain.
     3. Chain termination: The chain reaction stops when the cation is neutralized or undergoes a termination reaction.

  Example:

  

• Anionic polymerization

  1. Mechanism: Initiated by a nucleophile (e.g., NaNH2, BuLi), which reacts with alkenes to form an anionic propagating site.
  2. Steps:
     • Chain initiation: The nucleophile attacks the monomer, forming an anionic site.
     • Chain propagation: The anion reacts with more monomers, adding them to the chain.
     • Chain termination: The chain ends when the anion is neutralized.

  Example:

  

ii. Condensation polymerization

Condensation polymerization occurs when two bifunctional or trifunctional monomers react with the loss of small molecules such as water, hydrogen chloride, or ammonia. This type of polymerization typically forms high molecular weight polymers. Examples include:

1. Polyester (Terylene): Made by condensation of terephthalic acid and ethylene glycol.

   

2. Polyamides (e.g., Nylon 6,6): Made from hexamethylenediamine and adipic acid, commonly used in fabrics and engineering plastics.