Class 12 Chemistry Polymers Types of Polymerization Reactions

Types of Polymerization Reactions

  • There are  broadly 2 types of polymerization reactions:-
  1. Addition or chain growth polymerization
  2. Condensation or step growth polymerization


Addition or chain growth polymerization

  • The molecules of the same monomer or different monomers add together on a large scale to form a polymer.
  • The monomers used are unsaturated compounds, e.g., alkenes, alkadienes and their derivatives.
  • This mode of polymerisation leading to an increase in chain length or chain growth can take place through the formation of either free radicals or ionic species.
  • However, the free radical governed addition or chain growth polymerisation is the most common mode.
  1. Free Radical Mechanism:-
  • Many of alkenes or dienes and their derivatives are polymerized in the presence of a free radical (catalyst) like benzoyl peroxide.
  • The sequence of steps may be depicted as follows:
  • Chain initiation steps
  • Chain propagating step
  • Chain terminating step

 For example:-

  • The polymerisation of Ethene to polythene consists of heating or exposing to light a mixture of Ethene with a small amount of benzoyl peroxide initiator.
  • The process starts with the addition of phenyl free radical formed by the peroxide to the Ethene double bond thus generating a new and larger free radical. This step is called chain initiating step.


  • As this radical reacts with another molecule of Ethene, another bigger sized radical is formed. The repetition of this sequence with new and bigger radicals carries the reaction forward and the step is termed as chain propagating step.


  • Ultimately, at some stage the product radical thus formed reacts with another radical to form the polymerised product. This step is called the chain terminating step.


Chain terminating step

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