Types of Polymerization Reactions

Polymerization reactions are of two types: addition or chain-growth polymerization and condensation or step-growth polymerization.

Addition or chain-growth polymerization

In this type, the monomers are generally unsaturated (alkenes and alkynes) organic compounds. The molecules of the same or different monomers combine to form a single polymer.

The chain length is eventually increased through the formation of free radicals or ions.

The most common mechanism is a free radical mechanism in which monomers (unsaturated organic compounds) are polymerized in the presence of free-radical initiators.

The process starts with the formation of radicals from the initiator, this radical reacts with the double or triple bond of monomer to form larger radical (Chain initiating step).

There is the repetition of the reaction of radicals to form larger and larger radical moves the reaction forward (chain propagating step).

Finally, product radical reacts with another radical to form the polymerized product (chain-terminating step).

Example HDPE and LDPE (high and low-density polyethene) with benzoyl peroxide as initiator, Telfon.

Step growth or condensation polymerization

Step growth polymerization involves continuous condensation reaction between bi-functional monomers.

In this type, there is the loss of simple molecules like water, alcohol, etc.., and hence the formation of high molecular mass polymers.

In these reactions, the product undergoes a sequence of condensation reactions with bi-functional reactants.

Since each step produces a distinct functionalized species and is independent of each other; the process is also called a step-growth polymerization.

Examples include polyamides (Nylon 6 or Nylon 6,6), polyesters, melamine).

Co-polymerisation is the process in which not only 1 monomer is made to polymerize but a mixture of monomers is allowed to polymerize and form a copolymer. Due to this, we can see a mixture of properties is observed in a single polymer.

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