Polymerization reactions

Polymers are molecular compounds with high molar mass made up of repeating units.

How units of polymers come together?

a. Addition reaction (radical mechanism):

Addition reaction involves the addition of small unsaturated units to each other. If the monomers added together from the same type of monomer, the formed polymer is named homopolymer.

If there are two types of monomers used to form polymer the produced polymer is called copolymer. Lastly, if the involved monomers are from different monomers more than two, the formed polymer is named heteropolymer.

Example, polyethylene forms due to the addition of multiple ethylene monomers.  

In the addition reaction, the initiator is heated to produce two radicals:

R₂ → 2R^.

In the second step, the initiator attacks ethylene monomers, then the double bond breaks and further formation of radicals keep forming:

R^. + CH₂=CH₂ → R-CH₂-CH₂^.

The new formed radical attacks new ethylene monomer and so on:

R-CH₂-CH₂^. + CH₂=CH₂ → R-CH₂-CH₂- CH₂-CH₂^.

The process continues and a long chain of the ethylene is formed till termination reaction happen.

The termination reaction occurs due to the depletion of the used initiator and the radicals start to bind together:

R-(CH₂-CH₂)_n- CH₂-CH₂^. + R-(CH₂-CH₂)n- CH₂-CH₂^. →

R-(CH₂-CH₂)- CH₂-CH₂ – (CH₂-CH₂)- CH₂-CH₂ -R

b. Condensation reaction:

The second type of polymerization reactions is the condensation reaction. In this type of reaction, the acid reacts with alcohol or amine and water is eliminated.

In the first example of condensation reaction, the two monomers that react together are, a monomer that contains carboxylic group, and the other contains hydroxyl group, the reaction between the two units involve the elimination of water molecules.

Figure 1. Condensation reaction between terephthalic acid and ethylene glycol to form Dacron

In the above figure, the terephthalic acid reacts with 1,2-ethylene glycol where Dactron forms plus water.

Another condensation reaction is the reaction that results in the formation of nylon.

Figure 2. Formation of Nylone 6,6 by condensation reaction

In the reaction adipic acid react with hexamethylene diamine where water is eliminated and nylon is formed. The adipic contains six carbon atoms and hexamethylene contains another six carbon atoms, and the nylon formed in the above reaction is named nylon 6,6 (referring to the number of carbon atoms for the acid and the amine)