GCSE OCR Gateway Organic Chemistry C6.2e and f Condensation polymerisation

C6.2e To be able to explain the basic principles of condensation polymerisation

With reference to the functional groups of the monomers, the minimum number of functional groups within a monomer, the number of repeating units in the polymer, and simultaneous formation of a small molecule, e.g. a polyester or polyamide, using block diagrams to represent polymers

C6.2f To be able to describe practical techniques to make a polymer by condensation

This content is only a higher tier requirement.

Condensation polymerisation to form nylon

Nylon was one of the first synthetic polymers to be produced.  Carothers first formed nylon in the mid 1930’s.  It is said that the name nylon is the combination of two places New York and London—I’ll leave you to check that out for yourself.

Nylon is pretty indestructible and takes years to degrade in the ground and that’s why it last for even longer in the sea.  It is the cause of major sea pollution and hazardous to sea life.

All nylon is made from precursor molecules from crude oil.  But some things really ought not to be made from it when they are easily thrown away and then do not rot down. 

The use of nylon ought to be reserved for specialist applications. 

 Most types of nylon require two monomers to form.  One monomer is a diamine and the other is a dicarbonyl dichloride e.g. 1,6—diaminohexane NH₂(CH₂)₆NH₂ and hexandioyl dichloride ClOC(CH₂)₄COCl

 

Here is what happens in the reaction between these two compounds:

NH₂(CH₂)₆NH₂     +    ClOC(CH₂)₄COCl    ➞    H[HN(CH₂)₆NHCO(CH₂)₄CO]_nCl      +   HCl      

The two molecules add together and lose a small molecule of HCl. 

Addition followed by the elimination of a small molecule adds up to a condensation reaction.

 

The number of carbon atoms in the monomers determines the name of the nylon.  So in this case the nylon is nylon 6:6 because each monomer contains six carbon atoms. 

Note how there are square brackets around part of the polymer structure.  This is the repeat unit of the polymer and n is the number of repeat units involved on average in the polymer sample and e.g. it could be 500-1000.

It’s easy to place the brackets because outside them should be the atoms that form the small molecule eliminated from the two monomers.

Note that there are two functional groups in each monomer.

Preparing nylon 6:6 in the lab is fairly straightforward.  You need the two monomers in the appropriate solutions, the amine in alkaline sodium carbonate solution and the acid chloride in an organic solvent such as hexane.

Pour the amine solution into a small 100ml beaker and then carefully pour the hexane solution of the other monomer over the top.  Hexane having the lower density should float on the aqueous solution.

At the interface of the two solutions the polymer will form spontaneously and using tweezers you should be able to pull a continuous thread of nylon from there until the solutions are exhausted of monomer.

Here are a couple of pictures to illustrate this procedure one of which is more outrageous than the other!!