Addition Polymerisation

Addition Polymerisation: Polythene & PVC

Addition polymerisation is one of the most important addition reactions of alkenes which form the basis of the plastic industry
Addition polymerisation is the reaction in which many monomers containing at least one C-C double bond form long chains of polymers as the only product
- Just like in other addition reactions of alkenes, the π-bond in each C-C bond breaks and then the monomers link together to form new C-C single bonds
A polymer is a long-chain molecule that is made up of many repeating units
The small, reactive molecules that react together to form the polymer are called monomers
A polymerisation reaction can be represented by a general formula or by using displayed formulae
- E.g. poly(ethene) and poly(chloroethene) (also known as PVC) are polymers made up of the ethene and chloroethene monomers respectively and are commonly used in making plastics

polymerisation-general-formula

The general formulae of the addition polymerisation of ethene (1) and chloroethene (2)

polymerisation-displayed-formula

The displayed formulae of the addition polymerisation of ethene (1) and chloroethene (2)

Just like any other addition reaction of alkenes, addition polymerisation gives only one product

A repeat unit is the smallest group of atoms that when connected one after the other make up the polymer chain
- It is represented by square brackets in the displayed and general formula
In poly(alkenes) (such as poly(ethene)) and substituted poly(alkenes) (such as PVC) made of one type of monomer the repeating unit is the same as the monomer except that the C-C double bond is changed to a C-C single bond

The repeating units of poly(ethene) and poly(chloroethene) are similar to their monomer except that the C=C bond has changed into a C-C bond

Identify the monomers present in the given sections of addition polymer molecules:

polymers-worked-example

Answer 1:

When ethenol (CH(OH)=CH₂) is polymerised, the C-C double bond opens to produce a repeating unit of CH(OH)-CH₂.
This gives the polymer poly(ethenol)

Polymerisation Answer 1 Worked example - Identifying monomers, downloadable AS & A Level Chemistry revision notes

Answer 2:

Polymerisation Answer 2a Worked example - Identifying monomers, downloadable AS & A Level Chemistry revision notes

Since the repeating unit is now found, it can be concluded that the monomer is prop-2-enoic acid

Polymerisation Answer 2b Worked example - Identifying monomers, downloadable AS & A Level Chemistry revision notes

Answer 3:

Again, the repeating unit only has 2 carbons in the polymer chain which in this case are two carbon atoms that each contain one OH group
Thus, when ethene-1,2-diol (CH(OH)=CH(OH)) is polymerised, the C-C double bond opens to produce a repeating unit of CH(OH)-CH(OH) which gives the polymer poly(ethene-1,2-diol)

Polymerisation Answer 3 Worked example - Identifying monomers, downloadable AS & A Level Chemistry revision notes

The section of the polymer chain shown inside the square brackets by the structural or displayed formula is the repeat unit and not the monomer

The monomer is the same as the repeat unit except for that it has C=C bonds instead of C-C bonds