Polyesters and polyamides
Types of polymer
Polymers are long chains made up of lots of repeating units (monomers) bonded together. There are two types:
Addition polymers – formed from alkene monomers. The C=C bond breaks, allowing the carbon to attach to another alkene.
Condensation polymers – formed when two functional groups react. A bond is formed through the removal of water (a condensation reaction.)
Addition polymerisation
Addition polymers are formed by joining lots of alkene molecules together. The single alkene is called a monomer and when several monomers are connected, we call this a polymer. Polymerisation happens when the double carbon bond breaks, allowing another alkene to connect to the carbon. This happens multiple times until you end up with a polymer made up of hundreds or thousands of monomers.
Instead of drawing out a really long chain, it’s much easier to represent polymers by drawing a single subunit (the monomer) inside square brackets, with a little ‘n’ in the right hand corner to show that we have lots of them joined together. Remember to draw bonds sticking outside the square brackets to make it clear that the chain continues.
We name the polymer depending on the type of monomer it is made from and stick the word poly at the front. Let’s say we have a polymer made up of lots of ethene molecules joined together - this would be called poly(ethene), which we also refer to as polythene. Polythene is everywhere - you’ll find it in things like plastic water bottles, bin liners and hose pipes.
The diagram below shows the repeat units of the addition polymers formed from ethene, propene, chloroethene and tetrafluoroethene which are shown below.
Condensation polymerisation
Another way of forming polymers is by condensation polymerisation. This is when molecules join together in long chains by reacting together - during the reaction a connecting bond is formed and a molecule of water is lost.
Polyesters are a type of condensation polymer, which are used for making plastics and clothing. An ester is formed when a carboxylic acid reacts with an alcohol. A polyester forms when a molecule with two carboxylic acids groups (a dicarboxylic acid) reacts with a molecule with two alcohol groups (a diol). The reaction between the carboxylic acid group on one molecule and the alcohol group on another molecule forms an ester bond (-COO) to connect the two monomers, with the removal of water. This reaction happens several times until a long chain is formed, made up of monomers connected by ester bonds. In the example below, we can see a repeat unit of the polyester formed between ethanedioic acid and ethanediol.
Polyamides are another type of condensation polymer, formed when an amine reacts with a carboxylic acid. A molecule of water is removed, leaving behind the amide (-CONH) linkage between the monomers. The formation of both ester bonds and amide bonds are examples of condensation reactions.
Hydrolysis of polyesters and polyamides
We’ve seen how the formation of ester and amide bonds take place through the removal of water (condensation reactions). This means that the reverse reaction – addition of a water molecule in a hydrolysis reaction – will break apart the ester/amide linkage to form the original monomers.
Hydrolysis is usually done with the addition of an acid (for polyamide hydrolysis) or a base (for polyester hydrolysis) to speed up the reaction.