Polymers
Polymers are long molecules made up of hundreds or thousands of repeating subunits. They are really useful materials for everyday items, such as plastic containers and clothing, but they are a nuisance when it comes to throwing them away because they aren’t broken down easily.
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 together, 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.
Addition polymers are used to make plastics - a useful material but a pain in the arse to get rid of.
Disposing of addition polymers
Addition polymers are problematic when we want to dispose of them - they are extremely inert (unreactive) so they so not break down easily. This means that they are unable to biodegrade and take up space in landfill. This is why the use of plastic bags is such a big issue - they are made from addition polymers and are used in large numbers all over the world. America alone uses 100 billion plastic bags per year, each of which takes 500 years to degrade in landfill.
The other option is to burn the plastic, which also has its downsides. Plastic releases toxic gases when burnt which are damaging to health. The most sustainable option is to reuse and recycle the plastic as much as possible.
Condensation Polymerisation
Polyesters are useful for making different types of clothing and are commonly used in raincoats.
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 an 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 (read more about esters here). 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.
Biodegradable polyesters
Just like addition polymers, polyesters do not break down easily in the environment and can take up space in landfill for years. However, scientists have developed biodegradable polyesters (called biopolyesters) that can be broken down by microorganisms into carbon dioxide and water. These are much more environmentally friendly than non-biodegradable addition polymers which can persist in landfill sites for hundreds of years.
Did you know..
Scientists have been turning old clothes into building materials, in an attempt to reduce the amount of non-degradable polyesters ending up in landfill. They took bundles of unwanted clothes, removed any solid bits such as zips and buttons then shredded and compressed the material into square panels (pictured). The panels were very strong and thought to be suitable for building materials, such as floor tiles or wall panels. Image: Veena Sahajwalla, University of New South Wales