The physical property of a particular polymer depends upon four variables:
- Chain Length – generally, the longer the chains the stronger the polymer as long chains tend to get tangled up in each other and stick together far more than shorter chain polymers. This means that the longer the chain, the stronger and the higher the melting point.
- Side groups – if the side groups in a polymer are polar, it gives stronger attraction between polymer chains.
- Branching – straight, unbranched chains can pack together more closely than highly branched chains, giving polymers a higher density, which in turn makes them stronger.
- Cross Linking – If polymer chains are linked together by covalent bonds throughout the structure, the polymer is harder and has a higher melting point.
The monomers that are required to join together to make the polymer will not usually react with each other under standard conditions (i.e. room temperature and pressure) as they’re too stable. Condensation and addition polymerisation are two methods of polymerising monomers.
Condensation polymerisation involves making an extra product, usually water in addition to the required polymer. A common biological condensation reaction is the formation of cellulose from glucose molecules. When two glucose molecules react, a hydroxyl group from each molecule ‘condense’ to a water molecule, leaving an oxygen atom to link the two monomers together. This is repeated throughout a very large chain of monomers to create the polymer: cellulose. Esters can also be polymerised to create polyesters. These chains are made into fibres and then woven together to create many types of clothing. Polyester fabrics and fibres are very strong and are extremely durable (resistant to most chemicals, stretching and abrasion). The ester is first formed by the reaction between a carboxylic acid and an alcohol. As a result of a ‘waste’ product being produced, the atom economy of Condensation polymerisation reactions will not be 100% and are therefore less sustainable than addition polymerisation reactions. Also, the majority of synthetic polyesters are not biodegradable and pose a problem with disposal.
Addition polymerisation is slightly different. Monomers are added together but no other substance is formed apart from the required polymer. No waste products are produced because every atom that reacts is contained within the useful product, and so addition reactions have 100% atom economy. Examples of addition polymers include poly(ethene) and poly(tetrafluroethene) – PTFE. The diagram shows how chloroethene takes part in addition polymerisation to form poly(chloroethene).
Polyamides are polymers that contain repeating amide (-CO-NH-) linkages. The best known manufactured polyamides are often called nylons (a trade name given by the manufacturer, DuPont) and these are aliphatic polyamides. Polyamides are very important in the manufacture of clothing and carpets. Kevlar is another type of polyamide that is extremely useful as a result of its uses in ballistic and stab-resistant armour. Kevlar is formed from benzene-1,4-diamine and benzene-1,4-dicarboxylic acid.
Today, with a general opinion that global warming/climate change is being caused by humans, there is more focus than ever on renewable resources, especially in the plastics industry. There are many degradable polymers such as poly(lactic acid) which is used for waste sacks, which when put into a landfill site, will biodegrade. When a polymer is degradable, microorganisms break it down into a mixture of methane and other gases. Some polymers are photodegradable which means the reaction is catalysed by sunlight (UV radiation). Photodegradable polymers break up into small pieces that then do not biodegrade. However this is better than a completely non-biodegradable polymer. Polymers can also be hydrolysed by ‘adding’ a molecule of water at the ester or amide group to split the polymer. An increasing number of polymers can now be recycled. This reduces disposal problems and the amount of crude oil used to make new products. However, different polymers still need to be collected and separated before they can be recycled, which can prove time consuming and expensive. If incinerated, the polymers can produce toxic gases upon being burnt. For example, poly(vinyl chloride) produces HCl(g).
