Polymers

Polymers are long molecules formed when lots of smaller units called monomers join together. Polymers can be formed by joining alkenes in a chain or by reacting alcohols and carboxylic acids. They’re found in things like fabrics and plastic bags but you’ll also find polymers inside your cells - DNA and proteins are examples of natural polymers.

This topic is only included on the AQA triple science specification.

 
 

Addition polymerisation

Addition polymerisation involves the breaking of double carbon bonds within alkene molecules and stitching lots alkene molecules together to form a long polymer. In the polymerisation of ethene for example, the double bond between the two carbon atoms breaks and many ethene molecules bond together to form a long chain of ethene molecules known as poly(ethene) or ‘polythene’. Polythene is used to make thin plastics and can be found in plastic bags and shampoo bottles.

Rather than drawing out hundreds of ethene molecules connected together in one long chain, it’s far easier to draw a single monomer with brackets around it. We write a ‘n’ in the bottom right corner to show that there are lots of the same molecule joined together in a chain.

Condensation polymerisation

Condensation polymers are formed between two monomers which react through their functional groups to create a bond connecting them together. A molecule of water is removed in order to form the bond, which is why we describe them as condensation polymers. For example, when ethanediol and hexanedioic acid react together, an ester bond is formed through the removal of a water molecule. Since each monomer possesses two functional groups each, this reaction can keep happening to form a long chain which is known as a polyester.

Amino acids

Amino acids are the basic units (monomers) which make up proteins. A single amino acid contains amine (-NH2) and carboxylic acid (-COOH) functional groups. These functional groups can react with each other in a condensation reaction to form a polymer, which we refer to as a polyamide (in biology, you would call it a polypeptide). –OH is removed from the carboxylic acid functional group and –H is removed from the amine functional group to create a water molecule, leaving behind an amide (peptide) bond between the amino acids.

DNA and other natural polymers

DNA stands for deoxyribonucleic acid and is a molecule found in all living cells and viruses. It provides the instructions which allow organisms to develop and function, making it essential to the survival of living organisms.

DNA is a polymer made up of nucleotide monomers. The monomers are bonded together to form a long chain of repeating nucleotides. Each nucleotide consists of three components: a sugar group, a phosphate and one of four nucleotide bases (adenine, thymine, guanine or cytosine).

Other naturally occurring polymers include:

  • Cellulose – a polymer made up of glucose monomers

  • Starch – a polymer made up of glucose monomers

  • Proteins – a polymer made up of amino acid monomers

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Did you know…

Vulcanised tyres are basically just one huge molecule. Rubber is made up of addition polymers formed when lots of alkene molecules react together. In vulcanised tyres, these addition polymers are cross-linked with covalent bonds to form one ginormous molecule.