Polymers & Organic Reactions
What Are Polymers?
Separate Chemistry only. Combined Science asks only that you can say polymers are large molecules with long chains of carbon atoms, using poly(ethene) as the example. Addition polymerisation, repeating units, polymer naming and the bromine water test are not examined on Combined Science, so treat the rest of this lesson as background reading. A polymer is a very large molecule made from many smaller molecules called monomers. These monomers bond together in long chains through a process called polymerisation. The most familiar example is poly(ethene), which is made from ethene monomers and used in everyday plastic products such as bags, bottles and packaging.
Polymers consist of large molecules containing long chains of carbon atoms. They are formed when many small monomer molecules join together in a polymerisation reaction.
Addition Polymerisation
Addition polymerisation occurs when unsaturated monomers, such as alkenes, open their carbon–carbon double bonds and join together. No other substance is produced — the only product is the polymer. For example, many ethene molecules join to form poly(ethene):
\(n\text{CH}_2\text{=CH}_2 \rightarrow -(\text{CH}_2\text{-CH}_2)_n\text{-}\)
The double bond in each ethene monomer breaks, and each carbon atom forms a new single bond with the next monomer in the chain. The ‘n’ in the equation represents a very large number of repeating units.
To draw a repeating unit of an addition polymer, take the monomer and remove the double bond. Replace it with single bonds extending from each carbon. Show bonds going out of each side of the brackets to indicate the chain continues. The repeating unit retains the same atoms as the monomer, just with single bonds instead of the double bond.
Alkenes — The Monomers for Addition Polymers
Alkenes are unsaturated hydrocarbons because they contain a carbon–carbon double bond (\(\text{C=C}\)). This double bond makes them reactive and able to undergo addition reactions. The general formula for alkenes is \(\text{C}_n\text{H}_{2n}\). Common examples include ethene (\(\text{C}_2\text{H}_4\)) and propene (\(\text{C}_3\text{H}_6\)).
Alkenes contain a carbon–carbon double bond (\(\text{C=C}\)), making them unsaturated. This double bond allows them to undergo addition reactions, including addition polymerisation.
Testing for Alkenes
To test whether a hydrocarbon is an alkene, add bromine water. An alkene will decolourise bromine water, turning it from orange–brown to colourless. This happens because bromine adds across the double bond in an addition reaction. Alkanes do not decolourise bromine water because they have no double bond to react with.
Naming Addition Polymers
Addition polymers are named by placing the word ‘poly’ in front of the monomer name in brackets. Ethene becomes poly(ethene), propene becomes poly(propene), and chloroethene becomes poly(chloroethene) — better known by its common name PVC. The brackets around the monomer name are important because they distinguish the polymer from the monomer itself.
When you are given an unfamiliar polymer and asked to identify its monomer, reverse the process: take the repeating unit, replace the single bonds on each side with a double bond between the two carbon atoms, and you have the monomer. It is tested on the separate GCSE Chemistry papers, not on Combined Science.
Properties and Uses of Polymers
Different polymers have different properties depending on their monomer and structure. Poly(ethene) is flexible and waterproof, making it ideal for plastic bags and bottles. Poly(propene) is tougher and more heat-resistant, so it is used for food containers, ropes and carpets. Poly(chloroethene), commonly called PVC, is rigid and durable, widely used in window frames and pipes.
Not all polymers are synthetic. Natural polymers include DNA, starch, cellulose and proteins. DNA is a polymer made from nucleotide monomers. Starch and cellulose are both polymers of glucose, but their different structures give them very different properties. Proteins are polymers of amino acids. These natural polymers are essential for life.
Problems with Polymers
Most synthetic polymers are not biodegradable. This means they persist in landfill sites and the natural environment for hundreds of years, causing pollution. Burning polymers can release toxic gases such as carbon monoxide, hydrogen chloride and hydrogen cyanide. Recycling helps reduce waste, but different polymer types must be separated first, which adds cost and complexity.
Most synthetic polymers are not biodegradable. They take hundreds of years to decompose in landfill and can release toxic gases when burned. Recycling reduces waste but requires sorting different polymer types.
Cracking breaks down larger, less useful hydrocarbon molecules from crude oil into smaller, more useful ones. This process produces alkenes, which are the monomers used in addition polymerisation. Without cracking, there would not be enough alkene monomers to meet the demand for plastic production. Cracking also produces shorter-chain alkanes that are in higher demand as fuels.
When asked to draw the repeating unit of a polymer, remember to remove the double bond from the monomer and extend single bonds out of each side of the bracket. Always include the ‘n’ subscript outside the bracket to show many units are joined.