Addition polymers — AQA A-Level Chemistry
Test yourself on Addition polymers with AQA A-Level practice questions.
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Addition polymers explained
In addition polymerisation the π bond of each monomer opens and the carbons join end to end into a long chain.
Read the full explanation
Nothing else is formed, so the polymer has the same empirical formula as the monomer and the atom economy is one hundred per cent. Ethene gives poly(ethene), and substituted alkenes behave identically because the substituent is a spectator: chloroethene gives poly(chloroethene), propene gives poly(propene), and phenylethene gives poly(phenylethene). Write the equation with n monomer units on the left and the repeating unit in brackets with n outside on the right, with bonds drawn through the brackets to show the chain continues. The double bond must not survive into the product.
The repeating unit of addition polymers.
The repeating unit is the smallest section of chain that, repeated end to end, builds the whole polymer. For an addition polymer it contains exactly the same atoms as the monomer, with the former double bond now a single bond and two bonds drawn out through the brackets to show where the next unit joins. Going forwards, take the monomer, open the double bond, and enclose the two carbons. Going backwards from a section of chain, find the shortest length that repeats, cut the backbone at equivalent points, and then put the double bond back between the two backbone carbons to recover the monomer. Keep any substituent on the same carbon it occupied in the monomer, or the repeat will not match the drawn chain.
IUPAC rules for naming addition polymers.
The IUPAC name of an addition polymer is poly followed by the monomer name in brackets: poly(ethene), poly(propene), poly(chloroethene), poly(phenylethene). The brackets matter because they show where the monomer name begins and ends, and they are essential when the name contains more than one word or a locant, as in poly(methyl 2-methylpropenoate). Common trade names are not accepted in place of the IUPAC form, so polythene, PVC and polystyrene should be converted before you write them. To name a polymer from a drawn structure, work back to the monomer first, name that alkene correctly with the lowest locants, and only then wrap it in poly and brackets.
Addition polymers are unreactive.
Once the double bond has been used up, the chain contains only C–C and C–H bonds. These are strong and, because carbon and hydrogen have almost the same electronegativity, essentially non-polar, so there is no δ⁺ carbon for a nucleophile to attack and no δ⁻ site for an electrophile. With no functional group there is nothing for a reagent to grip, which is why poly(ethene) survives acids, alkalis and, importantly, the enzymes of decomposer organisms. That inertness makes polyalkenes useful containers and a disposal problem at the same time. Between chains the only forces are van der Waals forces; each is weak, but over a long chain there are so many contacts that the solid is tough, and longer or straighter chains pack better and melt higher.
Appreciate that knowledge and understanding of the production and properties of polymers has developed over time.
Polymer chemistry shows how understanding grows and changes what can be made. Poly(ethene) was first produced by accident in the 1930s at very high pressure with a trace of oxygen; the chains branched, packed badly, and gave a soft, low density material. In the 1950s catalysts were developed that let ethene polymerise at low pressure and gave straight chains, which pack closely, attract each other over more of their length and give the rigid, higher density poly(ethene) used for pipes and crates. Later work added plasticisers, co-polymers and, more recently, biodegradable and more easily recycled polymers. The pattern to learn is that controlling the structure of the chain, usually through the catalyst, controls the properties of the material.
Typical uses of poly(chloroethene), commonly known as PVC, and how its properties can be modified using a plasticiser.
Poly(chloroethene) carries a chlorine atom on every second carbon. The C–Cl bond is polar, so chains attract each other by permanent dipole–dipole forces as well as van der Waals forces, and the material is hard and rigid: window frames, drainpipes, guttering. A plasticiser is a small molecule added between the chains. It pushes the chains further apart, so the intermolecular forces between them act over a greater distance and are weaker, and the chains can then slide past one another. The same polymer becomes flexible, giving cable insulation, flooring and blood bags. Nothing is bonded to the chain and no covalent bond is broken; the plasticiser simply changes how strongly one chain is held to the next.
Your focus
- Write a balanced polymerisation equation from a given monomer, using n on both sides.
- Draw the polymer formed from a substituted alkene such as chloroethene or phenylethene.
- Explain why addition polymerisation has an atom economy of one hundred per cent.
Show all 18 objectives
- Draw the repeating unit from a given monomer structure, with brackets and continuation bonds.
- Deduce the monomer from a section of an addition polymer chain by restoring the double bond.
- Check a proposed repeating unit by reproducing a longer section of the chain from it.
- Name an addition polymer from its monomer using the poly(monomer) convention.
- Convert common trade names such as PVC into the correct IUPAC polymer name.
- Deduce and name the monomer from a drawn repeating unit, then name the polymer.
- Explain the unreactivity of poly(ethene) by reference to bond strength and the absence of polar sites.
- Describe the intermolecular forces between polyalkene chains and relate them to chain length.
- Explain why an unreactive addition polymer creates a waste disposal problem.
- Explain how catalyst development changed the structure and properties of poly(ethene).
- Relate branching in a polymer chain to its density, rigidity and melting point.
- Discuss one recent development in polymer science and the problem it was designed to solve.
- Explain, in terms of intermolecular forces, how a plasticiser makes poly(chloroethene) flexible.
- Give a use for rigid and for plasticised poly(chloroethene) and link each to the property involved.
- Identify the intermolecular forces between poly(chloroethene) chains and justify the choice.
Addition polymers exam tips
Marking Points
- one mark for n monomer units on the left of the equation
- one mark for the repeating unit in brackets with n outside on the right
- one mark for showing the C=C has become a C–C single bond
- one mark for the two continuation bonds passing through the brackets, with no other product formed
- one mark for a repeating unit containing the same atoms as the monomer
- one mark for the two continuation bonds drawn through the brackets
- one mark for a single carbon–carbon bond in the backbone with n outside the bracket
- one mark for restoring the C=C in the correct position when deducing a monomer from a section of chain
- one mark for the form poly followed by brackets around the monomer name
- one mark for the correct IUPAC name of the monomer inside the brackets
- one mark for deducing the monomer first when the polymer is given as a structure
- one mark for correct locants in the monomer name where the substituent position matters
- one mark for saying the chain contains only strong, non-polar C–C and C–H bonds
- one mark for saying there is no δ⁺ carbon, so nucleophiles are not attracted
- one mark for the consequence that the polymer is not biodegradable and persists in landfill
- one mark for naming van der Waals forces as the only forces acting between polyalkene chains
- one mark for describing a change in production, such as the move from high-pressure to catalysed low-pressure polymerisation
- one mark for linking that change to a change in chain structure, such as branched against straight chains
- one mark for linking the chain structure to a property such as density, rigidity or melting point
- one mark for naming a more recent development such as biodegradable polymers or improved recycling
- one mark for one use of rigid poly(chloroethene) and one use of the plasticised form
- one mark for saying plasticiser molecules get between the polymer chains
- one mark for saying the chains are forced further apart so the forces between them are weaker
- one mark for the consequence that the chains slide over each other, making the polymer flexible
Examiner Tips
- 💡Count the atoms in your repeating unit against the monomer; they must match exactly.
- 💡Draw the backbone as two carbons in a straight line and hang the substituents off them, so the repeat is obvious.
- 💡Write poly(monomer name) with brackets, not the trade name.
- 💡Test your answer by drawing the unit twice side by side; it should reproduce the chain in the question.
- 💡Repeating unit and monomer are different answers, so read which one the question wants.
- 💡Extend the bonds clearly past the brackets; a bond that stops short loses the mark.
- 💡AQA credits phonetic spelling generally but not where correct IUPAC nomenclature is required, so spell polymer names carefully.
- 💡Name the monomer on paper first; most lost marks here are monomer naming errors, not polymer ones.
- 💡Keep the brackets even when the monomer name is a single word.
- 💡Answer "unreactive" questions in two halves: the bonds are strong, and there is no polar site to attack.
- 💡Name the intermolecular force precisely; "intermolecular forces" alone rarely gains the mark.
- 💡Link non-biodegradability to the same lack of a functional group, since enzymes also need a site to act on.
- 💡Tie every historical point to a structural reason; the marks are for chemistry, not for dates.
- 💡One well-developed example scores better than a list of unexplained names.
- 💡Use branching and packing as your explanatory vocabulary for density and strength.
- 💡Use the full chain of reasoning: between the chains, further apart, weaker forces, chains slide, more flexible.
- 💡Give a use that clearly matches each form, rigid and flexible, rather than a general use of plastic.
- 💡Name the force as permanent dipole–dipole, since the C–Cl bond is polar.
Common Mistakes
- leaving a double bond inside the repeating unit
- omitting the n from one side of the equation so the equation does not balance
- stopping the bonds at the bracket instead of drawing them through it
- adding a small molecule such as water as a by-product, which belongs to condensation polymerisation
- drawing the monomer, complete with its double bond, when the repeating unit was asked for
- leaving out the continuation bonds so the unit looks like an isolated molecule
- choosing two monomer lengths as the repeating unit when one would do
- moving a substituent onto the wrong backbone carbon so the repeat does not reproduce the chain
- writing polyethylene, PVC or polystyrene when the IUPAC name is required
- dropping the brackets, giving polychloroethene
- naming the monomer as an alkane, for example chloroethane rather than chloroethene
- keeping the double bond in mind but numbering the chain from the wrong end, giving the wrong locant
- saying the chains are held together by strong intermolecular forces, when van der Waals forces are individually weak and merely numerous
- explaining inertness by the size of the molecule rather than by the absence of a functional group
- claiming covalent bonds join one chain to the next in an addition polymer
- naming hydrogen bonding between poly(chloroethene) chains, which have no hydrogen bonded to N, O or F
- writing only that polymers have improved over time, with no chemistry to support it
- saying high density poly(ethene) is denser because its molecules are heavier, rather than more closely packed
- offering a condensation polymer such as a polyester as an example of an addition polymer development
- describing the catalyst as changing the product's formula rather than the arrangement of its chains
- saying the plasticiser breaks covalent bonds in the polymer chain
- saying the plasticiser reacts with the polymer rather than sitting between the chains
- claiming poly(chloroethene) chains are held together by hydrogen bonds
- describing the plasticiser as a lubricant without mentioning intermolecular forces at all