Addition polymers and polymer disposalEdexcel A-Level Chemistry: Revision notes
Section 1
Addition polymerisation
Alkenes and substituted alkenes join together to form long chains in addition polymerisation. The C=C double bond in each monomer opens and the molecules bond to each other. There is only one product, so no atoms are lost.
n CH₂=CH₂ → –(CH₂–CH₂)ₙ– (poly(ethene))
The polymer chain is saturated: it has no C=C bonds.
Section 2
Repeat units: monomer to polymer and back
The repeat unit is the part of the chain that repeats. Keep the two carbon atoms of the former C=C in the chain and put every side group above or below.
- chloroethene CH₂=CHCl → repeat unit –CH₂–CHCl– (PVC)
- propene CH₃CH=CH₂ → –CH₂–CH(CH₃)–
- tetrafluoroethene CF₂=CF₂ → –CF₂–CF₂– (PTFE)
To find the monomer from a chain, take one repeat unit made of two chain carbons and put a double bond back between them.
Worked example. PVC of relative molecular mass 125 000: repeat unit C₂H₃Cl = 62.5, so 125 000 ÷ 62.5 = 2000 repeat units.
Never leave a C=C in a repeat unit. The double bond opens when the polymer forms.
Section 3
Why waste polymers are a problem
Addition polymers have a saturated carbon chain of strong, non-polar C–C and C–H bonds. They are chemically inert and are not broken down by micro-organisms, so they are not biodegradable. Waste polymers therefore remain in landfill and as litter for a very long time.
Section 4
Dealing with waste polymers
Waste polymers can be:
- Sorted and recycled: separated into types, then melted and reshaped. This saves crude oil and energy, but sorting is costly.
- Incinerated to release energy, which can generate electricity: C₃H₆ burns to CO₂ and H₂O. It also produces carbon dioxide and, with chlorine-containing polymers, toxic gases.
- Used as feedstock for cracking, to make shorter hydrocarbons that become new chemicals and polymers.
Section 5
Sustainable use and limiting disposal problems
Chemists contribute to more sustainable use of materials by considering the energy and resources used over the whole life cycle: raw materials, manufacture, use and disposal. They can use plant-based or recycled raw materials, reduce the energy used in manufacture, and make durable polymers that can be reused.
Disposal problems are limited by developing biodegradable polymers, which micro-organisms can break down, and by removing toxic gases from incineration. When PVC burns it forms hydrogen chloride, which is removed by passing the waste gases through an alkali such as calcium hydroxide: Ca(OH)₂ + 2HCl → CaCl₂ + 2H₂O.
That's the notes covered.
Carry on to the next subtopic.
Exam questions on Addition polymers and polymer disposal
- Poly(chloroethene), commonly called PVC, is an addition polymer used to make window frames and drainpipes.A sample of PVC has a relative molecular mass of 125 000. Calculate the number of repeat units in each polymer chain. (Ar: H = 1.0, C = 12.0, Cl = 35.5)2 marks
- A packaging company makes crates from poly(propene). A section of the polymer chain is drawn as –CH₂–CH(CH₃)–CH₂–CH(CH₃)–CH₂–CH(CH₃)–.Waste poly(propene) crates are burned in an incinerator with plenty of air. Write a balanced equation for the complete combustion of poly(propene), using C₃H₆ for the repeat unit, and state one advantage of incineration.2 marks
- A waste-treatment company incinerates mixed plastic waste, which includes poly(chloroethene), PVC, in order to generate electricity.Name the toxic gas formed when the PVC is burned that is not formed when poly(propene) is burned, and explain why it forms. Describe how the gas can be removed from the waste gases.3 marks
Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).