Addition polymers and polymer disposalEdexcel International A Level Chemistry: Revision notes

Alkenes are unsaturated: they contain a C=C double bond. In addition polymerisation thousands of small monomer molecules join to form one long polymer chain. One bond of each C=C breaks and the freed bonds link the monomers together. The polymer is the only product, so no atoms are lost.

The reaction is usually carried out at high pressure with a catalyst.

Poly(ethene): n CH₂=CH₂ → –(CH₂–CH₂)ₙ–

The polymer chain contains only single bonds, so it is saturated.

The repeat unit is the smallest group of atoms that repeats along the chain. To draw it from a monomer, CH₂=CHX:

1. Keep the two carbon atoms of the C=C in the chain, now joined by a single bond.
2. Leave every other group (X) hanging off as a side group.
3. Draw bonds extending out of each end through brackets, and put n after the bracket.

• ethene → –(CH₂–CH₂)ₙ–
• propene → –(CH₂–CH(CH₃))ₙ–
• chloroethene → –(CH₂–CHCl)ₙ– (poly(chloroethene), PVC)
• tetrafluoroethene → –(CF₂–CF₂)ₙ– (PTFE)
• phenylethene → –(CH₂–CH(C₆H₅))ₙ– (poly(phenylethene))

Reverse the process. Take the repeat unit, which has two carbon atoms in the chain, replace the single bond between them with a double bond, and delete the extension bonds.

Worked example: repeat unit –CH₂–CH(CH₃)– gives the monomer CH₂=CH–CH₃, propene.

If a section of chain showing several repeat units is given, find the repeating pattern first. A four-carbon section with a pattern repeated twice contains two repeat units, so each monomer has two chain carbons.

Because poly(alkene)s are saturated, with strong non-polar C–C and C–H bonds, they are unreactive. For example, poly(chloroethene) does not decolourise bromine water, but its monomer does.

The number of monomer units in a chain is found by dividing the average relative molecular mass of the polymer by the relative molecular mass of the repeat unit.

Example: poly(propene), Mr 42 000. Repeat unit C₃H₆ = 42, so n = 42 000 ÷ 42 = 1000.

Because addition polymers are unreactive, microorganisms cannot break them down: they are non-biodegradable. They persist in landfill and as litter.

Incineration reduces volume and can generate electricity, but burning produces gases:

• carbon dioxide (greenhouse gas) and, in limited air, toxic carbon monoxide
• hydrogen chloride from PVC and hydrogen fluoride from PTFE, which are toxic acidic gases

Removing toxic waste gases: the waste gases are passed through scrubbers containing an alkali, e.g. Ca(OH)₂ + 2HCl → CaCl₂ + 2H₂O, which neutralises the acidic gases.

Biodegradable polymers: chemists develop polymers, such as starch-based polymers, that contain bonds which microorganisms can break down, producing carbon dioxide and water. This reduces landfill and litter, but they may cost more, may be weaker, and need suitable conditions to break down.