Open the C=C double bond of an alkene and the molecules can join end to end, thousands at a time. Learn to draw the repeat unit from a monomer, work back from a polymer chain to its monomer, and see why the same unreactive chains that make plastics useful also make them so hard to get rid of.
In addition polymerisation many alkene molecules (the monomers) join to form one very long saturated chain (the polymer). The weaker π bond of each C=C breaks and the electrons form new C–C σ bonds to neighbouring monomers, so nothing else is produced and the polymer has the same empirical formula as the monomer. The repeat unit is the monomer with its C=C turned into C–C and a bond sticking out from each end of the 2-carbon backbone, written in brackets with a subscript n. To find the monomer from a chain, mark off 2-carbon units along the backbone and put the double bond back. Poly(alkenes) are saturated with strong, non-polar C–C and C–H bonds, so they are unreactive and non-biodegradable: they persist in landfill, and burning them releases CO, CO2 and, from PVC, toxic HCl.
Pick a monomer in the simulator and watch the π bonds open and the molecules join end to end. Then use the table to draw each repeat unit.
| Monomer | Formula | Polymer | Repeat unit | Typical use |
|---|---|---|---|---|
| ethene | CH2=CH2 | poly(ethene) | [–CH2–CH2–]n | bags, bottles, film |
| propene | CH2=CHCH3 | poly(propene) | [–CH2–CH(CH3)–]n | crates, ropes, carpets |
| chloroethene | CH2=CHCl | poly(chloroethene), PVC | [–CH2–CHCl–]n | pipes, window frames, cable insulation |
| tetrafluoroethene | CF2=CF2 | poly(tetrafluoroethene), PTFE | [–CF2–CF2–]n | non-stick coatings |
Use the counting view of the simulator to divide the mass of a chain by the mass of one monomer. The whole of each monomer ends up in the chain, so the ratio gives the number of units.
| Polymer sample | Average Mr of chain | Mr of monomer | Monomer units per chain |
|---|---|---|---|
| poly(ethene) | 1.40 × 105 | 28.0 | 5.00 × 103 |
| poly(propene) | 2.10 × 105 | 42.0 | 5.00 × 103 |
| PVC | 1.25 × 105 | 62.5 | 2.00 × 103 |
| PTFE | 4.00 × 104 | 100.0 | 400 |
Four quick checks. Each correct answer earns XP and records this skill on your profile.
What happens to the C=C double bond when ethene polymerises?
Which monomer is used to make PTFE?
Which is the repeat unit of poly(chloroethene)?
Why are poly(alkenes) non-biodegradable?
The properties that make poly(alkenes) so useful, being tough, waterproof and chemically inert, are the same ones that cause the disposal problem.
| Disposal route | What happens | Problem |
|---|---|---|
| landfill | chains are saturated, with strong non-polar C–C and C–H bonds; microorganisms cannot break them down | non-biodegradable: persists for hundreds of years and uses up land |
| combustion (incineration) | burns to CO2 and H2O, with CO and soot if oxygen is limited | CO2 adds to global warming; CO is toxic; PVC also gives toxic, acidic HCl |
| recycling | polymers are sorted, melted and remoulded | needs careful sorting by type; quality falls with each cycle |
Poly(ethene) is saturated, so never draw a C=C inside a repeat unit, and never write “polyethene contains double bonds”. Only the two C=C carbons form the backbone; side groups such as CH3 or Cl hang off it, so the repeat unit always has a 2-carbon backbone (per monomer) with bonds through the brackets and an n. Addition polymerisation produces no by-product such as water. Poly(alkenes) are non-biodegradable because they are unreactive, not because they are toxic, and it is specifically PVC that gives HCl on burning.
Unlocks once the four checks above are done. Worth more XP, written to AS Paper 1 and Paper 2 standard.
Which is the repeat unit of poly(propene)?
A sample of PTFE has an average Mr of 4.00 × 104. How many monomer units are in an average chain? (Ar: C 12.0, F 19.0)
A section of polymer chain is –CH(CH3)–CH(CH3)–CH(CH3)–CH(CH3)–. What is the monomer?
125 g of PVC is burned completely. What mass of hydrogen chloride is formed? (Mr: C2H3Cl 62.5, HCl 36.5)
This skill is now recorded on your profile. Keep the chain going.