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AS 20

Polymerisation

AS Level · Organic chemistry · Papers 1 and 2 · extended in A2 35

🎯What you need to be able to do

  • Describe addition polymerisation, using poly(ethene) and poly(chloroethene), PVC.
  • Deduce the repeat unit of an addition polymer from its monomer.
  • Identify the monomer or monomers in a given section of an addition polymer.
  • Explain why poly(alkene)s are difficult to dispose of: they are not biodegradable, and burning them produces harmful products.

📚The chemistry

Addition polymerisation

A polymer is a very long molecule made by joining together many small molecules, the monomers. In addition polymerisation the monomers are alkenes (or other molecules with a C=C). The π bond of each monomer opens, and the carbons link up into a long chain of single C–C bonds. No other product is formed: the polymer has the same empirical formula as the monomer, and its mass is the sum of the masses of all the monomers.

\[ \mathrm{n\,CH_2{=}CH_2 \rightarrow {-}\!\!\left[CH_2{-}CH_2\right]_n\!\!{-}} \quad \text{poly(ethene)} \] \[ \mathrm{n\,CH_2{=}CHCl \rightarrow {-}\!\!\left[CH_2{-}CHCl\right]_n\!\!{-}} \quad \text{poly(chloroethene), PVC} \]

Poly(ethene) is used for plastic bags, bottles and film. Poly(chloroethene), PVC (from its old name, polyvinyl chloride), is rigid and used for window frames, drainpipes and the insulation on electrical cables. n is very large — typically thousands.

From monomer to repeat unit

The repeat unit is the smallest part of the chain that repeats. For an addition polymer it is simply the monomer with its double bond opened up:

  1. Draw the two carbons of the C=C in a row, with a single bond between them.
  2. Keep every group that was attached to those carbons, above and below the chain — they become side groups.
  3. Add a bond sticking out from each end (the continuation bonds), and brackets with n if showing the whole polymer.

So propene, CH2=CHCH3, gives –CH2–CH(CH3)–: the CH3 is a side group, not part of the main chain. The main chain of an addition polymer only ever contains the two carbons that formed the double bond.

From polymer back to monomer

Work the other way:

  1. Find the repeat unit: the main chain of an addition polymer is all carbon, and the repeat unit is two main-chain carbons with their side groups.
  2. Put a double bond between those two carbons and remove the continuation bonds.

A section –CH2–CH(CN)–CH2–CH(CN)– repeats every two carbons, so the monomer is CH2=CHCN (propenenitrile). If the side groups along a chain do not repeat every two carbons, the polymer was made from more than one monomer (a copolymer): split it into two-carbon units and convert each.

Disposal

Poly(alkene)s are useful because they are unreactive: they are saturated chains of strong, non-polar C–C and C–H bonds, like very large alkanes. That same property makes them hard to get rid of:

  • They are not biodegradable. Bacteria and fungi have no enzymes that can break the chains down, so poly(alkene)s persist in landfill and in the environment for hundreds of years, and break into microplastics.
  • Burning them produces harmful products. Combustion releases carbon dioxide (a greenhouse gas), and incomplete combustion gives toxic carbon monoxide and soot. PVC is worse: burning it releases hydrogen chloride, a toxic and acidic gas, and can form highly toxic chlorinated compounds (dioxins).

That is why reducing use, reusing, and recycling by type matter so much for these materials, and why incinerators that burn plastic waste need scrubbers to remove acidic gases.

✏️Worked example

(a) Draw the repeat unit of poly(chloroethene) and write the equation for its formation. (b) A sample of PVC has an average Mr of 125 000. Calculate the average number of monomer units in each chain. (c) This section of a polymer chain is made from two different monomers: –CH2–CHCl–CH2–CH(CH3)–CH2–CHCl–. Identify both monomers. (d) Explain why PVC waste should not be burned in an ordinary incinerator. [Ar: C 12.0, H 1.0, Cl 35.5]

(a) Chloroethene is CH2=CHCl. Opening the double bond gives the repeat unit –CH2–CHCl–, with a continuation bond on each end and the Cl as a side group.

\[ \mathrm{n\,CH_2{=}CHCl \rightarrow {-}\!\!\left[CH_2{-}CHCl\right]_n\!\!{-}} \]

(b) Mr(C2H3Cl) = 2(12.0) + 3(1.0) + 35.5 = 62.5. Addition polymerisation loses no atoms, so each repeat unit also has mass 62.5:

\[ n = \frac{125\,000}{62.5} = 2000 \]

(c) Split the main chain into two-carbon units: –CH2–CHCl– | –CH2–CH(CH3)– | –CH2–CHCl–. Put a double bond back in each: the monomers are chloroethene, CH2=CHCl, and propene, CH2=CHCH3.

(d) Burning PVC releases hydrogen chloride, which is toxic and forms acid rain, and can produce toxic chlorinated organic compounds; incomplete combustion also gives carbon monoxide. The acidic gases would have to be removed from the flue gases.

Check it. Run it forwards: joining CH2=CHCl, CH2=CHCH3 and CH2=CHCl head to tail must rebuild the section exactly, and it does. An atom count confirms it: the section is C7H12Cl2, which is 2 × C2H3Cl + C3H6, with nothing left over — as it must be for an addition polymer.
Putting a side group into the main chain. Drawing poly(propene) as –CH2–CH2–CH2– turns the methyl side group into a chain carbon and loses the mark. The main chain contains only the two carbons that formed the C=C; every other atom hangs off it.

📝Practise

Work through these, then reveal the answer. Each question targets a different objective from the list above.

1. Draw the repeat units of the polymers formed from (a) tetrafluoroethene, CF2=CF2; (b) phenylethene, CH2=CHC6H5.
(a) –CF2–CF2–, the repeat unit of poly(tetrafluoroethene), PTFE (non-stick coatings). (b) –CH2–CH(C6H5)–, poly(phenylethene), also called polystyrene; the benzene ring is a side group. In each case: two carbons in the chain, single bond between them, all other groups kept as side groups, and a continuation bond at each end.
2. Identify the monomer of the polymer –CH(CH3)–CH(CH3)–CH(CH3)–CH(CH3)–.
The repeat unit is two main-chain carbons: –CH(CH3)–CH(CH3)–. Put a double bond between them: CH3CH=CHCH3, but-2-ene. Note it is not propene: propene would put a CH3 on only every other carbon.
3. Explain why addition polymerisation has an atom economy of 100%.
In addition polymerisation the monomers simply join by opening their C=C double bonds; no small molecule is eliminated and there is no other product. Every atom in the monomers ends up in the polymer, so the mass of polymer equals the mass of monomer used, and the atom economy is 100%. (Condensation polymerisation, at A Level, loses water or HCl at each link.)
4. Why are poly(alkene)s not biodegradable?
Poly(alkene)s are saturated molecules with a backbone of strong, non-polar C–C bonds and C–H bonds, like very long alkanes, so they are chemically unreactive. Microorganisms have no enzymes that can break these bonds (unlike, for example, the ester or amide links in natural polymers), so the polymers are not broken down by bacteria or fungi and persist in the environment.
5. A poly(ethene) sample has chains with an average of 5000 monomer units. Calculate its average Mr.
Mr(C2H4) = 2(12.0) + 4(1.0) = 28.0. No atoms are lost, so Mr of the chain = 5000 × 28.0 = 140 000. Because chains vary in length, a real sample has a range of Mr values; this is the average.
6. Give two harmful products of burning waste PVC, and one of burning waste poly(ethene) with limited air.
PVC: hydrogen chloride, HCl (toxic, and forms acidic rain), and toxic chlorinated compounds such as dioxins; also CO2. Poly(ethene) with limited air: carbon monoxide (toxic, binds to haemoglobin) and soot (particulates); with plenty of air, CO2, a greenhouse gas.

🔗Go deeper — other people’s work

These are external resources, not mine. If one stops working, tell me and everything above it on this page still stands.

  • Chemguide (Jim Clark) — the polymers pages on addition polymerisation
  • Royal Society of Chemistry — resources on plastics, recycling and their environmental impact