Synthetic polymers

A polymer is a giant molecule built by joining huge numbers of small units together, and once you know what to look for you will find them everywhere: polymers are large molecules consisting of many repeating units, and this chapter looks at two different ways that polymers can be made. Natural polymers run your body and your kitchen: proteins are polypeptides, and sugar is built from polysaccharides — chains you have already met in biology. Synthetic polymers run everything else: plastic bags, clothing, water pipes, non-stick pans. This lesson is about the synthetic kind, and it turns on one idea — a polymer grows from small starting molecules called monomers, and there are exactly two ways the joining can happen. addition-polymerisation builds polymers such as poly(ethene) and PVC: the monomers simply add on, one after another, and nothing is thrown away. condensation-polymerisation builds a polyester: two different monomers alternate down the chain, and each time a pair joins, a small molecule is expelled. Everything in this chapter hangs off that one distinction.

The specification splits this lesson down the same line, and the split decides which exam paper each half belongs to. Four statements are for everyone: knowing that an addition polymer is formed by joining up many small molecules called monomers (4.44); drawing repeat units for the four named polymers (4.45); deducing a monomer from a repeat unit and back again (4.46); and explaining the disposal problems (4.47). Three more statements carry the C suffix — condensation polymerisation (4.48C), writing a polyester's structural and displayed formula (4.49C), and biodegradable polyesters (4.50C) — and C-suffixed content is chemistry-only: it appears on Paper 2C, not on the double-award papers. If you sit Paper 1C only, the addition half of this lesson is your whole territory; the condensation half is flagged as you enter it.

The route through the chapter follows the chemistry. First the addition story: how a C=C double bond opens out and chains grow, what a repeat unit is, and how to write the polymerisation reaction starting from any alkene — ethene, propene, chloroethene and tetrafluoroethene in turn. Then the reverse skill: given a polymer's structure, work backwards to the monomer that built it. Then the bill that polymers present to the environment, because their great strength — inertness — is exactly what makes their disposal a problem. Finally the condensation story: a diol and a dicarboxylic acid joining with water expelled, the polyester family, and the biodegradable members that might solve the disposal problem the addition polymers created.