Polymers and disposal

Last time

Before we start: last time, in Nitriles and amines, you learned these points. Here they are again on one card. Keep them in mind, because today builds on them.

A plastic bag lasts longer than you do

A plastic bag can sit in the ground for centuries. It is made of poly(ethene), thousands of ethene molecules joined into one enormous chain. The same property that makes it useful, being chemically unreactive, makes it hard to dispose of. This lesson shows how the chains are built, and what we do with them afterwards.

Addition polymerisation

Addition polymerisation joins many alkene molecules, the monomers, into one long chain, the polymer. There is no other product. The C double bond C in each monomer opens, and each carbon bonds to the next monomer. The polymer has only single bonds in its backbone. The repeat unit is the part of the chain that repeats. Put square brackets round it, with n outside, and open bonds at each end. The polymer is saturated, so it is unreactive. It does not react with bromine water.

Drawing the repeat unit

Draw the monomer with the C double bond C horizontal and the substituents above and below. Change the double bond to a single bond, and add an open bond at each end. Check it has the same atoms as the monomer. The repeat unit has the same formula as the monomer.

Examples

Ethene gives poly(ethene): the repeat unit is CH two CH two. Propene gives poly(propene): the repeat unit is CH two then CH with a CH three branch. Chloroethene gives poly(chloroethene), PVC: the repeat unit is CH two then CHCl. Propenenitrile, CH two double bond CHCN, gives the polymer in carbon fibres.