Addition polymers

Last time

Before we start: last time, in Air pollutants, catalytic converters and global warming, you learned these points. Here they are again on one card. Keep them in mind, because today builds on them.

From gas to carrier bag

Ethene is a gas. But link thousands of ethene molecules into one very long chain and you get poly(ethene), the plastic of carrier bags, bottles and cling film. That chain-building reaction is addition polymerisation, and the rules for it are simple.

The vocabulary

A monomer is a small molecule that can join to others. For addition polymers it must have a C=C double bond. A polymer is a very long molecule made of many repeat units. In addition polymerisation, the double bonds open, and the monomers join end to end. Only one product is made.

Poly(ethene)

Ethene is CH 2 =CH 2. The double bond opens up, and each carbon bonds to the next molecule. We write: n CH 2 =CH 2 gives, in brackets, minus CH 2 minus CH 2 minus, with n at the bottom right. Nothing is lost: the polymer has the same atoms as the monomers.

Other monomers

Propene, CH 3 CH=CH 2, gives poly(propene), used for ropes and crates. Chloroethene, CH 2 =CHCl, gives poly(chloroethene), PVC, used for pipes and window frames. Tetrafluoroethene, CF 2 =CF 2, gives PTFE, the non-stick coating for frying pans.