As we have seen on the structure and properties page the alkene from which PTFE is produced is tetrafluoroethene. Let us first look at the synthseis of this starting material.
PTFE is formed by the free radical polymerisation rection, the mechanism of which is shown below:
In the first (initiation) step of this reaction, the -O-O- bond in the peroxide molecule ruptures forming two reactive radical molecules (note unpaired electron).
Next, one electron from the pi-bond of the alkene (CF2=CF2) forms a bond with the unpaired electron of the radical, leaving the remaining electron from the pi-bond unpaired. The product of this step is therefore a similarly reactive radical which goes on, in the next mechanistic step, to react in an identical manner with another alkene molecule, adding further to what will become a very long polymer chain. This is known as chain lengthening or propagation.
The rate of this chain-lengthening reaction is very fast, with thousands of additions per second possible.
NB. In this example, the radical used is an alkoxide radical. Organic peroxides are good sources of radicals because the weak -O-O- bond readily undergoes homolytic fission at relatively low temperatures. Alternatively this example could be illustrated using, for example, chlorine radicals or alkyl radicals.
When two radicals react, a stable molecule is formed and no further reactions occur. This termination step, resulting in the formation of the stable PTFE polymer molecule, is shown below.
Teflon(R) is a registered trademark of E.I. du Pont de Nemours and Co.
Tom Westgate - cha99tdw@sheffield.ac.uk (document modification date: 8th May 2000)