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Synthesis of Poly(tetrafluoroethene) (PTFE)
Poly(tetrafluoroethene) can be produced by the addition polymerisation of the monomer tetrafluoroethene, CF2=CF2.
The 2-dimensional structural formula of a molecule of tetrafluoroethene is shown below:
Tetrafluoroethene has a boiling point of −76°C so it is a gas at room tenperature and pressure.
The addition polymerisation of tetrafluoroethene results in the formation of a white, waxy solid with a melting point of 327°C. The structural formula of a short section of the polymer is often represented as shown below:
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F
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F
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F
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F
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F
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F
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| − |
C |
− |
C |
− |
C |
− |
C |
− |
C |
− |
C |
− |
| |
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F |
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F |
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F |
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F |
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F |
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F |
|
Based on the source of this polymer, that is the name of the monomer used to make this polymer, we can name this polymer poly(tetrafluoroethene).
The name of the monomer, tetrafluoroethene, is enclosed in round brackets and preceded by the prefix "poly".
Poly(tetrafluoroethene) is known as a source-based name, that is, the name of the polymer is based on the name of the monomer used to make it.
Notice the repeating unit in this polymer is −CF2− (we have highlighted this repeating unit in the diagram below):
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F
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F
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F
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F
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F
| |
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F
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| − |
C |
− |
C |
− |
C |
− |
C |
− |
C |
− |
C |
− |
| |
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F |
|
|
F |
|
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F |
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F |
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F |
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|
F |
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So we can represent poly(tetrafluoroethene) using the condensed structural formula(2) ( CF2 )n or, −(−CF2−)n−.
n is the number of repeating of repeating units, which for a polymer, is a very large, but unknown number(3).
The IUPAC name of this −CF2− repeating unit is difluoromethylene(4), so we can name the polymer based on its structure as poly(difluoromethylene).
This name is called a structure-based name because it uses the structure of the polymer to name the polymer.
In structure-based nomenclature of polymers, the name of the repeating structural unit is enclosed within round brackets and the prefix "poly" is added at the start.
We can represent the addition polymerisation reaction of tetrafluoroethene in a number of different, but equivalent, ways:
| |
monomer |
→ |
polymer |
| word equation (a): |
tetrafluoroethene |
→ |
poly(difluoromethylene) |
|---|
| word equation (b): |
tetrafluoroethene |
→ |
poly(tetrafluoroethene) |
|---|
chemical equation (a):
(recommended) |
nF2C=CF2 |
→ |
( CF2 )n |
|---|
chemical equation (b)(5):
(not recommended) |
nCF2=CF2 |
→ |
( CF2−CF2 )n |
Properties of Poly(tetrafluoroethene) (PTFE)
C-F bonds are too strong to be easily broken during the polymerisation of tetrafluoroethene, this means that the polymer grows as a long, linear chain with very little if any branching of the chain, and no noticeable crosslinking.
Poly(tetrafluoroethene) is therefore a highly crystalline polymer consisting of linear −CF2−CF2− chains.
The C-F bond is very strong because the fluorine atom is very electronegative (4.0) while the carbon atom is only moderately electronegative (2.5).
The difference in the electronegativity of C and F is 4.0 − 2.5 = 1.5 which means the C-F bond is very polar.
The F atom, being more electronegative, has a greater share of the bonding pair of electrons and takes on a partial negative charge; Fδ-.
The less electronegative C atom, has a lesser share of the bonding pair of electrons and takes on a partial positive charge; Cδ+
The covalent bond between C and F is very polar: Fδ-−Cδ+
The C and F atoms are much more greatly attracted to each other than they are to any other "outside" atoms, which gives poly(tetrafluoroethene) some very interesting properties.
Poly(tetrafluoroethene) is insoluble in most solvents. The attraction between poly(tetrafluoroethene) chains is much, much, greater than the attraction between these chains and any solvent molecules in general.(6)
Poly(tetrafluoroethene) is also resistant to attack by most corrosive agents.(7)
Only Group 1 metals (alkali metals), either molten or dissolved in ammonia, will attack poly(tetrafluoroethene).
Prolonged exposure to fluorine under pressure can also degrade poly(tetrafluoroethene).
Poly(tetrafluoroethene) is not hard. It has a slippery and waxy feel, and a low coefficient of friction, so, it is resistant to wear and has antistick properties.
Poly(tetrafluoroethene) will start to decompose at the chain ends at temperatures above about 250°C but random cleavage of the chains becomes sigmificant only at temperatures above about 350°C. It said to have good heat resistance.
With all the available valence electrons involved in strong covalent bonds, there are no mobile electrons (and no ions) present in the polymer so poly(tetrafluoroethene) does not conduct electricity, it is an electrical insulator.
Uses of Poly(tetrafluoroethene)
Most of the uses of poly(tetrafluoroethene) rely on one or more of the following properties:
- resistance to chemical attack (chemically inert)
- poor electrical conductor (electrical insulator)
- heat resistance
- low frictional coefficient
The table below outlines some uses of poly(tetrafluoroethene) and the properties that make it useful for this purpose:
| Uses of poly(tetrafluoroethene) |
|---|
| General application |
Use |
Advantageous properties |
|---|
| Electrical Applications |
⚛ wire and cable insulation
⚛ insulation for motors, generators, transformers, coils and capacitors
|
⚛ poor electrical conductor
⚛ heat resistant
⚛ resistant to chemical attack
|
|---|
| Chemical Equipment |
⚛ gaskets
⚛ pump and valve packings and parts
|
⚛ resistant to chemical attack
⚛ low coefficient of friction
|
|---|
| Low-friction and Antistick Applications |
⚛ nonlubricated bearings
⚛ baking tray linings and other food processing equipment
⚛ mold-release devices
⚛ covers for heat-sealer plates in packaging machines
⚛ dry lubricant (dispersed as aerosols)
|
⚛ low coefficient of friction
⚛ heat resistant
|
|---|
(1) Refer to the Compendium of Polymer Terminology and Nomenclature IUPAC Recommendations 2008.
The structure-based name is poly(difluoromethylene).
The traditional name is polytetrafluoroethylene.
The source-based name is poly(tetrafluoroethene). An acceptable alternative to this is polytetrafluoroethene because there can be no ambiguity about the structure of the polymer if the round brackets are omitted from the name.
Source-based nomenclature began when we didn't know much about the structure of polymers generally so the name of a polymer was made by adding "poly" to the start of the name of the monomer.
In the last 50 years or so enormous advances have been made in the determination of polymer structures and so the nomenclature of polymers is shifting towards a preference for structure-based names rather than source-based names.
(2) While either of these two condensed structural formula are acceptable, IUPAC prefers the first form in which there is a strike-through of the round brackets.
(3) Within the solid, polymer chains of different lengths will be present, hence we do not know the "length" of the polymer chains.
(4) −CH2− has the IUPAC name methylene, so, substitution of each H atom for an F atom, results in difluoromethylene.
(5) The correct IUPAC formula for poly(tetrafluoroethene) is ( CF2 ) however, ( CF2−CF2 ) is acceptable only because it has been commonly used in the past.
The first formula is more correct because it clearly shows the repeating unit is CF2, the second formula suggests the repeating unit is actually CF2−CF2 rather than CF2.
(6) Solvents to dissolve poly(tetrafluoroethene) have been found at temperatures just below its crystalline melting point.
(7) Poly(tetrafluoroethene) is the material used to make containers to hold "superacids".
For more information about superacids, read the March 2013 issue of AUS-e-NEWS.
