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Addition polymers
How alkene monomers join into long chains by free-radical addition polymerisation, why LDPE and HDPE behave differently, and the structures, properties and uses of PVC, polystyrene and PTFE.
Polyethylene was discovered by accident in 1933 at ICI in England, when a high-pressure experiment with ethene leaked and left behind a waxy white solid. The reaction only worked because a trace of oxygen had got in and started it. Today more than 100 million tonnes of polyethylene are made each year, more than any other plastic, and nearly all of it comes from that same idea: open the double bond and keep adding.
Monomers and polymers
A polymer is a very large molecule made of many small repeating units called monomers. In addition polymerisation the monomers each contain a C=C double bond. One bond of the double bond breaks and each carbon forms a new single bond to the next monomer. Nothing is lost, so the polymer has the same empirical formula as the monomer and the atom economy is 100%.
$$n\,\mathrm{CH_2{=}CH_2} \longrightarrow \mathrm{-\!\!\left[CH_2{-}CH_2\right]_{\!n}\!\!-}$$
The bracketed unit is the repeating unit. To draw it from a monomer: copy the two C=C carbons with their attached groups, make the double bond single, add bond lines through the brackets on each side, and write $n$ outside. To find the monomer from a polymer, pick out a two-carbon repeating unit and put the double bond back.
How the chain grows: free-radical polymerisation
LDPE, PVC and polystyrene are made by a chain reaction started by an initiator, usually an organic peroxide (R–O–O–R) that splits into two free radicals when heated. A radical has an unpaired electron, shown as a dot, and is very reactive.
- Initiation. The initiator splits: R–O–O–R → 2 RO•. The radical attacks a monomer, using one electron from the C=C and leaving an unpaired electron on the other carbon: RO• + CH₂=CH₂ → RO–CH₂–CH₂•.
- Propagation. The new radical adds another monomer, and the radical end moves along: RO–CH₂CH₂• + CH₂=CH₂ → RO–CH₂CH₂CH₂CH₂•. This repeats thousands of times in a fraction of a second.
- Termination. Two radical chain ends meet and pair their electrons, forming a bond. Growth stops for both chains.
Chain length depends on the conditions. More initiator means more chains start at once, so each one is shorter (more radicals, more termination). A high monomer concentration favours long chains.
LDPE and HDPE: same monomer, different plastics
Polyethylene comes in two main forms that differ in how the chains are shaped and packed:
| LDPE (low density) | HDPE (high density) | |
|---|---|---|
| How it's made | Free radical, ~1000–3000 atm, ~200 °C, peroxide initiator | Ziegler–Natta catalyst (Ti/Al compounds), low pressure, ~60 °C |
| Chain shape | Many short and long branches (radicals bite back along the chain) | Almost linear, few branches |
| Packing | Branches stop chains lining up: mostly amorphous | Chains pack closely in ordered crystalline regions |
| Density | ~0.92 g/cm³ | ~0.95–0.97 g/cm³ |
| Melting point | ~105–115 °C | ~130–135 °C |
| Properties | Soft, flexible, transparent in thin films | Stiffer, stronger, opaque, more chemical resistant |
| Uses | Cling wrap, squeeze bottles, bags, cable insulation | Milk bottles, crates, pipes, chopping boards, fuel tanks |
The chains are held together only by dispersion forces (and dipole–dipole forces in PVC). Both are proportional to how much chain surface is in close contact, which is why packing matters so much. Because the chains are not cross-linked, these plastics soften on heating and can be remoulded: they are thermoplastics.
PVC, polystyrene and PTFE
| Polymer | Monomer | Key structural feature | Properties | Uses |
|---|---|---|---|---|
| PVC, poly(chloroethene) | chloroethene (vinyl chloride), CH₂=CHCl | polar C–Cl bonds: dipole–dipole forces between chains | rigid and hard unless plasticisers are added; flame resistant; weatherproof | water and sewer pipes, window frames, (plasticised) cable insulation, hoses, flooring |
| Polystyrene (PS), poly(phenylethene) | phenylethene (styrene), CH₂=CH–C₆H₅ | bulky benzene rings stop chains packing or sliding | rigid, brittle, transparent; foamed with gas to make an insulator | CD cases, disposable cutlery, (expanded) foam packaging and cups, insulation panels |
| PTFE, poly(tetrafluoroethene) | tetrafluoroethene, CF₂=CF₂ | very strong C–F bonds; F atoms sheath the carbon backbone | chemically inert, very low friction, non-stick, high melting point (~327 °C), excellent electrical insulator | non-stick cookware, plumber's tape, bearings, lab ware, wire insulation, Gore-Tex membranes |
PVC is the classic case where additives change everything. Pure PVC is rigid (pipes). Adding a plasticiser, small molecules that sit between the chains and push them apart, weakens the interchain forces and makes it flexible (garden hoses, fake leather). Some phthalate plasticisers can leach out, and several are now restricted in children's toys in Australia and the EU.
PTFE is part of the PFAS family of fluorinated chemicals. PTFE itself is very stable, but some chemicals once used to make it, such as PFOA, are persistent in the environment and are now banned or phased out. Australia has a national PFAS environmental management plan, partly because firefighting foams contaminated land and water around several defence bases, including Williamtown (NSW) and Katherine (NT).
- Pick ethene and press Start with the initiator at Low. Note the average chain length when all the monomer is used up.
- Repeat with the initiator at High. Why are the chains shorter now?
- Switch the process to Ziegler–Natta and compare the branches per 100 carbons with the radical (high pressure) process.
- Change the monomer to tetrafluoroethene and read off the repeating unit and its molar mass. A chain of 1000 units then has a molar mass of about 100 000 g/mol.
Test yourself
Which is the repeating unit of the polymer made from propene?
Only the two C=C carbons go into the backbone; the CH₃ becomes a side group.
HDPE has a higher melting point than LDPE because HDPE
Same bonds, same monomer; packing is the difference.
In free-radical polymerisation, adding more initiator will
More radicals start more chains at once and they terminate sooner, so each uses fewer monomers.
Which property of PTFE is mainly due to the strength of the C–F bond?
The C–F bond is one of the strongest single bonds, so almost nothing reacts with PTFE.
Name the polymer made from each monomer.
LDPE or HDPE?
The first stage of free-radical polymerisation, when the peroxide splits, is . During the chain grows one monomer at a time. PVC is made from , and adding a makes it flexible. Polymers that soften when heated and can be remoulded are called .
Polyethylene can be made as LDPE or HDPE. Explain how differences in their structure lead to differences in their properties and uses.
LDPE is made by free-radical polymerisation at very high pressure and about 200 °C. Radicals transfer along chains, producing many branches. The branches stop chains lining up, so they can't pack closely; the contact area between chains is small and dispersion forces are weak. LDPE therefore has a low density (~0.92 g/cm³), a lower melting point (~110 °C) and is soft and flexible, suiting cling wrap and squeeze bottles.
HDPE is made at low pressure using a Ziegler–Natta catalyst, giving nearly linear chains. These pack closely into crystalline regions, increasing contact area and the strength of dispersion forces. HDPE is denser (~0.96 g/cm³), melts higher (~130 °C) and is rigid and strong, suiting milk bottles, crates and pipes.
A sample of PTFE has an average molar mass of $2.5 \times 10^{5}$ g mol⁻¹. Calculate the average number of monomer units per chain, and explain why PTFE is used to coat frying pans.
Monomer CF₂=CF₂: $M = 2(12.01) + 4(19.00) = 100.02$ g mol⁻¹. $n = \dfrac{2.5\times10^{5}}{100.02} \approx 2.5\times10^{3}$ monomer units.
The very strong C–F bonds and the sheath of fluorine atoms make PTFE chemically inert and give it very weak interactions with other substances, so food doesn't stick and the surface has very low friction. Its high melting point (~327 °C) means it stays solid at normal cooking temperatures.