Ethene molecules can open their double bonds and join to form polyethylene. Many other monomers join in the same way. They have the form \mathrm{CH_2{=}CHX}, in which one hydrogen atom of ethene is replaced by another atom or group of atoms, X.
The group \mathrm{CH_2{=}CH{-}} is called the vinyl group, so monomers of this kind are called vinyl compounds, as in the names vinyl chloride and vinyl acetate.
When they polymerize, the double bonds open and the carbon atoms join into a long chain, with a group X on every second carbon atom. The repeating unit is \mathrm{{-}CH_2{-}CHX{-}}, and the polymer is written \mathrm{{-}[CH_2{-}CHX]}_n\mathrm{{-}}.
The chain itself is the same in all these polymers, so their properties depend largely on the group X.
The table shows five monomers of this kind. A polymer is named by putting poly in front of the name of its monomer, usually the traditional name, as in polypropylene and poly(vinyl chloride).
Polypropylene is used for containers and ropes, poly(vinyl chloride) for pipes and hoses, and polystyrene for packaging and, as a foam, for heat insulation.
Polyacrylonitrile is spun into acrylic fibres, and poly(vinyl acetate) is used in adhesives and paints.
Some monomers have two groups in place of hydrogen atoms. Methyl 2-methylpropenoate (methyl methacrylate), \mathrm{CH_2{=}C(CH_3)COOCH_3}, gives a clear, hard polymer used as acrylic glass.
In tetrafluoroethene (tetrafluoroethylene), \mathrm{CF_2{=}CF_2}, all four hydrogen atoms are replaced by fluorine atoms. It gives polytetrafluoroethylene (PTFE), which is used for non-stick coatings.
Addition polymerization is usually started by a small amount of an initiator, a compound that easily splits into radicals. A radical is an atom or group of atoms with an unpaired electron, and it reacts very readily.
A radical uses its unpaired electron to open the double bond of a monomer and bond to one of its carbon atoms. The other carbon atom is left with an unpaired electron, so the end of the chain is again a radical.
The new radical end adds the next monomer in the same way, and the chain grows very quickly to thousands of repeating units. It stops growing when, for example, it meets the radical end of another chain and the two ends join.
Polyethylene made at high pressure with radicals has many short side branches. The branched chains cannot pack closely, so this low-density polyethylene is soft and flexible and is used for bags and films.
Polyethylene made at low pressure with special catalysts has almost no branches. Its chains pack closely into larger crystalline regions, so this high-density polyethylene is harder and stronger and is used for bottles and containers.
Both are made of the same repeating unit \mathrm{{-}CH_2{-}CH_2{-}}; they differ only in the shape of their chains.
Poly(vinyl alcohol) cannot be made from its own monomer, because vinyl alcohol, \mathrm{CH_2{=}CH{-}OH}, changes at once into its isomer ethanal (acetaldehyde), \mathrm{CH_3{-}CHO}.
Instead, vinyl acetate is polymerized first, and the poly(vinyl acetate) is then hydrolysed with sodium hydroxide. This changes each ester group \mathrm{{-}OCOCH_3} into a hydroxy group \mathrm{{-}OH}.
Poly(vinyl alcohol) has many hydroxy groups and dissolves in water. To make a fibre from it, it is treated with methanal (formaldehyde), which links pairs of neighbouring hydroxy groups into rings containing \mathrm{{-}O{-}CH_2{-}O{-}}.
This reaction is called acetalization, and it stops the fibre from dissolving. The fibre obtained is vinylon, developed in Japan in 1939. Some of its hydroxy groups are left unchanged, so it absorbs moisture well, like cotton.