Addition polymerisation of ethene

A polymer is a large molecule made by joining many smaller molecules called monomers. In addition polymerisation, many ethene monomers join to form a long chain called poly(ethene).

Each ethene molecule, $$\ce{CH2=CH2}$$, has a carbon–carbon double bond. During polymerisation, this double bond opens: it becomes a single bond, and each carbon can form a new bond to a carbon in a neighbouring unit. As more ethene molecules join, a long carbon chain forms.

The bracket shows the repeating unit of poly(ethene): the section of the chain derived from one ethene molecule. The bonds extending through the brackets show where the unit connects to its neighbours. The symbol $$n$$ represents the large number of monomers joined, and therefore the number of repeating units in the chain.

Environmental impacts of plastic disposal

Plastics are made from polymers, which are long molecules built from repeating units. Many plastics are durable and resist breaking down. These properties make them useful, but they also mean that plastic waste can remain in the environment for a long time.

Plastic waste in landfill and oceans

When plastic is sent to a landfill site, it may remain there for many years because it breaks down slowly. Plastic waste can also accumulate in oceans. Marine animals can become entangled in plastic or mistake it for food, causing injury or death.

Burning plastic waste

Burning is another way to dispose of plastic waste, but some plastics can produce toxic gases when burned. These gases can pollute the air and harm living organisms. The environmental challenge is therefore connected to the same durability that makes plastics useful: they are difficult to break down after use, while burning them can cause air pollution.