Small links, long chains

Section 1 of 4

A plastic bag, a nylon rope and a polyester shirt are all made of the same kind of molecule: a very long chain built from many small molecules joined together.

Each small molecule is a monomer (mono- means one). Thousands of monomers join end to end into one giant molecule, a polymer (poly- means many). Think of paper clips hooked into a chain, or beads on a string: each clip or bead is a monomer, and the whole chain is the polymer.

Ethene monomers join into a poly(ethene) chain Top: three separate ethene molecules, each with two carbon atoms joined by a double bond. One line of each double bond is marked in amber. An arrow labelled join points down to a long chain of carbon atoms, each with two hydrogen atoms, that carries on past both edges. The bonds that now join one ethene to the next are amber. monomers: ethene join polymer: poly(ethene)
Ethene monomers join end to end. One of the two bonds in each C=C opens (amber, top), and the new bonds between neighbouring molecules (amber, bottom) build one long chain.

The monomer here is ethene, C₂H₄, the two-carbon molecule with a C=C double bond. Its polymer is poly(ethene), also called polythene or polyethylene, the most widely made plastic in the world. The name is the monomer's name in brackets after poly-.

A single poly(ethene) molecule often has more than 10,000 carbon atoms in a row. That makes it thousands of times longer than it is wide: if it were as thick as a strand of spaghetti, it would be several metres long.

Quick check

Poly(ethene) is made by joining thousands of ethene molecules end to end. In this reaction, what is ethene?