Alkenes like ethene can undergo addition polymerisation.

They can join end to end …

… to make poly(ethene), the simplest addition polymer.

Ethene is the monomer for poly(ethene), also called polythene.

This is the repeating unit for poly(ethene).

Propene is the monomer for poly(propene), also called polypropylene.

This is the repeating unit for poly(propene).

Chloroethene (vinyl chloride) is the monomer for poly(chloroethene), also called polyvinylchloride or PVC.

This is the repeating unit for poly(chloroethene).

Polymer molecules can grow outwards from the surface of a Ziegler–Natta catalyst like a growing hair.

Let's look at poly(ethene), a polymer made from ethene monomers like this one.

An intermediate structure forms.

The ethene molecule bonds to the polymer chain and the catalyst.

The ethene molecule bonds to the polymer chain and the catalyst.

The conformation changes ready for the next ethene monomer molecule.

The conformation changes ready for the next ethene monomer molecule.

The conformation changes ready for the next ethene monomer molecule.

The conformation changes ready for the next ethene monomer molecule.

The conformation changes ready for the next ethene monomer molecule.

The conformation changes ready for the next ethene monomer molecule.

Another monomer molecule is ready to join the growing polymer chain.

An intermediate structure forms.

The conformation changes ready for the next ethene monomer molecule.

The conformation changes ready for the next ethene monomer molecule.

The conformation changes ready for the next ethene monomer molecule.

The conformation changes ready for the next ethene monomer molecule.