Alkenes can be used to make polymers.
Polymers are very large molecules made when many smaller molecules join together, end to end. The smaller molecules are called monomers.
lots of monomer molecules → a polymer molecule
The polymers formed are called addition polymers.
This slideshow shows how several chloroethene monomers can join end to end to make poly(chloroethene), also called PVC:
Alkenes can act as monomers because they are unsaturated: