How are alkenes used to make polymers?
Pressure and heat energy open the double bonds in the alkene and make it bond to form large polymer molecules.
Alkenes are unsaturated hydrocarbons, meaning they don't have all the hydrogen that they could do, because of the double bond between some of the carbons.
Under large amounts of energy - from heat and pressure specifically - the double bond in the alkene can open and bond to other now-opened alkenes in a polymerisation reaction.
You can see the double bond opening in the image to form polyethene, a common plastic.