Polyolefins are made from “olefins,” which you may know as alkenes. Olefin is an old term for an alkene that is still widely used in industry.
These compounds make up the majority of commercial polymers used today. If you consider the common recycled plastics, polyethylene (#2 and #4, depending on how the material is made), polyvinyl chloride (#3), polypropylene (#5), and polystyrene (#6). These are all examples. polyolefins
Polyolefins have been known for a long time, although it took about a century after they were first documented to be recognized as polymers. German chemists in the mid-19th century knew that the resins of certain trees eventually formed solids, including polystyrene.
If you compare the repeating structure of polystyrene to the structure of styrene, you can imagine that polystyrene is made up of a bunch of styrene molecules bonded together. Essentially, the double bond of the styrene has moved over and attached itself to the next molecule, the double bond there has done the same, and so on.
Polyolefins are formed in a similar manner from a wide range of alkenes, resulting in a variety of materials with properties well suited to unique applications.
Note that, like ring-opening polymerization, olefin polymerization relies on molecules interacting with other molecules that are very similar to them. This is unlikely to happen. They need something to come along and cause a reaction among them. So olefin polymerization is another case of monomers joining together via a chain reaction. To get the reaction going, an initiator is needed. This initiator potentially becomes a terminal group hanging over one end or the other of the polymer chain.
