In organic chemistry, an “olefin” or “alkene” is a molecule containing at least one double bond between carbon atoms. Polyethylene (PE) and polypropylene (PP) are “poly” olefins. The double bond is the means by which these molecules are joined together in a long polymer chain.
Ethylene “mers” can be strung together like beads on a string to form polyethylene. Different kinds of polyethylene can be made, depending on the number of “mers” in the “polymer.” A single polyethlylene chain may contain thousands or tens of thousands of carbon atoms.