"Polyethylene is a thermoplastic polymer consisting of long chains produced by combing the ingredient 
monomer ethylene (IUPAC name ethene), the name comes from the ingredient and not the actual chemical resulting. The ethylene actually converts to ethane as it takes its place in a polymer and straight sections of the polymer are the same structure as the simple chain hydrocarbons, e.g., propane, decane and other straight single-bonded carbon chains. As with any polymer, the structure of the resulting substance defies molecular description due to cross branching of the chains. The recommended scientific name polyethene is systematically derived from the scientific name of the monomer.[2][3] In certain circumstances it is useful to use a structure-based nomenclature; in such cases IUPAC recommends poly(methylene)[3] (poly(methanediyl) is a non-preferred alternative[4][5]). The difference in names between the two systems is due to the opening up of the monomer's double bond upon polymerization."