I understand that the monomer that polymerises to give polyethylene is an ethylene, but after polymerisation, the smallest repeating monomer in polyethylene seems to be a methyl and not an ethyl. So why don’t we call it polymethylene instead?
According to the Compendium of Polymer Terminology and Nomenclature (IUPAC Recommendations 2008, i.e. the ‘Purple Book’), three different types of names can be used for polymers:
- traditional names,
- structure-based names, and
- source-based names.
When traditional names fit into the general pattern of systematic nomenclature, they are retained, in this case: polyethylene (PE)
The systematic name for a polymer requires the naming of a preferred constitutional repeating unit (CRU). If necessary, this basic name is then modified by prefixes. The systematic structure-based name for polyethylene according to the Purple Book is indeed poly(methylene).
Polymers can also be named as being derived from a monomer (or precursors), which is named according to IUPAC rules. Such names are referred to as source-based names. The systematic source-based name for polyethylene according to the Purple Book is polyethene.
Polyethylene is prepared by polymerizing ethylene. Ethylene has a double bond, thanks to which the polymerization goes on. You could not do that with a methylene group. So the name comes after the smallest unit which is actually used to prepare the polymer.