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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?

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    $\begingroup$ Because methylene is not a compound in its own right. Who said we must name the polymer after the smallest repeating unit? $\endgroup$ – Ivan Neretin Jun 29 '18 at 7:15
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    $\begingroup$ Well that’s what my polymers professor said, and that seems to be the trend for all other polymers (as far as I have learned). $\endgroup$ – P. SN Jun 29 '18 at 7:16
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    $\begingroup$ For all other polymers, the smallest repeating unit is the same as the molecule that was polymerized. For polyethylene it is different. $\endgroup$ – Ivan Neretin Jun 29 '18 at 7:19
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    $\begingroup$ @IvanNeretin it is, actually, a compound in its own right, albeit a radical and thus hard to handle. $\endgroup$ – leftaroundabout Jun 29 '18 at 14:07
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    $\begingroup$ @IvanNeretin "Who said we must name the polymer after the smallest repeating unit?" – The IUPAC Recommendations 2008 said that. $\endgroup$ – Loong Jun 29 '18 at 15:35
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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.

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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.

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    $\begingroup$ Although this certainly is the reason, I have to remark that it's a bit weird to name a compound after how it's synthesized in practice. Who's to say that a better process isn't developed in the future that will start from, say, hex-1-ene? Surely the product shouldn't suddendly be renamed to polyhexylene, if it has the same chemical structure as polyethylene. And although I reckon it will never be feasible to polymerise this from $\mathrm{CH_2}$, it's certainly conceivable. $\endgroup$ – leftaroundabout Jun 29 '18 at 14:04
  • $\begingroup$ @leftaroundabout: I think it's a bit like the fact that "sodium bicarbonate" is so called because the ratio of carbonate to sodium is double that of sodium carbonate. In many cases, substances get named before they're fully understood, and the names stick whether or not they accurately "describe" the substance in question. $\endgroup$ – supercat Jun 29 '18 at 20:09

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