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The monomer in polyethene seems to drop the $\ce{C=C}$ bond and end up with a $\ce{C-C}$ bond. The polymer chain then looks like $\ce{[CH2-CH2]_{n}}$. If I were to look at that polymer chain I would probably call it polyethane not polythene (or I could even call it polymethane) because it doesn't have a double bond, so I wouldn't classify it as an alkene. Am I doing something wrong with my analysis here? Or is there some sort of rule about nomenclature that change when things become polymers and drop a functional bond (in this case the $\ce{C=C}$ bond)?

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2 Answers 2

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The reason it is called polyethene (or polyethylene) is because it was produced from the monomer of ethylene. To produce polyethylene, a radical initiator is added to ethylene, which attacks one carbon and opens up the double bond leaving a radical at the end. This radical attacks another ethylene molecule, etc. The chain stops forming when it encounters another radical. So, from ethylene, we get polyethylene (even though it looks like polymethylene).

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    $\begingroup$ So we name it based on the monomer it was made from, not what the repeating unit actually looks like? $\endgroup$
    – sion
    Commented Sep 16, 2014 at 3:16
  • $\begingroup$ In most cases, an exception is polyvinylalcohol; it is not made from ethylaldehyde (tautomer of vinyl alcohol). $\endgroup$
    – LDC3
    Commented Sep 16, 2014 at 3:18
  • $\begingroup$ @Sam, I would think that proper IUPAC names would always derive from the monomer, as you say. Common names can be wildly variant, though--for example, while polycarbonates do contain a $-\ce{CO3}-$ type moiety, their monomers always are more complex than just plain carbonate. $\endgroup$
    – hBy2Py
    Commented Jun 20, 2015 at 3:47
  • $\begingroup$ don't you mean polymethylene? :P $\endgroup$
    – DrAzulene
    Commented Sep 8, 2016 at 18:31
  • $\begingroup$ The IUPAC name is always derived from the repeating unit, and polyethylene is the only exception where not the smallest repeating unit is taken. A (formal) CH2 diradical is called "methylene", C2H4 "ethylene" in IUPAC nomenclature. $\endgroup$
    – Karl
    Commented Mar 26, 2020 at 23:14
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There is not such correct term for either of the terms you've come up with. There is no polyethene. There is no polyethane, not in polymer or macromolecular chemistry and thinking.

Polyethylene (PET) a thermoset polymer (plastic) on the other hand, is ubiquitous.

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    $\begingroup$ Polyethene most certainly does exist, and is the polymer about which OP is talking. But correct, there is no such thing as polyethane $\endgroup$
    – joe_young
    Commented Aug 14, 2016 at 12:48
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    $\begingroup$ The common abbreviation for polyethylene is PE. PET is polyethylene terephthalate. $\endgroup$
    – user7951
    Commented Sep 8, 2016 at 18:34

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