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In polymers the bonds between monomers are changed, usually from double to single1, to form a long-chain polymer with the now single bond joining another monomer. But in the monomers at the very ends of a polymer chain, there is an unfilled bond, a carbon atom with only three bonds filled. During the polymerization process are those unfilled bonds left unfilled, or is there some process that fills those bonds with, say, another hydrogen atom?


  1. I say this because I don't know if there are any polymers made where the bond inside the original monomer is a triple bond. In all the commonly used plastics (those that have recycling symbols) the original monomers have double bonds between the carbon atoms.
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  • $\begingroup$ You can polymerise ethine, but the problem is the same. You need to find out how the termination step of your polymerisation works. There are numerous possibilities for every of the different polymerisation types. $\endgroup$ – Karl Oct 22 '18 at 20:39
  • $\begingroup$ And initiation step for another end. $\endgroup$ – Mithoron Oct 22 '18 at 21:13
  • $\begingroup$ @Karl, I didn't realize there was a termination step involved in polymerization. Of course, that makes sense. $\endgroup$ – BillDOe Oct 23 '18 at 0:39
  • $\begingroup$ Apparently, this question got a couple of downvotes. I'd be curious as to why. Was the question worded in some way that made it ambiguous? If you thought it was stupid, why? If you thought it was ignorant, well...yeah. That's why I asked it. $\endgroup$ – BillDOe Oct 23 '18 at 0:52
  • $\begingroup$ The term "(un)filled bond" is a bit unorthodox. ;-) The term you are looking for is valence. en.wikipedia.org/wiki/Valence_(chemistry) $\endgroup$ – Karl Oct 23 '18 at 17:44
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You are asking about polymers made by radical polymerization.

But in the monomers at the very ends of a polymer chain, there is an unfilled bond, a carbon atom with only three bonds filled.

During polymerization this is true, the radical will continue to react with monomer and propagate the polymerization, but the radical does not sit in the bulk. There are a number of ways which an additional atom amy bond with the carbon radical in a process called termination. Typically this is done by combination or inhibition.

Combination is the process where two radicals come together to form a bond leaving no free radicals to continue the reaction. This may occur when two radical chains combine or when a selected radical is added to cease the reaction. This radical could be the other half of the initiator which began the radical polymerization.

Chain combination
Combination of two polymerizing chains.

Chain Radical Combination
Combination of polymerizing chain with radical

For inhibition, a radical combines with a molecule that can form a stable radical such as oxygen or butylated hydroxytoluene (BHT).

enter image description here
Radical chain combining with oxygen to form stable, non-ractive radical.

In these cases the radical will eventually find a stray radical or atom to bond with (like water), the unstable carbon radical will not exist permanently.


I say this because I don't know if there are any polymers made where the bond inside the original monomer is a triple bond. In all the commonly used plastics (those that have recycling symbols) the original monomers have double bonds between the carbon atoms.

Polyacetylene uses a triply bonded monomer in its production.

enter image description here

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  • $\begingroup$ The issue is not limited to radical polymerisation. All chain reactions have an active centre that needs to be terminated in the end. $\endgroup$ – Karl Oct 23 '18 at 19:38
  • $\begingroup$ @Karl yes, but because the OP said "* there is an unfilled bond, a carbon atom with only three bonds filled*", I'm lead to believe they are talking about radical polymerization. $\endgroup$ – A.K. Oct 23 '18 at 20:38
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All polymerisation reactions must be started (by creating an active centre), then the polymer chains grows (by attacking, in most cases, a C=C bond, which then becomes part of new active centre that get's shifted down the chain), and then the chain must terminate, either by recombination with another radical (in case of a radical polymerisation), or by transfer of the active centre (radical, anion, cation, metal complex) to another molecule.

The latter can lead to a new active (polymerising) chain (a chain transfer reaction) or to the formation of an inactive species (actual termination of the polymersiation reaction).

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