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In one of my classes, my teacher told me that benzene can be formed by cyclic polymerization of ethyne. But I think that benzene isn't a polymer because only 3 ethyne molecules are combining to form benzene whereas, thousands of monomers are combined to form a polymer in a polymerization reaction. So, after all, can benzene be called a polymer of ethyne?

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    $\begingroup$ To be precise, benzene is a cyclic trimer of ethyne. Trimer is also a polymer which has 3 constituent units or monomer. It is not necessary that polymer should contain thousand of monomer. $\endgroup$ – Nilay Ghosh Aug 13 at 13:35
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    $\begingroup$ This is essentially a question of "how many stones form a heap". To me, three is not many, hence a trimer is not a polymer. $\endgroup$ – Ivan Neretin Aug 13 at 13:36
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    $\begingroup$ @IvanNeretin trimer can be technically called a polymer whose degree of polymerization is 3. Now, it is a matter of convenience whether one wants to consider trimer a polymer. $\endgroup$ – Nilay Ghosh Aug 13 at 13:40
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    $\begingroup$ Oligo (few) perhaps; poly (many), I don't think so. Definitely trimer. $\endgroup$ – user55119 Aug 13 at 14:04
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    $\begingroup$ @NilayGhosh But a minimum of several dozen! Oligomers become polymers when the chemical properties of neighbouring $n$-mers are indistinguishable, partly because they become also unseparable. A trimer is a distinct chemical compound, not a polymer! $\endgroup$ – Karl Aug 13 at 20:04
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The scientist who coined the term polymer, Jöns Jacob Berzelius, used the word to refer to different substances which had the same empirical formula. In this sense, benzene is a polymer of ethyne (acetylene), because benzene $\ce{C6H6}$ and ethyne $\ce{C2H2}$ have the same empirical formula, namely $\ce{CH}$. In other words, all substances with molecular formula $\ce{(CH)_x}$ were polymers of the smallest unit in the family (in this case, $\ce{C2H2}$, because $\ce{CH}$ molecules are only known as extremely fleeting species on Earth, though they are abundant in space).

However, that was back in the 1830s, when we had no clue about atomic and molecular structure (in 1808, one of the eminent chemists of the time, John Dalton, defended $\ce{HO}$ as the formula for water). The meaning of the term has evolved over time to accommodate our increase in knowledge. The most current IUPAC recommendations on polymer nomenclature (Compendium of Polymer Terminology and Nomenclature - IUPAC Recommendations 2008) states the definition of polymer as:

A polymer is a substance composed of macromolecules.

And, fortunately, they also define a macromolecule as:

Molecule of high relative molecular mass, the structure of which essentially comprises the multiple repetition of units derived, actually or conceptually, from molecules of low relative molecular mass.

Note 1: In many cases, especially for synthetic polymers, a molecule can be regarded as having a high relative molecular mass if the addition or removal of one or a few of the units has a negligible effect on the molecular properties. This statement fails in the case of certain properties of macromolecules which may be critically dependent on fine details of the molecular structure, e.g., the enzymatic properties of polypeptides.

Note 2: If a part or the whole of the molecule has a high relative molecular mass and essentially comprises the multiple repetition of units derived, actually or conceptually, from molecules of low relative molecular mass, it may be described as either macromolecular or polymeric, or by polymer used adjectivally.

Note 3: In most cases, the polymer can actually be made by direct polymerization of its parent monomer but in other cases, e.g., poly(vinyl alcohol), the description ‘conceptual’ denotes that an indirect route is used because the nominal monomer does not exist.

As you can see, there is quite a bit of detailing to try to keep things consistent. Observe that the definition speaks of high relative molecular mass, and this point is expanded upon in Note 1. With these considerations, benzene is not a polymer - it is certainly a very small molecule, and adding or removing a portion of it (presumably cyclooctatetraene $\ce{C8H8}$ and cyclobutadiene $\ce{C4H4}$, respectively) would result in a substance with drastically different properties.

In some sense, Berzelius wasn't really wrong to call benzene a polymer, because for all he knew back then, benzene could well have the formula $\ce{C20000H20000}$ (actually, it was not widely believed that "large" molecules could exist at all, with some resistance to the idea existing as late as the 1920s-1930s). He did the best with what knowledge was available to him. But now we can do better, and it just is more convenient and self-consistent to not label benzene as a polymer.

This isn't to say that, however, that benzene and ethyne are completely chemically unrelated. There is in fact a close relationship, in that ethyne can efficiently undergo a [2+2+2] cycloaddition reaction to form benzene. This is not exploited industrially, however, as there are much cheaper (and safer) ways of making it.

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  • $\begingroup$ Here comes the question how small can a macromolecule of polymer be ? What is the least number of ethene molecules to create a macromolecule of polymer ? Like Ivan's number of stones for a heap. $\endgroup$ – Poutnik Aug 13 at 13:53
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    $\begingroup$ Ah yes, no matter where one draws the line, there will be arguments about what makes the cut and what doesn't. It's best to keep the boundary relatively soft and adjust as necessary depending on context. Regardless, this case is relatively clear in just about any reasonable context, so making a definitive statement is not that risky. $\endgroup$ – Nicolau Saker Neto Aug 13 at 13:58
  • $\begingroup$ So, if benzene isn't a polymer, how can this reaction be called a polymerization reaction? $\endgroup$ – Habib Aug 13 at 17:28
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    $\begingroup$ @Habib It is a very specific trimerisation reaction. Calling it a polymersiation is simply a misnomer. $\endgroup$ – Karl Aug 13 at 20:43
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Short answer: Yes and No, depending if we consider polymer(1) or polymer(2).

Long answer: The first step in decision "Is X A or not ?" must be clarification what we mean by "A".

Benzene is a cyclic trimer of ethyne. Trimers are a special case of polymers(1) as repeated monomer structures, being a superset of oligomers and polymers(2). Benzene is a polymer(1) with n=3.

OTOH, we consider polymers(2) as multiple repetitions of monomer unit, where repetition count $n \gg 1$. Such polymers(2) occur rather with a range of $n$ and no particular $n$ has a special, privileged status. Like a polymer plastic material may have $n$ in range 2000-4000.

In such a sense, benzene is not a polymer(2). Neither we say $\ce{N2O4}$ as the dimer of $\ce{NO2}$ is polymer(2), but we can still say it is a polymer(1).

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  • $\begingroup$ The IUPAC Gold Book defines "polymer: A substance composed of macromolecules." Nothing else. Your definition (1) is very unusual. . $\endgroup$ – Karl Aug 13 at 20:33
  • $\begingroup$ IUPAC definition is polymer(2) definition. Definition of macromolecule is very vague. $\endgroup$ – Poutnik Aug 13 at 20:50
  • $\begingroup$ Of course, but your definition (1) should not be used. It is a misnomer perpetuated by a few highschool teachers. $\endgroup$ – Karl Aug 13 at 20:52
  • $\begingroup$ It is not definition (1) versus (2). It is (1) and (2), a wider and a narrower sense. How do you call the superset of oligomers and polymers(2), what must be a useful term, as the boundary between the 2 subsets is very vague ? Perhaps polymer(1) has already assigned other name I am not aware of. $\endgroup$ – Poutnik Aug 13 at 21:19
  • $\begingroup$ There is no name for the superset (dimer-trimer-oligomer-polymer), or parts of it. I can also see no real use for such a term. Especially there is no use for an umbrella term that includes dimers/trimers and polymers. Polymers have end groups, specific n-mers do not. $\endgroup$ – Karl Aug 13 at 21:23
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A useful criterion I learned in organic chemistry: It is a polymer if removal of one or so monomers from the end does not change the properties, like viscosity significantly. You can use this rule of thumb to distinguish between oligomers and polymers in general.

So by this account, benzene is definitely not a polymer.

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  • $\begingroup$ This goes to the core without indulging in semantic and nomenclature. Conjugated polymers will be again special here... As far their electronic molecular properties are concerned the switch from oligomers to polymer will be clear as well. Perhaps it won't coincide with a rheological one, but in all cases there is physical significance. Plus one. $\endgroup$ – Alchimista Aug 22 at 10:18

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