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A polymer is sometimes defined as a macromolecule with MW larger than 1000 g/mol. So can graphene be considered a polymer?

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    $\begingroup$ If you have access, this review article might be a useful read - "Two-Dimensional Polymers: Just a Dream of Synthetic Chemists?". The definition of what constitutes a 2D polymer is still up for debate, but you can defend that graphene is indeed an example of one. $\endgroup$ Oct 2, 2015 at 8:23
  • $\begingroup$ According to your logic, every defect-free piece of metal (above the nanoscale) would also be a polymer… It consists of many (!) repeating units and its macroscopic order can be described very well. If this doesn't count for you, take silicon as it's used for computer chips in wafers. This would be a polymer either. Do you get my point? $\endgroup$
    – Nepumuk
    Jun 20, 2023 at 18:48
  • $\begingroup$ I have heard of graphite being called a polymer and this describes it as such shankharajsarkar.wordpress.com/2017/01/23/… "graphite is a polymer of graphene." $\endgroup$
    – barlop
    Dec 6, 2023 at 19:19

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Wow. Good question. Graphene is not considered a polymer in the traditional sense. While it is used a doping agent for other polymers it does not have the designation of being one itself, the allotropes of carbon, at least in my experience and that of my adviser, are typically in a class of their own. Unique structures such as buckminsterfullerene and diamond differ from graphene only by the angles between the carbon atoms, 3-d shape, (and number of carbons present). All of which are definitely not considered polymers. Also remember that a macromolecule is composed of polymers. http://www.macrogroup.org.uk/schools/polymer_chemistry.php

https://answers.yahoo.com/question/index?qid=20100906193125AAzb9T0

So the short answer would be no, graphene is not considered a polymer.

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    $\begingroup$ What is so special about the allotropes of carbon? Also, where would you put elementary silicon? Is it a polymer, or does it also form a class of its own? $\endgroup$ Oct 2, 2015 at 7:28
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The common usage of the word "polymer" does not quite match the definition of it as "a macromolecule composed of repeated subunits". Looks like that the definition contains a specification note, tacitly assumed by everybody but never stated explicitly, which says that the subunit in question (i.e., monomer) must exist as a molecule in its own right. That would include all plastics, but exclude covalent crystals like graphene, diamond, silicon, red phosphorus, boron nitride, silicon carbide, quartz, etc.

On the other hand, buckminsterfullerene is a molecule with double bonds, and if we break some of them and connect the molecules together, we obtain a thing which (while still being a carbon allotrope) is rightfully called polymerized fullerene.

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I think it boils down to semantics at the end of the day. While it does seem a bit unnatural, I don't really see a problem with saying that graphene is a 2D polymer of repeating "C" units.

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The version of definitions to molecules, macromolecules and polymers that I accept is the following.

Macromolecules are defined as molecules with molecular weight larger than $10^4$ D. The limiting value $10^4$ is not meant to be strictly sharp. This definition emphasizes more on the following points

  1. Macromolecules should be molecules. There are other types of pure substances that do not consist of molecules, and their "molecular weight" cannot be defined. Graphene fails at this point when it wants to be a macromolecule. This will be explained under the definition of molecules.
  2. The molecular weight of macromolecules should exceed some very large value so that their physical properties are qualitatively different, which require additional theories and calls for a new field of study. For example, solutions of macromolecules often exhibit non-Newtonian flowing behavior, which requires an independent field called rheology to the already existing field of fluid mechanics. This idea clear up many other arguments. For example, I saw someone asking whether benzene is an oligomer of ethylene. But distinguishing benzene as such does not cultivate any new study.

Molecules are electronically neutral entities formed by one or more atoms solely by covalent bonds. A molecule should be the smallest unit of a pure substance that can exist (at least theoretically) independently and can retain all the physical and chemical properties of that substance.

Although allotropes of elemental substances of carbon (e.g., diamond, graphite, fullerene, carbon nanotubes, graphene...) are all composed of covalent bonds, only a few types are molecular substances.

For example, graphene failed to be a molecule because its smallest unit that preserves the physical/chemical property of graphene (which may be three sp3 carbons cut from any graphene sheet) do not exist in the form of electronically neutral entity. Graphene does not "melt" into a liquid of such a thing.

In contrast, a molecule of H2O is the smallest existing unit that maintains the physical chemical property of the substance water.

This "smallest property-maintaining unit" constraint is not seen everywhere, but I think this is quite important and useful.

One can think of still many ways to increase the molecular weight of a molecule to the extent that it becomes a macromolecule, without making it not a molecule anymore. But the most feasible way is forming a covalently bonding chain of atoms long enough for its molecular weight to exceed $10^4$ (which defines the concept of long-chain molecules). DNA molecules (single-strand DNA) are often examples. But they are not polymers.

Polymers are long-chain molecules formed by repeating units.

And therefore single-strand DNA molecules are not polymers. DNA molecules are formed not by repeating, but according to a code of, the four nucleobases.

But let's see from an example why long-chain molecules are molecules in the sense that they are "the smallest unit of a pure substance that can exist independently and can retain all the physical and chemical properties" of a polymeric substance:

What smallest unit determines the melting point of a polymer? Does it need to be a whole chain of molecules? Or only the nature of its repeating units? The latter is correct at the first thought and excludes polymers from being molecules. But when we say that low-density polyethylene has a lower melting point (106°C to 112°C) than its high-density counterpart (120 to 130 °C), no difference in repeating unit is taking effect here.

However, the convention that polymeric "pure substance" does not strictly require all molecules to be the same molecular weight still creates some conceptual embarrassment in keeping polymers belonging to molecules.

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