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IUPAC defines a monomer as,

A molecule which can undergo polymerisation thereby contributing constitutional units to the essential structure of a macromolecule.

Bearing this in mind, I thought about molecules which exhibit hydrogen bonding. Take water for example: one water molecule bonds to another, and that one bonds to another, and another, etc. In the end, you'll have a long chain of water molecules bonded together. This process of water molecules coming together and hydrogen bonding sounds like they 'polymerise' in some way.

To explain my point, consider an infinite amount of water. Here, I'd imagine there to be an infinitely long chain of water molecules bonded together. This sounds like usual polymers I see, where there's an infinitely long chain of repeat units. However, the polymerisation is different for those.

Hence, I have two questions: could a single water molecule be described as a monomer? And, could a long chain of bonded water molecules be described as a polymer?


P.S. This doesn't just have to apply to water. The same could be said about a single alcohol molecule/a long chain of bonded alcohol molecules.

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    $\begingroup$ Hydrogen bonds are not actual bonds. $\endgroup$ – a-cyclohexane-molecule Jul 26 '18 at 21:24
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    $\begingroup$ @a-cyclohexane-molecule Of course hydrogen bonds aren't actual bonds, and I never said they were. That's why I said 'it sounds like they polymerise in some way'. However, water molecules still interact in such a way that they 'bond' together, hence my thinking where they could form a 'bonded' chain. $\endgroup$ – resno66 Jul 26 '18 at 21:36
  • $\begingroup$ I understand. Regardless, I pointed it out because there's an important distinction between bonds and strong intermolecular interactions. What you describe sounds like an unusual description of phase transitions; in your particular case, condensation of water from water vapor. $\endgroup$ – a-cyclohexane-molecule Jul 26 '18 at 21:52
  • $\begingroup$ @a-cyclohexane-molecule Phase transitions aren't what I was trying to describe, but I agree it does sound a bit like that. However my question was simply whether water molecules could form a very long chain due to hydrogen bonding, and if so, whether you could describe this as being polymer-like. $\endgroup$ – resno66 Jul 26 '18 at 22:07
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    $\begingroup$ There is no such thing as polywater but I'm old enough to remember the buzz. $\endgroup$ – MaxW Jul 26 '18 at 23:27
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There are actually two ways to answer this question: by lawyering or by trying to capture what a polymer is. I'll try both.

Answer by lawyer

The IUPAC defines the following terms as follows:

  • monomer: a molecule which can undergo polymerisation thereby contributing constitutional units to the essential structure of a macromolecule
  • macromolecule: A 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.
  • molecule: An electrically neutral entity consisting of more than one atom ($n > 1$). Rigorously, a molecule, in which $n > 1$ must correspond to a depression on the potential energy surface that is deep enough to confine at least one vibrational state.

Using the data from this paper and a harmonic oscillator approximation, I estimate that the zero-point energy of a harmonic oscillator with two waters and a hydrogen bond is just over 45 kJ/mol.

Since the energy contributed by hydrogen bonding in liquid water is estimated to be between 2kJ/mol and 6kJ/mol, I conclude that the hydrogen bond of water cannot confine a vibrational state. This means liquid water does not qualify as a single molecule.

Since a long chain of waters connected by hydrogen bonds does not qualify as a molecule, it cannot be a polymer, which means that water cannot be a monomer (at least, not in this sense).


A better answer

The real problem here isn't the technical stuff I wrote above (which, by the way, probably has a few holes large enough to sneak trucks through). It's that words have certain meanings, and we can't capture everything in a simple definition. For the same reason that you'd probably feel a bit weird calling "yellow" a "reddish-green", even though it is literally the result of mixing red and green light, it feels strange to call water a polymer.

Here are some common ideas that spring to mind when I see the word "polymer":

  • Made of organic monomers bonded together covalently
  • Structure of covalent bonds (e.g. branching) affects physical properties
  • Intermolecular forces affects physical properties

These are not strict, but they explain why quartz is not referred to as a polymer, even though it's made of covalently-bonded $\ce{SiO2}$ units: everything is covalently bonded and there aren't any intermolecular interactions to speak of! For another example, depending on the exact context, proteins are sometimes rarely thought of as polymers, even though they're made of repeating amino acids, because their structure is fixed (must be linear chain) and their intermolecular forces don't contribute much.

So the more sensible answer to this question that doesn't rely on lawyering is that liqid water just doesn't have the properties that most people think of when they hear the word "polymer" and so it isn't usually referred to as a polymer.

Supplemental Materials

The code I used to compute the harmonic oscillator energies

from math import pi,sqrt
# Compute the harmonic oscillator energy of two water molecules
# using a two-body QHO approximation
kg_per_amu = 1.66054e-27
watermass = 18.0 * kg_per_amu # Mass of water: close enough for now
reducedmass = watermass**2 / (2 * watermass)

hbar = 1.054571800e-34   # J-s
k = 845.4 # From https://www.princeton.edu/~fhs/paper79/paper79.pdf, N-m

E_zeropt = hbar * pi * sqrt(k / reducedmass) # for n = 0
E_zeropt_kjm = E_zeropt * 6.022e23 / 1000  # Scale from J/molecule to kJ/mol
print(E_zeropt_kjm)
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  • $\begingroup$ Nice answer. Basically, it wouldn't be wrong per se to say that a chain of hydrogen bonded water molecules could technically be a described as a polymer, but instead it's simply a 'bit weird' and rarely done due to commonplace ideas of what a polymer should be? $\endgroup$ – resno66 Jul 27 '18 at 10:57
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    $\begingroup$ @resno66 - Neither liquid water nor solid water is a polymer. Not at all. Stop trying to force fit water into some definition of as polymer. It just isn't. $\endgroup$ – MaxW Jul 27 '18 at 13:17
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    $\begingroup$ @MaxW Stop being arrogant. I'm not trying to 'force fit' anything. All I've done is discussed my thoughts on how water sounds like* it could form some long chained 'polymer', and not once did I say that's how water actually behaves. It was simply an interesting thought which I shared. There's no force fitting of any kind. P.S. The read on 'polywater' was interesting - I was meant to mention this above earlier. $\endgroup$ – resno66 Jul 27 '18 at 13:28
  • $\begingroup$ @resno66 The fundamental problem is that in a polymer, the bonds need to be covalent or near-covalent in nature, and hydrogen bonds just aren't strong enough to do that (in the "lawyer" section, I show that the bonds need to be 10x stronger to even come close to qualifying). So I'd say it's technically wrong to call a chain of water molecules a polymer...(cont) $\endgroup$ – chipbuster Jul 27 '18 at 18:09
  • $\begingroup$ I wrote the second part because there are a few issues in the technical section, and even if they turn out to be majorly problematic, I still would not call water a polymer, and those are the "more fuzzy" reasons why I would not do so. $\endgroup$ – chipbuster Jul 27 '18 at 18:11

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