Degree of polymerization with structure-based polymer naming

Question

Polymers may be named using either a structure-based or a source-based nomenclature, as detailed in the IUPAC Purple Book.[1] Therefore, the polymer descibed by $\require{enclose}\ce{\enclose{horizontalstrike}{\;(}CH2\enclose{horizontalstrike}{)_n}}$ may be called either poly(methylene) or polyethene (see also this recent question). Both names are perfectly fine and by themselves do not imply how the polymer was prepared, so polyethene is not necessarily the product of a polymerization reaction.

By considering a methylene group as the repeating unit within the macromolecules in poly(methylene), it was unclear to me how the degree of polymerization (DP), a rather common number used to describe a polymer, should be used in this case. The Purple Book defines the DP as:

Number of monomeric units in a macromolecule an oligomer molecule, a block, or a chain.

Now, what is a monomeric unit?

Largest constitutional unit contributed by a single monomer molecule to the structure of a macromolecule or oligomer molecule.

Looking for the definition of constitutional unit gives:

Atom or group of atoms (with pendant atoms or groups, if any) comprising a part of the essential structure of a macromolecule, an oligomer molecule, a block, or a chain.

...and a monomer molecule is:

Molecule which can undergo polymerization, thereby contributing constitutional units to the essential structure of a macromolecule.

All definitions are from the Purple Book. So, as I understand it, the term degree of polymerization is directly linked to the process of polymerization, and in case of poly(methylene) the monomer molecules, i.e. the molecules which can undergo polymerization, are ethene molecules. Also, the monomeric unit in the definition of the DP is not the methylene group.

The question is: What would be the correct DP for a poly(methylene) consisting of macromolecules with 10000 methylene groups on average? From the definitions, I would say that it must be 5000, but this would be extremely counterintuitive in combination with the structure-based name, and it would be silly to have to reformulate a structure-based name into a source-based name just to be able to understand the DP. On the other hand, it would also be strange to give different DPs for poly(methylene) and polyethene with identical average molar masses just because of the naming convention used. I would be happy if you could help me to resolve this slightly pedantic issue.

Reference

1. Compendium of Polymer Terminology and Nomenclature, IUPAC Recommendations 2008, IUPAC Purple Book, 2nd edition, prepared for publication by Richard G. Jones, Edward S. Wilks, W. Val Metanomski, Jaroslav Kahovec, Michael Hess, Robert Stepto, Tatsuki Kitayama, RSC Publishing, 2009 [ISBN 978-0-85404-491-7]; https://doi.org/10.1039/9781847559425.

Answer 1

I would like to share some of my random thoughts on your problem.

First, when we define something, sometimes we list the exceptional conventions that do not comply with the definition. In my opinion, exceptions are acceptable if they can be completely listed. For example, if the case of polyethylene is all the exceptions of the definition of degree of polymerization, that's all right for me.

Secondly, you provided a good point in your text, that "the term degree of polymerization is directly linked to the process of polymerization, and in case of poly(methylene) the monomer molecules, i.e. the molecules which can undergo polymerization, are ethene molecules. Also, the monomeric unit in the definition of the DP is not the methylene group." Then the DP can be differently counted for the same polymer due to different uses of raw monomer and polymerizing process. This is meaningful if the concept of DP is indeed designed for evaluating the efficiency of polymerizing process, but not a quantification of the molecular weight of a polymer.

Therefore, if you want to discuss another polymer, polymethylene, for which the degree of polymerization is counted by the number of methylene units, you should first report how you polymerize it from methylene.

This may not be meaningless. Poly(ethylene glycol) (PEG) and poly(ethylene oxide) (PEO) are the same polymer. The names reflect the difference in how they are synthesized. This may be one example that the monomer and polymerization process is distinguished in nomenclature even if the resultant polymer is the same.

Therefore, if there is a synthetic route to the polymer -[-CH2-CH2-]- from the polymerization of methylene, the polymer chemists might agree to name it differently as polymethylene and for which the DP should be differently counted from how it is done for polyethylene.