NOMENCLATURE FOR ORGANIC POLYMERS

Robert B. Fox and Edward S. Wilks

Organic polymers have traditionally been named on the basis of the monomer used, a hypothetical monomer or a semi-systematic structure. Alternatively, they may be named in the same way as organic compounds, i.e., on the basis of a structure as drawn. The former method, often called “source-based nomenclature” or “monomer-based nomenclature,” sometimes results in ambiguity and multiple names for a single material. The latter method, termed “structure-based nomenclature,” generates a sometimes cumbersome unique name for a given polymer, independent of its source. Within their limitations, both types of names are acceptable and well-documented. (Ref. 1) The use of stereochemical descriptors with both types of polymer nomenclature has been published. (Ref.2)

Traditional Polymer Names

Monomer-Based Names

“Polystyrene” is the name of a homopolymer made from the single monomer styrene. When the name of a monomer comprises two or more words, the name should be enclosed in parentheses, as in “poly(methyl methacrylate)” or “poly(4-bromostyrene)” to identify the monomer more clearly. This method can result in several names for a given polymer: thus, “poly(ethylene glycol),” “poly(ethylene oxide),” and “poly(oxirane)” describe the same polymer. Sometimes, the name of a hypothetical monomer is used, as in “poly(vinyl alcohol).” Even though a name like “polyethylene” covers a multitude of materials, the system does provide understandable names when a single monomer is involved in the synthesis of a single polymer. When one monomer can yield more than one polymer, e.g., 1,3-butadiene or acrolein, some sort of structural notation must be used to identify the product, and one is not far from a formal structure-based name.

Copolymers, Block Polymers, and Graft Polymers. When more than one monomer is involved, monomer-based names are more complex. Some common polymers have been given names based on an apparent structure, as with “poly(ethylene terephthalate).” A better system has been approved by the IUPAC.(Ref. 1) With this method, the arrangement of the monomeric units is introduced through use of an italicized connective placed between the names of the monomers. For monomer names represented by A, B, and C, the various types of arrangements are shown in Table 1.

Table 2 contains examples of common or semi-systematic names of copolymers. The systematic names of comonomers may also be used; thus, the polyacrylonitrile-block-polybutadiene-block-polystyrene polymer in Table 2 may also be named poly(prop-2-enenitrile)-block-polybuta-1,3-diene-block-poly(ethenylbenzene). IUPAC does not require alphabetized names of comonomers within a polymer name; many names are thus possible for some copolymers.

These connectives may be used in combination and with small, non-repeating (i.e., non-polymeric) junction units; see, for example, Table 2, line 8. A long dash may be used in place of the connective -block-; thus, in Table 2, the polymers of lines 7 and 8 may also be written as shown on lines 9 and 10.

IUPAC also recommends an alternative scheme for naming copolymers that comprises use of “copoly” as a prefix followed by the names of the comonomers, a solidus (an oblique stroke) to separate comonomer names, and the addition before “copoly” of any applicable connectives listed in Table 2 except -co-.

Table 3 gives the same examples shown in Table 2 but with the alternative format. Comonomer names need not be parenthesized.

TABLE 1. IUPAC Source-Based Copolymer Classification

TABLE 2. Examples of Source-Based Copolymer Nomenclature

TABLE 3. Examples of Source-Based Copolymer Nomenclature (Alternative Format)

Source-based nomenclature for nonlinear macromolecules and macromolecular assemblies is covered by a 1997 IUPAC document. (Ref. 11) The types of polymers in these classes, together with their connectives, are given in Table 4; the terms shown may be used as connectives, prefixes, or both to designate the features present.

TABLE 4. Connectives for Nonlinear Macromolecules and Macromolecular Assemblies

Nonlinear polymers are named by using the italicized connective as a prefix to the source-based name of the polymer component or components to which the prefix applies; some examples are listed in Table 5.

TABLE 5. Nonlinear Macromolecules

Macromolecular assemblies held together by forces other than covalent bonds are named by inserting the appropriate italicized connective between names of individual components; Table 6 gives examples.

TABLE 6. Examples of Polymer Blends and Nets

Structure-Based Polymer Nomenclature

Regular Single-Strand Polymers

Structure-based nomenclature has been approved by the IUPA) (Ref. 4) and is currently being updated; it is used by Chemical Abstracts.(Ref. 5) Monomer names are not used. To the extent that a polymer chain can be described by a repeating unit in the chain, it can be named “poly(repeating unit).” For regular single-strand polymers, “repeating unit” is a bivalent group; for regular double-strand (ladder and spiro) polymers, “repeating unit” is usually a tetravalent group. (Ref. 9)

Since there are usually many possible repeating units in a given chain, it is necessary to select one, called the “constitutional repeating unit” (CRU) to provide a unique and unambiguous name, “poly(CRU),” where “CRU” is a recitation of the names of successive units as one proceeds through the CRU from left to right. For this purpose, a portion of the main chain structure that includes at least two repeating sequences is written out. These sequences will typically be composed of bivalent subunits such as -CH₂-, -O-, and groups from ring systems, each of which can be named by the usual nomenclature rules.(Refs. 6 and 7)

Where a chain is simply one long sequence comprising repetition of a single subunit, that subunit is itself the CRU, as in “poly(methylene)” or “poly(1,4-phenylene).” In chains having more than one kind of subunit, a seniority system is used to determine the beginning of the CRU and the direction in which to move along the main chain atoms (following the shortest path in rings) to complete the CRU. Determination of the first, most senior, subunit is based on a descending order of seniority: (1) heterocyclic rings, (2) hetero atoms, (3) carbocyclic rings, and lowest, (4) acyclic carbon chains.

Within each of these classes, there is a further order of seniority that follows the usual rules of nomenclature.

Heterocycles: A nitrogen-containing ring system is senior to a ring system not containing nitrogen. (Refs. 4 and 9) Further descending order of seniority is determined by:

• (i) the highest number of rings in the ring system
• (ii) the largest individual ring in the ring system
• (iii) the largest number of hetero atoms
• (iv) the greatest variety of hetero atoms

Hetero atoms: The senior bivalent subunit is the one nearest the top right-hand corner of the Periodic Table; the order of seniority is: O, S, Se, Te, N, P, As, Sb, Bi, Si, Ge, Sn, Pb, B, Hg.

Carbocycles: Seniority (Ref. 4) is determined by:

• (i) the highest number of rings in the ring system
• (ii) the largest individual ring in the ring system
• (iii) degree of ring saturation; an unsaturated ring is senior to a saturated ring of the same size

Carbon chains: Descending order of seniority is determined by:

• (i) chain length (longer is senior to shorter)
• (ii) highest degree of unsaturation
• (iii) number of substituents (higher number is senior to lower number)
• (iv) ascending order of locants
• (v) alphabetical order of names of substituent groups

Among equivalent ring systems, preference is given to the one having lowest locants for the free valences in the subunit, and among otherwise identical ring systems, the one having least hydrogenation is senior. Lowest locants in unsaturated chains are also given preference. Lowest locants for substituents are the final determinant of seniority.

Direction within the repeating unit depends upon the shortest path, which is determined by counting main chain atoms, both cyclic and acyclic, from the most senior subunit to another subunit of the same kind or to a subunit next lower in seniority. When identification and orientation of the CRU have been accomplished, the CRU is named by writing, in sequence, the names of the largest possible subunits within the CRU from left to right. For example, the main chain of the polymer traditionally named “poly(ethylene terephthalate)” has the structure shown in Figure 1.

FIGURE 1. Structure-based name: poly(oxyethyleneoxyterephthaloyl); traditional name: poly(ethylene terephthalate).

The CRU in Figure 1 is enclosed in brackets and read from left to right. It is selected because (1) either backbone oxygen atom qualifies as the “most senior subunit,” (2) the shortest path length from either -O- to the other -O- is via the ethylene subunit. Orientation of the CRU is thus defined by (1) beginning at the -O- marked with an asterisk, and (2) reading in the direction of the arrow. The structure-based name of this polymer is therefore “poly(oxyethyleneoxyterephthaloyl),” not much longer than the traditional name and much more adaptable to the complexities of substitution. As organic nomenclature evolves, more systematic names may be used for subunits, e.g., “ethane-1,2-diyl” instead of “ethylene.” IUPAC still prefers “ethylene” for the -CH₂-CH₂- unit, however, but also accepts “ethane-1,2-diyl.”

Structure-based nomenclature can also be used when the CRU backbone has no carbon atoms. An example is the polymer traditionally named “poly(dimethylsiloxane),” which on the basis of structure would be named “poly(oxydimethylsilylene)” or “poly(oxydimethylsilanediyl).” This nomenclature method has also been applied to inorganic and coordination polymers⁸ and to double-strand (ladder and spiro) organic polymers. (Ref. 9)

Irregular Single-Strand Polymers

Polymers that cannot be described by the repetition of a single CRU or comprise units not all connected identically in a directional sense can also be named on a structure basis. (Ref. 10) These include copolymers, block and graft polymers, and star polymers. They are given names of the type “poly(A/B/C…),” where A, B, C, etc., are the names of the component constitutional units, the number of which are minimized. The constitutional units may include regular or irregular blocks as well as atoms or atomic groupings, and each is named by the method described above or by the rules of organic nomenclature.

The solidus denotes an unspecified arrangement of the units within the main chain. (Ref. 10) For example, a statistical copolymer derived from styrene and vinyl chloride with the monomeric units joined head-to-tail is named “poly(l-chloroethylene/l-phenylethylene).” A polymer obtained by 1,4-polymerization and both head-to-head and head-to-tail 1,2-polymerization of 1,3-butadiene would be named “poly(but-1-ene-l,4-diyl/l-vinylethylene/2-vinylethylene).” (Ref. 12) In graphic representations of these polymers, shown in Figure 2, the hyphens or dashes at each end of each CRU depiction are shown completely within the enclosing parentheses; this indicates that they are not necessarily the terminal bonds of the macromolecule.

FIGURE 2. Graphic representations of copolymers.

A long hyphen is used to separate components in names of block polymers, as in “poly(A)—poly(B)—poly(C),” or “poly(A)—X—poly(B)” in which X is a non-polymeric junction unit, e.g., dimethylsilylene.

In graphic representations of these polymers, the blocks are shown connected when the bonding is known (Figure 3, for example); when the bonding between the blocks is unknown, the blocks are separated by solidi and are shown completely within the outer set of enclosing parentheses (Figure 4, for example). (Refs. 10 and 13)

FIGURE 3. polystyrene—polyethylene—polystyrene.

FIGURE 4. poly[poly(methyl methacrylate)—polystyrene—poly(methyl acrylate)].

Graft polymers are named in the same way as a substituted polymer but without the ending “yl” for the grafted chain; the name of a regular polymer, comprising Z units in which some have grafts of “poly(A),” is “poly[Z/poly(A)Z].” Star polymers are treated as a central unit with substituent blocks, as in “tetrakis(polymethylene)silane.” (Refs. 10 and 13)

Other Nomenclature Articles and Publications

In addition to the Chemical Abstracts and IUPAC documents cited above and listed below, other articles on polymer nomenclature are available. A 1999 article lists significant documents on polymer nomenclature published during the last 50 years in books, encyclopedias, and journals by Chemical Abstracts, IUPAC, and individual authors. (Ref. 14) A comprehensive review of source-based and structure-based nomenclature for all of the major classes of polymers, (Ref. 15) and a short tutorial on the correct identification, orientation, and naming of most commonly encountered constitutional repeating units were both published in 2000. Ref. 16).

References and Notes

1. International Union of Pure and Applied Chemistry, Compendium of Macromolecular Nomenclature, Blackwell Scientific Publications, Oxford, 1991.
2. International Union of Pure and Applied Chemistry, Stereochemical Definitions and Notations Relating to Polymers (Recommendations 1980), Pure Appl. Chem., 53, 733–752 (1981). [https://doi.org/10.1351/pac198153030733]
3. International Union of Pure and Applied Chemistry, Source-Based Nomenclature for Copolymers (Recommendations 1985), Pure Appl. Chem., 57, 1427–1440 (1985). [https://doi.org/10.1351/pac198557101427]
4. International Union of Pure and Applied Chemistry, Nomenclature of Regular Single-Strand Organic Polymers (Recommendations 1975, Pure Appl. Chem., 48, 373–385 (1976). [https://doi.org/10.1351/pac197648030373]
5. Chemical Abstracts Service, Naming and Indexing of Chemical Substances for Chemical Abstracts, Appendix IV, Chemical Abstracts 1999 Index Guide.
6. International Union of Pure and Applied Chemistry, A Guide to IUPAC Nomenclature of Organic Compounds (1993), Blackwell Scientific Publications, Oxford, 1993.
7. International Union of Pure and Applied Chemistry, Nomenclature of Organic Chemistry, Sections A, B, C, D, E, F, and H, Pergamon Press, Oxford, 1979.
8. International Union of Pure and Applied Chemistry, Nomenclature of Regular Double-Strand and Quasi-Single-Strand Inorganic and Coordination Polymers (Recommendations 1984), Pure Appl. Chem., 57, 149–168 (1985).
9. International Union of Pure and Applied Chemistry, Nomenclature of Regular Double-Strand (Ladder and Spiro) Organic Polymers (Recommendations 1993), Pure Appl. Chem., 65, 1561–1580 (1993). [https://doi.org/10.1351/pac199365071561]
10. International Union of Pure and Applied Chemistry, Structure-Based Nomenclature for Irregular Single-Strand Organic Polymers (Recommendations 1994), Pure Appl. Chem., 66, 873–889 (1994). [https://doi.org/10.1351/pac199466040873]
11. International Union of Pure and Applied Chemistry, Source-Based Nomenclature for Non-Linear Macromolecules and Macromolecular Assemblies (Recommendations 1997). Pure Appl. Chem., 69, 2511–2521 (1997). [https://doi.org/10.1351/pac199769122511]
12. Poly(1,3-butadiene) obtained by polymerization of 1,3-butadiene in the so-called 1,4- mode is frequently drawn incorrectly in publications as -(CH₂-CH=CH-CH₂)_n-; the double bond should be assigned the lowest locant possible, i.e., the structure should be drawn as -(CH=CH-CH₂-CH₂)_n-.
13. International Union of Pure and Applied Chemistry, Graphic Representations (Chemical Formulae) of Macromolecules (Recommendations 1994). Pure Appl. Chem., 66, 2469–2482 (1994). [https://doi.org/10.1351/pac199466122469]
14. Wilks, E. S. Macromolecular Nomenclature Note No. 17: "Whither Nomenclature?" Polym. Prepr. 40(2), 6–11 (1999).
15. Wilks, E. S. Polymer Nomenclature: The Controversy Between Source-Based and Structure-Based Representations (A Personal Perspective). Prog. Polym. Sci. 25, 9–100 (2000). [https://doi.org/10.1016/S0079-6700(00)00007-1]
16. Wilks, E. S. Macromolecular Nomenclature Note No. 18: SRUs: Using the Rules. Polym. Prepr. 41(1), 6a–11a (2000).