notes on nomenclature the possible CRU's are
ROBERT B. FOX
- -
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Naval Research Laboratory Wa~hlngton.D C . 20375
Naming Organic Polymers II. Structure-Based Polymer Nomenclature In Part I' of this series, some of the limitations of conventional polymer nomenclature were discussed. It was clearlv demonstrated that the need exists for a rational polymer nomenclature through which, at the minimum, any (assumed) reeular uolvmer can be named. Within this re&riction, a noGenc~aiurksystem hased on structure was devised and published twenty years ago.Z Because of inherent limitations in detail hut not in concept, this system saw little application. Recently, this concept of regular polymer nomenclature has been hrought within the framework of ordinary organic nomenclature principles3 and ereatlv extended' to cover most reeular ~ o l v m e r strucGres that can be envisioned. The system is'flexihle and has been adopted for indexina- uumoses bv Chemical . . Abstracts; it mafhe expected to see increasing use in a t least the fundamental polymer journals. The recent IUPAC recommendations4 are limited to the nomenclature of regular single-strand organic polymers, i.e., polymer molecules whose constitutional units can he chosen so that they all have no more than two terminal atoms. This is a nomenclature based on the structure of the molecule, as is systematic organic nomenclature. These methods of naming chemical compounds are closely related, and the principles5 of organic nomenclature are followed as closely as possible in the polymer case. In concept, the system is simple^. Every regular singlestrand polymer molecule is a multiple of a bivalent constitutional (or structural) repeating unit (CRU) that can be written one or. more ways. The rules are designed to provide for each polymer structure a preferred CRU, the name of which when prefixed by "poly" provides a unique name for the polymer. The steps to he followed in sequence are (a) identify the CRU, (b) orient the CRU, and (c) name the CRU. This is best illustrated by example. If a fragment of a polymer chain can he written
-OCHCH20CHCH20CHCH20CHCH~OCHCH1-
I
F
I
F
I
F
I
F
I
F
R. B., J. CHEM. EDUC., 51,41(1974). 2J.Polymer. Sci.,8, 257 (1952). Macromolecules, 1,193 (1968). '"Nomenclature of Regular Single-Strand Organic Polymers," IUPAC Infarmstian Bulletin, Appendices on Tentative Nomenclature, Symbols, Units, and Standards,No. 29, November 1972. "Nomenclature of Oreanic Chemistrv." Sections A. B. and C Combined 3rd Ed. ~utterworths,ond don,-i971. 1 Fox,
Each CRU could be used to generate a name, but to avoid having six equally valid names for a single polymer, one CRU is given preference. The rules specify seniority among subunits to provide the point at which to begin writing the CRU as well as the direction along the chain in which to continue to the end of the CRU. In the above example, the subunit of highest seniority happens to be -0-, and therefore the parent (unsubstituted) CRU can or -CHzCHzO-. Further choice is be -0CHzCHzhased on the lowest locant for substitution, so that -OCHCH2-
I
is preferred to -OCH,CH-
I
F
F
and -CH,CHO-
I
is preferred to -CHCH,O-
I
Finally, the CRU is oriented to read from left to right. The preferred CRU is therefore
The name of this CRU is obtained by citing in order the names of the largest subunits in the CRU, beginning with subunit and proceeding to "l-fluo"oxy" for the -0roethylene" (for the parent unit -CHzCHzis preferred over -CH2because it is larger and as a unit has a name). The name of the corresponding polymer is therefore
The rules themselves are essentially detailed directions for the selection of the preferred bivalent CRU in a given polymer. Where there is a choice between bivalent and a higher-valent CRU, the bivalent CRU is always selected without consideration of the nature of subunits in the CRU. For a bivalent CRU, the starting point in writing this repeating unit will be at the single free valence adjacent to or nearest to the most senior suhunit, and citation will he in the direction of the shorter path toward the subunit or subunit combination of next highest seniority. The order of seniority among types of bivalent groups is: (a) heterocyclic rings, followed by (h) chains containing hetero atoms, (c) carbocyclic rings, and (d) chains containing only carbon, in that order. This order is unaffected by suhstituents on the main chain, even though these might he expressed as part of a trivial name or a bivalent group. The names of the subunits are formed according to Volume51, Number2 February 7974 / 113
the usual organic nomenclature rules,5 with due account being taken of the directionality requirement in the polymer names. Some examples of application of these principles follow. In the example and
IT
CHC-CH,
is preferred to
P"; 1 C-CHCH,
Poly(1-iodo-2-iminotrimethylene)
the heterocyclic pyridine ring is the mmt senior subunit and the imino group, -NH-, is the next senior suhunit. The shortest path between these units is through the carbon atom of the benzylidene group, rather than by way of the 4-atom path in the cyclohexylene group. Numbering of the pyridine ring is fixed by the nitrogen atom, and the free valences of the group are given lowest numbers before the CRU is named; 4,2- indicates the direction taken in writing the ring and the other units. An isomeric hut structurally different polymer might involve the 2,4-pyridinediyl unit. Another example is
+oCH,c€H,NHCH,CH2SCH1NHCH2CH21,
Here. the direction for writing (and eventuallv citing) the CRU has been determined h; the first point bf diffGence in senioritv. the ~ositionof a nitrogen atom in the chain. The a~ternitives&e 1 2 3 4 5 6 7 8 9 1 0 1 1 1 2 (I) -04-0-C-N-C4-S-C-N S > N, etc.) used in organic nomenclature dictates that the chain be written from 0 to 0 (the most senior atoms) to S. The path lengths are identical for the two possibilities that begin with 0 ; the first point of difference is the position of the next senior N atom numbered 5 in (I) and 10 in (II). Since the path length 3-4-5 in (I) is shorter than 1-12-11-10 in (11). CRU (I) is referred to (II). Once the CRU has been selecied, it remains only to citi the name. In this rase, orgunk nomcncla~ureoffers two reaionnlde alternatives: the first name given above is hased on conventional hivalent gmup names, cited in order; while the second name is hased on the principles of "replacement nomenclature,"5 in which the 12-atom hivalent group having the parent name dodecamethylene is named by replacing specific carbon atoms with hetero atoms, themselves denoted by the prefixes oxa (O), thia (S), and aza (N) along with their positional locants. Within a given class of hivalent groups, further orders or senioritv have been delineated bv the rules. An example involvkg hetero atoms has just been cited. Among the heterocvclic rinas. - . the order is that followed in conveutional organic nomenclature;5 a six-membered ring containing nitrogen is senior to any ring containing oxygen but no nitrogen, for example. Complexity is also the basis for the order of seniority among carbocyclic rings. Substitution determines seniority in a series of all-carbon acyclic hivalent groups; the descending order followed is (a) the chain with the lareest number of substituents. (h) the chain with suhstitu~nrshavmg lowest locanti, and rc) the alphnl~i,tiualorder oisuhstituents. For example 114
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Journal of Chemical Education
In the first case, the choice of the two-carbon subunit in pmceeding from 0 to N is dictated by the two phenyl substituents, which are ranked ahead of the single 0x0 suhstituent. In the second instance, alphabetical order controls the choice. These examples also illustrate the manner in which suhstituents are placed in the name itself, i.e., as prefixes to the name of the unit substituted rather than as prefixes to the name of the entire polymer. Provision in the rules is also made for naming a polymer molecule having a specific number of CRU's; e.g., deca(l,4phenylene) names a chain of 10 phenylene units bound to each other in the 1,4 positions. ,End groups are usually not cited in polymer names, hut if emphasis on the end groups is desired, they can he named by citing with the prefix alpha the group attached to the left side of the CRU as written; the other end group is designated with the prefix omega, as in
a-(Trichloromethyl)-w-chloropoly(l,4-phenylenemethylene)
The examples given here and most of the examples given in the recommended rules' are of fairly complex polymer structures. Obviously, polymers with relatively simple CRU's are easily named by these rules, and the names are translated back to structures with equal facility. Most of the best known polymers have simple CRU's. The IUPAC Commission recognizes that their conventional names are well-established by usage and does not expect that these names will he supplanted immediately by the structure-based names. An appendix comparing the conventional names with the structure-based names for common polymers is included in the rules. Some examples are
polystyrene poly(1-phenylethylene)
f CF,CF,k poly(tetrafluoroethylene)
poly(difluoromethylene) +c€H,CH2k poly(ethy1ene oxide) poly(oryethylene) fNH(CHJ,NHCO(CHJ4Wk poly(hexamethy1ene adipamide) poly(iminahexamethyleneiminoadipoy1)
It is clear that the structure-based names, if somewhat unfamiliar, are readily recognized for what they describe. In most cases, the description is far more precise-and this, after all, is what scientific communication is all about.
Limitations of Structure-Based Nomenclature for Polymers One must have no illusions regarding what a structurebased polymer nomenclature can or cannot do. It can he used to name unambiguously any single-strand regular organic polymer for which a unique bivalent constitutional or structural repeating unit can he written. The system is a t its best in naming precisely quite complex structures, most of which cannot he named descriptively and unambiguously by any other nomenclature method. Because it is rooted in approved organic nomenclature, the group names are familiar, and therefore the system is not difficult to utilize even with the most common polymers that have established names based on source. The one-to-one relationship between the chemical groups and their names results in facile translatability and adaptability for indexing and information retrieval. Changes in organic nomenclature will be readily assimilated in the structure-based polymer nomenclature system. At this writing, the recommendations are limited to single-strand polymers, i.e., those that can be reduced to a hivalent CRU. Further recommendations may be expected
to adapt the concepts of stereoregularity into the system. Double-strand or "ladder" polymers can he accommodated, hut the details are still under discussion. In all probability, certain types of inorganic or mixed organic-inorganic polymers may be included in the system in the future. The key to this system is structural regularity. With the exception of end-groups, no irregularity can be included in the name, although various descriptive phrases can always accompany a name. Thus, random unsaturation or branching is not included in a structure-based name, any more than it is in a source-based name. Random copolymers cannot he accommodated, hut a polymer made from two monomers, A and B, that has only the structure -ABABABcertainly is namahle since -ABis the CRU. It is possible that graft and block polymers may eventually be included in the system, but polymer blends and mixtures will not. An attempt has been made to devise a precise method for communicating in words the structure of polymers. It is hoped that most possible combinations of atoms have been covered in the rules. At the same time, it is also hoped that the chemist has been given another research tool for which he will find use for many years to come.
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