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Vol. 35 No. 4
July-August 2013

Nomenclature Notes | A column about the Principles of Chemical Nomenclature—A Guide to IUPAC Recommendations, 2011 Edition

Polymer Nomenclature

The aim of systematic IUPAC nomenclature is usually to introduce naming systems that define the structure of a molecule precisely, so that the reader can reproduce the exact structure of the molecule being discussed. The system works reasonably well for completely characterized small molecules, but cannot do so with the same level of precision for polymeric materials comprised of molecular chains (macromolecules), the structures of which are based on constitutional repeating units (CRUs). These repeat units may not be all of the same type and need not repeat in a regular fashion. A given polymer may consist of more than one chain and often of mixtures of different kinds of chain. In addition, there can be regular or irregular steric variations along the length of individual macromolecules and chains might be branched or linked to one another in diverse ways. For many polymers, the repetition may not be exactly regular, the material may consist of a mixture of chain macromolecules of different lengths, and the precise structure may not be known. Nevertheless, methods for naming such materials are necessary for general communication, and polymer chemists have been obliged to develop them. It is probably true that there is not yet a universal agreement among all polymer chemists as to how this should be done in every case, but there is a considerable consensus, and the new Principles presents its basic details.

A polymer is a substance composed of a collection of macromolecules of a range of molecular masses. As a consequence, it is characterized by an average molecular mass rather than a mass of a definite value, as typical of relatively small molecules. These macromolecules may consist of single strand, regular or irregular chains, or they may be double-stranded ladder-like structures or even sheets, the limit being a three-dimensional structure, which may be considered no longer to be within the province of polymers but better treated as a three-dimensional structure such as in a ceramic or glass. Finally, the polymers may be constituted of organic, organometallic, or even inorganic groups, including those of coordination type. Polymer nomenclature must attempt to describe all these types, and no satisfactory universal methodology has been developed.

Two basic methods have been developed to give names which are comprehensible and broadly consistent with the apparent structure. Neither method conveys all the details of the polymer structure, but one capable of doing so would probably be too long and complicated to be easily comprehensible, even to the informed reader. A shorter form is often adequate for many purposes.

Most polymers consisting of regular, single macromolecular chains may be named using structure-based nomenclature. Example (a) shows a generalized structure of such a polymer. A, B, C, and D represent groups of atoms comprising the main chain while E and R denote the chain end-groups and pendant groups, respectively. The CRU for this generalized structure, and the CRU and name and of a real polymer are also shown. Precise rules are necessary to govern the selection of the CRU.

Example (a)

Representative single-strand polymer structure
 
 
 
Constitutional repeating unit (CRU)
 
 
CRU of a polymer named poly[oxy(1-methylene)]

Methods have also been developed to name irregular single-strand polymers and polymers of other structures, and these are also mentioned in Principles. Nevertheless, a precise structure-based name may be impossible to devise for a variety of reasons, such as lack of enough structural information. By far, the most widely used and easily implemented method of naming polymers is source-based nomenclature and example (b) shows three such names.

Example (b)

polyacrylonitrile
polystyrene
poly(dimethylstannanediyl)

The first two names are based on the names of the reagents (which may be monomers) from which the polymers were synthesized, acrylonitrile and styrene in these cases, but they convey little information about structure. However, this use of a reagent name may not be always applicable. For example, the source reagent to synthesize the third polymer cannot be simply dimethylstannanediyl, even though the polymer name itself is easily comprehensible. All three source-based names are organic style, but for the third, which describes an inorganic single-strand polymer, an inorganic-style name is also available:

catena-poly[dimethyltin]

These are only very simple examples, but they hint at some of the complexities involved in naming polymers, which differ from some of the methods used for naming small molecules. Principles also describes how more complicated polymer structures can be named, and also abbreviations for names that are commonly used both in academe and in industry. As usual, a list of basic references is also provided.

The recommendations and advice of Professor Richard G. Jones (University of Kent, Canterbury, and chair of the IUPAC Subcommittee on Polymer Terminology) during the preparation of this note are gratefully acknowledged.

Jeffery Leigh is the editor and contributing author of Principles of Chemical Nomenclature—A Guide to IUPAC Recommendations, 2011 Edition (RSC 2011, ISBN 978-1-84973-007-5). Leigh is emeritus professor at the University of Sussex and has been active in IUPAC nomenclature since 1973.

www.iupac.org/publications/ci/indexes/nomenclature-notes.html

 


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