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== Basic Concepts ==
The terms polymer and macromolecule do not mean the same thing. A polymer is a substance composed of macromolecules. The latter usually have a range of molar masses (unit g mol<sup>-1</sup>), the distributions of which are indicated by dispersity (''Đ''). It is defined as the ratio of the mass-average molar mass (''M''<sub>m</sub>) to the number-average molar mass (''M''<sub>n</sub>) i.e. ''Đ'' = ''M''<sub>m</sub>/''M''<sub>n</sub>.<ref>Stepto, R. F. T.; Gilbert, R. G.; Hess, M.; Jenkins, A. D.; Jones, R. G.; Kratochvíl P. (2009). "[http://media.iupac.org/publications/pac/2009/pdf/8102x0351.pdf Dispersity in Polymer Science]" ''Pure Appl. Chem.'' '''81''' (2): 351–353. DOI:10.1351/PAC-REC-08-05-02.</ref> Symbols for physical quantities or variables are in italic font but those representing units or labels are in roman font.<br />
Polymer nomenclature usually applies to idealised representations; minor structural irregularities are ignored. A polymer can be named in one of two ways. Source-based nomenclature can be used when the monomer can be identified. Alternatively, more explicit structure-based nomenclature can be used when the polymer structure is proven. Where there is no confusion, some traditional names are also acceptable.<br />
Whatever method is used, all polymer names have the prefix poly, followed by enclosing marks around the rest of the name. The marks are used in the order: {[( )]}. Locants indicate the position of structural features, e.g., poly(4-chlorostyrene). If the name is one word and has no locants, then the enclosing marks are not essential, but they should be used when there might be confusion, e.g., poly(chlorostyrene) is a polymer whereas polychlorostyrene might be a small, multi-substituted molecule. End-groups are described with α- and ω-, e.g., α-chloro-ω-hydroxy-polystyrene.<ref name=purple />
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In the above example, the oxy subunits in the CRUs are heteroatom chains. From Figure 1, oxy subunits are senior to the acyclic carbon chain subunits, the largest of which are bromo-substituted -CH<sub>2</sub>-CH<sub>2</sub>- subunits. 1-Bromoethane-1,2-diyl is chosen in preference to 2- bromoethane-1,2-diyl as the former has a lower locant for the bromo-substituent. The preferred CRU is therefore oxy(1-bromoethane-1,2-diyl) and the polymer is thus named poly[oxy(1-bromoethane-1,2-diyl)]. Please note the enclosing marks around the subunit carrying the substituent.
Polymers that are not made up of regular repetitions of a single CRU are called irregular polymers. For these, each constitutional unit (CU) is separated by a slash, e.g., poly(but-1-ene-1,4-diyl/1-vinylethane-1,2-diyl).<ref>Fox, R. B.; Bikales, N. M.; Hatada, K.; Kahovec, J. (1994). "[http://www.iupac.org/publications/pac/pdf/1994/pdf/6604x0873.pdf Structure-Based Nomenclature for Irregular Single-Strand Organic Polymers]" ''Pure Appl. Chem.'' '''66''' (4): 873–889.</ref>
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