08824 IUPAC Gold Book - head-tail isomerism 10.1351/goldbook.08824 08824 current synonyms: HT-isomerism, regioisomerism 1 Constitutional isomerism of polymeric (oligomeric) chains composed of identical non symmetric repeating units consisting in different orientations of the units. In the repeating unit of a vinyl polymer: \(\ce{-CHR-CH2\bond{-}}\), the substituent-bearing group \(\ce{CHR}\) is denoted 'head' (H) while the other one 'tail' (T). There exist three modes how two such units can be linked in a chain: head-to-tail (HT), head-to-head (HH) and tail-to-tail (TT). The uniformity/non-uniformity of repeating units’ orientation determines the regularity/irregularity of macromolecules. regioirregular polymer https://goldbook.iupac.org//terms/view/08868 PAC, 2022, 94, 15. 'Glossary of terms relating to electronic, photonic and magnetic properties of polymers (IUPAC Recommendations 2021)' on page 34 (https://doi.org/10.1515/pac-2020-0501) https://goldbook.iupac.org/terms/view/08824/html https://goldbook.iupac.org/terms/view/08824/json https://goldbook.iupac.org/terms/view/08824/plain Citation: 'head-tail isomerism' in IUPAC Compendium of Chemical Terminology, 5th ed. International Union of Pure and Applied Chemistry; 2025. Online version 5.0.0, 2025. 10.1351/goldbook.08824 The IUPAC Gold Book is licensed under Creative Commons Attribution-ShareAlike CC BY-SA 4.0 International (https://creativecommons.org/licenses/by-sa/4.0/) for individual terms. If you are interested in licensing the Gold Book for commercial use, please contact the IUPAC Executive Director at executivedirector@iupac.org . The International Union of Pure and Applied Chemistry (IUPAC) is continuously reviewing and, where needed, updating terms in the Compendium of Chemical Terminology (the IUPAC Gold Book). Users of these terms are encouraged to include the version of a term with its use and to check regularly for updates to term definitions that you are using. 2026-07-12T23:15:41+00:00