14226 IUPAC Gold Book - racemic packing 10.1351/goldbook.14226 14226 current 1 Packing in a polymer crystal in which stems of opposite helicity coexist in equal numbers. Applicable to macromolecules that are chiral or adopt chiral conformations in the crystal. One example of the former case is the crystal structure in which poly(ʟ-lactide) and poly(ᴅ-lactide) stems coexist in equal numbers, i.e., the so-called polylactide stereocomplex. The second case is much more frequent: it applies to most isotactic vinyl polymer crystal polymorphs where the polymer chains are not intrinsically chiral but adopt helical, and thus chiral conformations. Left-handed helices and right-handed helices are, however, isoenergetic and equiprobable and coexist in equal numbers, often giving rise to centrosymmetric crystals. \(\ce{\upalpha\mbox{-}}\) and \(\ce{\upgamma\mbox{-}}\)crystal polymorphs of isotactic polypropylene the rhombohedral crystal polymorph \(R \bar 3 c\) of isotactic polystyrene. Left-handed helices https://goldbook.iupac.org//terms/view/14211 crystal polymorphs https://goldbook.iupac.org//terms/view/14219 right-handed helices https://goldbook.iupac.org//terms/view/14213 PAC, 2011, 83, 1831. 'Definitions of terms relating to crystalline polymers (IUPAC Recommendations 2011)' on page 1847 (https://doi.org/10.1351/PAC-REC-10-11-13) https://goldbook.iupac.org/terms/view/14226/html https://goldbook.iupac.org/terms/view/14226/json https://goldbook.iupac.org/terms/view/14226/plain Citation: 'racemic packing' 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.14226 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. 2026-06-29T00:31:43+00:00