chain carrier 

https://doi.org/10.1351/goldbook.C00949
A species, such as an atom or free radical, which is involved in chain-propagating reactions is known as a chain carrier.
Source:
PAC, 1996, 68, 149. (A glossary of terms used in chemical kinetics, including reaction dynamics (IUPAC Recommendations 1996)) on page 156 [Terms] [Paper]
Citation: 'chain carrier' in IUPAC Compendium of Chemical Terminology, 5th ed. International Union of Pure and Applied Chemistry; 2025. Online version 5.0.0, 2025. https://doi.org/10.1351/goldbook.C00949 RIS BibTex EndNote
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Last revised: February 24, 2014