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 (https://doi.org/10.1351/pac199668010149)