deactivator

in ATRP
https://doi.org/10.1351/goldbook.08949
Reagent that reacts reversibly with the initiating species or propagating species to form an initiator or a dormant chain, respectively, and an activator.
Source:
PAC, 2022, 94, 1093. (Terminology for chain polymerization (IUPAC Recommendations 2021)) on page 1119 [Terms] [Paper]