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 (https://doi.org/10.1515/pac-2020-1211)