secondary species

in a chain polymerization
https://doi.org/10.1351/goldbook.08969
Species formed by the rearrangement or fragmentation of a primary species that is capable of initiating a polymerization.
Note:
The term 'secondary species' is also used to designate a species centred on a secondary carbon atom, e.g., a propan-2-yl radical.
Source:
PAC, 2022, 94, 1093. (Terminology for chain polymerization (IUPAC Recommendations 2021)) on page 1125 [Terms] [Paper]