Term: initiating species
https://doi.org/10.1351/goldbook.08959

Definition:
Species to which monomer adds or becomes inserted to start a chain polymerization.

Notes:
1) An initiating species may be formed from an initiator or be the initiator itself.
2) In radical polymerization, initiating species formed directly from an initiator are called primary radicals.

Source: PAC, 2022, 94, 1093. 'Terminology for chain polymerization (IUPAC Recommendations 2021)' on page 1122 (https://doi.org/10.1515/pac-2020-1211)

Citation: 'initiating species' 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.08959

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