seed particle

https://doi.org/10.1351/goldbook.14376
Particle in a polymer colloid that is the locus of subsequent polymerization.
Note: A seed particle is either added to a polymerization mixture before the polymerization begins, or is formed in situ.
Source:
PAC, 2011, 83, 2229. (Terminology of polymers and polymerization processes in dispersed systems (IUPAC Recommendations 2011)) on page 2249 [Terms] [Paper]