RAFT-mediated semi-continuous emulsion polymerizations with controlled monomer feed have been studied by reaction calorimetry. This online monitoring technique provided detailed information about the onset of the nucleation period in semi-batch processes for emulsion polymerization of styrene and n-butyl acrylate. The reactions were carried out under controlled radical polymerization conditions by employing amphipathic macro-RAFT agents of various degrees of surface activity. For n-butyl acrylate as well as for styrene the more surface active polymeric macro-RAFT agents led to higher initial polymerization rates. Polymerizations with hydrophobic macro-RAFT agents also adapted more quickly to an increased monomer feed than polymerizations with hydrophilic macro-RAFT agents.This points to the presence of more particles when a hydrophobic macro-RAFT agent is used. Hydrophobic macro-RAFT agents demonstrated to be promising for producing latex products with a proper control of particle number and particle size distribution.