Reversible deactivation radical polymerization (RDRP) techniques regulate free radical polymerization through a process of reversible activation/deactivation of propagating radicals, which thus provides dormant polymer chains with unique capping moieties (alkoxyamine, halogen, thiocarbonylthio, etc.) as chain ends. The dormant chain ends are the reactive sites of initiation, propagation, and termination, and therefore their chemical structures are crucial for the mechanism and kinetics of RDRP. The investigation of stereochemical effects of dormant chain ends is challenging due to limited models that are highly accessible and able to represent long polymer chains. Herein, we employ a single unit monomer insertion (SUMI) technology to prepare ideal models (oligomeric macro-RAFT agents) for the investigation of the stereochemical effect on their photoactivation. Two diastereomeric macro-RAFT agents were prepared in high yield by SUMI, which were then photoactivated for photoinduced electron/energy transfer-reversible addition- fragmentation chain- transfer (PET-RAFT) polymerization under visible light. Discriminatory activation kinetics was observed with one diastereomer much faster than the other. The mechanistic explanation of such interesting behavior was suggested with the aid of quantum chemical calculations. Higher C-S bond dissociation energy and greater stability contributed to slower photoactivation of one of diastereomeric RAFT agents as compared to the other.