Catalyst-free photocontrolled reversible addition-fragmentation chain transfer (RAFT) polymerization avoids the side effects of photocatalysts but has the accompanying slow kinetics, thereby warranting more efficient photolysis and faster chain transfer. To understand the underlying mechanisms, both quantitative and qualitative interpretations are needed. Such a goal can be achieved by the iCAS (imposed automatic selection and localization of complete active spaces) approach [J. Chem. Theory Comput. 2021, 17, 4846], which maintains the same CAS and meanwhile provides localized orbitals along the whole reaction. Taking dithiobenzoate as a representative of RAFT agents, it is found here that electron-donating substitution (by methoxy) clearly outperforms both electron-standing (by methyl) and electron-withdrawing (by cyano) substitutions in facilitating photo-RAFT polymerization, by narrowing the gap between the π* and σ* orbitals, so as to facilitate the π* → σ* charge transfer dominating both the photolysis and chain transfer processes. Such findings are of general values.