The　polymer　dispersity　index　(PDI)　is　a　vital　metric　for　characterizing　molecular　weight　distribution,　significantly　influencing　polymer　material　performance.　However,　achieving　precise　control　over　PDI　in　polymer　synthesis　remains　a　substantial　challenge.　In　this　study,　we　introduce　an　approach　for　modifying　the　PDI　of　poly(methyl　acrylate)　(PMA)　in　switchable　reversible　addition-fragmentation　chain　transfer　radical　polymerizations　(RAFT).　The　devised　strategy　involves　the　utilization　of　photoresponsive　hexaarylbiimidazole　(HABI)　as　a　mediator,　coupled　with　temporal　programming,　to　cyclically　deactivate　and　reactivate　propagating　radicals　at　distinct　stages　throughout　the　polymerization　process.　The　precise　timing　of　the　light　stimulus　is　facilitated　through　computer-controlled　single-chip　microcomputer　technology,　ensuring　automatic　modulation　of　the　optical　state　and　mitigating　operational　inaccuracies.　By　manipulating　external　light　conditions,　the　PDI　of　PMAs　can　be　systematically　adjusted　within　the　range　of　1.80–2.59.　Validation　through　successful　chain-extension　experiments　and　analysis　via　MALDI-TOF　MS　confirm　the　preservation　of　good　chain-end　fidelity　across　PMAs　with　varying　PDIs　fabricated　through　this　methodology.　©　2024　Elsevier　Ltd