Novel water-soluble copolymers containing 4-difluoro-4-bora-3a,4a-diaza-s-indacene (BODIPY) dyes were synthesized by reversible addition-fragmentation chain transfer (RAFT) polymerization. The copolymers with both number-average molecular weight between 5.0 × 103 and 5.8 × 103 and narrow molecular weight distribution (Mw/Mn < 1.19) were obtained by the copolymerization of (2-dimethylamino)ethyl methacrylate (DMAEMA) and BODIPY-based methyl methacrylate (1) with 2,2′-azobis(isobutyronitrile) (AIBN) as an initiator in the presence of cumyl dithiobenzoate (CDB) as a chain transfer agent (CTA). The structures of the resulting copolymers were characterized by 1H, 13C, and 11B NMR spectroscopies, and the comonomer compositions were good consistent with the feed ratio. Characteristic optical properties of the obtained copolymers were investigated by UV-vis and PL spectroscopic methods. The copolymers composed of [DMAEMA]: [1] = 98.0:2.0 and 99.4:1.4 led to thermoresponsive polymers having phase separation temperatures at 32 °C and 40 °C, respectively, depending on the compositions of hydrophilic/hydrophobic balances. Further, the effective reversible decrease/increase of the emission intensity of the copolymers led to the reversible formation/inhibition of the H-aggregation between two BODIPY planes in the copolymers on heating and cooling across the border of LCST. © 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 48: 627–634, 2010