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 x 10(3) and 5.8 x 10(3) and narrow molecular weight distribution (M-w/M-n < 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 H-1, C-13, and B-11 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 degrees C and 40 degrees 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. (C) 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 48: 627-634, 2010