Molecular brushes with trans-4-methacryloyloxyazobenzene (MOAB) and 2-(dimethylamino)ethyl methacrylate ( DMAEMA) units in the side chains were successfully synthesized by grafting from a poly( 2-(2-bromopropionyloxy) ethyl methacrylate) (pBPEM) macroinitiator using atom transfer radical polymerization ( ATRP). Molecular weight, molecular weight distribution, and the DP of the side chains containing different distributions of DMAEMA and MOAB were determined by gel permeation chromatography (GPC) and H-1 NMR spectroscopy. The number-average molecular weight (M-n) of the brushes ranged from 4.7 x 10(5) to 1.1 x 10(6) depending on molecular architecture. The molecular weight distribution of the brushes was narrow (M-w/M-n = 1.23-1.36). UV-vis spectra of the brush copolymers in either chloroform or aqueous solution showed reversible isomerization of azobenzene units in the side chains upon irradiation with UV (365 nm) or visible light (442 nm). The transmission spectra of the aqueous solutions of the brush copolymers at 600 nm were measured as a function of temperature and showed that the lower critical solution temperature (LCST) can be affected by photoirradiation. The particle size in aqueous solution measured by dynamic light scattering ranged from 30 to 120 nm depending on temperature, concentration, and structural state of the azobenzene units. This variation of particle size gave additional evidence for photoresponsive thermal transition.