A photo-responsive amphiphilic triblock copolymer has been synthesized.The hydrophobic segment of copolymer was prepared by ring-opening polymerization of p-dioxanone(PDO) using 4

4'-bis-(hydroxyethylether) azobenzene (BHEAZO) as an initiator and stannous 2-ethylhexanoate ［Sn(Oct)2］ as a catalyst.The hydroxyl terminated poly(p-dioxanone)(PPDO) precursors were then coupling-reacted with the carboxyl group terminated hydroxymethyl polyethylene glycol(mPEG-COOH) to give the mPEG-BHEAZO-PPDO-mPEG triblock copolymer.The photo-responsive behavior of the copolymer was studied by UV-Vis spectroscopy

dynamic laser scattering (DLS) and transmission electron microscopy (TEM).The configurational change of the azobenzene group contained in the copolymer nano-aggregate was investigated by the UV-Vis spectroscopy.The absorbance of the nano-aggregate aqueous solution increased with the growth of the irradiation time.The size of nano-aggregate particle and particle size distribution after exposure to light with different wavelengths were measured by DLS.The size of the copolymer nano-aggregate after exposure to 450 nm light was getting bigger than that to 365 nm

and the size distribution was getting wider at the same time.The TEM images of the nano-aggregate aqueous solution were consistent with the DLS results.According to the analyses above

a hypothetic mechanism was proposed for explaining the changes of the nano-aggregates after exposure to lights with different wavelengths.A single nano-aggregate particle easily tended to aggregate with the particles nearby since the trans configuration of azobenzene groups in the copolymer was more likely to form a stable electron delocalization state of low level

which could lead to a stonger - stacking effect.