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Tri-arm star diblock copolymers, poly(2-hydroxyethyl methacrylate)-block-poly(N-isopropylacrylamide) [P-(HEMA-b-NIPAAm)] with PHEMA and PNIPAAm as separate inner and outer blocks were synthesized via a twostep ATRP at room temperature. The formation, molecular weight and distribution of polymers were examined, and the kinetics of the reaction was monitored. The PDI of PHEMA was shown to be lower, indicating well-controlled polymerization of trifunctional macro-initiator and resultant star copolymers. The thermoresponsive behavior of diblock copolymer aqueous solution were studied by DSC, phase diagrams, temperature-variable 1 H NMR, TEM and DLS. The results revealed that introducing a higher ratio of HEMA into copolymers could facilitate the formation of micelles and the occurrence of phase transition at lower temperatures. TEM images showed that I-(HEMA 40 -NIPAAm 320 ) 3 solutions developed into core-shell micelles with diameters of approximately 100 nm. I-(HEMA 40 -NIPAAm 320 ) 3 was used as a representative example to elucidate the mechanism underlying temperature-induced phase transition of copolymer solution. In this study we proposed a three-stage transition process: (1) separately dispersed micelles state at %17-22 8C; (2) aggregation and fusion of micelles at %22-29 8C; (3) sol-gel transition of PNIPAAm segments at %29-35 8C, and serious syneresis of shell layers.Molecular architecture of Poly(HEMA-b-NIPAAm).

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