Synthesis and photophysical characterization of amphiphilic dendritic-linear-dendritic block copolymers

Amphiphilic dendritic-linear-dendritic triblock copolymers based on hydroph ilic linear poly(ethylene oxide) (PEO) and hydrophobic dendritic carbosilan e were synthesized with a divergent approach at the allyl end groups of dia llyl-terminated PEG. Their micellar characteristics in an aqueous phase wer e investigated with dynamic light scattering, fluorescence techniques, and transmission electron microscopy. The block copolymer with the dendritic mo iety of a third generation could not be dispersed in water. The block copol ymers with the first (PEO-D-Si-1G) and second (PEO-n-Si-2G) generations of dendritic carbosilane blocks formed micelles in an aqueous phase. The criti cal micelle concentrations of PEO-D-Si-1G and PEO-D-Si-2G, determined by a fluorescence technique, were 27 and 16 mg/L, respectively. The mean diamete rs of the micelles of PEO-D-Si-1G and PEO-D-Si-2G, measured by dynamic ligh t scattering, were 170 and 190 nm, respectively, which suggests that the mi celles had a multicore-type structure. The partition equilibrium constants of pyrene in the micellar solution increased with the increasing size of th e dendritic block (e.g., 7.68 x 10(4) for PEO-n-Si-1G and 9.57 x 10(4) for PEO-D-Si-2G). The steady-state fluorescence anisotropy values (r) of 1,6-di phenyl-1,3,5-hexatriene were 0.06 for PEO-D-Si-1G and 0.09 for PEO-D-Si-2G. The r values were lower than those of the linear polymeric amphiphiles, su ggesting that the microviscosity of the dendritic micellar core was lower t han that of the linear polymeric analogues. (C) 2001 John Wiley & Sons, Inc .