Synthesis and characterization of branched fullerene-terminated poly(ethylene glycol)s

Poly(ethylene glycol) [1, PEG(similar to 4)(OH)2, M-n similar to 2001, glycerol ethoxylate [2, PEG(similar to 21)(OH)3, M-n similar to 1000] and pentaerythritol ethoxylate [3, PEG(similar to 15)(OH)(4), M-n similar to 797] react directly with phenyl-C-61-butyric acid methyl ester (PCBM), in the presence of dibutyltinoxide (DBTO) catalyst at 140 degrees C, to give a mixture of fullerene [C-60] end-capped PEGs via transesterification. Among these PEG linkers, only PEG(similar to 4)(OPCB)(2) (4a) (OPCB: ester oxygen linked phenyl-C-61-butyryl group) was successfully isolated from the crude product mixture in the fully end-capped form. Fully acylated PEG(similar to 21)(OPCB)(3) (5) and PEG(similar to 15)(OPCB)(4) (6) could not be separated chromatographically from incompletely reacted species due to the polydispersity in branch lengths. This purification challenge was overcome by using a monodisperse branched core, 1,3,5-tris-(Octagoloxymethyl)benzene [7, PEG(24)(OH)(3)] to give a monodisperse tris-fullerene homostar, PEG(24)(OPCB)(3) (8). The structures of the bis- and tris-fullerene products were confirmed by MALDI-TOF mass spectrometry and H-1 NMR spectroscopy with supporting FTIR and UV-vis spectroscopic analysis.