Synthesis of water-soluble star-block and dendrimer-like copolymers based on poly(ethylene oxide) and poly(acrylic acid)

Star-block copolymers (PEO3-b-PAA(3)) and dendrimer-like copolymers (PEO3-b-PAA(6)) consisting of three inner poly(ethylene oxide) (PEO) arms and either three or six peripheral poly(acrylic acid) (PAA) blocks were derived by a core-first approach. To this end, the OH end groups of three-arm PEO stars prepared anionically were derivatized into either three or six bromo-ester functions that served to grow the poly(tert-butyl acrylate) (PtBA) blocks by atom transfer radical polymerization (ATRP) in a controlled fashion. This could be achieved at 80 degreesC in toluene in the presence of CuBr/pentamethyldiethylenetriamine (CuBr/PMDETA) as the catalyst system. Characterization by size exclusion chromatography and NMR of star-block copolymers (PEO3-b-PtBA(3)) and dendrimer-like copolymers (PEO3-b-PtBA(6)) confirmed their well-defined character. Subsequent treatment with trifluoroacetic acid selectively hydrolyzed the PtBA blocks, leading to the targeted PEO3-b-PAA(3) and PEO3-b-PAA(6) compounds.. Alternatively, an arm-first methodology utilizing a divinylic comonomer as the linking agent was applied to access star-block copolymers incorporating an inner PAA part and a peripheral PEO layer. To this end, preformed PEO-b-PtBA diblock copolymers were reacted with divinylbenzene in anisole in the presence of CuBr/PMDETA. Some of the factors controlling the formation of (PtBA-b-PEO)(f) stars were examined. These included the molar ratio of the linking agent to the diblock precursor, the molar mass of the latter species, and the reaction time. Finally, selective hydrolysis led to the expected double hydrophilic star-block copolymers noted (PAA-b-PEO)(f).