Amphiphilic stars and dendrimer-like architectures based on poly(ethylene oxide) and polystyrene

Newly designed star-shaped and dendrimer-like copolymers made of poly(ethylene oxide) (PEO) and polystyrene (PS) were synthesized by sequential anionic polymerization of ethylene oxide and atom transfer radical polymerization (ATRP) of styrene, the switch from the first to the second mechanism being obtained by selective transformation of "living" oxanionic sites. First, tri- and tetrafunctional initiators were used to anionically polymerize ethylene oxide and produce tri- and tetraarmed PEO stars. Next, the OH end groups of PEO star branches were derivatized into 2-bromopropionate groups giving rise to the corresponding tri- and tetrabromoester ended-PEO stars; the latter served as macroinitiators for the ATRP of styrene at 100 degrees C in the presence of CuBr/2,2'-bipyridine catalyst system affording amphiphilic star block copolymers PEOn-b-PSn (n = 3 or 4). PEOn-b-PS2n (n = 3 or 4) dendrimer-like copolymers constituted of an inner PEO part and an outer PS layer were prepared by introducing a branching agent at the OH termini of the PEO arms before growing the PS generation by ATRP. The same branching agent was used in the preparation of miktoarmed PEO-b-PS2 copolymers and PS2-bPEO-b-PS2 H-type copolymers, starting from omega-OH and alpha,omega-di-OH PEG, respectively. The samples obtained were characterized by NMR spectroscopy and size exclusion chromatography equipped with a multiangle laser light scattering detector. These copolymers exhibited the expected structure, as confirmed after cleavage of the eater functions linking the PEO arm ends to PS moieties. Indeed, the hydrolyzed PS arms isolated were of low polydispersity index and their molar masses were in good agreement with the values calculated by NMR on the corresponding star-shaped and dendrimer-like copolymers.