Toward an easy access to asymmetric stars and miktoarm stars by atom transfer radical polymerization

AA'(2)-type asymmetric stars and AB(2)-type miktoarm star polymers were prepared by combination of atom transfer radical polymerization (ATRP) and chemical modification of the termini of ATRP-derived polymers. The first step involved the preparation of omega-bromopolystyrene (PS) chains by ATRP using ethyl 2-bromoisobutyrate as initiator. Styrene was polymerized in bulk at 100 degreesC in the presence of Cu(I)Br and pentamethyldiethylenetriamine (PMDETA) as catalytic system. Next, the bromo end groups of the resulting PS chains were derivatized into twice as many bromoisobutyrates in order to obtain omega,omega'-bis(bromo)-PS chains. The last step consisted of growing either two additional PS or two poly(tert-butyl acrylate) (PtBA) blocks by ATRP, following the same polymerization conditions as those mentioned above. This methodology enabled us to synthesize AA'(2) triarm PS stars with asymmetry in the molar mass of their branches and PS(PtBA)(2) stars with chemically different PS and PtBA arms. According to the evolution of molar masses with conversion during growth of both the precursors and the stars using size exclusion chromatography and characterization of the end groups by H-1 NMR and matrix-assisted laser desorption/ionization time-of-flight (MALDI-TOF) mass spectrometry, the structures are well-defined. Finally, the selective cleavage of tert-butyl groups from stars containing PtBA blocks was performed under acidic conditions. This resulted in the formation of novel amphiphilic PS(PAA)(2) miktoarm stars carrying one hydrophobic PS branch and two ionizable poly(acrylic acid) (PAA) arms. Characterization of the latter species by H-1 NMR and IR confirmed the expected structure.