Synthesis of miktoarm star and miktoarm star block copolymers via a combination of atom transfer radical polymerization and stable free-radical polymerization

A trifunctional initiator, 2-phenyl-2[(2,2,6,6-tetramethyl)-1-piperidinyloxy] ethyl 2,2-bis[methyl(2-bromopropionato)] propionate, was synthesized and used for the synthesis of miktoarm star AB₂ and miktoarm star block AB₂C₂ copolymers via a combination of stable free-radical polymerization (SFRP) and atom transfer radical polymerization (ATRP) in a two-step or three-step reaction sequence, respectively. In the first step, a polystyrene (PSt) macroinitiator with dual ω-bromo functionality was obtained by SFRP of styrene (St) in bulk at 125°C. Next, this PSt precursor was used as a macroinitiator for ATRP of tert-butyl acrylate (tBA) in the presence of Cu(I)Br and pentamethyldiethylenetriamine at 80°C, affording miktoarm star (PSt)(PtBA)₂ [where PtBA is poly(tert-butyl acrylate)]. In the third step, the obtained St(tBA)₂ macroinitiator with two terminal bromine groups was further polymerized with methyl methacrylate by ATRP, and this resulted in (PSt)(PtBA)₂(PMMA)₂-type miktoarm star block copolymer [where PMMA is poly(methyl methacrylate)] with a controlled molecular weight and a moderate polydispersity (weight-average molecular weight/number-average molecular weight < 1.38). All polymers were characterized by gel permeation chromatography and ¹H NMR.