Preparation of Linear and Star-Shaped Block Copolymers by ATRP Using Simultaneous Reverse and Normal Initiation Process in Bulk and Miniemulsion

Well-defined linear and star-shaped block copolymers were synthesized using halogenated ATRP macroinitiator via a simultaneous reverse and normal initiation (SR&NI) ATRP process in both bulk and stable aqueous miniemulsion. The miniemulsion was carried out with ∼20% solid (based on 100% conversion), 0.58 wt % nonionic surfactant, Brij 98 (based on water), and 3.6 wt % hexadecane (based on monomer). For example, Br-terminated 3-arm poly(methyl acrylate) ( M n = 9200 g/mol; M w / M n = 1.08) was extended with styrene to form star-shaped block copolymer, poly(methyl acrylate)- b -polystyrene, with M n = 24 700 g/mol and M w / M n = 1.41 in bulk condition (yield = 72%) and M n = 22 100 g/mol and M w / M n = 1.37 in miniemulsion (yield = 72%). This newly developed ATRP process requires a significantly reduced amount of highly active ATRP catalyst complex in its oxidatively stable high oxidation state (e.g., Cu II Br 2 /ligand), while the predominant initiating species was a normal halogen-containing macroinitiator. The compositional analysis of the resulting block copolymer using 2-dimensional chromatography indicated that the final product was essentially composed of block copolymers formed by chain extension of the halogenated macroinitiator via a normal ATRP mechanism. However, the presence of low amounts of linear homopolymers generated by AIBN-initiated species via a reverse ATRP mechanism was also detected.