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 similar to20% 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. = 24 700 g/mol and M-w/M-n = 1.41 in bulk condition (yield = 72%) and Mn = 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., (CuBr2)-Br-II/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.