Synthesis of Well-Defined Miktoarm Star Copolymers of Aromatic Polyether and Polystyrene by Chain-Growth Condensation Polymerization and Atom Transfer Radical Polymerization

Well-defined AB₂ and A₂B type miktoarm star copolymers consisting of aromatic polyether arms as the A segment and polystyrene arms as the B segment were synthesized from trifunctional initiators. Atom transfer radical polymerization (ATRP) of styrene was carried out in the presence of 3,5-bis(1-bromoethyl)-4′-fluorobenzophenone as a trifunctional initiator, and then the terminal C−Br bond of the polymer was reduced with Bu₃SnH. The obtained polystyrene macroinitiators were used for aromatic polyether segment construction via chain-growth condensation polymerization (CGCP). The GPC trace showed a clear shift toward the higher-molecular-weight region with retention of low polydispersity, and no peak was detected in the lower-molecular-weight region. Accordingly, the target AB₂ type miktoarm star copolymers were synthesized without step-growth polycondensation. The synthesis of A₂B type miktoarm star copolymers was carried out similarly using 1-bromomethyl-3,5-bis(4-fluorobenzoyl)benzene as a trifunctional initiator. The A₂B type miktoarm star copolymers, which contained two arms of aromatic polyether units with high crystallinity, exhibited higher solubility than AB type diblock copolymer or AB₂ type miktoarm star copolymers. As in the case of the AB type diblock copolymer, the AB₂ and A₂B type miktoarm star copolymers self-assembled to form spheres of 150−600 nm diameter when a THF solution of the copolymers was allowed to dry on a glass plate. Similar spherical aggregates were obtained from a THF-methanol solution of the A₂B type miktoarm star copolymer, whereas the AB₂ type afforded fiber-like structures under the same conditions.