Rod-coil type miktoarm star copolymers consisting of polyfluorene and polylactide : precise synthesis and structure-morphology relationship

A series of rod–coil miktoarm star copolymers consisting of poly[2,7-(9,9-dihexylfluorene)] (PF) and polylactide (PLA) arms as the rod and coil segments, respectively, were synthesized by combining the chain-growth Suzuki–Miyaura coupling polymerization and living ring-opening polymerization (ROP). First, PFs having a hydroxyl group at the α-chain end (PF–BnOH) were prepared by the polymerization of 2-(7-bromo-9,9-dihexyl-9H-fluorene-2-yl)4,4,5,5-tetramethyl-1,2,3-dioxaborolane using the initiating system of 4-(tetrahydropyran-2′-yloxymethyl)-iodobenzene/Pd2(dba)3/t-Bu3P. After a couple of chain end modifications, PFs having one, two, and three hydroxyl groups at the α-chain end (PF–OH, PF–(OH)2, and PF–(OH)3, respectively) were obtained with dispersity (Đ) values of less than 1.29. The obtained PF–OH, PF–(OH)2, and PF–(OH)3 were used as the initiators for the organic base-catalyzed ROP of rac-lactide to produce the AB type linear diblock (PF-b-PLA) and AB2 and AB3 type miktoarm star copolymers (PF-b-(PLA)2 and PF-b-(PLA)3, respectively). The self-assembled nanostructures in the thermally-annealed PF-b-(PLA)xs (x = 1, 2, and 3) were evaluated in the bulk by synchrotron small angle X-ray scattering (SAXS) and transmission electron microscopy (TEM). The PF-b-PLA, PF-b-(PLA)2, and PF-b-(PLA)3 with the PF weight fractions of 0.52–0.56 were found to form poorly ordered PLA cylindrical structures in the PF matrix due to a strong rod–rod interaction of the PF segment. On the other hand, these BCPs with the PF weight fractions of 0.22–0.23 exhibited the hexagonally close-packed cylinder (Hex) morphologies, in which the cylindrical PF phase was embedded in the PLA matrix. More interestingly, the domain-spacing (d) values for the Hex morphologies decreased with the increasing PLA arm number despite each BCP having a comparable molecular weight and PF weight fraction: d = 26.4 nm for PF-b-PLA, d = 21.7 nm for PF-b-(PLA)2, and d = 18.6 nm for PF-b-(PLA)3.