Raspberry-like Aggregate Formation of π-Stacked Block Copolymer of Dibenzofulvene and Methyl Methacrylate Synthesized by Atom-transfer Radical Polymerization

Block copolymerization of dibenzofulvene (DBF) forming a sequence having π-stacked conformation and methyl methacrylate (MMA) forming a flexible sequence was conducted by atom-transfer radical polymerization (ATRP) where MMA was polymerized first and DBF was then blocked. The block copolymer contained a fraction with number-averaged molar mass of 873400 (vs. polystyrene) which was found to be raspberry-like aggregates with sizes of hundreds of nanometers composed of smaller spherical particles with sizes of a few to low tens of nanometers. The aggregates are considered to have a core-shell structure where strong inter-chain interaction between poly(DBF) segments form the core and flexible poly(MMA) segments are the shell, and the formation appeared to be thermally driven. The aggregates having a rigid structure stabilized the π-stacked conformation of poly(DBF) segments both in the ground state and in excited states as found by NMR and fluorescence spectra, respectively, and restricted molecular motion of both the poly(DBF) segments and poly(MMA) segments.