Synthesis of Polystyrene-block-Poly(methyl methacrylate) with Fluorene at the Junction: Sequential Anionic and Controlled Radical Polymerization from a Single Carbon

Polystyrene - block - poly(methyl methacrylate) (PS-b-PMMA) has been synthesized by sequential anionic and reverse atom transfer radical polymerization (ATRP) or a variation of nitroxide mediated polymerization (NMP) from a single initiating site, specifically the 9-carbon on 2,7-dibromofluorene or fluorene. The addition of the second arm (PS) relied on thermal decomposition of 2,2'-azoisobutyronitrile (AIBN) to generate radicals, abstracting the 9-H on the polymer-bound fluorene species to form the initiating radical. Styrene was not present in the reaction mixture when AIBN was decomposed, preventing competition between addition across the monomeric alkene and hydrogen abstraction from the fluorene. After 1 h, styrene was introduced and mediation of the subsequent radical polymerization was achieved by the presence Of CuCl2/ligand or TEMPO. Characterization of the diblock copolymers by gel permeation chromatography (GPC) revealed substantial shifts in number average molecular weight ((M) over bar (n)) values compared to the anionically prepared PMMA macroinitiator, while polydispersity indices (PDI's) remained relatively low (typically < 1.5). Characterization by UV detection with GPC (at 310 nm) verified that the diblock polymer is chromophore-bound, which was further verified by UV-vis spectroscopy of the isolated diblock.