Synthesis and Characterization of Molecular Bottlebrushes Prepared by Iron-Based ATRP

Molecular bottlebrushes with hydrophobic poly(n-butyl acrylate) or polystyrene and hydrophilic poly(di(ethylene glycol) ethyl ether acrylate)) side chains were successfully synthesized by grafting from a poly(2-(2-bromoisobutyryloxy)ethyl methacrylate) macroinitiator using iron-based atom transfer radical polymerization (ATRP). Iron(II) bromide, iron(III) bromide, and tetrabutylammonium bromide catalyst was employed for an ATRP grafting-from reaction, resulting in brush macromolecules with a narrow molecular weight distribution (Mw/Mn = 1.18–1.28). Molecular weights measured by multiangle laser light scattering correlates well with the theoretical values for all bottlebrushes. Imaging of individual bottlebrushes by atomic force microscopy exhibited a wormlike conformation. Initiation efficiencies were calculated by cleaving the side chains by alcoholysis and then injecting to gel permeation chromatography. The initiation efficiencies were ca. 80–95%, showing relatively high values for a grafting from polymerization with an iron catalyst. These results indicate that iron-catalyzed ATRP allows well-controlled polymerization even when targeting dense grafting from procedures. The 0.1% (w/w) of water-soluble molecular bottlebrushes with poly(di(ethylene glycol) ethyl ether acrylate)) side chains displayed a lower critical solution temperature behavior in distilled water, and the average particle size started to increase above 8 °C due to intermolecular aggregation of the bottlebrushes. The slight decrease of the size in highly diluted solution (0.005% w/w) of the bottlebrush was observed as the temperature was increased, suggesting that intramolecular collapse of the individual molecules.