Synthesis, characterization and the paramagnetic properties of bottle-brush copolymers with shielding TEMPO radicals

Bottle-brush copolymers poly(2-hydroxyethyl methacrylate)(PHEMA)-g-{[(poly(acrylic acid)-g-2,2,6,6-tetramethylpiperidine-1-oxyl)]-b-poly(methyl acrylate)} (PHEMA-g-[(PAA-g-TEMPO)-b-PMA]) and PHEMA-g-(PAA-g-TEMPO) with shielding 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) radicals were synthesized, and their paramagnetic properties were compared in bulk and solution, respectively. Firstly, using a “grafting from” strategy, the atom transfer radical polymerization (ATRP) of tert-butylacrylate (tBA) permitted the successful synthesis of the bottle-brush copolymer PHEMA-g-PtBA from the macro-initiator poly[2-(2-bromoisobutyryloxy)ethyl methacrylate] (PBiBEMA) with a high density of initiating sites. Subsequently, following a second “grafting from” strategy and the ATRP of methyl acrylate (MA), the bottle-brush copolymer PHEMA-g-(PtBA-b-PMA) was further produced by employing PHEMA-g-PtBA as the macro-initiator. The poly(acrylic acid) (PAA) segment generated by the selective hydrolysis of the PtBA segment was used to introduce TEMPO radicals through the reaction between carboxyl groups on PAA and oxirane group on glycidoloxy-2,2,6,6-tetramethyl piperidine-1-oxyl (GTEMPO). With the shielding by the outer PMA segment in PHEMA-g-[(PAA-g-TEMPO)-b-PMA], superfine information of inner TEMPO radicals in the electron paramagnetic resonance (EPR) spectrum could be well discriminated. However, the PHEMA-g-(PAA-g-TEMPO) without any shielding showed a broad EPR spectrum, which was in accordance with a characteristic EPR spectrum of a polyradical polymer due to the strong interactions between radicals.