Structural mobility of molecular bottle-brushes investigated by NMR relaxation dynamics

The structural mobility of monomeric units of molecular bottle-brushes was studied by a systematic evaluation of NMR relaxation dynamics. The spin-spin relaxation time (T₂) was determined by Carr-Purcell-Meiboom-Gill (CPMG) NMR spectroscopic measurements. T₂ for protons that reside on the exterior and interior of the bottle-brush macromolecules varied with the grafting density and side chain length in bottle-brush copolymers. Poly((2-(2-bromopropionyloxy)ethyl methacrylate-stat-methyl methacrylate)-graft-butyl acrylate) (poly((BPEM-stat-MMA)-graft-PBA) was studied as a model brush copolymer. The T₂ values for protons of MMA units in the brush backbone significantly decreased with increasing side chain length and grafting density of PBA. The mobility and relaxation times T₂ for the side chain PBA protons decreased with grafting density. However, after initial increase, the relaxation times eventually decreased with PBA side chain length.