Structural mobility of molecular bottle-brushes investigated by NMR relaxation dynamics Joanna Pietrasik 1 , Brent S. Sumerlin 2 , Hyung-il Lee, Roberto R. Gil, Krzysztof Matyjaszewski * Center for Macromolecular Engineering, Department of Chemistry, Carnegie Mellon University, 4400 Fifth Avenue, Pittsburgh, PA 15213, United States Received 28 October 2006; received in revised form 28 November 2006; accepted 28 November 2006 Available online 15 December 2006 Abstract The structural mobility of monomeric units of molecular bottle-brushes was studied by a systematic evaluation of NMR relaxation dynamics. The spinespin relaxation time (T 2 ) was determined by CarrePurcelleMeiboomeGill (CPMG) NMR spectroscopic measurements. T 2 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 T 2 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 T 2 for the side chain PBA protons decreased with grafting density. However, after initial increase, the relaxation times eventually decreased with PBA side chain length. Ó 2006 Elsevier Ltd. All rights reserved. Keywords: Molecular bottle-brush; Graft copolymer; ATRP 1. Introduction Bottle-brush polymers have extended cylindrical shapes [1e6]. The chain-extended conformation is the result of intramolecular excluded volume interactions, a consequence of the high grafting density along the backbone of the co- polymer [7e9]. Therefore, variation in the grafting density along the backbone of the copolymer affects the conforma- tions of the bottle-brush polymers. There are several reports that correlate the shape of the bottle-brush macromolecules to their architectural parameters [9e11]. Numerous techniques have been used to characterize the solution properties of bottle-brush molecules, including sedimentation velocity [12] and viscosity [13,14] measurements, dynamic light scattering (DLS) [7,13,15], static light scattering (SLS) [13,15,16], small-angle neutron scattering (SANS) [17], and small-angle X-ray scattering (SAXS) [15,18]. Nuclear magnetic resonance has been extensively used to study the molecular dynamics of polymers in solution and in the solid state, since longitudinal (T 1 ) and transverse (T 2 ) relaxation times are extremely sensitive to chain motions. In solution, the relaxation of polymers is caused by the com- bination of rapid vibrational motions, gaucheetrans isomeri- zation, and even slower segmental motions. Conformational transformations of melted or dissolved polymers are quite rapid as compared to the tumbling and looping motions of entire chains. These localized molecular motions are very effective in causing relaxation, and even for high molecular weight polymers, the reorientation of the entire chain makes only a small contribution to the overall relaxation [19e21]. Modeling the molecular dynamics of polymers in solution requires that the relaxation times of distinctive nuclear pairs (i.e. 13 C- 1 H) be measured in solution as a function of magnetic field strength and temperature and then compare the fits to var- ious models. From these calculations, the motional correlation * Corresponding author. Tel.: þ1 412 268 3209; fax: þ1 412 268 6897. E-mail address: km3b@andrew.cmu.edu (K. Matyjaszewski). 1 Present address: Institute of Polymer & Dye Technology, Department of Chemistry, Stefanowskiego 12/16, 90 e 924 Lodz, Poland. 2 Present address: Department of Chemistry, Southern Methodist University, 3215 Daniel Avenue, Dallas, TX 75275-0314, United States. 0032-3861/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.polymer.2006.11.057 Polymer 48 (2007) 496e501 www.elsevier.com/locate/polymer