DNP NMR Studies of Crystalline Polymer Domains by Copolymerization with Nitroxide Radical Monomers

Abstract

Dynamic nuclear polarization (DNP) nuclear magnetic resonance (NMR) spectroscopy is increasingly recognized as a powerful and versatile tool for the characterization of polymers and polymer-based materials. DNP requires the presence of unpaired electrons, usually mono-or biradicals, and the method of incorporation of these groups and their distribution within the structure is crucial. Methods for covalently binding the radicals to the polymer and controlling their location (e.g., exclusively within a specific phase or at an interface) can allow the selective enhancement of a particular region or the measurement of domain sizes. We have prepared a series of polyurethanes by copolymerization of a nitroxide radical monomer with poly(ethylene glycol) (PEO) and diisocyanate linkers. The PEO is shown to form crystalline domains with the radical monomers in a separate phase, providing DNP enhancements of around 10 and allowing the domain size and morphology to be probed with the aid of X-ray scattering data. Additionally, electron paramagnetic resonance is used to estimate the inter-radical distances and density functional theory calculations are used to refine the PEO crystal structure.

Structural Investigations of Polymer Materials by Dynamic Nuclear Polarisation Solid-state NMR

“…This latter strategy was actually used recently in a very nice study by O'Dell and coworkers who synthesised nitroxides-containing polyurethanes with poly(ethylene oxide) (PEO) segments, which were subsequently studied by DNPenhanced 13 C CP MAS SSNMR. 85 While these authors obtained relatively modest DNP signal enhancements (<15), they could elegantly apply a theoretical model proposed by Emsley and coworkers to probe the size and morphology of the crystalline PEO domains within the material, 86 obtaining a remarkable agreement with independent X-ray scattering data for one of the samples that displayed a lamellar morphology. All together, however, it should be kept in mind that no universal method is nowadays available to properly prepare any polymer sample for DNP analysis, at least as long as one seeks to optimise both the sensitivity and the resolution of the experiment, especially for polymer samples with short 1 H spinlattice relaxation times.…”

Investigating the efficiency of silica materials with wall-embedded nitroxide radicals for dynamic nuclear polarisation NMR

“…1,2 In particular, when combined with magic-angle spinning (MAS), DNP can tremendously improve the sensitivity of high-field solidstate NMR (SSNMR) experiments, [3][4][5][6][7] and this has allowed for the detailed structural characterisation of a very broad range of samples, from biomolecular compounds [8][9][10][11][12][13][14] to inorganic [15][16][17][18][19][20] or hybrid materials, [21][22][23][24] organic crystals [25][26][27][28][29] and polymers. [30][31][32][33][34][35][36][37] More precisely, DNP enhances nuclear magnetisation through the microwave-driven transfer (usually at cryogenic temperatures) of electron spin polarisation to nuclei via exogenous paramagnetic centres referred to as polarising agents (PA). These polarising agents are stable paramagnetic species (e.g.…”

Porous functionalized polymers enable generating and transporting hyperpolarized mixtures of metabolites

“…Many materials containing PAs freely dispersed 31 or covalently bound onto polymers 32 , 33 and covalent organic frameworks 34 have been designed for MAS- or Overhauser-DNP (i.e. not requiring effective separation of polarizing medium and hyperpolarized analytes).…”