Category Archives: Polymorphism

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

Verde-Sesto, Ester, Nicolas Goujon, Haritz Sardon, Pauline Ruiz, Tan Vu Huynh, Fermin Elizalde, David Mecerreyes, Maria Forsyth, and Luke A. O’Dell. “DNP NMR Studies of Crystalline Polymer Domains by Copolymerization with Nitroxide Radical Monomers.” Macromolecules 51, no. 20 (October 23, 2018): 8046–53.

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.

Polymorphs of Theophylline Characterized by DNP Enhanced Solid-State NMR

Pinon, A.C., et al., Polymorphs of Theophylline Characterized by DNP Enhanced Solid-State NMR. Mol Pharm, 2015.

We show how dynamic nuclear polarization (DNP) enhanced solid-state NMR spectroscopy can be used to characterize polymorphs and solvates of organic solids. We applied DNP to three polymorphs and one hydrated form of the asthma drug molecule theophylline. For some forms of theophylline, sample grinding and impregnation with the radical-containing solution, which are necessary to prepare the samples for DNP, were found to induce polymorphic transitions or desolvation between some forms. We present protocols for sample preparation for solid-state magic-angle spinning (MAS) DNP experiments that avoid the polymorphic phase transitions in theophylline. These protocols include cryogrinding, grinding under inert atmosphere, and the appropriate choice of the impregnating liquid. By applying these procedures, we subsequently demonstrate that two-dimensional correlation experiments, such as 1H-13C and 1H-15N HETCOR or 13C-13C INADEQUATE, can be obtained at natural isotopic abundance in reasonable times, thus enabling more advanced structural characterization of polymorphs.

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