19F DOSY diffusion-NMR spectroscopy of fluoropolymers

A new pulse sequence for obtaining F-19 detected DOSY (diffusion ordered spectroscopy) spectra of fluorinated molecules is presented and used to study fluoropolymers based on vinylidene fluoride and chlorotrifluoroethylene. The performance of F-19 DOSY NMR experiments (and in general any type of NMR experiment) on fluoropolymers creates some unique complications that very often prevent detection of important signals. Factors that create these complications include: (1) the presence of many scalar couplings among H-1, F-19 and C-13; (2) the large magnitudes of many F-19 homonuclear couplings (especially (2)J(FF)); (3) the large F-19 chemical shift range; and (4) the low solubility of these materials (which requires that experiments be performed at high temperatures). A systematic study of the various methods for collecting DOSY NMR data, and the adaptation of these methods to obtain F-19 detected DOSY data, has been performed using a mixture of low molecular weight, fluorinated model compounds. The best pulse sequences and optimal experimental conditions have been determined for obtaining F-19 DOSY spectra. The optimum pulse sequences for acquiring F-19 DOSY NMR data have been determined for various circumstances taking into account the spectral dispersion, number and magnitude of couplings present, and experimental temperature. Pulse sequences and experimental parameters for optimizing these experiments for the study of fluoropolymers have been studied. Copyright (C) 2016 John Wiley & Sons, Ltd.