Use of 1H/13C/19F Triple Resonance 3D-NMR to Characterize the Stereosequences in Poly(vinyl fluoride).

Abstract

Tacticity has an enormous influence on the physical and chemical properties of polymers. There is considerable work using 1D NMR and empirical rules to study the stereosequences in polymers. This work shows that 1H/13C/19F 3D NMR experiments can provide superior resolution and atomic connectivity information, so that unambiguous resonance assignments can be made for poly(vinyl fluoride) (PVF). Compared to prior work on 3D NMR studies of stereosequence effects in fluoropolymers, the 3D NMR pulse sequence used in this work is based on single quantum coherence transfer, which eliminates the complicated splitting patterns resulting from evolution of multiple-quantum coherence. In addition, selective excitation of the 19F nuclei of interest significantly reduces the folding of peaks from other spectral regions. This greatly simplifies the spectra and makes the assignment of resonances much easier. Based on these results, it is possible to assign the 19F resonances to the pentad level. For example, consider the resonances of mm-centered sequences, which are not well resolved in 19F-19F COSY 2D NMR spectrum. 1H/13C/19F 3D NMR data provide clear evidence for all of the three pentad structures: mmmm, mmmr, and rmmr. Examples showing the resonance assignments of head-to-tail sequences are presented.