Simultaneous 'H and '9 broad-band decoupled l3C NMR spectra are recorded for poly(vinylidene fluoride) (PVFJ, poly(fluoromethy1ene) (PFM), poly(viny1 fluoride) (PVF), and poly(trifluoroethy1ene) (PF&). Observed 13C chemical shifts are compared to those calculated as a function of stereoregularity and/or defect structure, i.e., head-to-head to tail-to-tail (H-H:T-T) addition of monomers. Calculated chemical shifts are obtained through enumeration of the numbers and kinds of y effects (three-bond gauche arrangements) involving each carbon atom as dictated by the conformational characteristics of each polymer. Agreement between measured and predicted 13C chemical shifts is observed for each fluoro polymer when the following y effects are assumed: YCH,,CH, = -5.3, YCH~CF, = -2.2, YCH,CHF = -3.7, Y C H *~ = -3.8, YCHF,~ = -5.0, YCHF,F = -3.0, YCF*,C = -2.1, and YCF,,F = -1.4 ppm, where ya,b is the upfield shift (ppm) a t carbon a produced by atom or group b when in a gauche arrangement with a. This agreement permits detailed assignments of 13C NMR resonances in the spectra of all four fluoro polymers, including identification of those 13C resonances belonging to the carbon atoms in the H-H:T-T defects present in PVF2, PVF, and PF3E chains, whose abundance increases in this order. The effects of @fluorine substituents on the ' % chemical shifts in these polymers are also evaluated from the observed defect resonances.
IntroductionWe have recently demonstratedl that the 13C NMR spectra of vinyl homo-and copolymers can be predicted from analysis of the numbers and kinds of y interactions involving each carbon atom type in the polymer chain. Each nonhydrogen y substituent in a three-bond gauche arrangement (see Figure 1) with a given carbon atom produces an upfield chemical shift of that carbon resonance relative to its trans arrangement. The frequency with which such y gauche interactions occur can be evaluated from the conformational characteristics of the polymer chain as manifested by calculated bond rotation probabilities.The sensitivity of vinyl homo-and copolymer 13C chemical shifts to stereosequence and comonomer sequence results from the dependence of bond rotation probabilities on the same microstructural features, which lead via the y gauche effect to a dispersion of 13C chemical shifts reflecting the different possible microenvironments found along the vinyl polymer chain. All that is needed to calculate the effects of polymer microstructure on 13C NMR chemical shifts are the bond rotation probabilities obtained from the polymer's conformational characteristics and the magnitudes of the upfield y effects produced by the gauche arrangements of a given carbon atom with its substituents.This approach, which fully utilizes all the microstructural information provided by 13C NMR spectroscopy, has been successfully applied to the calculation of the 13C chemical shifts in a variety of vinyl homo-and copolymers.' Polypropylene 01igorners~~~ and homopolymer4 and its copolymers with ethylene5i6 and vinyl ~hloride,~ poly(...
