A Joined Theoretical−Experimental Investigation on the ¹H and ¹³C NMR Signatures of Defects in Poly(vinyl chloride)

¹H and ¹³C chemical shifts of PVC chains have been evaluated using quantum chemistry methods in order to evidence and interpret the NMR signatures of chains bearing unsaturated and branched defects. The geometrical structures of the stable conformers have been determined using molecular mechanics and the OPLS force field and then density functional theory with the B3LYP functional and the 6-311G(d) basis set. The nuclear shielding tensor has been calculated at the coupled-perturbed Kohn−Sham level (B3LYP exchange-correlation functional) using the 6-311+G(2d,p) basis set. The computational scheme accounts for the large number of stable conformers of the PVC chains, and average chemical shifts are evaluated using the Maxwell−Boltzmann distribution. Moreover, the chemical shifts are corrected for the inherent and rather systematic errors of the method of calculation by employing linear regression equations, which have been deduced from comparing experimental and theoretical results on small alkane model compounds containing Cl atoms and/or unsaturations. For each type of defect, PVC segments presenting different tacticities have been considered because it is known from linear PVC chains that the racemic (meso) dyads are characterized by larger (smaller) chemical shifts. NMR signatures of unsaturations in PVC chains have been highlighted for the internal CHCH and CHCCl units as well as for terminal unsaturations like the chloroallylic CHCHCH₂Cl group. In particular, the ¹³C chemical shifts of the two sp² C atoms are very close for the chloroallylic end group. The CH₂ and CHCl units surrounding an unsaturation present also specific ¹³C chemical shifts, which allow distinguishing them from the others. In the case of the proton, the CH₂ unit of the CHClCH₂CClCH segment presents a larger chemical shift (2.6−2.7 ppm), while some CHCl units close to the CHCH unsaturations appear at rather small chemical shifts (3.7 ppm). The CH₂Cl and CHClCH₂Cl branches also display specific signatures, which result in large part from modifications of the equilibrium conformations and their reduced number owing to the increased steric interactions. These branches lead to the appearance of ¹³C peaks at lower field associated either to the CH unit linking the CH₂Cl and CHClCH₂Cl branches (50 ppm) or to the CHCl unit of the ethyl branches (60 ppm). The corresponding protons resonate also at specific frequencies: 3.5−4.0 ppm for the CH₂Cl branch or 3.8−4.2 ppm for the terminal unit of the CHClCH₂Cl branch. Several of these signatures have been detected in the experimental ¹H and ¹³C NMR spectra and are consistent with the reaction mechanisms.