The Chemical Shifts of Poly(vinyl chloride) and Its Model Compounds

Abstract

ABSTRACT:The proton chemical shifts of meso and racemic 2, 4-dichloropentanes (DCP) were calculated by taking into account the diamagnetic shielding, bond anisotropy and polar effects, using preferred comformations of TG and G'T forms for meso DCP, and TT and GG forms for racemic DCP. The TT form was found to be more stable the GG one by 1.97 kcal/mol. It is predicted theoretically that the anti methylene proton would appear at higher field than the syn proton. Also the orders of the excess charges of the 2p electrons and the total charges on the carbon atoms of meso and racemic DCP's coincide with the order of the observed C-13 chemical shifts of those DCP's.The proton chemical shifts of poly(vinyl chloride) (PVC) were calculated assuming the preferred conformations, (TG)n and (G'T)n for isotactic PVC and (TT)n for syndiotactic PVC. The calculated chemical shifts of racemic methylene and isotactic methine protons appear at a higher field than the meso methylene and syndiotactic methine protons, respectively. As for the isotactic PVC the anti proton appears at higher field than the syn proton just as for DCP.KEY WORDS Poly(vinyl chloride) / Dichloropentane / Chemical Shift / Conformation / Extended Hiickel Method / The proton NMR spectrum of poly(vinyl chloride) (PVC) in solution has long been studied for the stereochemical structure of this polymer, sometimes using decoupling and selective deuteration techniques. 1 -16 Recently Heatley, 16 et al., have studied the vicinal coupling constants of PVC, using a 220-MHz NMR spectrometer and have suggested that in thermodynamically good solvents, the isotactic dyads are in approximate equilibrium between the conformations which result in a 31 helix, whereas syndiotactic dyads are probably in a planar zigzag structure. Also, as the model compounds of PVC, the molecular structure of meso and racemic 2, 4-dichloropentanes (DCP) and isotactic, heterotactic and syndiotactic 2, 3, 6-trichloroheptanes have been studied by infra-red and NMR methods 17 -24 and it has been proposed that the TG and G'T forms are preferred for meso DCP, whereas the preferred form for racemic DCP are predominantly TT, and also GG with some probabilities. Flory, et al.,25 have pointed out the existence of less-favoured conformations. However, these may be almost ignored in our discussion described below.As described above, the molecular structures of DCP and PVC have been studied in detail, but the relationship between the chemical shifts and molecular structures of these compounds have been scarcely studied theoretically. In this paper the calculation of the proton and C-13 chemical shifts of these model compounds in their preferred conformations are described.At first, in the cases of DCP's the extended Hiickel theory 26 was applied to calculate the total electron energies, the total net charges and the excess charges of 2p electrons. Then the proton chemical shifts of these compounds were calculated by taking into account the contributions from diamagnetic shielding, magnetic anisotropy and po...