C-Cl stretching vibrations in ethylene-vinyl chloride copolymers.

Shimanouchi, Krimm, and others have discussed the conformational dependence of the C-Cl stretching frequencies in PVC and its oligomers. They have demonstrated that the C-Cl stretching frequencies are separable and can be correlated with the various rotational isomers in the vicinity of the vibrating C-Cl bond. Since the backbone conformation adjacent to the C-Cl side chain bond depends on the local microstructure, the C-Cl stretching region of the IR spectra of ethylene-vinyl chloride (E-V) copolymers depends on the local comonomer- and stereosequence. We have analyzed the C-Cl stretching region in the IR spectra of our E-V copolymers by calculating the population of rotational isomers for the CHCl-CH(2) backbone bonds in all V-centered E-V triads, ie., X-V-Y, where X, Y =E or V. We predicted the composition dependence of the C-Cl vibration intensities observed in our E-V copolymers by weighting the population of rotational isomers in each V-centered E-V triad by the frequency of its occurrence, which is known from our previous C-13 NVR study. Though not quantitative, agreement between observed and calculated C-Cl stretching intensities, especially their dependence on E-V composition and microstructure, it satisfying and provides the underlying connection between E-V microstructure and the resulting pattern of C-Cl vibrations.