Tacticity of Polyvinylchloride

The study of stereochemical configuration by C-NMR has not been limited to the polyolefins. Schneider and co-workers [67] showed that the absorption around 14.49 pm was proportional to the number of isotactic diads, and in the region of 16.66-15.62 pm to syndiotactic diads. 

Based on this finding they proposed a method for determining the tacticity of amorphous samples of polyvinylchloride (PVC). As some samples cannot be easily transformed into an amorphous state Schneider and co-workers [68] devised an infrared method of tacticity determination which is independent of the crystallinity of the samples. From the temperature dependence of infrared spectra of PVC samples prepared by different methods, the intensity of the band at 14.40 pm (proportional to the number of isotactic diads in the sample), as well as that of the tacticity-independent C-Fl stretching band, was found to be independent of the crystallinity of the sample. These lines were applied for the tacticity determination in PVC, measured in the form of potassium bromide pellets. The numerical tacticity value was obtained from the known values of absorbance coefficients of Sch and S h type C-Cl stretching bands in solution, and from the shape of the spectrum. 

Abe and co-workers [69] have investigated the NMR spectra of model compounds of PVC in the hope that these investigations may offer useful information for the analysis of vinyl polymer spectra. They studied the NMR spectra of three stereoisomers of 2,4,6-trichloroheptane as model compounds of PVC  

Spectra were observed at 60 and 100 ppm both at room temperature and at high temperatures and spin-decoupling experiments were performed. The difference of the chemical shifts of the two meso methylene protons at 60 ppm was found to be approximately 7 counts per second for the isotactic three unit model while it was approximately 16 counts per second for the isotactic two-unit model or heterotactic three-unit model. The spectra of PVC can be interpreted reasonably on the basis of this result. Observed values of vicinal coupling constants of model compounds were interpreted as the weighted means of those for several conformations, and the stable conformations of the models were determined. 

Chemical shifts of PVC and model compounds such as meso- and racemic-2,4-dichloropentane have been measured from NMR spectra [70]. 

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