Tacticity of Polyvinyl Chloride PVC

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 

49 jam 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 PVC. Because some samples cannot be easily transformed into an amorphous state, Schneider and co-workers [68] devised an IR method of tacticity determination which is independent of sample crystallinity. From the temperature dependence of IR 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-H stretching band, was found to be independent of sample crystallinity. These lines were applied to the tacticity determination in PVC, measnred in potassium bromide pellets. The numerical tacticity value was obtained from the known values of absorbance coefficients of S h and S h type C-Cl stretching bands in solution, and from the shape of the spectrum. 

Abe and co-workers [69] 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 macrocycles/s and 100 macrocycles/s both at room temperature and at high temperatures, and spin-decoupling experiments were done. The difference in the chemical shifts of the two meso methylene protons at 60 macrocycles/s was found to be approximately 7 cycles/s for the isotactic three-unit model, whereas it was approximately 16 cycles/s for the isotactic two-unit model or heterotactic three-imit model. PVC spectra can be reasonably interpreted 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 determined. 

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