Composition and Microstructure of Acrylonitrile−Butadiene Copolymers by Pyrolysis-Photoionization Mass Spectrometry

The composition and microstructure of acrylonitrile−butadiene copolymers ranging from 19 to 51 wt % acrylonitrile were determined by pyrolysis-photoionization mass spectrometry. The high molecular weight samples chosen for this study (M w > 100 000) are relatively insoluble and therefore difficult or impossible to quantitatively characterize by NMR. A simple model based upon the average number of acrylonitrile units in a group of oligomers having the same total number of monomers but variable amounts of acrylonitrile and butadiene was found to give an accurate and precise determination of the copolymer composition. In addition, Bernoullian and first-order Markovian models were used to interpret the mass spectral data. Although the copolymer compositions calculated from each model agreed quantitatively with the reported compositions, the Markovian model gave slightly more precise fits to the mass spectra. Number-average sequence lengths were also determined from the Markovian model. The acrylonitrile sequence lengths determined by pyrolysis-photoionization were larger than those reported previously for other copolymer samples. It is not known whether these differences reflect differences in the samples analyzed or artifacts of the analytical techniques.