CRYSTALLINE-STRUCTURE OF ATACTIC POLY(ACRYLONITRILE)

Commercial poly(acrylonitrile) is an atactic polymer with a concentration of syndiotactic diads of 0.46-0.48(1,2) and shows a significant amount of crystallinity of about 30%.3 An explanation that accounts for this apparent paradox is proposed in terms of the formation of crystals from mixed tacticity chains, in which the isotactic sequences are in a ''shape emulating'' conformation which enables them, as long as they are present in sequences of even numbers of monomer units, to cocrystallize with planar zigzag syndiotactic units. The model is similar to that recently proposed to account for the crystallinity in poly(vinyl chloride) (PVC).4,5 The model is shown to be consistent with both the electron and X-ray diffraction data of PAN while, at the same time, providing an explanation of the comparatively high crystallinity and also a reason why it is significantly larger than PVC. The paper draws on the statistical basis developed in ref 5 and predicts a crystal thickness of 10 monomer units for PAN, a value similar to that in PVC but which is very difficult to measure from diffraction patterns because of the poor interchain register in the polymer.