Crystal Structure of Polyvinyl Alcohol

IT has often been said (by me1 as well as by others) that the molecular repeat distance (from X-ray diffraction photographs) of polyvinyl alcohol—2·52 A.2—indicates not only that the chain

has a simple plane zigzag configuration, but also that all the hydroxyl groups (in the section of a molecule in one crystallite) lie on the same side of the zigzag plane. This is not

necessarily true ; so far as X-ray diffraction is concerned, the observed repeat distance is equally compatible with a molecular structure in which hydroxyl groups are randomly placed in

left-and right-hand positions. This possibility was not considered until recently ; it was assumed that the high degree of crystallinity of polyvinyl alcohol implied regularity of molecular

structure. Recently, however, it has been found3 that interpolymers of ethylene and vinyl alcohol are crystalline ; thus hydroxyl groups may replace hydrogen atoms at random on a carbon

chain without destroying crystallinity ; the high crystallinity of polyvinyl alcohol cannot, therefore, be regarded as evidence against the idea of stereochemical irregularity in this

molecule.

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