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The structure and properties of two laterally ordered (directionally crystallized) polymers are discussed, namely, polyacrylonitrile and poly(vinyl trifluoroacetate). Evidence from x‐ray diagrams, infrared measurements, thermal expansion data, and fiber properties are interpreted in terms of molecular structure. It is concluded that the existence of only chain‐to‐chain order (lateral order) in polyacrylonitrile and poly(vinyl trifluoroacetate) is principally due to steric and dipolar intramolecular repulsions. These lead to a chain stiffening effect, regardless of tacticity, which is so great in the former case that the backbone chain is forced to assume a twisted, kinked configuration. The stiff molecular chains may be thought of as rigid rods which pack in a lattice array. The side groups, nitrile or trifluoroacetate, assume varying angles with the respect to the rod axis in order to minimize the steric and dipolar repulsion forces. This variation gives rise to diffuse off‐equatorial scattering in the x‐ray diagram of the oriented polymers and low values of the dichroic ratios of selected bands in the infrared. The chain stiffening effect is greater in polyacrylonitrile and this polymer exhibits a higher “melting point”, higher fiber tenacity at equivalent orientation, less solubility, more general diffuse scattering in the x‐ray diagram, and lower dichroic ratios in the polarized infrared spectrum than does poly(vinyl trifluoroacetate). In fact, polyacrylonitrile, in contrast to poly(vinyl trifluoroacetate), acts as if it were all ordered and contained no amorphous regions in the usual sense, as judged from thermal expansion data and the lack of a definite amorphous halo in the x‐ray diagram. Poly(vinyl trifluoroacetate) is concluded to be no more syndiotactic than conventional poly(vinyl acetate), since laterally ordered poly (vinyl trifluoroacetate) can be derived from commercial poly (vinyl alcohol) by esterification. Therefore, the observed lateral order in poly (vinyl trifluoroacetate) is not due to a predominantly syndiotactic structure. These laterally ordered polymers have a properties intermediate between those of normally crystalline polymers and those of amorphous polymers.

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