Molecular structure and rubber-like elasticity II. The stereochemistry of chain polymers

Abstract

A survey of the known structures of molecules containing two or more singly linked carbon atoms has shown that the staggered bond configuration is favoured. It is suggested that these molecules may be examples of a general rule—the ‘principle of staggered bonds’—which specifies the bond positions for successions of singly linked carbon atoms, just as the ‘tetrahedral principle’ specifies the bond positions on any one carbon atom. If this is true, it is possible to predict molecular configurations. The number of alternative configurations, all obeying the principle of staggered bonds, increases rapidly with the number of carbon atoms, but the,most probable configuration of a particular molecule can sometimes be predicted on geometrical grounds. The simpler types of carbon chain are described. The chain-type in a crystalline long-chain polymer may sometimes be deduced from a knowledge of the identity period of the molecules, which is readily found from X-ray or electron diffraction patterns. The principle of staggered bonds is also obeyed in molecules containing singly linked nitrogen, oxygen and sulphur atoms. The configurations of lietero-atomic chains containing these atoms are therefore of the same types as carbon chains. In molecules containing double bonds, the principle of staggered bonds may still be operative, a double bond being regarded in the familiar simple geometrical way as two distorted single bonds. The non-planar chain forms of gutta-percha, rubber and polychloroprene are attributed to the cooperation of two influences: the principle of staggered bonds, and the repulsion between methyl or chlorine side substituents and particular 0H o groups in the chains.