https://doi.org/10.1351/goldbook.09586
Pair of adjacent constitutionally equivalent stereogenic units that are of unlike configuration.
Figure 1: Diagrammatic representations of individual m and r diads as rotated Fischer projections (left) and as zigzag Natta projections (right). The zigzag line –/\/\/\– represents a connecting group or bond, which need not be symmetrically-constituted.
Figure 2: Diagrammatic representations of the configurational repeating unit or stereorepeating unit (upper), and the diad sequence (lower) for isotactic poly(oxypropylene), both as a rotated Fischer projections (left) and as a zigzag projection (right).
Note: Usage of the term racemo diad for describing structures with a symmetrically-constituted connecting group is no longer acceptable because of the potential for confusion with the similar term used in organic chemistry to describe small molecules.