Diad, configurational defined

A pair of consecutive, but not necessarily contiguous, tetrahedral stereoisomeric centers defines a diad [16]. Stereosequences terminating in tetrahedral stereoisomeric centers at both ends, and which comprise two, three, four, five, and so on, consecutive centers of that type, may be called diads, triads, tetrads, pentads, and so on, respectively. If the two units belonging to a diad have the same configuration, the diad is defined meso (m) and is characterized by a mirror plane of symmetry if the two units are enantiomorphous, the diad is defined racemo (r) and is characterized by a twofold rotation axis of symmetry (Fig. 2.2b). 

When it is necessary to specify the internal stereochemistry of the group, a prefix is required. In vinyl polymers there are meso (m) and racemic (r) diads and mm, mr, rr triads. The latter may be called isotactic, heterotactic and syndiotactic triads, respectively. Stereoregular vinyl polymers can be defined in terms of the regular sequences of diads thus an isotactic vinyl polymer consists entirely of m diads, i.e., it corresponds to the following succession of relative configuration -mmmmmm-, whereas a syndiotactic vinyl polymer consists entirely of r diads, corresponding to the sequence -rrrrrrr-. Similarly, a vinyl polymer consisting entirely of mr (= rm) triads is called a heterotactic polymer. 

In order to describe the stereosequence of such chains, meso or m diads with identical and racemic or r diads with opposite configuration of succeeding asymmetric carbon atoms are defined ... 

For macromolecules it is useful to consider the structure of the polymer resulting from monomer sequences that contain the asymmetric centre. Figure 1.5 shows the two possibilities for the addition of the repeat unit as sequences of d units or I units to give meso (m) diads (dd or //) when adjacent groups have the same configuration or racemic (r) diads (dl or Id) when they are opposite. If these sequences are repeated for a significant portion of the chain then we can define the tacticity of the polymer as being principally... 

The model shortly reviewed in the previous section will be the base for the simulation of the NMR spectrum. A sequence of three specified diads Xi and four well-defined dihedral angles will be necessary for the simulation (vide infra). As we want to investigate a high molar mass polymer and not a low molecular compound, we concentrate on a small sequence within an extended polymer chain of several hundred monomeric units (Fig. 15.b). We are interested in the probability distribution of the four dihedral angles t 2> if the configuration Xi, X2, Xs of the three central diads is given and... 

---