Racemic pentad configuration

This amorphous polypropylene, therefore, has a tendency toward short blocks of meso and racemic sequences. This structural conclusion is not readily apparent from a simple inspection of the pentad distribution although the dominant pentads are mmmm, mmmr, rrrr, rrrm and mmrr as indicated by the above formula for the average configurational structure. 

Carbon-13 nuclear magnetic resonance (NMR) is the most useful method of assessing tacticity. By C-13 NMR it is possible to assess the different sequential distributions of adjacent configurational units that are called dyads, triads, tetrads and pentads. The two possible dyads are shown in Fig. 1.5. A chain with 100% meso dyads is perfectly isotactic whereas a chain with 100% racemic dyads is perfectly syndiotactic. A chain with a 50/50 distribution of meso and racemic dyads is atactic. 

Tacticity in NMR is represented by the relative configuration of pair wise units. A typical C-NMR spectrum of an atactic vinyl polymer in solution consists of numerous relatively sharp lines, that are assigned to the various sequences of meso (m) and racemic (r) dyads. These new lines are a result of configurational splitting. Meso (m) or isotactic dyads have the same optical configurations (dd or 11). Racemic (r) or syndiotactic dyads are pairs of adjacent asymmetric centers that have opposite optical configuration (dl). Triads, tetrads, pentads, etc., are denoted by a succession of dyads. 

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