Atactic dyad sequence

II), while atactic structures have dyad sequences of the HH, TH, HT, and TT types (III) ... 

Poly(7-MeNBD) prepared with 10b and 11a contains between 47% and 52% trans C=C units, which are atactic. It is reported that the cis sequences in these samples comprise more r than m dyads. A model has been proposed suggesting that the change in polymer tacticity from syndiotactic to atactic is due to switching from a mode of coordination of the previously formed C=C double bond (similar to structure E in Scheme 20.15) to a mode of noncoordination. 

It is interesting to reconsider the case of a poly(acrylic acid) dehydration reaction where two different steps can be distinguished for the atactic polymer the first step is five to six times faster than the second one and obviously corresponds to dehydration of meso dyads (isotactic sequences). Hydrolysis of syndiotactic... 

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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