Syndiotactic dyad sequence

In polymers made of dis-symmetric monomers, such as, for example, poly(propylene), the stmcture may be irregular and constitutional isomerism can occur as shown in figure C2.1.1(a ). The succession of the relative configurations of the asymmetric centres can also vary between stretches of the chain. Configuration isomerism is characterized by the succession of dyads which are named either meso, if the two asymmetric centres have the same relative configurations, or racemo if the configurations differ (figure C2.1.1(b )). A polymer is called isotactic if it contains only one type of dyad and syndiotactic if the dyad sequence strictly alternates between the meso and racemo fonns. 

Consider the description of the sequence distribution of isotactic and syndiotactic placements in the polymerization of a monosubstituted ethylene. The approach is general and can be applied with appropriate modification to the 1,4-polymerization of a 1,3-diene. Dyad tac-ticity is defined as the fractions of pairs of adjacent repeating units that are isotactic or syndiotactic to one another. The isotactic and syndiotactic dyads (XV) are usually referred to as meso and racemic dyads. The horizontal line in XV represents a segment of the polymer... 

Since P and Pr are synonymous with isotactic and syndiotactic dyad fractions m and r, respectively, probabilities for isotactic, syndiotactic, and heterotactic triad sequences will be given by... 

If the vinyl monomer produces a stereogenic center upon incorporation into the polymer backbone, then additional structural permutations are possible. In the simplest case, we can consider the relative stereochemistry of adjoining monomer units in a polymer dyad. If these centers produce a mirror plane or a C2 axis of symmetry, then the dyad is said to have meso (m) or racemic (r) relative stereochemistry, respectively, as shown in Figure 1. The relative stereochemistries of higher n-ad sequences are defined by the stereochemistries of the component dyad sequences as shown for the triad components of isotactic (nun), syndiotactic (rr), and heterotactic (mr) polymers. 

NMR m and r dyad analysis of syndiotactic polystyrenes prepared with MAO activated CpTiCb or Cp TiCl3 systems was in agreement with the Bernoullian symmetric statistical model for stereoselective propagation. The polymer microstracture is a long sequence of syndiotactic dyads with only isolated m defects and no consecutive isotactic dyads (Figure 14.17), which is consistent with a chain-end control mechanism directed by the configuration of the tertiary carbon of the last inserted styrene unit (unlike 1,3-asymmetric induction). Thus, the syndiotactic preference exerted by the chain end arises from the phenyl-phenyl repulsive interaction between the last inserted... 

Note that in these equations the designations I or m (for meso) are used synonamously to designate isotactic dyads, and S or r (for racemic) for syndiotactic dyads. Accordingly to the m-r designation, an Isotactic triad would be an mm sequence, a syndiotactic triad an rr sequence and a heterotactic triad an mr (or rm) sequence (2). 

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. 

The number of racemic dyads in a sequence is the same as the number of syndiotactic units n. The number of meso dyads in a sequence is the same as the number of iso units nj. These can also be verified from structure [XVIII] above. 

Use the dyad and triad fractions in Table 7.9 to calculate the average lengths of isotactic and syndiotactic sequences for the polymers of Fig. 7.10. Comment on the results. 

Fig. 16. Relation between the syndiotactic triad fraction [rr] and the heterotactic dyad fraction of irregular propylene unit sequences [F01 + F10] in polypropylenes obtained at —78 °C with various soluble vanadium-based catalysts (from Ref. 95 )...

Second, the double bonds in the polymer may be cis or trans the double bond pair sequences may therefore be cc, ct or tt. Third, the rings may have one of two configurations ring dyads may therefore be m (isotactic) or r (syndiotactic) as represented in IP. Fourth, when the monomer is substituted in such a way that it does not have a plane of symmetry, there is the possibility of head-head (HH), head-tail (HT) or tail-tail (TT) structures in the polymer. 

Much interest therefore centres on the stereospecificity of the reaction since this provides clues to the mechanism. Additional information can be derived from the structural sequences in such polymers the double bonds may be present in cc, cl, or tt dyads successive rings may have an isotactic (m) or syndiotactic (r) relationship and the substituents may be oriented randomly to give head-head (HH), head-tail (HT), and tail-tail (TT) structures, or may align always in the same direction (HT) (see Ch. 11, 12 and 13). 

Free radical propagation is poorly stereocontrolled, with nearly equal proportion of meso and racemic dyads in polymerization of monosubstituted alkenes and a preference for syndiotactic placement for disubstituted monomers such as methacrylates (rr = 0.62, mm = 0.04). The sequence distrihution follows a first-order Markov model with a slight deviation from Bernoulian statistics. However, for very bulky substituents, as in polymerization of triphenylmethyl methacrylate, the preference for isotacticity was observed (mm = 0.64, rr = 0.12). Recently, complexation with Lewis acids and acidic solvents enabled to enhance stereocontrol in polymerization of vinyl esters and acrylamides, and to a smaller degree in polymerization of methacrylates (127-129). 

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... 

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