Isotactic 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. 

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. 

A simultaneous coordination of the two aldehydes prior to addition may explain the observed sequence of isotactic dyads. 

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. 

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. 

Structurally, plastomers straddle the property range between elastomers and plastics. Plastomers inherently contain some level of crystallinity due to the predominant monomer in a crystalline sequence within the polymer chains. The most common type of this residual crystallinity is ethylene (for ethylene-predominant plastomers or E-plastomers) or isotactic propylene in meso (or m) sequences (for propylene-predominant plastomers or P-plastomers). Uninterrupted sequences of these monomers crystallize into periodic strucmres, which form crystalline lamellae. Plastomers contain in addition at least one monomer, which interrupts this sequencing of crystalline mers. This may be a monomer too large to fit into the crystal lattice. An example is the incorporation of 1-octene into a polyethylene chain. The residual hexyl side chain provides a site for the dislocation of the periodic structure required for crystals to be formed. Another example would be the incorporation of a stereo error in the insertion of propylene. Thus, a propylene insertion with an r dyad leads similarly to a dislocation in the periodic structure required for the formation of an iPP crystal. In uniformly back-mixed polymerization processes, with a single discrete polymerization catalyst, the incorporation of these intermptions is statistical and controlled by the kinetics of the polymerization process. These statistics are known as reactivity ratios. 

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... 

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. 

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... 

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). 

It was shown in section 4.3 that often lengthens significantly for all carbons in isotactic chain segments in dissolved polymers [37]. This may be explained by the RIS analysis in the previous section, given the reasonable assumption that the transitions between conformations is rapid. Furthermore, the analysis shows that the relaxation parameters are insensitive to the rate of the local motions, within this limit. For example, an i-PMMA dyad must have two equally weighted conformations, loosely tg and gt, on symmetry grounds. The calculation described above also finds that tt has a very similar weighting. In contrast, an s-PMMA dyad has only one main conformation, tt, with a smaller contribution from gg. Thus more extensive local motion is to be anticipated in short i-PMMA sequences than in short s-PMMA sequences, in the same overall chain. Similar observations and calculations have been made with poly(acrylonitrile). 

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). 

NMR s ability to characterize the tacticity, or stereoisomerism, of polymers, such as polypropylene, polystyrene, or poly(vinyl chloride), is also significant. The tacticity of these, or of any other stereoirregular polymer, is analyzed as sequences of stereo pairs, or dyads. Each pair is designated as either meso (m) or racemic (r), depending on the stereostructure. In isotactic... 

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