Natta projection

Fig. 1.6. Tacticity in macromolecules of substituted vinyl monomers (Natta projection)...

The effective communication of chemical structure is essential for all chemists. Over the years many different types of structure representation have been developed. Before the use of computers, chemists drew structures manually, often using a linear text notation. As more sophisticated methods for drawing have become available, the trend has been toward two-dimensional stick structures, such as the zigzag Natta projection (Figure 8.1). 

Linear macromolecules having a constitutional repeating unit such as -CH2-CHX- (X 7 H) show two further stereoisomerisms, i.e., optical isomerism and tacticity. The stereoisomerism named tacticity has its origin in the different spatial arrangements of the substituents X. When we arrange the carbon atoms of the polymer main chain in a planar zigzag conformation in the paper plane, X is either above or below that plane ( Natta projection ). If the substituents X of the vinyl polymer are either all above or all below that plane (case I), the polymers are... 

Figure 3-5. Comparison of different projections of isotactic polymers with the base units (—CHR—) (I), (—CH2—CHR—) (II), and (—CH —CHR—O—) (III). From left to right Fischer projection (cis-eclipsed chain conformation), Natta projection (trans-staggered), Newman projection (truns-staggered), and Newman projection (ds-eclipsed).

CH2—CHR-, and - CH2—CHR—O-. Without going any further into individual types of conformations at this stage, it need only be mentioned that the Fischer projection corresponds to an eclipsed, and the Natta projection to a staggered conformation. Newman projections can represent both eclipsed and staggered conformations. As a rule, trans-staggered and as-eclipsed conformations are used as the basis of such projections. (For the significance of these expressions see Section 4.5.)... 

In the erythro-diisotactic configuration (eit), the substituents R and R all lie on the same side in a Fischer projection, whereas in a Natta projection all the R substituents are found on one side, but the R substituents on either side, of the plane of the paper. In the Newman projection of the eclipsed... 

Figure 1, Natta projections and Newmann diagrams for a meso rotational dyad. In the Natta projection R refers to an aromatic ring and C to one section of the remainder of the chain bonded to the dyad. In the Newmann diagram CH corresponds to the right asymmetric carbon of the Natta projection.

Figure 2. Natta projections of allowed meso and racemic dyads in the two state model for the aryl vinyl polymers. The values listed for R are the distances between the centers of mass of the substituent phenyl rings and those for are the orientation factors defined by Equation 3.

The pioneering work of Natta and co-workers introduced the concept of tacticity, i.e. the orderliness of the succession of configurational repeating units in the main chain of a polymer. For example, in polypropene (polypropylene), possible steric arrangements are (shown in Fischer projections displayed horizontally) ... 

Figure 3.14 Projection of the monoclinic unit cell of polypropylene along the chain-axis. (From Natta, G. and Corradini, R, Nuovo Cimento Supp/., 15(1), 40, 1960. With permission.)...

Tref has been used for many years in polymer reaction engineering investigations. For instance, Tref was one of the most important analytical techniques to determine the presence of multiple-site catalysts on heterogeneous Ziegler-Natta catalysts used for olefin polymerization, as previously illustrated in Fig. 18. Crystaf, with a much shorter analysis time than Tref, permits the routine determination of the CCD in polymer reaction engineering projects. 

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