Isotactic chains

For disubstituted ethylenes, the presence and type of tacticity depends on the positions of substitution and the identity of the substituents. In the polymerization of a 1,1-disubstituted ethylene, CH2=CRR, stereoisomerism does not exist if the R and R groups are the same (e.g., isobutylene and vinylidene chloride). When R and R are different (e.g., —CH3 and —COOCH3 in methyl methacrylate), stereoisomerism occurs exactly as in the case of a monosubstituted ethylene. The methyl groups can be located all above or all below the plane of the polymer chain (isotactic), alternately above and below (syndiotactic), or randomly (atactic). The presence of the second substituent has no effect on the situation since steric placement of the first substituent automatically fixes that of the second. The second substituent is isotactic if the first is isotactic, syndiotactic if the first substituent is syndiotactic, and atactic if the first is atactic. 

The 1.4-cis polymerization of 1.3-pentadiene offers a second type of steric control. The methyl group of the new monomer can be sterically oriented by the methyl groups at the end of the growing polymer chain. Isotactic cis polymers can be obtained by a planar six membered ring... 

Isotactic polymer—A polymer in which like groups bonded to the main chain are always on the same side of the chain isotactic polymers have a high degree of crystallinity (e.g., isotactic polypropylene). 

In sterically random propagation, the probability of meso diad generation Pm in each addition is equal to 0.5, and the ratio of triads in the chain is i h s = = 1 2 1. In an atactic polymer, the diads m and r are statistically distributed in the chain. Natta [81] called the simplest stereoregular chains isotactic —mmmm— and syndiotactic —rrrr—. Monomers with both a and / carbons unsymmetrically substituted can form diisotactic polymers... 

Figure 9 Structures of syndiotactic (regular alternating of stereochemistry along the polymer chain), isotactic (same stereochemistry across the polymer chain), and atactic (random stereochemistry along the polymer chain) poly-...

During polymerisation, it is possible to direct the way in which isotactic monomers join on to a growing chain. This means that side groups (X) may be placed randomly ( atactic ) or symmetrically along one side of the chain ( isotactic ) or in a regular alternating pattern along the chain ( syndiotactic ) as discussed below. 

Fig. 5. Different conformations of polypropylene chains isotactic (s(3/l)l group) and syndiotactic (s(2/l)2 and tern groups). ...

Figure 5.19. Packing of ternary symmetry chains isotactic polybut-1-ene (conformation 3i).

Figure 5.20. Packing of quaternary symmetry chains isotactic polyacetaldehyde (conformation 4i).

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