Syndiotactic polymers helical structures

Vinyl monomers, such as styrene, and alkenes with a side group, such as propylene, can polymerize in several molecular forms whose crystallization behaviors are quite different from each other. If the side groups are all on one side of the backbone, the structure is called isotactic, and if they are on alternating sides, it is called syndiotactic. If they are distributed in a random fashion, the polymer is said to be atactic. The isotactic and syndiotactic forms are crystallizable, often in a helical structure, while the atactic form does not crystallize and solidifies only at its glass transition temperature. Figure 2.3 illustrates the tacticities mentioned above for the case of polypropylene. It has been found that polypropylene tacticity can also have an important effect on chain dimensions [10] and on the rheological behaviour of the melt [11]. 

In the case of syndiotactic polymers with chains in helical conformation, as for sPP, the symbol s(2/l)2 indicates that neighboring structural units are repeated through the operation of twofold axes perpendicular to the chain axis and each pair of units is repeated according to a helix containing two pairs of monomeric units in one pitch. A line repetition group for sPP in the fully extended trans-planar conformation is tcm. In this case the repeating unit comprises two monomeric units [20]. 

The above reasoning regarding helical hand in the crystal rests on the assumption that the polymer melt is either made of random coils, or that, if helical stretches exist in the melt, both right- and left-handed helices exist for chiral but racemic polymers such as isotactic (or syndiotactic) polyolefins. For random coils, the conformation of the incoming chain would simply have to adapt to the crystalline substrate structure. When helical stretches do exist, the sorting-out process described above would have to be fully operative. 

Isotactic polypropylene is a rather stiff and tough solid material with a melting point of 164°C. Closely packed, CHs-studded helices (Figure 17), rigidly interwoven in crystalline domains (Figure 18), account for the mechanical and thermal resistance of isotactic polymers. Syndiotactic polypropylene has a related crystalline structure, but atactic polymers are amorphous and form oily or waxy materials depending on chain lengths. 

X-ray examination showed these polymers to be highly crystalline with the polymer assuming a helical configuration (Fig. 2). A soluble catalyst consisting of a combination of dialkyl aluminum halide and vanadium tetrachloride gives a regular structure with alternating d and 1 placements (syndiotactic) (18). Certain other soluble catalysts or those based on non-crystalline... 

The protective effects of syndiotactic and isotactic poly-2-vinylpyridine-l-oxide remarkably differ in the spatial arrangement of the structural units (Holt et al. 1970). The isotactic polymer has probably a helical conformation. When the cultures... 

The ordered structures of some polymers are governed by the influence of specific diluents. This involves a specific type of polymorphism, the more general aspects of which will be discussed in the chapter concerning thermodynamic quantities. Syndiotactic poly(styrene) is a polymer that is rich in compound formation with solvent mediated polymorphic behavior.( 126-130) The polymer can crystallize in four major crystalline modifications that involve two different chain conformations. In the a and p modifications the chains adopt an all trans planar zigzag conformation. These two modifications are formed by crystallization from the melt and, under special conditions, from solution. In contrast the y and 5 modifications are characterized by a helical conformation. The 5 polymorph can only be prepared in the presence of solvent. Its exact crystal structure depends on the nature of the solvent. Compound formation between the 5 form of the polymer and the solvent has been demonstrated. Complete elimination of the solvent results in the pure, helical y form. 

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