Chain , atactic syndiotactic

The NMR spectrum of the polymer 14.16 shows two singlets of approximately equal intensities that differ in chemical shift by only 0.17 ppm. This observation is attributed to the formation of equal amounts of isotactic (14.16a, both S=0 groups on the same side of the polymer chain) and syndiotactic (14.16b, S=0 groups on opposite sides of the polymer chain) forms of the polymer. The atacticity is confirmed by the H and NMR spectra. Two resonances are observed for the McaP groups of 14.16a, whereas 14.16b gives rise to only one resonance. 

Alkene polymerization can be carried out in a controlled manner using a Ziegler-Natta catalyst. Ziegler-Natta polymerization minimizes the amount of chain branching in the polymer and leads to stereoregular chains—either isotactic (substituents on the same side of the chain) or syndiotactic (substituents on alternate sides of the chain), rather than atactic (substituents randomly disposed). 

Neither PVC nor polystyrene is very crystalline and polystyrene often has poor mechanical strength. Both of these may be results of the stereorandom nature of the polymerization process. The substituents (Cl or Ph) are randomly to one side or other of the polymer chain and so the polymer is a mixture of many dlastereolsomers as well as having a range of chain lengths. Such polymers are called atactic. In some polymerizations, it is possible to control stereochemistry, giving (instead of atactic polymers) isotactic (where all substituents are on the same side of the zig-zag chain) or syndiotactic (where they alternate) polymers. 

If polymers of a-alkenes are regarded as infinite chains in their most symmetrical zigzag conformation, only atactic polymers can be chiral. Infinite isotactic polymers have a mirror plane along their chain and numerous mirror planes perpendicular to the chain. Infinite syndiotactic chains of ac-alkene polymers contain mirror planes in all tertiary carbon centers perpendicular to the chain. For convenience, the model of infinite polymer chains can be replaced by analogous cyclic systems (cyclopropanes for triades, cyclobutanes for tetrades, etc.). 

However, the most important defects which have an influence on the physical properties of the amorphous polyvinyls, are connected with the chirality of the unit. In fact, in (1.1.4), the carbon bearing the radical is an asymmetric carbon. Let us consider the skeleton of the polyvinylic chain in its configuration t, t, t, and so on (see Fig. 1.5). The side-groups R may indifferently appear on either side of the skeleton plane. The setting of the Rs with respect to this plane is called tacticity. If the Rs are always in the same side, the chain is isotactic if the Rs alternate, the chain is syndiotactic if the Rs are disordered, the chain is atactic. The polyvinyl samples whose physical properties are studied in this book are atactic. 

Figure 1-30 illustrates the three cases that can occur 1) isotactic, where all the R groups line up on one side of the chain (2) syndiotactic, where R s and H s alternate in some regular fashion and (3) atactic, where R and H positioning is completely random. In the foregoing cases, isotactic is the most crystalline, atactic is essentially amorphous, and syndiotactic somewhere between the other two. 

Figure 9 (a) Isotactic vinyl chain, (b) Syndiotactic chain, (c) Vinyl polymer chain with isotactic blocks, (d) Atactic vinyl chain. 

Polypropylene chains associate with one another because of attractive van der Waals forces. The extent of this association is relatively large for isotactic and syndiotactic polymers, because their stereoregularity permits efficient packing of the chains. Atactic polypropylene, on the other hand, does not associate as strongly and has a lower density and lower melting point than the stereoregular forms. The physical properties of stereoregular polypropylene are more useful for most purposes than those of atactic polypropylene. 

A model assuming that Cp substituents distal to the bridge experience steric non-bonded contacts with the monomer methyl group, perhaps mediated by the chain end, accounts for the specificity of the chiral metallocenes that produce isotactic, atactic, syndiotactic, hemiisotactic, and random or block cotactic polypropylenes. The tacticities as well as the microstructures of these polymers are accomodated by these simple concepts, the geometry of the metallocene ligands, and by generally accepted fundamental aspects of the polymerization and stereochemical control mechanisms. 

FIGURE 7 16 Poly mers of propene The mam chain IS shown in a zigzag conformation Every other carbon bears a methyl sub stituent and is a chirality center (a) All the methyl groups are on the same side of the carbon chain in isotactic polypropylene (b) Methyl groups alternate from one side to the other in syndiotactic polypropy lene (c) The spatial orienta tion of the methyl groups IS random in atactic polypropylene... 

Polymers of different tacticity have quite different properties, especially in the solid state. One of the requirements for polymer crystallinity is a high degree of microstructural regularity to enable the chains to pack in an orderly manner. Thus atactic polypropylene is a soft, tacky substance, whereas both isotactic and syndiotactic polypropylenes are highly crystalline. 

Figure 1.2 Sections of polymer chains of differing tacticity (a) isotactic (b) syndiotactic (c) atactic.

Any of the four monomer residues can be arranged in a polymer chain in either head-to-head, head-to-tail, or tail-to-tail configurations. Each of the two head-to-tail vinyl forms can exist as syndiotactic or isotactic stmctures because of the presence of an asymmetric carbon atom (marked with an asterisk) in the monomer unit. Of course, the random mix of syndiotactic and isotactic, ie, atactic stmctures also exists. Of these possible stmctures, only... 

---