Isotactic and syndiotactic configurations

A third configuration known as 7, 2 vinyl configuration, is also possible. In the case of 1, 2 vinyl configuration, the three types of optical isomerism, i.e., atactic, isotactic and syndiotactic configurations, are possible. 

Although the isotactic and syndiotactic configurations are regular and allow crystallization of the polymer chain to occur, most PVC is atactic, and the regularity necessary for chains to pack and therefore crystallize does not exist. Thus, PVC is not generally classified as a crystalline polymer. 

Isotactic and syndiotactic configurations having specific chain interactions between atoms highly favour the crystal structure. 

By conventional polymerization methods, polyacrylonitrile forms both isotactic and syndiotactic configurations in approximately equal proportion. However, primarily the isotactic polyacrylonitrile is formed in the polsonerization. 

Atactic polystyrene, as an amorphous material, has been known for centuries. In principle, polystyrene can occur with atactic, isotactic, and syndiotactic configurations. With the extensive studies concerning the stereoselective polymerization of olefins by Ziegler-Natta catalyst systems discovered... 

What is significant about these reactions is that only two possibilities exist addition with the same configuration (D -> DD or L LL) or addition with th< opposite configuration (D DL or L LD). We shall designate these isotactic (subscript i) or syndiotactic (subscript s) additions, respectively, and shal define the rate constants for the two steps kj and k. Therefore the rates o isotactic and syndiotactic propagation become... 

The synthesis of isotactic and syndiotactic polymers has been achieved for a number of polymers. For example poly (methyl methacrylate) can be prepared in either isotactic or syndiotactic configurations depending on the details of the polymerisation conditions. 

All the possible line repetition groups for cis and trans poly dienes compatible with the isotactic or syndiotactic configurations are reported in Figure 2.15,47,68 In order to consider only the possible conformations assumed in the crystalline state, the torsion angle of the central single bond is assumed to be 180° trans) in both the cis and trans polydienes. This condition produces conformations sufficiently extended to be packed in a crystalline lattice for each value of the torsion angles 0i and 02 (Figure 2.15). 

When A A or B B, the symmetry is lower and the only possible line repetition groups are s(M/N) and tc for isotactic and syndiotactic polymers, respectively, in both cis and trans configurations. In these cases, two independent torsion angles in the main chain define the regular conformation (Oi and 02 in Figure 2.15). 

Figure 29-8 Configuration of atactic, isotactic, and syndiotactic poly-propene. These configurations are drawn here to show the stereochemical relationships of the substituent groups and are not meant to represent necessarily the stable conformations of the polymer chains.

Figure 2-2 Polymer chain configurations of atactic, isotactic and syndiotactic polymers.

Isotactic and syndiotactic polymers will not have the same mechanical properties, because the different configurations affect the crystal structures of the polymers. Most highly stereoregular polymers of current importance are isotactic. 

Geometric isomerism. When there are unsaturated sites along a polymer chain, several different isomeric forms are possible. As illustrated in Fig. 14.14, conjugated dienes such as isoprene and chloroprene can be polymerized to give either 1,2-, 3,4, or 1,4-polymer. In the case of 1,4-polymers, both cis and trans configurations are possible. Also, stereoregular (i.e., isotactic and syndiotactic) polybutadienes can be produced in case of 1,2- and 3,4-polymerization. 

Section 7.15 Certain polymers such as polypropylene contain stereogenic centers, and the relative configurations of these centers affect the physical properties of the polymers. Like substituents appear on the same side of a zigzag carbon chain in an isotactic polymer, alternate along the chain in a syndiotactic polymer, and appear in a random manner in an atactic polymer. Isotactic and syndiotactic polymers are referred to as stereoregular polymers. 

Monosubstituted vinyl monomers can produce polymers with different configurations—two regular structures (isotactic and syndiotactic) and a random (atactic) form. For polymers with the same chemical makeup, those forms derived from regular structures exhibit greater rigidity, are higher melting, and are less soluble relative to the atactic form. 

Important information on the structure of substances of interest is provided by the yield and distribution of dimeric compounds, as their structure is representative of the addition of monomeric and comonomeric structural units in the polymer [257-261]. Dimbat [262] succeeded in applying Py-GC to determine the isotacticity and length of isotactic and syndiotactic blocks in polypropylene. This technique provides for the use of several calculation methods and does not require any calibration. It is based on the fact that the configuration of the pyrolysis products which contain asymmetric carbon atoms is different depending on whether they are formed from blocks (iso- or syndiotactic) or from block joints. 

Linear polymers made of symmetrical unsaturated monomers, such as polyethylene and polyvinylidene chloride, crystallize easily. Asymmetric linear polymers such as polypropylene (PP) crystallize only if the configuration is regular, so isotactic and syndiotactic PP can crystallize, while atactic PP is amorphous. 

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