Atactic polymers, chain conformation

Isotactic versus syndiotactic versus atactic sequences in a polymer chain Conformational nonuniformity given by rotation around single bonds Figure 1 Schematic description of possible nonuniformities in synthetic polymers. 

The struetural element of a eoumarone-indene resin is relatively similar to that for aromatie hydroearbon resins, as they differ only in the proportion of indene-type struetures which are present in higher eoneentration in the eoumarone-indene resins. The main monomers in the aromatie resins are styrene and indene. Styrene produces the atactic conformation of the resins, whereas indene introduees rigidity into the polymer chain. A typical structural element of an aromatie resin is given in Fig. 11. 

Although the definitions of isotactic, syndiotactic, and atactic polymers according to International Union of Pure and Applied Chemistry (IUPAC) rules are well established in terms of succession of mesa (m) or racemic (r) dyads,12 the symbolism of (+) and (—) bonds allows the easy treatments of possible configurations in cases of any complexity.1 Moreover, the (+) or (—) character of the bonds in a polymer chain is strictly related to the accessibility of gauche+ or gauche conformations of the bonds and, therefore, to the formation of right-handed or left-handed helical conformations.1... 

An atactic polymer can crystallize, for instance the case of polyacrylonitrile, whose crystallizability has been explained on the basis of local conformations, which produce extended and straight chains.140... 

This category includes such polymers as atactic polystyrene (25-291 or poly(vinylchloride) (30.31 and references therein). A closely related problem is the gelation of non-block copolymers (5), which share with atactic polymers the feature that chemically and conformationally homogeneous sequences may be relatively short, so that when two or more chains interact, large crystalline domains are prevented from forming. 

The ORD and CD curves of optically active polymers containing chromo-phoric groups show that the chromophores can be asymmetrically perturbed by the chirality of the substituents and of the main chain conformation. This is the case with poly( ec-butyl vinyl ketone) (377), which presents a Cotton effect at 292 nm, its intensity being greater in the prevalently isotactic polymer than in the atactic polymer. 

Commercial PAN is normally produced as an atactic polymer with strong hydrogen-bonded intermolecular forces. Because of repulsion between cyano pendant groups and intermolecular hydrogen bonds, the molecule assumes a crystallizable rodlike conformation. The hydrogen bonds between the rodlike chains create bundles of these chains. PAN may be spun into strong fibers. It has a Tg of 104 C... 

The configurational-conformational characteristics of PP are discussed by considering every polymer chain as constituted by the periodic repetition of a sequence of monomeric units in a given configuration. Calculations are presented for the special case in which mesa and racemic diads are distributed according to Bemoullian statistics. Numerical results show that the characteristic ratio of atactic PP reaches an asymptotic value of 5.34 when the size of the periodic sequence corresponds to six monomeric units. 

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.

Finally, we return to a practical problem, i. e., the chain conformation of stereo-specific polymers. Since Danusso and Moraglio (73), it has been repeatedly reported that the isotactic and atactic species of a given polymer show the same viscosity-molecular weight relationship but different second virial coefficients. This statement may be readily confirmed... 

Because atactic polymer has no ordered structure and shows only slight intramolecular interactions, the interactions between atactic polymers is the strongest (Fig. 10 a). The isotactic polymers may be stabilized by assuming the helix conformation reported for isotactic poly(methyl methacrylate)401. Nucleic add bases are situated outside the polymer chain so that they can form the complex, although the interaction is not so strong. On the other hand, the syndiotactic polymer may have a rod-like conformation that is supported by the low solubility of the polymer and by NMR spectra321. Tlierefoie, it is well understood that the complex formation ability of the syndiotactic polymers is very low. 

Considering again two adjacent carbons in the main chain of the polymer, six conformations are now possible because of the presence of an asymmetrically substituted carbon atom, as shown in Fig. 2.7. Forms 1 and 6 can be neglected for steric reasons so four different conformations are still possible for the polymer. Atactic polypropylene (see Stereoisomerism) has two trans forms (2 and 5) in the fully extended state (III)... 

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.). 

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