Atactic polymers crystallinity

Polymers without configurational regularity are called atactic. Configurationally regular polymers can fonn crystalline stmctures, while atactic polymers are almost always amorjihous. Many polymers consist of linear molecules, however, nonlinear chain architectures are also important (figure C2.1.2). 

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. 

In order to generate stereoregular (usually isotactic) polymers, the polymerization is conducted at low temperatures ia nonpolar solvents. A variety of soluble initiators can produce isotactic polymers, but there are some initiators, eg, SnCl, that produce atactic polymers under isotactic conditions (26). The nature of the pendant group can influence tacticity for example, large, bulky groups are somewhat sensitive to solvent polarity and can promote more crystallinity (14,27). 

The regular syndiotactic and isotactic structures are capable of crystallisation whereas the atactic polymer carmot normally do so. In the case of polypropylene the isotactic material is a crystalline fibre-forming material. It is also an important thermoplastic which can withstand boiling water for prolonged periods. Atactic polypropylene is a dead amorphous material. Polystyrene as commonly encountered is atactic and glass-like but the syndiotactic material... 

Non-crystalline polymers are those which include high levels of irregularity within their structure. Typical sources of such irregularity are copolymerisation with significant amounts of at least two co-monomers and also complete absence of stereoregularity, i.e. atactic polymers. 

The approximately random placement of side groups in atactic polymers prevents them from developing regular structures. For this reason, atactic polymers are non-crystalline and behave as rubbers or glasses, depending on whether they are above or below their glass transition temperature. 

However Iring et al. [47] found that the induction period for oxidation of solid isotactic PP is longer than that for atactic polymer. Their result is in accordance with the high crystallinity of isotactic PP and with the resistance of its crystalline zones to oxidation. 

Syndiotactic and isotactic polymers are much more crystalline than atactic polymers. 

As polystyrene obtained by free radical polymerisation technique is atactic it is therefore non-crystalline. The isotactic polystyrene is obtained by the use of Ziegler-Natta catalysts and n-butyl lithium. Isotactic polystyrene is having a high crystalline Melting point of 250°C. It is transparent. It is more brittle than the atactic polymer. 

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 amount of amorphous polymer, which is generally produced in small percentage (9-16%) contemporaneously with the non-atactic polymer, is independent of reaction time (see Table II). It is on the contrary closely connected with the nature of the catalytic system employed and changes, for instance, when the triethylaluminum is substituted by other metal alkyls (beryllium alkyls, propylaluminum, isobutylaluminum, etc.) 5,28). It also depends on the purity of the a-titanium trichloride, in particular increasing in the presence of other crystalline modifications of titanium trichloride [i.e. -TiCU (27)] and of titanium compounds obtained by reduction of titanium tetrachloride at low temperature with aluminum alkyls. 

Although atactic polymers with bulky pendant groups are usually amorphous, atactic polymers with small pendant groups, such as polyvinyl alcohol (PVA), may be made to be crystalline. Some polymers, such as PVC, may have long sequences of syndiotactic sequences in addition to atactic sequences, and hence are somewhat crystalline. 

Tsou, Magee and Malatesta (39) showed the effect of catalyst ratios on steric control m the polymerization of styrene with alkyllithium and titanium tetrachloride. These authors have shown that the isotactic polymer was produced when the butyllithium to titanium ratio was kept within the limits of 3.0 to 1.75. Outside of this critical range, amorphous polymers were produced. In the discussion of this paper, Friedlander (40) pointed out the cationic nature of the low-lithium-to-titanium-ratio-catalysts which also produced considerable rearrangement of the phenyl groups. Above 2.70 lithium to titanium ratio, an anionic type polymerization set in, which produced atactic polymer. At low ratios cationic catalysis also produced atactic polymer. Tsou and co-workers concluded that crystallinity of the catalyst is not important for steric order in the polymer. 

Stereochemistry also affects the crystallinity of a polymer. Stereoregular isotactic and syndiotactic polymers are generally more crystalline than atactic polymers. By careful choice of catalysts, we can make a linear polymer with either isotactic or syndiotactic stereochemistry. 

Neither PVC nor polystyrene is very crystalline and polystyrene often has poor mechanical strength. Both of these maybe 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 diastereoisomers 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 syn-diotactic (where they alternate) polymers. 

---