Melting 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. 

Commercial polystyrene, one of the most representative general-purpose resins, is an amorphous atactic polymer with no melting point (softening temperature ca 100 °C, glass transition temperature ca 70-100 °C). Since the discovery of Ziegler-Natta catalysts, many efforts to produce stereoregular polystyrenes have been made. 

Because of its ordered structure, an isotactic polymer has higher melting point and tensile strength than the atactic polymer. The melting points of isotactic polymers are typically between 165 and 171°C, while those of atactic polymers are less than 0°C. 

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. 

Because of their orderly arrangements, the chains of the tactic polymers (syndiotactic and isotactic) can lie closer together and the polymers are partially crystalline, whereas atactic polymers are amorphous and soft indicating the absence of all crystalline order. Isotactic pol q)ropy-lene is highly crystalline with a melting point of 160°C, whereas the atactic isomer is an amorphous (noncrystalline) soft polymer with a melting... 

It was noted in Section 1.1.3 that, when one moves to polymers more complex than polyethylene, the likelihood of the polymer being able to crystallize depends on the chemical composition, in particular whether the repeat unit has an asymmetric centre. When it does, then the ability to crystallize rapidly diminishes with the amount of atactic material in the polymer. Fully atactic polymers will generally be amorphous and the properties of the glass resulting from the cooling of an atactic polymer from the melt are discussed in the following section. 

The second example is the stereoregularity displayed by monosubstituted vinyl polymers of olefins. As we saw earlier, these types of polymers can occur in three forms of tacticity isotactic, syndiotactic, and atactic. Isotactic and syndiotactic polymers possess stereoregular structures. Generally these polymers are rigid, crystallizable, high melting, and relatively insoluble. On the other hand, atactic polymers are soft, low melting, easily soluble, and amorphous. 

As discussed above, there have been major improvements in catalyst efficiency and selectivity. The amount of atactic polymer has been reduced, and the number of pounds of polymer produced per pound of catalyst has been increased from five- to tenfold and more. Polymerization in the gas phase has been improved [73] in order that resin with low atactic content can be produced without solvent removal or polymer washing. The new processes [74] have reduced and, in some cases, eliminated the purification and solvent recovery steps, thereby simplifying the polymerization process and reducing the cost of a polymer plant. In addition, the new processes yield polymers with reduced catalyst residues and fewer gels, resulting in better filterability and improved pack life during fiber melt spinning. 

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