Atactic molecules

The steric configuration is extremely important in the polymer. Only isotactic polypropylene (iPP) has the properties necessary for forming fibers. The molecules are cross-linked only by Van der Waals forces, so it is important that they pack as closely as possible. The isotactic molecules form a 3, helix, as shown in Fig. 12.21,16 and exhibit a high crystallization rate. The atactic molecules, shown in the figure, do not pack well, and although the syndiotactic molecules can pack better and crystallize, this configuration is not a normal product of commonly used catalyst systems. 

Propylene, CH2=CHCH3, is obtained as a by-product of ethylene production from the cracking of petroleum fractions. Free radical polymerization yields only low-molecular-weight oils consisting of branched, atactic molecules. Isotactic poly (propylenes) were first made possible and came into commercial use through Natta s work with Ziegler catalysts. 

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). 

As with MAH, the extent of grafting varies dramatically vi ith the polyolefin substrate. Some differences have been attributed to variations in the type and amount of stabilizers present in the polyolefins substrate.326 In the case of isotactic PP, the maximum graft levels attained with 27 were found to correspond to only one unit of DEM per PP molecule 5 0 This would support a mechanism whereby grafts appear only at the chain ends. Higher graft levels were obtained with atactic PP. The higher reactivity of the atactic PP (and atactic sequences in... 

The majority of commercial polystyrene molecules consist of a backbone of carbon atoms with phenyl groups attached to half of the carbon atoms, as shown in Fig. 21.1. Free radical initiator residues terminate each end of the chain. Minor variants include chains terminated by anionic or cationic initiator residues. All commercial polystyrene products are atactic that is, the placement of the phenyl groups on either side of the chain is essentially random, as illustrated in Fig. 21.2. 

Commercially available polystyrene is mostly of the atactic variety and, hence, is amorphous in nature. Polystyrene consists generally of linear molecules and is chemically inert. Acids, alkalis, oxidising or reducing agents have little effect of it. 

Like polypropylene, PVC has the problem of stereospecificity. The carbon atom to which the chlorine atom is attached is asymmetrical. (See Figure 23-8.) As a result, PVC molecules can be iso tactic, syndiotactic, and atactic. Commercial PVC is only 5—10% crystalline—low percent isotactic. It is more dense, 1.3 to 1..8 g/cc, than the polyolefins, (fe Figure 23—9.)... 

Stereochemistry. The field of organic chemistry devoted, to three-dimensional spatial arrangements of molecules. Deals with stereoisomers, compounds having identical chemical formulas but different spatial arrangement of their atoms, such as geometric (cis/trans) isomers and optical (isotactic, atactic, and syndiotactic) isomers. 

An isotactic polymer has only one species of configurational base unit in a single sequential arrangement and a syndiotactic polymer shows an alternation of configurational base units that are enantiomeric, whereas in an atactic polymer the molecules have equal numbers of the possible configurational base units in a random sequence distribution. This can be generalized as follows in zig-zag and horizontal Fischer projections ... 

There are two types of Cs-symmetric metallocenes, XXX and XXXI (Table 8-5). Both types contain a mirror plane of symmetry—a horizontal plane in XXX, a vertical plane in XXXI. Both are achiral molecules, but they differ very significantly in stereoselectivity. XXX produces atactic polymer, while XXXI usually forms syndiotactic polymer. 

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