Atactic chains

Atactic. A characteristic of the spatial configuration of atoms or groups in a polymer chain. Atactic indicates a random distribution- of those atoms or groups, i.e., no symmetry to the spatial configuration. This characteristic is important, for example, in determining the properties of polypropylene. 

CHCHj-) where all the methyl (CH3) substituents are on the same side are shown schematically in Figure 2-12. (The methyl carbon atoms are shown in red to make them easier to see.) We call this arrangement isotactic, so this polymer is isotactic polypropylene. Because of steric repulsions between CH3 groups on adjacent units, the chain would not want to sit in this planar zigzag shape or conformation, but would fold into a different shape by rotating the polymer backbone bonds. More on this when we discuss conformations. Also shown in Figures 2-13 and 2-14 are two more polypropylene chains. One of these consists of units that are all racemic to one another and is called syndio-tactic. The other has a random arrangement of units and we call such chains atactic. 

Synthetic polymers can be classified as either chain-growth polymers or step-growth polymers. Chain-growth polymers are prepared by chain-atactic, 1267 reaction polymerization of vinyl monomers in the presence of a radical, an... 

Polypropylene chains associate with one another because of attractive van der Waals forces. The extent of this association is relatively large for isotactic and syndiotactic polymers, because their stereoregularity permits efficient packing of the chains. Atactic polypropylene, on the other hand, does not associate as strongly and has a lower density and lower melting point than the stereoregular forms. The physical properties of stereoregular polypropylene are more useful for most purposes than those of atactic polypropylene. 

The global dimensions of the chains in the bulk state were not revealed by experiment until the 1970s. The small-angle neutron scattering (SANS) technique was a revolution in this context. Using SANS, it was possible to determine the radius of gyration (s) of deuterated chains dissolved in a matrix of normal protonated chains. Atactic polystyrene is a well-documented fully amorphous polymer. It exhibits a value of s /M = 0.275 A moP glassy... 

Theodorou, D.N., Muller-Plathe, F., and Milano, G. (2007) Coarse-grained and reverse-mapped united-atom simulations of long-chain atactic polystyrene melts structure, thermodynamic properties, chain conformation, and entanglements. Macromolecules, 40, 3876-3885. 

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). 

FIGURE 7 16 Poly mers of propene The mam chain IS shown in a zigzag conformation Every other carbon bears a methyl sub stituent and is a chirality center (a) All the methyl groups are on the same side of the carbon chain in isotactic polypropylene (b) Methyl groups alternate from one side to the other in syndiotactic polypropy lene (c) The spatial orienta tion of the methyl groups IS random in atactic polypropylene... 

In an atactic arrangement, substituents are in an unordered sequence along the polymer chains. 

Atactic. Substituents are distributed at random along the chain, for example, -DDLDLLLDLDLL-. 

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. 

Figure 1.2 Sections of polymer chains of differing tacticity (a) isotactic (b) syndiotactic (c) atactic.

Any of the four monomer residues can be arranged in a polymer chain in either head-to-head, head-to-tail, or tail-to-tail configurations. Each of the two head-to-tail vinyl forms can exist as syndiotactic or isotactic stmctures because of the presence of an asymmetric carbon atom (marked with an asterisk) in the monomer unit. Of course, the random mix of syndiotactic and isotactic, ie, atactic stmctures also exists. Of these possible stmctures, only... 

Homopolymerization of butadiene can proceed via 1,2- or 1,4-additions. The 1,4-addition produces the geometrically distinguishable trans or cis stmctures with internal double bonds on the polymer chains, 1,2-Addition, on the other hand, yields either atactic, isotactic, or syndiotactic polymer stmctures with pendent vinyl groups (Eig. 2). Commercial production of these polymers started in 1960 in the United States. Eirestone and Goodyear account for more than 60% of the current production capacity (see Elastomers, synthetic-polybutadiene). 

For the 1,2- and 3,4-addition, a chiral carbon (marked by an asterisk) is formed which has an R or 3 configuration, but there is no net optical activity, because equal amounts of the R and S configurations are formed. The R and S configurations along the polymer chains lead to diastereomeric isomers called isotactic, syndiotactic, and atactic. In isotactic polyisoprene all monomer units have the same configuration as illustrated for isotactic... 

Figure 4.6. Relationship between isotactic, syndiotactic and atactic forms in head-to-tail vinyl polymers. (For simplicity of comparison the main chain in each case is shown stretched in the planar...

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