Chain configuration fully extended form

Polymers formed from monosubstituted ethylenes can exist in three configurations isotactic, syndiotactic, and atactic. An isotactic polymer has all of its substituents on the same side of the fully extended carbon chain. (Iso and taxis are Greek for the same and order, respectively.) In a syndiotactic polymer (syndic means alternating ), the substituents regularly alternate on both sides of the carbon chain. The substituents in an atactic polymer are randomly oriented. 

The application of these structural principles and the use of accurate values for interatomic distances and bond angles permitted the exact description of several possible configuration of the polypeptide chain, the alpha helix and the two pleated sheets. In particular, it was found that acceptable sheet structures of polypeptide chains could not be formed by fully extended polypeptide chains instead, the chains need to be contracted somewhat, and stiffened in the direction perpendicular to the fiber axis and the material hydrogen bonds. The predicted length of the two-residue unit of a completely extended polypeptide chain is 7.23A, that for the antiparallel chain pleated sheet is 7.00A, and that for the parallel chain pleated sheet is 6.6A. 

Once a polymer molecule has been formed, its configuration is fixed. However, it can take on an infinite number of shapes by rotation about the backbone bonds. The final shape that the molecule takes depends on the intramolecular and intermolecular forces, which, in turn, depend on the state of the system. For example, polymer molecules in dilute solution, melt phase, or solid phase would each experience different forces. The conformation of the entire molecule is first considered for semicrystalline solid polymers. Probably the simplest example is the conformation assumed by polyethylene chains in their crystalline lattice (planar zigzag), as illustrated in Fig. 1.4. A polymer molecule cannot be expected to be fully extended, and it actually assumes a chain-folded conformation, as... 

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