Steric Hindrance and Rotational Isomers

In this section, additional details on the conformations of macromolecules are given to further the understanding of flexible molecules. Details on computer simulation are presented to evaluate mobility. Increasingly stiff molecules are described to make the link to rigid macromolecules. 

Internal and Sterlc Hindrance on Rotation about a Cyllndrlcally Symmetric Covalent Bond 

Additional contributions to this expression can arise from electric charges and dipoles. The dichloroethane in Fig. 1.35 would need, because of the strongly polar C-Cl bonds an extra term accounting for the repulsion between parallel dipoles. 

The number of possible shapes a polyethylene molecule can now be calculated easily by assuming that each bond must be in one of these three rotational positions. Then the statistics of the molecule is based on the conformational entropy per rotatable bond of this type, k In 3, where k is Boltzmann s constant (see Sect. 2.2.4 and Fig. A.5.4). Per mole of bonds k is replaced by R, the gas constant. For a molecule with n rotatable bonds, Scontamation = n x 9.2 J K mol. The calculation at the bottom of Fig. 1.37 reveals that the different conformations possible for a macromolecule with 20,000 flexible backbone bonds and three rotational isomers per bond are more than astronomical. This large amount of disorder (entropy) is at the root of the special behavior of polymers. It permits melting of the macromolecule without breaking into small parts, and is the reason for entropy elasticity (viscoelasticity and 

---