Chemical Structure on the Staudinger Index

The Staudinger index is a measure of the macromolecular dimensions. Thus, for flexible macromolecules, chain skeleton parameters (bond lengths, valence angles, degree of polymerization, mass of the monomeric unit), 

9 Determination of Molecular Weight, Molecular-Weight Distribution 

Polymers of higher tacticity have more rigid chains than atactic polymers, since more base units are fixed in conformational sequences of the coil, which corresponds formally to an increase in the steric hindrance parameter a. With a predominantly iso tactic poly(methyl methacrylate), for example, a X = 0.087 cm /g was obtained in heptanone-3 (0 = 40.0°C), whereas an at-PMMA in the same solvent gave only K = 0.063 cm /g (0 = 33.7°C). At higher temperatures, potential barriers to rotation can be more readily surmounted. The K values then become practically identical 0.057 for it-PMMA (0 = 152.rC) and 0.058 for at-PMMA (0 = 159.7°C) in p-cymene. 

The Staudinger index is a function of the hydrodynamic volume, and this itself depends on the mass, shape, and density of the particle. The density of the particle is dependent on the flexibility of the molecular chain and the solvation. The flexibility and solvation of rigid particles such as spheres, ellipsoids, and rods vary only slightly with temperature. Thus, the Staudinger index of these particles is practically temperature independent. 

With flexible macromolecules, the coil expands with increasing temperature, probably because solvation increases with temperature. The expansion occurs, however, only up to a limiting value that is given by the skeletal parameters and the optimum degree of solvation. At still higher temperatures, the flexibility of the chain increases because of a decrease 

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