Macromolecular polydispersity

D.E. Koppel, Analysis of macromolecular polydispersity in intensity correlation spectroscopy method of cumulants, J. Chem. Phys. 57(1972) 4814-4820. 

A very wide distribution of molecular weights or hydrodynamic coil sizes are usually found within each EOR polymer solution. Because of this polydispersity, the molecular weights or hydrodynamic sizes determined by viscosity measurements are only averages. No information on macromolecular polydispersity is obtained from viscosity measurements. However, characterization of polydispersity is important because EOR polymers which have molecular weight or coil size distributions containing a large proportion of smaller... 

Koppel, D. E., Analysis of Macromolecular Polydispersity in Intensity Correlation Spectroscopy the Method ofCumulants, J. Chem. Phys. 1972, 57, 4814-4820. 

For a polydisperse solution—the hallmark of solutions of polysaccharides—s (and s ) will be a weight average [30,38,39]. If the solution contains more than one discrete (macromolecular) species—e.g. a mixture of different polysaccharides, the polydispersity will be manifested by asymmetry in the sedimenting boundary or, if the species have significantly different values for S2o,w. discrete boundaries are resolved (Fig. 2b [29]). 

Direct inversions of the concentration distribution profiles to obtain molecular weight distribution information are generally impossible because of comphcations involving non-ideality. Successful attempts have been given but only for simple discrete forms of polydispersity (two to three macromolecular species [93]). 

Dextrans are also attractive as macromolecular carriers of paramagnetic chelates because of their hydrophilicity, the different available molecular weights with narrow polydispersity, and the versatility of activation methods applicable. Several DTPA- or DOTA-loaded carboxymethyl dextran (CMD) derivatives have been prepared and tested in blood pool MRI.136-139 The relaxivities reported for these compounds are, however, relatively moderate. 

The fundamental difficulty is that polymeric substances cannot be obtained in a structurally and molecularly uniform state, unlike low-molecular-weight compounds. Thus, macromolecular materials of the same analytical composition may differ not only in their structure and configuration (see Sect. 1.2) but also in molecular size and molecular weight distribution they are polydisperse, i.e., they consist of mixtures of molecules of different size. Hence, it is understandable that the expression identical is not, in practice, applicable to macromolecules. Up to the present time, there is no possibility of preparing macromolecules of absolutely uniform structure and size. It follows, therefore, that physical measurements on polymers can only yield average values. The afore-... 

Other important pitfalls lie again in the low selectivity of SEC, which does not allow identifying small amounts of the macromolecular admixtures that is the minor components of polymer blends. The bell-shaped chromatograms with a broad base and a slim upper part are often erroneously proclaimed to signalize the narrow molar mass distribution of sample. On the other hand, the accumulation peaks due to presence of macromolecules excluded from the packing pores (Section 16.8.1) are interpreted as the sign of sample bimodality. The absolute detectors may also contribute to erroneous conclusions concerning sample polydispersity (Section 16.8.1). 

It is seen that free radical micromolecular or macromolecular initiators have been successfully employed for the synthesis of di-, tri- or multiblock copolymers. However, once again, the structure of these block copolymers depends upon the termination step of the polymerization, and especially on the recombination or disproportionation of macroradicals produced. Besides, such a method also generates homopolymers. Separation and purification of these different structures are usually very difficult or even impossible. Moreover, the copolymers obtained usually exhibit a broad polydispersity, a defect inherent in the classical radical process. 

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